Targeted overexpression of Bcl-2 in ovaries of transgenic mice leads to decreased follicle apoptosis, enhanced folliculogenesis, and increased germ cell tumorigenesis

We have generated transgenic mice overexpressing Bcl-2, an apoptosis suppression protein, in ovarian cells using the inhibin-cu gene promoter/enhancer. Ovarian apoptotic DNA fragmentation induced in immature animals by a low dose (2 IU) of PMSG was suppressed by greater than 55% in transgenic mice compared to their wild-type littermates. Morphological and in situ DNA end-labeling analyses showed that granulosa cells in large antral follicles of wild-type animals undergo apoptosis, but most follicles in transgenic animals are healthy. When the animals were treated with a high dose (4 IU) of PMSG to stimulate follicular growth, spontaneous ovulation was observed in 14 of 23 (61%) of the transgenic animals, but in only 3 of 18 (17%) of wild-type siblings. Furthermore, transgenic females had a larger litter size (9.07 +/- 0.25 pups/litter; n = 29) than wild-type controls (7.54 +/- 0.26 pups/litter; n = 28; P < 0.01). These data suggested that decreased ovarian apoptosis in transgenic animals could lead to enhanced folliculogenesis and ovulatory potential. Moreover, aging transgenic mice are susceptible to the development of benign cystic ovarian teratoma (4 in 20 transgenic animals and 0 in 26 wild-type controls). Some tumor tissues showed respiratory and intestinal cell types, whereas others showed the development of central nervous system-like structures. Because the bcl-2 transgene in these animals is overexpressed in somatic cells, but not oocytes, these findings suggest that enhanced survival of selected somatic cells in transgenic mice could lead to germ cell tumorigenesis. Thus, overexpression of Bcl-2 protein in the ovary leads to decreased ovarian somatic cell apoptosis, enhanced folliculogenesis, and increased susceptibility to ovarian germ cell tumorigenesis in transgenic animals. The present mouse model allows future studies on intracellular signal pathways regulating follicular atresia and on the potential role of ovarian somatic cell factors in germ cell tumorigenesis.