The DNA damage response is required for oocyte cyst breakdown and follicle formation in mice.

Mammalian oogonia proliferate without completing cytokinesis, forming cysts. Within these, oocytes differentiate and initiate meiosis, promoting double-strand break (DSBs) formation, which are repaired by homologous recombination (HR) causing the pairing and synapsis of the homologs. Errors in these processes activate checkpoint mechanisms, leading to apoptosis. At the end of prophase I, in contrast with what is observed in spermatocytes, oocytes accumulate unrepaired DSBs. Simultaneously to the cyst breakdown, there is a massive oocyte death, which has been proposed to be necessary to enable the individualization of the oocytes to form follicles. Based upon all the above-mentioned information, we hypothesize that the apparently inefficient HR occurring in the oocytes may be a requirement to first eliminate most of the oocytes and enable cyst breakdown and follicle formation. To test this idea, we compared perinatal ovaries from control and mutant mice for the effector kinase of the DNA Damage Response (DDR), CHK2. We found that CHK2 is required to eliminate similar to 50% of the fetal oocyte population. Nevertheless, the number of oocytes and follicles found in Chk2-mutant ovaries three days after birth was equivalent to that of the controls. These data revealed the existence of another mechanism capable of eliminating oocytes. In vitro inhibition of CHK1 rescued the oocyte number in Chk2(-/-) mice, implying that CHK1 regulates postnatal oocyte death. Moreover, we found that CHK1 and CHK2 functions are required for the timely breakdown of the cyst and to form follicles. Thus, we uncovered a novel CHK1 function in regulating the oocyte population in mice. Based upon these data, we propose that the CHK1- and CHK2-dependent DDR controls the number of oocytes and is required to properly break down oocyte cysts and form follicles in mammals. Author summary In mammals, female fertility is limited by the number of germ cells, i.e., the oocytes, females are born with. These oocytes associate with somatic cells to produce follicles that guarantee the production of a fertilizable egg and the synthesis of sex hormones. During fetal development, oocytes initiate meiosis to halve their ploidy. As part of the meiotic program, oocytes self-induce hundreds of DSBs into their genome, which have to be repaired. However, oocytes are not very efficient at repairing DSBs. Also, during fetal development, a great part of the oocytes are eliminated, in a process that has been linked to the formation of follicles. It has been described that the effector kinase of the DDR (CHK2) is responsible for eliminating oocytes with persistent DSBs. Thus, we hypothesized that the DDR may be responsible for eliminating oocytes. In agreement with our hypothesis, we found that CHK2 was responsible for eliminating most fetal oocytes and regulating follicle formation in the mouse. Unexpectedly, we found that there is a CHK1-dependent perinatal oocyte death occurring in Chk2(-/-) mice that alters follicle formation. These data suggest that the CHK2- and CHK1-dependent DDR control the oocyte pool and follicle formation in mammals.