Cyclin N-terminal domain-containing 1 (CNTD1) is critical for crossover designation during meiotic prophase I and for maintenance of ovarian reserve in mammalian oocytes

Abstract

ABSTRACTIn meiotic prophase I, hundreds of double-strand breaks (DSB) are induced to initiate homologous recombination, resulting in the formation of non-crossovers (NCO) and crossovers (CO). Of the 250-300 DSBs that form in mice, only ∼10% are resolved as COs, with all remaining DSB events resolving as NCO. Thus, both the frequency and distribution of COs must be tightly regulated to ensure their appropriate localization across the genome. Errors in crossover designation and maturation can result in nondisjunction, leading to aneuploidy, germ cell death, birth defects, or infertility. That these errors are more evident in female meiosis compared to males suggests that the events of meiotic prophase I are sexually dimorphic with respect to DSB processing, resolution, and/or surveillance. Therefore, it is essential to investigate how COs are designated and regulated during prophase I in the female germline. Previous studies in our lab showed that Cyclin N-Terminal Domain Containing 1 (CNTD1) protein plays a critical role in CO designation during male meiosis, co-localizing with other essential CO proteins such as MutLγ (MLH1/MLH3) and crossover site-associated cyclin-dependent kinase-2 (CDK2) during pachynema. In the current study, we investigated whether analogous roles for CNTD1 existed in female meiosis. Similar toCntd1-/-males,Cntd1-/-females are sterile, with no gross morphological ovarian defects in adults albeit with a depletion in ovarian follicle numbers. During prophase I, early DSB induction and repair occur normally, but oocytes fromCntd1-/-females fail to accumulate MutLγ in pachynema. Phenotypically, meiotic prophase I disruption inCntd1-/-females is similar to that seen inCntd1-/-males. Importantly, whereas theCaernohabditis elegansortholog of CNTD1, COSA-1, appears to interact with CDK-2 during pachynema in oocytes, CNTD1 in mouse oocytes lacks the CDK-interacting motif, suggesting that its function in oocytes does not involve a cyclin-like induction of CDK2 phosphorylation activity. In contradiction to the timing of prophase I disruption inCntd1-/-females at pachynema, we observe a dramatic depletion of primordial follicles, beginning shortly after birth and increasing dramatically through to adult ages. Those primordial follicles that persist, however, are able to develop through dictyate arrest into primary and larger follicular structures in pre-pubertal females, and only by adulthood were significant declines observed in all follicles. The early loss of oocytes inCntd1-/-females shortly after birth, considerably earlier than that observed for mice lacking MLH1 or MLH3, suggests a novel early role for CNTD1 in CO designation and/or in ensuring progression of oocytes into dictyate arrest at late pachynema.