21-hydroxylase deficiency

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency
Deficient 21-Hydroxylase can lead to accumulation of 17α-Hydroxyprogesterone
Classification and external resources
Specialty	endocrinology
ICD-10	E25.0
ICD-9-CM	255.2
OMIM	201910
DiseasesDB	1854
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Congenital adrenal hyperplasia due to 21-hydroxylase deficiency (21-OH CAH), in all its forms, accounts for over 95% of diagnosed cases of congenital adrenal hyperplasia, and "CAH" in most contexts refers to 21-hydroxylase deficiency. An overview of the other types of CAH is presented in the main article.

The condition can be classified into "salt-wasting", "simple virilizing", and "non-classical" forms.

The salt-wasting and simple virilizing types are sometimes grouped together as "classical".

The 21A2 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 17α-hydroxyprogesterone elevation after ACTH stimulation, or more recently by direct gene sequencing.

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Wikipedia