RSH/Smith-Lemli-Opitz syndrome is an autosomal recessive syndrome due to an inborn error of cholesterol metabolism and is characterized by developmental delay, facial anomalies, hypospadias, congenital heart defect (CHD), postaxial polydactyly, and 2–3 toe syndactyly. CHD is found in half of the propositi, and a specific association with atrioventricular canal defect (AVCD) and anomalous pulmonary venous return has been demonstrated.
We report on an additional patient with RSH/SLOS presenting with complete AVCD and anomalous pulmonary venous return, and discuss the possible relationship of the Sonic Hedgehog (SHH) pathway as causative factor of these CHDs and those in heterotaxia patients with postaxial polydactyly syndromes.
Anatomic similarities between heterotaxia and CHDs of several syndromes with postaxial polydactyly have been noted previously, considering the frequent association of AVCD with common atrium in these conditions. It is known that both CHDs of heterotaxia and postaxial polydactyly can be related to abnormalities of the SHH pathway. Cholesterol has a critical role in the formation of normally active hedgehog proteins. It could be hypothesized that specific types of CHDs in RSH/SLOS can be caused by modifications of the SHH protein related to the defect of cholesterol biosynthesis.
The RSH/Smith-Lemli-Opitz syndrome (RSH/SLOS) is an autosomal recessive syndrome characterized by developmental delay, growth retardation with feeding difficulties, microcephaly, characteristic facial anomalies, cataracts, cleft palate, hypospadias, congenital heart defect (CHD), frequent postaxial polydactyly, and even more common, 2–3 toe syndactyly (Opitz, 1999a, b, 2001, 2002; Kelley and Hennekam, 2000). RSH/SLOS is due to an inborn error of cholesterol metabolism (Irons et al., 1993; Tint et al., 1994). Deficient 7-dehydrocholesterol-Δ-7 reductase (DHCR7) activity results in reduced plasma and tissue cholesterol levels and elevated 7-dehydrocholesterol concentrations (Shefer et al., 1995; Honda et al., 1997). The human DHCR7 gene has been cloned and localized to chromosome region 11q12–13 (Moebius et al., 1998).
Congenital heart defect is reported in one-half of the patients with RSH/SLOS (Robinson et al., 1971; Johnson, 1975; Lin et al., 1997; Burn and Goodship, 2002). Atrioventricular canal defect (AVCD; also known as atrioventricular canal and endocardial defect) and septal defects are diagnosed most commonly (Lin et al., 1997; Burn and Goodship, 2002), and a strikingly specific association of RSH/SLOS with AVCD and anomalous pulmonary venous return was demonstrated recently (Lin et al., 1997).
We report on a patient with apparent RSH/SLOS, presenting with complete AVCD and anomalous pulmonary venous return. The involvement of abnormalities of cholesterol metabolism in the development of these cardiac defects is corroborated, and we discuss the possible relationship of the Sonic Hedgehog (SHH) pathway as causative factor of these CHDs and those in heterotaxia patients with postaxial polydactyly syndromes.
The female patient is the third child of a consanguineous Libyan couple who are first cousins. From age 21, the mother was treated for insulin-dependent diabetes. At the time of the birth of the proposita, the father was 44 years and the mother 32 years of age. The baby was born by cesarean section at the 38th week of an uneventful gestation. Birth weight was 3500 g; the perinatal period was considered normal. The patient was first evaluated by us at 6 months, when the baby was transferred to Rome from Libya for a suspected CHD. Weight was 6500 gm (3rd–10th centile), with a length of 62 cm (<3rd centile), and a head circumference (OFC) of 39 cm (<3rd centile). The baby had microcephaly, craniofacial asymmetry, bilateral ptosis, anteverted nostrils, micrognathia, large low-set ears (Fig. 1). There was bilateral 2–3 toe syndactyly and abnormal positioning of the toes. Genitalia were normal. The baby was hypotonic, psychomotor development was delayed, and a cataract was present in both eyes. Results of cerebral and renal ultrasonographies were normal. Echocardiography showed a very unusual cardiac malformation comprising visceroatrial situs solitus, d-loop of the ventricles, complete AVCD Rastelli type A with single papillary muscle of the left ventricle, and a supracardiac form of total anomalous pulmonary venous return (left pulmonary veins draining to a vertical vein into the superior vena cava, right pulmonary vein draining into a stenotic vertical vein and, partly, into a coronary sinus through a single venous channel) (Fig. 2) (Abbattista et al., 1994). The patient underwent a successful correction of the cardiac defect. Chromosome analysis on blood peripheral lymphocytes showed a 46,XX karyotype.
At a recent re-evaluation, RSH/SLOS was suspected and confirmed biochemically. Gas-chromatography/mass spectrometry analysis (Tint et al., 1994) showed elevated 7-dehydrocholesterol levels (367 μmol/l; normal <7) and a low cholesterol level (220 μmol/l, age-matched controls range 2100–5300).
CHDs are reported in almost 50% of RSH/SLOS propositi (Lin et al., 1997; Burn and Goodship, 2002), and the anatomic types were reviewed extensively by Lin et al. (1997). In that article, a surprisingly high prevalence of AVCD and anomalous pulmonary return, as isolated or associated defects, was detected in patients with RSH/SLOS. Several patients with spleen abnormalities and CHD were described, although there were no gross cases of heterotaxy. It has been speculated that the AVCD excess may reflect alterations in cell morphology and loss of cellular attachments, resulting in defective migration of cells during development of the endocardial cushions (Dehart et al., 1997; Kelley, 1997).
AVCD is a causally heterogeneous malformation (Digilio et al., 1999a). It can occur in Down syndrome (Rowe and Uchida, 1961; Marino, 1993) and in subjects with a great variety of chromosomal abnormalities, Mendelian syndromes and associations (De Biase et al., 1986; Marino et al., 1990; Carmi et al., 1992; Digilio et al., 1999a). AVCD can be a component of heterotaxia with situs ambiguus (Peoples et al., 1983; Phoon and Neill, 1994). Heterotaxia is a malformation complex comprising abnormal arrangement of abdominal and thoracic organs with complex CHDs including AVCD, common atrium, anomalous systemic and pulmonary venous drainage, persistent left superior vena cava with unroofed coronary sinus, and conotruncal defects (Webber et al., 1992; Phoon and Neill, 1994). Extracardiac anomalies can also be present in association with heterotaxia (Ticho et al., 2000).
An interesting finding consists of the presence of AVCD, particularly with common atrium, in several syndromes with postaxial polydactyly, including Ellis–van Creveld syndrome, short rib-polydactyly syndromes, oral-facial-digital syndromes, and hydrolethalus syndrome (Digilio et al., 1999b). A phenotypic overlap has been noted between oral-facial-digital syndromes, short rib-polydactyly syndromes, hydrolethalus syndrome, and Pallister–Hall syndrome. This observation suggested that these conditions, all with hand or foot postaxial polydactyly, may be causally related and prompted the denomination “oral-facial-skeletal syndromes” (Neri et al., 1995; Digilio et al., 1997). Anatomical similarities between the heart malformations of the “oral-facial-skeletal” syndromes with polydactyly and those of heterotaxia have been noted (Digilio et al., 1999b). In fact, the association of AVCD and common atrium found in patients with “oral-facial-skeletal” syndromes is a distinguishing manifestation of heterotaxia with polysplenia (Peoples et al., 1983).
Postaxial polydactyly is common in patients with RSH/SLOS (Kelley and Hennekam, 2000), mostly affecting hands, and, less commonly, the feet. As shown in the study by Lin et al. (1997), a specific association with AVCD is detectable also in RSH/SLOS, similar to that occurring in other conditions with polydactyly. Striking differences in severity are found in patients with RSH/SLOS, including those with the same genotype, so that a distinction between a mild–moderate Type I and a severe type II RSH/SLOS has fallen away. CHDs are more common (80%) in the more severe form (Curry et al., 1987; Verloes, 1995). Infants affected severely often manifest severe failure to thrive and markedly reduced survival.
It is known that several postaxial polydactyly syndromes share many manifestations with severely affected RSH/SLOS patients. For example, the intraoral anomalies of RSH/SLOS, such as cleft palate and anomalies of the tongue, which can be small, bifid, or grooved, with sublingual cysts (Ryan et al., 1998), recall those found in oral-facial-digital syndromes. Moreover, polydactyly, oral anomalies, epiglottic and laryngeal clefts, lung lobation anomalies, imperforate anus, ambiguous genitalia, short limbs and specific heart malformations are the component manifestations described in several genetic conditions that are probably related morphogenetically to the RSH/SLOS, such as hydrolethalus syndrome and Pallister–Hall hypothalamic hamartoblastoma syndrome (Hall et al., 1980; Salonen et al., 1981; Fraser et al., 1989; Neri et al., 1995).
Pallister-Hall syndrome is an autosomal dominant multiple congenital anomalies/mental retardation syndrome caused by mutations of the human GLI3 gene, a transcription factor critical to normal limb pattern development. GLI3 is defective also in the Greig cephalopolysyndactyly syndrome, in postaxial polydactyly Type A1 and B, and in pre-axial polydactyly Type IV (Kang et al., 1997; Radhakrishna et al., 1997; Wild et al., 1997). Most of the clinical characteristics of these four conditions are distinct, but all share polydactyly. It has been suggested that there is an association between the site of GLI3 protein truncation and the phenotypes in syndromes caused by GLI3 mutations (Biesecker, 1997). The GLI protein family composed by GLI1, GLI2, and GLI3 are vertebrate homologs of the Drosophila cubitus interruptus (Ci) protein, which is an important mediator of Hedgehog signaling (Johnson and Tabin, 1997). Hedgehog in Drosophila and its homologous gene in vertebrates, SHH, are important in regulating the anterior-posterior limb pattern formation and in specifying the fate of cells of the ventral aspect of the developing vertebrate nervous system (Echelard et al., 1993; Marigo et al., 1995; Goetz et al., 2001).
The GLI family of transcription factors plays a key role in transducing the SHH morphogen signal responsible for limb pattern formation in vertebrates, and GLI3 seems to play a role in SHH repression (Johnson and Tabin, 1997). An alterated GLI3 function could lead to both isolated and syndromic forms of polydactyly due to abnormal antagonistic action of GLI3 and SHH in anterior-posterior limb pattern formation.
Cholesterol has a critical role in the formation of normally active hedgehog proteins (Chamoun et al., 2001; Gingham, 2001). The formation of a cholesterol-modified hedgehog protein is apparently essential for the regional localization and concentration of hedgehog in various organizing centers of the developing Drosophila embryo (Porter et al., 1996). Mice homozygous for a disrupted SHH gene show absence of distal limb structures, foregut defects, absence of the spinal column and most of the ribs, defects in the development of midline neural structures, absence of ventral cells in the brain and craniofacial anomalies, including cyclopia and a proboscis-like nasal structure (Chiang et al., 1996; Litingtung et al., 1998). Because mice lacking SHH function manifest cyclopia (Chiang et al., 1996) and mutations of human SHH can be associated with classical autosomal dominant holoprosencephaly (Röessler et al., 1996), it has been suggested that the holoprosencephaly found in some patients with RSH/SLOS may be due to the abnormal function of SHH. In fact, an incomplete processing of the SHH protein and subsequent deficient cholesterol-modified SHH signaling could be due to low levels of cholesterol characteristic of RSH/SLOS patients (Kelley et al., 1996).
Considering that SHH has an important role in multiple embryonic tissues (Chiang et al., 2001; Gritli-Linde et al., 2001), abnormal processing of Hedgehog proteins secondary to abnormal cholesterol metabolism may have a role in the development of other malformations of RSH/SLOS.
In regard to cardiac defects, it has been found that SHH is involved in left-right axis development. In fact, SHH is required for the development of the midline and left-right asymmetry. The phenotype of SHH (−/−) mice includes CHDs in the setting of heterotaxia and left pulmonary isomerism (Chiang et al., 1996; Meyers and Martin, 1999; Tsukui et al., 1999). Because expression of left-side-specific genes is observed bilaterally, a failure of the abnormal midline to restrict contralateral spread of asymmetric signals is suspected to be involved in this phenotype. Normally expressed at the midline, lefty-1 is not expressed at all in SHH −/− mice, suggesting that SHH directly regulates expression of lefty-1. The SHH −/− phenotype is similar to that observed in lefty-1 −/− mice, suggesting that at least part of the function of SHH in left-right development is to induce lefty-1 expression (Chiang et al., 1996; Meyers and Martin, 1999; Tsukui et al., 1999).
As putatively occurring for holoprosencephaly in RSH/SLOS, specific types of CHD in this syndrome may be caused by modifications of the SHH protein related to the defect of cholesterol biosynthesis. The specific association of AVCD and anomalous pulmonary venous return (alone or in association) in patients with RSH/SLOS and the finding of AVCD ± common atrium in the oral-facial-skeletal syndromes leads to the hypothesis that heterotaxia due to SHH anomalies could be involved in a large spectrum of syndromes with polydactyly. A putative pathogenetic pathway involving SHH altered function is constructed in Figure 3. Moreover, a possible role of the neural crest cells has been suggested (Poelmann and Gittenberger-de Groot, 1999) for the development of the venous pole of the heart, but also for the morphogenesis of the AVCD.
In conclusion, the direct and indirect involvement of SHH in the development of both CHDs in the setting of heterotaxia and postaxial polydactyly is well established. CHDs included in the spectrum of heterotaxia as AVCD, common atrium, and anomalous pulmonary venous return are associated specifically with several genetic syndromes with postaxial polydactyly, such as short rib-polydactyly syndromes, oral-facial-digital syndromes and RSH/SLOS. It is hypothesized that perturbations of different components of the SHH pathway can lead to several developmental errors presenting with partially overlapping manifestations. Thus, sterol testing should be carried out in the oral-facial-skeletal syndromes.