A Molecular Method of Diagnosis of Congenital Adrenal Hyperplasia
Shoshana Israel, Chaim Brautbar
Tissue Typing Unit, Hadassah Medical Center, Jerusalem
Congenital adrenal hyperplasia (CAH) is caused mainly by deficiency of the 21-hydroxylase enzyme. The disease may appear in the classical salt-losing, simple virilizing forms or as a mild, nonclassical form. 21-hydroxylase is encoded by the CYP21B gene on the short arm of chromosome 6, in the midst of the human leukocyte antigen (HLA) complex, between HLA Class I and Class II regions.
We describe a method for identifying mutations in the CYP21B gene. It is based on amplification of the gene using the polymerase chain reaction and identification of mutations with sequence-specific oligo-probes. The mutations identified were: V281 and P30L responsible for nonclassical CAH, and I2 splice, Q318X, I172N, cluster E6, and a deletion including 8bP in the third exon (8bP del) responsible for the classical form of CAH.
We also analyzed 2 families affected with the classical form of CAH which demonstrate possible complications in genotyping. Typing for HLA haplotypes can be helpful in certain cases, as demonstrated in 1 of the families presented. In this case it was necessary to distinguish between 2 possible genotypes: 1 with the mutations in tandem on 1 chromosome and the other with the mutated genes on both chromosomes. HLA haplotyping enabled the assignment of the mutations to the relevant chromosomes and thus allowed correct genetic counseling.
The other family demonstrated the importance of CYP21B genotyping in individuals with the nonclassical form of CAH. This form may consist of 1 mild and 1 severe mutation, representing a serious potential for transmitting the classical form of CAH.