Departamento de Biología, Facultad de Ciencias, Universidad Autónoma de Madrid, Madrid, Spain.
PLoS Genet. 2019 Jan 22;15(1):e1007439. doi: 10.1371/journal.pgen.1007439. eCollection 2019 Jan.
Homologous recombination (HR) is the principal mechanism of DNA repair acting during meiosis and is fundamental for the segregation of chromosomes and the increase of genetic diversity. Nevertheless, non-homologous end joining (NHEJ) mechanisms can also act during meiosis, mainly in response to exogenously-induced DNA damage in late stages of first meiotic prophase. In order to better understand the relationship between these two repair pathways, we studied the response to DNA damage during male mouse meiosis after gamma radiation. We clearly discerned two types of responses immediately after treatment. From leptotene to early pachytene, exogenous damage triggered the massive presence of γH2AX throughout the nucleus, which was associated with DNA repair mediated by HR components (DMC1 and RAD51). This early pathway finished with the sequential removal of DMC1 and RAD51 and was no longer inducible at mid pachytene. However, from mid-pachytene to diplotene, γH2AX appeared as large discrete foci. This late repair pattern was mediated initially by NHEJ, involving Ku70 and XRCC4, which were constitutively present, and 53BP1, which appeared at sites of damage soon after irradiation. Nevertheless, 24 hours after irradiation, a HR pathway involving RAD51 but not DMC1 mostly replaced NHEJ. Additionally, we observed the occurrence of synaptonemal complex bridges between bivalents, most likely representing chromosome translocation events that may involve DMC1, RAD51 or 53BP1. Our results reinforce the idea that the early "meiotic" repair pathway that acts by default at the beginning of meiosis is replaced from mid-pachytene onwards by a "somatic-like" repair pattern. This shift might be important to resolve DNA damage (either endogenous or exogenous) that could not be repaired by the early meiotic mechanisms, for instance those in the sex chromosomes, which lack a homologous chromosome to repair with. This transition represents another layer of functional changes that occur in meiotic cells during mid pachytene, in addition to epigenetic reprograming, reactivation of transcription, changes in the gene expression profile and acquisition of competence to proceed to metaphase.
同源重组（HR）是减数分裂过程中 DNA 修复的主要机制，对于染色体的分离和遗传多样性的增加至关重要。然而，非同源末端连接（NHEJ）机制也可以在减数分裂过程中发挥作用，主要是在第一次减数前期的晚期响应外源诱导的 DNA 损伤。为了更好地理解这两种修复途径之间的关系，我们研究了γ辐射后雄性小鼠减数分裂过程中对 DNA 损伤的反应。我们清楚地分辨出处理后立即出现的两种反应类型。从细线期到早粗线期，外源损伤触发了大量 γH2AX 存在于整个细胞核中，这与 HR 成分（DMC1 和 RAD51）介导的 DNA 修复有关。这个早期途径随着 DMC1 和 RAD51 的顺序去除而结束，并且在中期粗线期不再诱导。然而，从中期粗线期到双线期，γH2AX 出现为大的离散焦点。这种晚期修复模式最初由 NHEJ 介导，涉及 Ku70 和 XRCC4，它们是组成性存在的，以及 53BP1，它在照射后很快出现在损伤部位。然而，照射后 24 小时，一种涉及 RAD51 但不涉及 DMC1 的 HR 途径主要取代了 NHEJ。此外，我们观察到二价体之间出现联会复合体桥，这很可能代表染色体易位事件，这些事件可能涉及 DMC1、RAD51 或 53BP1。我们的结果强化了这样一种观点，即在减数分裂开始时默认起作用的早期“减数分裂”修复途径从中期粗线期开始被一种“体样”修复模式所取代。这种转变对于解决无法通过早期减数分裂机制修复的 DNA 损伤（无论是内源性还是外源性的）可能很重要，例如那些在性染色体中，缺乏同源染色体进行修复。这种转变代表了中期粗线期减数分裂细胞中发生的另一个功能变化层，除了表观遗传重编程、转录重新激活、基因表达谱变化和获得进入中期的能力之外。
