Paul F. J. Koppens

Mapping of a de novo unequal crossover causing a deletion of the steroid 21-hydroxylase (CYP21A2) gene and a non-functional hybrid tenascin-X (TNXB) gene.

In the human genome, the major histocompatibility complex class III region on chromosome 6p21.3 stands out as an area of remarkably high gene density.1,2 Within this region, a section of particular complexity centres around the C4 genes, which encode the fourth component of complement.3–7 Centromeric to C4 lies the CYP21A2 gene, which encodes steroid 21-hydroxylase, a key enzyme in the biosynthesis of cortisol and aldosterone.4,8,9 The TNXB gene, which encodes the extracellular matrix protein tenascin-X, lies centromeric to CYP21A2 and is transcribed from the opposite strand.10–12 Telomeric to C4 lies the RP1 gene, encoding a putative serine/threonine kinase.13–15 A typical chromosome 6 carries a duplication of an area of approximately 30 kb encompassing the entire C4 and CYP21 genes4,8 plus small truncated sections of RP and TNX .10–14 This tandem repeat has been named the RCCX module after its four constituent genes.7,14,16,17 In most white populations, about 70% of all haplotypes have a bimodular arrangement similar to the one shown in fig 1. The complex genetics of this region, and the activities and clinical significance of the proteins encoded here, have been the subject of several recent reviews.7,18–21 Figure 1 Overview of a typical C4 / CYP21 area within the MHC class III region showing two RCCX modules as found on most chromosomes. TNXB is the full size 68 kb gene for tenascin-X. TNXA (also known as XA ) is a truncated pseudogene of 5.7 kb that not only lacks most of the coding sequence of TNXB but also has a 120 bp deletion (indicated by the small triangle) spanning an exon-intron boundary.10,12,17 CYP21A2 (also known as CYP21B ) is the active steroid 21-hydroxylase gene; CYP21A1P (also known as …

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 …

A rare TaqI polymorphism in a human complement C4 gene is caused by an additional restriction site in the first intron

We studied the configuration of the complement C4/CYP21 (steroid 21-hydroxylase) region of the human major histocompatibility complex in patients suffering from congenital adrenal hyperplasia (CAH) and in the general population in The Netherlands, using C4 and CYP21 probes and the restriction enzymes TaqI and Bg/II. We found a rare TaqI 3.9-kb restriction fragment in the mother of a CAH patient, and present evidence that this polymorphism is caused by an additional restriction site in the first intron of a complement C4 gene.

56 A RECOMBINATION EVENT CAUSING A DE NOVO DELETION OF THE STEROID 21-HYDROXYLASE GENE CYP21

Steroid 21-hydroxylase deficiency is the most prominent cause of congenital adrenal hyperplasia (CAH). Patients suffer from virilization, and in severe cases, from salt loss caused by lack of aldosterone. The CYP21 gene, encoding steroid 21-hydroxylase, and the highly homologous pseudogene CYP21P are located within the human MHC on chromosome 6p21.3, the usual arrangement being centromere - HLA-DP, -DQ, -DR - CYP21-complement C4B-CYP21P-complement C4A - tumour necrosis factor (TNF) genes - HLA-B. -C. -A. We investigated a CAH family (father, mother, patient, three healthy sibs) using 21-hydroxylase and complement C4 probes and allele specific oligonucleotides to detect specific mutations. Oligonucleotide hybridization showed that the patient inherited the Ile172->Asn mutation. characteristic of “simple virilizing” CAH. from the mother. On the other chromosome, the CYP21 gene was deleted: however, this defect was not found in DNA from the father. Paternity was confirmed using VNTR probes. Establishment of HLA-B. TNF and HLA-DQα markers showed that the patient had the HLA-B and TNF genes from one paternal chromosome and the HLA-DQα gene from the other. Apparently, a paternal meiotic recombination event has eliminated the CYP21 gene (as well as the adjacent C4B gene), contributing to steroid 21-hydroxylase deficiency in the patient.