The DNA damage repair complex MoMMS21-MoSMC5 is required for infection-related development and pathogenicity of Magnaporthe oryzae

The conserved DNA damage repair complex,MMS21-SMC5/6(Methyl methane sulfonate 21-Structural maintenance of chromosomes 5/6),has been extensively studied in yeast,animals,and plants.However,its role in phytopathogenic fungi,particularly in the highly destructive rice blast fungus Magnaporthe oryzae,remains unknown.In this study,we functionally characterized the homologues of this complex,MoMMS21 and MoSMC5,in M.oryzae.We first demonstrated the importance of DNA damage repair in M.oryzae by showing that the DNA damage inducer phleomycin inhibited vegetative growth,infection-related development and pathogenicity in this fungus.Additionally,we discovered that MoMMS21 and MoSMC5 interacted in the nuclei,suggesting that they also function as a complex in M.oryzae.Gene deletion experiments revealed that both MoMMS21 and MoSMC5 are required for infection-related development and pathogenicity in M.oryzae,while only MoMMS21 deletion affected growth and sensitivity to phleomycin,indicating its specific involvement in DNA damage repair.Overall,our results provide insights into the roles of MoMMS21 and MoSMC5 in M.oryzae,highlighting their functions beyond DNA damage repair.

LncRNA HOTAIR promotes DNA damage repair and radioresistance by targeting ATR in colorectal cancer

Long non-coding RNAs(lncRNAs)have been implicated in cancer progression and drug resistance development.Moreover,there is evidence that lncRNA HOX transcript antisense intergenic RNA(HOTAIR)is involved in colorectal cancer(CRC)progression.The present study aimed to examine the functional role of lncRNA HOTAIR in conferring radiotherapy resistance in CRC cells,as well as the underlying mechanism.The relative expression levels of HOTAIR were examined in 70 pairs of CRC tumor and para-cancerous tissues,as well as in radiosensitive and radioresistant samples.The correlations between HOTAIR expression levels and clinical features of patients with CRC were assessed using the Chi-square test.Functional assays such as cell proliferation,colony formation and apoptosis assays were conducted to determine the radiosensitivity in CRC cells with HOTAIR silencing after treatment with different doses of radiation.RNA pull-down assay andfluorescence in situ hybridization(FISH)were used to determine the interaction between HOTAIR and DNA damage response mediator ataxia-telangiectasia mutated-and Rad3-related(ATR).HOTAIR was significantly upregulated in CRC tumor tissues,especially in radioresistant tumor samples.The elevated expression of HOTAIR was correlated with more advanced histological grades,distance metastasis and the poor prognosis in patients with CRC.Silencing HOTAIR suppressed the proliferation and promoted apoptosis and radiosensitivity in CRC cells.HOTAIR knockdown also inhibited the tumorigenesis of CRC cells and enhanced the sensitivity to radiotherapy in a mouse xenograft model.Moreover,the data showed that HOTAIR could interact with ATR to regulate the DNA damage repair signaling pathway.Silencing HOTAIR impaired the ATR-ATR interacting protein(ATRIP)complex and signaling in cell cycle progression.Collectively,the present results indicate that lncRNA HOTAIR facilitates the DNA damage response pathway and promotes radioresistance in CRC cells by targeting ATR.

In Vivo Improvements in Facial Appearance and in Vitro Changes in Gene Expression Using a Topical Formulation Designed to Repair Environmentally Induced DNA Damage

Background: While sunscreen has been accepted as a mainline defence against photodamage from ultraviolet, visible light and near-infrared radiation, there appears to be a lack of research into photorepair. The concept of protecting the skin during the day and repairing cellular damage at night is intuitive, yet specific strategies revolving around combinations of proven reparative active ingredients remain unelucidated. Purpose: To investigate the efficacy of a solar repair Formulation following ultraviolet and environmental exposure in order to improve overall skin health and appearance through three hypotheses: The Formulation increases expression of DNA repair mechanisms markers;The Formulation enhances overall skin appearance through reducing signs of inflammation, elevating hydration, reinforcing skin firmness and amplifying radiance;In-Vivo efficacy test results are aligned with measured gene expression changes. Methods: The Formulation (#6NIC1.V1.1-1) was tested for: In-vitro LDH cytotoxicity activity, In-vitro qPCR gene expression with and without ultraviolet exposure on a reconstructed 3-dimensional skin model, and In-Vivo efficacy study on a panel of 22 participants objectively and subjectively. Results: Skin radiance, firmness, hydration, redness, and inflammation are significantly improved after In-Vivo skin exposure to the Formulation and environmental challenges such as ultraviolet radiation. These outcomes were confirmed by in-vitro genetic testing on a reconstructed human skin model. Conclusion: The studies allowed us to identify and group results in four main skin functions that were significantly enhanced following the application of the Formulation: firmness, hydration, radiance and soothing.

DNA Damage and Repair of Two Ecotypes of Phragmites communis Subjected to Water Stress

In order to thoroughly understand the mechanism Of drought resistance in plants at DNA level, the DNA damage of two ecotypes of reeds (Phragmites communis T.) stressed by PEG 6000 was analyzed by means of fluorescence analysis of DNA unwinding (FADU). The results showed that the residual double strand DNA percentages (dsDNA%) in dune reed (DR) were significantly higher than those in swamp reed (SR) treated with either 20% or 30% PEG 6000. This meant that the DNA of DR was less damaged in comparison with SR. Similarly, DR resisted DNA damage more strongly than SR as reactive oxygen species (ROS) increased by adding ROS producers diethyldithio carbamate (DDC), H2O2 and Fe2+ of different concentrations. Meanwhile, treating PEG stressed SR with ROS scavengers such as dimethyl sulphoxide (DMSO) and ascorbic acid (Vc) resulted in the reduction of DNA damage, suggesting that ROS could cause DNA damage. In addition, the DNA repair for water-stressed reeds indicated that DR repaired DNA damage much faster and more completely. This might be the first indication that drought stress led to DNA damage in plants and that drought resistance of plants was closely related to DNA damage and repair.

DNA损伤修复相关通路的合成致死靶点研究及其在卵巢癌中的应用和前景

DNA损伤引发细胞启动一系列DNA损伤应答(DNA damage response,DDR),包括DNA损伤修复、细胞周期检查点激活、细胞周期阻滞、各种细胞内信号转导途径的活化和细胞凋亡等。DNA损伤修复(DNA damage repair)是细胞维持基因组稳定性的重要机制,于2015年获得诺贝尔化学奖。DNA损伤修复途径主要包括:碱基切除修复(base-excision repair,BER)、核苷酸切除修复(nucleotide excision repair,NER)、错配修复(mismatch repair,MMR)、同源重组(homologous recombination,HR)和非同源末端连接(non-homologous end joining,NHEJ)等,分别在DNA单链断裂(single-strand break,SSB)或双链断裂(double-strand break,DSB)等损伤修复中发挥重要作用。DNA损伤修复缺陷与肿瘤发生发展密切相关,同时也是肿瘤治疗的重要靶点。DNA损伤修复通路的多聚ADP核糖聚合酶(poly-ADP-ribose polymerase,PARP)与乳腺癌易感基因BRCA 1/2等存在合成致死(synthetic lethality)作用,使PARP抑制剂(PARP inhibitor,PARPi)成为第一个也是目前唯一上市的肿瘤治疗合成致死靶药。PARPi在卵巢癌及多种实体瘤治疗中疗效良好,使DNA损伤修复及相关DDR通路的合成致死靶药研发成为热点,其他在研靶点主要包括:共济失调毛细血管扩张突变蛋白(ataxia telangiectasia-mutated protein,ATM)、共济失调毛细血管扩张与RAD3相关蛋白(ataxia telangiectasia and Rad3 related protein,ATR)、DNA依赖性蛋白质激酶催化亚单位(DNA-dependent protein kinase catalytic subunit,DNA-PKcs)、细胞周期检测点激酶1(checkpoint kinase1,CHK1)、细胞周期检测点激酶2(checkpoint kinase 2,CHK2)、阻止有丝分裂的蛋白质激酶WEE1等。PARPi与其他DDR靶药、抗血管生成药物及免疫检查点抑制剂的联用,有可能成为克服PARPi耐药、提高疗效的有效手段和发展前景。本文针对DNA损伤修复及相关DDR通路的关键分子和潜在肿瘤治疗靶点进行综述,阐述了DNA损伤修复相关通路的合成致死靶点研究及在卵巢癌的应用和前景,为基础研究及临床应用提供指导。

Regulation of DNA double-strand break repair pathway choice

DNA double-strand breaks （DSBs） are critical lesions that can result in cell death or a wide variety of genetic alterations including largeor small-scale deletions, loss of heterozygosity, translocations, and chromosome loss. DSBs are repaired by non-homologous end-joining （NHEJ） and homologous recombination （HR）, and defects in these pathways cause genome instability and promote tumorigenesis. DSBs arise from endogenous sources including reactive oxygen species generated during cellular metabolism, collapsed replication forks, and nucleases, and from exogenous sources including ionizing radiation and chemicals that directly or indirectly damage DNA and are commonly used in cancer therapy. The DSB repair pathways appear to compete for DSBs, but the balance between them differs widely among species, between different cell types of a single species, and during different cell cycle phases of a single cell type. Here we review the regulatory factors that regulate DSB repair by NHEJ and HR in yeast and higher eukaryotes. These factors include regulated expression and phosphorylation of repair proteins, chromatin modulation of repair factor accessibility, and the availability of homologous repair templates. While most DSB repair proteins appear to function exclusively in NHEJ or HR, a number of proteins influence both pathways, including the MRE11/RAD50/NBS1（XRS2） complex, BRCA1, histone H2AX, PARP-1, RAD18, DNA-dependent protein kinase catalytic subunit （DNA-PKcs）, and ATM. DNA-PKcs plays a role in mammalian NHEJ, but it also influences HR through a complex regulatory network that may involve crosstalk with ATM, and the regulation of at least 12 proteins involved in HR that are phosphorylated by DNA-PKcs and/or ATM.

KRAS and BRAF gene mutations and DNA mismatch repair status in Chinese colorectal carcinoma patients

AIM:To investigate gene mutations and DNA mismatch repair(MMR) protein abnormality in Chinese colorectalcarcinoma(CRC) patients and their correlations with clinicopathologic features.METHODS:Clinical and pathological information for 535 patients including 538 tumors was reviewed and recorded.Mutation analyses for exon 2 of KRAS gene and exon 15 of BRAF gene were performed by Sanger sequencing except that in 9 tumors amplification refractory mutation system PCR was used.Expression of MMR proteins including MHL1,MSH2,MSH6 and PMS2 was evaluated by immunohistochemistry.Correlations of KRAS and BRAF mutation status and the expression status of MMR proteins with age,gender,cancer stage,location,and histology were analyzed.Correlations between KRAS or BRAF mutations and MMR protein expression were also explored.RESULTS:The overall frequencies of KRAS and BRAF mutations were 37.9% and 4.4%,respectively.KRAS mutations were more common in patients ≥ 50 years old(39.8% vs 22% in patients < 50 years old,P < 0.05).The frequencies of BRAF mutants were higher in tumors from females(6.6% vs males 2.8%,P < 0.05),located in the right colon(9.6% vs 2.1% in the left colon,1.8% in the rectum,P < 0.01),with mucinous differentiation(9.8% vs 2.8% without mucinous differentiation,P < 0.01),or being poorly differentiated(9.5% vs 3.4% well/moderately differentiated,P < 0.05).MMR deficiency was strongly associated with proximal location(20.5% in the right colon vs 9.2% in the left colon and 5.1% in the rectum,P < 0.001),early cancer stage(15.0% in stages Ⅰ-Ⅱ vs 7.7% in stages Ⅲ-Ⅳ,P < 0.05),and mucinous differentiation(20.2% vs 9.2% without mucin,P < 0.01).A higher frequency of MLH1/PMS2 loss was found in females(9.2% vs 4.4% in males,P < 0.05),and MSH2/MSH6 loss tended to be seen in younger(<50 years old) patients(12.0% vs 4.0% ≥ 50 years old,P < 0.05).MMR deficient tumors were less likely to have KRAS mutations(18.8% vs 41.7% in MMR proficient tumors,P < 0.05) and tumorswith abnormal MLH1/PMS2 tended to harbor BRAF mutations(15.4% vs 4.2% in MMR proficient tumors,P < 0.05).CONCLUSION:The frequency of sporadic CRCs having BRAF mutation,MLH1 deficiency and MSI in Chinese population may be lower than that in the Western population.

Epigenetic regulation of DNA repair machinery in Helicobacter pylori-induced gastric carcinogenesis

Although thousands of DNA damaging events occur in each cell every day,efficient DNA repair pathways have evolved to counteract them. The DNA repair machinery plays a key role in maintaining genomic stability by avoiding the maintenance of mutations. The DNA repair enzymes continuously monitor the chromosomes to correct any damage that is caused by exogenous and endogenous mutagens. If DNA damage in proliferating cells is not repaired because of an inadequate expression of DNA repair genes,it might increase the risk of cancer. In addition to mutations,which can be either inherited or somatically acquired,epigenetic silencing of DNA repair genes has been associated with carcinogenesis. Gastric cancer represents the second highest cause of cancer mortality worldwide. The disease develops from the accumulation of several genetic and epigenetic changes during the lifetime. Among the risk factors,Helicobacter pylori(H. pylori) infection is considered the main driving factor to gastric cancer development. Thus,in this review,we summarize the current knowledge of the role of H. pylori infection on the epigenetic regulation of DNA repair machinery in gastric carcinogenesis.

Perspectives on the combination of radiotherapy and targeted therapy with DNA repair inhibitors in the treatment of pancreatic cancer

Pancreatic cancer is highly lethal. Current research that combines radiation with targeted therapy may dramatically improve prognosis. Cancerous cells are characterized by unstable genomes and activation of DNA repair pathways, which are indicated by increased phosphorylation of numerous factors, including H2 AX, ATM, ATR, Chk1, Chk2, DNA-PKcs, Rad51, and Ku70/Ku80 heterodimers. Radiotherapy causes DNA damage. Cancer cells can be made more sensitive to the effects of radiation(radiosensitization) through inhibition of DNA repair pathways. The synergistic effects, of two or more combined non-lethal treatments, led to coadministration of chemotherapy and radiosensitization in BRCA-defective cells and patients, with promising results. ATM/Chk2 and ATR/Chk1 pathways are principal regulators of cell cycle arrest, following DNA doublestrand or single-strand breaks. DNA double-stranded breaks activate DNA-dependent protein kinase, catalytic subunit(DNA-PKcs). It forms a holoenzyme with Ku70/Ku80 heterodimers, called DNA-PK, which catalyzes the joining of nonhomologous ends. This is the primary repair pathway utilized in human cells after exposure to ionizing radiation. Radiosensitization, induced by inhibitors of ATM, ATR, Chk1, Chk2, Wee1, PP2 A, or DNA-PK, has been demonstrated in preclinical pancreatic cancer studies. Clinical trials are underway. Development of agents that inhibit DNA repair pathways to be clinically used in combination with radiotherapy is warranted for the treatment of pancreatic cancer.

转录因子MYB转录调控MTFR2通过DNA损伤修复促进胃癌细胞化疗耐药性

目的探究v-myb禽成髓细胞病病毒癌基因同源物(MYB)转录调控线粒体裂变调节因子2(MTFR2)对胃癌(GC)细胞顺铂(DDP)耐药性的影响及分子作用机制。方法TCGA数据库分析GC中差异mRNA并预测上游调控分子,qRT-PCR检测MTFR2和MYB的表达,双荧光素酶和染色质免疫共沉淀(ChIP)实验验证MTFR2和MYB的调控关系,细胞计数盒8(CCK-8)检测细胞活力并计算IC_(50)值,流式细胞术检测细胞周期和细胞凋亡,彗星实验检测DNA损伤,蛋白质免疫印迹法检测DNA损伤相关蛋白(γ-H2AX、ATM、p-ATM)的表达。结果MTFR2在GC组织和细胞中显著高表达,敲低MTFR2能够降低细胞增殖,阻滞S期,诱导细胞凋亡,促进DNA损伤和DDP敏感性。生信预测MTFR2存在上游转录因子MYB,MYB在GC组织和细胞中的表达显著上调,双荧光素酶和ChIP验证了MTFR2启动子区域与MYB的结合关系。回复实验发现进一步过表达MTFR2能够逆转敲低MYB对GC细胞增殖和DDP耐药性的抑制作用。结论MYB上调MTFR2的表达通过DNA损伤途径促进GC细胞增殖和DDP耐药,表明靶向MYB/MTFR2调控轴可能是克服GC DDP耐药性的潜在途径。

A brief history of the DNA repair field

The history of the repair of damaged DNA can be traced to the mid-1930s. Since then multiple DNA repair mechanisms, as well as other biological responses to DNA damage, have been discovered and their regulation has been studied. This article briefly recounts the early history of this field.

Assessment of Human DNA Repair (NER) Capacity With DNA Repair Rate (DRR) by Comet Assay

Objective Alkaline comet assay was used to evaluate DNA repair (nucleotide excision repair, NER) capacity of human fresh lymphocytes from 12 young healthy non-smokers (6 males and 6 females). Methods Lymphocytes were exposed to UV-C (254 nm) at the dose rate of 1.5 J/m2/sec. Novobiocin (NOV) and aphidicolin (APC), DNA repair inhibitors, were utilized to imitate the deficiency of DNA repair capacity at the incision and ligation steps of NER. Lymphocytes from each donor were divided into three grougs: UVC group, UVC plus NOV group, and UVC plus APC group. DNA single strand breaks were detected in UVC irradiated cells incubated for 0, 30, 60, 90, 120, 180, and 240 min after UVC irradiation. DNA repair rate (DRR) served as an indicator of DNA repair capacity. Results The results indicated that the maximum DNA damage (i.e. maximum tail length) in the UVC group mainly appeared at 90 min. The ranges of DRRs in the UVC group were 62.84%-98.71%. Average DRR value was 81.84%. The DRR difference between males and females was not significant (P<0.05). However, the average DRR value in the UVC plus NOV group and the UVC plus APC group was 52.98% and 39.57% respectively, which were significantly lower than that in the UVC group (P<0.01). Conclusion The comet assay is a rapid, simple and sensitive screening test to assess individual DNA repair (NER) capacity. It is suggested that the time to detect DNA single strand breaks in comet assay should include 0 (before UV irradiation), 90 and 240 min after exposure to 1.5 J·m-2 UVC at least. The DRR, as an indicator, can represent the individual DNA repair capacity in comet assay.

Homologous recombination in DNA repair and DNA damage tolerance

Homologous recombination （HR） comprises a series of interrelated pathways that function in the repair of DNA double-stranded breaks （DSBs） and interstrand crosslinks （ICLs）. In addition, recombination provides critical support for DNA replication in the recovery of stalled or broken replication forks, contributing to tolerance of DNA damage. A central core of proteins, most critically the RecA homolog Rad51, catalyzes the key reactions that typify HR： homology search and DNA strand invasion. The diverse functions of recombination are reflected in the need for context-specific factors that perform supplemental functions in conjunction with the core proteins. The inability to properly repair complex DNA damage and resolve DNA replication stress leads to genomic instability and contributes to cancer etiology. Mutations in the BRCA2 recombination gene cause predisposition to breast and ovarian cancer as well as Fanconi anemia, a cancer predisposition syndrome characterized by a defect in the repair of DNA interstrand crosslinks. The cellular functions of recombination are also germane to DNA-based treatment modalities of cancer, which target replicating cells by the direct or indirect induction of DNA lesions that are substrates for recombination pathways. This review focuses on mechanistic aspects of HR relating to DSB and ICL repair as well as replication fork support.

Early steps in the DNA base excision/single-strand interruption repair pathway in mammalian cells

Base excision repair （BER） is an evolutionarily conserved process for maintaining genomic integrity by eliminating several dozen damaged （oxidized or aikylated） or inappropriate bases that are generated endogenously or induced by genotoxicants, predominantly, reactive oxygen species （ROS）. BER involves 4-5 steps starting with base excision by a DNA glycosylase, followed by a common pathway usually involving an AP-endonuclease （APE） to generate 3＇ OH terminus at the damage site, followed by repair synthesis with a DNA polymerase and nick sealing by a DNA iigase. This pathway is also responsible for repairing DNA single-strand breaks with blocked termini directly generated by ROS. Nearly all glycosylases, far fewer than their substrate lesions particularly for oxidized bases, have broad and overlapping substrate range, and could serve as back-up enzymes in vivo. In contrast, mammalian cells encode only one APE, APEI, unlike two APEs in lower organisms. In spite of overall similarity, BER with distinct subpathways in the mammals is more complex than in E. coli. The glycosylases form complexes with downstream proteins to carry out efficient repair via distinct subpathways one of which, responsible for repair of strand breaks with 3＇ phosphate termini generated by the NEIL family glycosylases or by ROS, requires the phosphatase activity of polynucleotide kinase instead of APE1. Different complexes may utilize distinct DNA polymerases and iigases. Mammalian glycosylases have nonconserved extensions at one of the termini, dispensable for enzymatic activity but needed for interaction with other BER and non-BER proteins for complex formation and organeile targeting. The mammalian enzymes are sometimes covalently modified which may affect activity and complex formation. The focus of this review is on the early steps in mammalian BER for oxidized damage.

Epigenetic reduction of DNA repair in progression to gastrointestinal cancer

Deficiencies in DNA repair due to inherited germ-line mutations in DNA repair genes cause increased risk of gastrointestinal(GI) cancer. In sporadic GI cancers, mutations in DNA repair genes are relatively rare. However, epigenetic alterations that reduce expression of DNA repair genes are frequent in sporadic GI cancers. These epigenetic reductions are also found in field defects that give rise to cancers. Reduced DNA repair likely allows excessive DNA damages to accumulate in somatic cells. Then either inaccurate translesion synthesis past the un-repaired DNA damages or error-prone DNA repair can cause mutations. Erroneous DNA repair can also cause epigenetic alterations(i.e., epimutations, transmitted through multiple replication cycles). Some of these mutations and epimutations may cause progression to cancer. Thus, deficient or absent DNA repair is likely an important underlying cause of cancer. Whole genome sequencing of GI cancers show that between thousands to hundreds of thousands of mutations occur in these cancers. Epimutations that reduce DNA repair gene expression and occur early in progression to GI cancers are a likely source of this high genomic instability. Cancer cells deficient in DNA repair are more vulnerable than normal cells to inactivation by DNA damaging agents. Thus, some of the most clinically effective chemotherapeutic agents in cancer treatment are DNA damaging agents, and their effectiveness often depends on deficient DNA repair in cancer cells. Recently, at least 18 DNA repair proteins, each active in one of six DNA repair pathways, were found to be subject to epigenetic reduction of expression in GI cancers. Different DNA repair pathways repair different types of DNA damage. Evaluation of which DNA repair pathway(s) are deficient in particular types of GI cancer and/or particular patients may prove useful in guiding choice of therapeutic agents in cancer therapy.

The characteristics of DNA repair synthesis induced by DNA polymerase β in hepatoma cells after γ rays irradiation

AIM To investigate the effects of DNA repair synthesis induced by DNA polymerase β in hepatoma cells after γ ray irradiation. METHODS Cell nuclei were prepared from SMMC LTNM hepatoma which is a transplanted human liver cancer born on nude mice. Samples were irradiated with 60 Co γ rays at different doses or dose rates. N ethylmaleimide (NEM) and ddTTP were used as selective inhibitors to DNA polymerases. The reaction of DNA repair synthesis was carried out with the selective inhibitor test. RESULTS It was found that the 3H TTP incorporation in irradiated nuclei or calf thymus DNA was significantly higher than that in the non irradiated ones, under the conditions of DNA polymerase α or γ being inhibited. When NEM and ddTTP which selectively inhibits DNA polymerase β both existed in the DNA repair synthesis reaction mixture, the 3H TTP incorporation in irradiated DNA did not significantly increased. Furthermore, 3H TTP incorporation into DNA of SMMC LTNM hepatoma nuclei was higher than that of normal hepatocyte nuclei ( P <0 01). The DNA repair synthesis induced by DNA polymerase β reacted more fast in hepatoma nuclei than in hepatocyte nuclei. CONCLUSION The effects of DNA repair synthesis induced by DNA polymerase β in some tumor cells might be stronger than that in normal cells, which may facilitate the cells to repair DNA damages from radiation.

Preliminary Studies on Base Substitutions and Repair of DNA Mismatch Damage Stimulated by Low Energy N^+ Ion Beam Implantation in Escherichia coli

Ever since the low energy N+ ion beam has been accepted that the mutation effects of ionizing radiation are attributed mainly to direct or indirect damage to DNA. Evidences based on naked DNA irradiation in support of a mutation spectrum appears to be consistent, but direct proof of such results in vivo are limited. Using mutS, dam and/or dcm defective Eschericha coli imitator strains, an preliminary experimental system on induction of in vivo mutation spectra of low energy N+ ion beam has been established in this study. It was observed that the mutation rates of rifampicin resistance induced by N+ implantation were quite high, ranging from 9.2 x 10~8 to 4.9× 10~5 at the dosage of 5.2×1014 ions/cm2. Strains all had more than 90-fold higher mutation rate than its spontaneous mutation rate determined by this method. It reveals that base substitutions involve in induction of mutation of low energy nitrogen ion beam implantation. The mutation rates of mutator strains were nearly 500-fold (GM2929), 400-fold (GM5864) and 6-fold larger than that of AB1157. The GM2929 and GM5864 both lose the ability of repair DNA mismatch damage by virtue of both dam and dcm pathways defective (GM2929) or failing to assemble the repair complex (GM5864) respectively. It may explain the both strains had a similar higher mutation rate than GM124 did. It indicated that DNA cytosine methylase might play an important role in mismatch repair of DNA damage induced by N+ implantation. The further related research were also discussed.

Effects of temperature on UV-B-induced DNA damage and photorepair in Arabidopsis thaliana

DNA damage in the form of cyclobutane pyrimidine dimers(CPDs) and (6-4) photoproducts(6-4PPs) induced by UV-B radiation in Arabidopsis thaliana at different temperatures was investigated using ELISA with specific monoclonal antibodies. CPDs and 6-4PPs increased during 3 h UV-B exposure, but further exposure led to decreases. Contrary to the commonly accepted view that DNA damage induced by UV-B radiation is temperature-independent because of its photochemical nature, we found UV-B-induction of CPDs and 6-4PPs in Arabidopsis to be slower at a low than at a high temperature. Photorepair of CPDs at 24℃ was much faster than that at 0℃ and 12℃, with 50% CPDs removal during 1 h exposure to white light. Photorepair of 6-4PPs at 12℃ was very slow as compared with that at 24℃, and almost no removal of 6-4PPs was detected after 4 h exposure to white light at 0℃. There was evidence to suggest that temperature-dependent DNA damage and photorepair could have important ecological implications.

DNA repair and synthetic lethality

Tumors often have DNA repair defects, suggesting additional inhibition of other DNA repair pathways in tumors may lead to synthetic lethality. Accumulating data demonstrate that DNA repair-defective tumors, in particular homologous recombination （HR）, are highly sensitive to DNA-damaging agents. Thus, HR-defective tumors exhibit potential vulnerability to the synthetic lethality approach, which may lead to new therapeutic strategies. It is well known that poly （adenosine diphosphate （ADP）-ribose） polymerase （PARP） inhibitors show the synthetically lethal effect in tumors defective in BRCA1 or BRCA2 genes encoded proteins that are required for efficient HR. In this review, we summarize the strategies of targeting DNA repair pathways and other DNA metabolic functions to cause synthetic lethality in HR-defective tumor cells.

The endless tale of non-homologous end-joining

DNA double-strand breaks （DSBs） are introduced in cells by ionizing radiation and reactive oxygen species. In addition, they are commonly generated during V（D）J recombination, an essential aspect of the developing immune system. Failure to effectively repair these DSBs can result in chromosome breakage, cell death, onset of cancer, and defects in the immune system of higher vertebrates. Fortunately, all mammalian cells possess two enzymatic pathways that mediate the repair of DSBs： homologous recombination and non-homologous end-joining （NHEJ）. The NHEJ process utilizes enzymes that capture both ends of the broken DNA molecule, bring them together in a synaptic DNA-protein complex, and finally repair the DNA break. In this review, all the known enzymes that play a role in the NHEJ process are discussed and a working model for the co-operation of these enzymes during DSB repair is presented.