Mutations in the 3 beta-hydroxysterol Delta(24)-reductase gene cause desmosterolosis, an autosomal recessive disorder of cholesterol biosynthesis

Desmosterolosis is a rare autosomal recessive disorder characterized by mul tiple congenital anomalies. Patients with desmosterolosis have elevated lev els of the cholesterol precursor desmosterol, in plasma, tissue, and cultur ed cells; this abnormality suggests a deficiency of the enzyme 3 beta -hydr oxysterol Delta (24)-reductase (DHCR24), which, in cholesterol biosynthesis , catalyzes the reduction of the Delta (24) double bond of sterol intermedi ates. We identified the human DHCR24 cDNA, by the similarity between the en coded protein and a recently characterized plant enzyme-DWF1/DIM, from Arab idopsis thaliana-catalyzing a different but partially similar reaction in s teroid/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-termin al secretory-signal sequence as well as at least one putative transmembrane helix, and is a member of a recently defined family of flavin adenine dinu cleotide (FAD)-dependent oxidoreductases. Conversion of desmosterol to chol esterol by DHCR24 in vitro is strictly dependent on reduced nicotinamide ad enine dinucleotide phosphate and is increased twofold by the addition of FA D to the assay. The corresponding gene, DHCR24, was identified by database searching, spans similar to 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 mutatio ns, which were shown, by functional expression, in yeast, of the patient al leles, to be disease causing. Our data demonstrate that desmosterolosis is a cholesterol-biosynthesis disorder caused by mutations in DHCR24.