A new face for an old syndrome

Carrier frequency and incidence estimation of Smith-Lemli-Opitz syndrome in East Asian populations by Genome Aggregation Database (gnomAD) based analysis

Background

Smith–Lemli–Opitz syndrome (SLOS) is an autosomal, recessively inherited congenital malformation syndrome characterized by multiple congenital anomalies such as microcephaly with mental defects, distinctive facial features, genital abnormalities, and 2–3 syndactyly of the toes. SLOS is caused by defective 7-dehydrocholesterol reductase, which is encoded by the DHCR7 gene. This study aimed to analyze the carrier frequency and expected incidence of SLOS in East Asians and Koreans using exome data from the Genome Aggregation Database (gnomAD) through the 2015 American College of Medical Genetics and Genomics and the Association for Molecular Pathology guideline (2015 ACMG-AMP guideline).

Methods

We analyzed 9197 exomes for East Asian populations from gnomAD, comprising 1909 Korean, 76 Japanese, and 7212 other East Asian populations. All identified variants were classified according to the 2015 ACMG-AMP guideline.

Results

According to the 2015 ACMG-AMP guideline, 15 pathogenic variant/likely pathogenic variant (PV/LPV) cases were identified in 33 East Asian individuals (33/9191 = 0.4%). Among them, four PVs/LPVs were identified in 19 Korean individuals (19/1909 = 1.0%). The predicted incidence, based upon the carrier rates of PV/LPV of DHCR7 alleles, is 1 in 310,688 in East Asians and l in 40,380 in Koreans.

Conclusions

This study is the first to identify carrier frequencies in East Asians and Koreans using gnomAD. It was confirmed that East Asians (0.4%) had a lower carrier frequency than did other ethnicities (1–3%) and Koreans (1.0%) had similar or lower carrier frequencies than other ethnicities. The variant spectrums of DHCR7 in East Asian and Korean populations differed greatly from those of other ethnic groups.

Normal IQ is possible in Smith-Lemli-Opitz syndrome

Children with Smith-Lemli-Opitz syndrome (SLOS) are typically reported to have moderate to severe intellectual disability. This study aims to determine whether normal cognitive function is possible in this population and to describe clinical, biochemical and molecular characteristics of children with SLOS and normal intelligent quotient (IQ). The study included children with SLOS who underwent cognitive testing in four centers. All children with at least one IQ composite score above 80 were included in the study. Six girls, three boys with SLOS were found to have normal or low-normal IQ in a cohort of 145 children with SLOS. Major/multiple organ anomalies and low serum cholesterol levels were uncommon. No correlation with IQ and genotype was evident and no specific developmental profile were observed. Thus, normal or low-normal cognitive function is possible in SLOS. Further studies are needed to elucidate factors contributing to normal or low-normal cognitive function in children with SLOS.

Pulmonary vein stenosis in patients with Smith-Lemli-Opitz syndrome

OBJECTIVE

To describe a group of children with co-incident pulmonary vein stenosis and Smith-Lemli-Opitz syndrome and to generate hypotheses as to the shared pathogenesis of these disorders.

DESIGN

Retrospective case series.

PATIENTS

Five subjects in a pulmonary vein stenosis cohort of 170 subjects were diagnosed with Smith-Lemli-Opitz syndrome soon after birth.

RESULTS

All five cases were diagnosed with Smith-Lemli-Opitz syndrome within 6 weeks of life, with no family history of either disorder. All cases had pathologically elevated 7-dehydrocholesterol levels and two of the five cases had previously reported pathogenic 7-dehydrocholesterol reductase mutations. Smith-Lemli-Opitz syndrome severity scores ranged from mild to classical (2-7). Gestational age at birth ranged from 35 to 39 weeks. Four of the cases were male by karyotype. Pulmonary vein stenosis was diagnosed in all cases within 2 months of life, earlier than most published cohorts. All cases progressed to bilateral disease and three cases developed atresia of at least one vein. Despite catheter and surgical interventions, all subjects' pulmonary vein stenosis rapidly recurred and progressed. Three of the subjects died, at 2 months, 3 months, and 11 months. Survival at 16 months after diagnosis was 43%.

CONCLUSIONS

Patients with pulmonary vein stenosis who have a suggestive syndromic presentation should be screened for Smith-Lemli-Opitz syndrome with easily obtainable serum sterol tests. Echocardiograms should be obtained in all newly diagnosed patients with Smith-Lemli-Opitz syndrome, with a low threshold for repeating the study if new respiratory symptoms of uncertain etiology arise. Further studies into the pathophysiology of pulmonary vein stenosis should consider the role of cholesterol-based signaling pathways in the promotion of intimal proliferation.

A placebo-controlled trial of simvastatin therapy in Smith-Lemli-Opitz syndrome

Background

Smith-Lemli-Opitz syndrome (SLOS) is a multiple malformation/cognitive impairment syndrome characterized by the accumulation of 7-dehydrocholesterol (7DHC), a precursor sterol of cholesterol. Simvastatin, an HMG-CoA reductase inhibitor that crosses the blood-brain-barrier, has been proposed for treatment of SLOS based on in vitro and in vivo studies suggesting that simvastatin increases expression of hypomorphic DHCR7 alleles.

Methods

Safety and efficacy of simvastatin therapy in 23 mild to typical SLOS patients was evaluated in a randomized, double-blind, placebo-controlled trial. The cross-over trial consisted of two 12 month treatment phases separated by a 2 month wash-out period.

Results

No safety issues were identified in this study. Plasma dehydrocholesterol levels decreased significantly 8.9 ± 8.4% on placebo to 6.1 ± 5.5% on simvastatin (p<0.005) and we observed a trend toward decreased cerebral spinal fluid dehydrocholesterol levels. A significant improvement (p=0.017, paired t-test) was observed in the Aberrant Behavior Checklist-C Irritability when subjects were on simvastatin.

Conclusions

This paper reports the first randomized, placebo-controlled trial designed to test the safety and efficacy of simvastatin therapy in SLOS. Simvastatin appears to be relatively safe in SLOS patients, improves the serum dehydrocholesterol/total sterol ratio, and significantly improves irritability symptoms in mild to classical SLOS patients.

Pathogenesis, Epidemiology, Diagnosis and Clinical Aspects of Smith-Lemli-Opitz Syndrome

INTRODUCTION

Smith-Lemli-Opitz Syndrome (SLOS) is a malformation syndrome inherited in an autosomal recessive fashion. It is due to a metabolic defect in the conversion of 7-dehydrocholesterol to cholesterol, which leads to an accumulation of 7-dehydrocholesterol and frequently a deficiency of cholesterol. The syndrome is characterized by typical dysmorphic facial features, multiple malformations, and intellectual disability.

AREAS COVERED

In this paper we provide an overview of the clinical phenotype and discuss how the manifestations of the syndrome vary depending on the age of the patients. We then explore the underlying biochemical defect and pathophysiological alterations that may contribute to the many disease manifestations. Subsequently we explore the epidemiology and succinctly discuss population genetics as they relate to SLOS. The next section presents the diagnostic possibilities. Thereafter, the treatment and management as is standard of care are presented.

EXPERT OPINION

Even though the knowledge of the underlying molecular mutations and the biochemical alterations is being rapidly accumulated, there is currently no efficacious therapy addressing neurological dysfunction. We discuss the difficulty of treating this disorder, which manifests as a combination of a malformation syndrome and an inborn error of metabolism. A very important factor in developing new therapies is the need to rigorously establish efficacy in controlled trials.

Biochemical and Physiological Improvement in a Mouse Model of Smith-Lemli-Opitz Syndrome (SLOS) Following Gene Transfer with AAV Vectors

Smith-Lemli-Opitz syndrome (SLOS) is an inborn error of cholesterol synthesis resulting from a defect in 7-dehydrocholesterol reductase (DHCR7), the enzyme that produces cholesterol from its immediate precursor 7-dehydrocholesterol. Current therapy employing dietary cholesterol is inadequate. As SLOS is caused by a defect in a single gene, restoring enzyme functionality through gene therapy may be a direct approach for treating this debilitating disorder. In the present study, we first packaged a human DHCR7 construct into adeno-associated virus (AAV) vectors having either type-2 (AAV2) or type-8 (AAV2/8) capsid, and administered treatment to juvenile mice. While a positive response (assessed by increases in serum and liver cholesterol) was seen in both groups, the improvement was greater in the AAV2/8-DHCR7 treated mice. Newborn mice were then treated with AAV2/8-DHCR7 and these mice, compared to mice treated as juveniles, showed higher DHCR7 mRNA expression in liver and a greater improvement in serum and liver cholesterol levels. Systemic treatment did not affect brain cholesterol in any of the experimental groups. Both juvenile and newborn treatments with AAV2/8-DHCR7 resulted in increased rates of weight gain indicating that gene transfer had a positive physiological effect.

Relation between biomarkers and clinical severity in patients with Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS), a multiple congenital anomaly with severe mental retardation, is caused by decreased activity of 7-dehydrocholesterol reductase. Fifteen Hungarian patients were diagnosed with SLOS on the basis of clinical symptoms, serum cholesterol, 7-dehydrocholesterol, and molecular genetic testing. Their age at the time of diagnosis in mild SLOS (n = 4, clinical score <20) was 0.5-18 years, cholesterol was 2.37 ± 0.8 mmol/L, and 7DHC was 0.38 ± 0.14 mmol/L. In the group of typical SLOS (n = 7, score 20-50), the diagnosis was set up earlier (age of 0.1-7 years); t-cholesterol was 1.47 ± 0.7 mmol/L, and 7DHC was 0.53 ± 0.20 mmol/L. Patients with severe SLOS (n = 4, clinical score > 50) died as newborns and had the lowest t-cholesterol (0.66 ± 0.27 mmol/L), and 7DHC was 0.47 ± 0.14 mmol/L. Correlation coefficient with clinical severity was 0.74 for initial t-cholesterol and 0.669 for Cho/7DHC. Statistically significant difference was between the initial t-cholesterol of mild and severe SLOS (p = 0.01), and between the Cho/7DHC ratios of groups (p = 0.004). In severe SLOS, the percentage of α-lipoprotein was significantly lower than in typical (p = 0.003) and mild SLOS (p = 0.004). Although serum albumin, total bilirubin, and hemostasis parameters remained in the reference range during cholesterol supplementation (n = 10) combined with statin therapy (n = 9), increase of aspartate aminotransferase and alanine aminotransferase in 50 % of the patients probably refers to a reversible alteration of liver function; therefore, statin therapy was suspended.

CONCLUSION

life expectancy is fundamentally determined by the initial t-cholesterol, but dehydrocholesterol and α-lipoprotein have prognostic value. Accumulation of hepatotoxic DHC may inhibit the synthesis of α-lipoproteins, decreasing the reverse cholesterol transport. During statin therapy, we suggest monitoring of lipid parameters and liver function.

Challenging behavior in Smith-Lemli-Opitz syndrome: initial test of biobehavioral influences

OBJECTIVE

To study challenging behavior (destruction, aggression, self-injury, stereotypy) in children with Smith-Lemli-Opitz syndrome (SLOS) using a biobehavioral model that helps distinguish biological from socially mediated variables influencing the behavior.

BACKGROUND

SLOS is an autosomal-recessive syndrome of multiple malformations and intellectual disability resulting from a genetic error in cholesterol synthesis in all cells and tissues, including brain. The exact cause of the challenging behavior in SLOS is unclear, but defective brain cholesterol synthesis may contribute. Because the precise genetic and biochemical etiology of SLOS is known, this disorder is a good model for studying biological causes of challenging behavior.

METHOD

In a preliminary application of a biobehavioral model, we studied the association between cholesterol levels (as a biochemical indicator of disease severity) and behavior subtype ("biological" vs "learned") in 13 children with SLOS. Parents completed a questionnaire that categorized challenging behavior as influenced primarily by social or nonsocial (thus, presumably biological) factors.

RESULTS

The severity of the cholesterol synthesis defect correlated significantly with behavior subtype classification for 1 of 2 challenging behaviors. Greater severity of the cholesterol synthesis defect was associated with behavior being classified as primarily influenced by biological factors.

CONCLUSION

The interplay between challenging behavior and defective cholesterol synthesis in SLOS may help explain biological influences on the behavior. Our findings have implications for research on the effectiveness of behavioral and medical treatments for behavioral difficulties in SLOS and other neurodevelopmental disorders.

The RSH/"Smith-Lemli-Opitz" syndrome: historical footnote

Thirty years after its clinical delineation in humans and its teratologic simulation in rats, a Garrodian error of metabolism was discovered in the autosomal recessive RSH/SLO syndrome, namely defective conversion of 7-dehydrocholesterol to cholesterol due to the mutant 7-dehydrocholesterol reductase. This opened the door to the study of several other defects of sterol biosynthesis in humans and the creation of animal "models." The gross discrepancy between expected and observed birth prevalence suggests high embryolethality. The discovery of the role of cholesterol in the synthesis of the morphogen sonic hedgehog has greatly advanced our understanding of mammalian development.

Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology

Smith-Lemli-Opitz syndrome (SLOS) is a congenital multiple anomaly/intellectual disability syndrome caused by a deficiency of cholesterol synthesis resulting from a deficiency of 7-dehydrocholesterol (7DHC) reductase encoded by DHCR7. SLOS is inherited in an autosomal recessive pattern. It is characterized by prenatal and postnatal growth retardation, microcephaly, a variable degree of intellectual disability that encompasses normal intelligence to severe intellectual deficiency, and multiple major and minor malformations. External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system. The clinical spectrum is wide, and rare individuals have been described with normal development and only minor malformations. The clinical diagnosis of SLOS is confirmed by demonstrating an abnormally elevated concentration of the cholesterol precursor, 7DHC, in serum or other tissues, or by the presence of two DHCR7 mutations. The enzymatic deficiency results in decreased cholesterol and increased 7DHC levels, both during embryonic development and after birth. The malformations found in SLOS may result from decreased cholesterol, increased 7DHC or a combination of these two factors. This review discusses the physical and behavioral phenotype of SLOS, the diagnostic approaches, the natural history from the prenatal period to adulthood, and current understanding of the pathophysiology of SLOS.

Treatment of Smith-Lemli-Opitz syndrome and other sterol disorders

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive genetic condition with a broad phenotype that results from deficiency of the final enzyme of the cholesterol synthesis pathway. This defect causes low or low-normal plasma cholesterol levels and increased 7- and 8-dehydrocholesterol (DHC) levels. Many therapies for SLOS and other disorders of sterol metabolism have been proposed, and a few of them have been undertaken in selected patients, but robust prospective clinical trials with validated outcome measures are lacking. We review the current literature and expert opinion on treatments for SLOS and other selected sterol disorders, including dietary cholesterol therapy, statin treatment, bile acid supplementation, medical therapies, and surgical interventions, as well as directions for future therapies and treatment research.

Growth charts for individuals with Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a rare multiple congenital anomaly neurodevelopmental syndrome of impaired cholesterol synthesis. Growth restriction and developmental delay are very common clinical manifestations of SLOS. The degree, etiology, and consequences of growth restriction in SLOS remain an area of limited knowledge to the scientific community. There have been no studies describing the growth parameters and providing reference growth charts for individuals with SLOS. Our longitudinal data from 78 patients between the ages of 0.1 and 16 years with SLOS show a growth restriction of about two standard deviations below the Centers for Disease Control (CDC) norms for age. This study represents comprehensive anthropometric data from the largest cohort available, and proposes growth charts for widespread use in the management and study of individuals with SLOS.

Malformation syndromes caused by disorders of cholesterol synthesis

Cholesterol homeostasis is critical for normal growth and development. In addition to being a major membrane lipid, cholesterol has multiple biological functions. These roles include being a precursor molecule for the synthesis of steroid hormones, neuroactive steroids, oxysterols, and bile acids. Cholesterol is also essential for the proper maturation and signaling of hedgehog proteins, and thus cholesterol is critical for embryonic development. After birth, most tissues can obtain cholesterol from either endogenous synthesis or exogenous dietary sources, but prior to birth, the human fetal tissues are dependent on endogenous synthesis. Due to the blood-brain barrier, brain tissue cannot utilize dietary or peripherally produced cholesterol. Generally, inborn errors of cholesterol synthesis lead to both a deficiency of cholesterol and increased levels of potentially bioactive or toxic precursor sterols. Over the past couple of decades, a number of human malformation syndromes have been shown to be due to inborn errors of cholesterol synthesis. Herein, we will review clinical and basic science aspects of Smith-Lemli-Opitz syndrome, desmosterolosis, lathosterolosis, HEM dysplasia, X-linked dominant chondrodysplasia punctata, Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects Syndrome, sterol-C-4 methyloxidase-like deficiency, and Antley-Bixler syndrome.

Direct analysis of sterols from dried plasma/blood spots by an atmospheric pressure thermal desorption chemical ionization mass spectrometry (APTDCI-MS) method for a rapid screening of Smith-Lemli-Opitz syndrome

Here is proposed a rapid and sensitive method involving atmospheric pressure thermal desorption chemical ionization mass spectrometry (APTDCI-MS) for specific laboratory screening of the Smith-Lemli-Opitz syndrome (SLOS), an inherited defect of cholesterol biosynthesis. Biochemical findings in the blood of SLOS patients are low cholesterol (Chol), high 7- and 8-dehydrocholesterol (DHCs) levels and high DHCs/Chol ratios. The APTDCI proposed method is able to ionize sterols for qualitative and quantitative analysis directly from dried plasma/blood spots. Critical APTDCI parameters--desolvation gas flow and temperature--were optimized analyzing Chol, 7-DHC and cholesteryl stearate standards spotted onto a glass slide acquiring the full scan spectra in positive ion mode. Chol levels in dried plasma spots of unaffected controls (n = 23) obtained by the proposed method were compared with those of the enzymatic method (y = 0.9166x + 0.3811; r = 0.8831) while Chol and DHCs of SLOS patients (n = 9) were compared with the gas chromatography flame ionization detection (GC-FID) method (y = 0.8214x + 0.7388; r = 0.8288). The APTDCI-MS method is also able to differentiate normal from SLOS samples directly analyzing whole blood and washed red cells spotted on paper. In conclusion, the intrinsic analytical high-throughput of APTDCI-MS method for sterol analysis could be useful to screen SLO syndrome.

A novel DHCR7 mutation in a Smith-Lemli-Opitz syndrome infant presenting with neonatal cholestasis

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive malformation syndrome caused by a defect in cholesterol biosynthesis. The incidence is very low in Asians and only one case has been reported in Korea thus far. Recently, we found an infant with neonatal cholestasis. He had microcephaly, ambiguous genitalia, cleft palate, syndactyly of toes, patent ductus arteriosus and hypertrophic pyloric stenosis. The serum cholesterol was decreased and serum 7-dehydrocholesterol was markedly elevated. Genetic analysis of the DHCR7 gene identified a novel missense mutation (Pro227Ser) as well as a known mutation (Gly303Arg) previously identified in a Japanese patient with SLOS. Although rare in Korea, SLOS should be considered in the differential diagnosis of neonatal cholestasis, especially in patients with multiple congenital anomalies and low serum cholesterol levels.

Molecular screening of Smith-Lemli-Opitz syndrome in pregnant women from the Czech Republic

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive metabolic disorder. SLOS is caused by the mutations in the gene for 3beta-hydroxysterol Delta(7) reductase (DHCR7; EC 1.3.1.21), which maps to chromosome 11q12-13. DHCR7 catalyses the final step in cholesterol biosynthesis-the reduction of 7-dehydrocholesterol to cholesterol. Clinical severity ranges from mild dysmorphism to severe congenital malformation and intrauterine lethality. Pregnant women are offered a biochemical screening test for Down syndrome in the second trimester, where the suspicion for SLOS could be registered, when the unconjugated estriol (uE3) level appears low. A group of 456 fetuses with a high risk for SLOS were examined by DNA analysis. We confirmed SLOS in 5 fetuses and 11 fetuses were carriers. One novel mutation (p.G30A) was detected. The most frequently found mutations, c.964-1G > C and p.W151X, are also the most severe ones. At least one of these mutations was detected in each fetus with SLOS. This suggests that the biochemical screening of pregnant women probably uncovers mainly more severely affected fetuses. We confirmed SLOS also in two patients whose prenatal screening was negative. Both of them had nonsense mutation on one allele. It stands to reason that some modifying factors may play a role in the reduction of the uE3 level in the mother's serum.

Increased nonsterol isoprenoids, dolichol and ubiquinone, in the Smith-Lemli-Opitz syndrome: effects of dietary cholesterol

Smith-Lemli-Opitz syndrome (SLOS) is an inherited autosomal recessive cholesterol deficiency disorder. Our studies have shown that in SLOS children, urinary mevalonate excretion is normal and reflects hepatic HMG-CoA reductase activity but not ultimate sterol synthesis. Hence, we hypothesized that in SLOS there may be increased diversion of mevalonate to nonsterol isoprenoid synthesis. To test our hypothesis, we measured urinary dolichol and ubiquinone, two nonsterol isoprenoids, in 16 children with SLOS and 15 controls, all fed a low-cholesterol diet. The urinary excretion of both dolichol (P < 0.002) and ubiquinone (P < 0.02) in SLOS children was 7-fold higher than in control children, whereas mevalonate excretion was comparable. In a subset of 12 SLOS children, a high-cholesterol diet decreased urinary mevalonate excretion by 61% (P < 0.001), dolichol by 70% (P < 0.001), and ubiquinone by 67% (P < 0.03). Our hypothesis that in SLOS children, normal urinary mevalonate excretion results from increased diversion of mevalonate into the production of nonsterol isoprenoids is supported. Dietary cholesterol supplementation reduced urinary mevalonate and nonsterol isoprenoid excretion but did not change the relative ratios of their excretion. Therefore, in SLOS, a secondary peripheral regulation of isoprenoid synthesis may be stimulated.

DHCR7 mutation carrier rates and prevalence of the RSH/Smith-Lemli-Opitz syndrome: Where are the patients?

RSH/Smith-Lemli-Opitz (SLOS) is an inborn error of metabolism with protean manifestations. Its exact incidence and prevalence are not known; however, the carrier rate for the most frequently occurring mutation, the null mutation IVS8-1G > C, is approximately 1 in 100 for the Caucasian population in North America (1%) and possibly as high as 1 in 50 to 1 in 30 in Central European populations (2-3.3%). Based on the allele frequencies and the proportion of this mutation observed in various patient populations, the expected incidence of RSH/SLOS in those populations was calculated and reported to be between 1 in 1,590 and 1 in 17,000. However, around the world the observed prevalence and incidence are much lower than those calculated from the individual mutation carrier rates observed in any given population. The discrepancy between the expected incidence and prevalence can be explained only in part by the neonatal and infancy deaths of the most severely affected children with RSH/SLOS and the under ascertainment of mild and atypical cases at the mild end of the spectrum. RSH/SLOS may be responsible for a high number of miscarriages. Recent observations estimate the prevalence of SLOS at 16 weeks of gestation as similar to that observed at birth (approximately 1 in 60,000) suggesting that either reduced fertility of carrier couples or losses of affected embryos or fetuses in the first trimester play a significant role in reducing the second trimester prevalence of RSH/SLOS. It is possible that the estimates of carrier rates based on population screening for the most commonly occurring mutations may not reflect the true carrier rates in the population. In order to reconcile the above-mentioned paradoxes, we propose a model based on a higher than observed carrier frequency of the most common mutation and on very high fetal loss of homozygotes for that mutation.

A membrane defect in the pathogenesis of the Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is an often lethal birth defect resulting from mutations in the gene responsible for the synthesis of the enzyme 3beta-hydroxy-steroid-Delta7-reductase, which catalyzes the reduction of the double bond at carbon 7 on 7-dehydrocholesterol (7-DHC) to form unesterified cholesterol. We hypothesize that the deficiency in cholesterol biosynthesis and subsequent accumulation of 7-DHC in the cell membrane leads to defective composition, organization, dynamics, and function of the cell membrane. Using skin fibroblasts obtained from SLOS patients, we demonstrate that the SLOS membrane has increased 7-DHC and reduced cholesterol content and abnormal membrane fluidity. X-ray diffraction analyses of synthetic membranes prepared to mimic SLOS membranes revealed atypical membrane organization. In addition, calcium permeability is markedly augmented, whereas membrane-bound Na+/K+ATPase activity, folate uptake, inositol-1,4,5-trisphosphate signaling, and cell proliferation rates are markedly suppressed. These data indicate that the disturbance in membrane sterol content in SLOS, likely at the level of membrane caveolae, directly contributes to the widespread tissue abnormalities in this disease.

Identifying Smith–Lemli–Opitz syndrome in conjunction with prenatal screening for Down syndrome

BACKGROUND

Smith-Lemli-Opitz syndrome (SLOS) is a rare hereditary disorder of cholesterol metabolism. We examine the feasibility of identifying SLOS as a part of a routine prenatal screening and evaluate diagnostic testing in maternal urine (or serum), in addition to amniotic fluid.

METHODS

Our SLOS risk algorithm utilized three Down syndrome screening markers (estimated 62% detection rate; 0.3% screen-positive rate). Fifteen North American prenatal screening programs implemented this algorithm.

RESULTS

SLOS risk was assigned to 1 079 301 pregnancies; 3083 were screen-positive (0.29%). Explanations were found for 1174, including 914 existing fetal deaths. Among the remaining pregnancies, 739 were screen-positive only for SLOS; 1170 were also screen-positive for other fetal disorders. Five of six SLOS pregnancies (83%) were screen-positive. All six had sonographic findings, were biochemically confirmed, and were terminated. Maternal urine steroid measurements were confirmatory in four cases tested. Second-trimester prevalence among Caucasians was 1 in 101 000 (1 in 130 000 overall; no cases in other racial groups). Among 739 pregnancies screen-positive only for SLOS, two cases were identified; another 69 had major fetal abnormalities.

CONCLUSIONS

Although SLOS occurred less often than previously reported, many other major abnormalities were detected. Implementing the algorithm as an adjunct to Down syndrome screening may be feasible.

R352Q mutation of the DHCR7 gene is common among Japanese Smith-Lemli-Opitz syndrome patients

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive malformation syndrome characterized by microcephaly, syndactyly of toes, ambiguous genitalia, and mental retardation. The underlying DHCR7 gene has been identified and a wide variety of distinct mutations were reported in USA and European SLOS patients. A significant difference has been suggested in the frequency of SLOS among different ethnic populations. Here, we report mutational analysis of seven Japanese SLOS patients. Five mutations, R352Q, R242H, G303R, X476Q, and S192F, were identified, and R352Q appeared most frequent, since nine out of the 13 mutations of Japanese origin were the same R352Q. These results suggest that R352Q is a predominant founder mutation in Japanese SLOS patients.

3beta-hydroxysterol Delta7-reductase and the Smith-Lemli-Opitz syndrome

In the final step of cholesterol synthesis, 7-dehydrocholesterol reductase (DHCR7) reduces the double bond at C7-8 of 7-dehydrocholesterol to yield cholesterol. Mutations of DHCR7 cause Smith-Lemli-Opitz syndrome (SLOS). Over 100 different mutations of DHCR7 have been identified in SLOS patients. SLOS is a classical multiple malformation, mental retardation syndrome, and was the first human malformation syndrome shown to result from an inborn error of cholesterol synthesis. This paper reviews the biochemical, molecular, and mutational aspects of DHCR7.

Antenatal manifestations of Smith-Lemli-Opitz (RSH) syndrome: A retrospective survey of 30 cases

Smith-Lemli-Opitz (SLO) syndrome or RSH syndrome is an autosomal recessive multiple malformation, and mental retardation syndrome ascribed to 7-dehydrocholesterol reductase deficiency, and usually diagnosed in the early postnatal period. Reviewing a series of 30 cases of SLO, we have investigated the variable antenatal expression of the disorder. Intrauterine growth retardation (IUGR) was the most frequent detectable trait (20/30). IUGR was either isolated (9/20) or associated with at least one other anomaly (11/20), including nuchal edema, renal, cardiac, cerebral malformations, genital anomalies, or polydactyly. In this last group, 3/11 presented with multiple malformations (> or =3 anomalies). In 5/30 cases, isolated nuchal edema (3/30), and isolated cardiac (1/30) or renal malformations (1/30) were the only detectable anomalies. Ultrasound findings were considered normal in 5/30 cases and were abnormal in 25/30 cases (83%), but early detection of multiple malformations was rare (3/30, 10%). We suggest giving consideration to a more systematic sterol analysis when dealing with IUGR, especially when associated anomalies are detected.

Human malformation syndromes due to inborn errors of cholesterol synthesis

PURPOSE OF REVIEW

This review covers a group of human malformation syndromes, which are caused by inborn errors of cholesterol synthesis. The Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive, multiple malformation, and mental retardation syndrome that is the prototypical example of this group of disorders. In the 10 years since the biochemical cause of SLOS was identified, other malformation syndromes have been shown to result from defects in this pathway. These include desmosterolosis, lathosterolosis, X-linked dominant chondrodysplasia punctata type 2 (CDPX2), congenital hemidysplasia with ichthyosiform erythroderma and limb defects (CHILD syndrome), hydrops-ectopic calcification-moth-eaten skeletal dysplasia (HEM dysplasia), and some cases of Antley-Bixler syndrome. These disorders represent the first true merging of dysmorphology with biochemical genetics.

RECENT FINDINGS

Recent studies report the identification of human lathosterolosis patients, indicate that SLOS is a relatively common genetic disorder that may be a major unrecognized cause of fetal loss, suggest that correction of the biochemical defect can improve central nervous system function, and show that perturbed sonic hedgehog signaling due to decreased sterol levels likely underlies some of the malformations in SLOS and lathosterolosis.

SUMMARY

Recognition of the biochemical defect in these syndromes has given insight into the role that cholesterol plays during normal development, into understanding the pathophysiological processes that underlie the clinical problems found in these disorders, and into developing therapeutic interventions.

Specific congenital heart defects in RSH/Smith-Lemli-Opitz syndrome: postulated involvement of the sonic hedgehog pathway in syndromes with postaxial polydactyly or heterotaxia

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

The specific association of AVCD and anomalous pulmonary venous return in patients with RSH/SLOS and the finding of AVCD +/- common atrium in several syndromes with polydactyly leads to the hypothesis that heterotaxia due to SHH anomalies could be involved in a large spectrum of conditions. Perturbations in different components of the SHH pathway could lead to several developmental errors presenting with partially overlapping clinical manifestations.

Desmosterolosis presenting with multiple congenital anomalies and profound developmental delay

Desmosterol (cholesta-5,24-dien-3beta-ol) is a minor sterol that forms as an intermediate in the cholesterol biosynthetic pathway when the 24-unsaturated sterol bond is reduced as the last step rather than earlier in the conversion of lanosterol to cholesterol. In 1998, FitzPatrick et al. reported a premature infant who died shortly after birth and had marked tissue elevations of desmosterol and a strikingly abnormal phenotype. We describe here the first living patient with desmosterolosis and show biochemical evidence in plasma and cultured lymphoblasts for an autosomal recessive deficiency of 24-dehydrocholesterol reductase (DHCR24). The infant has severe microcephaly, agenesis of the corpus callosum, downslanting palpebral fissures, micrognathia, submucous cleft palate, clubfoot, and a persistent patent ductus arteriosus. Plasma sterol quantification in the patient at age 2 years demonstrated a normal cholesterol level, but a 100-fold increased level of desmosterol (60 mcg/ml; nl 0.5 +/- 0.3 mcg/ml (SD)) suggesting deficient activity of 24-dehydrocholesterol (desmosterol) reductase (DHCR24). Both parents had mildly increased levels of desmosterol in plasma (mother: 1.4 mcg/ml; father: 1.8 mcg/ml), consistent with heterozygosity for DHCR24 deficiency. Analysis of sterol metabolism in cultured transformed lymphoblasts showed a 100-fold increased level of desmosterol and a moderately decreased level of cholesterol in the patient's cells and a 10-fold elevation of desmosterol in the mother's cells. At the age of 3.5 years, the patient stands but does not walk, uses a 5-word vocabulary, and lacks any major medical problems. This unique patient broadens the spectrum of inborn errors of cholesterol biosynthesis and suggests additional candidate clinical phenotypes associated with abnormal cholesterol metabolism.

Cholesterol storage defect in RSH/Smith-Lemli-Opitz syndrome fibroblasts

The RSH/Smith-Lemli-Opitz syndrome (SLOS) is a multiple malformation/mental retardation syndrome caused by an inborn error of cholesterol synthesis. Mutations in the 3beta-hydroxysteroid Delta(7)-reductase gene result in impaired enzymatic reduction of 7-dehydrocholesterol (7-DHC) to cholesterol. Cells obtain cholesterol by either de novo synthesis or from exogenous sources by the binding and uptake of low density lipoprotein (LDL) particles. Because de novo synthesis of cholesterol is impaired in SLOS, current investigational therapy for SLOS consists of dietary cholesterol supplementation. However, the potential effects of elevated intracellular levels of 7-DHC on intracellular LDL metabolism have not been described. We now report that in addition to the primary defect in de novo cholesterol synthesis, SLOS fibroblasts have a secondary defect of LDL cholesterol metabolism. Staining of fibroblasts with filipin, a fluorescent polyene antibiotic which binds unesterified sterols, shows that SLOS fibroblasts accumulate unesterified sterols. Further studies show that this increased filipin staining was due to an abnormal accumulation of LDL derived cholesterol rather than due to storage of endogenously synthesized 7-dehydrocholesterol (7-DHC). We have also found that SLOS fibroblasts failed to degrade LDL at a normal rate, and examination of SLOS fibroblasts by electron microscopy demonstrated the formation of lysosomal inclusions similar to that seen in Niemann-Pick type C (NPC) cells. We propose that 7-DHC may directly or indirectly inhibit the function of the NPC protein through its sterol-sensing domain (SSD), and that 7-DHC may perturb the function of other SSD containing proteins in SLOS.

Genetic disorders of cholesterol biosynthesis in mice and humans

Over the past few years, the number of identified inborn errors of cholesterol biosynthesis has increased significantly. The first inborn error of cholesterol biosynthesis to be characterized, in the mid 1980s, was mevalonic aciduria. In 1993, Irons et al. ( 1 ) (M. Irons, E. R. Elias, G. Salen, G. S. Tint, and A. K. Batta, Lancet 341:1414, 1993) reported that Smith-Lemli-Opitz syndrome, a classic autosomal recessive malformation syndrome, was due to an inborn error of cholesterol biosynthesis. This was the first inborn error of postsqualene cholesterol biosynthesis to be identified, and subsequently additional inborn errors of postsqualene cholesterol biosynthesis have been characterized to various extent. To date, eight inborn errors of cholesterol metabolism have been described in human patients or in mutant mice. The enzymatic steps impaired in these inborn errors of metabolism include mevolonate kinase (mevalonic aciduria as well as hyperimmunoglobulinemia D and periodic fever syndrome), squalene synthase (Ss-/- mouse), 3beta-hydroxysteroid Delta14-reductase (hydrops-ectopic calcification-moth-eaten skeletal dysplasia), 3beta-hydroxysteroid dehydrogenase (CHILD syndrome, bare patches mouse, and striated mouse), 3beta-hydroxysteroid Delta8,Delta7-isomerase (X-linked dominant chondrodysplasia punctata type 2, CHILD syndrome, and tattered mouse), 3beta-hydroxysteroid Delta24-reductase (desmosterolosis) and 3beta-hydroxysteroid Delta7-reductase (RSH/Smith-Lemli-Opitz syndrome and Dhcr7-/- mouse). Identification of the genetic and biochemical defects which give rise to these syndromes has provided the first step in understanding the pathophysiological processes which underlie these malformation syndromes.

DHCR7 genotypes of cousins with Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive disorder of cholesterol biosynthesis caused by mutations of the 7-dehydrocholesterol reductase gene (DHCR7). We report on three cousins with SLOS, all of whom were found to be compound heterozygotes for the common splice site mutation IVS8-1G-->C and the missense mutation T289I. DNA analysis of one set of parents demonstrated that the father carried the missense mutation and the mother carried the IVS8-1G-->C mutation. By extension, the two unrelated mothers were both heterozygous for IVS8-1G-->C. This finding supports the notion of a high carrier frequency of the IVS8-1G-->C null mutation in Northern European Caucasians.

Mutations in the human DHCR7 gene

The Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive metabolic disorder characterized by variable congenital malformations, facial dysmorphism, and mental retardation. Mutations in the DHCR7 gene have been identified in SLOS patients. This gene encodes for the enzyme Delta7-sterol reductase which catalyses the last step of cholesterol biosynthesis. Among the 73 different mutations observed so far, including 10 novel mutations reported in this review, the majority are missense mutations (65) which cluster in three domains of the protein: in the transmembrane domain (TM mutations), in the fourth cytoplasmic loop (4L mutations), and at the C-terminus (CT mutations). Two nonsense mutations, one splice site mutation, two single nucleotide insertions, and three deletions which likely all represent null mutations were also described. Expression studies have demonstrated a decreased protein stability for all analyzed missense mutations. By comparing clinical severity scores, biochemical data, and mutations in SLOS patients a genotype-phenotype correlation has been established. The null and 4L mutations are associated with a severe clinical phenotype, and TM and CT mutations are associated with a mild clinical phenotype. DHCR7 mutational spectra in SLOS patients of British, German, Italian, and Polish origin demonstrate significant geographic frequency differences of common DHCR7 mutations.

RSH/Smith-Lemli-Opitz syndrome: a multiple congenital anomaly/mental retardation syndrome due to an inborn error of cholesterol biosynthesis

The RSH/Smith-Lemli-Opitz syndrome (RSH/SLOS) is an autosomal recessive multiple congenital anomaly/mental retardation syndrome caused by an inborn error of cholesterol biosynthesis. The RSH/SLOS phenotypic spectrum is broad; however, typical features include microcephaly, ptosis, a small upturned nose, micrognathia, postaxial polydactaly, second and third toe syndactaly, genital anomalies, growth failure, and mental retardation. RSH/SLOS is due to a deficiency of the 3beta-hydroxysterol Delta(7)-reductase, which catalyzes the reduction of 7-dehydrocholesterol (7-DHC) to cholesterol. This inborn error of cholesterol biosynthesis results in elevated serum and tissue 7-DHC levels. The 3beta-hydroxysterol Delta(7)-reductase gene (DHCR7) maps to chromosome 11q12-13, and to date 66 different mutations of this gene have been identified in RSH/SLOS patients. Identification of the biochemical basis of RSH/SLOS has led to development of therapeutic regimens based on dietary cholesterol supplementation and has increased our understanding of the role cholesterol plays during embryonic development.

Cholesterol supplementation with egg yolk increases plasma cholesterol and decreases plasma 7-dehydrocholesterol in Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS), an autosomal recessive condition comprising multiple malformations, mental retardation, and growth failure, results from reduced activity of the final enzyme in cholesterol biosynthesis, 7-dehydrocholesterol Delta(7)-reductase (DHCR7). Reduced plasma and tissue cholesterol concentrations and accumulation of cholesterol precursors including 7-dehydrocholesterol (7-DHC) are characteristic biochemical abnormalities. While it is still unclear what role these potentially toxic precursors have in the pathogenesis of this disorder, the accumulation of 7-DHC in the brain has been associated with impaired learning in rats and oxidized 7-DHC has been shown to induce growth retardation in cultured rat embryos. We hypothesized that supplemental dietary cholesterol would increase plasma cholesterol levels and suppress synthesis of 7-DHC and other abnormal sterols in individuals with SLOS. After baseline sterol levels were obtained, patients were provided supplemental cholesterol as egg yolk. Plasma sterols were analyzed by capillary-column gas chromatography over time in four children with SLOS. When evaluated at 4-8 weeks after the initiation of cholesterol supplementation, there was a marked increase in mean plasma cholesterol, from 53 mg/dl to 82 mg/dl. While the percent of total sterols as 7-DHC decreased from 15% to 10%, there was no change in total plasma 7-DHC levels. However, when evaluated 35-90 weeks after the institution of cholesterol supplementation, mean plasma 7-DHC decreased, from 11.3 mg/dl to 3.5 mg/dl (-67%, P < 0.05), along with an increase in mean plasma cholesterol from 53 mg/dl to 114 mg/dl (+116%, P < 0.05). These results support the hypothesis that over time dietary cholesterol supplementation from egg yolk increases the plasma cholesterol levels and decreases levels of 7-DHC which may be toxic. These data have important therapeutic implications in the management of SLOS.

The Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is one of the archetypical multiple congenital malformation syndromes. The recent discovery of the biochemical cause of SLOS and the subsequent redefinition of SLOS as an inborn error of cholesterol metabolism have led to important new treatment possibilities for affected patients. Moreover, the recent recognition of the important role of cholesterol in vertebrate embryogenesis, especially with regard to the hedgehog embryonic signalling pathway and its effects on the expression of homeobox genes, has provided an explanation for the abnormal morphogenesis in the syndrome. The well known role of cholesterol in the formation of steroid hormones has also provided a possible explanation for the abnormal behavioural characteristics of SLOS.

Evidence for requirement of NADPH-cytochrome P450 oxidoreductase in the microsomal NADPH-sterol Delta7-reductase system

Rabbit antibodies raised against the hydrophilic part of microsomal NADPH-cytochrome P450 oxidoreductase (denoted fpT) demonstrated a marked ability to inhibit NADPH-sterol Delta7-reductase activity. In addition, trypsin and proteinase K treatment of microsomes removed almost all microsomal electron transfer constituents from the microsomes, but the Delta7-reductase activity could be reconstituted by adding detergent-solubilized NADPH-cytochrome P450 oxidoreductase (denoted OR). Furthermore, after solubilization from microsomes, the Delta7-reductase activity could be reconstituted with OR in a DEAE-cellulose column chromatography eluate fraction, which contained little OR activity. In the microsomal system, carbon monoxide, ketoconazole, and miconazole, specific inhibitors of cytochrome P450, had no effect on Delta7-reductase activity. These results provide the first evidence of an essential requirement of OR, which is distinct from cytochrome P450, in the NADPH-sterol Delta7-reductase system. EDTA, o-phenanthroline and KCN markedly lowered Delta7-reductase activity in a dose-dependent manner. Among metal ions tested, only ferric ion restored the reductase activity in the EDTA-treated microsomes. These results sugguest that NADPH-sterol Delta7-reductase is membrane-bound iron-dependent protein embedded in the microsomal lipid bilayer.

Mutational spectrum in the Delta7-sterol reductase gene and genotype-phenotype correlation in 84 patients with Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS), an autosomal recessive malformation syndrome, ranges in clinical severity from mild dysmorphism and moderate mental retardation to severe congenital malformation and intrauterine lethality. Mutations in the gene for Delta7-sterol reductase (DHCR7), which catalyzes the final step in cholesterol biosynthesis in the endoplasmic reticulum (ER), cause SLOS. We have determined, in 84 patients with clinically and biochemically characterized SLOS (detection rate 96%), the mutational spectrum in the DHCR7 gene. Forty different SLOS mutations, some frequent, were identified. On the basis of mutation type and expression studies in the HEK293-derived cell line tsA-201, we grouped mutations into four classes: nonsense and splice-site mutations resulting in putative null alleles, missense mutations in the transmembrane domains (TM), mutations in the 4th cytoplasmic loop (4L), and mutations in the C-terminal ER domain (CT). All but one of the tested missense mutations reduced protein stability. Concentrations of the cholesterol precursor 7-dehydrocholesterol and clinical severity scores correlated with mutation classes. The mildest clinical phenotypes were associated with TM and CT mutations, and the most severe types were associated with 0 and 4L mutations. Most homozygotes for null alleles had severe SLOS; one patient had a moderate phenotype. Homozygosity for 0 mutations in DHCR7 appears compatible with life, suggesting that cholesterol may be synthesized in the absence of this enzyme or that exogenous sources of cholesterol can be used.

Genetics of limb anomalies in humans

The limbs have an essential function in all vertebrates. In animals, the key genes that are involved in the growth and patterning of the limb buds, and of the development of the complex extremities, have been identified and their interactions recognized. Aided by these discoveries, human genetics has also been able to identify, or at least localize, certain genes responsible for anomalies of the limbs. These malformations are isolated or associated with anomalies of other developmental fields, according to the expression domain of the gene involved. Increasing knowledge of the embryology and genes involved has lead to a regrouping of malformation manifestations in genetics terms. Clear genotype-phenotype correlations are difficult to establish owing to the interlinking network of genetic signals underlying limb development.

Molecular genetics of the Smith-Lemli-Opitz syndrome and postsqualene sterol metabolism

The Smith-Lemli-Opitz syndrome is a disorder of morphogenesis resulting from an enzymatic defect in the last step of cholesterol metabolism (reduction of 7-dehydrocholesterol). Analysis of the defective gene and identification of mutations therein have paved the way for the study of the molecular genetics of the disorder which is caused by numerous different mutations. Future efforts should identify a postulated intracellular signalling activity of sterol intermediates, isolate proteins that govern the sterol traffic between intracellular compartments, structurally characterize the enzyme delta 7-sterol reductase defective in the Smith-Lemli-Opitz syndrome and investigate the pathomechanism of sterol depletion-induced dysmorphogenesis.

Variant RSH/Smith-Lemli-Opitz syndrome with atypical sterol metabolism

The RSH/Smith-Lemli-Opitz syndrome (RSH/SLOS) is an autosomal recessive malformation syndrome comprising microcephaly, developmental and growth retardation, characteristic facial anomalies, midline cleft palate, and genital and limb anomalies. Recently, biochemical evidence of an inborn error of cholesterol biosynthesis at the level of 7-dehydrocholesterol (7DHC) reductase was reported in children and adults with RSH/SLOS. We report on two sibs with a variant form of RSH/SLOS whose sterol metabolism in cultured lymphoblasts is abnormal but differs from that of patients with classical RSH/SLOS. The children have relatively mild physical and developmental abnormalities, but a phenotype still consistent with the diagnosis of RSH/SLOS. Their plasma cholesterol levels are only mildly depressed, and they have less markedly increased plasma levels of 7DHC than most patients with classical RSH/SLOS. Cultured lymphoblasts from our patients accumulated 7DHC to the same degree as classical RSH/SLOS lymphoblast when grown with cholesterol-depleted fetal calf serum. However, unlike other RSH/SLOS cells, the increase in cellular 7DHC levels was not suppressed when the cells were grown in the presence of cholesterol from untreated fetal calf serum. The parents' sterol metabolism was also strikingly abnormal in that the levels of 7DHC in their lymphoblasts were markedly elevated compared with those of lymphoblasts from other RSH/SLOS parents. Our findings suggest that these mildly affected RSH/SLOS sibs may have a genetic disorder of sterol metabolism that is related to but biochemically different from classical RSH/SLOS, possibly one affecting intracellular transport of sterols.

Smith-Lemli-Opitz syndrome is caused by mutations in the 7-dehydrocholesterol reductase gene

Smith-Lemli-Opitz syndrome is a frequently occurring autosomal recessive developmental disorder characterized by facial dysmorphisms, mental retardation, and multiple congenital anomalies. Biochemically, the disorder is caused by deficient activity of 7-dehydrocholesterol reductase, which catalyzes the final step in the cholesterol-biosynthesis pathway-that is, the reduction of the Delta7 double bond of 7-dehydrocholesterol to produce cholesterol. We identified a partial transcript coding for human 7-dehydrocholesterol reductase by searching the database of expressed sequence tags with the amino acid sequence for the Arabidopsis thaliana sterol Delta7-reductase and isolated the remaining 5' sequence by the "rapid amplification of cDNA ends" method, or 5'-RACE. The cDNA has an open reading frame of 1,425 bp coding for a polypeptide of 475 amino acids with a calculated molecular weight of 54.5 kD. Heterologous expression of the cDNA in the yeast Saccharomyces cerevisiae confirmed that it codes for 7-dehydrocholesterol reductase. Chromosomal mapping experiments localized the gene to chromosome 11q13. Sequence analysis of fibroblast 7-dehydrocholesterol reductase cDNA from three patients with Smith-Lemli-Opitz syndrome revealed distinct mutations, including a 134-bp insertion and three different point mutations, each of which was heterozygous in cDNA from the respective parents. Our data demonstrate that Smith-Lemli-Opitz syndrome is caused by mutations in the gene coding for 7-dehydrocholesterol reductase.

Smith-Lemli-Opitz syndrome: a variable clinical and biochemical phenotype

We have reviewed all known UK cases of Smith-Lemli-Opitz syndrome. Among 49 cases with proven 7-dehydrocholesterol reductase deficiency, half had been terminated or had died in infancy. The minimum incidence is 1 in 60,000. The frequent occurrence of hypospadias may account for 71% of recognised cases being male. Important common features which emerged include short thumbs, severe photosensitivity, aggressive behaviour, and atrioventricular septal defect. The typical facial appearance becomes less obvious with age and 20% of cases did not have 2/3 toe syndactyly. Biochemical measurements of serum 7-dehydrocholesterol did not correlate with clinical severity.

Sex in the 90s: SRY and the switch to the male pathway

In mammals the male sex determination switch is controlled by a single gene on the Y chromosome, SRY. SRY encodes a protein with an HMG-like DNA-binding domain, which probably acts as a local organizer of chromatin structure. It is believed to regulate downstream genes in the sex determination cascade, although no direct targets of SRY are clearly known. More genes in the pathway have been isolated through mutation approaches in mouse and human. At least three genes, SRY itself, SOX9, and DAX1, are dosage sensitive, providing molecular evidence that the sex determination step operates at a critical threshold. SRY initiates development of a testis from the bipotential cells of the early gonad. The dimorphic male and female pathways present a rare opportunity to link a pivotal gene in development with morphogenetic mechanisms that operate to pattern an organ and the differentiation of its cells. Mechanisms of testis organogenesis triggered downstream of SRY include pathways of cell signaling controlling cell reorganization, cell proliferation, cell migration, and vascularization.

Neurometabolic disease

Advances in knowledge of neurometabolic disease continues. Of great interest to the neurologist are the definitions of the molecular defects in Niemann-Pick C disease, Refsum disease, and five of the disorders of peroxisome biogenesis, including rhizomelic chondrodysplasia punctata. Duplication of the proteolipid protein gene is the most common molecular abnormality in Pelizaeus Merzbacher disease. Therapies for guanidinoacetate methyltransferase deficiency and for 3-phosphoglycerated dehydrogenase deficiency appear promising. Animal models have been developed for X-linked adrenoleukodystrophy, metachromatic leukodystrophy and Zellweger syndrome and will aid in the understanding of pathogenesis and the evaluation of therapy.