Thongsroy Jirapan, Patchsung Maturada, Pongpanich Monnat, Settayanon Sirapat, Mutirangura Apiwat
School of Medicine, Walailak University, Nakhon Si Thammarat, Thailand.
Center for Excellence in Molecular Genetics of Cancer and Human Diseases, Chulalongkorn University, Bangkok, Thailand.
FASEB J. 2018 May 29:fj201800218RR. doi: 10.1096/fj.201800218RR.
The mechanism that causes genomic instability in nondividing aging cells is unknown. Our previous study of mutant yeast suggested that 2 types of replication-independent endogenous DNA double-strand breaks (RIND-EDSBs) exist and that they play opposing roles. The first type, known as physiologic RIND-EDSBs, were ubiquitous in the G phase of both yeast and human cells in certain genomic locations and may act as epigenetic markers. Low RIND-EDSB levels were found in mutants that lacked chromatin-condensing proteins, such as the high-mobility group box (HMGB) proteins and Sir2. The second type is referred to as pathologic RIND-EDSBs. High pathological RIND-EDSB levels were found in DSB repair mutants. Under normal physiologic conditions, these excess RIND-EDSBs are repaired in much the same way as DNA lesions. Here, chronological aging in yeast reduced physiological RIND-EDSBs and cell viability. A strong correlation was observed between the reduction in RIND-EDSBs and viability in aging yeast cells ( r = 0.94, P < 0.0001). We used galactose-inducible HO endonuclease (HO) and nhp6a∆, an HMGB protein mutant, to evaluate the consequences of reduced physiological RIND-EDSB levels. The HO-induced cells exhibited a sustained reduction in RIND-EDSBs at various levels for several days. Interestingly, we found that lower physiologic RIND-EDSB levels resulted in decreased cell viability ( r = 0.69, P < 0.0001). Treatment with caffeine, a DSB repair inhibitor, increased pathological RIND-EDSBs, which were distinguished from physiologic RIND-EDSBs by their lack of sequences prior to DSB in untreated cells [odds ratio (OR) ≤1]. Caffeine treatment in both the HO-induced and nhp6a∆ cells markedly increased OR ≤1 breaks. Therefore, physiological RIND-EDSBs play an epigenetic role in preventing pathological RIND-EDSBs, a type of DNA damage. In summary, the reduction of physiological RIND-EDSB level is a genomic instability mechanism in chronologically aging cells.-Thongsroy, J., Patchsung, M., Pongpanich, M., Settayanon, S., Mutirangura, A. Reduction in replication-independent endogenous DNA double-strand breaks promotes genomic instability during chronological aging in yeast.
导致非分裂衰老细胞基因组不稳定的机制尚不清楚。我们之前对突变酵母的研究表明，存在两种类型的复制非依赖性内源性DNA双链断裂（RIND-EDSBs），且它们发挥着相反的作用。第一种类型，称为生理性RIND-EDSBs，在酵母和人类细胞的G期某些基因组位置普遍存在，可能作为表观遗传标记。在缺乏染色质凝聚蛋白的突变体中发现低水平的RIND-EDSBs，如高迁移率族蛋白盒（HMGB）蛋白和Sir2。第二种类型被称为病理性RIND-EDSBs。在DSB修复突变体中发现高水平的病理性RIND-EDSBs。在正常生理条件下，这些过量的RIND-EDSBs与DNA损伤的修复方式大致相同。在这里，酵母的时序性衰老降低了生理性RIND-EDSBs和细胞活力。在衰老酵母细胞中，观察到RIND-EDSBs的减少与活力之间存在很强的相关性（r = 0.94，P < 0.0001）。我们使用半乳糖诱导型HO内切酶（HO）和nhp6a∆（一种HMGB蛋白突变体）来评估生理性RIND-EDSBs水平降低的后果。HO诱导的细胞在几天内不同水平的RIND-EDSBs持续减少。有趣的是，我们发现较低的生理性RIND-EDSBs水平导致细胞活力下降（r = 0.69，P < 0.0001）。用咖啡因（一种DSB修复抑制剂）处理增加了病理性RIND-EDSBs，在未处理细胞中，它们与生理性RIND-EDSBs的区别在于DSB之前缺乏序列[优势比（OR）≤1]。在HO诱导细胞和nhp6a∆细胞中进行咖啡因处理均显著增加了OR≤1的断裂。因此，生理性RIND-EDSBs在预防病理性RIND-EDSBs（一种DNA损伤类型）方面发挥着表观遗传作用。总之，生理性RIND-EDSBs水平的降低是时序性衰老细胞中的一种基因组不稳定机制。-通斯罗伊，J.，帕奇松，M.，蓬帕尼奇，M.，塞塔亚农，S.，穆蒂兰古拉，A.复制非依赖性内源性DNA双链断裂的减少促进酵母时序性衰老过程中的基因组不稳定。
