OR06-1 Cyclophosphamide Depletes Primordial Follicles via the p63 Apoptotic Pathway

Cancer therapies often cause serious side effects, affecting the quality of life for cancer survivors. The ovary can be affected by cancer therapies, causing premature ovarian insufficiency, leading to endocrine dysfunction and infertility. Thus, maintaining ovarian function against cancer treatment is an unmet need for female cancer patients. Cyclophosphamide (CPA), a common chemotherapeutic agent, is metabolized in vivo and forms DNA crosslinks, inducing apoptosis in rapidly proliferating tumor cells. However, the underlying mechanism of the CPA-induced oocyte death in ovarian reserve remains unclear. This study aims to investigate the mechanism of oocyte death in primordial follicles by generating oocyte-specific Abl1 and p63 knockout and Pik3ca* knockin mouse models using Gdf9-iCre+. Postnatal day 7 female mice were i.p. injected w/wo CPA. Notably, we found CPA drastically reduced the number of primordial follicles without increasing the number of growing follicles. The quantification of surviving follicles validated that 90% of the primordial follicles from oocyte-specific Abl1 knockout mice were lost following CPA treatment in vivo and in vitro. Concurrently, high expression of CHK2 was detected in the oocytes of the ovary cultured with CPA metabolite in vitro. This implied CHK2 rather than ABL is a possible tyrosine kinase for CPA-induced oocyte death. Most of all, p63 was a key player for CPA-induced oocyte death, supported by the result that p63 knockout oocytes were rescued after CPA treatment. To clarify whether the ovaries lose follicles via activation or apoptosis pathway, PIK3CA* mice were examined with CPA. As expected, primordial follicles and primordial follicles with activated oocytes in the ovary of PIK3CA* mice survived against CPA due to constitutive PI3K activity in them. This indicates activated follicles resist gonadotoxic reagents, although p63 is expressed inside their oocytes. Accordingly, the apoptosis markers, BAX and cleaved PARP were highly induced with CPA injection in a time-dependent manner in the ovaries from wild-type female mice but not from the p63 knockout. The double-strand break also occurred in the nucleus of oocytes post CPA administration as γH2AX was detected. Interestingly, OPA1, a protein required for mitochondrial fusion, was highly induced inside the oocyte cytoplasm of p63 knockout, while oocytes in wild-type mice time-dependently lost the OPA1 expression by CPA treatment. This indicates that oocytes without p63 in the nucleus survive and induce mitochondrial fusion to escape apoptosis by mitochondrial damages. In conclusion, CPA induces depletion of oocytes of primordial follicles through the CHK2-TAp63 apoptotic pathway. Presentation: Saturday, June 11, 2022 11:30 a.m. - 11:45 a.m.