PRENATAL DIAGNOSIS OF CONGENITAL ADRENAL HYPERPLASIA DUE TO 21-HYDROXYLASE DEFICIENCY

Abstract

Prenatal diagnosis of the classic forms of the congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency has progressed at the same time as the advances made in the field of both endocrinology and molecular genetics. In 1975, prenatal diagnosis of CAH was first made in the second trimester on the basis of raised levels of 17-hydroxyprogesterone (17-OHP) in the amniotic fluid (AF). Later an indirect prenatal diagnosis based on the genetic tracking of CAH was greatly facilitated by its linkage to HLA. By 1979, to obviate the need for corrective genital surgery the virilized genitalia in CAH female fetuses, a prenatal treatment was proposed by giving dexametfiasone to the mother in early pregnancy, and later was shown to be successful. Rapidly, there was a demand for an early prenatal diagnosis of the affection. This was rendered possible by the advances in molecular biology associated with the practice of chorionic villus biopsy. Prenatal diagnosis was then done in the first trimester of the pregnancy, by either HLA typing using HLA-DNA probes or direct studies of the genetic abnormalities of the C4-CYP21 locus. In practice, four methods have been, and can still be used : measurement of AF levels of 17-OHP, HLA typing, molecular studies of the C4-CYP21 genes, all in association with fetal sex determination (measurement of AF testosterone levels, karyotype and more recently detection of Y-chromosome specific sequences). Steroid analysis remains a simple and safe method in the absence of prenatal treatment and the only informative method when there is no previous family study. Nowadays, an early and accurate prenatal diagnosis can be achieved by molecular genetic studies on DNA extracted from chorion villus samples, but it requires the previous study of the nuclear family. The choice between HLA typing and direct C4-CYP21 loci studies depend on the possibilities and expertise of a given laboratory. HLA typing is still more often used with the improvement of using molecular studies instead of serological methods because almost all families are then informative. Nevertheless, HLA typing is unreliable in some circumstances: in case of a recombination event (1 % in our studies of over 250 families) or de novo mutation(s) and when the index case is deceased or not available. Studies of the C4-CYP21 gene locus by Southern blotting and the CYP21B gene mutations by PCR method have not these disadvantages. In our hands, Southern blotting study of the duplicated C4-CYP21 region has shown some complex genetic rearrangements and is fully informative method for prenatal diagnosis in about 60% of CAH families. The 9 most common point mutations of the CYP21B gene causing CAH (and also present in the CYP21A pseudogene) can be detected after specific PCR amplification of the functional CYP21B gene. In these conditions, almost all families are informative in our experience. Moreover, using this direct genetic analysis associated with the possibility to detect the heterozygotes in a non related-CAH population, a prenatal diagnosis could be done in a family without a previous CAH affected child. A prenatal diagnosis in the first trimester is required in order to stop unnecessary prenatal treatments (if unaffected female fetuses and all males fetuses), or when the parents wish to interrupt the pregnancy in case of a CAH affected fetus. In our experience using molecular genetics of the CYP21-C4 loci and sex determination by PCR method (SRY, Y-chromosome repeated sequences) or karyotype, prenatal diagnosis has been successfully done in 23 pregnancies at risk 1:4. In one case, there was no previous family study because the index c;>se was dead; in an other family, the only informations were the state of heterozygoty of the parents: one being a relative of a characterized CAH family, the other being detected by a systematic ACTH test and confirmed by mutation studies.