PCR-based Detection of CYP21 Deletions

We read with interest the Technical Brief by Lee et al. (1), in which the authors describe a novel method to detect C4-CYP21 deletions in patients with steroid 21-hydroxylase deficiency. Such deletions result from an unequal crossover in the RCCX module ( RP-C4-CYP21-TNX ) on chromosome 6. In most cases, chromosome 6 carries two RCCX modules, one with a CYP21P ( CYP21A1P ) pseudogene and a truncated XA pseudogene, and one with a functional CYP21 ( CYP21A2 ) gene (encoding steroid 21-hydroxylase) and a functional TNXB gene (encoding tenascin-X). Meiotic misalignment and recombination may occur at several locations and create a chromosome with a single chimeric RCCX module. The PCR described by Lee et al. uses one primer in the 5′ flanking sequence of CYP21 and CYP21P (2), whereas the other primer is positioned in a 120-bp sequence of TNXB that is not present in the XA pseudogene (3). Although this PCR is indeed suitable for the detection of chimeric CYP21P / CYP21 genes, it would fail to detect any RCCX chimera in which the pseudogene-like region includes the 120-bp deletion of XA (4), as …