ABSTRACT
Mutations of the lamin B receptor (LBR) have been shown to cause HEM dysplasia in humans and ichthyosis in mice. LBR is a bifunctional protein with both a lamin B binding and a sterol Delta(14)-reductase domain. It previously has been proposed that LBR is the primary sterol Delta(14)-reductase and that HEM dysplasia and ichthyosis are inborn errors of cholesterol synthesis. However, DHCR14 also encodes a sterol Delta(14)-reductase and could provide enzymatic redundancy with respect to cholesterol synthesis. To test the hypothesis that LBR and DHCR14 both function as sterol Delta(14)-reductases, we obtained ichthyosis mice (Lbr(-/-)) and disrupted Dhcr14. Heterozygous Lbr and Dhcr14 mice were intercrossed to test for a digenic phenotype. Lbr(-/-), Dhcr14(Delta4-7/Delta4-7) and Lbr(+/-):Dhcr14(Delta4-7/Delta4-7) mutant mice have distinct physical and biochemical phenotypes. Dhcr14(Delta4-7/Delta4-7) mice are essentially normal, whereas Lbr(+/-):Dhcr14(Delta4-7/Delta4-7) mice are growth retarded and neurologically abnormal. Neither of these mutants resembles the ichthyosis mouse and biochemically, no sterol abnormalities were detected in either liver or kidney tissue. In contrast, relatively small transient elevations of Delta(14)-sterols were observed in Lbr(-/-) and Dhcr14(Delta4-7/Delta4-7) brain tissue, and marked elevations were seen in Lbr(+/-):Dhcr14(Delta4-7/Delta4-7) brain. Pathological evaluation demonstrated vacuolation and swelling of the myelin sheaths in the spinal cord of Lbr(+/-):Dhcr14(Delta4-7/Delta4-7) mice consistent with a demyelinating process. This was not observed in either Lbr(-/-) or Dhcr14 (Delta4-7/Delta4-7) mice. Our data support the conclusions that LBR and DHCR14 provide substantial enzymatic redundancy with respect to cholesterol synthesis and that HEM dysplasia and ichthyosis are laminopathies rather than inborn errors of cholesterol synthesis.
Subject(s)
Abnormalities, Multiple/genetics , Bone Diseases, Developmental/genetics , Ichthyosis/genetics , Oxidoreductases/deficiency , Receptors, Cytoplasmic and Nuclear/genetics , Abnormalities, Multiple/metabolism , Abnormalities, Multiple/pathology , Animals , Bone Diseases, Developmental/metabolism , Bone Diseases, Developmental/pathology , Brain/metabolism , Calcinosis/genetics , Calcinosis/metabolism , Calcinosis/pathology , Cholesterol/biosynthesis , Disease Models, Animal , Female , Humans , Hydrops Fetalis/genetics , Hydrops Fetalis/metabolism , Ichthyosis/metabolism , Ichthyosis/pathology , Lipid Metabolism, Inborn Errors/genetics , Lipid Metabolism, Inborn Errors/metabolism , Lipid Metabolism, Inborn Errors/pathology , Male , Mice , Mice, Inbred C57BL , Mice, Knockout , Mice, Mutant Strains , Mutation , Oxidoreductases/genetics , Phenotype , Receptors, Cytoplasmic and Nuclear/metabolism , Sterols/metabolism , Syndrome , Lamin B ReceptorABSTRACT
Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive, multiple congenital anomaly syndrome caused by deficiency of 7-dehydrocholesterol reductase (DHCR7), which catalyzes the last step of endogenous cholesterol synthesis. Surveys of SLOS patients have identified more than one hundred point mutations of the DHCR7 gene, most of which are missense mutations. Here, we report the identification of nine novel missense mutations of the DHCR7 gene.
Subject(s)
Mutation, Missense/genetics , Oxidoreductases Acting on CH-CH Group Donors/genetics , Smith-Lemli-Opitz Syndrome/genetics , DNA Mutational Analysis , Humans , Smith-Lemli-Opitz Syndrome/enzymologyABSTRACT
Smith-Lemli-Opitz syndrome (RSH/SLOS) is an autosomal recessive, malformation syndrome caused by mutations in the 3beta-hydroxysterol delta7-reductase gene (DHCR7). DHCR7 catalyzes the reduction of 7-dehydrocholesterol (7DHC) to cholesterol. We report the mutation analysis and determination of residual cholesterol synthesis in 47 SLOS patients, and the effects of treatment of SLOS skin fibroblasts with simvastatin. Using deuterium labeling we have quantified the amount of synthesized cholesterol and 7DHC in homozygote, heterozygote, and control fibroblast cell lines. In SLOS fibroblasts, the fraction of synthesized cholesterol to total sterol synthesis ranged from undetectable to over 50%. This establishes that different mutant alleles encode enzymes with varying degrees of residual activity. There was a correlation between increased phenotypic severity and decreased residual cholesterol synthesis (r(2)=0.45, p<0.0001). Simvastatin treatment of SLOS fibroblasts with residual DHCR7 enzymatic activity decreased 7DHC levels and increased cholesterol synthesis. This increase in cholesterol synthesis is due to increased expression of a mutant allele with residual function. Determination of residual enzymatic activity for specific DHCR7 mutant alleles will help in understanding the processes underlying the broad phenotypic spectrum found in this disorder and will be useful in identifying patients who may benefit from simvastatin therapy.
