Daphne L. Davis

Molecular genetics of steroid 5 alpha-reductase 2 deficiency.

Two isozymes of steroid 5 alpha-reductase encoded by separate loci catalyze the conversion of testosterone to dihydrotestosterone. Inherited defects in the type 2 isozyme lead to male pseudohermaphroditism in which affected males have a normal internal urogenital tract but external genitalia resembling those of a female. The 5 alpha-reductase type 2 gene (gene symbol SRD5A2) was cloned and shown to contain five exons and four introns. The gene was localized to chromosome 2 band p23 by somatic cell hybrid mapping and chromosomal in situ hybridization. Molecular analysis of the SRD5A2 gene resulted in the identification of 18 mutations in 11 homozygotes, 6 compound heterozygotes, and 4 inferred compound heterozygotes from 23 families with 5 alpha-reductase deficiency. 6 apparent recurrent mutations were detected in 19 different ethnic backgrounds. In two patients, the catalytic efficiency of the mutant enzymes correlated with the severity of the disease. The high proportion of compound heterozygotes suggests that the carrier frequency of mutations in the 5 alpha-reductase type 2 gene may be higher than previously thought.

Identification of a new inborn error in bile acid synthesis: mutation of the oxysterol 7alpha-hydroxylase gene causes severe neonatal liver disease.

We describe a metabolic defect in bile acid synthesis involving a deficiency in 7alpha-hydroxylation due to a mutation in the gene for the microsomal oxysterol 7alpha-hydroxylase enzyme, active in the acidic pathway for bile acid synthesis. The defect, identified in a 10-wk-old boy presenting with severe cholestasis, cirrhosis, and liver synthetic failure, was established by fast atom bombardment ionization-mass spectrometry, which revealed elevated urinary bile acid excretion, a mass spectrum with intense ions at m/z 453 and m/z 510 corresponding to sulfate and glycosulfate conjugates of unsaturated monohydroxy-cholenoic acids, and an absence of primary bile acids. Gas chromatography-mass spectrometric analysis confirmed the major products of hepatic synthesis to be 3beta-hydroxy-5-cholenoic and 3beta-hydroxy-5-cholestenoic acids, which accounted for 96% of the total serum bile acids. Levels of 27-hydroxycholesterol were > 4,500 times normal. The biochemical findings were consistent with a deficiency in 7alpha-hydroxylation, leading to the accumulation of hepatotoxic unsaturated monohydroxy bile acids. Hepatic microsomal oxysterol 7alpha-hydroxylase activity was undetectable in the patient. Gene analysis revealed a cytosine to thymidine transition mutation in exon 5 that converts an arginine codon at position 388 to a stop codon. The truncated protein was inactive when expressed in 293 cells. These findings indicate the quantitative importance of the acidic pathway in early life in humans and define a further inborn error in bile acid synthesis as a metabolic cause of severe cholestatic liver disease.

Expression and Regulation of Steroid 5α-Reductase 2 in Prostate Disease

AbstractThe androgen dihydrotestosterone is synthesized by the enzyme steroid 5α-reductase, and it is required for growth and development of the prostate. We used immunohistochemistry to examine the expression of the type 2 isozyme of 5α-reductase in benign prostatic hyperplasia and prostate cancer. The type 2 isozyme is highly expressed within stromal cells in both disease states. No type 2 isozyme is detectable in a lymph node metastasis. Immunoblotting studies show that androgen ablation therapies substantially decrease isozyme expression in the epididymis but have a lesser effect on expression in the prostate. Finasteride therapy (2 weeks to 3 years) did not abolish expression of the prostatic type 2 isozyme nor did this drug treatment induce expression of the type 1 isozyme.

The bile acid synthetic gene 3β-hydroxy-Δ5-C27-steroid oxidoreductase is mutated in progressive intrahepatic cholestasis

We used expression cloning to isolate cDNAs encoding a microsomal 3β-hydroxy-Δ5-C27-steroid oxidoreductase (C27 3β-HSD) that is expressed predominantly in the liver. The predicted product shares 34% sequence identity with the C19 and C21 3β-HSD enzymes, which participate in steroid hormone metabolism. When transfected into cultured cells, the cloned C27 3β-HSD cDNA encodes an enzyme that is active against four 7α-hydroxylated sterols, indicating that a single C27 3β-HSD enzyme can participate in all known pathways of bile acid synthesis. The expressed enzyme did not metabolize several different C19/21 steroids as substrates. The levels of hepatic C27 3β-HSD mRNA in the mouse are not sexually dimorphic and do not change in response to dietary cholesterol or to changes in bile acid pool size. The corresponding human gene on chromosome 16p11.2-12 contains six exons and spans 3 kb of DNA, and we identified a 2-bp deletion in the C27 3β-HSD gene of a patient with neonatal progressive intrahepatic cholestasis. This mutation eliminates the activity of the enzyme in transfected cells. These findings establish the central role of C27 3β-HSD in the biosynthesis of bile acids and provide molecular tools for the diagnosis of a third type of neonatal progressive intrahepatic cholestasis associated with impaired bile acid synthesis.

The Molecular Genetics of Steroid 5α-Reductases

Publisher Summary This chapter discusses the molecular genetics of steroid 5α-reductases. 5α-reductase plays a central role in androgen action by catalyzing the conversion of testosterone into the more potent hormone dihydrotestosterone. Like other steroid and thyroid hormones, testosterone and dihydrotestosterone activate responsive genes by binding to the androgen receptor—a member of the steroid hormone receptor family of transcriptional activator proteins. Despite this mechanistic similarity, androgen action differs from other steroid hormones in that two different steroids interact with the same receptor to bring about different physiological effects. Testosterone bound to the androgen receptor is responsible for the regulation of gonadotropin production, spermatogenesis, and the formation of the internal male genitalia—epididymis, seminal vesicles, and vas deferens—from wolffian duct anlagen during phenotypic sexual differentiation in the male embryo. In contrast, dihydrotestosterone bound to the same androgen receptor brings about the formation of the male external genitalia—penis and scrotum—and prostate from urogenital sinus primordium in the male embryo and is required for sexual maturation at puberty.

Steroid 5α-reductase 2 deficiency: virilization in early infancy may be due to partial function of mutant enzyme

Male pseudohermaphroditism due to steroid 5α‐reductase deficiency is the consequence of mutations in the gene encoding the type 2 isoenzyme. Most (60%) affected subjects have homozygous mutations, and the remainder are compound heterozygotes or presumed compound heterozygotes. We report an Italian subject with phenotypic and endocrine features of 5α‐reductase 2 deficiency who is homozygous for a substitution mutation (H231R). Although close consanguinity is not present, genealogical data demonstrated that the parents are distantly related, and both parents and the maternal grandmother are heterozygous carriers of the mutation. The fact that this particular mutation results in the formation of an enzyme with considerable residual activity may explain in part the significant degree of virilization that took place in this subject in early infancy. This same mutation (H231R) is present in heterozygous form in two other families, an African‐American family and an American family of northern European descent.