Congenital Adrenal Hyperplasia (CAH) due to 11-β-Hydroxylase Deficiency (11-OHD) via the CYP11B1 Gene

Congenital adrenal hyperplasia (CAH) is a group of autosomal recessive disorders each due to defects in a single gene involved in different steps of cortisol biosynthesis (Nimkarn et al. 2008. PubMed ID: 18294861; Hannah-Shmouni et al. 2017. PubMed ID: 28476231). CAH represents a continuous phenotypic spectrum depending on causative gene, genotype, residual activity and age of presentation. Over 95% of CAH cases are caused by 21- hydroxylase deficiency (21-OHD). The second most common form of CAH is due to 11-hydroxylase deficiency (11-OHD) which accounts for approximately 5% of cases with an incidence of 1 in 100,000 live births.

The classic form of 11-OHD consists of two groups: (1) virilization of the external genitalia in 46,XX newborn females; and (2) precocious pseudopuberty in both sexes. The non-classic form is rare and presents with hyperandrogenism during childhood.

CAH due to 11-OHD is an autosomal recessive disorder caused by defects in the CYP11B1 gene (White et al. 1991. PubMed ID: 2022736). Documented pathogenic variants throughout the entire CYP11B1 gene include truncating changes (nonsense, splice variants and frame-shifting small deletion/insertions), missense substitutions and intragenic large deletions (Human Gene Mutation Database). Pathogenic variants clustered in exons 2, 6, 7 and 8 are evident. In addition, due to high sequence similarity to the CYP11B2 gene, which encodes aldosterone synthase, rare chimeric CYP11B2/CYP11B1 genes can also result in CAH due to 11-OHD (Hampf et al. 2001. PubMed ID: 11549691; Duan et al. 2018. PubMed ID: 29703198; Portrat et al. 2001. PubMed ID: 11443188).

The 11-hydroxylase (CYP11B1) enzyme is a P450 type I mitochondrial enzyme responsible for the conversion of 11-deoxycortisol to cortisol, and 11-deoxycorticosterone (DOC) to corticosterone.

In a study of 28 patients from 24 Turkish families affected by 11-hydroxylase deficiency (26 classic and 2 late-onset), using the strategy of specifically amplifying CYP11B1 gene fragments, all patients received a molecular diagnosis (Kandemir et al. 2017. PubMed ID: 26956189). Similar clinical sensitivity was observed by Bas et al. (2018. PubMed ID: 29626607). These studies indicated intragenic large deletions and chimeric CYP11B2/CYP11B1 genes are rare. Therefore, our test is expected to be able to detect the vast majority of pathogenic variants.

This test involves bidirectional Sanger sequencing of all coding exons and splice sites of the CYP11B1 gene. The full coding sequence of each exon plus ~10 bp of flanking DNA on either side are sequenced. We will also sequence any single exon (Test #100) or pair of exons (Test #200) in family members of patients with known mutations or to confirm research results.

By using specifically amplifying the CYP11B1 gene (to avoid the paralogous gene CYP11B2), our strategy is NOT designed to detect rare chimeric CYP11B2/CYP11B1 genes. However, our assay is expected to capture the majority of small gene-conversion events.