[Genetic aspects of congenital adrenal hyperplasia due to 21-hydroxylase deficiency].

The variability of clinical and biological expression of the 21 OH hydroxylase deficiency is likely to be related to genetical variability. Beside the well known autosomic recessive mode of inheritance the frequencies of the different forms of the disease, especially the classical and late onset form, have been more precisely defined through neonatal screening programs for the classical form which lead to a frequency of about 1 case/20,000 with a calculated gene frequency around 1/140. The linkage with the major histocompatibility complex allows the location of the putative locus of the 21 OH ase on the short arm of the chromosome 6 in the class III of the MHC. This linkage has made possible a better fetal diagnosis even if some pitfalls as recombination must be kept in mind. On the basis of clinical conditions the abnormal genes are likely to be considered as an allelic series with a least two main types of pathological alleles: the "severe" and "moderate". During the last two years, taking advantages of molecular gene biology, the structure of the normal human 21 OH ase gene has been studied. It exists as duplicate genes in close relation with the gene of the fourth component of the complement. A deletion of one of the copy has been demonstrated in the form associated with the BW47 MHC haplotype. It is likely that during the coming years genetical heterogeneity will be demonstrated as it has been for other genetic diseases as thalassemia.