Structure-function relationships of 3 beta-hydroxysteroid dehydrogenase: contribution made by the molecular genetics...

The transformation of delta 5-3 beta-hydroxysteroids into the corresponding delta 4-3-keto-steroids is an essential step for the biosynthesis of all classes of active steroids: progesterone, mineralocorticoids, glucocorticoids, androgens, and estrogens. These steroid hormones play a crucial role in the differentiation, development, growth, and physiological function of most human tissues. The structures of several cDNAs encoding 3 beta-HSD isoenzymes have been characterized in human and several other vertebrate species: human types I and II; macaque; bovine; rat types I, II, III, and IV; mouse types I, II, III, IV, V and VI; hamster types I, II, and III; and rainbow trout. Their transient expression reveals that 3 beta-HSD and delta 5-delta 4-isomerase activities reside within a single protein. Distinct approaches have been used for a better understanding of the structure-function relationships of these 3 beta-HSD enzymes: i) affinity radiolabeling studies of the human type I 3 beta-HSD; ii) identification and the functional consequences of the human type-II 3 beta-HSD mutations detected in patients with 3 beta-HSD deficiency. Taken together, all of these data were examined to determine whether the relationship between the genotype and the phenotype of these patients were consistent with in vitro mutagenesis studies. 3 beta-HSD deficiency, transmitted in an autosomic recessive disorder, is characterized by varying degrees of salt wasting; in genetic males, fetal testicular 3 beta-HSD deficiency causes an undervirilized male genitalia (male pseudohermaphroditism); females exhibit either normal sexual differentiation or mild virilization. All mutations were detected in the type II 3 beta-HSD gene, which is expressed almost exclusively in the adrenals and gonads. No mutation was detected in the type I 3 beta-HSD gene, which is expressed in peripheral tissues. The finding of a normal type I 3 beta-HSD gene explains the elevated delta 5-steroids and mild virilization of affected girls at birth. To date, 24 mutations have been identified in 25 distinct families with 3 beta-HSD deficiencies. All nonsense and frameshift mutations introducing a premature termination codon were associated with the classical salt-losing form. The locations of these nonsense mutations suggest that at least the first 318 amino acids out of 371 are required for 3 beta-HSD activity. The consequences of the missense mutations on some domains of the 3 beta-enzyme, such as membrane-spanning domains, cofactor-binding site, and steroid-binding site, were reviewed. The future crystallization of the overexpressed normal and mutant-type II-3 beta-HSD enzymes should contribute to a better understanding of the structure-function relationships of this enzyme, especially for missense mutations located outside the putative functional regions.