Smith-Lemli-Opitz-syndrome

Smith-Lemli-Opitz syndrome is an autosomal recessively inherited disorder. A severe defect in cholesterol biosynthesis has been identified leading to abnormally low plasma cholesterol levels and elevated levels of the cholesterol precursor 7-dehydrocholesterol, the result of deficiency of 7-dehydrocholesterol reductase. We describe one such child with Smith-Lemli-Opitz syndrome. This child had clinical features similar to Smith-Lemli-Opitz syndrome like facial dysmorphism and cardiac and renal anomalies with failure to thrive.     

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.     

Identification of a yeast artificial chromosome clone spanning a translocation breakpoint at 7q32.1 in a Smith-Lemli-Opitz syndrome patient.

Smith-Lemli-Opitz syndrome (SLOS) is a mental retardation/multiple congenital anomaly syndrome. The gene(s) involved has not been mapped or cloned, but, recently, a biochemical abnormality in cholesterol biosynthesis has been shown to occur in most SLOS patients. The defect is suspected to occur in the penultimate step of the cholesterol pathway, involving the enzyme 7-dehydrocholesterol reductase, which has not been isolated. On the basis of the hypothesis that a de novo balanced translocation [t(7;20)(q32.1;q13.2)] in an SLOS patient directly interrupts the SLOS gene, positional cloning techniques are being employed to localize and identify the SLOS gene. We report the identification of a chromosome 7-specific YAC that spans the translocation breakpoint, as detected by FISH. This is the first study narrowing a candidate SLOS region and placing it on physical and genetic maps of the human genome.     

Possible abnormalities of steroid secretion in children with Smith-Lemli-Opitz syndrome and their parents

In early infancy, two unrelated children with Smith-Lemli-Opitz syndrome were found to have elevated levels of androgen sulfates. When the steroid conjugates in the serum of normal infants were hydrolyzed and chromatographed on Sephadex LH-20, 4 androgen containing peaks (I, II, III, IV) were found. In the serum from these two infants with Smith-Lemli-Opitz syndrome, Peaks I and III were increased, but Peaks II and IV were absent. The parents of the two children, and of three additional unrelated children with Smith-Lemli-Opitz syndrome, had exaggerated 17-hydroxyprogesterone responses to an intravenous bolus of ACTH. These findings suggest that a defect in steroid metabolism may be linked to the Smith-Lemli-Opitz syndrome.     

The function of cholesterol in embryogenesis

Cholesterol is critical in embryonic development. Inhibition of cholesterol synthesis in experimental animals has caused a birth defect called holoprosencephaly (HPE), which is evidenced by cyclopia (one eye in the middle of the face), monorhinia (protruding single nose above the eye), absence of the pituitary gland, and central nervous system (CNS) abnormalities. In humans, an inherited defect in the cholesterol-synthesizing enzyme 7-dehydrocholesterol reductase depletes cholesterol and results in human HPE, termed Smith-Lemli-Opitz syndrome. In its most severe form, the syndrome leads to cyclopia, monorhinia, and lack of separation of cerebral hemispheres. The cause of the syndrome is a defect in a protein coded by the gene Sonic hedgehog (SHH). The protein SHH is expressed in the notochord of the CNS in the early embryo and is activated by being cleaved autocatalytically, with simultaneous covalent attachment of cholesterol to the N-terminal fragment, which is secreted by cells of the mesoderm layer, signaling the establishment of the neural midline cells. Thus, cholesterol is essential for proper signaling in the development of the normal embryo.     

Cholesterol biosynthesis from birth to adulthood in a mouse model for 7-dehydrosterol reductase deficiency (Smith-Lemli-Opitz syndrome)

Smith-Lemli-Opitz syndrome (SLOS) is caused by deficiency in the terminal step of cholesterol biosynthesis, which is catalyzed by 7-dehydrocholesterol reductase (DHCR7). The disorder exhibits several phenotypic traits including dysmorphia and mental retardation with a broad range of severity. Pathogenesis of SLOS is complex due to multiple roles of cholesterol and may be further complicated by unknown effects of aberrant metabolites that arise when 7-dehydrocholesterol (7-DHC), the substrate for DHCR7, accumulates. A viable mouse model for SLOS has recently been developed, and here we characterize cholesterol metabolism in this model with emphasis on changes during the first few weeks of postnatal development. Cholesterol and 7-DHC were measured in "SLOS" mice and compared with measurements in normal mice. SLOS mice had measurable levels of 7-DHC at all ages tested (up to 1 year), while 7-DHC was below the threshold for detection in normal mice. In perinatal to weaning age SLOS mice, cholesterol and 7-DHC levels changed dramatically. Changes in brain and liver were independent; in brain cholesterol increased several fold while 7-DHC remained relatively constant, but in liver cholesterol first increased then decreased again while 7-DHC first decreased then increased. In older SLOS animals the ratio of 7-DHC/cholesterol, which is an index of biochemical severity, tended to approach, but not reach, normal. While these mice provide the best available genetic animal model for the study of SLOS pathogenesis and treatment, they probably will be most useful at early ages when the metabolic effects of the mutations are most dramatic. To correlate any experimental treatment with improved sterol metabolism will require age-matched controls. Finally, determining the mechanism by which these "SLOS" mice tend to normalize may provide insight into the future development of therapy.     

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.     

Sterol balance in the Smith-Lemli-Opitz syndrome. Reduction in whole body cholesterol synthesis and normal bile acid production

The Smith-Lemli-Opitz syndrome (SLOS) is a multiple malformation/mental retardation syndrome caused by a deficiency of the enzyme 7-dehydrocholesterol Delta(7)-reductase. This enzyme converts 7-dehydrocholesterol (7-DHC) to cholesterol in the last step in cholesterol biosynthesis. The pathology of this condition may result from two different factors: the deficiency of cholesterol itself and/or the accumulation of precursor sterols such as 7-DHC. Although cholesterol synthesis is defective in cultured SLOS cells, to date there has been no evidence of decreased whole body cholesterol synthesis in SLOS and only incomplete information on the synthesis of 7-DHC and bile acids. In this first report of the sterol balance in SLOS, we measured the synthesis of cholesterol, other sterols, and bile acids in eight SLOS subjects and six normal children. The diets were very low in cholesterol content and precisely controlled. Cholesterol synthesis in SLOS subjects was significantly reduced when compared with control subjects (8.6 vs. 19.6 mg/kg per day, respectively, P < 0.002). Cholesterol precursors 7-DHC, 8-DHC, and 19-nor-cholestatrienol were synthesized in SLOS subjects (7-DHC synthesis was 1.66 +/- 1.15 mg/kg per day), but not in control subjects. Total sterol synthesis was also reduced in SLOS subjects (12 vs. 20 mg/kg per day, P < 0.022). Bile acid synthesis in SLOS subjects (3.5 mg/kg per day) did not differ significantly from control subjects (4.6 mg/kg per day) and was within the range reported previously in normals. Normal primary and secondary bile acids were identified.This study provides direct evidence that whole body cholesterol synthesis is reduced in patients with SLOS and that the synthesis of 7-DHC and other cholesterol precursors is profoundly increased. It is also the first reported measure of daily bile acid synthesis in SLOS and provides evidence that bile acid supplementation is not likely to be necessary for treatment. These sterol balance studies provide basic information about the biochemical defect in SLOS and strengthen the rationale for the use of dietary cholesterol in its treatment.     

Biochemical and genetic aspects of 7-dehydrocholesterol reductase and Smith-Lemli-Opitz syndrome

In recent years, several inherited human disorders caused by defects in cholesterol biosynthesis have been identified. These are characterized by malformations, multiple congenital anomalies, mental and growth retardation and/or skeletal and skin abnormalities indicating a pivotal role of cholesterol in morphogenesis and embryonic development. The first recognized and most common of these developmental disorders is Smith-Lemli-Opitz syndrome, an autosomal recessive trait caused by mutations in the DHCR7 gene resulting in a deficiency of the encoded sterol Delta(7)-reductase, alternatively called 7-dehydrocholesterol reductase (EC 1.3.1.21). This enzyme catalyzes the final step in cholesterol biosynthesis, which is the reduction of the Delta(7) double bond of 7-dehydrocholesterol to produce cholesterol.     

Mutations in the human sterol delta7-reductase gene at 11q12-13 cause Smith-Lemli-Opitz syndrome.

The Smith-Lemli-Opitz syndrome (SLOS; also known as "RSH syndrome" [MIM 270400]) is an autosomal recessive multiple malformation syndrome due to a defect in cholesterol biosynthesis. Children with SLOS have elevated serum 7-dehydrocholesterol (7-DHC) levels and typically have low serum cholesterol levels. On the basis of this biochemical abnormality, it has been proposed that mutations in the human sterol Delta7-reductase (7-DHC reductase; E.C.1.3.1.21) gene cause SLOS. However, one could also propose a defect in a gene that encodes a protein necessary for either the expression or normal function of sterol Delta7-reductase. We cloned cDNA encoding a human sterol Delta7-reductase (DHCR7) on the basis of its homology with the sterol Delta7-reductase from Arabidopsis thaliana, and we confirmed the enzymatic function of the human gene product by expression in SLOS fibroblasts. SLOS fibroblasts transfected with human sterol Delta7-reductase cDNA showed a significant reduction in 7-DHC levels, compared with those in SLOS fibroblasts transfected with the vector alone. Using radiation-hybrid mapping, we show that the DHCR7 gene is encoded at chromosome 11q12-13. To establish that defects in this gene cause SLOS, we sequenced cDNA clones from SLOS patients. In three unrelated patients we have identified four different mutant alleles. Our results demonstrate both that the cDNA that we have identified encodes the human sterol Delta7-reductase and that mutations in DHCR7 are responsible for at least some cases of SLOS.     

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.     

The Smith-Lemli-Opitz syndrome with a G303R/R352W mutation: in an extremely irritable child responsive to cholesterol supplementation

Smith-Lemli-Opitz syndrome(SLOS) is a hereditary disorder of cholesterol biosynthesis. It is characterized by growth deficiency, mental retardation and multiple congenital anomalies, including a typical facial appearance, cleft palate, syndactyly, and a variety of central nervous system and/or major congenital malformations. There are very few reports of the Smith-Lemli-Opitz syndrome in Korean children. Here, we report on a girl with a G303R/R352W mutation. The patient was extremely irritable; assessment of the severity of the irritability included severity scoring and the amount of sleep. Cholesterol supplementation was started with egg yolks, and the irritability improved.     

Disorders of post-squalene cholesterol biosynthesis leading to human dysmorphogenesis.

Insights in molecular developmental biology in animals and humans are facilitating the understanding of pathophysiologic mechanisms in dysmorphogenesis or abnormalities in normal embryologic structural development. A milestone was recognition of the role of shh in morphogenesis of craniofacial structures, especially the development of holoprosencephaly. The dependence of hedgehog morphogens on cholesterol modification for normal hedgehog signaling function has particular relevance to disorders of cholesterol synthesis which manifest dysmorphogenesis. Four human disorders of morphogenesis (Smith-Lemli-Opitz syndrome, desmosterolosis, X-linked chondrodysplasia punctata, CHILD syndrome) have recently been shown to be caused by sterol abnormalities resulting from cholesterol biosynthesis enzyme deficiencies. This review summarizes the clinical, biochemical and molecular data in these disorders with an emphasis on understanding the pathophysiology of dysmorphogenesis.     

The roles of cis-inactivation by Notch ligands and of neuralized during eye and bristle patterning in Drosophila

Background  

Normal post-squalene cholesterol biosynthesis is important for mammalian embryonic development. Neonatal mice lacking functional dehydrocholesterol Δ7-reductase (Dhcr7), a model for the human disease of Smith-Lemli-Opitz syndrome, die within 24 hours of birth. Although they have a number of biochemical and structural abnormalities, one cause of death is from apparent respiratory failure due to developmental pulmonary abnormalities.     

Oxysterols: functional significance in fetal development and the maintenance of normal retinal function

PURPOSE OF REVIEW: Recent findings extend the biologic activities of oxysterols as ligands for nuclear receptors to a role in morphogenesis during fetal development and to a role in the metabolism of photooxidation products of cholesterol in the retina. RECENT FINDINGS: A 1000-fold increase of the 27-hydroxy metabolite of 7-dehydrocholesterol in the plasma of children with Smith-Lemli-Opitz syndrome imply that intermediates in cholesterol synthesis follow alternate pathways of metabolism that generate novel oxysterols. A mouse model also finds an increase in sterol intermediates as the proximate cause of dysmorphisms. A role for oxysterols in the effects of Sonic hedgehog protein focuses on their role in normal fetal development. Both CYP27A1 and CYP46A1 are expressed in primate retina indicating that local metabolism of 7-ketocholesterol to nontoxic derivatives is important for preventing retinal degeneration. SUMMARY: Recent data expand the functional roles of oxysterols to fetal development and to the detoxification of oxidation products of cholesterol. This review shifts the focus of attention from studies of their ligand-binding activity to studies of animal models that indicate a number of important biologic effects during fetal development and during the aging process.     

The effects of 7-dehydrocholesterol on the structural properties of membranes

Smith-Lemli-Opitz syndrome, a congenital and developmental malformation disease, is typified by abnormal accumulation of 7-dehydrocholesterol (7DHC), the immediate precursor of cholesterol (CHOL), and depletion thereof. Knowledge of the effect of 7DHC on the biological membrane is, however, still fragmentary. In this study, large-scale atomistic molecular dynamics simulations, employing two distinct force fields, have been conducted to elucidate differences in the structural properties of a hydrated dimyristoylphosphatidylcholine bilayer due to CHOL and 7DHC. The present series of results indicate that CHOL and 7DHC possess virtually the same ability to condense and order membranes. Furthermore, the condensing and ordering effects are shown to be strengthened at increasing sterol concentrations.     

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.     

Formation of 7-dehydrocholesterol-containing membrane rafts in vitro and in vivo, with relevance to the Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a recessive disease typified by 7-dehydrocholesterol (7DHC) accumulation and depletion of cholesterol. Because cholesterol is a primary component of detergent-resistant membrane domains ("rafts"), we examined the compatibility of 7DHC with raft formation. Liposomes containing bovine brain phosphatidylcholine, sphingomyelin, cerebrosides, and either cholesterol, 7DHC, or coprostanol (the latter being incompatible with raft formation) were prepared. 7DHC was indistinguishable from cholesterol in its ability to become incorporated into membrane rafts, as judged by physical and chemical criteria, whereas coprostanol did not form rafts. The in vivo compatibility of 7DHC with raft formation was evaluated in brains of rats treated with trans-1,4-bis(2-dichlorobenzylamino-ethyl)cyclohexane dihydrochloride (AY9944), which mimics the SLOS biochemical defect. 7DHC/cholesterol ratios in rafts and whole brains from AY9944-treated rats were similar, indicating comparable efficiency of 7DHC and cholesterol incorporation into brain rafts. In contrast, dolichol (a nonsterol isoprenoid incompatible with raft formation) was greatly depleted in brain rafts relative to whole brain. Although brain raft fractions prepared from AY9944-treated and control rats yielded similar sterol-protein ratios, their gel electrophoresis profiles exhibited multiple differences, suggesting that altered raft sterol composition perturbs raft protein content. These results are discussed in the context of the SLOS phenotype, particularly with regard to the associated central nervous system defects.     

Biological activities of 7-dehydrocholesterol-derived oxysterols: implications for Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a metabolic and developmental disorder caused by mutations in the gene encoding the enzyme 7-dehydrocholesterol reductase (Dhcr7). This reductase catalyzes the last step in cholesterol biosynthesis, and levels of 7-dehydrocholesterol (7-DHC), the substrate for this enzyme, are elevated in SLOS patients as a result of this defect. Our group has previously shown that 7-DHC is extremely prone to free radical autoxidation, and we identified about a dozen different oxysterols formed from oxidation of 7-DHC. We report here that 7-DHC-derived oxysterols reduce cell viability in a dose- and time-dependent manner, some of the compounds showing activity at sub-micromolar concentrations. The reduction of cell survival is caused by a combination of reduced proliferation and induced differentiation of the Neuro2a cells. The complex 7-DHC oxysterol mixture added to control Neuro2a cells also triggers the gene expression changes that were previously identified in Dhcr7-deficient Neuro2a cells. Based on the identification of overlapping gene expression changes in Dhcr7-deficient and 7-DHC oxysterol-treated Neuro2a cells, we hypothesize that some of the pathophysiological findings in the mouse SLOS model and SLOS patients might be due to accumulated 7-DHC oxysterols.