Abnormal sterol metabolism in patients with Conradi-Hünermann-Happle syndrome and sporadic lethal chondrodysplasia punctata

Lathosterolosis, a novel multiple-malformation/mental retardation syndrome due to deficiency of 3beta-hydroxysteroid-delta5-desaturase

We report the clinical, biochemical, and molecular characterization of a patient with a novel defect of cholesterol biosynthesis. This patient presented with a complex phenotype, including multiple congenital anomalies, mental retardation, and liver disease. In the patient's plasma and cells, we found increased levels of lathosterol. The biosynthesis of cholesterol in the patient's fibroblasts was defective, showing a block in the conversion of lathosterol into 7-dehydrocholesterol. The activity of 3beta-hydroxysteroid-Delta(5)-desaturase (SC5D), the enzyme involved in this reaction, was deficient in the patient's fibroblasts. Sequence analysis of the SC5D gene in the patient's DNA, showing the presence of two missense mutations (R29Q and G211D), confirmed that the patient is affected by a novel defect of cholesterol biosynthesis.

Abnormal sterol metabolism in a patient with Antley-Bixler syndrome and ambiguous genitalia

Antley-Bixler syndrome (ABS) is a rare multiple anomaly syndrome comprising radiohumeral synostosis, bowed femora, fractures of the long bones, premature fusion of the calvarial sutures, severe midface hypoplasia, proptosis, choanal atresia, and, in some, ambiguous genitalia. Of fewer than 40 patients described to date, most have been sporadic, although reports of parental consanguinity and affected sibs of both sexes suggests autosomal recessive inheritance in some families. Known genetic causes among sporadic cases of ABS or ABS-like syndromes are missense mutations in the IgII and IgIII regions of FGFR2, although the assignment of the diagnosis of ABS to such children has been disputed. A third cause of an ABS-like phenotype is early in utero exposure to fluconazole, an inhibitor of lanosterol 14-alpha-demethylase. The fourth proposed cause of ABS is digenic inheritance combining heterozygosity or homozygosity for steroid 21-hydroxylase deficiency with effects from a second gene at an unknown locus. Because fluconazole is a strong inhibitor of lanosterol 14-alpha-demethylase (CYP51), we evaluated sterol metabolism in lymphoblast cell lines from an ABS patient without a known FGFR2 mutation and from a patient with an FGFR2 mutation and ABS-like manifestations. When grown in the absence of cholesterol to stimulate cholesterol biosynthesis, the cells from the ABS patient with ambiguous genitalia but without an FGFR2 mutation accumulated markedly increased levels of lanosterol and dihydrolanosterol. Although the abnormal sterol profile suggested a deficiency of lanosterol 14-alpha-demethylase, mutational analysis of its gene, CYP51, disclosed no obvious pathogenic mutation in any of its 10 exons or exon-intron boundaries. Sterol metabolism in lymphoblasts from the phenotypically unaffected mother was normal. Our results suggest that ABS can occur in a patient with an intrinsic defect of cholesterol biosynthesis at the level of lanosterol 14-alpha-demethylase, although the genetic nature of the deficiency remains to be determined.

Inherited disorders of cholesterol biosynthesis

For many decades, cholesterol has been considered an important structural component of cellular membranes and myelin, and a precursor of steroid hormones and bile acids. Moreover, the recognition that high cholesterol levels (hypercholesterolemia) are a major risk factor for the development of heart disease and atherosclerosis has gained enormous attention not only in medicine, medical and pharmacological research, but also from the general public. The discovery of a crucial role of cholesterol in human embryogenesis and the recent identification of a number of inherited disorders of cholesterol biosynthesis also show that low cholesterol levels (hypocholesterolemia) may have severe consequences for human health and development. In the past few years, seven distinct inherited disorders have been linked to different enzyme defects in the cholesterol biosynthetic pathway by the finding of abnormally increased levels of intermediate metabolites in patients followed by the demonstration of disease-causing mutations in genes encoding the implicated enzymes. Patients afflicted with these disorders are characterized by multiple morphogenic and congenital anomalies including internal organ, skeletal and/or skin abnormalities.

Gas chromatography-mass spectrometry and molecular genetic studies in families with the Conradi-Hünermann-Happle syndrome

The Conradi-Hünermann-Happle syndrome is an X-linked dominant disease that is due to mutations in the gene for emopamil binding protein. Emopamil binding protein is a Delta8-Delta7 sterol isomerase and plays a pivotal role in the final steps of cholesterol biosynthesis. We wanted to know to what extent this X-linked dominant enzyme defect has functional consequences at the biochemical level and whether it is possible to predict the clinical phenotype from serum sterol measurements. Therefore we performed sterol biochemical studies in 11 Conradi-Hünermann-Happle syndrome families and compared the results obtained to the clinical and molecular genetic findings. To assess disease severity a score considering bone and skin involvement and further features was used. For evaluation of the functional consequences we studied serum samples using gas chromatography-mass spectrometry analysis. For mutation screening we analyzed the emopamil binding protein gene using polymerase chain reaction, heteroduplex analysis of all exons, direct sequencing, and restriction enzyme analysis. Mutations in the emopamil binding protein gene were found in all 11 families including seven novel mutations affecting exons 2, 4, and 5. Gas chromatography-mass spectrometry analysis revealed markedly elevated levels of 8-dehydrocholesterol and of cholest-8(9)-en-3beta-ol and helped to identify somatic mosaicism in a clinically unaffected man. The extent of the metabolic alterations in the serum, however, do not allow prediction of the clinical phenotype, nor the genotype. This lack of correlation may be due to differences in X-inactivation between different tissues of the same patient and/or loss of the mutant clone by outgrowth of proficient clones after some time.

Teratogenesis of holoprosencephaly

Teratogenic causes of holoprosencephaly are critically assessed. A brief general review of holoprosencephaly is followed by four tables summarizing etiologic factors. Subjects evaluated here include: 1) maternal diabetes; 2) ethyl alcohol; 3) retinoic acid; 4) mutated genes and teratogens involving the sonic hedgehog signaling network and cholesterol biosynthesis; and 5) cholesterol trafficking, sterol adducts, target tissue response, and sterol sensing domain.

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.

Inborn errors of sterol biosynthesis

The known disorders of cholesterol biosynthesis have expanded rapidly since the discovery that Smith-Lemli-Opitz syndrome is caused by a deficiency of 7-dehydrocholesterol. Each of the six now recognized sterol disorders-mevalonic aciduria, Smith-Lemli-Opitz syndrome, desmosterolosis, Conradi-Hünermann syndrome, CHILD syndrome, and Greenberg dysplasia-has added to our knowledge of the relationship between cholesterol metabolism and embryogenesis. One of the most important lessons learned from the study of these disorders is that abnormal cholesterol metabolism impairs the function of the hedgehog class of embryonic signaling proteins, which help execute the vertebrate body plan during the earliest weeks of gestation. The study of the enzymes and genes in these several syndromes has also expanded and better delineated an important class of enzymes and proteins with diverse structural functions and metabolic actions that include sterol biosynthesis, nuclear transcriptional signaling, regulation of meiosis, and even behavioral modulation.

Molecular cloning and structural analysis of human sterol C5 desaturase

Sterol C5 desaturase (SC5D) converts lathosterol to 7-dehydrocholesterol in cholesterol biosynthesis. In this study, we investigated the genome structure of SC5D and transcription of the human SC5D gene to try to elucidate the mechanism by which cholesterol synthesis is regulated. The SC5D gene had a structural gene from a single copy from genome DNA that contained five exons and four introns. The human SC5D was found to be located in chromosome 11q24.2-24.3 by fluorescence in situ hybridization mapping. Human SC5D mRNA transcripts, of which the major transcript was a 2 kb and the minor transcripts were 8 kb and 1.4 kb mRNA transcripts, were detected in almost all of the tissues examined. The human SC5D gene contained a GC box instead of a TATA box upstream of the transcript start sites. Human SC5D transcription started from several transcription start sites, and the first start site was located 31 bp upstream of the translation start site (ATG). The expression level of SC5D mRNA extracted from human liver carcinoma cells decreased as the amount of cholesterol added to the culture medium was increased. Inhibition of SC5D transcription was ascribed to the suppression of promoter activity of SC5D.

7-Dehydrocholesterol-dependent proteolysis of HMG-CoA reductase suppresses sterol biosynthesis in a mouse model of Smith-Lemli-Opitz/RSH syndrome

Smith-Lemli-Opitz/RSH syndrome (SLOS), a relatively common birth-defect mental-retardation syndrome, is caused by mutations in DHCR7, whose product catalyzes an obligate step in cholesterol biosynthesis, the conversion of 7-dehydrocholesterol to cholesterol. A null mutation in the murine Dhcr7 causes an identical biochemical defect to that seen in SLOS, including markedly reduced tissue cholesterol and total sterol levels, and 30- to 40-fold elevated concentrations of 7-dehydrocholesterol. Prenatal lethality was not noted, but newborn homozygotes breathed with difficulty, did not suckle, and died soon after birth with immature lungs, enlarged bladders, and, frequently, cleft palates. Despite reduced sterol concentrations in Dhcr7(-/-) mice, mRNA levels for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-controlling enzyme for sterol biosynthesis, the LDL receptor, and SREBP-2 appeared neither elevated nor repressed. In contrast to mRNA, protein levels and activities of HMG-CoA reductase were markedly reduced. Consistent with this finding, 7-dehydrocholesterol accelerates proteolysis of HMG-CoA reductase while sparing other key proteins. These results demonstrate that in mice without Dhcr7 activity, accumulated 7-dehydrocholesterol suppresses sterol biosynthesis posttranslationally. This effect might exacerbate abnormal development in SLOS by increasing the fetal cholesterol deficiency.

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.

Applications of mass spectrometry in the study of inborn errors of metabolism

During the twentieth century, and particularly in its last decade, there have been major advances in mass spectrometry (MS). As a result, MS remains one of the most powerful tools for the investigation of genetic metabolic disease. Analysis of organic acids by gas chromatography-mass spectrometry (GC-MS) and analysis of acylcarnitines by tandem mass spectrometry are still leading to the discovery of new disorders. Tandem mass spectrometry is increasingly being used for neonatal screening. New methods for lipid analysis have opened up the fields of inborn errors of cholesterol synthesis, of bile acid synthesis and ofleukotriene synthesis. The latest developments in MS allow it to be used for determination of the amino acid sequence and posttranslational modifications of proteins. There are still some major hurdles to be overcome, but soon it should be possible to detect mutant proteins directly rather than by cDNA or genomic DNA analysis. Measurement of which proteins are overexpressed and underexpressed ('proteomics') should provide further information on the pathogenesis of complications of inborn errors, e.g. hepatic cirrhosis. The use of stable isotopes in conjunction with MS allows us to probe metabolic pathways. As an example, evidence is presented to support the contention that vitamin E and its oxidation product are catabolized by peroxisomal beta-oxidation. Mass spectrometry also has a major role in monitoring new forms of treatment for inborn errors.

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.

[X-linked recessive chondrodysplasia punctata. Cytogenetic study and role of molecular biology]

Among the many acquired or constitutional causes of chondrodysplasia punctata, the X-linked recessive form is well individualized.

CASE REPORT

A male newborn presented a dysmorphic syndrome with a marked nasal hypoplasia, a macroglossia and a short neck. The diagnosis of chondrodysplasia punctata was made by radiography whereas the chromosomal chart revealed the existence of an additional Y fragment in Xpter, effectuating a partial disomy Yp and a monosomy Xpter. Molecular biology showed a deletion of very small size, isolated and located between the gene missing aryl sulfatase E and the microsatellite DXS 1233, sping gene MRX2 (non-specific gene of mental retardation), and making it possible to give the reassuring elements as regards the psychomotor prognosis, sometimes compromised in this disorder.

CONCLUSION

In case of chondrodysplasia punctata with dysmorphy, it is important to execute a chromosomal chart in the search for a chromosomal reorganization on the X and a study in molecular biology.

Cholesterol biosynthesis from lanosterol: molecular cloning, chromosomal localization, functional expression and liver-specific gene regulation of rat sterol delta8-isomerase, a cholesterogenic enzyme with multiple functions

Sterol Delta(8)-isomerase (SI) (EC 5.3.3.5), also known as emopamil binding protein or sigma receptor, catalyses the conversion of the 8-ene isomer into the 7-ene isomer in the cholesterol biosynthetic pathway in mammals. Recently, mutations of SI have been found to be associated with Conradi-Hünermann syndrome in humans. To investigate the in vitro and in vivo modes of molecular regulation of SI and its role in cholesterol biosynthesis in mammals, we isolated a full-length cDNA encoding rat SI. The deduced amino-acid sequence of rat SI predicts a 230-residue protein (26737 Da) with 87% and 80% amino-acid identity to mouse and human counterparts. The rat SI gene was mapped to chromosome 12q1.2 using fluorescence in situ hybridization (FISH). The biological function of the cloned rat SI cDNA was verified by overexpressing recombinant Myc-SI in Saccharomyces cerevisiae. It showed a characteristic pattern of inhibition on exposure to trans-2-[4-(1,2-diphenylbuten-1-yl)phenoxy]-N,N-dimethylethylamine (tamoxifen; IC(50)=11.2 microM) and 3beta-[2-(diethylamino)ethoxy]androst-5-en-17-one (U18666A; IC(50)=4.2 microM), two well known potent inhibitors of SI. Northern-blot analysis of 3-week-old rats compared with 2-year-old rats showed that SI mRNA expression in both age groups was restricted to liver, where a 70% reduction in mRNA levels was observed in 2-year-old rats. The FISH studies revealed ubiquitous expression of SI mRNA in rat hepatocytes. The in vitro studies showed that the SI mRNA was highly suppressed by 25-hydroxycholesterol in H4IIE cells. Treatment of H4IIE cells grown in medium supplemented with fetal bovine serum with tamoxifen for 24 h resulted in a dose-dependent induction of SI mRNA, with a concomitant suppression of sterol regulatory element binding protein-1 mRNA. Interestingly, this effect was not seen in emopamil-treated cells. The in vivo experiments also indicate that both mRNA expression and enzymic activity of SI in liver were induced approx. 3-fold in rats fed 5% (w/w) cholestyramine plus 0.1% (w/w) lovastatin in normal chow for 2 weeks. With this newly cloned rat SI cDNA, it becomes possible to gain molecular understanding of previously unknown and tamoxifen-mediated gene regulation of SI that is involved in cholesterol metabolism, ischaemia and genetic diseases.

Lethal form of chondrodysplasia punctata with normal plasmalogen and cholesterol biosynthesis

We present a male autopsied case of chondrodysplasia punctata with abnormal face, symmetrical proximal limb shortness, severe psychomotor developmental delay, respiratory muscle weakness, and death at the age of 2 years. Although his clinical manifestations were similar to those of rhizomelic chondrodysplasia punctata (RCDP), biochemical studies using skin fibroblasts did not document the peroxisomal dysfunction described in RCDP. In addition, the sterol profile, for which abnormalities have recently been reported in cases of X-linked dominant form chondrodysplasia punctata (CDPX2), was normal both in the liver and in the fibroblasts. This patient may represent a new lethal form of chondrodysplasia punctata.

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.

Cholesterol metabolsim defect associated with Conradi-Hunerman-Happle syndrome

We present a 6-week-old black girl with Conradi-Hunerman-Happle syndrome (CHS). The mother had no past medical history of illness, and the pregnancy progressed normally to a spontaneous vaginal delivery at 36 weeks. There was no known significant family history. A diagnosis of chondrodysplasia punctata was made at birth from physical examination and X-ray findings. On physical examination at 6 weeks, a koala face, a saddle nose, and a right-sided cataract were noted (Fig. 1a,b). There was unilateral left-sided ichthyosis well demarcated at the midline, with whorled brown fine scale following Blashko's lines on the patient's right side. Orthopedic complications were bilateral but were more pronounced on the left side. There was bilateral shortening of the humerus, with polydactyly of the right hand, arachnodactyly of the left fingers, bilateral clubbing, and mild contractures of the feet. X-Rays showed multiple calcifications along the spine, proximal and distal femoral epiphysis, and proximal humeral epiphysis (Fig. 2). The patient was treated with emollients (aquaphor) twice daily with continuing improvement in ichthyosis. The clubbed feet were treated with splinting and the polydactyly was corrected by surgery. Ophthalmology was to follow the patient for her right-sided cataract. At the patient's 4-month follow-up, the ichthyosis showed a marked improvement with some residual hypo- pigmented atrophoderma noted. The distribution remained unchanged. Biopsies taken of ichthyotic lesions showed compact hyperkeratosis and follicular plugging. Vesicles within the stratum corneum contained amorphous material (Fig. 3a,b). The granular cell layer was thickened with retained oval nuclei. The epidermal and adnexal epithelium were disorganized. Increased apoptotic/dyskeratotic keratinocytes were seen within the epidermis, but were most evident within the follicular epithelium. Ultrastructural studies showed saccular dilations of the acellular space within the stratum corneum. These acellular spaces were filled with unprocessed lamellated pleated sheets and vesicle complexes and processed lamellae. Dyskeratotic cells were seen within the stratum spinosum. Red blood cell (RBC) plasmalogen levels and polyunsaturated fatty acids (PUFA), including decosahexaenoic acid (DHA), were within normal limits. Plasma very long chain fatty acids (VLCFA), including C26 : 0/C22 : 0 ratios, phytanic and pristanic acids, plasmalogen, and phytanic/pristanic ratios, trihydroxycholestanic acid (THCA) and dihydroxycholestanoic acid (DHCA) including their ratios, THCA/cholic acid and DHCA/chenodeoxycholic acid, and PUFAs including DHA were within normal limits. Urine organic acids and piecolic acid were within normal limits. Despite these normal values, there was an increase in cholest-8(9)-en-3beta-ol of 6.8 microg/mL (normal, 0.01-0.10 microg/mL) and an increase in 8-dehydrocholesterol (5.1 microg/mL) (normal, <0.10 microg/mL).

Novel and recurrent EBP mutations in X-linked dominant chondrodysplasia punctata

Chondrodysplasia punctata (CDP) is a heterogeneous group of skeletal dysplasias characterized by stippled epiphyses. A subtype of CDP, X-linked dominant chondrodysplasia punctata (CDPX2), known also as Conradi-Hünermann-Happle syndrome, is a rare skeletal dysplasia characterized by short stature, craniofacial defects, cataracts, ichthyosis, coarse hair, and alopecia. The cause of CDPX2 was unknown until recent identification of mutations in the gene encoding Delta(8),Delta(7) sterol isomerase emopamil-binding protein (EBP). Twelve different EBP mutations have been reported in 14 patients with CDPX2 or unclassified CDP, but with no evidence of correlation between phenotype and nature of the mutation. To characterize additional mutations and investigate possible phenotype-genotype correlation, we sequenced the entire EBP gene in 8 Japanese individuals with CDP; 5 of them presented with a CDPX2 phenotypes. We found EBP mutations in all 5 CDPX2 individuals, but none in non-CDPX2 individuals. Three of these CDPX2 individuals carried novel nonsense mutations in EBPand the other two, separate missense mutations that had been reported also in different ethnic groups. Our results, combined with previous information, suggest all EBP mutations that produce truncated proteins result in typical CDPX2, whereas the phenotypes resulted from missense mutations are not always typical for CDPX2. Patients with nonsense mutations showed abnormal sterol profiles consistent with a defect in Delta(8), Delta(7) sterol isomerase. X-inactivation patterns of the patients showed no skewing, an observation that supports the assumption that inactivation of the EBP gene occurs at random in affected individuals.

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 in signal transduction

Membrane cholesterol impinges on signal transduction in several ways, which is highlighted in particular by the Hedgehog signaling pathway. In Hedgehog signaling, cholesterol is important for ligand biogenesis, as well as for signal transduction in receiving cells. Hedgehog ligands are post-translationally modified by cholesterol, and the Hedgehog receptor, Patched, is structurally similar to the Niemann-Pick C1 protein, which functions in intracellular lipid transport. Although the exact role of cholesterol in Hedgehog signal transduction remains elusive and is probably multifaceted, studies over the past year have implicated raft membrane subdomains, cholesterol transport and a link between protein and lipid trafficking in endocytic compartments.

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.

CHILD syndrome caused by deficiency of 3beta-hydroxysteroid-delta8, delta7-isomerase

CHILD (congenital hemidysplasia, ichthyosis, and limb defects) syndrome is a rare, usually sporadic disorder associated with unilateral distribution of ichthyosiform skin lesions, limb defects, punctate calcifications of cartilaginous structures, and visceral anomalies. CHILD syndrome shares some manifestations with X-linked dominant Conradi-Hünermann syndrome (CDPX2), although the skeletal defects and skin lesions in CDPX2 are bilateral and asymmetric. Because CDPX2 patients have abnormal 8-dehydrosterol metabolism caused by mutations in 3beta-hydroxysteroid-delta8,delta7-isomerase, we measured plasma sterols in a patient with CHILD syndrome and found levels of 8-dehydrocholesterol and 8(9)-cholestenol increased to the same degree as in CDPX2 patients. Subsequently, we identified a nonsense mutation in exon 3 of the patient's 3beta-hydroxysteroid-delta8,delta7-isomerase gene. We speculate that at least some cases of CHILD syndrome are allelic with CDPX2 caused by 3beta-hydroxysteroid-delta8,delta7-isomerase deficiency.

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.

Mutations in a delta 8-delta 7 sterol isomerase in the tattered mouse and X-linked dominant chondrodysplasia punctata. jderry@immunex.com

Tattered (Td) is an X-linked, semi-dominant mouse mutation associated with prenatal male lethality. Heterozygous females are small and at 4-5 days of age develop patches of hyperkeratotic skin where no hair grows, resulting in a striping of the coat in adults. Craniofacial anomalies and twisted toes have also been observed in some affected females. A potential second allele of Td has also been described. The phenotype of Td is similar to that seen in heterozygous females with human X-linked dominant chondrodysplasia punctata (CDPX2, alternatively known as X-linked dominant Conradi-Hünermann-Happle syndrome) as well as another X-linked, semi-dominant mouse mutation, bare patches (Bpa). The Bpa gene has recently been identified and encodes a protein with homology to 3beta-hydroxysteroid dehydrogenases that functions in one of the later steps of cholesterol biosynthesis. CDPX2 patients display skin defects including linear or whorled atrophic and pigmentary lesions, striated hyperkeratosis, coarse lusterless hair and alopecia, cataracts and skeletal abnormalities including short stature, rhizomelic shortening of the limbs, epiphyseal stippling and craniofacial defects (MIM 302960). We have now identified the defect in Td mice as a single amino acid substitution in the delta8-delta7 sterol isomerase emopamil binding protein (Ebp; encoded by Ebp in mouse) and identified alterations in human EBP in seven unrelated CDPX2 patients.

Mutations in the gene encoding 3 beta-hydroxysteroid-delta 8, delta 7-isomerase cause X-linked dominant Conradi-Hünermann syndrome

X-linked dominant Conradi-Hünermann syndrome (CDPX2; MIM 302960) is one of a group of disorders with aberrant punctate calcification in cartilage, or chondrodysplasia punctata (CDP). This is most prominent around the vertebral column, pelvis and long bones in CPDX2. Additionally, CDPX2 patients may have asymmetric rhizomesomelia, sectorial cataracts, patchy alopecia, ichthyosis and atrophoderma. The phenotype in CDPX2 females ranges from stillborn to mildly affected individuals identified in adulthood. CDPX2 is presumed lethal in males, although a few affected males have been reported. We found increased 8(9)-cholestenol and 8-dehydrocholesterol in tissue samples from seven female probands with CDPX2 (ref. 4). This pattern of accumulated cholesterol intermediates suggested a deficiency of 3beta-hydroxysteroid-delta8,delta7-isomerase (sterol-delta8-isomerase), which catalyses an intermediate step in the conversion of lanosterol to cholesterol. A candidate gene encoding a sterol-delta8-isomerase (EBP) has been identified and mapped to Xp11.22-p11.23 (refs 5,6). Using SSCP analysis and sequencing of genomic DNA, we found EBP mutations in all probands. We confirmed the functional significance of two missense alleles by expressing them in a sterol-delta8-isomerase-deficient yeast strain. Our results indicate that defects in sterol-delta8-isomerase cause CDPX2 and suggest a role for sterols in bone development.

The gene mutated in bare patches and striated mice encodes a novel 3beta-hydroxysteroid dehydrogenase

X-linked dominant disorders that are exclusively lethal prenatally in hemizygous males have been described in human and mouse. None of the genes responsible has been isolated in either species. The bare patches (Bpa) and striated (Str) mouse mutations were originally identified in female offspring of X-irradiated males. Subsequently, additional independent alleles were described. We have previously mapped these X-linked dominant, male-lethal mutations to an overlapping region of 600 kb that is homologous to human Xq28 (ref. 4) and identified several candidate genes in this interval. Here we report mutations in one of these genes, Nsdhl, encoding an NAD(P)H steroid dehydrogenase-like protein, in two independent Bpa and three independent Str alleles. Quantitative analysis of sterols from tissues of affected Bpa mice support a role for Nsdhl in cholesterol biosynthesis. Our results demonstrate that Bpa and Str are allelic mutations and identify the first mammalian locus associated with an X-linked dominant, male-lethal phenotype. They also expand the spectrum of phenotypes associated with abnormalities of cholesterol metabolism.