Peter Clayton

Mutational Spectrum in the Δ7-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 Δ7-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.

Impaired insulin and IGF2 signalling in the primordial growth disorder 3-M syndrome

 Relative phosphorylation of AKT was assessed by western blotting and comparisons made both within and between cell lines. In view of this altered growth factor signalling, we have investigated the activation of AKT by IGF-2 and Insulin.  Relative pAKT was reduced in 3-M fibroblasts compared to normal cells when stimulated with either insulin (p = 0.048) or IGF-2 (p = 0.033) (Fig. 2).  For both treatments, CUL7-/and OBSL1-/cells had a markedly lower activation of pAKT than did CCDC8-/cells.  CUL7-/cells had an earlier activation in response to insulin (p = 0.016) than other cell types.