Naturally-Occurring Mutations in Steroidogenic Factor-1 (Sf-1) Provide Insight into Dose-Dependent Regulation of Gene Transcription by Monomeric Orphan Nuclear Receptors

Abstract

Steroidogenic factor-I (SF-I, NR5A1) is an orphan nuclear receptor that regulates the transcription of multiple target genes involved in steroidogenesis, reproduction and male sexual differentiation. Unlike most nuclear receptors, SF-I binds to the response elements of these genes as a monomer and recognizes variations on the extended half site, PyCA AGGTCA. Recently, we described a heterozygous G35E mutation in the P-box of SF-I in a patient with severe primary adrenal failure, complete XY sex-reversal and persistent Miillerian structures (Nat Genet 1999;22: 125-6). The P-box amino-acid sequence is critical for recognizing the core half-site element of target genes (AGGTCA) and for determining DNA-binding specificity. Consequently, the G35E mutation impairs binding to SFI responsive promoters, particularly when “imperfect” binding sites are present, and a heterozygous mutation is sufficient to cause a severe phenotype in this patient. We have now identified a homozygous R92Q mutation in the A-box of SF-I in a baby with a similar phenotype and consanguineous parents. This A-box arginine residue is highly conserved among nuclear receptors that bind to DNA as monomers (e.g., SF-I, LRH, ERR, NGFIB) and is thought to intcract primarily with the 5’ flanking sequence within the minor groove of DNA (PyCA) to stabilize the receptor-DNA complex. Functional studies reveal that the R92Q mutant has partial loss of binding to and transactivation of several SF-I target genes when compared to the G35E P-box change. Further, heterozygous carriers of the R92Q mutation have a normal phenotype and adrenal function. Thus, a homozygous R92Q A-box mutation is necessary for full phenotypic penetrance and autosomal dominant inheritance is seen. Taken together, these two naturally-occurring SF-I mutations reveal the relative importance of the P-box and A-box regions for monomeric binding by nuclear receptors, demonstrating that functional gene dosage effects are important when one factor controls the transcription of many different target genes in humans.