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

Characterization of large deletions in the DHCR7 gene

Pathogenic variants in the DHCR7 gene cause Smith-Lemli-Opitz syndrome (SLOS), a defect of cholesterol biosynthesis resulting in an autosomal recessive congenital metabolic malformation disorder. In approximately 4% of patients, the second mutation remains unidentified. In this study, 12 SLOS patients diagnosed clinically and/or by elevated 7-dehydrocholesterol (7-DHC) have been investigated by customized multiplex ligation-dependent probe amplification (MLPA) analysis, because only one DHCR7 sequence variant has been detected. Two unrelated patients of this cohort carry different large deletions in the DHCR7 gene. One patient showed a deletion of exons 3-6. The second patient has a deletion of exons 1 and 2 (non-coding) and lacks the major part of the promoter. These two patients show typical clinical and biochemical phenotypes of SLOS. Second disease-causing mutations are p.(Arg352Trp) and p.(Thr93Met), respectively. Deletion breakpoints were characterized successfully in both cases. Such large deletions are rare in the DHCR7 gene but will resolve some of the patients in whom a second mutation has not been detected.

Distribution and functions of sterols and sphingolipids

Sterols and sphingolipids are considered mainly eukaryotic lipids even though both are present in some prokaryotes, with sphingolipids being more widespread than sterols. Both sterols and sphingolipids differ in their structural features in vertebrates, plants, and fungi. Interestingly, some invertebrates cannot synthesize sterols de novo and seem to have a reduced dependence on sterols. Sphingolipids and sterols are found in the plasma membrane, but we do not have a clear picture of their precise intracellular localization. Advances in lipidomics and subcellular fractionation should help to improve this situation. Genetic approaches have provided insights into the diversity of sterol and sphingolipid functions in eukaryotes providing evidence that these two lipid classes function together. Intermediates in sphingolipid biosynthesis and degradation are involved in signaling pathways, whereas sterol structures are converted to hormones. Both lipids have been implicated in regulating membrane trafficking.

Nuclear receptor DHR96 acts as a sentinel for low cholesterol concentrations in Drosophila melanogaster

All eukaryotic cells have to maintain cholesterol concentrations within defined margins in order to function normally. Perturbing cholesterol homeostasis can result in a wide range of cellular and systemic defects, including cardiovascular diseases, as well as Niemann-Pick and Tangier diseases. Here, we show that DHR96 is indispensable for mediating the transcriptional response to dietary cholesterol and that it acts as a key regulator of the Niemann-Pick type C gene family, as well as of other genes involved in cholesterol uptake, metabolism, and transport. DHR96 mutants are viable and phenotypically normal on a standard medium but fail to survive on diets that are low in cholesterol. DHR96 mutants have aberrant cholesterol levels, demonstrating a defect in maintaining cholesterol homeostasis. Remarkably, we found that a high-cholesterol diet phenocopied the genomic profile of the DHR96 mutation, indicating that DHR96 resides at the top of a genetic hierarchy controlling cholesterol homeostasis in insects. We propose a model whereby DHR96 is activated when cellular cholesterol concentrations drop below a critical threshold in order to protect cells from severe cholesterol deprivation.

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.

Neoplasms and pathology of sexual developmental disorders (intersex)

Continuing new insights into the biology of sexual development and advances in chromosome analysis have led to early identification and prompt treatment of the intersexual patient, the results of which facilitate a more normal life for affected individuals. Based on these advances, a classification of abnormal sexual development has been developed and refined that correlates the gonadal and genital anatomy with the chromosomal findings and specific genetic or metabolic defects. In a shift from a classification anchored on whether the intersex revolves about a specific gene or whole chromosomal abnormality, the current classification is organised by broader categories into which the intersexual disorders are divided into 'abnormalities of genital differentiation', due largely to the abnormal production or sensitivity of a single hormone, or 'abnormalities in sex determination', due to abnormal gonadal differentiation, usually testicular, with or without chromosomal aberration. The current classification is an integrated approach to this complex group of disorders and is organised according to the manner by which patients present as well as on the pathophysiological basis of the defect. The classification also groups patients who are at high risk for development of gonadal neoplasia.

Peroxisomal cholesterol biosynthesis and Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS), caused by 7-dehydrocholesterol-reductase (DHCR7) deficiency, shows variable severity independent of DHCR7 genotype. To test whether peroxisomes are involved in alternative cholesterol synthesis, we used [1-(14)C]C24:0 for peroxisomal beta-oxidation to generate [1-(14)C]acetyl-CoA as cholesterol precursor inside peroxisomes. The HMG-CoA reductase inhibitor lovastatin suppressed cholesterol synthesis from [2-(14)C]acetate and [1-(14)C]C8:0 but not from [1-(14)C]C24:0, implicating a peroxisomal, lovastatin-resistant HMG-CoA reductase. In SLOS fibroblasts lacking DHCR7 activity, no cholesterol was formed from [1-(14)C]C24:0-derived [1-(14)C]acetyl-CoA, indicating that the alternative peroxisomal pathway also requires this enzyme. Our results implicate peroxisomes in cholesterol biosynthesis but provide no link to phenotypic variation in SLOS.

A comparison of the behavior of cholesterol and selected derivatives in mixed sterol-phospholipid Langmuir monolayers: a fluorescence microscopy study

Eukaryotic cells require sterols to achieve normal structure and function of their plasma membranes, and deviations from normal sterol composition can perturb these features and compromise cellular and organism viability. The Smith-Lemli-Opitz syndrome (SLOS) is a hereditary metabolic disease involving cholesterol (CHOL) deficiency and abnormal accumulation of the CHOL precursor, 7-dehydrocholesterol (7DHC). In this study, the interactions of CHOL and the related sterols desmosterol (DES) and 7DHC with l-alpha-dipalmitoylphosphatidylcholine (DPPC) monolayers were compared. Pressure-area isotherms and fluorescence microscopy were used to study DPPC monolayers containing 0, 10, 20, or 30 mol% sterol. Similar behavior was noted for CHOL- and DES-containing DPPC monolayers with both techniques. However, while 7DHC gave isotherms similar to those obtained with the other sterols, microscopy indicated limited domain formation with DPPC, indicating that 7DHC packs somewhat differently in DPPC membranes compared to CHOL and DES. These results are discussed in relation to SLOS pathobiology.

Transport of cholesterol across a BeWo cell monolayer: implications for net transport of sterol from maternal to fetal circulation

The placental transport of various compounds, such as glucose and fatty acids, has been well studied. However, the transport of cholesterol, a sterol essential for proper fetal development, remains undefined in the placenta. Therefore, the purpose of these studies was to examine the transport of cholesterol across a placental monolayer and its uptake by various cholesterol acceptors. BeWo cells, which originated from a human choriocarcinoma, were grown on transwells for 3 days to form a confluent monolayer. The apical side of the cells was radiolabeled with either free cholesterol or LDL cholesteryl ester. After 24 h, the radiolabel was removed and cholesterol acceptors were added to the basolateral chamber. Cholesterol was found to be taken up by the apical surface of the placental monolayer, transported to the basolateral surface of the cell, and effluxed to fetal human serum, fetal HDL, or phospholipid vesicles, but not to apolipoprotein A-I. In addition, increasing the cellular cholesterol concentration further increased the amount of cholesterol transported to the basolateral acceptors. These are the first studies to demonstrate the movement of cholesterol across a placental cell from the maternal circulation (apical side) to the fetal circulation (basolateral side).

Smith-Lemli-Opitz syndrome and the DHCR7 gene

Smith-Lemli-Opitz syndrome, a severe developmental disorder associated with multiple congenital anomalies, is caused by a defect of cholesterol biosynthesis. Low cholesterol and high concentrations of its direct precursor, 7-dehydrocholesterol, in plasma and tissues are the diagnostic biochemical hallmarks of the syndrome. The plasma sterol concentrations correlate with severity and disease outcome. Mutations in the DHCR7 gene lead to deficient activity of 7-dehydrocholesterol reductase (DHCR7), the final enzyme of the cholesterol biosynthetic pathway. The human DHCR7 gene is localised on chromosome 11q13 and its structure has been characterized. Ninety-one different mutations in the DHCR7 gene have been published to date. This paper is a review of the clinical, biochemical and molecular genetic aspects.

Feedback inhibition of the cholesterol biosynthetic pathway in patients with Smith-Lemli-Opitz syndrome as demonstrated by urinary mevalonate excretion

Smith-Lemli-Opitz syndrome (SLOS) is a genetic disorder characterized by low plasma cholesterol and high 7-dehydrocholesterol (7-DHC). Synthesis of cholesterol and 7-DHC and its metabolites is regulated by HMG-CoA reductase, whose activity can be measured by 24-h excretion of its product mevalonate. We devised a simple, non-invasive method for collecting 24-h urine in our subjects. With a background of a very low cholesterol diet, mean mevalonate excretion did not differ between controls and SLOS children, indicating that SLOS subjects have normal HMG-CoA reductase activity. In a short term feeding study, the effects of a high cholesterol diet in SLOS subjects include a significant 55% increase in plasma cholesterol levels and 39% decrease in mevalonate excretion and no change in plasma 7-DHC levels. However, in four SLOS subjects, fed a high cholesterol diet for 2-3 years, plasma cholesterol levels continued to increase, urinary mevalonate excretion remained low and total 7-DHC decreased significantly, likely from decreased total sterol synthesis. Thus, in SLOS subjects, HMG-CoA reductase activity was normal and was subject to normal cholesterol induced feedback inhibition. However, total sterol synthesis in SLOS may still be decreased because of increased diversion of mevalonate into the shunt pathway away from sterol synthesis.

A colorimetric assay for 7-dehydrocholesterol with potential application to screening for Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS; MIM 270400) is a genetic disorder characterized by hypocholesterolemia and elevated 7-dehydrocholesterol (7DHC) levels resulting from mutations affecting 7-dehydrocholesterol reductase. We describe a colorimetric assay for 7DHC with potential application to large-scale screening for SLOS. Reaction of 7DHC and its esters with the Liebermann-Burchard reagent resulted in a brief initial absorbance at 510 nm (pink color) followed by an absorbance at 620 nm (blue color) after 2 min, while cholesterol samples were essentially colorless. The assay could identify typical SLOS blood samples by their pink color and increased absorbance at 620 nm after 2 min. Colorimetric identification of mild SLOS cases requires monitoring of the transient absorbance at 510 nm, which must be detected immediately after rapid, consistent mixing of the reagents. The need for special mixing devices and rigorous validation precludes sporadic use of the assay for diagnosing suspected SLOS cases. We also studied the stability of 7DHC in dried SLOS blood spots on Guthrie cards, which are widely used for archiving neonatal blood. Decomposition of 7DHC was effectively retarded by storage at low temperature and by precoating of the cards with antioxidants. The combined results provide a foundation for development of a simple, automated test for SLOS screening.

The origins and roles of cholesterol and fatty acids in the fetus

The fetus grows at a rate that is unparalleled by that at any other stage of life. Significant amounts of cholesterol and fatty acids are required to maintain membrane growth. Recent studies have shown that these lipids are also necessary mediators of processes that are essential for proper development.

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.

Smith-Lemli-Opitz syndrome and other sterol disorders among Finns with developmental disabilities

Smith-Lemli-Opitz syndrome (SLOS) is an inherited disorder of cholesterol metabolism in which 7- and 8-dehydrocholesterols are accumulated in blood and tissues. Diagnosis of SLOS and other disorders in cholesterol metabolism (eg, cerebrotendinous xanthomatosis, phytosterolemia, desmosterolosis, and X-linked dominant Conradi-Hünermann-Happle syndrome) can be performed by gas-liquid chromatographic analysis of serum sterols. To elucidate their involvement in developmental disability, we evaluated serum sterols in two study groups: developmentally disabled subjects in long-term care (N = 322) and newborns and young children (N = 49) with features of SLOS in the Finnish population of 5 million. Only 1 SLOS case (type II) was found from among the 49 children. Seven additional adult cases (type I) with a wide range of clinical features and the serum sterol abnormalities characteristic of SLOS were detected from among the developmentally disabled subjects. The frequency of SLOS in the latter group was relatively high (7 in 322). No other hereditary sterol disorders were found, but two subgroups with low serum cholesterol precursor sterols and high serum plant sterols were identified. Several subjects, including the 7 SLOS patients, used ample medication and had abnormalities in serum sterol concentrations. Thus, among the subjects taking melperone, a high serum delta8-cholestenol level suggests an interference by the drug with cholesterol synthesis. Our results emphasize the importance of analyzing the serum sterols of developmentally disabled subjects to diagnose SLOS and of finding putative undiagnosed disorders in sterol metabolism associated with these clinical conditions.

Homozygosity for the W151X stop mutation in the delta7-sterol reductase gene (DHCR7) causing a lethal form of Smith-Lemli-Opitz syndrome: retrospective molecular diagnosis

Smith-Lemli-Opitz syndrome (SLOS) is a multiple congenital anomalies syndrome caused by an abnormality in cholesterol metabolism. The clinical severity may vary from very mild to lethality in utero, making diagnosis difficult at both ends of the spectrum. Patients with severe SLOS might often escape diagnosis because they die before the correct diagnosis is made. We describe an Austrian family whose first child died neonatally with multiple congenital anomalies. The second pregnancy was terminated because the fetus showed similar severe anomalies ultrasonographically. A further pregnancy ended in a spontaneous first trimester abortion. Clinical diagnosis of SLOS was not considered until the autopsy of the fetus of the terminated pregnancy. Because no material for biochemical testing was available we performed mutational analysis of the DHCR7 gene from paraffin-embedded tissue and a Guthrie card focusing on mutations known to cause a severe SLOS phenotype. This demonstrated homozygosity for the mutation W151X, which has been demonstrated to be a functional null mutation. Our data confirm the concept that homozygosity for functional null alleles of the DHCR7 locus results in intrauterine or perinatal lethality. Furthermore, our findings suggest the usefulness of molecular studies of stored material in similarly affected cases where no material for biochemical analysis is available.

Genetic defects in postsqualene cholesterol biosynthesis

In humans and mice, four different genetic defects in the nine biosynthetic steps from lanosterol to cholesterol have been identified. They impair the activity of a putative C3-sterol dehydrogenase (Nshdl, X-linked dominant bare patches/striated mutation in mice), the sterol delta 8-delta 7 isomerase/EBP (Ebp, X-linked dominant tattered mutation in mice; chondrodysplasia punctata (CDPX2) in humans), the delta 24-sterol reductase (autosomal recessive desmosterolosis) and the delta 7-sterol reductase (DHCR7 gene, autosomal recessive Smith-Lemli-Opitz syndrome in humans). These inborn errors in postsqualene cholesterol metabolism result in dysmorphogenetic syndromes of variable severity. The X-linked dominant mutations result in mosaicism in females, as a result of X-inactivation, and midgestational lethality in males. The mechanisms by which the depletion of cholesterol or the accumulation of intermediates impair morphogenetic programs are unclear. So far, no cellular processes that require an intact cholesterol biosynthetic pathway have been identified, although the morphogenetic hedgehog-patched signaling cascade is a candidate.

Detection of a common mutation in the RSH or Smith-Lemli-Opitz syndrome by a PCR-RFLP assay: IVS8-G-->C is found in over sixty percent of US propositi

The RSH or Smith-Lemli-Opitz syndrome (SLOS) is a relatively common autosomal recessive disorder of cholesterol biosynthesis resulting from a deficiency of the enzyme 7-dehydrocholesterol delta7-reductase (7-DHCR). Mutations in 7-DHCR gene cause SLOS. Among these, a G-->C transversion in the splice acceptor site of exon 9 (IVS8-1G-->C) was suspected to be a frequent mutation, having been detected in about 18% of SLOS patients so far. This mutation results in the elimination of a AlwN1 restriction endonuclease site. We report a simple PCR-RFLP assay to detect the IVS8-1G-->C mutation. Using this method, we identified the IVS8-1G-->C mutation in 21 of 33 SLOS propositi. This mutation was detected in one of 90 normal adult Caucasian Americans; but not among 121 Africans from Sierra Leone, 120 Caucasians from Finland, 95 Chinese or 103 Japanese adults. The results of this study provide further evidence that IVS8-1G-->C transversion is a very common mutation in SLOS patients from the US and that the carrier rate in US caucasians may be high. The simple PCR-RFLP assay developed makes identification of this mutation convenient for diagnosis and for carrier detection.

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.