Microsatellite instability is associated with genetic alteration but not with low levels of expression of the human mismatch repair proteins hMSH2 and hMLH1

Cold atmospheric plasma stabilizes mismatch repair for effective, uniform treatment of diverse colorectal cancer cell types

Mismatch Repair (MMR) mechanisms play a pivotal role in rectifying DNA replication errors and maintaining the stability of DNA microsatellite structure. Colorectal cancer (CRC) can be characterized into microsatellite stability (MSS) and microsatellite instability (MSI) subtypes based on the functionality of MMR. MSI CRC notably exhibits enhanced chemotherapy resistance, attributable to diminished MMR-related protein expression. Cold atmospheric plasma (CAP) has emerged as a promising treatment modality, demonstrating efficacy in inducing apoptosis in various cancer cells. However, the therapeutic impact of CAP on MSI colorectal cancer, and the underlying mechanisms remain elusive. In this study, we investigated the effects of CAP on MSI (MC38, HCT116, and LOVO) and MSS (CT26 and HT29) CRC cell lines. We are probing into the products of CAP treatment. Our findings indicate that CAP treatment induces comparable effects on apoptosis, reactive oxygen species (ROS), and reactive nitrogen species (RNS), as well as the expression of apoptosis-related proteins in both MSI and MSS cells. Mechanistically, CAP treatment led to an elevation in the expression of mismatch repair proteins (MLH1 and MSH2), particularly in MSI cells, which notably have been proven to facilitate the activation of apoptosis-related proteins. Collectively, our study reveals that CAP enhances apoptotic signaling and induces apoptosis in MSI colorectal cancer cells by upregulating the expression of MMR-related proteins, thereby reinforcing MMR stabilization.

Upper tract urothelial carcinoma has a luminal-papillary T-cell depleted contexture and activated FGFR3 signaling

Upper tract urothelial carcinoma (UTUC) is characterized by a distinctly aggressive clinical phenotype. To define the biological features driving this phenotype, we performed an integrated analysis of whole-exome and RNA sequencing of UTUC. Here we report several key insights from our molecular dissection of this disease: 1) Most UTUCs are luminal-papillary; 2) UTUC has a T-cell depleted immune contexture; 3) High FGFR3 expression is enriched in UTUC and correlates with its T-cell depleted immune microenvironment; 4) Sporadic UTUC is characterized by a lower total mutational burden than urothelial carcinoma of the bladder. Our findings lay the foundation for a deeper understanding of UTUC biology and provide a rationale for the development of UTUC-specific treatment strategies.

DNA Repair Capacity in Multiple Pathways Predicts Chemoresistance in Glioblastoma Multiforme

Cancer cells can resist the effects of DNA-damaging therapeutic agents via utilization of DNA repair pathways, suggesting that DNA repair capacity (DRC) measurements in cancer cells could be used to identify patients most likely to respond to treatment. However, the limitations of available technologies have so far precluded adoption of this approach in the clinic. We recently developed fluorescence-based multiplexed host cell reactivation (FM-HCR) assays to measure DRC in multiple pathways. Here we apply a mathematical model that uses DRC in multiple pathways to predict cellular resistance to killing by DNA-damaging agents. This model, developed using FM-HCR and drug sensitivity measurements in 24 human lymphoblastoid cell lines, was applied to a panel of 12 patient-derived xenograft (PDX) models of glioblastoma to predict glioblastoma response to treatment with the chemotherapeutic DNA-damaging agent temozolomide. This work showed that, in addition to changes in O⁶-methylguanine DNA methyltransferase (MGMT) activity, small changes in mismatch repair (MMR), nucleotide excision repair (NER), and homologous recombination (HR) capacity contributed to acquired temozolomide resistance in PDX models and led to reduced relative survival prolongation following temozolomide treatment of orthotopic mouse models in vivo Our data indicate that measuring the combined status of MMR, HR, NER, and MGMT provided a more robust prediction of temozolomide resistance than assessments of MGMT activity alone. Cancer Res; 77(1); 198-206. ©2016 AACR.

Minor Changes in Expression of the Mismatch Repair Protein MSH2 Exert a Major Impact on Glioblastoma Response to Temozolomide

Glioblastoma (GBM) is often treated with the cytotoxic drug temozolomide, but the disease inevitably recurs in a drug-resistant form after initial treatment. Here, we report that in GBM cells, even a modest decrease in the mismatch repair (MMR) components MSH2 and MSH6 have profound effects on temozolomide sensitivity. RNAi-mediated attenuation of MSH2 and MSH6 showed that such modest decreases provided an unexpectedly strong mechanism of temozolomide resistance. In a mouse xenograft model of human GBM, small changes in MSH2 were sufficient to suppress temozolomide-induced tumor regression. Using The Cancer Genome Atlas to analyze mRNA expression patterns in tumors from temozolomide-treated GBM patients, we found that MSH2 transcripts in primary GBM could predict patient responses to initial temozolomide therapy. In recurrent disease, the absence of microsatellite instability (the standard marker for MMR deficiency) suggests a lack of involvement of MMR in the resistant phenotype of recurrent disease. However, more recent studies reveal that decreased MMR protein levels occur often in recurrent GBM. In accordance with our findings, these reported decreases may constitute a mechanism by which GBM evades temozolomide sensitivity while maintaining microsatellite stability. Overall, our results highlight the powerful effects of MSH2 attenuation as a potent mediator of temozolomide resistance and argue that MMR activity offers a predictive marker for initial therapeutic response to temozolomide treatment.

Low frequency of HNPCC-associated microsatellite instability and aberrant MMR protein expression in early-onset bladder cancer

OBJECTIVES

Recently, it was shown that patients with Lynch syndrome due to an MSH2 mutation are at increased risk for the development of bladder cancer. To further this discussion, we screened the largest investigated cohort of patients with early-onset bladder cancer for microsatellite instability (MSI) and mismatch repair (MMR) deficiency to determine a possible role of Lynch syndrome in young patients with bladder cancer.

METHODS

A total of 109 cases of bladder tumors from young patients (aged <45 years) were examined for MSI (Bethesda consensus panel). Expression of MMR proteins (hMLH1, hMSH2, and hMSH6) was evaluated by immunohistochemistry using a tissue microarray. Results were compared with a series of unselected consecutive bladder tumors (n = 95).

RESULTS

Regarding the frequency of MSI high (1% vs 0%) or abnormal expression of MMR proteins (2% vs 6.5%), no significant difference between the early-onset and unselected patient group was found.

CONCLUSIONS

In young patients with bladder tumors, MSI and defects in MMR protein expression were not more frequent than in a series of consecutive bladder tumors. Most bladder tumors in young patients are not to be attributed to Lynch syndrome.

Comprehensive genome- and transcriptome-wide analyses of mutations associated with microsatellite instability in Korean gastric cancers

Microsatellite instability (MSI) is a critical mechanism that drives genetic aberrations in cancer. To identify the entire MS mutation, we performed the first comprehensive genome- and transcriptome-wide analyses of mutations associated with MSI in Korean gastric cancer cell lines and primary tissues. We identified 18,377 MS mutations of five or more repeat nucleotides in coding sequences and untranslated regions of genes, and discovered 139 individual genes whose expression was down-regulated in association with UTR MS mutation. In addition, we found that 90.5% of MS mutations with deletions in gene regions occurred in UTRs. This analysis emphasizes the genetic diversity of MSI-H gastric tumors and provides clues to the mechanistic basis of instability in microsatellite unstable gastric cancers.

Genetic and structural variation in the gastric cancer kinome revealed through targeted deep sequencing

Genetic alterations in kinases have been linked to multiple human pathologies. To explore the landscape of kinase genetic variation in gastric cancer (GC), we used targeted, paired-end deep sequencing to analyze 532 protein and phosphoinositide kinases in 14 GC cell lines. We identified 10,604 single-nucleotide variants (SNV) in kinase exons including greater than 300 novel nonsynonymous SNVs. Family-wise analysis of the nonsynonymous SNVs revealed a significant enrichment in mitogen-activated protein kinase (MAPK)-related genes (P < 0.01), suggesting a preferential involvement of this kinase family in GC. A potential antioncogenic role for MAP2K4, a gene exhibiting recurrent alterations in 2 lines, was functionally supported by siRNA knockdown and overexpression studies in wild-type and MAP2K4 variant lines. The deep sequencing data also revealed novel, large-scale structural rearrangement events involving kinases including gene fusions involving CDK12 and the ERBB2 receptor tyrosine kinase in MKN7 cells. Integrating SNVs and copy number alterations, we identified Hs746T as a cell line exhibiting both splice-site mutations and genomic amplification of MET, resulting in MET protein overexpression. When applied to primary GCs, we identified somatic mutations in 8 kinases, 4 of which were recurrently altered in both primary tumors and cell lines (MAP3K6, STK31, FER, and CDKL5). These results demonstrate that how targeted deep sequencing approaches can deliver unprecedented multilevel characterization of a medically and pharmacologically relevant gene family. The catalog of kinome genetic variants assembled here may broaden our knowledge on kinases and provide useful information on genetic alterations in GC.

High polymorphism in the trisomic portion of a gastric cancer cell line

BACKGROUND

Genetic instability is a hallmark of malignancy, and microsatellite instability is a widely appreciated mechanism of generating genetic changes. We have recently observed four markers clustered on chromosome 20 that showed the effects of microsatellite instability in the gastric adenocarcinoma cell line SNU-1. Each affected marker had alleles of three different sizes. The aim of this study was to investigate the origin for this high-density polymorphism on a single chromosome.

METHODS

The high polymorphism located on chromosome 20 was confirmed using 37 additional markers. To further evaluate this finding, 15 clones of the cell line were generated and then assayed with the triallelic markers.

RESULTS

All told, almost a third of the markers on chromosome 20 had triallelic patterns, but only 0.3% of the markers not on chromosome 20 showed this result. The number of clones showing allelic variation was an average of 50% greater for chromosome 20 markers than for markers elsewhere. A karyotype analysis showed that the progenitor cell line of SNU-1 was trisomic for chromosome 20, and the high polymorphism on that chromosome is almost certainly due to the trisomy.

CONCLUSIONS

Not only are there more chromosome copies and therefore more gene copies subject to mutation in cells containing trisomy, but also more mutations may be passed on to the progeny. This elevated polymorphism increases the repertoire of genetic changes that could affect cellular growth, and may independently increase genomic instability.

Immunohistochemical expression of mismatch repair genes: A screening tool for predicting mutator phenotype in liver fluke infection-associated intrahepatic cholangiocarcinoma

AIM: To clarify possible contributions of DNA mismatch repair (MMR) system in carcinogenesis of liver fluke infection-associated intrahepatic cholangiocarcinoma (ICC) by using immunohistochemical assay.

METHODS: A total of 29 ICC samples, which had been assessed for genomic instability by a PCR-based method, were used for study. They were examined immunohistochemically to demonstrate protein expression of two MMR genes, hMSH2 and hMLH1. Results obtained were compared with their mutator phenotype assessed previously.

RESULTS: Either hMSH2 or hMLH1 protein was obviously expressed in 28 of 29 (96.6%) ICC samples. Positive nuclear localization of hMSH2 or hMLH1 protein was observed in 86.2% (25/29) or 93.1% (27/29) ICC cases, respectively, while their negative nuclear reactivity was only detected in 13.8% (4/29) or 6.9% (2/29) ICC cases analyzed, respectively.

CONCLUSION: Our study, probably for the first time, showed through immunohistochemical detection of hMSH2 and hMLH1 gene that DNA MMR system does not play a prominent role in liver fluke infection-associated cholangiocarcinogenesis. These results confirm previous findings on mutational status of these genes assessed through a PCR-based method. The immunohistochemical analysis has proven to be an effective and sensitive approach for screening MMR deficiency regardless of somatic inactivation or promoter hypermethylation of hMSH2 and/or hMLH1 gene. Furthermore, immunohistochemistry is more advantageous compared to mutator phenotyping assay in terms of simplicity, less time consuming and cost effectiveness for screening possible involvements of target MMR genes in tumorigenesis.

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.

Biology of SNU cell lines

SNU (Seoul National University) cell lines have been established from Korean cancer patients since 1982. Of these 109 cell lines have been characterized and reported, i.e., 17 colorectal carcinoma, 12 hepatocellular carcinoma, 11 gastric carcinoma, 12 uterine cervical carcinoma, 17 B-lymphoblastoid cell lines derived from cancer patients, 5 ovarian carcinoma, 3 malignant mixed Mllerian tumor, 6 laryngeal squamous cell carcinoma, 7 renal cell carcinoma, 9 brain tumor, 6 biliary tract, and 4 pancreatic carcinoma cell lines. These SNU cell lines have been distributed to biomedical researchers domestic and worldwide through the KCLB (Korean Cell Line Bank), and have proven to be of value in various scientific research fields. The characteristics of these cell lines have been reported in over 180 international journals by our laboratory and by many other researchers from 1987. In this paper, the cellular and molecular characteristics of SNU human cancer cell lines are summarized according to their genetic and epigenetic alterations and functional analysis.

Genetic instability in primary leiomyosarcoma of bone

Primary leiomyosarcoma (LMS) of bone is an exceedingly rare entity on which to date no molecular data have been reported. In a series of 6 tumors (5 grade IIB, 1 grade IIA), we assessed the prevailing genetic stability by microsatellite analysis at 7 loci. The IIB tumors demonstrated a rate of genomic loss as high as 90%, accompanied by an intratumoral heterogeneity in 30% of conspicuous markers. High microsatellite instability in the severe type was not observed, although hMLH1 immunostaining was consistently negative. We assume that intraosseous LMS pertains to "deletor phenotype" tumors. We did observe a locus-specific MSI in our marker linked with hMSH2. Immunostaining and allelotyping indicated a knock-out of pRb in all cases, confirming its major role in sarcomas. Only the stage IIB tumors (4 of 5) pointed to p53 inactivation. In addition, the human telomerase subunit-linked markers exhibited high rates of chromosomal loss. The stage IIA tumor still confined to the bone displayed no genetic instability. Moreover, the proliferation index made a clear distinction between the IIA and IIB tumors (5% vs 30%). We propose to further investigate the usefulness of loss of heterozygosity as a progression marker in this entity.

Detection of frameshift mutations of RIZ in gastric cancers with microsatellite instability

AIM: To study the frameshift mutations of the retinoblastoma protein-interacting zinc finger gene RIZ in gastric cancer with microsatellite instability, and to identify two coding polyadenosine tracts of RIZ.

METHODS: Frameshift mutations at (A)₈ and (A)₉ tracts of RIZ were detected in 70 human gastric cancer (HGC) specimens by DHPLC and DNA sequencing. Microsatellite instability (MSI) status was assessed by two mononucleotide markers, BAT26 and BAT25, by means of denaturing high-performance liquid chromatography (DHPLC).

RESULTS: In 70 HGC samples, 8 (11.4%) were found positive for instabilities at BAT26 and BAT25. In 7 of the 8 cases with instabilities at both BAT26 and BAT25 (MSI-H), 1 was unstable at BAT26 but stable at BAT25. Frameshift mutations were identified in 4 (57.1%) of the 7 samples with MSI-H in the (A)₉ tract of RIZ without mutations in the (A)₈ tract. In contrast, frameshift mutations were found in neither of the polyadenosine tracts in 63 samples of MSI-L or MSI stable tumors. Pro704 LOH detection in 4 cases with frameshift mutations did not find LOH in these cases.

CONCLUSION: Frameshift mutations of RIZ may play an important role in gastric cancers with MSI.

Alterations of DNA mismatch repair proteins and microsatellite instability levels in gastric cancer cell lines

Alterations in DNA mismatch repair (MMR) proteins result in microsatellite instability (MSI), increased mutation accumulation at target genes and cancer development. About one-third of gastric cancers display high-level microsatellite instability (MSI-High) and low-level microsatellite instability (MSI-Low) is frequently detected. To determine whether variations in the levels of MMR proteins or mutations in the main DNA MMR genes are associated with MSI-Low and MSI-High in gastric cancer cell lines, the MSI status (MSI-High, MSI-Low or MS-Stable (MSS)) of 14 gastric cancer lines was determined using multiple clone analysis with a panel of five microsatellite markers. Protein levels of hMLH1, hMSH2, hMSH6, hPMS2 and hPMS1 were determined by Western blot. Sequence analysis of hMLH1 and hMSH2 was performed and the methylation status of the hMLH1 promoter was examined. The cell lines SNU1 and SNU638 showed MSI-High, decreased to essentially absent hMLH1 and hPMS2 and reduced hPMS1 and hMSH6 protein levels. The hMLH1 promoter region was hypermethylated in SNU638 cells. The MKN28, MKN87, KATOIII and SNU601 cell lines showed MSI-Low. The MMR protein levels of cells with MSI-Low status was similar to the levels detected in MSS cells. A marked decrease in the expression levels of MutL MMR proteins (hMLH1, hPMS2 and hPMS1) is associated with high levels of MSI mutations in gastric cancer cells. Gastric cancer cell lines with MSI-Low status do not show significant changes in the levels of the main DNA MMR proteins or mutations in the DNA mismatch repair genes hMSH2 and hMLH1. These well-characterized gastric cancer cell lines are a valuable resource to further our understanding of DNA MMR deficiency in cancer development, progression and prognosis.

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Differential expression of hMLH1 and hMSH2 is related to bladder cancer grade, stage and prognosis but not microsatellite instability

Defects in the DNA mismatch repair proteins result in microsatellite instability and malignancy in hereditary non-polyposis colorectal carcinoma (HNPCC). However, the role of mismatch repair (MMR) proteins and microsatellite instability (MSI) in transitional cell carcinoma of the bladder is less clear. In our study, the expression of 2 MMR proteins and the frequency of MSI in Transitional cell carcinoma of the bladder (TCC) were investigated. One hundred eleven patients with TCC of the bladder were studied, with complete clinicopathological data (median follow up of 5 years, range 5-16 years). Immunohistochemistry was used to detect the expression levels of hMLH1 and hMSH2. Microsatellite analysis for 14 loci (10 loci from the Bethesda consensus panel and the repeats in the TGFbetaR2, BAX, hMSH3 and hMSH6 genes) was performed on 84 tumors. Reduced expression of either MMR protein was seen in 26 of 111 tumors (23%). Reduced expression was seen more commonly in muscle invasive (p<0.03) and high grade TCC (p<0.03) than in superficial, low grade tumors. By 5 years, reduced expression of either MMR protein was associated with fewer recurrences of superficial tumors (p=0.015) and fewer relapses in all tumors (p=0.03), compared to tumors with normal expression. Nine tumors had reduced expression of both MMR proteins, analysis which suggests a synergistic reduction in expression (p=0.001). MMR expression was related to patient age, younger patients being more likely to have reduced MMR expression than older patients (p<0.01). MSI was seen at multiple loci in 1 tumor (1%) and at a single locus in 6 tumors (7%). MSI was not associated with MMR expression. Our findings indicate that reduced expression of the MMR proteins may have an important contribution in the development of a subset of TCCs and suggest a potential role for MMR expression as prognostic indicators.

Lymphocyte-rich gastric cancer: associations with Epstein-Barr virus, microsatellite instability, histology, and survival

Lymphocyte-rich gastric carcinomas may have a better prognosis than cancers without a pronounced host inflammatory response. Two subsets of gastric cancer-Epstein-Barr virus-positive and microsatellite instability high-have been associated with a lymphocyte-rich phenotype. We assessed relationships between tumor-infiltrating lymphocytes, Epstein-Barr virus status, microsatellite instability status, and cancer-specific survival in 110 resected gastric cancers. Seven patients had Epstein-Barr virus-positive cancer, including 4 (3.7%) of 107 consecutive patients. Tumors from 17 patients (16%) were designated microsatellite instability high on the basis of negative immunohistochemical staining for MLH1; all tumors had intact expression of MSH2 and MSH6. Epstein-Barr virus-positive cancers had increased tumor-infiltrating lymphocytes compared with Epstein-Barr virus-negative cancers (median 450/10 HPF versus 21/10 HPF, P <.001). Microsatellite instability-high cancers also had increased tumor-infiltrating lymphocytes compared with non-microsatellite instability-high cancers (median 150/10 HPF versus 20/HPF, P <.001). Microsatellite instability-high cancers affected older patients and were more likely to be intestinal in the Lauren classification and expanding in the Ming classification. By univariate analysis, decreased risk of death from gastric cancer was significantly associated with low tumor stage, expanding growth pattern, increasing tumor-infiltrating lymphocyte count, and microsatellite instability-high status. High tumor-infiltrating lymphocyte count and microsatellite instability-high status retained statistical significance as favorable prognostic factors after adjustment for tumor stage in multivariate analysis. Tumor-infiltrating lymphocyte count retained statistical significance as a favorable prognostic factor after adjustment for microsatellite instability-high status; but microsatellite instability-high status did not remain a significant independent prognosticator after adjustment for tumor-infiltrating lymphocyte count. The association between microsatellite instability-high cancers and high tumor-infiltrating lymphocyte counts may account for the association of microsatellite instability-high gastric cancers with improved survival.

Studies on microsatellite instability in p16 gene and expression of hMSH2 mRNA in human gastric cancer tissues

AIM: To detect the loss of heterozygosity (LOH) frequency of microsatellite sites D9s171, D9s1604 of p16 gene and expression of hMSH2 mRNA in various differentiated types of gastric cancer, adjacent cancer tissues and normal gastric mucosa.

METHODS: LOH was detected by polymerase chain reaction (PCR)-denaturing polyacrylamide gel electrophoresis-silver staining. The expression of hMSH2 mRNA was examined with in situ hybridization.

RESULTS: The frequency rate of LOH was significantly higher in gastric cancers than that in adjacent cancer tissues (P = 0.032). No significant difference was noted among various differentiated types and various clinical stages of gastric cancers. The significantly reduced expression of hMSH2 mRNA positive signal cells exhibited in gastric cancers, in comparison with that in the adjacent cancer tissues and normal gastric mucosa, respectively (P = 0.001). No significant difference was noted among various clinical stages of gastric cancers (P > 0.05). The difference of positive signal cells in poorly differentiated cancers and those in well and moderately differentiated cancers were significant (P < 0.001).

CONCLUSION: The frequencies of LOH in two microsatellite sites, D9s171 and D9s1604, in p16 genome were associated with development of gastric cancer and no significant correlation was demonstrated between the LOH frequency and the cell differentiated types of tumor cells or clinical stages. There was a positive relationship between the expression of hMSH2 mRNA and the differentiated types of gastric cancer.

High level of microsatellite instability but not hypermethylation of mismatch repair genes in therapy-related and secondary acute myeloid leukaemia and myelodysplastic syndrome

Microsatellite instability (MSI) is associated with defects in the DNA mismatch repair (MMR) system, such as mutation or epigenetic silencing of the genes by promoter hypermethylation. We investigated the presence of MSI and promoter hypermethylation of hMLH1 and hMSH2 genes in 82 patients (68 acute myeloid leukaemia, AML; 14 myelodysplastic syndromes, MDS). Twelve separate microsatellite loci, including three mononucleotide repeat markers, were used. Mutator phenotype (RER+) was detected in 20 AML (29.4%) and 3 MDS (21.4%) patients. RER+ rate was much higher in the therapy-related and secondary cases compared with the de novo cases. Three out of 7 (42.9%) secondary (s-AML) and 8 out of 17 (47.1%) therapy-related (t-AML) showed RER+ in comparison with 9 out of 44 (20.5%) de novo cases. Similar rates were detected in MDS patients (2/2 therapy-related and 1/12 de novo). The promoter hypermethylation was found in three hMLH1 (3.7%) and two hMSH2 (2.4%) genes. All these five patients had AML and were older than 60 years of age. Two of them had s-AML and one had t-AML. RER+ was detected in three of these five patients. Our data suggest that genetic instability is associated with AML and MDS, especially t-AML and s-AML. In addition, our results indicate that the hMSH2 and hMLH1 promoter hypermethylation is not a common event in these malignancies, but may play a role in the development of AML in elderly patients.

Methylation tolerance in mismatch repair proficient cells with low MSH2 protein level

Loss of DNA mismatch repair has been found in tumors associated with the familial cancer predisposition syndrome HNPCC (hereditary non-polyposis colorectal cancer) and a subset of sporadic cancers. MSH2 deficiency abolishes the action of the mismatch repair system, resulting in a phenotype which is characterized by an increased accumulation of base substitutions and frameshifts, enhanced recombination between homologous but non-identical DNA sequences, and tolerance to the cytotoxic effects of methylating agents. In this study we describe an embryonic stem cell line in which the level of MSH2 protein is 10-fold reduced compared to that in wild-type cells. Remarkably, these MSH2-low cells were as resistant to killing by methylating agents as cells completely lacking MSH2, while they had retained almost maximal mismatch repair capacity as judged from their anti-mutagenic and anti-recombinogenic capacity and the absence of microsatellite instability. In contrast, MSH2-low cells were highly sensitive to methylation-damage induced mutagenesis. Thus, 10-fold reduced MSH2 protein levels render cells resistant to the toxic and highly sensitive to the mutagenic effects of methylating agents. This condition is not manifested by microsatellite instability and may have implications for both the etiology and treatment of cancer.

Building bridges in cancer: mismatch repair and microsatellite instability

Mismatch repair (MMR) defects and microsatellite instability (MSI) are two genetic alterations that have been documented in a wide variety of human cancers, including some that involve the skin. Available evidence indicates that these two features are sometimes directly related, although their connection seems to be indirect or nonexistent in other instances. The purposes of this review are to summarize the variable relations between MMR and MSI as deduced from analysis of a diverse array of human neoplasms and to give brief insights as to the other molecular mechanisms potentially involved in the maintenance of genomic stability.

High-density allelotype of the commonly studied gastric cancer cell lines

Gastric cancer is one of the leading causes of death from cancer throughout the world, and studies to elucidate the genetic defects found in this type of cancer are growing in number. Increasingly sophisticated techniques and the sequencing of the human genome have had an impact on the scope of such studies. While the use of tumor specimens remains popular, more emphasis is being placed on cell lines as model systems where specific data can be directly combined with results from other studies. This article describes a genetic survey of the most widely used gastric adenocarcinoma cell lines. The allelotype at 351 polymorphic loci in 14 cell lines was obtained, and the results from the 4,900 polymerase chain reactions are displayed. In addition to confirming loss of heterozygosity on chromosome arms 6p, 7q, 17p, and 18, additional deletions on arm 5p and the pericentromeric regions of chromosomes 1 and 10 were detected. Areas that might contain homozygous deletions or amplifications also were mapped. The rate of microsatellite instability was quantified and shown to vary greatly among the different cell lines. Most important, this study serves as a genetic scaffold for the integration of past and future studies on the nature of the genetic defects in gastric cancer.

Alterations of mismatch repair protein expression in benign melanocytic nevi, melanocytic dysplastic nevi, and cutaneous malignant melanomas

Immunoperoxidase-staining methods were used to examine the expression of hMLH1, hMSH2, and hMSH6 mismatch repair (MMR) proteins in 50 melanocytic lesions. Microsatellite instability (MSI), screened previously in these lesions by polymerase chain reaction-based microsatellite assay, showed low-level microsatellite instability (MSI-L) in 11 of 22 melanocytic dysplastic nevi (MDN) and two of nine primary cutaneous malignant melanomas (CMMs) but not in the benign melanocytic nevi (BN). Mismatch repair proteins were widely expressed in the epidermis and adnexal structures. All lesions showed positive immunoreactivity with a gradual decrease in the MMR staining values during the progression from BN to MDN to CMMs. The average percentage of positively (PP) stained cells for hMLH1, hMSH2, and hMSH6 in BN was 85.50 +/- 1.95, 77.90 +/- 4.50, and 87.11 +/- 1.85, respectively. The PP cell values in CMMs were significantly reduced as compared with BN (75.22 +/- 3.57, p= 0.01; 56.11 +/- 8.73, p= 0.02; 65.22 +/- 6.47, p = 0.0002 for hMLH1, hMSH2, and hMSH6, respectively). No comparable significant difference was found between microsatellite stable and MSI-L lesions (p = 0.173, p = 0.458, and p = 0.385), suggesting a lack of correlation between MMR expression and MMR function. There was a direct correlation between PP cell values of hMSH2 and hMSH6 (R = 0.39, p = 0.008), implying that their expression could be regulated by a common mechanism. Thus, an important finding of these studies was the reduction of MMR protein levels in CMMs; whether this reflects underlying genetic or epigenetic mechanisms is still to be determined.