Female Germ Cell Biology Group, Department of Clinical Studies, Center for Animal Transgenesis and Germ Cell Research, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, 382 West Street Road, Kennett Square, PA 19348, USA;
Genes (Basel). 2011;2(1):59-80. doi: 10.3390/genes2010059.
Polycomb group proteins (PcG) are major epigenetic regulators, essential for establishing heritable expression patterns of developmental control genes. The mouse PcG family member M33/Cbx2 (Chromobox homolog protein 2) is a component of the Polycomb-Repressive Complex 1 (PRC1). Targeted deletion of Cbx2/M33 in mice results in homeotic transformations of the axial skeleton, growth retardation and male-to-female sex reversal. In this study, we tested whether Cbx2 is involved in the control of chromatin remodeling processes during meiosis. Our analysis revealed sex reversal in 28.6% of XY(-/-) embryos, in which a hypoplastic testis and a contralateral ovary were observed in close proximity to the kidney, while the remaining male mutant fetuses exhibited bilateral testicular hypoplasia. Notably, germ cells recovered from Cbx2((XY-/-)) testes on day 18.5 of fetal development exhibited premature meiosis onset with synaptonemal complex formation suggesting a role for Cbx2 in the control of meiotic entry in male germ cells. Mutant females exhibited small ovaries with significant germ cell loss and a high proportion of oocytes with abnormal synapsis and non-homologous interactions at the pachytene stage as well as formation of univalents at diplotene. These defects were associated with failure to resolve DNA double strand breaks marked by persistent γH2AX and Rad51 foci at the late pachytene stage. Importantly, two factors required for meiotic silencing of asynapsed chromatin, ubiquitinated histone H2A (ubH2A) and the chromatin remodeling protein BRCA1, co-localized with fully synapsed chromosome axes in the majority of Cbx2((-/-)) oocytes. These results provide novel evidence that Cbx2 plays a critical and previously unrecognized role in germ cell viability, meiosis onset and homologous chromosome synapsis in the mammalian germline.
多梳抑制复合物蛋白（PcG）是主要的表观遗传调控因子，对于建立发育控制基因的可遗传表达模式至关重要。小鼠 PcG 家族成员 M33/Cbx2（同源盒蛋白 2）是多梳抑制复合物 1（PRC1）的组成部分。在小鼠中靶向敲除 Cbx2/M33 会导致轴骨骼的同源转化、生长迟缓以及雄性到雌性的性别反转。在这项研究中，我们测试了 Cbx2 是否参与了减数分裂过程中染色质重塑过程的控制。我们的分析显示，在 28.6%的 XY(-/-)胚胎中出现了性别反转，在这些胚胎中，一个发育不良的睾丸和一个对侧卵巢紧邻肾脏，而其余的雄性突变胎儿则表现为双侧睾丸发育不良。值得注意的是，从 Cbx2((XY-/-))胎儿第 18.5 天的睾丸中回收的生殖细胞表现出早熟减数分裂的起始，伴随着联会复合体的形成，这表明 Cbx2 在控制雄性生殖细胞减数分裂进入方面发挥作用。突变雌性表现出小的卵巢，伴随着显著的生殖细胞丢失，并且在粗线期阶段有大量的卵母细胞具有异常的联会和非同源相互作用，以及在双线期形成单价体。这些缺陷与未能解决 DNA 双链断裂有关，这些断裂在晚期粗线期阶段通过持续的 γH2AX 和 Rad51 焦点标记。重要的是，两个在减数分裂中沉默未联会染色质所必需的因素，泛素化组蛋白 H2A（ubH2A）和染色质重塑蛋白 BRCA1，在大多数 Cbx2((-/-))卵母细胞的完全联会染色体轴上共定位。这些结果提供了新的证据，表明 Cbx2 在哺乳动物生殖细胞中发挥着至关重要的、以前未被认识的作用，对于生殖细胞的存活、减数分裂的起始以及同源染色体的联会。
