ENHANCED EXPRESSION OF THE MURINE FMR1 GENE DURING GERM-CELL PROLIFERATION SUGGESTS A SPECIAL FUNCTION IN BOTH THE MALE AND THE FEMALE GONAD

To elucidate the function of the FMR1 gene, we applied RNA in situ hyb ridization to cryosections of mice from different developmental stages . The murine Fmr-l was found transcribed in a ubiquitous manner with a n expression pattern similar to glyceraldehyd phosphate dehydrogenase, Gapdh, which was used as a control gene. A significant difference in the Fmr-l expression pattern, however, was markedly enhanced expressio n specifically confined to the testis and the fetal ovary. In the imma ture and mature testis an elevated level of Fmr-l expression is found in type A, spermatogonia. Expression in the testis is observed in feta l life, reaches the highest level in the immature testis, and declines early in adult life. In the mature ovary no specific Fmr-l expression signal was found but enhanced levels were seen in the fetal ovary. At this developmental stage proliferation of oogonia takes place. It is suggested that FMR1 serves a special function during germ cell prolife ration in males and females. These findings are discussed in the light of the current observation that fragile X patients produce only sperm with a premutation sized allele. Two hypotheses are put forward: (1) In males lack of FMR1 function results in a premeiotic defect preventi ng spermatogonia with a full mutation to reach meiosis. A fragile X mu tation can be passed on to offsprings only as a premutation (selection hypothesis). (2) Transition of a premutation allele to full mutation occurs in a postzygotic stage after separation of the germ line and is restricted to soma cells (restriction hypothesis). Expression of FMR1 in proliferating germ cells is in line with both hypothesis.