Genetic Prevalence
The CYP21A2 gene for the P450c21 enzyme (also known as 21-hydroxylase) is at 6p21.3, amid genes HLA B and HLA DR coding for the major human histocompatibility loci (HLA). CYP21A2 is paired with a nonfunctional pseudogene CYP21A1P. Scores of abnormal alleles of CYP21A2 have been documented, most arising from recombinations of homologous regions of CYP21A2 and CYP21A1P. Differences in residual enzyme activity of the various alleles account for the various degrees of severity of the disease. Inheritance of all forms of 21-hydroxylase CAH is autosomal recessive.
Persons affected by any forms of the disease have two abnormal alleles, and both parents are usually heterozygotes (or carriers). When both parents carry an abnormal allele, each child has a 25% chance of having the disease, a 50% chance of being a carrier like the parents, and a 25% chance of having two normal genes.
It is now possible to test for heterozygosity by measuring 17-hydroxyprogesterone elevation after ACTH stimulation, or more recently by direct gene sequencing.
The incidence of 21-hydroxylase deficient CAH detectable in childhood is about 1 in 15,000 births. The severe salt-wasting form accounts for the majority of these cases, which is high enough that many states and countries routinely include it in mandated newborn screening tests. The incidence of simple virilizing CAH is about 1 in 60,000 children.
Read more about this topic: Congenital Adrenal Hyperplasia Due To 21-hydroxylase Deficiency
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