Approximately 5% of CAH cases are caused by 11β- hydroxylase (CYP11B1) deficiency (11OHD, OMIM + 202010) leading to impaired cortisol biosynthesis. Accumulation of the mineralocorticoid precursor 11- deoxycorticosterone leads to transactivation of the mineralocorticoid receptor resulting in arterial hypertension. Excess steroid hormone precursors are shunted into androgen synthesis and cause hyperandrogenism. Classic 11OHD occurs 1 in 100 000 to 1 in 200 000 live births. A higher incidence has been reported in Israel (1 in 30 000 to 1 in 40 000 live births), in particular in Israeli Jews of Moroccan origin (1 in 5000 to 1 in 7000 live births). The clinical presentation of classic 11OHD includes glucocorticoid deficiency, severe virilization of external genitalia in 46,XX neonates, precocious pseudopuberty in both sexes, and, in two- thirds of patients, hypertension. Non- classic 11OHD (NC11OHD) is much rarer than NC21OHD, but appears to be more frequent than previously thought (Table 1). Patients with NC11OHD present with signs and symptoms of androgen excess during childhood.
Table1. Differential diagnosis of congenital adrenal hyperplasia— clinical, biochemical, and genetic characteristics
Mutations in the 11β- hydroxylase gene (CYP11B1; GeneID: 1584, GenbankID NC_ 000008.9) are the underlying cause of 11OHD. CYP11B1 is localized on chromosome 8q21, approximately 40 kb apart from the highly homologous aldosterone synthase gene (CYP11B2). The CYP11B1 gene consists of nine exons (Figure 1) and encodes for a protein of 503 amino acids. CYP11B1- inactivating mutations are distributed over the entire coding region consisting of 9 exons. A cluster is reported in exons 2, 6, 7, and 8, but real hot spots do not exist. A broad variety of mutations have been reported to cause either classic or non- classic 11OHD. Most molecular analysis approaches amplify the CYP11B1 gene in 3 fragments avoiding amplification of the highly homologous CYP11B2 gene (Figure1). Since no hotspots exist in the general population, direct DNA sequencing is the most suitable approach for molecular genetic analysis. Large rearrangements, such as large gene deletions and chimeric genes can be detected by Southern Blot analysis or MLPA.
Fig1. Genomic organization of genes causing different forms of congenital adrenal hyperplasia. (1) Genes encoding steroidogenic Cytochrome P450 type II enzymes: (a) The 21- hydroxylase (CYP21A2) gene consists of 10 exons and it is typically amplified in two overlapping fragments. (b) The 17- hydroxylase (CYP17A1) gene consists of 8 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 11- hydroxylase (CYP11B1) gene consists of 9 exons and is usually amplified in three overlapping fragments, although non- overlapping strategies have been described. (d) The P450 side chain cleavage (CYP11A1) gene consists of 9 exons and it is usually amplified in small non- overlapping fragments, although different PCR strategies have been described. (e) The aldosterone synthase (CYP11B2) gene consists of 9 exons normally amplified in either two overlapping fragments or three non- overlapping fragments. (3) Genes encoding hydroxysteroid dehydrogenases: (f) The hydroxysteroid dehydrogenase type 2 (HSD3B2) gene has 4 exons; exon 1 and the 5- prime part of exon 2 are not translated. (4) Gene encoding the electron donor of steroidogenic cytochrome P450 type II: (g) The P450 oxidoreductase (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 steroid acute regulatory protein (StAR) gene consists of 7 exons commonly amplified in five fragments.
