Restoration of the conversion of desmosterol to cholesterol in L-cells after hybridization with human fibroblasts

Mutations in the 3beta-hydroxysterol Delta24-reductase gene cause desmosterolosis, an autosomal recessive disorder of cholesterol biosynthesis

Desmosterolosis is a rare autosomal recessive disorder characterized by multiple congenital anomalies. Patients with desmosterolosis have elevated levels of the cholesterol precursor desmosterol, in plasma, tissue, and cultured cells; this abnormality suggests a deficiency of the enzyme 3beta-hydroxysterol Delta24-reductase (DHCR24), which, in cholesterol biosynthesis, catalyzes the reduction of the Delta24 double bond of sterol intermediates. We identified the human DHCR24 cDNA, by the similarity between the encoded protein and a recently characterized plant enzyme--DWF1/DIM, from Arabidopsis thaliana--catalyzing a different but partially similar reaction in steroid/sterol biosynthesis in plants. Heterologous expression, in the yeast Saccharomyces cerevisiae, of the DHCR24 cDNA, followed by enzyme-activity measurements, confirmed that it encodes DHCR24. The encoded DHCR24 protein has a calculated molecular weight of 60.1 kD, contains a potential N-terminal secretory-signal sequence as well as at least one putative transmembrane helix, and is a member of a recently defined family of flavin adenine dinucleotide (FAD)-dependent oxidoreductases. Conversion of desmosterol to cholesterol by DHCR24 in vitro is strictly dependent on reduced nicotinamide adenine dinucleotide phosphate and is increased twofold by the addition of FAD to the assay. The corresponding gene, DHCR24, was identified by database searching, spans approximately 46.4 kb, is localized to chromosome 1p31.1-p33, and comprises nine exons and eight introns. Sequence analysis of DHCR24 in two patients with desmosterolosis revealed four different missense mutations, which were shown, by functional expression, in yeast, of the patient alleles, to be disease causing. Our data demonstrate that desmosterolosis is a cholesterol-biosynthesis disorder caused by mutations in DHCR24.

Cholesterol biosynthesis from lanosterol: development of a novel assay method and characterization of rat liver microsomal lanosterol delta 24-reductase

The membrane-bound sterol delta 24-reductase (24-reductase) catalyses anaerobic reduction of the 24(25)-enes of lanosterol and other obligatory intermediates of cholesterol biosynthesis from lanosterol. A novel assay method and properties of the 24-reductase are described. More than a 120-fold induction of the 24-reductase activity was achieved by feeding rats a diet containing 5% cholestyramine plus 0.1% lovastatin in chow and by modulating diurnal variation. With this enzyme induction condition, lanosterol was converted efficiently into dihydrolanosterol in both intact hepatic microsomes and freshly isolated hepatocytes only when either miconazole or CO was added to inhibit 14 alpha-demethylation of lanosterol. AR45 cells, which are deficient in 14 alpha-methyl demethylase (14 alpha-DM), exhibit lanosterol 24-reductase activity without addition of either CO or miconazole. Conversely, inhibition of the 24-reductase was not required for the expression of 14 alpha-DM activity. Studies on the substrate specificities for the 24-reductase using different 24(25)-enes showed that the most reactive substrate was 5 alpha-cholesta-7,24-dien-3 beta-ol, which exhibited a maximal 18-fold higher kcat than that of lanosterol without the aid of the 14 alpha-DM inhibitor. In addition, both the kinetic behaviour of lanosterol substrate in relation to the 24-reductase and a non-competitive inhibition mode of U18666A (Ki 0. 157 microM) as well as Triparanol (Ki 0.523 microM), two well-known 24-reductase inhibitors, were determined. On the basis of our new findings on the preferred substrate and on the negative effect of 14 alpha-DM on the 24-reductase, we suggest that C-24 reduction of sterols takes place straight after sterol delta 8-->7 isomerization of zymosterol, which occurs several steps after C-32 demethylation of lanosterol in the 19-step pathway of cholesterol biosynthesis from lanosterol.

The map of chromosome 20

The number of gene assignments to human chromosome 20 has increased slowly until recently. Only seven genes and one fragile site were confirmed assignments to chromosome 20 at the Ninth Human Gene Mapping Workshop in September 1987 (HGM9). One fragile site, 13 additional genes, and 10 DNA sequences that identify restriction fragment length polymorphisms (RFLPs), however, were provisionally added to the map at HGM9. Five mutated genes on chromosome 20 have a relation to disease: a mutation in the adenosine deaminase gene results in a deficiency of the enzyme and severe combined immune deficiency; mutations in the gene for the growth hormone releasing factor result in some forms of dwarfism; mutations in the closely linked genes for the hormones arginine vasopressin and oxytocin and their neurophysins are probably responsible for some diabetes insipidus; and mutations in the gene that regulates both alpha-neuraminidase and beta-galactosidase activities determine galactosialidosis. The gene for the prion protein is on chromosome 20; it is related to the infectious agent of kuru, Creutzfeld-Jacob disease, and Gertsmann-Straussler syndrome, although the nature of the relationship is not completely understood. Two genes that code for tyrosine kinases are on the chromosome, SRC1 the proto-oncogene and a gene (HCK) coding for haemopoietic kinase (an src-like kinase), but no direct relation to cancer has been shown for either of these kinases. The significance of non-random loss of chromosome 20 in the malignant diseases non-lymphocytic leukaemia and polycythaemia vera is not understood. Twenty-four additional loci are assigned to the chromosome: five genes that code for binding proteins, one for a light chain of ferritin, genes for three enzymes (inosine triphosphatase, s-adenosylhomocysteine hydrolase, and sterol delta 24-reductase), one for each of a secretory protein and an opiate neuropeptide, a cell surface antigen, two fragile sites, and 10 DNA sequences (one satellite and nine unique) that detect RFLPs.