Ovarian differentiation and gonadal failure

Ovarian failure can result from several different genetic mechanisms-X chromosomal abnormalities, autosomal recessive genes causing various types of XX gonadal dysgenesis, and autosomal dominant genes. The number and precise location of loci on the X are still under investigation, but it is clear that, in aggregate, these genes are responsible for ovarian maintenance, given that monosomy X shows germ cells that undergo accelerated atresia. Despite recent hypotheses, at present there is no evidence for a gene directing primary ovarian differentiation; this process may be constitutive. Phenotypic/karyotypic correlation and limited molecular confirmation have long shown that proximal Xp and proximal Xq contain regions of the most importance to ovarian maintenance. Terminal deletions at Xp11 result in 50% primary amenorrhea and 50% premature ovarian failure or fertility. Deletions at Xq13 usually produce primary amenorrhea. Terminal deletions nearer the telomeres on either Xp of Xq bring about premature ovarian failure more often than complete ovarian failure. The X-linked zinc finger gene (ZFX) and diaphanous 2 Drosophila homologue (DIAPH2) are the only candidate genes for ovarian maintenance that map to the X chromosome. Additional, as yet unidentified, genes along the X chromosome must be involved. The search for these genes in humans is hampered by the lack of candidate genes that map to the X chromosome, the scarcity of patients with fortuitous autosomal translocations, and small pedigrees, which hinder mapping of the loci. In addition, difficulties with human germ cell research also make it challenging to dissect genes important to ovarian development. Autosomal genes also are involved in ovarian differentiation and gonadal failure. Follicle-stimulating hormone receptor and ataxia telangiectasia are examples of autosomal genes known to cause human ovarian failure. Transgenic mouse models point to many other candidate autosomal genes, and sequencing of the human homologues in affected women should lead to the discovery of new genes responsible for human ovarian failure. Identification, functional analysis, and mapping of novel genes specifically expressed in the ovary of mice and women eventually should lead to fruitful dissection of essential genes in mammalian ovarian development and maintenance.