Co-ordination of base excision repair and genome stability

Base excision repair (BER) is a major DNA repair pathway employed in mammalian cells that is required to maintain genome stability, thus preventing several human diseases, such as ageing, neurodegenerative diseases and cancer. This is achieved through the repair of damaged DNA bases, sites of base loss and single strand breaks of varying complexity that are continuously induced endogenously or via exogenous mutagens. Whilst the enzymes involved in BER are now well known and characterised, the role of the co-ordination of BER enzymatic activities in the cellular response to DNA damage and the mechanisms regulating this process are only now being revealed. Post-translational modifications of BER proteins, including ubiquitylation and phosphorylation, are increasingly being identified as key processes that regulate BER. In this review we will summarise recent evidence discovering novel mechanisms that are involved in maintaining genome stability by regulation of the key BER proteins in response to DNA damage.     

Genomic instability and aging-like phenotype in the absence of mammalian SIRT6

The Sir2 histone deacetylase functions as a chromatin silencer to regulate recombination, genomic stability, and aging in budding yeast. Seven mammalian Sir2 homologs have been identified (SIRT1-SIRT7), and it has been speculated that some may have similar functions to Sir2. Here, we demonstrate that SIRT6 is a nuclear, chromatin-associated protein that promotes resistance to DNA damage and suppresses genomic instability in mouse cells, in association with a role in base excision repair (BER). SIRT6-deficient mice are small and at 2-3 weeks of age develop abnormalities that include profound lymphopenia, loss of subcutaneous fat, lordokyphosis, and severe metabolic defects, eventually dying at about 4 weeks. We conclude that one function of SIRT6 is to promote normal DNA repair, and that SIRT6 loss leads to abnormalities in mice that overlap with aging-associated degenerative processes.     

The spliceosome-associated protein Nrl1 suppresses homologous recombination-dependent R-loop formation in fission yeast

The formation of RNA–DNA hybrids, referred to as R-loops, can promote genome instability and cancer development. Yet the mechanisms by which R-loops compromise genome instability are poorly understood. Here, we establish roles for the evolutionarily conserved Nrl1 protein in pre-mRNA splicing regulation, R-loop suppression and in maintaining genome stability. nrl1Δ mutants exhibit endogenous DNA damage, are sensitive to exogenous DNA damage, and have defects in homologous recombination (HR) repair. Concomitantly, nrl1Δ cells display significant changes in gene expression, similar to those induced by DNA damage in wild-type cells. Further, we find that nrl1Δ cells accumulate high levels of R-loops, which co-localize with HR repair factors and require Rad51 and Rad52 for their formation. Together, our findings support a model in which R-loop accumulation and subsequent DNA damage sequesters HR factors, thereby compromising HR repair at endogenously or exogenously induced DNA damage sites, leading to genome instability.     

SIRT1 Deacetylates TopBP1 and Modulates Intra-S-Phase Checkpoint and DNA Replication Origin Firing

SIRT1, the mammalian homolog of yeast Sir2, is a founding member of a family of 7 protein and histone deacetylases that are involved in numerous biological functions. Previous studies revealed that SIRT1 deficiency results in genome instability, which eventually leads to cancer formation, yet the underlying mechanism is unclear. To investigate this, we conducted a proteomics study and found that SIRT1 interacted with many proteins involved in replication fork protection and origin firing. We demonstrated that loss of SIRT1 resulted in increased replication origin firing, asymmetric fork progression, defective intra-S-phase checkpoint, and chromosome damage. Mechanistically, SIRT1 deacetylates and affects the activity of TopBP1, which plays an essential role in DNA replication fork protection and replication origin firing. Our study demonstrated that ectopic over-expression of the deacetylated form of TopBP1 in SIRT1 mutant cells repressed replication origin firing, while the acetylated form of TopBP1 lost this function. Thus, SIRT1 acts upstream of TopBP1 and plays an essential role in maintaining genome stability by modulating DNA replication fork initiation and the intra-S-phase cell cycle checkpoint.     

Deciphering the BRCA1 Tumor Suppressor Network

The BRCA1 tumor suppressor protein is a central constituent of several distinct macromolecular protein complexes that execute homology-directed DNA damage repair and cell cycle checkpoints. Recent years have borne witness to an exciting phase of discovery at the basic molecular level for how this network of DNA repair proteins acts to maintain genome stability and suppress cancer. The clinical dividends of this investment are now being realized with the approval of first-in-class BRCA-targeted therapies for ovarian cancer and identification of molecular events that determine responsiveness to these agents. Further delineation of the basic science underlying BRCA network function holds promise to maximally exploit genome instability for hereditary and sporadic cancer therapy.     

The Budding Yeast Ubiquitin Protease Ubp7 Is a Novel Component Involved in S Phase Progression

DNA damage must be repaired in an accurate and timely fashion to preserve genome stability. Cellular mechanisms preventing genome instability are crucial to human health because genome instability is considered a hallmark of cancer. Collectively referred to as the DNA damage response, conserved pathways ensure proper DNA damage recognition and repair. The function of numerous DNA damage response components is fine-tuned by posttranslational modifications, including ubiquitination. This not only involves the enzyme cascade responsible for conjugating ubiquitin to substrates but also requires enzymes that mediate directed removal of ubiquitin. Deubiquitinases remove ubiquitin from substrates to prevent degradation or to mediate signaling functions. The Saccharomyces cerevisiae deubiquitinase Ubp7 has been characterized previously as an endocytic factor. However, here we identify Ubp7 as a novel factor affecting S phase progression after hydroxyurea treatment and demonstrate an evolutionary and genetic interaction of Ubp7 with DNA damage repair pathways of homologous recombination and nucleotide excision repair. We find that deletion of UBP7 sensitizes cells to hydroxyurea and cisplatin and demonstrate that factors that stabilize replication forks are critical under these conditions. Furthermore, ubp7Δ cells exhibit an S phase progression defect upon checkpoint activation by hydroxyurea treatment. ubp7Δ mutants are epistatic to factors involved in histone maintenance and modification, and we find that a subset of Ubp7 is chromatin-associated. In summary, our results suggest that Ubp7 contributes to S phase progression by affecting the chromatin state at replication forks, and we propose histone H2B ubiquitination as a potential substrate of Ubp7.     

New players in the BRCA1-mediated DNA damage responsive pathway

DNA damage checkpoint is an important self-defense mechanism for the maintenance of genome stability. Defects in DNA damage signaling and repair lead to various disorders and increase tumor incidence in humans. In the past 10 years, we have identified many components involved in the DNA damage-signaling pathway, including the product of breast cancer susceptibility gene 1 (BRCA1). Mutations in BRCA1 are associated with increased risk of breast and ovarian cancers, highlighting the importance of this DNA damage-signaling pathway in tumor suppression. While it becomes clear that BRCA1 plays a crucial role in the DNA damage responsive pathway, exactly how BRCA1 receives DNA damage signals and exerts its checkpoint function has not been fully addressed. A series of recent studies reported the discovery of many novel components involved in DNA damage-signaling pathway. These newly identified checkpoint proteins, including RNF8, RAP80 and CCDC98, work in concern in recruiting BRCA1 to DNA damage sites and thus regulate BRCA1 function in G2/M checkpoint control. This review will summarize these recent findings and provide an updated view of the regulation of BRCA1 in response to DNA damage.     

The checkpoint response to replication stress

Genome instability is a hallmark of cancer cells, and defective DNA replication, repair and recombination have been linked to its etiology. Increasing evidence suggests that proteins influencing S-phase processes such as replication fork movement and stability, repair events and replication completion, have significant roles in maintaining genome stability. DNA damage and replication stress activate a signal transduction cascade, often referred to as the checkpoint response. A central goal of the replication checkpoint is to maintain the integrity of the replication forks while facilitating replication completion and DNA repair and coordinating these events with cell cycle transitions. Progression through the cell cycle in spite of defective or incomplete DNA synthesis or unrepaired DNA lesions may result in broken chromosomes, genome aberrations, and an accumulation of mutations. In this review we discuss the multiple roles of the replication checkpoint during replication and in response to replication stress, as well as the enzymatic activities that cooperate with the checkpoint pathway to promote fork resumption and repair of DNA lesions thereby contributing to genome integrity.     

The role of post-translational modifications in fine-tuning BLM helicase function during DNA repair

RecQ-like helicases are a highly conserved family of proteins which are critical for preserving genome integrity. Genome instability is considered a hallmark of cancer and mutations within three of the five human RECQ genes cause hereditary syndromes that are associated with cancer predisposition. The human RecQ-like helicase BLM has a central role in DNA damage signaling, repair, replication, and telomere maintenance. BLM and its budding yeast orthologue Sgs1 unwind double-stranded DNA intermediates. Intriguingly, BLM functions in both a pro- and anti-recombinogenic manner upon replicative damage, acting on similar substrates. Thus, BLM activity must be intricately controlled to prevent illegitimate recombination events that could have detrimental effects on genome integrity. In recent years it has become evident that post-translational modifications (PTMs) of BLM allow a fine-tuning of its function. To date, BLM phosphorylation, ubiquitination, and SUMOylation have been identified, in turn regulating its subcellular localization, protein–protein interactions, and protein stability. In this review, we will discuss the cellular context of when and how these different modifications of BLM occur. We will reflect on the current model of how PTMs control BLM function during DNA damage repair and compare this to what is known about post-translational regulation of the budding yeast orthologue Sgs1. Finally, we will provide an outlook toward future research, in particular to dissect the cross-talk between the individual PTMs on BLM.     

Inhibition of SIRT6 in prostate cancer reduces cell viability and increases sensitivity to chemotherapeutics

SI RT6 is an important histone modifying protein that regulates DNA repair, telomere maintenance, energy metabolism, and target gene expression. Recently SIRT6 has been identifi ed as a tumor suppressor and is downregulated in certain cancer types, but not in other cancers. From deposited gene profi ling studies we found that SIRT6 was overexpressed in prostate tumors, compared with normal or paratumor prostate tissues. Tissue microarray studies confi rmed the higher levels of SIRT6 in both prostate tumor tissues and prostate cancer cells than in their normal counterparts. Knockdown of SIRT6 in human prostate cancer cells led to sub-G1 phase arrest of cell cycle, increased apoptosis, elevated DNA damage level and decrease in BCL2 gene expression. Moreover, SIRT6-deficiency reduced cell viability and enhanced chemotherapeutics sensitivity. Taken together, this study provides the fi rst evidence of SIRT6 overexpression in human prostate cancer, and SIRT6 regulation could be exploited for prostate cancer therapy.     

DNA Damage Checkpoints and Cancer

DNA damage checkpoint is one of the surveillance systems to maintain genomic integrity. Checkpoint systems sense the DNA damage and execute cell cycle arrest through inhibiting the activity of cell cycle regulators. This pathway is essential for the maintenance of genome stability and prevention of tumor development. Recent studies have showed that the cellular responses towards DNA damage, such as cell cycle arrest, DNA repair, chromatin remodeling, and apoptosis are well coordinated. Here we describe the molecular mechanisms of checkpoint activation in response to DNA damage and the correlation between checkpoint gene mutation and genomic instability.     

去乙酰化转移酶SIRT7的作用及机制研究进展

SIRT7是哺乳动物Sirtuins家族中的一员,定位于核仁,是一种高度特异性的H3K18Ac(组蛋白H3的乙酰化18位赖氨酸残基)去乙酰化酶。近年来的研究发现SIRT7可通过多种途径参与调控核糖体RNA转录、细胞代谢、细胞应激以及DNA损伤修复等生理过程。此外,SIRT7还与衰老、心脏疾病及脂肪肝等密切相关。特别是SIRT7在多种肿瘤如肝癌、胃癌、乳腺癌、膀胱癌、结直肠癌、胰腺癌和头颈鳞状细胞癌等发生发展中起着重要的调节作用。文中综述了SIRT7的细胞及分子生物学作用,并系统总结了其在人类疾病中的研究现状。     

Trending topics of SIRT1 in tumorigenicity

BackgroundCarcinogenesis is governed by a series of genetic alterations and epigenetic changes that lead to aberrant patterns in neoplastic cells. Sirtuin-1(SIRT1), an NAD⁺-dependent protein deacetylase, is capable of deacetylating histones and non-histone substrates that regulate various physiological activities during tumorigenesis. Recent studies have identified the role of SIRT1 in different stages of cancer, including genome instability, tumor initiation, proliferation, metabolism, and therapeutic response. However, the action of SIRT1 has been reported to be both oncogenic and tumor suppressive during carcinogenesis. Consequently, the biological functions of SIRT1 in cancer remain controversial.Scope of reviewWe highlight the most recent findings on SIRT1 in different stages of tumorigenesis, and update the current status of SIRT1 small molecule modulators in clinical application of cancer treatment.Major conclusionBy targeting both tumor suppressors and oncogenic proteins, SIRT1 has a bifunctional role at different stages of tumorigenesis. The impact of SIRT1 on tumorigenesis is also distinct at different stages and is dependent on its dosages. SIRT1 suppresses tumor initiation through its functions in promoting DNA repair, increasing genome stability, and inhibiting inflammation at the pre-cancer stage. However, SIRT1 enhances tumor proliferation, survival, and drug resistance through its roles in anti-apoptosis, pro-tumor metabolism, and anti-inflammation (inhibition of anti-tumor immunity) at the stages of tumor progression, metastasis, and relapse. Consequently, both SIRT1 inhibitors and activators have been explored for cancer treatment.General significanceBetter understanding the dose- and stage-dependent roles of SIRT1 in each cancer type can provide new avenues of exploration for therapy development.