Histone H2AFX Links Meiotic Chromosome Asynapsis to Prophase I Oocyte Loss in Mammals

Chromosome abnormalities are common in the human population, causing germ cell loss at meiotic prophase I and infertility. The mechanisms driving this loss are unknown, but persistent meiotic DNA damage and asynapsis may be triggers. Here we investigate the contribution of these lesions to oocyte elimination in mice with chromosome abnormalities, e.g. Turner syndrome (XO) and translocations. We show that asynapsed chromosomes trigger oocyte elimination at diplonema, which is linked to the presence of phosphorylated H2AFX (γH2AFX). We find that DNA double-strand break (DSB) foci disappear on asynapsed chromosomes during pachynema, excluding persistent DNA damage as a likely cause, and demonstrating the existence in mammalian oocytes of a repair pathway for asynapsis-associated DNA DSBs. Importantly, deletion or point mutation of H2afx restores oocyte numbers in XO females to wild type (XX) levels. Unexpectedly, we find that asynapsed supernumerary chromosomes do not elicit prophase I loss, despite being enriched for γH2AFX and other checkpoint proteins. These results suggest that oocyte loss cannot be explained simply by asynapsis checkpoint models, but is related to the gene content of asynapsed chromosomes. A similar mechanistic basis for oocyte loss may operate in humans with chromosome abnormalities.染色体异常在人类群体中较为高发，可引发减数分裂I期前期（meiotic prophase I）生殖细胞丢失与不孕不育。目前此类丢失的驱动机制尚未阐明，但持续性减数分裂DNA损伤与联会异常（asynapsis）或为其潜在诱因。本研究以染色体异常模型小鼠——包括特纳综合征（Turner syndrome，XO型）与染色体易位（translocations）模型——为研究对象，探究了上述损伤在卵母细胞清除过程中的作用。 研究结果显示，联会异常的染色体可在双线期（diplonema）触发卵母细胞清除，该过程与磷酸化H2AFX（γH2AFX）的存在密切相关。我们观察到，在粗线期（pachynema）阶段，联会异常染色体上的DNA双链断裂（DNA double-strand break，DSB）灶会逐渐消失，这一发现排除了持续性DNA损伤作为潜在诱因的可能性，并证实哺乳动物卵母细胞中存在针对联会异常相关DNA双链断裂的修复通路。 尤为关键的是，敲除或点突变H2afx基因可将XO型雌性小鼠的卵母细胞数量恢复至野生型（wild type，XX型）水平。出乎意料的是，尽管额外染色体（supernumerary chromosomes）富集有γH2AFX与其他检验点蛋白（checkpoint proteins），但其并不会引发减数分裂I期前期的卵母细胞丢失。 上述结果表明，卵母细胞丢失无法仅通过联会异常检验点模型进行解释，而是与联会异常染色体的基因组成密切相关。染色体异常人群的卵母细胞丢失可能存在与之相似的机制基础。