Unrestrained poly-ADP-ribosylation provides insights into chromatin regulation and human disease Prokhorova et al.

ARH3/ADPRHL2 and PARG are the primary enzymes reversing ADP-ribosylation in vertebrates, yet their functions in vivo remain unclear. ARH3 is the only hydrolase able to remove serine-linked mono(ADP-ribose) (MAR) but is much less efficient than PARG against poly(ADP-ribose) (PAR) chains in vitro. Here, by using ARH3-deficient cells, we demonstrate that endogenous MARylation persists on chromatin throughout the cell cycle, including mitosis, and is surprisingly well tolerated. Conversely, persistent PARylation is highly toxic and has distinct physiological effects, in particular on active transcription histone marks such as H3K9ac and H3K27ac. Furthermore, we reveal a synthetic lethal interaction between ARH3 and PARG and identify loss of ARH3 as a mechanism of PARP inhibitor resistance, both of which can be exploited in cancer therapy. Finally, we extend our findings to neurodegeneration, suggesting that patients with inherited ARH3 deficiency suffer from stress-induced pathogenic increase in PARylation that can be mitigated by PARP inhibition.ARH3/ADPRHL2与PARG是脊椎动物体内介导ADP核糖基化（ADP-ribosylation）逆转的核心酶类，但其在活体中的功能仍有待阐明。ARH3是唯一可清除丝氨酸连接型单（ADP-核糖）修饰（mono(ADP-ribose), MAR）的水解酶，但在体外环境中，其降解多（ADP-核糖）链（poly(ADP-ribose), PAR）的效率远低于PARG。本研究借助ARH3缺陷细胞展开实验，证实内源性MAR修饰会在包括有丝分裂（mitosis）在内的整个细胞周期中持续存在于染色质（chromatin）上，且意外地可被细胞良好耐受。与之相反，持续存在的PAR修饰具有高度细胞毒性，并会引发独特的生理效应，尤其会对H3K9ac、H3K27ac这类活性转录相关组蛋白修饰标记产生显著影响。此外，本研究揭示了ARH3与PARG之间的合成致死（synthetic lethal）相互作用，并确认ARH3缺失是PARP抑制剂（PARP inhibitor）耐药的潜在机制，上述发现均可应用于癌症治疗策略开发。最后，本研究将结论拓展至神经退行性疾病领域，提示遗传性ARH3缺陷患者会出现应激诱导的PAR修饰病理性升高，此类病症可通过PARP抑制剂治疗得到缓解。