Mutation groups have been established based on the basis of the residual 21- hydroxylase activity of the mutant proteins as observed in vitro (Figure 1c). CYP21A2 deficiency is an autosomal recessive condition. About 65– 75% of the CAH patients are compound heterozygous (e.g. they are affected), but carry different mutations on each chromosome. The clinical phenotype of CAH correlates well with the less severely mutated allele, and consequently with the allele encoding for the higher residual in vitro activity of 21- hydroxylase. This has major implications for genetic counselling in patients with NCCAH. If a patient with NCCAH is compound heterozygous for a mild and a severe mutation, the risk of having a child with classic CAH increases significantly to about 1 in 400 (1/ 50 × 1/ 2 × 1/ 4), assuming a heterozygous rate of 1/ 50 for classic mutations in the general population. The correlation with glucocorticoid and mineralocorticoid deficiency is strong (Figure1c). However, divergence between genotype and phenotype occurs in some cases. Although a trend exists, the correlation between the genotype and degree of 46,XX DSD classified assessed by Prader genital stage is less pronounced (Figure1c). This implies the importance of factors modifying clinical androgen effects. Several potential modifiers have been studied. However, no analysed candidate has sufficiently explained the phenotypic variability in 21OHD. Long- term health outcomes in adults do not necessarily show a good correlation with the genotype. However, girls and women with CAH with more severe genotypes appear to have an increased risk for psychiatric disorders.
Fig1. CYP21A2 gene locus, most common CYP21A2 gene mutations and genotype- phenotype correlation. (a) Organization of the functional CYP21A2 gene and its non- functional CYP21A1P pseudogene in the RCCX module Grey arrows indicate for the approximate position of MLPA probes. (b) Nine out of 10 common mutations are transferred by microconversions from the CYP21A1P gene into CYP21A2. Grey arrows show the five MLPA probes specific for the CYP21A2 gene and the three probes specific for the CYP21A1P pseudogene. (c) 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 virilizing (SV) form, and group C with the non- classic (NC) form. Positive predictive values are calculated from the cited publications. The variability in the degree of virilization of the female external genitalia in the different mutation groups (grading 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 splice refers to the c.293- 13A/ C>G mutation (other names: i2G, I2G, IVS2- 13A/ C>G): Δ8bp refers to the p.Gly110ValfsX21 mutation.
CYP21A2 gene deletions extending into TNXB lead to a contiguous gene syndrome consisting of CAH and Ehlers– Danlos syn drome, which is rarely observed in patients with severe SWCAH. Up to 10% of patients with SWCAH show haploinsufficiency of TNX and heterozygosity for TNXB mutations. This extended phenotype has been termed as CAH- X syndrome. CAH- X is associated with joint hypermobility, chronic arthralgia, joint subluxations, hernias, and cardiac defects. Further investigations in patients with CAH- X should be considered.
Novel insights into the underlying molecular pathology have been gained by the analysis of the CYP21A2 protein crystal structure. Null and A mutations commonly disrupt haeme- and/ or substrate- binding domains, the anchoring of the protein to the membrane, or impair protein stability. Mutations categorized as group B partially impair membrane anchoring or affect conserved hydrophobic clusters within the protein. Milder mutation (group C) results in less severe alterations, often interfering with electron transfer from POR, salt- bridge and hydrogen- bonding networks, and non- conserved hydrophobic clusters.
