An H2A Histone Isotype, H2ac, Associates with Telomere and Maintains Telomere Integrity

Telomeres are capped at the ends of eukaryotic chromosomes and are composed of TTAGGG repeats bound to the shelterin complex. Here we report that a replication-dependent histone H2A isotype, H2ac, was associated with telomeres in human cells and co-immunoprecipitates with telomere repeat factor 2 (TRF2) and protection of telomeres protein 1 (POT1), whereas other histone H2A isotypes and mutations of H2ac did not bind to telomeres or these two proteins. The amino terminal basic domain of TRF2 was necessary for the association with H2ac and for the recruitment of H2ac to telomeres. Depletion of H2ac led to loss of telomeric repeat sequences, the appearance of dysfunctional telomeres, and chromosomal instability, including chromosomal breaks and anaphase bridges, as well as accumulation of telomere-associated DNA damage factors in H2ac depleted cells. Additionally, knockdown of H2ac elicits an ATM-dependent DNA damage response at telomeres and depletion of XPF protects telomeres against H2ac-deficiency-induced G-strand overhangs loss and DNA damage response, and prevents chromosomal instability. These findings suggest that the H2A isotype, H2ac, plays an essential role in maintaining telomere functional integrity.

端粒（Telomeres）锚定在真核染色体的末端，由结合于shelterin复合物的TTAGGG重复序列构成。本研究报道，一种依赖复制的组蛋白H2A亚型H2ac可与人细胞中的端粒相结合，并可与端粒重复因子2（telomere repeat factor 2, TRF2）及端粒保护蛋白1（protection of telomeres protein 1, POT1）发生免疫共沉淀；而其余组蛋白H2A亚型及H2ac的突变体则无法与端粒或这两种蛋白结合。TRF2的氨基端碱性结构域是其与H2ac相互作用、并将H2ac招募至端粒的必要结构基础。敲除H2ac会导致端粒重复序列丢失、功能异常端粒的出现以及染色体不稳定，具体表现包括染色体断裂、有丝分裂后期桥（anaphase bridges），同时在H2ac敲除的细胞中会出现端粒相关DNA损伤因子的积累。此外，敲低H2ac可在端粒处引发依赖于ATM（ataxia-telangiectasia mutated）的DNA损伤反应；而敲除XPF则能够保护端粒免受H2ac缺陷诱导的G链悬垂丢失及DNA损伤反应，并可抑制染色体不稳定的发生。上述研究结果表明，组蛋白H2A亚型H2ac在维持端粒功能完整性中发挥着不可或缺的核心作用。