Genome Damage and Stability Centre, University of Sussex, East Sussex BN1 9RQ, United Kingdom.
DNA Repair (Amst). 2010 Dec 10;9(12):1273-82. doi: 10.1016/j.dnarep.2010.09.013. Epub 2010 Oct 30.
DNA non-homologous end-joining (NHEJ) and homologous recombination (HR) represent the major DNA double strand break (DSB) pathways in mammalian cells, whilst ataxia telangiectasia mutated (ATM) lies at the core of the DSB signalling response. ATM signalling plays a major role in modifying chromatin structure in the vicinity of the DSB and increasing evidence suggests that this function influences the DSB rejoining process. DSBs have long been known to be repaired with two (or more) component kinetics. The majority (∼85%) of DSBs are repaired with fast kinetics in a predominantly ATM-independent manner. In contrast, ∼15% of radiation-induced DSBs are repaired with markedly slower kinetics via a process that requires ATM and those mediator proteins, such as MDC1 or 53BP1, that accumulate at ionising radiation induced foci (IRIF). DSBs repaired with slow kinetics predominantly localise to the periphery of genomic heterochromatin (HC). Indeed, there is mounting evidence that chromatin complexity and not damage complexity confers slow DSB repair kinetics. ATM's role in HC-DSB repair involves the direct phosphorylation of KAP-1, a key HC formation factor. KAP-1 phosphorylation (pKAP-1) arises in both a pan-nuclear and a focal manner after radiation and ATM-dependent pKAP-1 is essential for DSB repair within HC regions. Mediator proteins such as 53BP1, which are also essential for HC-DSB repair, are expendable for pan-nuclear pKAP-1 whilst being essential for pKAP-1 formation at IRIF. Data suggests that the essential function of the mediator proteins is to promote the retention of activated ATM at DSBs, concentrating the phosphorylation of KAP-1 at HC DSBs. DSBs arising in G2 phase are also repaired with fast and slow kinetics but, in contrast to G0/G1 where they all DSBs are repaired by NHEJ, the slow component of DSB repair in G2 phase represents an HR process involving the Artemis endonuclease. Results suggest that whilst NHEJ repairs the majority of DSBs in G2 phase, Artemis-dependent HR uniquely repairs HC DSBs. Collectively, these recent studies highlight not only how chromatin complexity influences the factors required for DSB repair but also the pathway choice.
DNA 非同源末端连接（NHEJ）和同源重组（HR）是哺乳动物细胞中主要的 DNA 双链断裂（DSB）途径，而共济失调毛细血管扩张突变（ATM）位于 DSB 信号反应的核心。ATM 信号在改变 DSB 附近染色质结构方面发挥着重要作用，越来越多的证据表明，这种功能会影响 DSB 重接过程。DSB 长期以来一直被认为以两种（或更多）组成动力学进行修复。大多数（∼85%）DSB 以快速动力学方式修复，主要是非 ATM 依赖性方式。相比之下，∼15%的辐射诱导 DSB 通过一种需要 ATM 和那些介导蛋白（如 MDC1 或 53BP1）的过程以明显较慢的动力学修复，这些介导蛋白在电离辐射诱导的焦点（IRIF）中积累。以缓慢动力学修复的 DSB 主要定位于基因组异染色质（HC）的外围。事实上，越来越多的证据表明，染色质复杂性而不是损伤复杂性赋予了缓慢的 DSB 修复动力学。ATM 在 HC-DSB 修复中的作用涉及到关键的 HC 形成因子 KAP-1 的直接磷酸化。在辐射后，KAP-1 的磷酸化（pKAP-1）以全核和焦点的方式出现，ATM 依赖性的 pKAP-1 对于 HC 区域内的 DSB 修复是必不可少的。介导蛋白，如 53BP1，对于 HC-DSB 修复也是必不可少的，它们对于全核 pKAP-1 是可有可无的，而对于 IRIF 中 pKAP-1 的形成则是必不可少的。数据表明，介导蛋白的关键功能是促进激活的 ATM 在 DSB 处的保留，将 KAP-1 的磷酸化集中在 HC DSB 处。在 G2 期产生的 DSB 也以快速和缓慢动力学修复，但与 G0/G1 不同，在 G0/G1 中，所有 DSB 都通过 NHEJ 修复，G2 期 DSB 修复的缓慢成分代表涉及 Artemis 内切酶的 HR 过程。结果表明，虽然 NHEJ 修复了 G2 期大多数 DSB，但 Artemis 依赖性 HR 独特地修复了 HC DSB。总的来说，这些最近的研究不仅强调了染色质复杂性如何影响 DSB 修复所需的因素，还强调了途径选择。
