Flexibility in the order of action and in the enzymology of the nuclease, polymerases, and ligase of vertebrate non-homologous DNA end joining： relevance to cancer, aging, and the immune system

Nonhomologous DNA end joining （NHEJ） is the primary pathway for repair of double-strand DNA breaks in human cells and in multicellular eukaryotes. The causes of double-strand breaks often fragment the DNA at the site of damage, resulting in the loss of information there. NHEJ does not restore the lost information and may resect additional nucleotides during the repair process. The ability to repair a wide range of overhang and damage configurations reflects the flexibility of the nuclease, polymerases, and ligase of NHEJ. The flexibility of the individual components also explains the large number of ways in which NHEJ can repair any given pair of DNA ends. The loss of information locally at sites of NHEJ repair may contribute to cancer and aging, but the action by NHEJ ensures that entire segments of chromosomes are not lost.

Establishment of genomic library technology mediated by non-homologous end joining mechanism in Yarrowia lipolytica

Genomic variants libraries are conducive to obtain dominant strains with desirable phenotypic traits.The non-homologous end joining(NHEJ),which enables foreign DNA fragments to be randomly integrated into different chromosomal sites,shows prominent capability in genomic libraries construction.In this study,we established an efficient NHEJ-mediated genomic library technology in Yarrowia lipolytica through regulation of NHEJ repair process,employment of defective Ura marker and optimization of iterative transformations,which enhanced genes integration efficiency by 4.67,22.74 and 1.87 times,respectively.We further applied this technology to create high lycopene producing strains by multi-integration of heterologous genes of CrtE,CrtB and CrtI,with 23.8 times higher production than rDNA integration through homologous recombination(HR).The NHEJ-mediated genomic library technology also achieved random and scattered integration of loxP and vox sites,with the copy number up to 65 and 53,respectively,creating potential for further application of recombinase mediated genome rearrangement in Y.lipolytica.This work provides a high-efficient NHEJ-mediated genomic library technology,which enables random and scattered genomic integration of multiple heterologous fragments and rapid generation of diverse strains with superior phenotypes within 96 h.This novel technology also lays an excellent foundation for the development of other genetic technologies in Y.lipolytica.

Functional non-homologous end joining patterns triggered by CRISPR/Cas9 in human cells

CRISPR/Cas9-mediated genome engineering technologies are now widely applied in various organisms,including mouse and human cells（Cong et al.,2013;Mali et al.,2013;Yang et al.,2013;Hsu et al.,2014）.The most widely used customized CRISPR/Cas9（Sp Cas9）is derived from Streptococcus pyogenes（Cong et al.,2013）.

The phage T4 DNA ligase mediates bacterial chromosome DSBs repair as single component non-homologous end joining

DNA double-strand breaks(DSBs)are one of the most lethal forms of DNA damage that is not efficiently repaired in prokaryotes.Certain microorganisms can handle chromosomal DSBs using the error-prone non-homologous end joining(NHEJ)system and ultimately cause genome mutagenesis.Here,we demonstrated that Enterobacteria phage T4 DNA ligase alone is capable of mediating in vivo chromosome DSBs repair in Escherichia coli.The ligation efficiency of DSBs with T4 DNA ligase is one order of magnitude higher than the NHEJ system from Mycobacterium tuberculosis.This process introduces chromosome DNA excision with different sizes,which can be manipulated by regulating the activity of host-exonuclease RecBCD.The DNA deletion length reduced either by inactivating recB or expressing the RecBCD inhibitor Gam protein fromλphage.Furthermore,we also found single nucleotide substitutions at the DNA junction,suggesting that T4 DNA ligase,as a single component non-homologous end joining system,has great potential in genome mutagenesis,genome reduction and genome editing.

DNA end binding activity and Ku70/80 heterodimer expression in human colorectal tumor

AIM： TO determine the DNA binding activity and protein levels of the Ku70/80 heterodimer, the functional mediator of the NHEJ activity, in human colorectal carcinogenesis. METHODS： The Ku70/80 DNA-binding activity was determined by electrophoretic mobility shift assays in 20 colon adenoma and 15 colorectal cancer samples as well as matched normal colonic tissues. Nuclear and cytoplasmic protein expression was determined by immunohistochemistry and Western blot analysis. RESULTS： A statistical found in both adenomas y significant difference was and carcinomas as compared to matched normal colonic mucosa （P〈0.00）. However, changes in binding activity were not homogenous with approximately 50% of the tumors showing a clear increase in the binding activity, 30% displaying a modest increase and 15% showing a decrease of the activity.Tumors, with increased DNA-binding activity, also showed a statistically significant increase in Ku70 and Ku86 nuclear expression, as determined by Western blot and immunohistochemical analyses （P〈0.001）. Cytoplasmic protein expression was found in pathological samples, but not in normal tissues either from tumor patients or from healthy subjects. CONCLUSION： Overall, our DNA-binding activity and protein level are consistent with a substantial activation of the NHEJ pathway in colorectal tumors. Since the NHEJ is an error prone mechanism, its abnormal activation can result in chromosomal instability and ultimately lead to tumorigenesis.

小分子化合物靶向DNA连接酶Ⅳ提高CRISPR/Cas9基因编辑效率的研究进展

基因编辑工具能够在DNA链上制造特定位点双链断裂(Double-strand breaks,DSB)。细胞内具有修复DSB的2种不同途径,即同源重组(HDR)和非同源末端连接(NHEJ)。NHEJ与HDR之间相互竞争,互为替代DNA修复途径。DNA连接酶Ⅳ参与NHEJ途径并发挥重要作用。DNA连接酶Ⅳ抑制剂可以抑制DNA修复通路中NHEJ的效率,同时可以提高HDR的效率。为了优选小分子化合物实现更有效的基因定点插入,综述了SCR7、NU7026、白藜芦醇、L755507这4种不同的小分子化合物在CRISPR/Cas9基因编辑效率上的研究,归纳了这4种小分子化合物在其中发挥的作用,并用生物信息学软件模拟所用小分子化合物与DNA连接酶Ⅳ的对接。在此基础上,对这4种小分子化合物提高基因编辑效率的研究前景进行了展望。

G_1和G_2期细胞对X射线引起双链DNA断裂修复的影响

目的:探讨X射线对G_1和G_2期细胞双链DNA断裂损伤修复的动力学影响。方法:采用3%多聚甲醛或70%乙醇固定受X射线放射细胞的M059J、M059K和Hela,用流式细胞仪分选G_1和G_2期Hela细胞,以脉冲场电泳分析双链DNA断裂修复动力学。结果:乙醇固定不影响X射线放射引起的双链DNA断裂修复动力学,可用于固定Hela细胞G_1和G_2期的分选,且G_2期细胞双链DNA断裂修复速率显著慢于G_1期细胞。结论:G_1和G_2期细胞中的双链DNA断裂可能采用不同的修复方式。G_1期细胞主要采用快速的末端连接修复方式,而G_2期细胞主要采用相对较慢的同源性重组修复方式。

组蛋白去乙酰化酶抑制剂对DNA双链断裂修复路径的作用

DNA双链断裂(double strand breaks,DSBs)对细胞生存是致命的.细胞内非同源末端连接(NHEJ)、重组修复(HDR)、单链退火修复(SSA)和微同源序列末端连接(MMEJ)等通路可竞争性修复DNA双链断裂损伤.在肿瘤细胞DNA中制造难以修复的基因损伤,诱导肿瘤细胞周期中止、坏死和凋亡是临床放、化疗的主要策略.组蛋白去乙酰化酶(histone deacetylase)作为抗肿瘤治疗的新靶标,其抑制剂(histone deacetylase inhibitors,HDACi)可显著降低肿瘤细胞DSBs修复能力,增强肿瘤细胞的放、化疗敏感性.研究显示,HDACi抑制了肿瘤细胞中具有正确修复倾向的HDR和经典NHEJ通路,具有错误修复倾向的SSA和MMEJ路径也可能牵涉其中.目前,HDACi作用于DSBs修复通路的分子机制已取得较大进展,但仍有许多问题有待阐明.

Ku基因在微生物中的研究进展

ku基因介导的非同源末端连接(NHEJ)途径是DNA双链断裂(DSBs)的一种修复机制,它不依赖于同源重组,且通过与之竞争而削弱同源重组。由于ku基因在生物进化过程中的高度保守性,其功能在很多微生物中已经得到研究,尤其在丝状真菌中,将ku基因敲除,在NHEJ途径缺陷的背景下,同源重组发挥主要作用,基因敲除的频率大为提高,从而方便了对基因功能的研究。

BRCA1与DSB修复路径取向

乳腺癌易感基因1(BRCA1)是一个肿瘤抑制基因.BRCA1参与DNA末端切除、细胞周期调控以及染色体修饰等来维护基因组的稳定性.有研究表明,它能够促进正确的DNA双链断裂(DSBs)修复,如同源重组修复(HDR)和经典的非同源末端连接(C-NHEJ);而抑制错误性的DSB修复,如单链退火修复(SSA)和非经典的末端连接(A-EJ);其机制是通过与某些DNA修复相关蛋白质的相互作用来引导DSB修复.目前,BRCA1在DSB修复通路中的作用机制尚未完全明确,仍有待进一步的研究.本文主要阐述BRCA1在DSB各修复通路中是如何发挥其引导作用的.

DNA Double-Strand Breaks,Potential Targets for HBV Integration

Hepatitis B virus(HBV)-induced hepatocellular carcinoma(HCC) is one of the most fre-quently occurring cancers.Hepadnaviral DNA integrations are considered to be essential agents which can promote the process of the hepatocarcinogenesis.More and more researches were designed to find the relationship of the two.In this study,we investigated whether HBV DNA integration occurred at sites of DNA double-strand breaks(DSBs),one of the most detrimental DNA damage.An 18-bp I-SceI homing endonuclease recognition site was introduced into the DNA of HepG2 cell line by stable DNA transfection,then cells were incubated in patients’ serum with high HBV DNA copies and at the same time,DSBs were induced by transient expression of I-SceI after transfection of an I-SceI expression vector.By using nest PCR,the viral DNA was detected at the sites of the break.It appeared that integra-tion occurred between part of HBV x gene and the I-SceI induced breaks.The results suggested that DSBs,as the DNA damages,may serve as potential targets for hepadnaviral DNA insertion and the integrants would lead to widespread host genome changes necessarily.It provided a new site to investi-gate the integration.

植物基因组编辑检测方法

以CRISPR/Cas9技术为代表的基因组编辑在生物领域的革命性应用使得生命科学研究迈入新篇章。该技术以其灵活性、易用性且扩展性强等优势,大大加快了基因工程研究,也加速了植物分子育种的步伐。但是,遗传转化过程中产生大量潜在的基因编辑植株,使得早期高通量快速筛选和检测目标编辑植株面临很大挑战。本文综述了近年来植物基因组编辑检测的各种方法,比较了其优缺点和适用范围;同时,还对近几年植物基因组编辑检测方法的发展趋势进行了深入分析和展望,以期对基因组编辑技术在植物中的应用提供参考。

Genome engineering using the CRISPR/Cas system

Recently, an epoch-making genome engineering technology using clustered regularly at interspaced short palindromic repeats(CRISPR) and CRISPR associated(Cas) nucleases, was developed. Previous technologies for genome manipulation require the time-consuming design and construction of genome-engineered nucleases for each target and have, therefore, not been widely used in mouse research where standard techniques based on homologous recombination are commonly used. The CRISPR/Cas system only requires the design of sequences complementary to a target locus, making this technology fast and straightforward. In addition, CRISPR/Cas can be used to generate mice carrying mutations in multiple genes in a single step, an achievement not possible using other methods. Here, we review the uses of this technology in genetic analysis and manipulation, including achievements made possible to date and the prospects for future therapeutic applications.

Frederick W.Alt received the 2015 Szent-Gyrgi Prize for Progress in Cancer Research

The Szent-Gyorgyi Prize for Progress in Cancer Research is a prestigious scientific award established by the National Foundation for Cancer Research(NFCR)—a leading cancer research charitable organization in the United States that is committed to supporting scientific research and public education relating to the prevention,early diagnosis,better treatments,and ultimately,a cure for cancer.Each year,the Szent-Gyorgyi Prize honors an outstanding researcher,nominated by colleagues or peers,who has contributed outstanding,significant research to the fight against cancer,and whose accomplishments have helped improve treatment options for cancer patients.The Prize also promotes public awareness of the importance of basic cancer research and encourages the sustained investment needed to accelerate the translation of these research discoveries into new cancer treatments.This report highlights the pioneering work led by the 2015 Prize winner,Dr.Frederick Alt.Dr.Alt's work in the area of cancer genetics over four decades has helped to shape the very roots of modern cancer research.His work continues to profoundly impact the approaches that doctors around the globe use to diagnose and treat cancer.In particular,his seminal discoveries of gene amplification and his pioneering work on molecular mechanisms of DNA damage repair have helped to usher in the era of genetically targeted therapy and personalized medicine.

New insights into tumor dormancy:Targeting DNA repair pathways

Over the past few decades, major strides have advanced the techniques for early detection and treatment of cancer. However, metastatic tumor growth still accounts for the majority of cancer-related deaths worldwide. In fact, breast cancers are notorious for relapsing years or decades after the initial clinical treatment, and this relapse can vary according to the type of breast cancer. In estrogen receptor-positive breast cancers, late tumor relapses frequently occur whereas relapses in estrogen receptor-negative cancers or triple negative tumors arise early resulting in a higher mortality risk. One of the main causes of metastasis is tumor dormancy in which cancer cells remain concealed, asymptomatic, and untraceable over a prolonged period of time. Under certain conditions, dormant cells can re-enter into the cell cycle and resume proliferation leading to recurrence. However, the molecular and cellular regulators underlying this transition remain poorly understood. To date, three mechanisms have been identified to trigger tumor dormancy including cellular, angiogenic, and immunologic dormancies. In addition, recent studies have suggested that DNA repair mechanisms may contribute to the survival of dormant cancer cells. In this article, we summarize the recent experimental and clinical evidence governing cancer dormancy. In addition, we will discuss the role of DNA repair mechanisms in promoting the survival of dormant cells. This information provides mechanistic insight to explain why recurrence occurs, and strategies that may enhance therapeutic approaches to prevent disease recurrence.

聚腺苷二磷酸-核糖聚合酶1与DNA双链断裂修复的相关性

DNA双链断裂(double strand breaks,DSBs)是细胞最严重的DNA损伤形式。细胞通过同源重组(homologous recombination,HR)和非同源末端连接(non-homologous end joining,NHEJ)途径修复DNA双链断裂损伤。聚腺苷二磷酸核糖基化(poly(ADP-ribosyl)ation,PARylation)是蛋白质翻译后修饰过程,这个过程由聚腺苷二磷酸-核糖聚合酶家族(poly(ADP-ribose)polymerases,PARPs)催化完成。PARP1作为PARPs家族最重要的成员,其在DNA损伤应答方面发挥重要作用。研究显示,PARP1在DSBs修复过程中发挥关键作用,参与DSBs的早期应答反应及其具体修复途径,可依据KU蛋白的存在与否发挥不同的特定作用。本文较全面地综述了PARP1在DNA双链断裂修复方面的潜在作用,将为临床疾病的诊治提供新的思路。

The Role of DNA Mismatch Repair and Recombination in the Processing of DNA Alkylating Damage in Living Yeast Cells

It is proposed that mismatch repair (MMR) mediates the cytotoxic effects of DNA damaging agents by exerting a futile repair pathway which leads to double strand breaks (DSBs). Previous reports indicate that the sensitivity of cells defective in homologous recombination (HR) to DNA alkylation is reduced by defects in MMR genes. We have assessed the contribution of different MMR genes to the processing of alkylation damage in vivo. We have directly visualized recombination complexes formed upon DNA damage using fluorescent protein (FP) fusions. We find that msh6 mutants are more resistant than wild type cells to MNNG, and that an msh6 mutation rescues the sensitivity of rad52 strains more efficiently than an msh3 mutation. Analysis of RAD52-GFP tagged strains indicate that MNNG increases repair foci formation, and that the inactivation of the MHS2 and MSH6 genes but not the MSH3 gene result in a reduction of the number of foci formed. In addition, in the absence of HR, NHEJ could process the MNNG-induced DSBs as indicated by the formation of NHEJ-GFP tagged foci. These data suggest that processing of the alkylation damage by MMR, mainly by MSH2-MSH6, is required for recruitment of recombination proteins to the damage site for repair.

Analysis of Up-Regulation of DNA-PKcs and Its Mechanism in Human Gliomas

OBJECTIVE To detect the differences in gene expression of nonhomologous end-joining pathway including Ku70, Ku80, ERCC4, lig4 and DNA-PKcs between human primary gliomas and normal brain tissues, and furthermore, to explore the underlying mechanism for the expression alteration.METHODS The expression levels of Ku70, Ku80, ERCC4, lig4 and DNA-PKcs in 36 specimens of glioma and 12 specimens of normal brain tissue were measured using SYBR green-based real- time quantitative PCR. Methylation of DNA-PKcs was detected through methylation-specific PCR （MSP）.RESULTS There was no significant difference in expression of Ku70, Ku80, ERCC4 and lig4 between human primary gliomas and normal brain tissues （P 〈 0.05）, while DNA-PKcs were significantly up-regulated （P = 0.002）. The expression of DNA- PKcs was significantly higher in patients with grade III and IV diseases compared to patients with grade II disease or in normal brain tissues （P 〈 0.05）. Moreover, glioma tissue showed weaker methvlation than normal brain tissue.CONCLUSION The up-regulation of the DNA-PKcs may be associated with pathogenesis of glioma. Demethylation of DNA- PKcs promoter is an important reason for its up-regulation.

Di-and tri-methylation of histone H3K36 play distinct roles in DNA double-strand break repair

Histone H3 Lys36(H3K36)methylation and its associated modifiers are crucial for DNA double-strand break(DSB)repair,but the mechanism governing whether and how different H3K36 methylation forms impact repair pathways is unclear.Here,we unveil the distinct roles of H3K36 dimethylation(H3K36me2)and H3K36 trimethylation(H3K36me3)in DSB repair via non-homologous end joining(NHEJ)or homologous recombination(HR).Yeast cells lacking H3K36me2 or H3K36me3 exhibit reduced NHEJ or HR efficiency.y Ku70 and Rfa1 bind H3K36me2-or H3K36me3-modified peptides and chromatin,respectively.Disrupting these interactions impairs y Ku70 and Rfa1 recruitment to damaged H3K36me2-or H3K36me3-rich loci,increasing DNA damage sensitivity and decreasing repair efficiency.Conversely,H3K36me2-enriched intergenic regions and H3K36me3-enriched gene bodies independently recruit y Ku70 or Rfa1 under DSB stress.Importantly,human KU70 and RPA1,the homologs of y Ku70 and Rfa1,exclusively associate with H3K36me2 and H3K36me3 in a conserved manner.These findings provide valuable insights into how H3K36me2 and H3K36me3 regulate distinct DSB repair pathways,highlighting H3K36 methylation as a critical element in the choice of DSB repair pathway.

cGAS guards against chromosome endto-end fusions during mitosis and facilitates replicative senescence

As a sensor of cytosolic DNA, the role of cyclic GMP-AMP synthase (cGAS) in innate immune response is well established, yet how its functions in different biological conditions remain to be elucidated. Here, we identify cGAS as an essential regulator in inhibiting mitotic DNA double-strand break (DSB) repair and protecting short telomeres from end-to-end fusion independent of the canonical cGAS-STING pathway. cGAS associates with telomeric/subtelomeric DNA during mitosis when TRF1/TRF2/POT1 are deficient on telomeres. Depletion of cGAS leads to mitotic chromosome end-to-end fusions predominantly occurring between short telomeres. Mechanistically, cGAS interacts with CDK1 and positions them to chromosome ends. Thus, CDK1 inhibits mitotic non-homologous end joining (NHEJ) by blocking the recruitment of RNF8. cGAS-deficient human primary cells are defective in entering replicative senescence and display chromosome end-to-end fusions, genome instability and prolonged growth arrest. Altogether, cGAS safeguards genome stability by controlling mitotic DSB repair to inhibit mitotic chromosome end-to-end fusions, thus facilitating replicative senescence.