How a patient homozygous for a 30-kb deletion of the C4-CYP 21 genomic region can have a nonclassic form of 21-hydroxylase deficiency

A case of nonclassic (NC) 21-hydroxylase deficiency, with a moderately elev ated 17-hydroxyprogesterone level (145 nmol/L in filter paper blood spot), was detected in newborn screening. The newborn's phenotype was female, with no sign of virilization. Confirmatory diagnosis revealed elevated serum le vels of 17-hydroxyprogesterone and of 21-desoxycortisol, whereas cortisol, PRA, and electrolytes were normal. Hydrocortisone substitution was consider ed at the age of 6 months, when virilization became obvious. For clinical r easons, this case had to be classified as late-onset congenital adrenal hyp erplasia (CAH) with unusually early manifestation. However, the diagnosis o f classic 21-hydroxylase deficiency was obtained by Southern blotting studi es, showing that she was homozygous for the 30-kb deletion, including the 3 ' end of CYP21P pseudogene, the C4B gene, and the 5' end of the functional CYP21 gene. Further studies, using PCR and sequencing, were conducted to ex plain the discrepancy between this genotype, usually associated with a clas sic salt-wasting form, and the girl's phenotype. Typically, patients homozy gous for the 30-kb deletion encoding classic CAH possess a unique CYP21P/21 hybrid gene with the junction site located after the third exon, yielding a nonfunctional pseudogene. The girl in question, however, was heterozygous for the 8-bp deletion, suggesting that the chimeric pseudogene on one alle le had a junction site before the third exon. She was compound heterozygous for a 30-kb deletion encoding classic CAH on the paternal allele, and a 30 -kb deletion encoding NC CAH on the maternal allele. This novel maternal CY P21P/21 hybrid gene is characterized by a junction site before intron 2 and differs from the normal CYP21 gene only by the P30L mutation in exon 1 and the promoter region of the CYP21P pseudogene. Because the P30L mutation ha s been described to result in an enzyme with 30-60% activity of the normal P450c21 enzyme, and the CYP21P promoter reduced the transcription to 20% of normal, this puzzling phenotype of a NC CAH with early onset may be fully explained by the genotype of the patient and considered as an intermediate form between the simple virilizing and NC form.