DNA mismatch repair proteins promote apoptosis and suppress tumorigenesis in response to UVB irradiation: an in vivo study

DNA mismatch repair (MMR) proteins are integral to the maintenance of genomic stability and suppression of tumorigenesis due to their role in repair of post-replicative DNA errors. Recent data also support a role for MMR proteins in cellular responses to exogenous DNA damage that does not involve removal of DNA adducts. We have demonstrated previously that both Msh2- and Msh6-null primary mouse embryonic fibroblasts are significantly less sensitive to UVB (ultraviolet B)-induced cytotoxicity and apoptosis than wild-type control cells. In order to ascertain the physiological relevance of the data we have exposed MMR-deficient mice to acute and chronic UVB radiation. We found that MMR-deficiency was associated with reduced levels of apoptosis and increased residual UVB-induced DNA adducts in the epidermis 24-h following acute UVB exposure. Moreover, Msh2-null mice developed UVB-induced skin tumors at a lower level of cumulative UVB exposure and with a greater severity of onset than wild-type mice. The Msh2-null skin tumors did not display microsatellite instability, suggesting that these tumors develop via a different tumorigenic pathway than tumors that develop spontaneously. Therefore, we propose that dysfunctional MMR promotes UVB-induced tumorigenesis through reduced apoptotic elimination of damaged epidermal cells.     

The mammalian mismatch repair protein MSH2 is required for correct MRE11 and RAD51 relocalization and for efficient cell cycle arrest induced by ionizing radiation in G2 phase

In yeast, MSH2 plays an important role in mismatch repair (MMR) and recombination, whereas the function of the mammalian MSH2 protein in recombinational repair is not completely established. We examined the cellular responses of MSH2-deficient mouse cells to X-rays to clarify the role of MSH2 in recombinational repair. Cell survival, checkpoint functions and relocalization of the recombination-related proteins MRE11 and RAD51 were analysed in embryonic fibroblasts derived from MSH2(+/+) and MSH2(-/-) mice, and in MSH2-proficient and deficient mouse colorectal carcinoma cells. Loss of MSH2 function was found to be associated with reduction in cell survival following radiation, absence of either MRE11 or RAD51 relocalization and a higher level of X-ray-induced chromosomal damage specifically in G2-phase cells. Finally, MSH2(-/-) cells showed an inefficient early G2/M checkpoint, being arrested only transiently after irradiation before progressing into mitosis. Consistent with the premature release from the G2-phase arrest, activation of CHK1 was transient and CHK2 was not phosphorylated in synchronized MSH2-null cells. Our data suggest that an active MSH2 is required for a correct response to ionizing radiation-induced DNA damage in the G2 phase of the cell cycle, possibly connecting DSB repair to checkpoint signalling.     

Signalling cell cycle arrest and cell death through the MMR System

Loss of DNA mismatch repair (MMR) in mammalian cells, as well as having a causative role in cancer, has been linked to resistance to certain DNA damaging agents including clinically important cytotoxic chemotherapeutics. MMR-deficient cells exhibit defects in G2/M cell cycle arrest and cell killing when treated with these agents. MMR-dependent cell cycle arrest occurs, at least for low doses of alkylating agents, only after the second S-phase following DNA alkylation, suggesting that two rounds of DNA replication are required to generate a checkpoint signal. These results point to an indirect role for MMR proteins in damage signalling where aberrant processing of mismatches leads to the generation of DNA structures (single-strand gaps and/or double-strand breaks) that provoke checkpoint activation and cell killing. Significantly, recent studies have revealed that the role of MMR proteins in mismatch repair can be uncoupled from the MMR-dependent damage responses. Thus, there is a threshold of expression of MSH2 or MLH1 required for proper checkpoint and cell-death signalling, even though sub-threshold levels are sufficient for fully functional MMR repair activity. Segregation is also revealed through the identification of mutations in MLH1 or MSH2 that provide alleles functional in MMR but not in DNA damage responses and mutations in MSH6 that compromise MMR but not in apoptotic responses to DNA damaging agents. These studies suggest a direct role for MMR proteins in recognizing and signalling DNA damage responses that is independent of the MMR catalytic repair process. How MMR-dependent G2 arrest may link to cell death remains elusive and we speculate that it is perhaps the resolution of the MMR-dependent G2 cell cycle arrest following DNA damage that is important in terms of cell survival.     

BRCA1 activates a G2-M cell cycle checkpoint following 6-thioguanine-induced DNA mismatch damage

Human DNA mismatch repair (MMR) is involved in the response to certain chemotherapy drugs, including 6-thioguanine (6-TG). Consistently, MMR-deficient human tumor cells show resistance to 6-TG damage as manifested by a reduced G(2)-M arrest and decreased apoptosis. In this study, we investigate the role of the BRCA1 protein in modulating a 6-TG-induced MMR damage response, using an isogenic human breast cancer cell line model, including a BRCA1 mutated cell line (HCC1937) and its transfectant with a wild-type BRCA1 cDNA. The MMR proteins MSH2, MSH6, MLH1, and PMS2 are similarly detected in both cell lines. BRCA1-mutant cells are more resistant to 6-TG than BRCA1-positive cells in a clonogenic survival assay and show reduced apoptosis. Additionally, the mutated BRCA1 results in an almost complete loss of a G(2)-M cell cycle checkpoint response induced by 6-TG. Transfection of single specific small interfering RNAs (siRNA) against MSH2, MLH1, ATR, and Chk1 in BRCA1-positive cells markedly reduces the BRCA1-dependent G(2)-M checkpoint response. Interestingly, ATR and Chk1 siRNA transfection in BRCA1-positive cells shows similar levels of 6-TG cytotoxicity as the control transfectant, whereas MSH2 and MLH1 siRNA transfectants show 6-TG resistance as expected. DNA MMR processing, as measured by the number of 6-TG-induced DNA strand breaks using an alkaline comet assay (+/-z-VAD-fmk cotreatment) and by levels of iododeoxyuridine-DNA incorporation, is independent of BRCA1, suggesting the involvement of BRCA1 in the G(2)-M checkpoint response to 6-TG but not in the subsequent excision processing of 6-TG mispairs by MMR.     

Mismatch repair, G(2)/M cell cycle arrest and lethality after DNA damage

The role of the mismatch repair pathway in DNA replication is well defined but its involvement in processing DNA damage induced by chemical or physical agents is less clear. DNA repair and cell cycle control are tightly linked and it has been suggested that mismatch repair is necessary to activate the G(2)/M checkpoint in the presence of certain types of DNA damage. We investigated the proposed role for mismatch repair (MMR) in activation of the G(2)/M checkpoint following exposure to DNA-damaging agents. We compared the response of MMR-proficient HeLa and Raji cells with isogenic variants defective in either the hMutLalpha or hMutSalpha complex. Different agents were used: the cross-linker N-(2-chloroethyl)-N'-cyclohexyl-N-nitrosourea (CCNU), gamma-radiation and the monofunctional methylating agent N-methyl-N-nitrosourea (MNU). MMR-defective cells are relatively sensitive to CCNU, while no differences in survival between repair-proficient and -deficient cells were observed after exposure to gamma-radiation. Analysis of cell cycle distribution indicates that G(2) arrest is induced at least as efficiently in MMR-defective cells after exposure to either CCNU or ionizing radiation. As expected, MNU does not induce G(2) accumulation in MMR-defective cells, which are known to be highly tolerant to killing by methylating agents, indicating that MNU-induced cell cycle alterations are strictly dependent on the cytotoxic processing of methylation damage by MMR. Conversely, activation of the G(2)/M checkpoint after DNA damage induced by CCNU and gamma-radiation does not depend on functional MMR. In addition, the absence of a simple correlation between the extent of G(2) arrest and cell killing by these agents suggests that G(2) arrest reflects the processing by MMR of both lethal and non-lethal DNA damage.     

Time course for early adaptive responses to ultraviolet B light in the epidermis of SKH-1 mice.

Hairless SKH-1 mice were exposed once to UVB light (180 mJ/cm2), and mechanistically important early adaptive responses in the epidermis were evaluated by immunohistochemical and morphological methods. Interrelationships in the time course for these UVB-induced responses were examined. The number of epidermal cells with DNA strand breaks (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling-positive cells) or with thymine dimers increased to maximal levels within 30 min after UVB. The number of cells with DNA strand breaks located specifically in the basal layer of the epidermis was increased substantially by 3-30 min after UVB and gradually increased further over the next 5.5 hours. DNA strand breaks specifically in the basal layer of the epidermis were increased maximally at 6 h after UVB. The number of epidermal cells with DNA strand breaks or thymine dimers decreased markedly between 12 and 36 h. Pyrimidine (6-4) pyrimidone photodimers (6-4 photoproducts) in isolated epidermal DNA were increased immediately after irradiation of the mice with UVB and decreased markedly during the next 6 h. Exposure to UVB caused a rapid 8-fold increase in the number of epidermal cells with the DNA mismatch repair protein, MSH2 (within 30-60 min), and the level of MSH2-positive cells remained elevated for at least 48 h. These observations suggest a possible role of MSH2 in the repair of UVB-induced DNA damage. The number of epidermal cells with wild-type p53 protein started to increase at 1 h after UVB exposure and reached maximal levels by 8-12 h. The number of p53-positive cells fell markedly between 24 and 48 h. The time course for UVB-induced increases in the number of p53-positive cells was paralleled very closely by the time course for UVB-induced increases in the number of cells with p21(WAF1/CIP1), increases in morphologically distinct apoptotic sunburn cells, and decreases in the number of epidermal cells with bromodeoxyuridine (BrdUrd) incorporation into DNA. Although the start of UVB-induced increases in the number of p21(WAF1/CIP1)-positive cells was similar to that for the increase in p53-positive cells and very high levels of p21(WAF1/CIP1)-positive cells were observed at 8-12 h, maximal increases in p21(WAF1/CIP1)-positive cells were not achieved until 24 h after UVB irradiation (approximately 12 h after the peak value for p53). Myeloperoxidase-positive epidermal cells started to increase by 30 min after UVB exposure, and maximal numbers of myeloperoxidase-positive epidermal cells were observed at 2 h after UVB (18-fold higher than in nonirradiated control mice). An increased level of epidermal peroxidase enzyme activity in the epidermis was also observed from 1 to 24 h after exposure of the mice to UVB. Although neutrophil infiltration into the epidermis was not seen after exposure to UVB, neutrophil infiltration into the dermis (inflammatory response) was observed from 4 to 144 h after UVB exposure. In contrast to the marked inhibitory effect of UVB on BrdUrd incorporation into the DNA of epidermal cells observed at 8-12 h after UVB irradiation (>90% inhibition), BrdUrd incorporation into the DNA of epidermal cells was markedly increased (approximately 30-fold increase in the number of BrdUrd-positive cells) at 48 h after UVB exposure, and increases in epidermal cell layers and epidermal thickness (hyperplasia) were also observed. These later effects were associated with regeneration of the damaged epidermis.     

Silibinin prevents ultraviolet B radiation-induced epidermal damages in JB6 cells and mouse skin in a p53-GADD45α-dependent manner

Better preventive strategies are required to reduce ultraviolet (UV)-caused photodamage, the primary etiological factor for non-melanoma skin cancer (NMSC). Accordingly, here we examined the preventive efficacy of silibinin against UVB-induced photodamage using mouse epidermal JB6 cells and SKH1 hairless mouse epidermis. In JB6 cells, silibinin pretreatment protected against apoptosis and accelerated the repair of cyclobutane pyrimidine dimers (CPD) induced by moderate dose of UVB (50 mJ/cm(2)), which we are at risk of daily exposure. Silibinin also reversed UVB-induced S phase arrest, reducing both active DNA synthesizing and inactive S phase populations. In mechanistic studies, UVB-irradiated cells showed a transient upregulation of both phosphorylated (Ser-15 and Ser-392) and total p53, whereas silibinin pretreatment led to a more sustained upregulation and stronger nuclear localization of p53. Silibinin also caused a marked upregulation of GADD45α, a downstream target of p53, implicated in DNA repair and cell cycle regulation. Importantly, under p53 and GADD45α knockdown conditions, cells were more susceptible to UVB-induced apoptosis without any significant S phase arrest, and protective effects of silibinin were compromised. Similar to the in vitro results, topical application of silibinin prior to or immediately after UVB irradiation resulted in sustained increase in p53 and GADD45α levels and accelerated CPD removal in the epidermis of SKH1 hairless mice. Together, our results show for the first time that p53-mediated GADD45α upregulation is the key mechanism by which silibinin protects against UVB-induced photodamage and provides a strong rationale to investigate silibinin in reducing the risk and/or preventing early onset of NMSC.     

Cell cycle regulation in response to DNA damage in mammalian cells: A historical perspective

Summary Cell cycle delay has long been known to occur in mammalian cells after exposure to DNA-damaging agents. It has been hypothesized that the function of this delay is to provide additional time for repair of DNA before the cell enters critical periods of the cell cycle, such as DNA synthesis in S phase or chromosome condensation in G2 phase. Recent evidence that p53 protein is involved in the delay in G1 in response to ionizing radiation has heightened interest in the importance of cell cycle delay, because mutations in p53 are commonly found in human cancer cells. Because mammalian cells defective in p53 protein show increased genomic instability, it is tempting to speculate that the instability is due to increased chromosome damage resulting from the lack of a G1 delay. Although this appears at first glance to be a highly plausible explanation, a review of the research performed on cell cycle regulation and DNA damage in mammalian cells provides little evidence to support this hypothesis. Studies involving cells treated with caffeine, cells from humans with the genetic disease ataxiatelangiectasia, and cells that are deficient in p53 show no correlation between G1 delay and increased cell killing or chromosome damage in response to ionizing radiation. Instead, G1 delay appears to be only one aspect of a complex cellular response to DNA damage that also includes delays in S phase and G2 phase, apoptosis and chromosome repair. The exact mechanism of the genomic instability associated with p53, and its relationship to the failure to repair DNA before progression through the cell cycle, remains to be determined.     

Cell cycle phase dependent role of DNA polymerase beta in DNA repair and survival after ionizing radiation.

PURPOSE: The purpose of the present study was to determine the role of DNA polymerase beta in repair and response after ionizing radiation in different phases of the cell cycle. METHODS AND MATERIALS: Synchronized cells deficient and proficient in DNA polymerase beta were irradiated in different phases of the cell cycle as determined by BrdU/flow cytometry. Cell kill and DNA repair were assessed by colony formation and alkaline comet assays, respectively. RESULTS: We first demonstrated delayed repair of ionizing radiation induced DNA damage in confluent polymerase beta deficient cells. Cell synchronization experiments revealed a cell cycle phase dependence by demonstrating radiation hypersensitivity of polymerase beta-deficient cells in G1, but not in the S-phase. Complementing polymerase beta-deficient cells with polymerase beta reverted the hypersensitivity in G1. Ionizing radiation damage repair was found to be delayed in beta-deficient cells when irradiated in G1, but not in S. CONCLUSIONS: The data show a differential role of DNA polymerase beta driven base excision and single strand break repair throughout the cell cycle after ionizing radiation damage.     

PTEN positively regulates UVB-induced DNA damage repair

Nonmelanoma skin cancer is the most common cancer in the United States, where DNA-damaging ultraviolet B (UVB) radiation from the sun remains the major environmental risk factor. However, the critical genetic targets of UVB radiation are undefined. Here we show that attenuating PTEN in epidermal keratinocytes is a predisposing factor for UVB-induced skin carcinogenesis in mice. In skin papilloma and squamous cell carcinoma (SCC), levels of PTEN were reduced compared with skin lacking these lesions. Likewise, there was a reduction in PTEN levels in human premalignant actinic keratosis and malignant SCCs, supporting a key role for PTEN in human skin cancer formation and progression. PTEN downregulation impaired the capacity of global genomic nucleotide excision repair (GG-NER), a critical mechanism for removing UVB-induced mutagenic DNA lesions. In contrast to the response to ionizing radiation, PTEN downregulation prolonged UVB-induced growth arrest and increased the activation of the Chk1 DNA damage pathway in an AKT-independent manner, likely due to reduced DNA repair. PTEN loss also suppressed expression of the key GG-NER protein xeroderma pigmentosum C (XPC) through the AKT/p38 signaling axis. Reconstitution of XPC levels in PTEN-inhibited cells restored GG-NER capacity. Taken together, our findings define PTEN as an essential genomic gatekeeper in the skin through its ability to positively regulate XPC-dependent GG-NER following DNA damage.     

DNA mismatch repair protein MSH2 dictates cellular survival in response to low dose radiation in endometrial carcinoma cells

DNA repair and G2-phase cell cycle checkpoint responses are involved in the manifestation of hyper-radiosensitivity (HRS). The low-dose radioresponse of MSH2 isogenic endometrial carcinoma cell lines was examined. Defects in cell cycle checkpoint activation and the DNA damage response in irradiated cells (0.2 Gy) were evaluated. HRS was expressed solely in MSH2+ cells and was associated with efficient activation of the early G2-phase cell cycle checkpoint. Maintenance of the arrest was associated with persistent MRE11, γH2AX, RAD51 foci at 2 h after irradiation. Persistent MRE11 and RAD51 foci were also evident 24 h after 0.2 Gy. MSH2 significantly enhances cell radiosensitivity to low dose IR.     

HER-3基因表达对HER-2阳性型乳腺癌细胞放射敏感性的影响及作用机制

目的:探讨人表皮生长因子受体-3（human epidermal growth factor receptor-3,HER-3）表达对HER-2阳性型乳腺癌细胞放射敏感性的影响。方法:使用慢病毒颗粒感染HER-2阳性型乳腺癌细胞系（AU-565、SKBR-3）构建HER-3基因敲低细胞系模型,蛋白质印迹法（Western blot）及实时定量PCR(RT-qPCR)技术检测感染的效率。克隆形成实验测定不同组别乳腺癌细胞放射增敏效果。将HER-3敲低组与对照组细胞分别给予X线照射（6 Gy,6 MV,源皮距100 cm）,流式细胞术测定不同组别细胞凋亡率以及细胞周期分布的变化。免疫荧光法检测细胞辐射后不同时间段的γH2AX焦点数,评估辐射后DNA损伤情况,蛋白质印迹法检测细胞周期调控相关蛋白CyclinB1的表达。结果:克隆形成实验结果显示,HER-3敲低组乳腺癌细胞放射敏感性增强。流式细胞术结果显示,HER-3敲低后,HER-2阳性型乳腺癌细胞辐射后凋亡率明显增加,细胞周期G2期分布比例提高。免疫荧光结果显示,HER-3敲低后,HER-2阳性型乳腺癌细胞γH2AX焦点数目在辐射后先增加后降低,较对照组峰值更高,降低时趋势更慢,提示HER-3敲低可增强辐射诱导的DNA损伤,减少DNA修复。蛋白质印迹法结果显示,细胞周期相关驱动蛋白CyclinB1表达降低。结论:HER-3基因敲低后一方面增加了HER-2阳性型乳腺癌细胞辐射后DNA的损伤并减缓其修复能力,促进细胞凋亡;另一方面通过下调细胞周期蛋白CyclinB1的表达,增加G2期细胞分布,提升了放射敏感性。     

Hypersensitivity to camptothecin in MSH2 deficient cells is correlated with a role for MSH2 protein in recombinational repair.

DNA mismatch repair (MMR) corrects DNA polymerase insertion errors that have escaped proofreading in order to avoid the accumulation of deleterious mutations. While the role of MMR in the correction of replication errors is well established, its involvement in the processing of DNA damage induced by chemical and physical agents is less clear. A role for some of the MMR proteins, such as MSH2, in the repair of double strand break (DSBs) through recombination has also been envisaged. Why MMR- deficient cells are sensitive to agents causing replication fork stalling and thus DSBs remains unclear. To verify a possible role of MSH2 in homologous recombinational repair, we have treated cells from knockout mice for the MSH2 gene and mouse colorectal carcinoma cells also defective for MSH2 with different doses of camptothecin, an agent known to interfere with DNA replication. In the absence of MSH2, we found a reduced survival rate accompanied by higher levels of chromosomal damage and SCE induction. Furthermore, MSH2(-/-) cells displayed an elevated spontaneous RAD51 focus-forming activity and a higher induction of RAD51 foci following camptothecin treatment. Thus, the absence of MSH2 could result in both spontaneous DNA damage and uncontrolled recombination events leading to the observed higher yield of chromosomal damage and the higher induction of RAD51 foci following CPT treatment. Therefore, our results suggest an involvement of MSH2 in the early events leading to correct RAD51 relocalization after the formation of DSBs specifically produced at the blocked replication fork.     

Evidence for the direct binding of phosphorylated p53 to sites of DNA breaks in vivo

Despite a clear link between ataxia-telangiectasia mutated (ATM)-dependent phosphorylation of p53 and cell cycle checkpoint control, the intracellular biology and subcellular localization of p53 phosphoforms during the initial sensing of DNA damage is poorly understood. Using G0-G1 confluent primary human diploid fibroblast cultures, we show that endogenous p53, phosphorylated at Ser15 (p53Ser15), accumulates as discrete, dose-dependent and chromatin-bound foci within 30 minutes following induction of DNA breaks or DNA base damage. This biologically distinct subpool of p53Ser15 is ATM dependent and resistant to 26S-proteasomal degradation. p53Ser15 colocalizes and coimmunoprecipitates with gamma-H2AX with kinetics similar to that of biochemical DNA double-strand break (DNA-dsb) rejoining. Subnuclear microbeam irradiation studies confirm p53Ser15 is recruited to sites of DNA damage containing gamma-H2AX, ATM(Ser1981), and DNA-PKcs(Thr2609) in vivo. Furthermore, studies using isogenic human and murine cells, which express Ser15 or Ser18 phosphomutant proteins, respectively, show defective nuclear foci formation, decreased induction of p21WAF, decreased gamma-H2AX association, and altered DNA-dsb kinetics following DNA damage. Our results suggest a unique biology for this p53 phosphoform in the initial steps of DNA damage signaling and implicates ATM-p53 chromatin-based interactions as mediators of cell cycle checkpoint control and DNA repair to prevent carcinogenesis.     

UCN-01去除放射后肿瘤细胞G2期阻滞及其相关机制

背景与目的:辐射对肿瘤细胞的影响常表现为细胞周期的改变。本研究观察X线照射后肿瘤细胞G2期阻滞及药物UCN鄄01去除此阻滞的情况,并进一步研究其相关机制。方法:选用已知p53突变的人鼻咽癌CNE鄄1细胞系和人肺腺癌973细胞系进行研究,并以p53功能正常的人纤维母细胞瘤HT鄄1080细胞系作为对照。采用细胞培养和流式细胞仪技术分析X线照射对以上细胞系的细胞周期的影响,并观察UCN鄄01对放射后细胞G2期阻滞的影响;应用蛋白印迹法(Westernblot)测定在UCN鄄01去除CNE鄄1细胞G2期阻滞的过程中,细胞内磷酸化CDC2鄄Tyr15含量的相应变化。结果:X线照射明显导致CNE鄄1细胞和973细胞G2阻滞,照射2Gy后两组的G2期细胞比例分别由18.4%和14.8%上升至43.6%和42.8%,两组细胞的G1期阻滞不显著,衡量G1期阻滞的指标“S期消耗率”均较低,分别为14.8%和-1.2%,明显低于p53功能正常的HT鄄1080细胞(57.0%)。UCN鄄01能够去除放射后CNE鄄1细胞和973细胞G2阻滞,两组的G2期细胞比例分别从单纯照射组的63.5%和35.4%降至16.1%和16.3%。CNE鄄1细胞照射后随着细胞G2期阻滞增加,磷酸化CDC2鄄Tyr15的含量相应增加,而UCN鄄01能够降低照射后细胞内磷酸化CDC2鄄Tyr15的含量,并与该药去除细胞G2期阻滞的过程相一致。结论:对于p53突变的人鼻咽癌CNE     

NPM-ALK mediates phosphorylation of MSH2 at tyrosine 238, creating a functional deficiency in MSH2 and the loss of mismatch repair

The vast majority of anaplastic lymphoma kinase-positive anaplastic large cell lymphoma (ALK+ALCL) tumors express the characteristic oncogenic fusion protein NPM-ALK, which mediates tumorigenesis by exerting its constitutive tyrosine kinase activity on various substrates. We recently identified MSH2, a protein central to DNA mismatch repair (MMR), as a novel binding partner and phosphorylation substrate of NPM-ALK. Here, using liquid chromatography–mass spectrometry, we report for the first time that MSH2 is phosphorylated by NPM-ALK at a specific residue, tyrosine 238. Using GP293 cells transfected with NPM-ALK, we confirmed that the MSH2^Y238F mutant is not tyrosine phosphorylated. Furthermore, transfection of MSH2^Y238F into these cells substantially decreased the tyrosine phosphorylation of endogenous MSH2. Importantly, gene transfection of MSH2^Y238F abrogated the binding of NPM-ALK with endogenous MSH2, re-established the dimerization of MSH2:MSH6 and restored the sensitivity to DNA mismatch-inducing drugs, indicative of MMR return. Parallel findings were observed in two ALK+ALCL cell lines, Karpas 299 and SUP-M2. In addition, we found that enforced expression of MSH2^Y238F into ALK+ALCL cells alone was sufficient to induce spontaneous apoptosis. In conclusion, our findings have identified NPM-ALK-induced phosphorylation of MSH2 at Y238 as a crucial event in suppressing MMR. Our studies have provided novel insights into the mechanism by which oncogenic tyrosine kinases disrupt MMR.