SEARCH

SEARCH BY CITATION

Keywords:

  • microtia;
  • anotia;
  • craniofacial development;
  • craniofacial microsomia;
  • hemifacial microsomia;
  • OAVS (oculo-auriculo-vertebral spectrum)

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EMBRYOLOGY: OUTER EAR DEVELOPMENT
  5. GENETICS OF MICROTIA
  6. CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS
  7. FUTURE DIRECTIONS
  8. CONCLUSIONS
  9. REFERENCES

Microtia is a congenital anomaly of the ear that ranges in severity from mild structural abnormalities to complete absence of the ear, and can occur as an isolated birth defect or as part of a spectrum of anomalies or a syndrome. Microtia is often associated with hearing loss and patients typically require treatment for hearing impairment and surgical ear reconstruction. The reported prevalence varies among regions, from 0.83 to 17.4 per 10,000 births, and the prevalence is considered to be higher in Hispanics, Asians, Native Americans, and Andeans. The etiology of microtia and the cause of this wide variability in prevalence are poorly understood. Strong evidence supports the role of environmental and genetic causes for microtia. Although some studies have identified candidate genetic variants for microtia, no causal genetic mutation has been confirmed. The application of novel strategies in developmental biology and genetics has facilitated elucidation of mechanisms controlling craniofacial development. In this paper we review current knowledge of the epidemiology and genetics of microtia, including potential candidate genes supported by evidence from human syndromes and animal models. We also discuss the possible etiopathogenesis in light of the hypotheses formulated to date: Neural crest cells disturbance, vascular disruption, and altitude. © 2011 Wiley Periodicals, Inc.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EMBRYOLOGY: OUTER EAR DEVELOPMENT
  5. GENETICS OF MICROTIA
  6. CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS
  7. FUTURE DIRECTIONS
  8. CONCLUSIONS
  9. REFERENCES

The vertebrate ear is divided into the outer, middle, and inner ear. This review focuses on malformations of the external ear, and more specifically of the auricles, which are collectively termed microtia. However, other components of the external (acoustic meatus and tympanic membrane), middle, and inner ear are also frequently impacted, as are other craniofacial and extra cranial anomalies.

Microtia encompasses a spectrum of congenital anomalies of the auricle that range in severity from mild structural abnormalities to complete absence of the ear (anotia) [Carey et al., 2006]. There is no consensus regarding the terminology used for these external ear malformations. Some authors prefer to use the term “microtia” [Castilla and Orioli, 1986; Suutarla et al., 2007; Alasti and Van Camp, 2009; Hunter et al., 2009a] while others use “microtia-anotia” or “microtia/anotia” [Mastroiacovo et al., 1995; Harris et al., 1996; Shaw et al., 2004; Forrester and Merz, 2005; Stevenson, 2006; Canfield et al., 2009]. In this paper, the term “microtia” includes anotia as the most severe end of the microtia spectrum.

The occurrence of microtia is of public health importance in part due to the psychosocial sequelae, including the stigma associated with malformations of the ear and the burden of undergoing multiple surgeries [Du et al., 2007; Li et al., 2010; Steffen et al., 2010]. In addition, greater than 90% of individuals with microtia experience conductive hearing loss on the affected side [Bassila and Goldberg, 1989; Calzolari et al., 1999; Carey et al., 2006; Ishimoto et al., 2007; Suutarla et al., 2007]. Although there has been no recent review of the average medical cost associated with treatment of microtia and the associated health issues, the costs are expected to be considerable given that hearing impairment care and multiple surgical procedures for ear reconstruction are often necessary.

Microtia occurs more frequently in males, with an estimated 20–40% increased risk compared to females. Microtia can occur bilaterally, although 77–93% of affected individuals have unilateral involvement [Nelson and Berry, 1984; Castilla and Orioli, 1986; Mastroiacovo et al., 1995; Shaw et al., 2004; Forrester and Merz, 2005; Suutarla et al., 2007; Canfield et al., 2009]. The right ear is affected in approximately 60% of individuals with unilateral microtia [Castilla and Orioli, 1986; Mastroiacovo et al., 1995; Harris et al., 1996; Forrester and Merz, 2005; Suutarla et al., 2007; González-Andrade et al., 2010]. A higher proportion of bilateral microtia is found in cases with associated anomalies not directly related to the ear abnormality [Mastroiacovo et al., 1995; Harris et al., 1996; Shaw et al., 2004; Canfield et al., 2009]. Microtia may occur as an isolated condition, or as part of a spectrum of anomalies or a syndrome. The most common anomalies associated with microtia include: Vertebral anomalies, macrostomia, oral clefts, facial asymmetry, renal abnormalities, cardiac defects, microphthalmia, holoprosencephaly, and polydactyly [Kaye et al., 1989; Mastroiacovo et al., 1995; Harris et al., 1996; Carey et al., 2006]. Most of these anomalies are also associated with oculo-auriculo-vertebral spectrum (OAVS), a condition notable for wide clinical variability and for which the etiologies remain unknown.

Existing data indicate that Mendelian inheritance is more likely in syndromic and familial cases of microtia, whereas multifactorial or polygenic causes are more probable in sporadic cases. Several non-genetic factors have been consistently associated with microtia. Although no genes have been associated with isolated microtia, a number of genes have now been identified on syndromes associated with microtia. The purpose of this paper is to review the current literature regarding the genetics and epidemiology of microtia, and discuss the etiological and pathogenetic mechanisms proposed for this condition.

Prevalence

Population-based studies on microtia prevalence conducted in Italy, France, Sweden, Finland, and United States show prevalence rates ranging between 0.83 and 4.34 per 10,000 births [Harris et al., 1996; Shaw et al., 2004; Forrester and Merz, 2005; Suutarla et al., 2007; Canfield et al., 2009] (Table I). This wide range in prevalence may be due to variation among the studies in case inclusion criteria and case ascertainment. Microtia is an external anomaly that can be identified on physical examination of newborns; however, the less severe forms of microtia may not be recognized or described as a form of microtia in medical records or the term may be used for deformities of the ears. This could lead to under- or over-reporting of microtia in reports of prevalence.

Table I. Prevalence (per 10,000) of Microtia Reported in the Literature From 1960 to 2010
StudyAge of ascertainmentPrevalence microtia-anotiaPrevalence anotiaPrevalence microtiaTypes of microtia includedaRefs.
  • LB, Livebirth; SB, Stillbirth; ETOP, Elective termination of pregnancy; nr, not reported.

  • a

    According to Marx Classification.

  • b

    Argentina, Brazil, Chile, Ecuador, Peru, Uruguay, and Venezuela.

  • c

    Excluded cases with known chromosomal anomalies.

Population-based
 Central-East FranceLB + SB0.80.40.4I–IV

Harris et al. [1996

] c
 California (USA)LB + SB2.00.21.8I–IV

Harris et al. [1996

] c
 SwedenLB + SB2.40.22.1I–IV

Harris et al. [1996

] c
 California (USA)LB + SB2.2nrnrnr

Shaw et al. [2004

] c
 Hawaii (USA)LB + SB + ETOP3.80.33.5II–IV

Forrester and Merz [2005

]
 FinlandLB + SB4.30.24.1I–IV

Suutarla et al. [2007

]
 Texas (USA)LB + SB + ETOP2.8nrnrnr

Husain et al. [2008

]
 Texas (USA)LB + SB + ETOP2.90.22.7II–IV

Canfield et al. [2009

]
Hospital-based
 Navajo (USA)>21 years old9.7nrnrnr

Jaffe [1969

]
 New Mexico (USA)Any age1.3nrnrI–IV

Aase and Tegtmeier [1977

]
 Navajo (USA)4–14 years old12.0nrnrnr

Nelson and Berry [1984

]
 South AmericabLB3.2nrnrI–IVCastilla et al. [1986]
 ItalyLB + SB1.50.31.2I–IV

Mastroiacovo et al. [1995

]
 ChinaLB + SB1.4nrnrnr

Zhu et al. [2000

]
 ChileLB + SB8.80.58.3I–IV

Nazer et al. [2006

]

Population-based studies performed in the United States, however, consistently report variations in prevalence according to race/ethnicity, with a higher risk for individuals of Asian heritage [Harris et al., 1996; Shaw et al., 2004; Forrester and Merz, 2005], Pacific Islanders [Forrester and Merz, 2005], and individuals of Hispanic descent [Harris et al., 1996; Shaw et al., 2004; Yang et al., 2004; Canfield et al., 2009] when compared to Caucasians and African-Americans. Studies conducted using non-population-based data reported higher prevalence for Ecuadorians [Castilla and Orioli, 1986; González-Andrade et al., 2010], Chileans, and among Native Americans in the US [Jaffe, 1969; Aase and Tegtmeier, 1977; Nelson and Berry, 1984]. More comprehensive studies are required to investigate the racial/ethnic differences in prevalence of microtia and the etiology of this variability. For instance, the prevalence of microtia is three to eight times higher in Chile and Ecuador than previously reported worldwide, which may be at least in part due to genetic variation, environmental factors (such as diet) or a combination of gene-environment interactions.

Human Disorders With Microtia

Approximately 20–60% of children with microtia have associated anomalies or an identifiable syndrome (Table II) [Castilla and Orioli, 1986; Kaye et al., 1989; Mastroiacovo et al., 1995; Shaw et al., 2004]; therefore, individuals with microtia should be examined for other dysmorphic features. Microtia is a common feature of craniofacial microsomia, Townes–Brocks syndrome and the mandibulofacial dysostoses (e.g., Treacher Collins and Nager syndrome) and these conditions should be considered among the differential diagnosis when evaluating an individual with microtia.

Table II. Human Disorders With Microtia (Except Chromosomopathies)
SyndromeMicrotia (%)aGene(s) identified
  • a

    Gorlin et al. [2001].

  • b

    Hemizigozity: Only inner ear anomalies.

Bixler (Hypertelorism-microtia-clefting)100
Bosley–Salih–Alorainy33HOXA1
Branchiooculofacial (BOF)20TFAP2A
Branchiootic (BO)80–90EYA1, SIX1
Branchiootorenal (BOR)30–60EYA1, SIX5
CHARGEReportedCHD7, SEMA3E
Congenital deafness, inner ear agenesis, microtia, microdontia100FGF3b
Craniofacial microsomia (CFM)65
FraserReportedFRAS1, FREM2
Kabuki80MLL2
Klippel–FeilReportedGDF6
Lacrimoauriculodentodigital20FGFR2, FGFR3, FGF10
Mandibulofacial dysostosis100HOXD
Meier–Gorlin (Ear-patella-short stature)100ORC1, ORC4, ORC6, CDT1, CDC6
Microtia, hearing impairment, and cleft palate100HOXA2
Miller100DHODH
Nager80
Oculo-auricular100HMX1
Pallister HallReportedGLI3
Townes–Brocks20SALL1
Treacher Collins60–80TCOF1
Wildervanck (Cervicooculoacoustic)Reported

OAVS is characterized by facial asymmetry, microtia, ear and facial tags, epibulbar dermoids, microphthalmia, and macrostomia [Heike and Hing, 2009]. Craniofacial, or hemifacial, microsomia, and Goldenhar syndrome are included in this spectrum. Extracranial features include renal, cardiac, and vertebral anomalies. There is no agreement about minimal diagnostic criteria for OAVS. Most cases of OAVS are sporadic; however, autosomal dominant or, less commonly, recessive inheritance have been reported.

Microtia and OAVS share the following characteristics: (1) variable phenotypic expression, (2) asymmetric involvement of facial structures, (3) right side preponderance, (4) male predilection, and (5) familial occurrence of microtia or related anomalies such as preauricular tags and pits. Based on these observations, it has been suggested that isolated microtia represents a milder phenotype of OAVS [Rollnick and Kaye, 1983; Llano-Rivas et al., 1999; Tasse et al., 2005]. This has led to the controversial concept that most (or all) cases presenting with apparent isolated microtia are actually cases of OAVS. This controversy remains unsettled. In many cases, the occurrence of microtia associated with chromosomal abnormalities and in single gene disorders supports a complex genetic regulatory network coordinating morphogenesis of the external ear. Therefore, although the clinical expression of microtia and OAVS overlap and likely share many common genetic mechanisms, each should be considered as a separate entity. In this review, we cite the literature referring to microtia as a separate condition from OAVS.

Known Risk Factors

To date there have been few published case-control studies on microtia [Castilla and Orioli, 1986; Mastroiacovo et al., 1995; Correa et al., 2008; Zhang et al., 2009; Ma et al., 2010]. The risk factors that were identified in these studies include low birth weight, higher maternal parity, maternal acute illness, and use of medications (specific acute maternal conditions or medications were not identified in these studies), and maternal diabetes mellitus. Multiple births, advanced maternal age, low maternal education, and Hispanic ethnicity have also been reported as risk factors for microtia in cross-sectional, population-based studies. More recently, periconceptional intake of folic acid-containing supplements has been associated with reduced risk of microtia among non-obese women [Ma et al., 2010]. A summary of the risk factors reported in the literature, in case-control and cross-sectional studies, is presented in Table III.

Table III. Risk Factors for Microtia Reported in the Literature
Risk factorsAuthor and year of publicationa
  • a

    Case-control studies: Castilla and Orioli [1986]; Mastroiacovo et al. [1995]; Zhang et al. [2009].

  • b

    No specific maternal condition identified.

  • c

    No specific drug identified.

  • d

    Controls were not defined.

  • e

    Controls and data on pollution were not defined.

General
 Male sex

all authors, except Zhu et al. [2000

]
 Low birthweight

Castilla and Orioli [1986

], Forrester and Merz [2005

], Mastroiacovo et al. [1995

]
 First parity

Mastroiacovo et al. [1995

]
 High parity

Castilla and Orioli [1986

], Harris et al. [1996

], Mastroiacovo et al. [1995

]
 Multiple births

Forrester and Merz [2005

], Shaw et al. [2004

]
 Maternal acute illnesses

Castilla and Orioli, 1986

], Okajima et al. [1996

], Zhang et al. [2009

] b
 Maternal insulin dependent diabetes

Correa et al. [2008

], Mastroiacovo et al. [1995

], Stevenson [2006

]
 Maternal use of medications

Castilla and Orioli [1986

], Zhang et al. [2009

] c
 Advanced paternal age

Castilla and Orioli [1986

]
 Advanced maternal age

Forrester and Merz [2005

], Harris et al. [1996

]
 Low maternal education

Harris et al. [1996

], Shaw et al. [2004

], Zhang et al. [2009

]
 Maternal exposure to altitude

Castilla et al. [1999

]
 Maternal residence in an urban area

Zhu et al. [2000

]
 Maternal residence in a rural area

Zhang et al. [2009

] d
 Maternal exposure to air pollution

Zhang et al. [2009

] e
Race/Ethnicity
 Native ethnicity

Aase and Tegtmeier [1977

], Jaffe [1969

]
 Hispanic ethnicity

Harris et al. [1996

], Husain et al. [2008

], Shaw et al. [2004

]
 Ecuadorian

Castilla and Orioli [1986

]
 Chilean

Nazer et al. [2006

]
 Asian, Philippine, Pacific Islander

Forrester and Merz [2005

]
Teratogens
 Retinoic acid

Carey et al. [2006

], Lammer et al. [1985

], Stern et al. [1984

]
 Thalidomide

Carey et al. [2006

]
 Alcohol

Carey et al. [2006

]
 Mycophenolate mofetil

Anderka et al. [2009

], Merlob et al. [2009

], Perez-Aytes et al. [2008

]

Strong evidence supports the association between gestational exposure to specific medications and microtia, including well-known teratogens such as retinoids, thalidomide, and the immunosuppressant, mycophenolate mofetil [Anderka et al., 2009; Klieger-Grossmann et al., 2010]. Alcohol has been inconsistently reported as a risk factor [Carey et al., 2006]. The mechanisms by which these exposures cause microtia have not been fully elucidated.

High altitude, usually defined as above 2,500 meters or 8,200 feet, has been associated with microtia in two independent studies in South America [Castilla et al., 1999; González-Andrade et al., 2010], which is inhabited by the largest populations living at high altitudes in the world. The observed association may be related to altitude or altitude could be a confounder. For example, the true association may be related to ethnicity, given the high proportion of Native American ancestry in regions of high altitude, or to differences in diet between low and high altitude populations.

Overview of Classification Systems

The degree of phenotypic variability of congenital anomalies of the ear makes the development of a meaningful classification system challenging (Fig. 1). Nevertheless, classification systems can facilitate diagnosis, treatment, and standardized data collection in multi-center studies. Hermann Marx [1926] published the first system, named the Marx classification, in 1926 and it remains one of the most frequently used systems. Tanzer [1978] classified ear abnormalities correlating with the surgical approach. Weerda [1988] modified the Marx and Tanzer definitions based on embryologic development as well as surgical steps and included all congenital abnormalities of the external ear (i.e., deformities and minor anomalies). The American Journal of Medical Genetics has recently published a collection of articles in an effort to standardize external ear terminology in the clinical genetics field [Hunter et al., 2009a]. The Weerda classification system was chosen as the basis for the standardized terminology used for microtia. These classification systems, commonly cited in studies of microtia, are summarized in Table IV.

thumbnail image

Figure 1. Photographs of individuals with different types of microtia. A: typical ear; B-D: first degree dysplasia; E: second degree dysplasia; F-J: third degree dysplasia (Classification proposed by Hunter et al., 2009a).

Download figure to PowerPoint

Table IV. Classification Systems for Microtia

Marx [1926

]

Tanzer [1978

]
 Grade I. Abnormal auricle with all identifiable landmarks Type 1. Anotia
 Grade II. Abnormal auricle without some identifiable landmarks Type 2. Completely hypoplastic ear (microtia)
 Grade III. Very small auricular tag or anotia  a. With atresia of the external auditory canal
Rogers [1974] proposed a fourth grade classification:  b. Without atresia of the external auditory canal
 Grade IV. Anotia Type 3. Hypoplasia of the middle third of the auricle
  Type 4. Hypoplasia of the superior third of the auricle
   a. Constricted (cup and lop) ear
   b. Cryptoptia
   c. Hypoplasia of entire superior third
  Type 5. Prominent ear

Weerda [1988

]

Hunter et al. [2009a

]
 First degree dysplasia. Most structures of a normal auricle are recognizable (minor deformities) Microtia, First Degree. Presence of all the normal ear components and the median longitudinal length more than 2 SD below the mean
  A. Macrotia  E. Small deformities Microtia, Second Degree. Median longitudinal length of the ear more than 2 SD below the mean in the presence of some, but not all, parts of the normal ear
  B. Protruding ears  F. Colobomata Microtia, Third Degree. Presence of some auricular structures, but none of these structures conforms to recognized ear components
  C. Cryptoptia  G. Lobule deformities Anotia. Complete absence of the ear
  D. Absence of upper helix  H. Cup ear deformities 
 Second degree dysplasia. Some structures of a normal auricle are recognizable 
  A. Cup ear deformity type III 
  B. Mini ear 
 Third degree dysplasia. None of the structures of a normal auricle are recognizable 
  A. Unilateral 
  B. Bilateral 
  C. Anotia (Peanut ears are included in this group) 

Most published studies on microtia report the presence or absence of microtia and/or anotia without further detail regarding severity. This is likely due, in part, to the fact that many prevalence studies of microtia rely on birth defect registries, which incorporate the International Classification for Diseases (ICD) coding system. The ICD system has only one code for microtia and one code for anotia and no information on severity or laterality.

As the genetic control of embryonic tissue morphogenesis is better understood, we may discover that the existing classification systems are too simplistic to be used in the study of normal and abnormal ear development. Detailed description of the malformation of each component of the ear, and acquisition of corresponding images should be the standard for recording information on microtia and other birth defects, regardless of the classification system chosen. Epidemiological and genetic studies could benefit from more detailed phenotypic information that would enable subclassification and grouping of malformations with shared characteristics. As we develop new genomic approaches for the study of birth defects, the importance of detailed phenotypic description has become clear. For example, in the discovery of the causative gene for Kabuki syndrome, reassessment of the images and clinical description was crucial when the first attempt of exome sequencing was unsuccessful [Ng et al., 2010]. Likewise, subphenotype analysis indicates that at least a subgroup of isolated cleft lip may be etiologically distinct from isolated cleft lip and palate [Jugessur et al., 2011]. Detailed description of external ear malformations would enable future reassessment of this information and reclassification if necessary to aggregate cases in multiple ways. The feasibility of this type of approach has been demonstrated in a study that performed systematic examination of ears of individuals with Cornelia de Lange syndrome and controls, using standardized 2D photographs [Hunter et al., 2009b].

EMBRYOLOGY: OUTER EAR DEVELOPMENT

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EMBRYOLOGY: OUTER EAR DEVELOPMENT
  5. GENETICS OF MICROTIA
  6. CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS
  7. FUTURE DIRECTIONS
  8. CONCLUSIONS
  9. REFERENCES

Vertebrate embryos develop a series of paired outgrowths on the ventro-lateral surface at their rostral end called the pharyngeal or branchial arches, which give rise to structures of the head and neck [Schoenwolf and Larsen, 2009b]. The pharyngeal arches are composed of mesenchymal cells of mesodermal and cranial neural crest origin. The neural crest cells (NCC) are a transitory group of pluripotent cells that originate from the dorsal part of the embryonic neural tube: The ectodermal-neurectodermal boundary. During early development, many of these cells collectively transform to a mesenchymal phenotype and assume new morphological characteristics distinct from their epithelial neighbors, segregate from the neural tube and emigrate through specific routes to contribute to a wide variety of tissues and structures throughout the vertebrate body [Engleka et al., 2008]. In the cranial region, reciprocal signaling between NCC (ectomesenchyme) and other embryonic cell types (e.g., endothelia and craniofacial ectoderm) play an important role in driving facial outgrowth and morphogenesis, including that of the external ear [Noden and Trainor, 2005].

The outer ear consists of the ear pinna (i.e., auricle, external ear), the external acoustic meatus (i.e., ear canal), and the outer layer of the tympanic membrane (i.e., eardrum). Outer ear development is driven by the mesenchyme of the first and second pharyngeal arches and is controlled, at least in part, by genes that determine first and second pharyngeal arch identity.

The auricle is formed from several protuberances in the first and second arches known as auricular hillocks (i.e., hillocks of His). These hillocks surround the first pharyngeal cleft, which is the space between the first and second arches. Each of the hillocks contributes to a specific component of the pinna, and those in the second arch form most of the ear structure [Mallo, 2003]. The auricular hillocks grow, fuse, and undergo morphogenesis to produce an appendage that funnels airborne vibrations into the meatus and along the canal to the tympanic membrane.

The outer ear begins its development during the fifth week, and the hillocks are first identifiable during the sixth week of embryogenesis. The development of the auricular hillocks into an auricle progresses slowly over several months and takes place largely during fetal stages. From their initially low position on the embryonic neck the auricles re-position progressively dorsalward [Schoenwolf and Larsen, 2009a]. As with more general facial growth [Hu and Marcucio, 2009], the overlying pharyngeal ectoderm may play a key role in determining the overall morphology or form of the auricle.

The auditory canal and tympanic membrane are derived from ectoderm of the pharyngeal cleft that separates the first and second pharyngeal arches. The cleft invaginates to form the meatus; this process is controlled and coordinated by a C-shaped skeletal structure, the tympanic ring, which develops from the first arch mesenchyme. As the ring grows, the invaginated external acoustic meatus starts to flatten down in the plane defined by the ring and becomes apposed to the endoderm of the middle ear cavity [Mallo, 2003]. The ring progressively integrates into the temporal bone at postnatal stages to serve as the attachment of the tympanic membrane.

Auricular and external acoustic meatus development must be tightly coordinated in order to be functional. Evidence from two mouse models (Gsc and Prx1 mutant lines), however, suggests that auricular and external acoustic meatus development is regulated by independent mechanisms, as both the Gsc and Prx1 mutants present with absent external acoustic meatus but exhibit fairly normal auricles [Martin et al., 1995; Yamada et al., 1995].

GENETICS OF MICROTIA

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EMBRYOLOGY: OUTER EAR DEVELOPMENT
  5. GENETICS OF MICROTIA
  6. CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS
  7. FUTURE DIRECTIONS
  8. CONCLUSIONS
  9. REFERENCES

Investigators have used a variety of genetic approaches to study microtia, including linkage analysis, direct sequencing of DNA from affected individuals, the study of single gene disorders that occur with microtia, identification of cytogenetic rearrangements in cases, and the study of animal models.

Evidence for a significant genetic contribution to microtia is based on: (1) Higher concordance in monozygotic twins than in dizygotic twins; 38.5 and 4.5%, respectively [Artunduaga et al., 2009]; (2) reported familial cases with autosomal recessive or dominant modes of inheritance with variable expression and incomplete penetrance [Ellwood et al., 1968; Konigsmark et al., 1972; Balci, 1974; Guizar-Vazquez et al., 1978; Zankl and Zang, 1979; Schmid et al., 1985; Strisciuglio et al., 1986; Orstavik et al., 1990; Gupta and Patton, 1995; Balci et al., 2001; Klockars et al., 2007; Alasti et al., 2008; Chafai Elalaoui et al., 2010]; (3) estimates of familial cases ranging from 3 to 34% [Castilla and Orioli, 1986; Mastroiacovo et al., 1995; Okajima et al., 1996; Llano-Rivas et al., 1999]; (4) more than 18 different microtia-associated syndromes for which single-gene defects or chromosomal aberrations have been reported; and 5) mouse models demonstrating that mutations in specific genes result in microtia. We discuss below the most relevant existing data for candidate genes from studies on animal models and humans.

Animal Models Studies

The application of novel strategies (analytical, genetic, imaging, etc) in developmental biology and genetics has begun to facilitate elucidation of mechanisms controlling craniofacial development in animal models. Murine models in particular are commonly used to study developmental mechanisms involved in the formation of the head and face. Defects in outer ear development in mutant mice range from hypomorphisms to the complete absence of structural elements. In Table V, we have included models with abnormalities in the structure of the external ear to be consistent with the definition of microtia. Mouse models with only inner ear anomalies were not included. In this section, we discuss in further detail some of the most promising models.

Table V. Summary of Genes Associated With Microtia in Mice and Comparison With ear Phenotypes in Humans
GeneMouse ear phenotypeHuman ear phenotype
  • nr, not reported; EAM, external acoustic meatus.

  • a

    According to Marx Classification.

Bmp5Small outer ear/Normal EAMnr
Chuk (Ikka)Anotianr (MIM:613630- no ear anomalies in the two cases with IKKA mutations)
Cyp26b1Small outer earnr
Dlx5/Dlx6Absent outer ear Severely affected middle earnr
Edn1Very small outer ear/absent EAM/absent middle earnr
EdnraSmall outer earnr
Eya1AnotiaEYA1: 30% with outer ear anomalies (overfolded/deformed)a
Fgf8Small outer ear/middle ear anomaliesnr
Fgf10Small outer earnr
Frem2Anotianr
GscSlightly reduced outer ear/EAM absent/inner ear anomaliesnr
Hfm locusSmall outer ear/anotia/absent EAM/middle ear anomaliesCorrelated gene not identified in humans
Hoxa1Small outer ear/middle and inner ear anomaliesDeformed ears (present in few cases)
Hoxa1/Hoxb1Anotia (with no residual auricle)nr
Hoxa2“Anotia” (poorly defined protuberance)Microtia I and IIa
Hox2.2Small outer earnr (humans: HOXB6)
Irf6Anotianr
Pax8Anotia/small outer ear/EAM stenotic or atretic/middle and inner ear anomaliesnr
Prrx1/Prrx2Small outer ear/middle and inner ear anomaliesnr (humans: PMX1)
PrkraSmall outer ear/middle ear anomaliesnr
RarSmall outer ear/anotianr
Sall1No affected earsMicrotia types I-IIa/preauricular tags
Six1/Six4Anotianr
Tbx1Small outer ear/anotia/middle and inner ear anomaliesMicrotia Ia (uncommon)
Tcfap2aAnotianr
Tcof1Cup-shaped ears/middle ear anomaliesMicrotia and ear canal atresia.
Wnt5aSmall outer earnr

Hox Genes

Homeobox genes are involved in the development of the pharyngeal arches. They encode highly conserved transcription factors that control positional identity of cells (body patterning) and morphogenesis throughout development, as well as switch on cascades of other genes. The Hox gene family is clustered within the genome and is ordered on the chromosome in the sequence in which they are expressed during development; this highly ordered pattern of gene expression might constitute part of a mechanism whereby morphogenetic specification is established [Kmita and Duboule, 2003]. Inactivation of Hoxa1 in mice results in hypoplastic external ears and abnormalities of the middle and inner ear, whereas compound Hoxa1/Hoxb1 mutants present with complete anotia [Gavalas et al., 1998]. In contrast, Hoxa2 seems to be required for defining second pharyngeal arch identity and thus the initial steps of pinna formation, and is strongly expressed in the pinna of mice. Consistent with this, Hoxa2 knockout mice present with microtia, described as “a small protuberance with no recognizable shape” [Gendronmaguire et al., 1993]. Hoxb6 and Hoxa7 deficient mice present with microtia in addition to open-eyes and cleft palate [Balling et al., 1989; Kaur et al., 1992].

Six and Eya Genes

In vertebrates, members of the SIX homeobox gene family (SIX1–6) have also been implicated in external ear development [Kawakami et al., 2000]. SIX genes are homologs of sine oculis (six) gene in the vinegar fly, Drosophila melanogaster. SIX function seems conserved across evolution since knockdown of Six1 in frogs, chicks, and mice result in craniofacial abnormalities [Laclef et al., 2003; Brugmann et al., 2004; Christophorou et al., 2009] while misexpression of Six2 leads to frontonasal dysplasia in mice [Fogelgren et al., 2008]. Six1/Six4 mice present with microtia, whereas Six1 deficiency alone is associated with normal external and middle ears [Laclef et al., 2003], suggesting some redundancy in function within this gene family. Other Six mutants have not been reported to have ear abnormalities.

EYA1 is the human homolog of the Drosophila eyes absent (eya) gene. EYA forms a complex with SIX (EYA-SIX) to regulate the development of several tissues and organs in vertebrates and in flies. Natural target genes of the EYA-SIX complex include SIX2 and SALL1. Studies on Eya1 expression have shown a major role in pinna development apparently related to cartilage formation; the knockout mice for Eya1 present anotia. Sall1 is expressed in craniofacial tissue but the knockout animals have normal ears. As for the SIX genes, there are additional SALL genes in mammals (SALL2-4) and so redundancy in function may also mask or modulate the phenotypic presentation.

Recently, Sipl1 (Shank-interacting protein-like 1) and Rbck1 (RBCC protein interacting with PKC1) were identified as novel Eya1-interacting proteins. Both Sipl1 and Rbck1 are expressed together with Eya1 in many tissues in mouse and zebrafish to direct the development of the inner ear and the pharyngeal arches as well as other organs. In fact, both Sipl1 and Rbck1 act as cofactors for the Eya-Six complex [Landgraf et al., 2010]. Further experiments regarding the functional consequences of the interaction of Sipl1 or Rbck1 with Eya1 should clarify the importance of the respective interaction for outer ear development in mammals.

Tbx1

In mice, mutations in Tbx1, a member of the T-box gene family of transcription factors, result in failure of middle and outer ear development and in hypoplasia of the inner ear sensory organs. A similar phenotype was also seen following inactivation of Tbx1 exclusively in pharyngeal arch endoderm, indicating a primary role for this gene in pharyngeal arch morphogenesis [Arnold et al., 2006]. Of interest, Tbx1 heterozygosity is associated with chronic otitis media, but not morphological defects, and does not interfere with the formation of the outer, middle and inner ear structures [Liao et al., 2004].

Irf6 and Chuk (Ikka)

Mice homozygous null for Irf6 lack external ears in addition to exhibiting abnormal skin, limbs, and both shorter snouts and jaws. Ectopic epidermal adhesions at several sites, including the oral cavity, between the tail and hindlimbs, and in the esophagus were observed, although not specifically reported for the ear. A similar phenotype was observed in mice deficient for Chuk (also known as Ikka). The authors speculate, based on histological and gene expression analyses, that the abnormalities in the Irf6 and Chuk mice are secondary to defects in epidermal differentiation or cell proliferation [Hu et al., 1999; Ingraham et al., 2006].

Signaling Pathways

Signaling pathways involved in the outer ear development include bone morphogenetic proteins (Bmps), Wingless/INT (Wnts), fibroblast growth factors (Fgfs), and retinoic acid. Dysregulation of these signaling pathways triggered by genetic or environmental factors constitutes a potential source of under- or maldevelopment. While NCC likely receives patterning signals during migration, much of the signaling necessary for patterning within an arch comes from signals received after their arrival at the arches [Knight and Schilling, 2006].

The Bmp genes, especially Bmp5, have been considered as candidate genes for microtia in humans; however, studies in mice have shown that Bmp5 is apparently more related to growth than the early pattern of differentiation and formation of the external ear. The Bmp5 mutant mice usually present with short ears attributed to defective auricular cartilage framework. Over two dozen viable radiation- and chemically-induced alleles have been isolated at the Bmp5 locus [Russell, 1971; Russell et al., 1989; Kingsley et al., 1992; Marker et al., 1997]. The different mutations produce an apparent gradient of effects on the size of the external ear; mutants completely missing the Bmp5 gene have the shortest ears.

FGF signaling, involving different Fgf ligands and their receptors, Fgfr1-3, plays various roles in pinna development [Abu-Issa et al., 2002; Wright and Mansour, 2003] as evidenced by specific mutant phenotypes; Fgf8 and Fgf10 mutant mice present with small outer ears [Abu-Issa et al., 2002; Mouse Genomic Database (MGD, 2011) http://www.informatics.jax.org] and mice homozygous for a hypomorphic Fgfr1 allele present with very small ears and abnormal external auditory canals [Partanen et al., 1998]. However, it is not clear when these signaling components are required, nor whether these particular ligands and receptors are expressed in the pinna during late gestation.

Members of the Wnt family have been implicated in NCC formation and development, but their independent roles have been difficult to determine due to overlapping expression and functional redundancy. It has been shown that Wnt5a is expressed in the mesenchyme of the developing outer ear, and indeed Wnt5a knockout mice present with small ears [Yamaguchi et al., 1999]. However, microtia has not been described in any other Wnt mutant.

Mouse lines harboring a mutation in endothelin or endothelin receptors also present with various ear malformations. The endothelin pathway has a well-established role in regulating neural crest proliferation and migration, and therefore it is plausible that mutations in this pathway could be involved in microtia in humans. In this regard, the transcription factor Goosecoid (Gsc), a downstream target of endothelin signaling, is expressed in the pharyngeal mesenchyme around the first pharyngeal cleft and has been implicated in outer and middle ear development through mutational analyses in patients (see below).

Human Genetics Studies

Microtia has been reported in individuals with autosomal trisomies, such as trisomy 18 (Fig. 2), 21, and 22, as well as with mosaicism of trisomy 13 and 18 [Giannatou et al., 2009; Griffith et al., 2009]; and aneusomies, as in deletion of 4p, 5p, and 18p, 18q, and 22q11.2. Chromosomal translocations involving the 6p24 region have been associated with orofacial clefting and bilateral microtia [Davies et al., 1998]. Several cases reports of mosaicism 46,X,der(Y)t(Y;1)(q12;q21)/46,XY describe the presence of microtia associated with anomalies such as kyphoscoliosis, oligodactyly, joint contractures, central nervous system malformations, omphalocele, diaphragmatic hernia, cardiac defects, and urogenital malformation [Watson et al., 1990; Zeng et al., 2003; Scheuerle et al., 2005; Li, 2010]. Microtia has been associated with abnormalities in each of the chromosomes [POSSUM, 2010] confirming Schinzel's [2001] observation that malformations confined to one or very few chromosome aberrations are suspicious for single gene deletions, whereas, malformations frequent in chromosome aberrations are caused by deficiency of a step in organogenesis. For the purpose of this review we have only cited cytogenetic rearrangements recurrently reported involving microtia.

thumbnail image

Figure 2. External ear morphology in a 122d anencephalic fetus. 3D rendered image of a microcomputed tomography (microCT) scan showing external ear morphology. Note the microtic appearance of the right ear (right image) compared to the normal left ear (left image).

Download figure to PowerPoint

Microtia is a clinical finding in several well-established human single gene disorders. For example, mutations in SIX1 and EYA1 have been shown to cause Branchio-otic (BO) syndrome, while mutations in SIX5 and EYA1 can cause Branchio-oto renal (BOR) syndrome, both are associated with microtia, among other craniofacial defects [Abdelhak et al., 1997; Kumar et al., 1997; Rodriguez Soriano, 2003; Hoskins et al., 2007]. Other familial cases with syndromic microtia have also been reported. Table II summarizes the human genes involved in syndromes that are associated with microtia.

Few studies have focused on the genetic causes of isolated microtia. Sequence analysis of GSC exons in 121 individuals with isolated microtia revealed a missense mutation in exon 3 in two cases. In the same study, screening of the BMP5 locus revealed a missense mutation in four patients. None of these mutations were detected in control subjects, suggesting a causative role. Individuals with incomplete clinical data, inadequate quantity of blood samples, or unsatisfactory genetic analyses were excluded and thus the total number of cases and controls included in the analysis is not clear [Zhang et al., 2010].

Monks et al. [2010] did not identify mutations in exons of HOXA2 or SIX2, which acts downstream of HOXA2 during development, in 12 patients of Hispanic and African descent with isolated microtia. In another study, the methylation status of the EYA1 gene promoter was analyzed in 64 individuals with microtia and 36 healthy controls. The authors reported that the methylation levels at this locus were significantly lower in individuals with microtia than in controls and suggested that hypomethylation may be related to the pathogenesis of this condition [Lin et al., 2009]. Further studies are needed to validate these conclusions.

In summary, although some studies have found genetic variants potentially associated with microtia, no causal genetic mutation has been confirmed to date.

CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EMBRYOLOGY: OUTER EAR DEVELOPMENT
  5. GENETICS OF MICROTIA
  6. CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS
  7. FUTURE DIRECTIONS
  8. CONCLUSIONS
  9. REFERENCES

Microtia is both etiologically and pathogenetically heterogeneous. As discussed above, single gene mutations are associated with microtia in syndromic and familial cases, whereas a multifactorial (genetic and environmental) or polygenic cause is likely in sporadic cases. Current hypotheses favor disturbance of NCC as the likely underlying cause, although the exact mechanism(s) remain unknown. However, given the clinical heterogeneity, it is possible that different pathogenetic processes lead to the different types of microtia. We discuss several hypotheses for the occurrence of this condition below.

Neural Crest Cells Disturbance

Defects in NCC function have been associated with numerous craniofacial syndromes [Passos-Bueno et al., 2009]. In Treacher Collins syndrome, TCOF1 mutations result in haploinsufficiency of the protein Treacle (encoded by TCOF1) leading to insufficient ribosome genesis, diminished cell proliferation, and increased neuroepithelial apoptosis. The mechanism proposed is that this results in depletion of NCC precursors with reduced number of cells migrating into the first and second pharyngeal arches leading to the complete craniofacial phenotype observed in the syndrome that includes severe, bilateral microtia [Trainor, 2010].

Strong evidence for the role of NCC in the occurrence of microtia derives also from the recent studies on the causative mechanisms of various teratogens associated with this condition. Retinoid and diabetic embryopathy have been associated with apoptosis of NCC before migration into the pharyngeal arches, and disturbance of NCC differentiation after arrival in the pharyngeal arches. In diabetes, hyperglycemia has been recently associated with down regulation of Pax3, which encodes a transcription factor critical for early NCC survival and migration [Zabihi and Loeken, 2010]. Conversely, retinoid exposure appears to disrupt the endothelin signaling pathway; which in turn regulates Hox gene expression. Hox genes are hypothesized to govern positional identity of NCC before and during migration from the neural folds. Mallo [2003] suggested that the more severe forms of microtia could result from a loss of second arch identity, since most of the definitive pinna derives from the hillocks of this arch. This conclusion is supported for the phenotype observed on the Hoxa2 mouse mutant (anotia), a gene that is strongly expressed in the second pharyngeal arch. The effects of thalidomide could include down regulation of Fgf8 [Hansen et al., 2002] and Bmp signaling [Knobloch et al., 2007; Ito et al., 2010], though direct anti-angiogenic effects and oxidative stress are also postulated as independent mechanisms [Parman et al., 1999; Ito et al., 2010].

From an embryological and developmental biology perspective, defects or insults affecting NCC delamination, proliferation, apoptosis, or migration, or their reciprocal interactions with mesoderm, endoderm, or overlying ectoderm are feasible explanations for the impairment in auricular hillock growth, repositioning, or cartilage development seen in patients with various forms of microtia. In view of this, investigations into the pathogenesis of 22q11.2 deletion syndrome (22q11.2DS) initially focused on intrinsic abnormalities in NCC. TBX1 is a gene located in the typically deleted region in this condition and is considered to be a candidate gene for several of the malformations associated with 22q11.2DS. TBX1 encodes a transcription factor, which presumably affects the expression of a secreted or cell surface molecule; however, TBX1 appears not to be expressed by the NCC, but to be expressed throughout the non-crest pharyngeal mesenchyme and in regions of pharyngeal endoderm. Individuals with 22q11.2DS often have small ears and there are reports of some presenting with microtia [Digilio et al., 2009]; Tbx1 mice mutants also can present with small or absent ears [Liao et al., 2004]. Thus, an indirect interaction between non-crest mesoderm or endoderm and neural crest can alter NCC fate and result in craniofacial malformations that might include microtia.

Vascular Disruption

Vascular disruption can occur via several mechanisms, including (1) occlusion of an artery that interrupts blood flow to previously formed tissue, (2) vasoconstriction and diminished arterial blood flow, or (3) underdevelopment of the arterial system required for adequate blood supply to developing tissues. Vascular disruption has been proposed to cause microtia by disruption in the development of the blood vascular system in the head and neck, resulting in localized ischemia and tissue necrosis, although this is still heavily debated [Sadler and Rasmussen, 2010].

The concept of vascular involvement in microtia comes from various observations. The greater prevalence of unilateral cases of microtia suggests a more localized effect during embryogenesis, which could feasibly result in occlusion of a single vessel. This hypothesis is mainly supported by studies in the 1970s [Poswillo, 1973, 1975] in which mice exposed to triazine and monkeys exposed to thalidomide showed ipsilateral hematomas at the junction of the pharyngeal and hyoid arteries with associated unilateral ear and mandibular defects. Additionally, Otani et al. [1991] and Naora et al. [1994] reported a phenotype resembling craniofacial microsomia (unilateral microtia, abnormal biting, anomalies in the EAM and middle ear, and cranial base) in a transgenic mouse line carrying a non-expressed transgene. Notably, the authors reported rupture of the vasculature of the second pharyngeal arch with histologically confirmed hemorrhage and subsequent phagocytosis. They concluded that integration of the transgene on mouse chromosome 10 interrupted an endogenous gene that has a critical role in craniofacial morphogenesis. The causative gene in this important mouse model has not been identified.

Arguing against a primary role for vascular disruption, Johnston and Bronsky [1995] reassessed Poswillo's original experiments and concluded that the hematomas appeared too late (after 2 days) in relation to the exposure to this drugs and that there was already severe underdevelopment of the mandibular arches and brain at the time of hemorrhage. Consistent with this, a recent review by Sadler and Rasmussen [2010] concluded that there was not enough epidemiological or experimental data to support the vascular disruption hypothesis for OAVS or microtia. They also emphasized the fact that even malformations caused by genetic alterations occur unilaterally and that other factors that act through nonvascular mechanisms can also cause microtia. In addition, the vascular hypothesis does not explain the abnormalities of OAVS occurring in other non-craniofacial structures (e.g., kidney and vertebra) or the cumulative evidence showing the frequent bilaterality of this condition. Likewise, this hypothesis cannot adequately explain the bilateral cases of isolated microtia. In addition, the epidemiologic data on the association of OAVS and vascular defects have not been conclusive.

Of further interest is the lack of occurrence of microtia in misoprostol-induced embryopathy. Misoprostol, a synthetic analog of prostaglandin E1, is a vasoconstrictive agent known to cause uterine hypoperfusion. Maternal exposure to misoprostol has been reported for individuals presenting mainly with transverse-limb defects, Moebius sequence and arthrogryposis [Gonzalez et al., 1993; Orioli and Castilla, 2000; Vargas et al., 2000; da Silva Dal Pizzol et al., 2006].

Alternatively, vascular disruptions may simply be an indirect consequence of excessive mesenchymal cell death, perturbed NCC migration or premature NCC differentiation. For example, studies with Frem2 deficient mice have shown frequent hematomas, yet Frem2 itself is not expressed in the embryonic vasculature [Timmer et al., 2005]. However, it is expressed in many cell types that contact the vasculature. Frem2, and its related proteins, Fras1 and Frem1, have been implicated in the regulation of extracellular matrix structure. Thus, loss of any of these proteins is thought not only to impact tissue morphogenesis but also to increase tissue fragility [Vrontou et al., 2003]. In the uterine environment, the external surface of the developing embryo is constantly in contact with the uterine wall. In this context, increased tissue fragility or reduced cell-specific adhesiveness may increase the embryos susceptibility to physical or mechanical trauma [Vrontou et al., 2003], thus resulting in local vascular disruptions and transient focal tissue ischemia.

Altitude

Castilla and Orioli [1986] reported a fivefold higher prevalence of microtia in Quito, Ecuador (located at 2,850 m or 9,350 ft) compared with countries in low altitudes of South America. The authors proposed that this difference was related to the high altitude. Their analysis did not detect differences in occurrence of microtia among mothers who identified themselves as having Native American ancestry and those that did not. A subsequent study that included Quito and the other two other large high altitude cities of South America, La Paz (Bolivia) and Bogota (Colombia), also revealed a higher prevalence of microtia as well as oral clefts, congenital heart disease, and limb defects [Castilla et al., 1999]. The relationship between altitude and microtia is further supported by a recent study using data from vital statistics from Ecuador [González-Andrade et al., 2010], although data on ethnicity in this study were also obtained through self-reporting only. Our literature review failed to identify studies on the prevalence of birth defects in other high-altitude areas such as Tibet.

Intra-uterine growth restriction and increased frequency of pre-eclampsia and stillbirths are more common in populations living at high altitude than those at low altitude. The uterine artery undergoes remodeling during pregnancy to accommodate the rise in maternal uterine artery blood flow and facilitates delivery of oxygen and nutrients to the feto-placental circulation. The chronic hypoxia associated with residence at high altitude impairs maternal vascular adaptation to pregnancy by reducing the increase in the uterine artery diameter and rise in its blood flow by about one-third. Furthermore, circulating levels of catecholamines and inflammatory cytokines increase during pregnancy in women residing at high altitude [Coussons-Read et al., 2002]. Therefore, high altitude may constrain fetal growth through exposure to low oxygen levels ([Zamudio et al., 2006]. Nonetheless, the effects of hypoxia on the developing embryo are not well understood. Evidence from experimental studies suggest that periods of severe hypoxia in the first trimester can cause birth defects, such as transverse limb defects, heart defects, cleft lip, and maxillary hypoplasia. However, these studies do not report on anomalies of the ear.

Notably, populations with many generations of residence at high altitude, such as Andean or Tibetan people, are relatively protected against this altitude-associated reduction in fetal growth, providing further support for direct biologic altitude effects [Julian et al., 2007; Bennett et al., 2008; Julian et al., 2009]. The mechanisms responsible for the ancestry-associated differences are unclear but could provide important insight into the genetic contributions to microtia. An important confounder, however, is that altitude may constrain agricultural production and thus increase costs of transporting fresh food products. This could feasibly result in maternal nutritional deficiencies that in turn could be the cause for some of the birth defects observed in this population [Niermeyer et al., 1995; Cook et al., 2005; Niermeyer et al., 2009].

A number of genes and respective pathways and environmental factors are required for normal development of the ear. The present challenge is to understand (1) how they integrate to result in the formation of the ear, (2) how their disruption can cause microtia, and (3) how to study new risk factors that may cause microtia.

FUTURE DIRECTIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EMBRYOLOGY: OUTER EAR DEVELOPMENT
  5. GENETICS OF MICROTIA
  6. CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS
  7. FUTURE DIRECTIONS
  8. CONCLUSIONS
  9. REFERENCES

Genetic Studies on Humans

The prevalence of microtia varies by region. However, even in the higher prevalence regions, and considering the clinical heterogeneity of this condition, the collection of cases requires many years to accomplish a sample size necessary to study this condition. National and international consortiums that include data from birth defect registries and/or craniofacial centers could facilitate prospective, standardized data collection for individuals with microtia. Such data would increase the likelihood of success for larger studies and thus, advance the knowledge of the etiology of microtia.

Recent advances offer multiple methodologies to study the genetics of microtia. The four most common methods include genome-wide association studies (GWAS), exome sequencing, linkage studies in large families, and copy number variation investigations. The success of such investigations requires high-quality phenotypic data.

Given that the prevalence of microtia appears to be higher in some ethnic groups, we would expect at least in these groups, that the genetic variants associated with microtia are common (i.e., present in more than 5% of the population) and, therefore, GWAS could be a suitable and cost-effective approach. Although GWAS studies are typically not feasible for studies of birth defects given the need for large sample sizes, a successful GWAS study with 111 cases was recently performed in oral clefting [Grant et al., 2009]. New GWAS arrays also allow for the clarification of ethnicity through the identification of ancestry informative markers and the identification of copy number variations providing information regarding cytogenetic diagnoses.

Exome sequencing offers promise as a technique to study microtia, particularly in isolated cases from ethnic groups that apparently have a lower risk for microtia. In such cases the genetic effects seem to be rare and therefore they could potentially represent sporadic variants. Exome sequencing can identify coding variants specific to each individual studied and some functional annotation can usually be ascribed to the findings. In contrast, functional annotation in GWAS studies usually has to be inferred via linkage disequilibrium as assessed variants are not necessarily functional variants. Another advantage of exome sequencing is the option to study fewer cases (such as case-parent trios) and to identify genetic variation, although this technology has not yet been proven to be effective for complex diseases. Exome sequencing techniques are limited by the fact that they do not identify functional noncoding, nor structural, mutations; however, the recent development of analysis of copy number variation data derived from exome sequencing could partially overcome this limitation. Identification of mutations in segmentally duplicated regions of the genome with short read sequencing also remains challenging in exome sequencing.

Animal Models

There are likely many murine models with ear abnormalities not yet described in the literature. Identification of microtia in animal models can be challenging for the following reasons: (1) Mild types of microtia can easily go unnoticed to an unfamiliar handler; (2) experiments that are focused on other phenotypes may not report abnormalities of the ear; (3) even if noticed, the likelihood of having it reported and/or published is low. This is supported by our own investigations of numerous existing mutant mouse lines that have shown many striking yet previously unreported craniofacial malformations including microtia (T. Cox, unpublished data). In addition, auditory canal and tympanic membrane abnormalities are more difficult to identify in mouse models than pinna defects.

A concerted effort assessing the large repositories of spontaneous, chemically-induced and gene targeted mouse lines such as the Jackson Laboratories (Mouse Genomic Database (MGD, 2011) http://www.informatics.jax.org) may ultimately uncover many new and important genes involved in external ear development and hence candidates for microtia in humans.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EMBRYOLOGY: OUTER EAR DEVELOPMENT
  5. GENETICS OF MICROTIA
  6. CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS
  7. FUTURE DIRECTIONS
  8. CONCLUSIONS
  9. REFERENCES

The genetic and cellular mechanisms underlying normal morphogenesis of the external ear is not completely understood. Further insight into the mechanisms of normal ear development will contribute to an understanding of abnormal ear development that results in microtia and other ear abnormalities. Identification and characterization of the primary and secondary factors that lead to microtia on the other hand will be important for the delineation of the molecular pathways involved in craniofacial development. In addition, such studies will likely open new strategies for treatment for individuals with microtia. In conjunction with well-designed clinical research, continued application of novel technologies and models is essential to fully understand the pathogenesis of isolated microtia and the exact role that individual genes play in the development of the external ear.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EMBRYOLOGY: OUTER EAR DEVELOPMENT
  5. GENETICS OF MICROTIA
  6. CURRENT HYPOTHESES FOR THE ETIOLOGY AND PATHOGENESIS
  7. FUTURE DIRECTIONS
  8. CONCLUSIONS
  9. REFERENCES
  • Aase JM, Tegtmeier RE. 1977. Microtia in New Mexico: Evidence for multifactorial causation. Birth Defects Orig Artic Ser 13: 113116.
  • Abdelhak S, Kalatzis V, Heilig R, Compain S, Samson D, Vincent C, Levi-Acobas F, Cruaud C, Le Merrer M, Mathieu M, Konig R, Vigneron J, Weissenbach J, Petit C, Weil D. 1997. Clustering of mutations responsible for branchio-oto-renal (BOR) syndrome in the eyes absent homologous region (eyaHR) of EYA1. Hum Mol Genet 6: 22472255.
  • Abu-Issa R, Smyth G, Smoak I, Yamamura K, Meyers EN. 2002. Fgf8 is required for pharyngeal arch and cardiovascular development in the mouse. Development 129: 46134625.
  • Alasti F, Sadeghi A, Sanati MH, Farhadi M, Stollar E, Somers T, Van Camp G. 2008. A mutation in HOXA2 is responsible for autosomal-recessive microtia in an Iranian family. Am J Hum Genet 82: 982991.
  • Alasti F, Van Camp G. 2009. Genetics of microtia and associated syndromes. J Med Genet 46: 361369.
  • Anderka MT, Lin AE, Abuelo DN, Mitchell AA, Rasmussen SA. 2009. Reviewing the evidence for mycophenolate mofetil as a new teratogen: Case report and review of the literature. Am J Med Genet Part A 149A: 12411248.
  • Arnold JS, Braunstein EM, Ohyama T, Groves AK, Adams JC, Brown MC, Morrow BE. 2006. Tissue-specific roles of Tbx1 in the development of the outer, middle and inner ear, defective in 22q11DS patients. Hum Mol Genet 15: 16291639.
  • Artunduaga MA, Quintanilla-Dieck MD, Greenway S, Betensky R, Nicolau Y, Hamdan U, Jarrin P, Osorno G, Brent B, Eavey R, Seidman C, Seidman JG. 2009. A classic twin study of external ear malformations, including microtia. N Engl J Med 361: 12161218.
  • Balci S. 1974. Familial microtia with meatal atresia in father and son. Turk J Pediatr 16: 140143.
  • Balci S, Boduroglu K, Kaya S. 2001. Familial microtia in four generations with variable expressivity and incomplete penetrance in association with type I syndactyly. Turk J Pediatr 43: 362365.
  • Balling R, Mutter G, Gruss P, Kessel M. 1989. Craniofacial abnormalities induced by ectopic expression of the homeobox gene Hox-1.1 in transgenic mice. Cell 58: 337347.
  • Bassila MK, Goldberg R. 1989. The association of facial palsy and/or sensorineural hearing loss in patients with hemifacial microsomia. Cleft Palate J 26: 287291.
  • Bennett A, Sain SR, Vargas E, Moore LG. 2008. Evidence that parent-of-origin affects birth-weight reductions at high altitude. Am J Hum Biol 20: 592597.
  • Brugmann SA, Pandur PD, Kenyon KL, Pignoni F, Moody SA. 2004. Six1 promotes a placodal fate within the lateral neurogenic ectoderm by functioning as both a transcriptional activator and repressor. Development 131: 58715881.
  • Calzolari F, Garani G, Sensi A, Martini A. 1999. Clinical and radiological evaluation in children with microtia. Br J Audiol 33: 303312.
  • Canfield MA, Langlois PH, Nguyen LM, Scheuerle AE. 2009. Epidemiologic features and clinical subgroups of anotia/microtia in Texas. Birth Defects Res A Clin Mol Teratol 85: 905913.
  • Carey JC, Park AH, Muntz HR. 2006. External ear. In: Stevenson RE, editor. Human malformations and related anomalies. Oxford, New York: Oxford University Press. pp. 329338.
  • Castilla EE, Lopez-Camelo JS, Campana H. 1999. Altitude as a risk factor for congenital anomalies. Am J Med Genet 86: 914.
  • Castilla EE, Orioli IM. 1986. Prevalence rates of microtia in South America. Int J Epidemiol 15: 364368.
  • Chafai Elalaoui S, Cherkaoui Jaouad I, Rifai L, Sefiani A. 2010. Autosomal dominant microtia. Eur J Med Genet 53: 100103.
  • Christophorou NA, Bailey AP, Hanson S, Streit A. 2009. Activation of Six1 target genes is required for sensory placode formation. Dev Biol 336: 327336.
  • Cook JD, Boy E, Flowers C, Daroca Mdel C. 2005. The influence of high-altitude living on body iron. Blood 106: 14411446.
  • Correa A, Gilboa SM, Besser LM, Botto LD, Moore CA, Hobbs CA, Cleves MA, Riehle-Colarusso TJ, Waller DK, Reece EA. 2008. Diabetes mellitus and birth defects. Am J Obstet Gynecol 199: 237e1239.
  • Coussons-Read ME, Mazzeo RS, Whitford MH, Schmitt M, Moore LG, Zamudio S. 2002. High altitude residence during pregnancy alters cytokine and catecholamine levels. Am J Reprod Immunol 48: 344354.
  • da Silva Dal Pizzol T, Knop FP, Mengue SS. 2006. Prenatal exposure to misoprostol and congenital anomalies: Systematic review and meta-analysis. Reprod Toxicol 22: 666671.
  • Davies AF, Imaizumi K, Mirza G, Stephens RS, Kuroki Y, Matsuno M, Ragoussis J. 1998. Further evidence for the involvement of human chromosome 6p24 in the aetiology of orofacial clefting. J Med Genet 35: 857861.
  • Digilio MC, McDonald-McGinn DM, Heike C, Catania C, Dallapiccola B, Marino B, Zackai EH. 2009. Three patients with oculo-auriculo-vertebral spectrum and microdeletion 22q11. Am J Med Genet Part A 149A: 28602864.
  • Du JM, Zhuang HX, Chai JK, Liu GF, Wang Y, Guo WH. 2007. Psychological status of congenital microtia patients and relative influential factors: Analysis of 410 cases. Zhonghua Yi Xue Za Zhi 87: 383387.
  • Ellwood LC, Winter ST, Dar H. 1968. Familial microtia with meatal atresia in two sibships. J Med Genet 5: 289291.
  • Engleka KA, Lang D, Brown CB, Antonucci NB, Epstein JA. 2008. Neural crest formation and craniofacial development. In: Epstein CJ, Erickson RP, Wynshaw-Boris AJ, editors. Inborn errors of development, 2nd edition. Oxford, New York: Oxford University Press. pp. 6978.
  • Fogelgren B, Kuroyama MC, McBratney-Owen B, Spence AA, Malahn LE, Anawati MK, Cabatbat C, Alarcon VB, Marikawa Y, Lozanoff S. 2008. Misexpression of Six2 is associated with heritable frontonasal dysplasia and renal hypoplasia in 3H1 Br mice. Dev Dyn 237: 17671779.
  • Forrester MB, Merz RD. 2005. Descriptive epidemiology of anotia and microtia, Hawaii, 1986–2002. Congenit Anom (Kyoto) 45: 119124.
  • Gavalas A, Studer M, Lumsden A, Rijli FM, Krumlauf R, Chambon P. 1998. Hoxa1 and Hoxb1 synergize in patterning the hindbrain, cranial nerves and second pharyngeal arch. Development 125: 11231136.
  • Gendronmaguire M, Mallo M, Zhang M, Gridley T. 1993. Hoxa-2 mutant mice exhibit homeotic transformation of skeletal elements derived from cranial neural crest. Cell 75: 13171331.
  • Giannatou E, Leze H, Katana A, Kolialexi A, Mavrou A, Kanavakis E, Kitsiou-Tzeli S. 2009. Unilateral microtia in an infant with trisomy 18 mosaicism. Genet Couns 20: 181187.
  • González-Andrade F, López-Pulles R, Espín VH, Paz-y-Miño C. 2010. High altitude and microtia in Ecuadorian patients. J Neonatal Perinatal Med 3: 109116.
  • Gonzalez CH, Vargas FR, Perez AB, Kim CA, Brunoni D, Marques-Dias MJ, Leone CR, Correa Neto, Llerena J, Junior JC, de Almeida JC. 1993. Limb deficiency with or without Mobius sequence in seven Brazilian children associated with misoprostol use in the first trimester of pregnancy. Am J Med Genet 47: 5964.
  • Gorlin RJ, Cohen MM, Hennekam RCM. 2001. Syndromes of the head and neck. Oxford, New York: Oxford University Press.
  • Grant SF, Wang K, Zhang H, Glaberson W, Annaiah K, Kim CE, Bradfield JP, Glessner JT, Thomas KA, Garris M, Frackelton EC, Otieno FG, Chiavacci RM, Nah HD, Kirschner RE, Hakonarson H. 2009. A genome-wide association study identifies a locus for nonsyndromic cleft lip with or without cleft palate on 8q24. J Pediatr 155: 909913.
  • Griffith CB, Vance GH, Weaver DD. 2009. Phenotypic variability in trisomy 13 mosaicism: Two new patients and literature review. Am J Med Genet Part A 149A: 13461358.
  • Guizar-Vazquez J, Arredondo-Vega F, Rostenberg I, Manzano C, Armendares S. 1978. Microtia and meatal atresia in mother and son. Clin Genet 14: 8082.
  • Gupta A, Patton MA. 1995. Familial microtia with meatal atresia and conductive deafness in five generations. Am J Med Genet 59: 238241.
  • Hansen JM, Gong SG, Philbert M, Harris C. 2002. Misregulation of gene expression in the redox-sensitive NF-kappab-dependent limb outgrowth pathway by thalidomide. Dev Dyn 225: 186194.
  • Harris J, Kallen B, Robert E. 1996. The epidemiology of anotia and microtia. J Med Genet 33: 809813.
  • Heike CL, Hing AV. 2009. Craniofacial microsomia overview. In: Pagon RABT, Dolan CR, Stephens K, editors. GeneReviews, 2010/03/20 edition. Seattle: University of Washington.
  • Hoskins BE, Cramer CH, Silvius D, Zou D, Raymond RM, Orten DJ, Kimberling WJ, Smith RJ, Weil D, Petit C, Otto EA, Xu PX, Hildebrandt F. 2007. Transcription factor SIX5 is mutated in patients with branchio-oto-renal syndrome. Am J Hum Genet 80: 800804.
  • Hu D, Marcucio RS. 2009. Unique organization of the frontonasal ectodermal zone in birds and mammals. Dev Biol 325: 200210.
  • Hu Y, Baud V, Delhase M, Zhang P, Deerinck T, Ellisman M, Johnson R, Karin M. 1999. Abnormal morphogenesis but intact IKK activation in mice lacking the IKKalpha subunit of IkappaB kinase. Science 284: 316320.
  • Hunter A, Frias JL, Gillessen-Kaesbach G, Hughes H, Jones KL, Wilson L. 2009a. Elements of morphology: Standard terminology for the ear. Am J Med Genet Part A 149A: 4060.
  • Hunter AG, Collins JS, Deardorff MA, Krantz ID. 2009b. Detailed assessment of the ear in Cornelia de Lange syndrome: Comparison with a control sample using the new dysmorphology guidelines. Am J Med Genet Part A 149A: 21812192.
  • Husain T, Langlois PH, Sever LE, Gambello MJ. 2008. Descriptive epidemiologic features shared by birth defects thought to be related to vascular disruption in Texas, 1996–2002. Birth Defects Res A Clin Mol Teratol 82: 435440.
  • Ingraham CR, Kinoshita A, Kondo S, Yang B, Sajan S, Trout KJ, Malik MI, Dunnwald M, Goudy SL, Lovett M, Murray JC, Schutte BC. 2006. Abnormal skin, limb and craniofacial morphogenesis in mice deficient for interferon regulatory factor 6 (Irf6). Nat Genet 38: 13351340.
  • Ishimoto S, Ito K, Karino S, Takegoshi H, Kaga K, Yamasoba T. 2007. Hearing levels in patients with microtia: Correlation with temporal bone malformation. Laryngoscope 117: 461465.
  • Ito T, Ando H, Suzuki T, Ogura T, Hotta K, Imamura Y, Yamaguchi Y, Handa H. 2010. Identification of a primary target of thalidomide teratogenicity. Science 327: 13451350.
  • Jaffe BF. 1969. Incidence of ear diseases in Navajo Indians. Laryngoscope 79: 21262134.
  • Johnston MC, Bronsky PT. 1995. Prenatal craniofacial development: New insights on normal and abnormal mechanisms. Crit Rev Oral Biol Med 6: 368422.
  • Jugessur A, Shi M, Gjessing HK, Lie RT, Wilcox AJ, Weinberg CR, Christensen K, Boyles AL, Daack-Hirsch S, Nguyen TT, Christiansen L, Lidral AC, Murray JC. 2011. Fetal genetic risk of isolated cleft lip only versus isolated cleft lip and palate: A subphenotype analysis using two population-based studies of orofacial clefts in Scandinavia. Birth Defects Res A Clin Mol Teratol 91: 8592.
  • Julian CG, Vargas E, Armaza JF, Wilson MJ, Niermeyer S, Moore LG. 2007. High-altitude ancestry protects against hypoxia-associated reductions in fetal growth. Arch Dis Child Fetal Neonatal Ed 92: F372F377.
  • Julian CG, Wilson MJ, Lopez M, Yamashiro H, Tellez W, Rodriguez A, Bigham AW, Shriver MD, Rodriguez C, Vargas E, Moore LG. 2009. Augmented uterine artery blood flow and oxygen delivery protect Andeans from altitude-associated reductions in fetal growth. Am J Physiol Regul Integr Comp Physiol 296: R1564R1575.
  • Kaur S, Singh G, Stock JL, Schreiner CM, Kier AB, Yager KL, Mucenski ML, Scott WJ Jr, Potter SS. 1992. Dominant mutation of the murine Hox-2.2 gene results in developmental abnormalities. J Exp Zool 264: 323336.
  • Kawakami K, Sato S, Ozaki H, Ikeda K. 2000. Six family genes—structure and function as transcription factors and their roles in development. Bioessays 22: 616626.
  • Kaye CI, Rollnick BR, Hauck WW, Martin AO, Richtsmeier JT, Nagatoshi K. 1989. Microtia and associated anomalies: Statistical analysis. Am J Med Genet 34: 574578.
  • Kingsley DM, Bland AE, Grubber JM, Marker PC, Russell LB, Copeland NG, Jenkins NA. 1992. The mouse short ear skeletal morphogenesis locus is associated with defects in a bone morphogenetic member of the TGF beta superfamily. Cell 71: 399410.
  • Klieger-Grossmann C, Chitayat D, Lavign S, Kao K, Garcia-Bournissen F, Quinn D, Luo V, Sermer M, Riordan S, Laskin C, Matok I, Gorodischer R, Chambers C, Levi A, Koren G. 2010. Prenatal exposure to mycophenolate mofetil: An updated estimate. J Obstet Gynaecol Can 32: 794797.
  • Klockars T, Suutarla S, Kentala E, Ala-Mello S, Rautio J. 2007. Inheritance of microtia in the Finnish population. Int J Pediatr Otorhinolaryngol 71: 17831788.
  • Kmita M, Duboule D. 2003. Organizing axes in time and space; 25 years of colinear tinkering. Science 301: 331333.
  • Knight RD, Schilling TF. 2006. Cranial neural crest and development of the head skeleton. Adv Exp Med Biol 589: 120133.
  • Knobloch J, Shaughnessy JD Jr, Ruther U. 2007. Thalidomide induces limb deformities by perturbing the Bmp/Dkk1/Wnt signaling pathway. FASEB J 21: 14101421.
  • Konigsmark BW, Nager GT, Haskins HL. 1972. Recessive microtia, meatal atresia, and hearing loss. Report of a sibship. Arch Otolaryngol 96: 105109.
  • Kumar S, Deffenbacher K, Cremers CW, Van Camp G, Kimberling WJ. 1997. Branchio-oto-renal syndrome: Identification of novel mutations, molecular characterization, mutation distribution, and prospects for genetic testing. Genet Test 1: 243251.
  • Laclef C, Souil E, Demignon J, Maire P. 2003. Thymus, kidney and craniofacial abnormalities in Six 1 deficient mice. Mech Dev 120: 669679.
  • Lammer EJ, Chen DT, Hoar RM, Agnish ND, Benke PJ, Braun JT, Curry CJ, Fernhoff PM, Grix AW Jr, Lott IT, Richard JM, Sun SC. 1985. Retinoic acid embryopathy. N Engl J Med 313: 837841.
  • Landgraf K, Bollig F, Trowe MO, Besenbeck B, Ebert C, Kruspe D, Kispert A, Hanel F, Englert C. 2010. Sipl1 and Rbck1 are novel Eya1-binding proteins with a role in craniofacial development. Mol Cell Biol 30: 57645775.
  • Li C. 2010. A prenatally recognizable malformation syndrome associated with a recurrent post-zygotic chromosome rearrangement der(Y)t(Y;1)(q12:q21). Am J Med Genet Part A 152A: 23392341.
  • Li D, Chin W, Wu J, Zhang Q, Xu F, Xu Z, Zhang R. 2010. Psychosocial outcomes among microtia patients of different ages and genders before ear reconstruction. Aesthetic Plast Surg 34: 570576.
  • Liao J, Kochilas L, Nowotschin S, Arnold JS, Aggarwal VS, Epstein JA, Brown MC, Adams J, Morrow BE. 2004. Full spectrum of malformations in velo-cardio-facial syndrome/DiGeorge syndrome mouse models by altering Tbx1 dosage. Hum Mol Genet 13: 15771585.
  • Lin L, Pan B, Jiang HY, Zhuang HX, Zhao YY, Yang QH, He LR, Han J, Wang SJ. 2009. Study of methylation of promoter of EYA1 gene in microtia. Zhonghua Zheng Xing Wai Ke Za Zhi 25: 436439.
  • Llano-Rivas I, Gonzalez-del Angel A, del Castillo V, Reyes R, Carnevale A. 1999. Microtia: A clinical and genetic study at the National Institute of Pediatrics in Mexico City. Arch Med Res 30: 120124.
  • Ma C, Carmichael SL, Scheuerle AE, Canfield MA, Shaw GM. 2010. Association of Microtia With Maternal Obesity and Periconceptional Folic Acid Use. Am J Med Genet Part A 152A: 27562761.
  • Mallo M. 2003. Formation of the outer and middle ear, molecular mechanisms. Curr Top Dev Biol 57: 85113.
  • Marker PC, Seung K, Bland AE, Russell LB, Kingsley DM. 1997. Spectrum of Bmp5 mutations from germline mutagenesis experiments in mice. Genetics 145: 435443.
  • Martin JF, Bradley A, Olson EN. 1995. The paired-like homeo box gene MHox is required for early events of skeletogenesis in multiple lineages. Genes Dev 9: 12371249.
  • Marx H. 1926. Die Missbildungen des ohres. In: Denker A, Kahler O, editor. Handbuch der Spez Path Anatomie Histologie. Berlin, Germany: Springer. pp. 131.
  • Mastroiacovo P, Corchia C, Botto LD, Lanni R, Zampino G, Fusco D. 1995. Epidemiology and genetics of microtia-anotia: A registry based study on over one million births. J Med Genet 32: 453457.
  • Merlob P, Stahl B, Klinger G. 2009. Tetrada of the possible mycophenolate mofetil embryopathy: A review. Reprod Toxicol 28: 105108.
  • Monks DC, Jahangir A, Shanske AL, Samanich J, Morrow BE, Babcock M. 2010. Mutational analysis of HOXA2 and SIX2 in a Bronx population with isolated microtia. Int J Pediatr Otorhinolaryngol 74: 878882.
  • Mouse Genome Database (MGD). Mouse Genome Informatics website. In: Laboratory TJ, editor. Bar Harbor, Maine: World Wide Web. Available at: http://www.informatics.jax.org.
  • Naora H, Kimura M, Otani H, Yokoyama M, Koizumi T, Katsuki M, Tanaka O. 1994. Transgenic mouse model of hemifacial microsomia: Cloning and characterization of insertional mutation region on chromosome 10. Genomics 23: 515519.
  • Nazer J, Lay-Son G, Cifuentes L. 2006. Prevalence of microtia and anotia at the maternity of the University of Chile Clinical Hospital. Rev Med Chil 134: 12951301.
  • Nelson SM, Berry RI. 1984. Ear disease and hearing loss among Navajo children–a mass survey. Laryngoscope 94: 316323.
  • Ng SB, Bigham AW, Buckingham KJ, Hannibal MC, McMillin MJ, Gildersleeve HI, Beck AE, Tabor HK, Cooper GM, Mefford HC, Lee C, Turner EH, Smith JD, Rieder MJ, Yoshiura K, Matsumoto N, Ohta T, Niikawa N, Nickerson DA, Bamshad MJ, Shendure J. 2010. Exome sequencing identifies MLL2 mutations as a cause of Kabuki syndrome. Nat Genet 42: 790793.
  • Niermeyer S, Andrade Mollinedo P, Huicho L. 2009. Child health and living at high altitude. Arch Dis Child 94: 806811.
  • Niermeyer S, Yang P, Shanmina, Drolkar, Zhuang J, Moore LG. 1995. Arterial oxygen saturation in Tibetan and Han infants born in Lhasa, Tibet. N Engl J Med 333: 12481252.
  • Noden DM, Trainor PA. 2005. Relations and interactions between cranial mesoderm and neural crest populations. J Anat 207: 575601.
  • Okajima H, Takeichi Y, Umeda K, Baba S. 1996. Clinical analysis of 592 patients with microtia. Acta Otolaryngol Suppl 525: 1824.
  • Orioli IM, Castilla EE. 2000. Epidemiological assessment of misoprostol teratogenicity. BJOG 107: 519523.
  • Orstavik KH, Medbo S, Mair IW. 1990. Right-sided microtia and conductive hearing loss with variable expressivity in three generations. Clin Genet 38: 117120.
  • Otani H, Tanaka O, Naora H, Yokoyama M, Nomura T, Kimura M, Katsuki M. 1991. Microtia as an autosomal dominant mutation in a transgenic mouse line: A possible animal model of branchial arch anomalies. Anat Anz 172: 19.
  • Parman T, Wiley MJ, Wells PG. 1999. Free radical-mediated oxidative DNA damage in the mechanism of thalidomide teratogenicity. Nat Med 5: 582585.
  • Partanen J, Schwartz L, Rossant J. 1998. Opposite phenotypes of hypomorphic and Y766 phosphorylation site mutations reveal a function for Fgfr1 in anteroposterior patterning of mouse embryos. Genes Dev 12: 23322344.
  • Passos-Bueno MR, Ornelas CC, Fanganiello RD. 2009. Syndromes of the first and second pharyngeal arches: A review. Am J Med Genet Part A 149A: 18531859.
  • Perez-Aytes A, Ledo A, Boso V, Saenz P, Roma E, Poveda JL, Vento M. 2008. In utero exposure to mycophenolate mofetil: A characteristic phenotype? Am J Med Genet Part A 146A: 17.
  • POSSUM: A dysmorphology database of multiple malformations, metabolic, teratogenic, chromosomal and skeletal syndromes and their images - for learning diagnosis 2010. Melbourne, Australia: Murdoch Childrens Research Institute.
  • Poswillo D. 1973. The pathogenesis of the first and second branchial arch syndrome. Oral Surg Oral Med Oral Pathol 35: 302328.
  • Poswillo D. 1975. Hemorrhage in development of the face. Birth Defects Orig Artic Ser 11: 6181.
  • Rodriguez Soriano J. 2003. Branchio-oto-renal syndrome. J Nephrol 16: 603605.
  • Rogers B. Anatomy, embryology, and classification of auricular deformities. In: Tanzer R, Edgerton M, editors. Symposium on reconstruction of the auricle, Vol. 10. St. Louis: CV Mosby; 1974. p. 311.
  • Rollnick BR, Kaye CI. 1983. Hemifacial microsomia and variants: Pedigree data. Am J Med Genet 15: 233253.
  • Russell LB. 1971. Definition of functional units in a small chromosomal segment of the mouse and its use in interpreting the nature of radiation-induced mutations. Mutat Res 11: 107123.
  • Russell LB, Hunsicker PR, Cacheiro NL, Bangham JW, Russell WL, Shelby MD. 1989. Chlorambucil effectively induces deletion mutations in mouse germ cells. Proc Natl Acad Sci U S A 86: 37043708.
  • Sadler TW, Rasmussen SA. 2010. Examining the evidence for vascular pathogenesis of selected birth defects. Am J Med Genet Part A 152A: 24262436.
  • Scheuerle A, Heller K, Elder F. 2005. Complete trisomy 1q with mosaic Y;1 translocation: A recurrent aneuploidy presenting diagnostic dilemmas. Am J Med Genet Part A 138A: 166170.
  • Schinzel A. 2001. Catalogue of unbalanced chromosome aberrations in man. Berlin: Walter de Gruyter.
  • Schmid M, Schroder M, Langenbeck U. 1985. Familial microtia, meatal atresia, and conductive deafness in three siblings. Am J Med Genet 22: 327332.
  • Schoenwolf GC, Larsen WJ. 2009a. Development of the ears and eyes. In: Livingstone/Elsevier C, editor. Larsen's human embryology. Philadelphia: Churchill Livingstone/Elsevier.
  • Schoenwolf GC Larsen WJ 2009b Development of the pharyngeal apparatus and face. In: Livingstone/Elsevier C, editor. Larsen's Human Embryology. Philadelphia: Churchill Livingstone/Elsevier.
  • Shaw GM, Carmichael SL, Kaidarova Z, Harris JA. 2004. Epidemiologic characteristics of anotia and microtia in California 1989–1997. Birth Defects Res A Clin Mol Teratol 70: 472475.
  • Steffen A, Wollenberg B, Konig IR, Frenzel H., 2010. A prospective evaluation of psychosocial outcomes following ear reconstruction with rib cartilage in microtia. J Plast Reconstr Aesthet Surg 63: 14661473.
  • Stern RS, Rosa F, Baum C. 1984. Isotretinoin and pregnancy. J Am Acad Dermatol 10: 851854.
  • Stevenson RE. 2006. Human malformations and related anomalies. Oxford New York: Oxford University Press.
  • Strisciuglio P, Ballabio A, Parenti G. 1986. Microtia with meatal atresia and conductive deafness: Mild and severe manifestations within the same sibship. J Med Genet 23: 459460.
  • Suutarla S, Rautio J, Ritvanen A, Ala-Mello S, Jero J, Klockars T. 2007. Microtia in Finland: Comparison of characteristics in different populations. Int J Pediatr Otorhinolaryngol 71: 12111217.
  • Tanzer RC. 1978. Microtia. Clin Plast Surg 5: 317336.
  • Tasse C, Bohringer S, Fischer S, Ludecke HJ, Albrecht B, Horn D, Janecke A, Kling R, Konig R, Lorenz B, Majewski F, Maeyens E, Meinecke P, Mitulla B, Mohr C, Preischl M, Umstadt H, Kohlhase J, Gillessen-Kaesbach G, Wieczorek D. 2005. Oculo-auriculo-vertebral spectrum (OAVS): Clinical evaluation and severity scoring of 53 patients and proposal for a new classification. Eur J Med Genet 48: 397411.
  • Timmer JR, Mak TW, Manova K, Anderson KV, Niswander L. 2005. Tissue morphogenesis and vascular stability require the Frem2 protein, product of the mouse myelencephalic blebs gene. Proc Natl Acad Sci USA 102: 1174611750.
  • Trainor PA. 2010. Craniofacial birth defects: The role of neural crest cells in the etiology and pathogenesis of Treacher Collins syndrome and the potential for prevention. Am J Med Genet Part A 152A: 29842994.
  • Vargas FR, Schuler-Faccini L, Brunoni D, Kim C, Meloni VF, Sugayama SM, Albano L, Llerena JC Jr, Almeida JC, Duarte A, Cavalcanti DP, Goloni-Bertollo E, Conte A, Koren G, Addis A. 2000. Prenatal exposure to misoprostol and vascular disruption defects: A case-control study. Am J Med Genet 95: 302306.
  • Vrontou S, Petrou P, Meyer BI, Galanopoulos VK, Imai K, Yanagi M, Chowdhury K, Scambler PJ, Chalepakis G. 2003. Fras1 deficiency results in cryptophthalmos, renal agenesis and blebbed phenotype in mice. Nat Genet 34: 209214.
  • Watson WJ, Katz VL, Albright SG, Rao KW, Aylsworth AS. 1990. Monozygotic twins discordant for partial trisomy 1. Obstet Gynecol 76: 949951.
  • Weerda H. 1988. Classification of congenital deformities of the auricle. Facial Plast Surg 5: 385388.
  • Wright TJ, Mansour SL. 2003. Fgf3 and Fgf10 are required for mouse otic placode induction. Development 130: 33793390.
  • Yamada G, Mansouri A, Torres M, Stuart ET, Blum M, Schultz M, De Robertis EM, Gruss P. 1995. Targeted mutation of the murine goosecoid gene results in craniofacial defects and neonatal death. Development 121: 29172922.
  • Yamaguchi TP, Bradley A, McMahon AP, Jones S. 1999. A Wnt5a pathway underlies outgrowth of multiple structures in the vertebrate embryo. Development 126: 12111223.
  • Yang J, Carmichael SL, Kaidarova Z, Shaw GM. 2004. Risks of selected congenital malformations among offspring of mixed race-ethnicity. Birth Defects Res A Clin Mol Teratol 70: 820824.
  • Zabihi S, Loeken MR. 2010. Understanding diabetic teratogenesis: Where are we now and where are we going? Birth Defects Res A Clin Mol Teratol 88: 779790.
  • Zamudio S, Baumann MU, Illsley NP. 2006. Effects of chronic hypoxia in vivo on the expression of human placental glucose transporters. Placenta 27: 4955.
  • Zankl M, Zang KD. 1979. Inheritance of microtia and aural atresia in a family with five affected members. Clin Genet 16: 331334.
  • Zeng S, Patil SR, Yankowitz J. 2003. Prenatal detection of mosaic trisomy 1q due to an unbalanced translocation in one fetus of a twin pregnancy following in vitro fertilization: A postzygotic error. Am J Med Genet Part A 120A: 464469.
  • Zhang Q, Zhang J, Yin W. 2010. Pedigree and genetic study of a bilateral congenital microtia family. Plast Reconstr Surg 125: 979987.
  • Zhang QG, Zhang J, Yu P, Shen H. 2009. Environmental and genetic factors associated with congenital microtia: A case-control study in Jiangsu, China, 2004 to 2007. Plast Reconstruct Surg 124: 11571164.
  • Zhu J, Wang Y, Liang J, Zhou G. 2000. An epidemiological investigation of anotia and microtia in China during 1988–1992. Zhonghua Er Bi Yan Hou Ke Za Zhi 35: 6265.