SIRT2 orchestrates the DNA damage response

SIRT2 regulates a network of proteins in the DDR

Sirtuin 2 (SIRT2), is a member of the sirtuin family of NAD⁺ dependent deacetylases, which are implicated in diverse biological processes, including metabolism, aging, genome maintenance, and tumor suppression. SIRT2 is critical for tumor suppression and genome maintenance. Sirt2 knockout mice develop breast, liver, and other cancers, and SIRT2 expression is reduced in human breast and liver cancers.¹ These cancers also express genome instability, suggesting that SIRT2's role in tumor suppression may at least in part be due to its functions in protecting against DNA damage. The DNA damage response (DDR) is a signaling network, which recognizes DNA lesions and mobilizes cellular events to maintain genome integrity. Thus, the DDR is a critical barrier against genomic instability and carcinogenesis. Recent studies have demonstrated SIRT2's crucial function in the DDR through protein deacetylation. SIRT2 regulates anaphase promoting complex/cyclosome (APC/C) activity during mitosis by deacetylating APC/C co-activators CDH1 and CDC20.¹ SIRT2 also deacetylates histone H3 on lysine 56, a signature chromatin mark for packaging DNA into chromatin upon DNA replication and repair.² Likewise, we have found that SIRT2 deacetylates CDK9 on lysine 48, thereby promoting recovery from replication stress.³ Most recently, we found that SIRT2 deacetylates ATRIP at lysine 32, which drives ATR activation by facilitating binding to RPA-coated single stranded-DNA in response to replication stress.⁴ The regulation of CDK9 and ATRIP-ATR by SIRT2 deacetylation is unlikely redundant since ATR depletion impairs hydroxyurea (HU) regulated deacetylation of CDK9, suggesting that ATR regulation is epistatic to that of CDK9.³ Overall, our study revealed that SIRT2 deacetylase activity is essential for the ATR checkpoint pathway and helps explain how loss of Sirt2 results in compromised genomic stability and tumorigenesis. Our findings also suggest that SIRT2 is capable of deacetylating multiple key components within the ATR signaling pathway, perhaps to amplify or fine-tune the signal. Our studies, together with others, implicate SIRT2 in controlling various aspects and levels of the DDR, suggesting that SIRT2 functions as a master regulator of the DDR. To fully characterize the comprehensive landscape of the SIRT2 acetylome in the DDR is thus of great interest for elucidating the mechanism by which SIRT2 promotes genome integrity and prevents cancer.

Regulation of SIRT2 by DNA damage

Genome maintenance proteins are often regulated by DNA damage. Consistently, SIRT2 function has been shown to be regulated by posttranslational modification (PTM). SIRT2 is phosphorylated by CDK1 and dephosphorylated by CDC14A/CDC14B on serine 368.⁵ In addition, SIRT2 is acetylated by P300, which results in reduced SIRT2 deacetylase activity.⁶ Moreover, SIRT2 may also be regulated through oligomerization as SIRT2 has been purified as a homotrimer.⁷ These studies imply that SIRT2 is extensively modified after translation, and most PTMs identified to date seem to influence SIRT2 deacetylase activity. It is not clear whether these PTMs are regulated in response to DNA damage, and if so, what their contributions are to SIRT2's function in the DDR. We have found an increase in SIRT2 phosphorylation following HU treatment (unpublished data). Additionally, our mass spectrometry analysis of purified SIRT2 also identified a number of potential upstream regulators, including several kinases. We also found that SIRT2 can form oligomers, and that oligomerization is likely to be regulated by DNA damage (unpublished data). Meanwhile, in vitro deacetylation assays utilizing SIRT2 purified from cells treated with HU, demonstrated increased deacetylation of purified ATRIP as a substrate, suggesting that SIRT2 enzymatic activity is upregulated in response to replication stress.⁴ Further investigation is needed to determine the precise mechanism by which SIRT2 is regulated in the DDR.

Acetylome as a key regulator of the DDR

Protein phosphorylation cascades were widely believed to be the canonical manner for DDR regulation. However, increasing evidence suggest that other PTMs, such as methylation, ubiquitylation, sumoylation and acetylation, also play critical roles in most biological processes. Acetylation, for example, has increasingly been recognized as a major PTM as quantitative, high-throughput, high-resolution mass spectrum acetyl peptide detection has become widely available with advances in proteomics technology. The new paradigm is that acetylation is a ubiquitous phenomenon. Numerous proteins, including those in the DDR, have been found to be acetylated. It is easy to hypothesize that acetylation/deacetylation constitutes another layer of control, coordinating with other PTMs such as phosphorylation to collectively achieve more sophisticated and precise regulation of the DDR. Proteome-wide quantitative acetylation detection will be of great value to decipher these sophisticated levels of regulation. Indeed, quantitative acetylome analyses for sirtuins as a whole, and individual sirtuins, such as SIRT1 and SIRT3 are now widely available. However, the investigation of the SIRT2 acetylome remains incomplete. Conducting proteome-wide analyses of lysine acetylation in a SIRT2 loss-of-function model, particularly in the context of DNA damage, will provide a comprehensive depiction of the SIRT2-dependent DDR acetylome. Furthermore, elucidating the global landscape of the SIRT2 acetylome under different DNA damage conditions may provide insights into how to exploit DDR protein deacetylation by SIRT2 as novel biomarkers or therapeutic targets for cancer treatment.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Figure 1.

Schematic map depicts the role of SIRT2 in DNA Damage Response. SIRT2 regulates a network of DDR proteins such as APC^CDH1/CDC20, H3K56, CDK9 and ATRIP through deacetylation to maintain genome integrity and prevent tumorigenesis. SIRT2 is regulated through PTMs such as phosphorylation by CDKs and acetylation by P300 and probably oligomerization, which are speculated to affect SIRT2's activity in the DDR.