A rare duplicated 21-hydroxylase haplotype and a de novo mutation: a family analysis

21-Hydroxylase (21-OH) genotyping was performed in clinically unaffected family members of a congenital adrenal hyperplasia (CAH) index patient (Prader stage 3), who is a compound heterozygous carrier of the I172N (exon 4) and the intron2 splicing mutations. Whereas the latter mutation could be traced to the father, the exon 4 aberration represents a de novo mutation (accounting for 1% of CAH alleles) harbored on an unaffected allele, which was inherited from the mother. Although clinically and biochemically unaffected, the patient's brother was found to be compound heterozygous for intron2splice (paternal allele) and Q318X in exon 8 (maternal allele). As shown by PCR-based sequence and Southern blot analysis, the maternal haplotype, inherited by the brother, has a duplicated CYP21B (functional) gene, one of which carries a Q318X mutation. This duplicated Q318X-affected haplotype is the first of its kind among 800 alleles screened for 21-OH deficiency in our laboratory and has to date been reported only in three Swedish CAH patients, all of them bearing an intron2splice and a Q318X mutation. This family analysis highlights the complexity of the CYP21/CYP21P(pseudogene) loci and the difficulties of 21-OH genotyping.