Molecular Genetics of Congenital Adrenal Hyperplasia

Abstract

Congenital adrenal hyperplasia (CAH) is a group of autosomal recessive disorders causing an impairment of cortisol biosynthesis. The phenotypic expression of different CAH forms depends on the underlying enzymatic defect. Steroid 21‐hydroxylase (CYP21A2) and 11β‐hydroxylase (CYP11B1) deficiencies only affect adrenal steroidogenesis, whereas 17α‐hydroxylase (CYP17A1) and 3β‐hydroxysteroid dehydrogenase type 2 (HSD3B2) also impair gonadal steroid biosynthesis. P450 oxidoreductase deficiency (PORD) manifests with apparent combined CYP17A1–CYP21A2 deficiency. In contrast to other CAH forms, PORD also causes skeletal malformations and genital ambiguity in both sexes. Three additional enzymatic defects have been traditionally classified as CAH. Steroidogenic acute regulatory protein (StAR) deficiency results in congenital lipoid adrenal hyperplasia (CLAH), and has the unique feature of adrenal and gonadal lipid accumulation. P450 side‐chain cleavage (CYP11A1) deficiency resembles the CLAH phenotype, but patients have normal‐sized or absent adrenals. Aldosterone synthase (CYP11B2) deficiency manifests with isolated aldosterone deficiency and normal cortisol synthesis.

Key Concepts:

  • Congenital adrenal hyperplasia (CAH) is one of the most common inherited metabolic disorders and comprises a group of autosomal recessive conditions.

  • Molecular genetic analysis of the CYP21A2 gene is challenging due to a high rate of conversions and a multitude of possible complex rearrangements.

  • Common mutations in CYP21A2 account for the majority of cases including gene deletions, chimeric genes, seven single point mutations, an eight base‐pair deletions and a cluster of point mutations.

  • CYP21A2 gene duplications have to be considered in carriers of the p.Gln318X mutation to provide the correct molecular genetic diagnosis.

  • Other forms of CAH are less often caused by common mutations, which are only found in specific populations.

  • The genotype correlates overall well with the adrenal phenotype in all CAH forms, CLAH, CYP11A1 and CYP11B2 deficiencies.

  • Although a trend exists, correlation between genotype and genital development is weaker, and it is particularly poor in CYP11A1 or 3β‐hydroxysteroid dehydrogenase type 2 deficiencies.

  • Mutations in POR and CYP17A1 genes can be associated with clinically isolated 17,20‐lyase deficiency.

  • CYP11A1 deficiency and CLAH due to StAR deficiency can manifest with a phenotype resembling familial glucocorticoid deficiency.

  • Molecular genetic diagnosis is essential to provide the correct diagnosis and allow for appropriate clinical and genetic counselling.

Keywords: CAH; congenital adrenal hyperplasia; CYP21A2; CYP17A1; CYP11B1; POR; StAR; CYP11A1; adrenal insufficiency; disorder of sex development

Figure 1.

Organisation of the RCCX module at chromosome 6p21. Representative Copy Number Variants at the RCCX locus nondisease causative (a) or associated with 21‐hydroxylase deficiency (b). C4A and C4B: complement component C4A and C4B genes; CYP21A1P: steroid 21‐hydroxylase pseudogene; CYP21A2: steroid 21‐hydroxylase gene; TNXB: tenascin‐X gene; TNXA: tenascin‐X pseudogene; RP1: serine/threonine kinase 19 gene (other names: STK19); RP2: serine/threonine kinase 19 pseudogene (other names: STK19P); intron 2 splice: c.293‐13A/C>G, other names: i2G, I2G, IVS2‐13A/C>G.

Figure 2.

Three‐dimensional molecular model of CYP21A2. Localisation of the most common pseudogene‐derived CYP21A2 mutations associated with classic* (a) and nonclassic 21‐hydroxylase deficiency** (b). N‐term, amino terminus; C‐term, carboxyl terminus; E6 cluster refers to the p.Ile236Asn, p.Val237Glu and p.Met239Leu mutation cluster at exon 6; Δ8 bp refers to the p.Gly110ValfsX21 mutation. The I helix is coloured in blue. The X‐ray structure of the mammalian cytochrome CYP2C5 (PDB accession code IDT6) was used as a template for the three‐dimensional modelling of human CYP21A2. The structural representations were generated using Molsoft ICM Browser Pro (Molsoft L.L.C, La Jolla, CA). Notes: *The common intron 2 splice site mutation (c.293‐13A/C<G, other names: i2G, I2G, IVS2‐13A/C<G) is not shown. **The p.Pro453Ser mutation is not derived from the pseudogene but it has been included because it is commonly found in patients with nonclassic 21‐hydroxylase deficiency.

Figure 3.

Localisation of the CYP21A2 gene at the RCCX module, Southern blot analysis, most common CYP21A2 gene mutations and genotype–phenotype correlation. (a) Organisation of the functional CYP21A2 gene and its nonfunctional CYP21A1P pseudogene in the RCCX module. Red arrows indicate the approximate position of MLPA probes. (b) Restriction fragment lengths after digestion of genomic DNA with the restriction enzymes most commonly used for Southern blot, TaqI or BglII. Digested DNA is used for hybridisation with probes for CYP21 genes and C4 genes. (c) Nine out of 10 common mutations are transferred by micro‐conversions from the CYP21A1P gene into CYP21A2. Red arrows show the five MLPA probes specific for the CYP21A2 gene and the three probes specific for the CYP21A1P pseudogene. (d) Genotype–phenotype correlations in CAH due to 21‐hydroxylase deficiency based on in vitro CYP21A2 activity. Mutation groups Null and A are associated with the salt wasting (SW) form of 21‐hydroxylase deficiency (21OHD), group B with the simple virilising (SV) form, and group C with the nonclassic (NC) form. Positive predictive values are calculated from the cited publications. The variability in the degree of virilisation of the female external genitalia in the different mutation groups (classification according to Prader genital stages) is shown in the lower panel. Modal values are provided in brackets where possible. E6 cluster refers to the p.Ile236Asn, p.Val237Glu and p.Met239Leu mutation cluster at exon 6; intron 2 splice refers to the c.293‐13A/C>G mutation (other names: i2G, I2G, IVS2‐13A/C>G): Δ8 bp refers to the p.Gly110ValfsX21 mutation.

Figure 4.

Genomic organisation and PCR amplification strategies for the CYP21A2, CYP17A1, CYP11B1, CYP11A1, CYP11B2, HSD3B2, POR and StAR genes. (1) Genes encoding steroidogenic Cytochrome P450 type II enzymes: (a) The CYP21A2 gene consists of 10 exons and it is typically amplified in two overlapping fragments. (b) The CYP17A1 gene consists of eight exons and different strategies have been employed either amplifying the gene in five or in two fragments. (2) Genes encoding steroidogenic Cytochrome P450 type I enzymes: (c) The CYP11B1 gene consists of nine exons and is usually amplified in three overlapping fragments. However, nonoverlapping strategies have been described. (d) The CYP11A1 gene consists of nine exons and it is usually amplified in small nonoverlapping fragments. (e) The CYP11B2 gene consists of nine exons normally amplified in either two overlapping fragments or three nonoverlapping fragments. (3) Genes encoding hydroxysteroid dehydrogenases: (f) The HSD3B2 gene has four exons; exon 1 and the 5′ part of exon 2 are not translated. (4) Gene encoding the electron donor of steroidogenic Cytochrome P450 type II: (g) The POR gene has 15 translated exons and an untranslated exon (1U). PCR amplification is performed in several small fragments. (5) Gene encoding for a cholesterol transporter: (h) The StAR gene consists of seven exons normally amplified in five fragments.

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Parajes, Silvia, and Krone, Nils(Mar 2012) Molecular Genetics of Congenital Adrenal Hyperplasia. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023590]