Loss Of DNA Mismatch Repair Research Articles

High fidelity DNA ligation prevents single base insertions in the yeast genome

Finalization of eukaryotic nuclear DNA replication relies on DNA ligase 1 (LIG1) to seal DNA nicks generated during Okazaki Fragment Maturation (OFM). Using a mutational reporter in Saccharomyces cerevisiae, we previously showed that mutation of the high-fidelity magnesium binding site of LIG1Cdc9 strongly increases the rate of single-base insertions. Here we show that this rate is increased across the nuclear genome, that it is synergistically increased by concomitant loss of DNA mismatch repair (MMR), and that the additions occur in highly specific sequence contexts. These discoveries are all consistent with incorporation of an extra base into the nascent lagging DNA strand that can be corrected by MMR following mutagenic ligation by the Cdc9-EEAA variant. There is a strong preference for insertion of either dGTP or dTTP into 3–5 base pair mononucleotide sequences with stringent flanking nucleotide requirements. The results reveal unique LIG1Cdc9-dependent mutational motifs where high fidelity DNA ligation of a subset of OFs is critical for preventing mutagenesis across the genome.

Prevalence of mismatch repair mutations in colorectal carcinoma patients in Mangalore

Colorectal cancer (CRC) is highly prevalent throughout the world and represents the 3rd most common cancer in men and the 2nd in women worldwide. Microsatellite instability (MSI) is a term used to denote a hypermutable phenotype caused by the loss of DNA mismatch repair (MMR) activity, and is a phenomenon now linked to the pathways of colorectal carcinogenesis. Compounding its importance is its integral association with Lynch syndrome, the most common cause for CRCs in young individuals. In the present study, we aimed to analyse the proportion of patients with risk of microsatellite instability by checking for loss of immunostaining for mismatch repair (MMR) proteins.From January 2016 to December 2016 and May 2017 to October 2017, 40 consecutive newly diagnosed cases of colorectal cancer were included in the study. The expression of MMR proteins in the tumour tissue using IHC for MSH2, MSH6, MLH1 and PMS2 was studied.Among the 40 cases, 3 (7.5%) demonstrated loss of MMR proteins and 37 (92.5%) cases had intact nuclear expression. Out of the three cases with MMR loss, one showed concurrent loss of MLH1 and PMS2, the second showed concurrent loss of MSH2 and MSH6 and the third showed an isolated loss of MSH6.Colorectal carcinomas showing MMR mutations are seen in the Mangalorean population. However, the incidence in our study was relatively low compared to most other studies, probably due to a variation in ethnicity.

Tissue-specific reduction in MLH1 expression induces microsatellite instability in intestine of Mlh1+/− mice

Tumors of Lynch syndrome (LS) patients display high levels of microsatellite instability (MSI), which results from complete loss of DNA mismatch repair (MMR), in line with Knudson’s two-hit hypothesis. Why some organs, in particular those of the gastrointestinal (GI) tract, are prone to tumorigenesis in LS remains unknown. We hypothesized that MMR is haploinsufficient in certain tissues, compromising microsatellite stability in a tissue-specific manner before tumorigenesis. Using mouse genetics, we tested how levels of MLH1, a central MMR protein, affect age- and tissue-specific microsatellite stability in vivo and whether elevated MSI is detectable prior to loss of MMR function and to neoplastic growth.To assess putative tissue-specific MMR haploinsufficiency, we determined relevant molecular phenotypes (MSI, Mlh1 promoter methylation status, MLH1 protein and RNA levels) in jejuna of Mlh1+/− mice and compared them to those in spleen, as well as to MMR-proficient and -deficient controls (Mlh1+/+ and Mlh1−/− mice). While spleen MLH1 levels of Mlh1+/− mice were, as expected, approximately 50 % compared to wildtype mice, MLH1 levels in jejunum varied substantially between individual Mlh1+/− mice and moreover, decreased with age. Mlh1+/− mice with soma-wide Mlh1 promoter methylation often displayed severe MLH1 depletion in jejunum. Reduced (but still detectable) MLH1 levels correlated with elevated MSI in Mlh1+/− jejunum. MSI in jejunum increased with age, while in spleens of the same mice, MLH1 levels and microsatellites remained stable. Thus, MLH1 expression levels are particularly labile in intestine of Mlh1+/− mice, giving rise to tissue-specific MSI long before neoplasia. A similar mechanism likely also operates also in the human GI epithelium and could explain the wide range in age-of-onset of LS-associated tumorigenesis.

Evaluation of immunohistochemical expression of mismatch repair proteins in endometrioid and seromucinous borderline ovarian tumors a 10-year experience in a large health care institution.

e17557 Background: Borderline tumors (BT) are atypical proliferation of epithelium in the ovary in the absence of destructive stromal invasion, representing for 15% of all epithelial ovarian cancers. [1] Around 10% of the ovarian tumors are hereditary, and approximately 10% of all the hereditary forms of epithelial ovarian tumors are result of a loss of DNA mismatch repair (MMR). [2] Endometrioid borderline tumor (EBT) and Seromucinous borderline tumors (SMBT) are rare tumors in ovary and there is limited literature available on immunohistochemical (IHC) expression of Mismatch repair proteins(MMRP)in these tumors.[3, 4] The aim of this study is to evaluate IHC expression of MMRP in EBT and SMBT of ovary. Methods: Pathology database was searched for ovarian Endometrioid borderline tumor (EBT) and Seromucinous borderline tumor (SMBT) for a 10-year period (2010-2020). The cohort consisted of 10 EBT (6 of which had focal microinvasion or carcinoma) and 12 SMBT(2 of which had focal carcinoma ). For comparison, 1 borderline Brenner. 15 serous borderline tumors (SBT) and 15 mucinous borderline tumors (MBT) were also included. After reviewing slides, a block with adequate borderline tumor was selected for IHC stains. For the cases with carcinoma, two different blocks with each component were selected. In all selected blocks, IHC stains for four MMRP (MLH1, PMS2, MSH2, MSH6) were performed. The complete absence of nuclear staining in tumor cells was considered as “loss” of the MMRP expression. Any “weak” or “focal” nuclear staining was considered intact. Results: Total 53 cases were evaluated for MMRP IHC. All cases had intact MMRP expression. In cases with carcinoma, both components (BT and carcinoma) have intact MMR IHC expression. See table. Conclusions: Our study did not show loss of MMRP IHC expression in EMT or SMBT. However, our study consisted of a small number of cases. Multi Institutional study with a large number of cases can be helpful in future to further evaluate the role of MMRP IHC in EMT and SMBT. [Table: see text]

Abstract IA008: Mismatch repair deficiency and survival in stage I endometrial cancer

Abstract The importance of testing for DNA mismatch repair defects and high levels of tumor microsatellite instability (MSI) is established for screening colorectal and endometrial cancers for Lynch Syndrome and advanced solid tumor malignancies for treatment with checkpoint inhibitors. Therefore, accurate clinical assays are necessary to identify DNA mismatch repair defects and patients with cancers with high levels of MSI. While defects in DNA mismatch repair identified by immunohistochemistry may often overlap with tumor microsatellite instability as identified by PCR-based assays, the two molecular events are not synonymous and can be discordant in individual cancers. Cancers outside of the GI tract may have loss of DNA mismatch repair proteins but may have more subtle microsatellite instability or even be microsatellite stable. The College of American Pathologists has established an expert guideline committee to help navigate some of the clinical testing issues when trying to detect defects in DNA mismatch repair and MSI in cancers. In addition to these established clinical uses of DNA mismatch repair and MSI, their use as a prognostic biomarker for endometrial cancer is emerging. Data suggest that specific environmental factors may interact with DNA mismatch repair, causing a more variable phenotype in cancers with loss of DNA mismatch repair. Citation Format: Russell R. Broaddus. Mismatch repair deficiency and survival in stage I endometrial cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference: Endometrial Cancer: New Biology Driving Research and Treatment; 2020 Nov 9-10. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(3_Suppl):Abstract nr IA008.

Histologic Appearance and Immunohistochemistry of DNA Mismatch Repair Protein and p53 in Endometrial Carcinosarcoma: Impact on Prognosis and Insights Into Tumorigenesis.

Endometrial carcinosarcoma (ECS) is a rare and aggressive mixed-type epithelial and mesenchymal tumor. This study focused on the histologic appearance, loss of DNA mismatch repair (MMR) protein expression, and aberrant p53 expression in the epithelial component, and overall prognosis of 57 cases with ECS. Histologically, 21 and 36 cases exhibited low-grade (endometrioid grade 1 and 2) and high-grade (others) epithelial components, respectively. In a Kaplan-Meier analysis, patients with a high-grade epithelial component exhibited worse progression-free survival (PFS), compared with those with a low-grade component. Although the former group also exhibited worse overall survival, the difference was not significant. Thirty-six cases exhibited aberrant p53 expression. Of these, 5 cases exhibited focally aberrant p53 expression in carcinomatous components with diffuse aberrant p53 expression in mesenchymal components. Aberrant expression of p53 did not show significant association with prognosis. Six patients with MMR deficiency exhibited relatively better PFS. In conclusion, a low-grade epithelial component is a superior predictor of the PFS of ECS, compared with MMR protein and p53 expression status. In some cases of ECS, TP53 mutation may be a late event associated with histogenesis of the sarcomatous component.

Usefulness of immunohistochemistry for mismatch repair protein and microsatellite instability examination in adenocarcinoma and background endometrium of sporadic endometrial cancer cases.

Microsatellite instability (MSI), which reflects loss of DNA mismatch repair (MMR) activity, and immunohistochemistry (IHC) for MMR proteins are employed as screening examinations for Lynch syndrome (LS). Recent studies revealed that there is a population of MSI-high tumors in sporadic endometrial cancer (EC). However, MSI data for Japanese EC patients are scarce. Furthermore, sporadic estrogen-dependent EC (type I) is generally considered to arise from hyperplasia. Because LS is usually associated with type I EC, we hypothesized that MSI might be involved in the oncogenic process in some sporadic EC. We conducted MSI testing to reveal MSI status in sporadic Japanese EC. IHC for MMR proteins was also performed. Ninety-eight tissue samples of sporadic ECs from Japanese patients were used for IHC and MSI examinations. We also evaluated MMR protein expressions in the background normal endometrium. Microsatellite instability-high was observed in 10.2% of 98 cases with sporadic EC, a lower percentage than that in Western studies. Loss of some MMR proteins was observed in 23 cases (23.5%) and there was a significant correlation with MSI-high status (P < 0.001). Concerning the background endometrium, two cases showed partial loss of MLH1 and PMS2, corresponding to adjacent EC lesions, suggesting that MMR deficiency may already be present in the background endometrium. The MSI-high rate was low in our Japanese cohort. Our data confirmed the usefulness of MMR protein assessment for MSI screening in Japanese EC patients. Furthermore, IHC of the background endometrium might reveal the mechanism of MSI-high tumorigenesis.

EPCAM mutation update: Variants associated with congenital tufting enteropathy and Lynch syndrome.

The epithelial cell adhesion molecule gene (EPCAM, previously known as TACSTD1 or TROP1) encodes a membrane‐bound protein that is localized to the basolateral membrane of epithelial cells and is overexpressed in some tumors. Biallelic mutations in EPCAM cause congenital tufting enteropathy (CTE), which is a rare chronic diarrheal disorder presenting in infancy. Monoallelic deletions of the 3′ end of EPCAM that silence the downstream gene, MSH2, cause a form of Lynch syndrome, which is a cancer predisposition syndrome associated with loss of DNA mismatch repair. Here, we report 13 novel EPCAM mutations from 17 CTE patients from two separate centers, review EPCAM mutations associated with CTE and Lynch syndrome, and structurally model pathogenic missense mutations. Statistical analyses indicate that the c.499dupC (previously reported as c.498insC) frameshift mutation was associated with more severe treatment regimens and greater mortality in CTE, whereas the c.556‐14A>G and c.491+1G>A splice site mutations were not correlated with treatments or outcomes significantly different than random simulation. These findings suggest that genotype–phenotype correlations may be useful in contributing to management decisions of CTE patients. Depending on the type and nature of EPCAM mutation, one of two unrelated diseases may occur, CTE or Lynch syndrome.

Abstract 421: Improved detection and identification of microsatellite instability features in colorectal cancer: Implications for immunotherapy

Abstract Colorectal cancers with loss of DNA mismatch repair display microsatellite instability (MSI). These tumors have a better prognosis and an improved response rate to immune check point therapies compared to non-MSI tumors. MSI-positive tumors demonstrate a surprising range of somatic alterations in repetitive sequence tracts. For example, elevated microsatellite alterations at selected tetranucleotide repeats (EMAST) that may be related to functional loss of MSH3. Current MSI tests are based on PCR and capillary electrophoresis. This conventional approach is suboptimal because i) only a small number of microsatellite markers are interrogated per assay, and ii) sensitivity is insufficient for low allelic fraction alleles in heterogeneous tumors. Next generation sequencing permits the inclusion of more microsatellite markers, but these NGS method still rely on PCR methods, which is the major source of artifacts that obscure the true somatic alterations. In this study, we analyzed a series of colorectal cancers using a new sequencing technology that sequences in parallel more than 200 microsatellite regions - we evaluate mono-, di- , tri- and tetranucleotide repeats to provide a broader spectrum of MSI events. This deep sequencing technology provides greater than 2,000X coverage, uses in vitro CRISPR-Cas9 segmentation, and eliminates the need for PCR amplification. Because every read corresponded to a single DNA molecule, we eliminated the artifacts introduced by PCR that obscured true MSI events. Moreover, we improved the sensitivity of detecting minor allele fractions to as low as 0.1% by measuring microsatellite haplotypes. Included in our analysis was new method for somatic copy number changes in 82 cancer genes, by which we can infer chromosomal instability (CIN) status. Among our cases, 10 cancers had MSI when considering both mononucleotide and tetranucleotide repeats. Interestingly, diagnostic testing of two of these tumors were false negatives and were demonstrated to be MSI-high. There were three major types among the 10 MSI cancers: MSI-high, EMAST, and the cancers having both. However, such types were distinguishable only by microsatellites with intermediate length (10 - 20 bp). MSI-high tumors (n = 3) showed no sign of CIN while all the EMAST tumors (n = 4) also showed CIN phenotype. Our results suggest that the current gold standard MSI diagnostic test may have issues with both false negatives and false positives, indicating that NGS approaches are better suited for evaluating MSI status of tumors and consideration of immunotherapy. Citation Format: Giwon Shin, HoJoon Lee, Susan M. Grimes, Matthew A. Kubit, Hanlee P. Ji. Improved detection and identification of microsatellite instability features in colorectal cancer: Implications for immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 421.

Molecular genetics of colorectal cancer

Colorectal cancer is the end result of an accumulation of destabilizing mutations and other genetic events, which occur in clones of colonocytes over many years. While each colorectal cancer is genetically unique, there are at least three distinct mechanisms by which the process occurs. The commonest is chromosomal instability, producing microsatellite stable, aneuploid cancers. The second is DNA promoter methylation that underlies CpG island methylation phenotype cancers, either microsatellite stable or unstable, and the third is loss of DNA mismatch repair, causing microsatellite unstable, diploid cancers. Cancers arising from these three mechanisms are biologically different and the differences have clinical implications.

Impact of Microsatellite Instability in Signet-Ring Cell and Mucinous Components in Patients With Colorectal Carcinoma.

Impact of Microsatellite Instability in Signet-Ring Cell and Mucinous Components in Patients With Colorectal Carcinoma.

Causes and consequences of microsatellite instability in gastric carcinogenesis.

Loss of DNA mismatch repair (MMR) function, due to somatic or germline epi/genetic alterations of MMR genes leads to the accumulation of numerous mutations across the genome, creating a molecular phenotype known as microsatellite instability (MSI). In gastric cancer (GC), MSI occurs in about 15% to 30% of the cases. This review summarizes the current knowledge on the molecular mechanisms underlying the acquisition of MSI in GC as well as on the clinic, pathologic and molecular consequences of the MSI phenotype. Additionally, current therapeutic strategies for GC and their applicability in the MSI subset are also discussed.

Clinicopathologic significance of DNA mismatch repair protein defects and endometrial cancer in women 40 years of age and younger

Background The hereditary basis of endometrial cancer is apparent in young women with endometrial cancer. The objective of this study was to examine risk factors and outcomes in patients 40 years of age and younger with endometrial cancer. Methods We performed a retrospective cohort study of patients aged 40 years or less who were diagnosed with endometrial carcinoma between 1/93 and 5/08. Clinical and pathologic data were extracted from medical records. Paraffin-embedded slides from hysterectomy specimens were obtained and DNA mismatch repair (MMR) immunohistochemistry was performed. Cases were analyzed according to the presence of DNA MMR protein defects. Standard two-sided statistical tests were performed. Results Of the 56 identified patients, the median age was 36 years (range, 24–40). The majority of the endometrial carcinomas were of endometrioid histology (91%), stage I (71%), and FIGO grade 1 (59%). Abnormal DNA MMR was found in 9 cases (16%). Cases with abnormal DNA MMR had lower body mass index (BMI) ( P = 0.05), and had a family history suggestive of Lynch syndrome ( P = 0.001). Tumors were more likely to have advanced stage disease ( P < 0.001), be high grade ( P < 0.001), have deep myometrial invasion ( P < 0.001), and have lymphovascular invasion ( P = 0.002). Cases with abnormal DNA MMR had significantly worse overall survival ( P = 0.028) and progression-free survival ( P = 0.042). Conclusions Endometrial cancer is rare in women aged 40 years or less. In this group of patients, loss of DNA MMR was associated with lower BMI, worse clinicopathologic factors, and worse outcome. These results may have implications when young women diagnosed with endometrial cancer are counseled regarding prognosis.

Loss of DNA Mismatch Repair Imparts a Selective Advantage in Planarian Adult Stem Cells

Lynch syndrome (LS) leads to an increased risk of early-onset colorectal and other types of cancer and is caused by germline mutations in DNA mismatch repair (MMR) genes. Loss of MMR function results in a mutator phenotype that likely underlies its role in tumorigenesis. However, loss of MMR also results in the elimination of a DNA damage-induced checkpoint/apoptosis activation barrier that may allow damaged cells to grow unchecked. A fundamental question is whether loss of MMR provides pre-cancerous stem cells an immediate selective advantage in addition to establishing a mutator phenotype. To test this hypothesis in an in vivo system, we utilized the planarian Schmidtea mediterranea which contains a significant population of identifiable adult stem cells. We identified a planarian homolog of human MSH2, a MMR gene which is mutated in 38% of LS cases. The planarian Smed-msh2 is expressed in stem cells and some progeny. We depleted Smed-msh2 mRNA levels by RNA-interference and found a striking survival advantage in these animals treated with a cytotoxic DNA alkylating agent compared to control animals. We demonstrated that this tolerance to DNA damage is due to the survival of mitotically active, MMR-deficient stem cells. Our results suggest that loss of MMR provides an in vivo survival advantage to the stem cell population in the presence of DNA damage that may have implications for tumorigenesis.

The immune biology of microsatellite unstable cancer

Colorectal cancers (CRC) develop through 2 major pathways of genetic instability. In contrast to the majority of CRCs, which are characterized by chromosomal instability, high-level microsatellite unstable (MSI-H) CRCs arise as a consequence of the loss of DNA mismatch repair (MMR) functions and show accumulation of insertion and deletion mutations particularly in microsatellite sequences. MSI-H occurs in about 15% of CRCs, and virtually all CRCs occurring in the context of the hereditary cancer-predisposing Lynch syndrome. These tumors are characterized by a comparably good prognosis and a low frequency of distant metastases. Because of the expression of a defined set of tumor-specific antigens, MSI-H CRCs elicit a strong local and systemic antitumoral immune response of the host and therefore use different strategies to evade the control of the immune system. At the conference we will summarize novel molecular mechanisms that at the same time drive pathogenesis, immunogenicity and immune evasion during the development and progression of MSI-H CRCs. We will focus on the current knowledge about alterations in human leukocyte antigen (HLA) antigen presentation and discuss how immune evasion-while offering protection against local antitumoral immune responses-paradoxically might interfere with the ability of the tumor to form distant organ metastases.

Immune evasion of microsatellite unstable colorectal cancers

Colorectal cancers (CRC) develop through 2 major pathways of genetic instability. In contrast to the majority of CRCs, which are characterized by chromosomal instability, high-level microsatellite unstable (MSI-H) CRCs arise as a consequence of the loss of DNA mismatch repair (MMR) functions and show accumulation of insertion and deletion mutations particularly in microsatellite sequences. MSI-H occurs in about 15% of CRCs, and virtually all CRCs occurring in the context of the hereditary cancer-predisposing Lynch syndrome. These tumors are characterized by a comparably good prognosis and a low frequency of distant metastases. Because of the expression of a defined set of tumor-specific antigens, MSI-H CRCs elicit a strong local and systemic antitumoral immune response of the host and therefore use different strategies to evade the control of the immune system. In this review, we will summarize novel molecular mechanisms that at the same time drive pathogenesis, immunogenicity and immune evasion during the development and progression of MSI-H CRCs. We will focus on the current knowledge about alterations in human leukocyte antigen (HLA) antigen presentation and discuss how immune evasion-while offering protection against local antitumoral immune responses-paradoxically might interfere with the ability of the tumor to form distant organ metastases.

DNA mismatch repair protein defects and endometrial cancer in women age 40 and younger

5579 Background: The significance of the hereditary basis of endometrial cancer is apparent in young women with endometrial cancer. The objective of this study was to describe the incidence and prognostic implications of DNA mismatch repair protein defects in a series of patients age 40 years and younger with endometrial cancer. Methods: We performed a retrospective cohort study of patients age 40 years or less who were diagnosed with endometrial carcinoma between 1/93 and 5/08. Clinical and pathologic data were extracted from medical records. Paraffin-embedded slides from hysterectomy specimens were obtained and DNA mismatch repair (MMR) immunohistochemistry (IHC) was performed. Cases were analyzed according to presence of DNA MMR protein defects. Standard two-sided statistical tests were performed. Results: Of the 71 identified patients, the median age was 37 years (range, 24–40), with a median follow-up of 47 months (range, 1–178). The majority were of endometrioid histology (94%), stage I (73%), and FIGO grade 1 (61%). IHC was performed with available blocks (n = 56), and loss of DNA MMR was found in 9 cases (16%). Cases with loss of DNA MMR were more likely to have high-grade (FIGO 2 or 3) tumors (p &lt; 0.001), be advanced stage (III or IV) at the time of diagnosis (p &lt; 0.001), and have a family history suggestive of hereditary non-polyposis colorectal cancer (p = 0.01). There was no difference between the groups in histology, obesity (BMI&gt;30), parity, or history of infertility. Analysis of clinical outcomes revealed that cases with loss of DNA MMR had significantly worse overall survival (median survival not reached, log rank p = 0.018). At the time of last follow-up, 2 (22%) patients with loss of DNA MMR were dead of disease compared with 2 (4%) patients with retained DNA MMR. Conclusions: Endometrial cancer is rare in young women age 40 years or less. In these patients, loss of DNA MMR was associated with worse clinicopathologic factors (high-grade and advanced-stage tumors) and worse outcome. Other clinical risk factors associated with endometrial cancer were not associated with loss of DNA MMR. The data suggest that MMR testing in women 40 years of age or younger with endometrial cancer may have clinically useful prognostic information. No significant financial relationships to disclose.

The mismatch repair‐mediated cell cycle checkpoint response to fluorodeoxyuridine

The loss of DNA mismatch repair (MMR) is responsible for hereditary nonpolyposis colorectal cancer and a subset of sporadic tumors. Acquired resistance or tolerance to some anti-cancer drugs occurs when MMR function is impaired. 5-Fluorouracil (FU), an anti-cancer drug used in the treatment of advanced colorectal and other cancers, and its metabolites are incorporated into RNA and DNA and inhibit thymidylate synthase resulting in depletion of dTTP and incorporation in DNA of uracil. Although the MMR deficiency has been implicated in tolerance to FU, the mechanism of cell killing remains unclear. Here, we examine the cellular response to fluorodeoxyuridine (FdU) and the role of the MMR system. After brief exposure of cells to low doses of FdU, MMR mediates DNA damage signaling during S-phase and triggers arrest in G2/M in the first cell cycle in a manner requiring MutSalpha, MutLalpha, and DNA replication. Cell cycle arrest is mediated by ATR kinase and results in phosphorylation of Chk1 and SMC1. MutSalpha binds FdU:G mispairs in vitro consistent with its being a DNA damage sensor. Prolonged treatment with FdU results in an irreversible arrest in G2 that is independent of MMR status and leads to the accumulation of DNA lesions that are targeted by the base excision repair (BER) pathway. Thus, MMR can act as a direct sensor of FdU-mediated DNA lesions eliciting cell cycle arrest via the ATR/Chk1 pathway. However, at higher levels of damage, other damage surveillance pathways such as BER also play important roles.

Preferential loss of mismatch repair function in refractory and relapsed acute myeloid leukemia: potential contribution to AML progression

Acute myeloid leukemia (AML) is an aggressive hematological cancer. Despite therapeutic regimens that lead to complete remission, the vast majority of patients undergo relapse. The molecular mechanisms underlying AML development and relapse remain incompletely defined. To explore whether loss of DNA mismatch repair (MMR) function is involved in AML, we screened two key MMR genes, MSH2 and MLH1, for mutations and promoter hypermethylation in leukemia specimens from 53 AML patients and blood from 17 non-cancer controls. We show here that whereas no amino acid alteration or promoter hypermethylation was detected in all control samples, 18 AML patients exhibited either mutations in MMR genes or hypermethylation in the MLH1 promoter. In vitro functional MMR analysis revealed that almost all the mutations analyzed resulted in loss of MMR function. MMR defects were significantly more frequent in patients with refractory or relapsed AML compared with newly diagnosed patients. These observations suggest for the first time that the loss of MMR function is associated with refractory and relapsed AML and may contribute to disease pathogenesis.

Microsatellite Instability in Pediatric and Adult High‐grade Gliomas

About 15% of sporadic gastrointestinal and endometrial tumors show the microsatellite instability (MSI) phenotype because of loss of DNA mismatch repair (MMR) function. The incidence of MSI in tumors of the central nervous system still remains controversial. Previous studies reported a particular high frequency of MSI (approximately 25%) in young patients suffering from high-grade gliomas. Based on these data and the fact that in different tumor entities MMR deficiency defines a subgroup of tumors with distinct pathogenesis and particular clinicopathological features that may have impact on prognosis and therapy, we screened 624 gliomas from 71 young and 553 adult patients for MMR deficiency by MSI analysis using three highly sensitive diagnostic markers. Alterations of MMR protein expression was examined by immunohistochemistry. A malignant glioma from an adult patient displayed MSI and concomitant loss of nuclear MSH2 and MSH6 protein expression (0.16%; 1/619). No evidence for MSI or loss of MMR protein expression was observed in 71 gliomas from young patients (0%; 0/71) including 41 high-grade astrocytic tumors. Overall, we observed a much lower incidence of MSI among high-grade pediatric gliomas than initially reported and suggest that MMR deficiency does not play a major role in the pathogenesis of glial neoplasms.