A lacZ transgenic mouse assay for the detection of mutations in follicular granulosa cells

There is ongoing concern that an assay for germ cell effects in female animals is not available. While transgenic mutation detection systems provide unprecedented access to numerous rodent tissues, studies on the induction of gene mutations in oocytes are still not possible because sufficient numbers of cells cannot be harvested. However, following stimulation of an ovarian follicle, the granulosa cells contained therein divide rapidly, increasing substantially in numbers. Since these granulosa cells share the same environment as the ovum, they may serve as suitable surrogates for the study of exposure of female germ cells to mutagens. Female lacZ transgenic mice (MutaMouse) were treated by intraperitoneal injection of N-ethylnitrosourea (ENU) and subsequently with pregnant mare serum gonadotropin (PMSG, 5IU/animal, i.p.) to induce follicular growth. Animals were sacrificed 48 h after the administration of PMSG and granulosa cells and bone marrow were harvested. A comparable dose-related increase in the mutant frequency (MF) of both granulosa and bone marrow cells was observed. The highest dose caused a decrease in the MF of granulosa cells, but not in the bone marrow, suggesting possible greater susceptibility of granulosa cells to ENU toxicity. Doubling dose estimates for bone marrow and granulosa cells were lower than those derived from the literature on oocyte mutation frequency using the Russell specific locus assay, suggesting that both cell types are more sensitive to ENU-induced mutation than oocytes. The results indicate that transgene mutations in granulosa cells may provide a sensitive pre-screening tool for potential genotoxic germ cell effects of exposed oocytes.