Genetics of congenital adrenal hyperplasia

Abstract

Congenital adrenal hyperplasia (CAH) is one of the most common inherited metabolic disorders. It comprises a group of autosomal recessive disorders caused by the deficiency of one of four steroidogenic enzymes involved in cortisol biosynthesis or in the electron donor enzyme P450 oxidoreductase (POR) that serves as electron donor to steroidogenic cytochrome P450 (CYP) type II enzymes. The biochemical and clinical phenotype depends on the specific enzymatic defect and the impairment of specific enzyme activity. Defects of steroid 21-hydroxylase (CYP21A2) and 11beta-hydroxylase (CYP11B1) only affect adrenal steroidogenesis, whereas 17alpha-hydroxylase (CYP17A1) and 3beta-hydroxysteroid dehydrogenase type 2 (HSD3B2) deficiency also impact on gonadal steroid biosynthesis. Inactivating POR gene mutations are the cause of CAH manifesting with apparent combined CYP17A1-CYP21A2 deficiency. P450 oxidoreductase deficiency (ORD) has a complex phenotype including two unique features not observed in any other CAH variant: skeletal malformations and severe genital ambiguity in both sexes.

Figure 1

The CYP21A2 gene and its duplicated pseudogene (CYP21A1P) are located 30kb apart in the human leukocyte antigen (HLA) class III region in the major histocompatibility (MHC) locus on the short arm of chromosome 6 (band 6p21.3), and share approximately 98% sequence homology. Both genes are arranged in tandem repeat with the C4 (C4A and C4B) genes. C4/CYP21A is flanked by telomeric RP (RP1 and RP2) and centromeric tenascin (TNXA and TNXB) genes. RP1 encodes a serine/threonine nuclear protein kinase, C4 encodes for the immune effector protein complement component, and TNX encodes an extracellular matrix protein. With the exception of C4, each of the other functional genes (RP1, CYP21A2 and TNXB) has a corresponding highly homologous pseudogene (RP2, CYP21A1P and TNXA). The RP, TNX, C4 and CYP21 genes together compose the RCCX bimodule (RP1-C4-CYP21A1P-TNXA-RP2-C4-CYP21A2-TNXB). Enlarged area represents the 10 exons of CYP21A2. Active genes are solid colors. Pseudogenes are grey, outlined with the color of the corresponding active gene.

Figure 2

The high degree of sequence homology between CYP21A2 and its pseudogene CYP21A1P allows for recombination events. Unequal crossing-over through intergenic recombination results in large deletions, and the transfer of deleterious pseudogene sequence to the active gene. The majority of mutations found in CAH due to 21-OHD are pseudogene derived or large 30kb deletions resulting in chimeric genes. The most common pseudogene-derived mutations found in the CYP21A2 are shown: p.P30L; IVS2-13A/C>G; 8bp deletion; p.I172N; exon 6 cluster (p.I236N, p.V237E, p.M239K); p.V281L; p.Leu307fs; p.Q318X; p.R356W; p.P453SW.

Figure 3

Junction sites of CYP21A1P/CYP21A2 chimeras. Misalignment during meiosis can result in a 30kb gene deletion, which produces a chimeric CYP21A1P/CYP21A2 gene. Nine different CYP21A1P/CYP21A2 chimeras have been reported with unique junction sites. These chimeras are numbered chronologically, CH-1 through CH-9, based on when the junction sites were reported. The location of the junction site is clinically meaningful. Seven chimeras are nonfunctional and are associated with a classic salt-wasting phenotype. Two chimeras, CH-4 and CH-9, carry the p.P30L mutation and along with a weak CYP21A1P promoter, only partially impair the gene. These chimera are referred to as attenuated and are associated with a milder phenotype.

Figure 4

Formation of CYP21A1P/CYP21A2 and TNXA/TNXB chimeras. Due to the high degree of sequence homology between CYP21A2 and CYP21A1P, recombination events occur. The deletions associated with CAH-X syndrome, a condition characterized by having a combination of CAH and a connective tissue dysplasia, are TNXA/TNXB chimeras. The formation of a TNXA/TNXB chimera results in deletion of the CYP21A2 gene.

Figure 5

Genetic risk in the offspring of a patient with CAH. A) If the partner of a patient with classic CAH is unaffected, then all of the children will be carriers of classic CAH. B) If the partner of a patient with classic CAH is a carrier of a classic mutation, then there is a 50% chance that a pregnancy will result in a child affected with classic CAH. Phenotype is determined by the most functional allele. C) If the patient has nonclassic CAH, but carries a classic gene, and the partner is a carrier of a classic mutation, then there is a 25% chance of having a child with classic CAH.