Abstract 818 Clinical Genetics/Dysmorphology Platform, Tuesday, 5/4

The Smith-Lemli-Opitz Syndrome (SLOS) is an autosomal recessive malformation syndrome due to mutations in the 7-dehydrocholesterol reductase gene. In the penultimate step of endogenous cholesterol biosynthesis 7-dehydrocholesterol reductase catalyzes the conversion of 7-dehydrocholesterol to cholesterol. Thus individuals with SLOS have elevated levels of 7-dehydrocholesterol. The molecular, biochemical and cellular mechanisms by which this inborn error of metabolism cause the SLOS phenotype have not been determined. In order to establish a genotype/phenotype correlation for this malformation syndrome we have cloned the human 7-dehydrocholesterol reductase gene and identified mutations in patients with SLOS. The most common mutation is G to C splice acceptor mutation. When homozygous, this mutation gives rise to a severe phenotype. All of the mutations result in elevated levels of 7-dehydrocholesterol. These abnormal levels of 7-DHC appear to perturb the function of proteins that contain sterol sensing domains. The sonic hedgehog receptor Patched and the Neiman Pick type C gene both encode sterol sensing domains. Previously it has been shown that inhibitors of cholesterol biosynthesis impair the signaling of sonic hedgehog through Patched. SLOS has phenotypic overlap with sonic hedgehog mutations. Thus abnormal sterol biosynthesis in SLOS may perturb Patched signalling during development and thereby may explain the genesis of some of the developmental malformations. We now show that SLOS fibroblasts have a secondary defect in intracellular LDL cholesterol transport similar to that seen in Niemann Pick type C (NPC) cells. Like NPC cells, SLOS fibroblasts show increased filipin staining, show decreased intracellular LDL degradation, and have lysosomal inclusion bodies similar to those seen in NPC. We propose that these findings are due to inhibition of NPC1 function through its sterol sterol sensing domains. Clinically, this secondary defect in LDL cholesterol transport may limit the effectiveness of the current dietary therapy.