Loss of ovarian reserve by cisplatin and radiation and prevention

The immature oocytes in ovarian reserve are exquisitely sensitive to damage-induced apoptosis. Thus, radiation and chemotherapy cause ovarian insufficiency due to loss of ovarian reserve. Previous mouse studies have established that radiation and cisplatin induce apoptosis in oocytes through activation of TAp63α transcription factor. It has also been demonstrated that imatinib and GNF-2, ABL kinase inhibitors, protect immature oocytes from cisplatin-induced apoptosis. Therefore, the phosphorylation/activation of TAp63α by c-ABL is considered the regulatory mechanism in damage-induced oocyte death. However, we discovered imatinib and GNF-2 were ineffective in protection of primordial oocytes from ionizing irradiation. Furthermore, hyper-phosphorylation of TAp63α was not induced by cisplatin and ABL kinase inhibitors did not block the hyper-phosphorylation of TAp63α induced by irradiation.Thus cisplatin and radiation activate TAp63α through different mechanisms, and c-ABL is not the kinase that phosphorylates TAp63α in radiation-induced oocyte apoptosis. To further investigate the molecular mechanism of TAp63αactivation by cisplatin, we generated oocyte-specific conditional knockout mice for Abl1 and Abl2. Surprisingly, oocyte-specific ablation of c-ABL/ABL1 or ARG/ABL2 did not protect primordial follicles from cisplatin, indicating the ABL kinase inhibitors do not target oocytic ABL1 and ABL2 to protect the ovarian reserve from cisplatin. The different mechanism of cisplatin and radiation in the activation of TAp63α was further indicated by the differential efficacy of inhibitors for ATM and ATR against cisplatin and x-ray in ovarian reserve protection. Inhibition of ATM protected primordial follicles from radiation but not cisplatin, whereas ATR inhibitors protected the ovarian reserve only from cisplatin but not radiation. Thus, pharmacological fertility preservation options must be specific to the type of anti-cancer therapy – in this case, cisplatin and radiation. Nevertheless, our screening of kinase inhibitors identified CHK2 Inhibitor II hydrate (C2II) as an agent that protects primordial follicles from both radiation and cisplatin by totally blocking the DNA damage response in primordial oocytes. Accordingly, we tested the efficacy of C2II for fertility preservation against cisplatin in female mice. When neonatal female CD-1 mice were exposed to cisplatin, they became sterile by 6 months after giving birth to 2.2 litters on average. However, co-administration of C2II reversed the gonadotoxicity of cisplatin, and the female mice maintained their fertility until the end of fertility test (9 months) producing 6 litters on average. Hence C2II can be a potent neo-adjuvant for fertility preservation against cisplatin. Since C2II is expected to be effective for radiation, the efficacy test of C2II in fertility preservation against x-irradiation is ongoing.