The lifespan regulator protein SIRT6 is an NAD + -dependent deacetylase that plays an early role in cellular responses to DNA damage. In a recent study from Liu, Xu and team (2020), the authors describe a novel lysine acetylation/deacetylation switch on SIRT6 that modulates its ability to polymerize and sense DNA breaks. They also identified SIRT1 as the major deacetylase that regulates this lysine modification, unveiling a novel SIRT1-SIRT6 axis that drives the DNA damage response.
A DNA double-strand break (DSB) is considered the most critical DNA lesion because it causes cell death and severe mutations if it is not repaired or repaired incorrectly. Accumulating evidence has shown that the majority of DSBs are repaired by DNA non-homologous end joining (NHEJ), the first utilized repair pathway in human cells. In contrast, the repair pathway is sometimes diverted into using homologous recombination (HR), which has increased precision under specific circumstances: e.g., when DSBs are generated at transcriptionally active loci or are not readily repaired due to the complexity of damage at the DSB ends or due to highly compacted chromatin. DSB end resection (resection) is considered the most critical turning point for directing repair towards HR. After resection, the HR process is finalized by RAD51 loading and recombination. Thus, understanding the process of resection is critically important to understand the regulation of the choice of DSB repair pathway. In addition, resection is also an important factor influencing DNA damage signaling because unresected ends preferentially activate ATM, whereas longer resected ends activate ATR. Thus, DSB end resection is a key relay point that determines the repair pathway and the signal balance. In this review, we summarize the mechanism underlying DSB end resection and further discuss how it is involved in cancer therapy.
DNA damage occurs frequently resulting from both exogenous and endogenous factors, which induce a series of downstream responses including autophagy and DNA damage repair. In the past few years, increasing evidence has indicated that the interplay between autophagy and DNA damage repair is essential for maintaining genome stability as well as cellular homeostasis, and have significant effects on cell fate. On one hand, autophagy is induced during the process of DNA damage repair, and can act as an upstream factor of DNA damage repair as well. On the other hand, autophagy plays a rather dual role in regulating DNA damage repair, as mild and repairable DNA damage repair can be restored with facilitation of autophagy, hyperactivation of autophagy can be cytotoxic and have negative impact on DNA damage repair. In this article, we review current understandings about the cross talk between autophagy and DNA damage repair, with particular attention to their significance to genome integrity and effects on cell fate.
Homologous recombination (HR) repairs double-strand breaks (DSBs) occurring in sister chromatids using the intact sisters as the repair template. HR is initiated by DSB resection, which generates 3′ single-strand DNA (ssDNA). RAD51 recombinase polymerizes on the ssDNA and undergoes strand exchange with intact sister chromatids, generating junction molecules (JMs). The separation of JMs completes HR-dependent DSB repair. Defective resolution of JMs not only leaves DSBs unrepaired but also has the broken sisters remain entangled with the intact sisters, leading to the formation of isochromatid-type breaks, where both sister chromatids are broken at the same sites, in mitotic chromosome spreads. The MRE11 nuclease plays a key role in HR, and it is generally believed that MRE11 does so by initiating DSB resection. We here showed that the loss of MRE11 reduced the efficiency of HR in human TK6 cells without affecting DSB resection, indicating a role for MRE11 in HR also at a post-resection step. MRE11-deficient TK6 cells showed proficient induction of RAD51 foci by ionizing-radiation (IR) and olaparib but significantly delayed their resolution. Although exposure of G2-phase cells to IR cleaves only one of two sister chromatids, the loss of the MRE11-nuclease activity increased the number of isochromosome-type breaks in subsequent M phase. The overexpression of GEN1 resolvase suppressed the formation of IR-induced isochromatid-type breaks in MRE11-nuclease-deficient TK6 cells. These data indicate that MRE11 plays an important role in HR by processing JMs. We propose the dual roles of MRE11 in HR at DSB resection and post-resection steps.
The Ataxia Telangiectasia Mutated (ATM) protein kinase is mutated in several human cancers, presenting potential opportunities for targeted cancer therapy. We previously reported that the poly-ADP-ribose polymerase (PARP) inhibitor olaparib induces transient G2 arrest but not cell death in ATM-deficient lung cancer cells, while the combination of olaparib with the ATM- and Rad3-related (ATR) inhibitor VE-821 induced cell death. Here, we show that combination of olaparib plus the clinically relevant ATR inhibitor AZD6738 also induces cell death in ATM-deficient lung, prostate and pancreatic cancer cells with little effect on their ATM-proficient counterparts. Together, our data suggest that lung, prostate and pancreatic patients whose tumours exhibit loss or inactivation of ATM may benefit from combination of a PARP inhibitor plus an ATR inhibitor.