A NOVEL MISSENSE MUTATION IN THE HSD3B2 GENE, UNDERLYING NONSALT-WASTING CONGENITAL ADRENAL HYPERPLASIA. NEW INSIGHT INTO THE STRUCTURE-FUNCTION RELATIONSHIPS OF 3β-HYDROXYSTEROID DEHIDROGENASE TYPE II.

[congenital adrenal hyperplasia]

Context : 3 βHSD 2 is a bifunctional microsomal NAD +-dependent enzyme crucial for adrenal and gonad steroid biosynthesis , converting Δ 5 -steroids to Δ 4 -steroids . 3 βHSD 2 deficiency is a rare cause of congenital adrenal hyperplasia caused by recessive loss -of-function HSD 3 B 2 mutations . Objective : The aim was to define the pathogenic consequences of a novel missense mutation in HSD 3 B 2 gene . Patient : We report a 7 -month -old 46 ,XX girl referred because of precocious pubarche and postnatal clitoromegaly . Hormonal profile showed inadequate glucocorticoid levels , increased 17 OHP and renin levels and very high DHEAS levels , suggestive of compensated nonsalt-losing 3 βHSD 2 deficiency . Design and results : Direct sequencing revealed a novel , homozygous , pG 250 V HSD 3 B 2 mutation . In vitro analysis in intact COS -7 cells , showed impaired enzymatic activity for the conversion of pregnenolone to progesterone and dehydroepiandrosterone to androstenedione (20 % and 27 % of WT at 6 h , respectively ). G 250 V-3 βHSD 2 decreased the Vmax for progesterone synthesis without affecting the Km for pregnenolone . Western blot and immunofluorescence suggested that p .G 250 V mutation has no effect on the expression and intracellular localization of the mutant protein . Molecular homology modeling predicted that mutant V 250 affect a L 239 -Q 251 loop next to a β-sheet structure in the NAD +-binding domain . Conclusions : We identified a novel p .G 250 V mutation of HSD 3 B 2 which causes an incomplete loss of enzymatic activity , explaining the compensated non-salt loss phenotype . In vitro and in silico experiments provided insight into the structure-function relationship of the 3 βHSD 2 protein suggesting the importance of L 239 -Q 251 loop for the catalytic activity of otherwise stable 3 βHSD 2 enzyme .