Fragile X syndrome is caused by the expansion of a "triplet repeat" in a gene (FMR1) that is important for signaling in the brain. In fragile X syndrome, the triplet repeat -- three DNA letters (CGG) repeated many times -- forces the gene to shut off.
For a woman who carries the premutation, the triplet repeat is not large enough to shut the gene off. There is a risk that it will expand in her children enough to generate fragile X syndrome. In addition, the triplet repeat appears to have an effect on the woman's ovaries, independently from its influence on the FMR1 gene.
Jin says studying mice that have an analogous genetic alteration will help scientists understand what's happening to the ovaries in POI. It appears that the RNA coming from the premutation impairs development of the ovarian follicles, the structures in which eggs/oocytes mature.
The research team found that a quarter of premutation-carrying female mice are infertile. When they are housed with male mice, those that do have pups have them a month later on average (12.5 weeks of age compared to 8.5 weeks), and they have fewer pups.
Puberty occurs at roughly five weeks of age in mice, and the premutation mice have alterations in their ovaries already before puberty. At 25 days of age, there are a reduced number of mature follicles in ovaries of the female mice carrying the premutation. Those mice also have altered levels of hormones resembling those of women with POI, such as elevated FSH (follicle stimulating hormone).
The research team found that in the ovaries of mice with the fragile X premutation, ovulation-related genes are less active. In addition, two cellular signaling pathways (Akt/mTOR) are less active in the ovaries,
|Contact: Quinn Eastman|