Gene Conversion in Salt-Losing Congenital Adrenal Hyperplasia with Absent Complement C4B Protein*

Abstract

Two of four siblings expressed the salt-losing form of congenital adrenal hyperplasia due to 21-hydroxylase deficiency (CAH) and had identical human lymphocyte antigen (HLA) and complement C4 (fourth component of complement) types (HLA-A3,C4,B35,C4A3,C4BQO,DR1/A2,C-,B18,C4A3, C4BQO,DR6). The father and one unaffected sibling were heterozygous carriers of CAH, as determined by a 30-min iv ACTH stimulation test and HLA typing. In addition, the iv ACTH stimulation test revealed that the mother and the other unaffected sibling also carried an allele for an attenuated form of CAH. Restriction endonuclease digests of genomic DNA obtained from members of this family and from normal unrelated subjects were hybridized with cDNA probes encoding human 21-hydroxylase and C4. With the 21-hydroxylase probe, Southern blots prepared from control DNA samples revealed two major restriction fragments in each of four restriction endonuclease digests; TaqI produced major bands at 3.7 and 3.2 kilobases (kb), KpnI at 4.0 and 2.9 kb, EcoRI at 18 and 13 kb, and BglII at 15 and 12.5 kb. Southern blots prepared from DNA of the two patients lacked the 3.7-kb TaqI and 2.9-kb KpnI fragments, but had increased hybridization intensity (relative to control DNA samples) in the 3.2-kb TaqI and 4.0-kb KpnI fragments. By contrast, blots with EcoRI or BglII had two large hybridization fragments not different from control DNA samples. These data indicate the presence of two different 21-hydroxylase genes. Additional mapping studies revealed that the two genes had the restriction pattern of the inactive 21-hydroxylase gene. When genomic DNA that had been isolated from all members of this family and from normal subjects was hybridized with the human C4 cDNA probe, the restriction fragment hybridization patterns for all four endonuclease digests were similar in the two groups. Hence, our results suggest that the 21-hydroxylase deficiency of our patients is due to conversion of the active 21-hydroxylase gene to the inactive gene. This gene conversion was associated with absence of functional C4B protein, without any detectable alterations in the restriction fragment pattern of the C4 genes.

CYP21 and CYP21P variability in steroid 21-hydroxylase deficiency patients and in the general population in the Netherlands

“…Analysis of TaqI and BglII restriction patterns is commonly used to determine the gross arrangement of the CYP21/C4 region in steroid 21-hydroxylase deficiency patients and their family members. 1,[9][10][11][12][13][14][15] This approach allows the definition of CYP21/C4 haplotypes, some of which are associated with steroid 21-hydroxylase deficiency. [14][15][16][17][18][19][20][21][22] .…”

“…10,12,13 (c) Haplotypes where a gene with a CYP21-like restriction pattern is present (by exclusion).…”

Characterization of a De Novo Conversion in Human Complement C4 Gene Producing a C4B5-Like Protein

“…However, gene conversions and unequal crossover events can also promote homogeneity by transferring sequence motifs between homologous loci. Both mechanisms are active in the MHC class III region (31,32). Homoexpression of two C4B genes on a bimodular RCCX is extremely rare (0.67%) compared with that of C4A-C4A haplotypes (13.3%) in the Caucasian population (33).…”

Clinical and biochemical description of a novel CYP21A2 gene mutation 962_963insA using a new 3D model for the P450c21 protein

“…Activity of the Q318X protein has been reported to be completely lost (3). As Q318X is present in the CYP21A1 pseudogene, this mutation in the CYP21A2 gene is thought to be formed through gene conversion during meiosis (22,23). Thus, Q318X is a common mutation identified in patients with 21OHase deficiency from all over the world (24).…”