Linkage between I172N mutation, a marker of 21-hydroxylase deficiency, and a single nucleotide polymorphism in Int6 of CYP21B gene: A genetic study of Sardinian family

Congenital adrenal hyperplasia (CAH) is a genetic disorder due to 21-hydroxylase deficiency. More than 90% of CAH cases are caused by mutations of CYP21B gene, most of which are the result of microconversion events between the functional gene and its pseudogene. Using a combination of RFLP and direct sequencing analysis, in this paper we describe the genetic study of a Sardinian family carrying I172N mutation in linkage with a SNP namely 1636 Int6. The characterization of new polymorphisms in CYP21B can improve the analysis of segregation of CYP21B mutated alleles especially in genetic familial studies. The analysis of linkage between specific mutations and this SNPs, especially if focused on genetic familial studies, may improve the quality of genetic analysis of 21-hydroxylase deficiency.

Multiplex Minisequencing of the 21-Hydroxylase Gene as a Rapid Strategy to Confirm Congenital Adrenal Hyperplasia

Background: Congenital adrenal hyperplasia (CAH) is a frequent autosomal recessive disease, with a wide range of clinical manifestations, most commonly attributable to mutations in the 21-hydroxylase gene (CYP21). Large gene deletions, large gene conversions, a small 8-basepair deletion, and eight point mutations in CYP21 account for ∼95% of all enzyme deficiencies. We developed a new strategy for a rapid CYP21 analysis.

Methods: DNA samples from 40 CAH patients previously genotyped by direct DNA sequencing were reanalyzed by allele-specific amplification of the functional CYP21 gene followed by a multiplex minisequencing reaction using 13 primers. In addition, a second PCR that amplified a part of exon 3 was used to demonstrate the presence or absence of at least one functional gene.

Results: The assay detected the P453S mutation and nine of the most common mutations (P30L, intron 2 splice, Δ8bp, I172N, exon 6 cluster, V281L, F306+t, Q318X, and R356W) caused by microconversions from the CYP21P pseudogene. The concordance was 100% for detecting these mutations, including gene deletions and large gene conversions. The 40 patient DNA samples were analyzed in 1.5 working days by one technician (actual hands-on time, 3.5 h). The material cost for analyzing one sample was approximately €10.00 (US $9.00).

Conclusions: This novel mutation screening strategy rapidly detects 90–95% of all mutations associated with CAH and appears applicable as a tool for confirmation of increased 17-hydroxyprogesterone found in neonatal CAH screening.

Classical and nonclassical 21-hydroxylase deficiency: a molecular study of Argentine patients

OBJECTIVE

To characterize the molecular basis of the 21-hydroxylase deficiency in a group of Argentine patients presenting the classical and nonclassical forms of the disease.

DESIGN

To analyse the frequency of point mutations in the CYP21 gene by DNA amplification and mutation detection.

PATIENTS

Forty-one patients from 36 nonrelated families: 25 nonclassical (NC), 11 salt-wasting (SW) and five simple virilizing (SV). A total of 27 parents and 13 nonaffected siblings were also analysed.

MEASUREMENTS

Basal steroid hormones and 17-hydroxyprogesterone levels following adrenal stimulation with adrenocorticotrophic hormone were measured, together with an analysis of 10 point mutations in the CYP21 gene.

RESULTS

A total of 83% and 74.4% classical and nonclassical chromosomes, respectively, were characterized. The intron 2 mutation was the most prevalent among classical alleles. In addition, a high frequency for R356W was observed in both groups (13.3 and 6.9%, respectively), while V281L was the most frequent mutation among the nonclassical patients with a frequency of 39.5%. No alleles containing P30L were observed, and one de novo mutation (R356W) was found. A total of 68.3% patients were fully genotyped, and all but one showed no genotype/phenotype discrepancy. Though the cut-off value for post-ACTH 17-hydroxyprogesterone stimulation was 30.25 nmol/l (10.00 microg/l), the lowest value observed in the fully genotyped nonclassical group was 42.35 nmol/l (14.00 microg/l).

CONCLUSIONS

The high number of unidentified alleles in the nonclassical group suggests that less frequent mutations, or the presence of new ones, might be the cause of the disease in the Argentine population. Alternatively, the cut-off value in the ACTH-stimulated 17-hydroxyprogesterone test might overestimate the diagnosis of the nonclassical form by including some patients with heterozygous status.