Separase Phosphosite Mutation Leads to Genome Instability and Primordial Germ Cell Depletion during Oogenesis Juan Xu1., Meizhi Wang2., Xinxing Gao1, Bian Hu 1 , Yinan Du1, Jiankui Zhou 1 , Xuemei Tian3*, Xingxu 1 Huang * 1 Model Animal Research Center, Nanjing University, Nanjing, China, 2 Department of Anatomy, Histology and Embryology, Southern Medical University, Guangzhou, China, 3 School of Life Science, South China Normal University, Guangzhou, China Abstract To ensure equal chromosome segregation and the stability of the genome during cell division, Separase is strictly regulated primarily by Securin binding and inhibitory phosphorylation. By generating a mouse model that contained a mutation to the inhibitory phosphosite of Separase, we demonstrated that mice of both sexes are infertile. We showed that Separase deregulation leads to chromosome mis-segregation, genome instability, and eventually apoptosis of primordial germ cells (PGCs) during embryonic oogenesis. Although the PGCs of mutant male mice were completely depleted, a population of PGCs from mutant females survived Separase deregulation. The surviving PGCs completed oogenesis but produced deficient initial follicles. These results indicate a sexual dimorphism effect on PGCs from Separase deregulation, which may be correlated with a gender-specific discrepancy of Securin. Our results reveal that Separase phospho-regulation is critical for genome stability in oogenesis. Furthermore, we provided the first evidence of a pre-zygotic mitotic chromosome segregation error resulting from Separase deregulation, whose sex-specific differences may be a reason for the sexual dimorphism of aneuploidy in gametogenesis. Citation: Xu J, Wang M, Gao X, Hu B, Du Y, et al. (2011) Separase Phosphosite Mutation Leads to Genome Instability and Primordial Germ Cell Depletion during Oogenesis. PLoS ONE 6(4): e18763. doi:10.1371/journal.pone.0018763 Editor: Laszlo Orban, Temasek Life Sciences Laboratory, Singapore Received July 2, 2010; Accepted March 17, 2011; Published April 11, 2011 Copyright: 2011 Xu et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work is supported by the National Natural Science Foundation of China (30871429) and the 973 program (2010CB945101). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: xmtian05@yahoo.com (XT); huangxingxu@yahoo.com (XH) . These authors contributed equally to this work. Introduction The most fundamental feature of mitotic and meiotic cell division is the equal transmission of the duplicated genome into two daughter cells. This is achieved by the separation of sister chromatids, a process mainly executed by a cysteine protease called Separase [1,2]. This process occurs with high fidelity by accurate intrinsic chromosome segregation machinery and the activity of a spindle assembly checkpoint [3]. However, errors during cell division do occur, resulting in chromosome instability and leading to aneuploidy, a condition with an abnormal number of chromosomes. Aneuploidy is considered to be the leading genetic cause of human fertility failure. Approximately 10–30% of human zygotes and 50% of spontaneously aborted fetuses have an abnormal number of chromosomes [4]. Genetic etiology studies revealed that the aneuploidy mainly originated from gametogen- esis and early embryogenesis during development [5]. Gameto- genesis starts from primordial germ cells (PGCs), which undergo mitotic and meiotic cell division to generate gametes with a diploid or a haploid karyotype [6,7]. It is now generally accepted that the main cytogenetic events that lead to chromosome segregation errors are non-disjunction in meiosis I (MI), premature chromosome segregation in meiosis II (MII) [8,9] and post-zygotic mitosis [5]. Yet, the molecular mechanisms underlying chromosome segregation errors remain unclear due to the lack of appropriate model systems [10]. There have been several recent successful analyses using mouse models, which demonstrated that the abnormalities in the connections between homologous chromosomes and sister chromatids and related events, including pairing, synapsis, and recombination, can yield chromosome segregation errors [11–15]. Moreover, it has been reported that deletion of the meiosis-specific Cohesin component SMC1b causes meiotic chromosome mis-segregation [12,13]. Cohesin is a key molecular link between female aging and chromosome mis-segregation during MI [16]. Separase functions in arm cohesion dissociation, chiasma resolution, and meiosis I exit [17,18]. These data strongly suggest that Separase deregula- tion may cause chromosome segregation errors during gama- togenesis. To ensure proper chromosome segregation, Separase is strictly controlled by two elaborate mechanisms in mammals. First, Separase is inhibited through association with Securin, which is degraded by the proteasome after anaphase-promoting complex (APC)-mediated polyubiquitination at the metaphase/anaphase transition [19–23]. Second, inhibitory phosphorylation of Ser1126 and Thr1326 (Ser1121 and Thr1321 in the mouse, respectively) of Separase [24], which allow binding with Cyclin B1 and result in the inhibition of Separase activity [25]. In addition, Clift and PLoS ONE | www.plosone.org 1 April 2011 | Volume 6 | Issue 4 | e18763