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Keywords:

  • polysplenia;
  • asplenia;
  • heterotaxy;
  • Mendelian inheritance;
  • neural tube defect;
  • left-right asymmetry

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CLINICAL REPORTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Disturbances of the normal asymmetric placement of organs, such as polysplenia or situs inversus, have been defined traditionally as laterality defects. However, there is compelling evidence from vertebrate models and human birth defects to hypothesize that defects of the midline, isolated congenital heart defects, and laterality defects are etiologically related. We present the clinical characteristics of three families that exhibit a variety of midline defects and isolated heart defects in addition to laterality defects. These observations suggest that the phenotypic consequences of mutations causing laterality defects include defects of the midline as well as isolated heart defects. To further explore the relationship between midline, heart, and laterality defects, it is imperative that detailed phenotyping of individuals and families with laterality defects be done and a classification system created to facilitate identification of genes causing human laterality disorders. © 2001 Wiley-Liss, Inc.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CLINICAL REPORTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

All vertebrates have a body plan with a midline that defines left and right halves of the body that are symmetric for most external features. However, many internal organs have asymmetric placement with respect to the midline. For example, normally the heart is positioned in the left hemithorax with its apex pointed toward the left. Additionally, the spleen and stomach are placed on left side of the abdominal cavity, and the small bowel loops in a counterclockwise direction. Alterations of the left–right (LR) asymmetry may cause randomization of the position of organs (i.e., heterotaxia or situs ambiguus) or they may produce reversal of LR organ position (i.e., situs inversus). Disturbances of LR asymmetry may also be limited to a single asymmetric organ such as the heart in individuals with isolated dextrocardia. Collectively, we refer to any abnormality of LR asymmetry as a laterality defect.

It has long been recognized that there is a genetic basis for some disturbances of LR asymmetry in humans. Although most cases are considered sporadic, families with laterality defects segregating in autosomal dominant [Casey et al., 1996], autosomal recessive [Debrus et al., 1997], and X-linked recessive [Gebbia et al., 1997] inheritance patterns have been reported. Thus, multiple genes exist that are responsible for human laterality defects, although mutations of single genes may cause laterality defects that segregate in Mendelian patterns.

Our understanding of the molecular basis of the development of LR asymmetry in a variety of model organisms has substantially increased during the last 5 years. Genes controlling LR asymmetry have been identified in animals such as Drosophila, roundworm, zebrafish, frog, chick, and mouse. Nevertheless, identification of genes causing laterality defects in humans has remained challenging.

During the last few years, it has become increasingly clear that model organisms with disturbances of LR asymmetry also have defects of the midline and/or isolated heart defects more commonly than expected by chance. In humans, congenital midline and isolated heart defects are common in children with disturbances of LR asymmetry, and the spectrum of cardiovascular anomalies found in individuals with laterality defects is wide. Collectively, these observations suggest that the phenotypic consequences of mutations causing laterality defects include defects of the midline as well as isolated heart defects. To this end, we present a clinical analysis of three families, each of which consists of individuals with either midline, laterality, and/or heart defects.

CLINICAL REPORTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CLINICAL REPORTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

All studies were performed with the approval of the institutional review board of the University of Utah. All living individuals were evaluated by undergoing a physical examination, a chest radiograph, an echocardiogram, and an abdominal ultrasound scan. Specific diagnostic criteria for inclusion were formulated prospectively. Major diagnostic criteria included: vascular structures with abnormal LR positioning, randomization of organ placement, left or right isomerism, reversal of individual organ symmetry, abnormal rotation of the small bowel, and reversal or symmetric lobation of the lungs. Minor diagnostic criteria included: isolated congenital heart defects, neural tube defects, anal atresia, omphalocele, and hypospadias. Each of the findings included in the minor criteria has been reported previously in families in which laterality defects segregate in Mendelian patterns. Individuals with one major criterion or one minor criterion and a family history of a laterality defect were considered to be affected. Individuals with a specific syndrome diagnosis (e.g., trisomy 18) and a laterality defect were excluded. First-degree relatives of each proband, known affected relatives, and all living family members intervening between affected individuals were evaluated. Abbreviated pedigrees were constructed showing only members of a family who underwent full evaluation.

Family 1

Figure 1 illustrates the pedigree of a family with individuals having laterality defects, isolated heart defects, and neural tube defects. The proband, IV-3, presented at birth with multiple congenital heart defects including dextrocardia, a left-dominant unbalanced atrioventricular septal defect, l-transposition of the great arteries, aortic coarctation status post repair, and anomalous pulmonary venous return to the right atrium. In addition, she had asplenia, gastrointestinal malrotation, bilateral trilobed lungs, a right aortic arch, and bilateral superior venae cavae. She is currently 6 years old status post a Glenn procedure and doing relatively well.

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Figure 1. Pedigree of family 1.

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Her family history revealed that her sister (IV-4) died at 1 year of age from a complex congenital heart defect consisting of dextrocardia, a single ventricle, and pulmonary outflow obstruction. Additionally, IV-4 had asplenia. A maternal half-second cousin (IV-7) had isolated tricuspid atresia and is now in her 20s and doing well after a Fontan procedure. Another half-second cousin (IV-11) had an isolated lumbar meningomyelocele. Of the five individuals intervening IV-3 and IV-7, two had isolated laterality defects. Individual III-6, the mother of the girl with a meningomyelocele, was found to have a right-sided abdominal aorta and a midline inferior vena cava, and her mother (II-4) had a midline abdominal aorta. Thus, disturbances of LR asymmetry were found in the apparently “normal” parents of offspring with midline defects.

Based on our diagnostic criteria, the laterality defects in this family appeared to be segregating in an autosomal dominant pattern with reduced penetrance. Subsequently, a genome-wide screen for genes causing laterality defects was performed using an autosomal dominant model of transmission with penetrance varied between .67 and .95. A maximal two-point LOD score of 3.05 (θ = 0.0) was obtained with marker D3S1261 and a penetrance value of .85. The maximal multipoint LOD score was 4.05. Fine mapping defined a critical interval of 22 cM between D3S2329 and D3S1663, corresponding to chromosome bands 3p12–3p21.

Family 2

Figure 2 illustrates the pedigree of a family in which multiple individuals have a variety of isolated disturbances of LR patterning of the heart. The proband (III-12) was diagnosed prenatally with a congenital heart defect and a right-sided stomach. At birth, she was found to have pulmonary atresia, a common atrioventricular valve with an atrioventricular septal defect, and secundum and primum atrial septal defects. She also had abdominal situs ambiguus with asplenia, a right aortic arch, and bilateral superior venae cavae. She underwent a central shunt in the newborn period and is status post a Glenn procedure at 7 months of age.

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Figure 2. Pedigree of family 2.

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A paternal aunt (II-4) had died at 6 years of age with transposition of the great arteries and congenital heart block. A first cousin (III-3) of the proband was found at birth to have mitral atresia, a functional single ventricle with left atrioventricular valve atresia, pulmonary valve stenosis, and d-transposition of the great arteries. She also underwent a Glenn procedure at 14 months of age. On screening evaluation, another first cousin (III-7) to the proband had a bicuspid aortic valve, and her mother (II-6) had a midline abdominal aorta. Linkage to chromosome 3p12-3p21 was excluded in this family.

Family 3

In the pedigree in Figure 3, the proband (IV-1) was born with a single atrium, an atrioventricular septal defect with a cleft mitral valve, and polysplenia. He underwent repair of his defects at 4 years of age and subsequently required pacemaker placement. His brother (IV-3) was found at birth to have dysplastic and stenotic aortic and pulmonary valves and an anomalous left coronary artery arising from the single right coronary artery. He also had hyperbilirubinemia and neonatal hepatitis, although biliary atresia was excluded. He died at 1 month of age with cardiac failure and multiple cardiac infarcts. At autopsy no other findings of laterality defects were noted. A second cousin (IV-9) of the proband died at 2 days of age with a hypoplastic left heart, mitral atresia, and an atretic aortic arch. His father (III-3) was found to have an interrupted inferior vena cava and a midline liver, although he was an otherwise healthy 53-year-old man who was unaware of any anatomical abnormality before these studies. Another second cousin (IV-11) of the proband had an aortic coarctation but was otherwise normal.

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Figure 3. Pedigree of family 3.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CLINICAL REPORTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

A considerable amount of information has been collected about the genetic control of developmental pathways coordinating LR asymmetry in animal models. Somewhat unexpectedly, these studies have made it apparent that some of the mediators of LR development are conserved among even distantly related organisms such as Drosophila and mouse. The success of this strategy has inspired the search, in homologous human genes, for mutations causing disturbances of LR asymmetry. However, identification of mutations causing human laterality defects has lagged considerably behind the rapid discoveries of genes causing defects in model organisms.

There are several possible explanations for this observation. Mutations of the same gene may cause different phenotypes in humans and nonhuman models. For example, a mutation in the gene encoding Connexin43 gap junction protein was found in some children with disturbances of LR asymmetry and congenital heart defects [Britz-Cunningham et al., 1995]. However, inactivation of connexin43 in mice produced obstruction of the right ventricular outflow tract [Reaume et al., 1995], a phenotype different than that observed in humans. It is also becoming evident that mutations in the same gene may cause substantially different phenotypes among humans, and that similar phenotypes may be produced by mutations in different genes. Thus, further definition of the molecular pathways controlling human LR asymmetry will likely require the exhaustive collection of phenotypic data, explicit definition of what findings constitute laterality defects, and stepwise inclusion criteria for diagnosis. In this article, we have presented the phenotypic characteristics of three families that we think move forward the discussion about whether some isolated heart defects and midline defects are etiologically related to disturbances of LR asymmetry.

In each of the pedigrees presented here, individuals with midline defects or isolated heart defects are found segregating in the same family as individuals who have abnormalities that would be defined traditionally as disturbances of LR asymmetry. Similar pedigrees have been reported previously but with less emphasis on the possible etiological relationship between midline and laterality defects. For example, in the families with mutations in ZIC3 [Gebbia et al., 1997], there is clearly a broader phenotype than only laterality defects. Indeed, nearly 50% of the affected males had lumbosacral or anal anomalies, and two of three affected females had anal atresia.

Additionally, of the 15–50% of individuals with laterality defects who also have other malformations, the most common defects observed are anomalies of the midline [Martinez-Frias, 1995]. In our retrospective review of more than 100 probands with laterality defects, ∼ 15% had anomalies of the midline [Morelli et al., 1998b], including tracheoesophageal fistula with esophageal atresia, scoliosis, neural tube defects, omphalocele, and anal atresia. Individuals with severe early midline defects such as anencephaly, sirenomelia, or holoprosencephaly are also reported to have laterality defects [Morelli et al., 1998a; Bonneau et al., 1999]. In 1995, Martinez-Frias demonstrated that the risk of having a laterality defect is approximately three times higher in children with midline anomalies compared to children with nonmidline anomalies, and ∼ 100 times higher than the background population risk.

The spectrum of cardiovascular anomalies found in individuals with laterality defects is wide, ranging from asymptomatic malformations (e.g., a left-sided inferior vena cava) to complex and life-threatening defects (e.g., right-sided isomerism with total anomalous pulmonary venous return). Furthermore, mutations in genes controlling LR asymmetry have been found recently in individuals with isolated transposition of the great arteries [Bamford et al., 2000]. Overall, these data suggest that isolated heart defects may be caused by mutations in genes controlling midline and the development of LR asymmetry.

The genes involved in LR asymmetry are being elucidated rapidly in vertebrate models, and the importance of the midline in generation and maintenance of LR asymmetry is also emerging. Many of these genes are expressed in the embryonic midline (e.g., nodal and lefty1) [Ramsdell and Yost, 1998]. It is possible that anomalies at the midline may alter expression of these genes and lead to laterality defects or that altered expression of these genes causes anomalies of the midline. Experiments by Danos and Yost [1996] showed that extirpation of the presumptive notochord and floorplate during open neural plate stages causes cardiac reversals and bilateral expression of nodal in lateral plate mesoderm. Thus, at certain periods of development, the midline may function as a physical barrier to prevent spread of signaling molecules. Anomalies of the midline in humans may lead to laterality defects via a similar mechanism.

It was noted several years ago that at least 21 known zebrafish midline mutants also exhibit laterality defects [Chen et al., 1997]. Bisgrove et al. [2000] described an elegant analysis looking at nine of these zebrafish mutants, noting the type of midline defect and the expression patterns of asymmetric markers in the brain, heart, and gut. Four classes of defects were defined and correlated the midline phenotype with the expression pattern of asymmetric markers. They hypothesized that distinct genetic pathways are expressed in specific domains along the anterior–posterior axis of the embryo and regulate distinct steps in the development of LR asymmetry in the brain, heart, and gut. Thus, it may be possible to correlate the location and type of human midline defects with specific laterality defects, and create a classification of human laterality defects based on the developmental programs that are perturbed.

In summary, some laterality defects in humans may be caused primary disturbances of the embryonic midline with or without producing an observable midline anomaly. In some cases, early midline defects such as anencephaly may result simply in a lack of integrity of the normal structural components of the midline that leaves the developmental pathways for LR asymmetry vulnerable to perturbation. In other cases it may be that abnormalities of the midline are caused by alterations of a molecule that plays a role both in midline development and in the establishment of LR asymmetry. Molecules that subsequently control the asymmetric placement of organs or control individual organ asymmetry may be responsible for isolated organ defects (e.g., congenital heart defects). To further explore the relationship between midline, heart, and laterality defects, it is imperative that detailed phenotyping of individuals and families with laterality defects be done and a classification system be created to facilitate the identification of genes causing human laterality disorders.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CLINICAL REPORTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

We thank the families for their participation, generosity, and patience. We also thank Joe Yost and Ed Clark for discussion and comments. We thank Stacy Maxwell, Eric Rosenthal, Laura Newren, Heidi Pollard, Chuck Williams, and the members of the Division of Pediatric Cardiology for their assistance. This work was supported by grants from the Shriners Hospitals for Children (8510 and 8520), the General Clinical Research Center at the University of Utah (PHS MO1-00064), the International Clinical Genetics Research Program, and the Primary Children's Foundation (S.M.). Susan Morelli is a Howard Hughes Medical Institute Physician Postdoctoral Fellow.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CLINICAL REPORTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  • Bamford RN, de la Cruz J, Roessler E, Saplakoglu U, Burdine R, Goldmuntz E, Shen M, Schier A, Casey B, Muenke M. 2000. Mutations in the EGF-CFC gene, CRYPTIC, cause human left-right axis abnormalities and transposition of the great arteries. Am J Hum Genet 67: A10.
  • Bisgrove BW, Essner JJ, Yost HJ. 2000. Multiple pathways in the midline regulate concordant brain, heart, and gut left-right asymmetry. Development 127: 35673579.
  • Bonneau D, Marechaud M, Odent S, Piegay I, Godard A, Amati P. 1999. Heterotaxy-neural rube defect and holoprosencephaly occurring independently in two sib fetuses. Am J Med Genet 84: 373376.
  • Britz-Cunningham SH, Shah MM, Zuppan CW, Fletcher WH. 1995. Mutations of the Connexin43 gap-junction gene in patients with heart malformations and defects of laterality. N Engl J Med 332: 13231329.
  • Casey B, Cuneo BF, Vitali C, van Hecke H, Barrish J, Hicks J, Ballabio A, Hoo JJ. 1996. Autosomal dominant transmission of familial laterality defects. Am J Med Genet 61: 325328.
  • Chen JN, van Eeden FJ, Warren KS, Chin A, Nusslein-Volhard C, Haffter P, Fishman MC. 1997. Left-right pattern of cardiac BMP4 may drive asymmetry of the heart in zebrafish. Development 124: 43734382.
  • Danos MC, Yost HJ. 1996. Role of notochord in specification of cardiac left-right orientation in zebrafish and Xenopus. Dev Biol 177: 96103.
  • Debrus S, Sauer U, Gilgenkrantz S, Jost W, Jesberger HJ, Bouvagnet P. 1997. Autosomal recessive lateralization and midline defects: blastogenesis recessive 1. Am J Med Genet 68: 401404.
  • Gebbia M, Ferrero GB, Pilia G, Bassi MT, Aylsworth A, Penman-Splitt M, Bird LM, Bamforth JS, Burn J, Schlessinger D, Nelson DL, Casey B. 1997. X-linked situs abnormalities result from mutations in ZIC3. Nat Genetics 17: 305308.
  • Martinez-Frias ML. 1995. Primary midline developmental field. I. Clinical and epidemiological characteristics. Am J Med Genet 56: 374381.
  • Morelli SH, Pysher TJ, Gilbert EF, Opitz JM, Viskochil DH. 1998a. Pathogenesis of sirenomelia with anencephaly and other midline defects. Pediatr Res 43: 65A.
  • Morelli SH, Ruttenberg H, Yost HJ, Bamshad M. 1998b. Spectrum of birth defects in children with laterality disorders. Am J Hum Genet 68: A114.
  • Morelli SH, Pagotto LT, Reid B, Ruttenberg H, Yost HJ, Bamshad M. 1999. A gene for laterality defects maps to 3p14.2-3p12. Am J Hum Genet 65: A31.
  • Ramsdell AF, Yost HJ. 1998. Molecular mechanisms of vertebrate left-right development. Trends Genet 14: 459465.
  • Reaume AG, de Sousa PA, Kulkarni S, Langille BL, Zhu D, Davies TC, Juneja SC, Kidder GM, Rossant J. 1995. Cardiac malformation in neonatal mice lacking connexin43. Science 267: 18311834.