Identification of rare alleles in an Italian population of 284 patients with 21- hydroxylase deficiency by complete sequencing of the CYP21 gene

The screening of the usually tested CYP21 gene alterations (the large gene deletion/conversion and the P30L, IVS2-13A/C>G, Ex 3 D8nt, Ex6 cluster, I172N, V281L, 1766-1767insT, Q318X, R356Q, P453S) by means of Southern blotting and Allele-Specific PCR in an Italian population of 284 patients with 21-hydroxylase deficiency led us to characterize 461 of the total 506 different alleles (91.1%), while 45 (8,9 %) remained uncharacterized. Some incongruent genotype/phenotype correlation was also observed. In order to reduce the number of uncharacterized alleles and possibly to identify additional alterations in some patients’ alleles, we reanalysed the entire population (128 classic and 156 nonclassic forms selected for stimulated 17-HP level>10 ng/ml) by complete sequencing of the CYP21 gene, promoter included. Identified mutations were verified in the available parents to confirm allele segregation. Result a: of the 461 characterized alleles, 10 present an additional/different known mutation from that previously identified (2.1% false positive/negative results); 7 showed a conversion extending from the promoter to the P30L mutation (3 cases) or the IVS2 (4 cases). Result b: of the 34 uncharacterized alleles, 16 showed rare mutations (1 M283V, 1 R316X, 4 R341P, 4 R356Q, 1 R426H, 4 P482S, 1 R483P); 7 showed as far as we know unique mutations, 2 (W19X, L480Xfs) with an obvious implication in the phenotype, and 5 affecting residues L142, I171, R341, V358, L446 currently being studied; 11 remained uncharacterized. A total of 43 (23 of pseudogene origin) SNPs, 15 in the promoter, 3 in the 3’ UTR and 25 in introns were identified: further studies will verify their frequency in controls and the possible implication in the phenotypes. The high frequence of complex, rare or unique alleles in the Italian population underlines the importance to routinely analyse the CY21 gene by complete sequencing to avoid false/incomplete results, identify new mutations or sequence variations and to improve the genotype/phenotype correlation, genetic counselling and treatment.