Steroid 21-hydroxylase deficiency (congenital adrenal hyperplasia).

Congenital adrenal hyperplasia, caused by any one of a number of inborn errors of steroidogenesis in which cortisol is not sufficiently produced by the adrenal cortex, is in most cases due to a deficiency of the enzyme steroid 21-hydroxylase. Classic 21-hydroxylase deficiency occurs in about 1 in 14,000 live births. In classic 21-hydroxylase deficiency, the most common cause of genital ambiguity in females, prenatal exposure to excess androgens results in virilization of the female fetus. Newborn males have normal genitalia. Postnatally, untreated females as well as males present with signs of androgen excess. Three fourths of classic 21-hydroxylase deficiency cases do not effectively synthesize aldosterone and are salt-wasting, a condition that is potentially fatal. An allelic variant of classic 21-hydroxylase deficiency, termed nonclassic 21-hydroxylase deficiency, is associated with a milder enzymatic defect and no genital ambiguity at birth, but postnatal virilization may be seen. The 21-hydroxylase enzyme, a cytochrome P450 hemeprotein (cytochrome P450c21), is encoded by the gene CYP21, which has a closely neighboring homologous pseudogene, CYP21P. Mutations in the CYP21 gene, causing 21-hydroxylase deficiency, are common and occur owing to two mechanisms: gene deletion and apparent gene conversion. Prenatal diagnosis is important to identify a fetus affected with 21-hydroxylase deficiency. Genital ambiguity in affected females can be prevented by proper administration of dexamethasone to the pregnant mother. Postnatally, the treatment of 21-hydroxylase deficiency is lifelong hormonal replacement. With carefully supervised medical treatment, congenital adrenal hyperplasia patients have the capacity for normal puberty and fertility.