Mutational spectrum in the Delta7-sterol reductase gene and genotype-phenotype correlation in 84 patients with Smith-Lemli-Opitz syndrome

Abstract

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.

Figure 1

Distribution of SLOS-causing mutations in the DHCR7 gene. Exons are drawn to scale; introns are not drawn to scale. The ATG initiation codon is indicated. The hatched boxes with roman numerals denote the transmembrane domains. The cytosolic loop in exon 9 (cytosol) and the C-terminus, assumed to be located in the lumen of the ER, are indicated.

Figure 2

A, Correlation between severity score and DHC fraction. R_S and linear regression coefficients are shown. B, Box-plot diagram illustrating the distribution of DHC fraction values within genotypes. C, Box-plot diagram illustrating the distribution of severity scores within genotypes. Black lines within boxes in B and C represent medians. Boxes represent the range, accommodating 95% of values, and vertical lines represent the total range for the group.