Inherited congenital adrenal hyperplasia in the rabbit: Absent cholesterol side-chain cleavage cytochrome P450 gene expression

We investigated adrenal steroidogenic enzymes, their activity and mRNA expression, and in vitro biosynthesis of an enzyme in rabbits with congenital adrenal hyperplasia (CAH; weight: CAH, 19 ± 5 mg/ adrenal; normal, 2.7 ± 1.0 mg/adrenal). Serum pregnenolone (Δ5-P) levels in CAH newborn rabbits (12-36 h) were normal (mean/range, 438/51-2191 ng/dl), but corticosterone levels were low [0.05 ± 0.05 μg/ dl; P < 0.001 vs. normal (0.66 ± 0.57)]. Serum Na⁺ levels in CAH newborn rabbits were in the normal range (143 ± 30 meq/liter), but K⁺ levels were elevated [7 ± 1.1 meq/liter; P < 0.05 vs. normal (5.9 ± 0.6 meq/liter)]. Minced normal adrenal tissue incubated with [³H] cholesterol (30-100 pmol/flask) and ACTH (100 mU/flask) produced [³H]Δ5-P (newborn, 21 and 45 fmol/100 mg; adult, 3 and 5 fmol/100 mg) and [³H]corticosterone (newborn, 23 fmol/100 mg; adult, 11.3 fmol/100 mg), but CAH adrenals produced no product (<1.3 fmol/100 mg). Adrenal mitochondria from normal newborn rabbits produced Δ5-P (4.4-7 nmol/mg protein), but CAH adrenals did not, while CAH adrenal mitochondria demonstrated over 4 times greater 11β-hydroxylase activity. A Western blot of adrenal homogenate from normal newborn rabbits revealed a cholesterol side-chain cleavage cytochrome P450 (P450scc)-immunoreactive species (mol wt, 53 × 10³), but this species was absent in CAH adrenals; CAH adrenals had a normal adrenodoxin and intensified 17α-hydroxylase cytochrome P450 (P450_17α) band compared to normal adrenals. In vitro translation of RNA in a cell-free rabbit reticulocyte lysate system containing [³⁵S] methionine yielded a precursor P450scc protein (mol wt, 58.5 × 10³) with normal adrenal RNA, but not with CAH adrenal RNA. P450scc mRNA was detected in all normal adrenals, but was not detected in all CAH adrenals. 21-Hydroxylase cytochrome P450 mRNA expression was detected at a similar level in both normal and CAH adrenals. We conclude that CAH in the rabbit is caused by inherited absent P450scc gene expression. The clinical, pathological, and biochemical manifestations of P450scc deficiency in the rabbit are nearly identical to the human disorder. Increased 11β-hydroxylase activity and increased P450_17α on Western blot of CAH adrenals indicate altered gene expression of other steroidogenic enzymes due to CAH. Further molecular analysis of the P450scc gene in this animal CAH model will facilitate understanding of P450scc deficiency CAH.