HEM dysplasia and ichthyosis are likely laminopathies and not due to 3β-hydroxysterol Δ14-reductase deficiency

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(Delta 4-7/Delta 4-7) and Lbr(+/-):Dhcr14(Delta 4-7/Delta 4-7) mutant mice have distinct physical and biochemical phenotypes. Dhcr14(Delta 4-7/Delta 4-7) mice are essentially normal, whereas Lbr(+/-):Dhcr14(Delta 4-7/Delta 4-7) 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(Delta 4-7/Delta 4-7) brain tissue, and marked elevations were seen in Lbr(+/-):Dhcr14(Delta 4-7/Delta 4-7) brain. Pathological evaluation demonstrated vacuolation and swelling of the myelin sheaths in the spinal cord of Lbr(+/-):Dhcr14(Delta 4-7/Delta 4-7) mice consistent with a demyelinating process. This was not observed in either Lbr(-/-) or Dhcr14(Delta 4-7/Delta 4-7) 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.