Oncogenic pathway of sporadic colorectal cancer with novel germline missense mutations in the hMSH2 gene

The deficiency of the DNA mismatch repair (MMR) system is involved in tumorigenesis of either familial or sporadic colorectal cancers showing microsatellite instability (MSI). To investigate the involvement of the mutated hMSH2 gene in carcinogenesis, we searched for alteration of the gene in 15 MSI tumors of Japanese patients with sporadic colorectal cancer by a polymerase chain reaction (PCR)-single strand conformation polymorphism (SSCP) and DNA sequencing analyses. We found 20 alterations including 7 novel mutations, 6 germline and one somatic. To assume an oncogenic pathway of tumor of two patients carrying germline missense mutations, G40S located in an evolutionarily conserved amino-terminal motif and Y619C in a domain interacting with either hMSH3 or hMSH6, somatic mutations in 9 target genes of the MMR defect and in the p53 and K-ras genes and loss of heterozygosity (LOH) at the hMLH1 and p53 gene loci were then studied. In the tumor carrying G40S, other somatic hMSH2 mutations, G203R and 687delA in the (A)(7) repeat, and 5 one-bp deletions in the target genes were found, while no mutation in the p53 and K-ras genes. These results indicate that G40S may affect the hMSH2 function and the tumor may be developed by a typical MSI pathway. In another tumor with Y619C, LOH at the hMLH1 gene locus, no mutation in MMR target genes, and two-hit inactivation of the p53 gene were detected. This MSI tumor seems to be developed by another than MSI pathway. These results indicate that there are different oncogenic pathways in the MSI sporadic colorectal cancers with germline missense mutations in the hMSH2 gene. We conclude that familial colorectal cancer-suspected cases exist in a small population of sporadic colorectal cancers.     

Mutations of hMLH1 and hMSH2 in patients with suspected hereditary nonpolyposis colorectal cancer: correlation with microsatellite instability and abnormalities of mismatch repair protein expression.

PURPOSE: The relationship between germ-line mutations of hMSH2 and hMLH1, microsatellite instability (MSI), and loss of DNA mismatch repair (MMR) gene expression were studied to formulate an effective selection protocol for patients with suspected hereditary nonpolyposis colorectal cancer who should be offered genetic testing. PATIENTS AND METHODS: Patients eligible for germ-line analysis of hMLH1 and hMSH2 were selected. Tumor specimens were obtained to assess MSI and loss of MMR gene expression. RESULTS: Among 37 patients who participated in the study, two hMSH2 and two hMLH1 missense mutations (11%) were detected, none of which was found in a panel of 60 healthy volunteers. High MSI was found in five tumors (19%) and low MSI in 10 tumors (39%); 12 tumors (46%) were microsatellite stable. Four tumors demonstrated loss of hMLH1, and three tumors demonstrated loss of hMSH2 protein expression. CONCLUSION: No relationship was found between MMR gene mutations and MSI; low or no MSI was found in the four patients with germ-line mutations, and none of the five patients with high MSI demonstrated abnormalities of MMR genes. On the contrary, loss of hMLH1 or hMSH2 expression was found in the tumors from three of the four patients demonstrating germ-line mutations. These data suggest that germ-line mutations of the MMR gene can occur in people with MSI-negative tumors. Sensitive clinical criteria and the study of MMR gene expression may be useful to identify this subset of patients.     

错配修复基因与大肠癌相关性研究进展

目的 MMR系统是一组高度保守基因及其产物蛋白构成,其主要功能是纠正DNA复制、重组及基因损伤时出现的碱基配对错误.本研究总结国内外错配修复基因与大肠癌相关性的研究进展.方法 检索Pubmed和万方数据库,以“错配修复基因、微卫星不稳定、大肠癌”等为关键词,检索1997-01-2016-06的相关文献共102篇.纳入标准:(1)错配修复基因与微卫星不稳定的关系.(2)错配修复基因大肠癌的发生及预后的关系.根据纳入标准,最终选取49篇文献纳入分析.结果 错配修复系统是一种广泛存在于机体细胞内的自我修复机制,对防止基因突变和维持基因组稳定性方面起着重要作用.错配修复基因突变与微卫星不稳定关系密切,错配修复基因发生突变时,使机体肿瘤易感性明显增加.结论 错配修复系统是大肠癌的发生、发展的重要预后指标.     

Prognosis in DNA Mismatch Repair Deficient Colorectal Cancer: are all MSI Tumours Equivalent?

Microsatellite instability (MSI) in colorectal tumours is the hallmark of defective DNA mismatch repair (MMR) and high level MSI can be detected in up to 15% of incident colorectal cancers. MSI in sporadic colorectal tumours is primarily due to epigenetic silencing of MLH1 while MSI is almost universal in tumours from HNPCC family members due to germline MMR gene mutation with loss or mutational inactivation of the second copy as a somatic event. There is evidence that tumour MSI is associated with a better outcome than the generality of large bowel malignancy. However, although MSI occurs in both sporadic colorectal cancer and in tumours arising in patients with germline MMR gene mutations, cancer survival should not be considered to be equivalent for these two groups with MSI tumours simply because both exhibit similarities in molecular phenotype. Here, we review the evidence on prognosis in patients with sporadic MSI tumours compared to those who have inherited a germline DNA MMR repair gene defect. In addition, we explore whether there are variables that afford opportunity to distinguish three groups on the basis of MSI status, namely: sporadic MSI tumours; MSI tumours in carriers of germline MMR gene defects; microsatellite stable (MSS) tumours. Differences in prognosis between these three groups is important because it underpins the rationale for surveillance and early identification of tumours in MMR gene carriers, as well as refining understanding of the influence of MSI on cancer progression. Furthermore, we discuss the effect of MSI on the effectiveness of chemotherapy regimens.     

Infrequent widespread microsatellite instability in hepatocellular carcinomas

Widespread or high-frequency microsatellite instability (MSI) due to the defective DNA mismatch repair (MMR) occurs in the majority of hereditary non-polyposis colorectal cancer and a subset of sporadic malignant tumors. The incidence of MSI and underlying DNA MMR defects have been well characterized in gastrointestinal carcinogenesis, but not in hepatocarcinogenesis. To address the issue, we analyzed 55 Japanese hepatocellular carcinomas using several indicators of DNA MMR defects, such as microsatellite analysis, loss of heterozygosity (LOH) and mutation analysis of MMR genes, methylation of hMLH1 promoter, and frameshift mutations of mononucleotide repeat sequences within possible target genes. Mutation of beta2-microglobulin gene, which is presumably involved in MSI-positive tumor cell escape from immune surveillance was also examined. Some of these analyses were also carried out in 9 human liver cancer cell lines. None of the 3 quasi-monomorphic mononucleotide markers sensitive for MSI, BAT26, BAT25, and BAT34C4 presented shortened unstable alleles in any of the carcinoma, cirrhosis, chronic hepatitis tissues, or cell lines. LOH at MMR genes was infrequent (4.4 approximately 7.1%), and no mutations were detected. Neither hMLH1 hypermethylation nor frameshift mutation in the target genes was detected. No mutations were found in beta2-microglobulin. Widespread MSI due to the defective DNA MMR appears to play little if any part in Japanese hepatocarcinogenesis.     

Thymidylate synthase expression in colon carcinomas with microsatellite instability.

PURPOSE: Colon cancer cells with high-frequency microsatellite instability (MSI-H) display resistance to 5-fluorouracil (5-FU) that can be reversed by restoring DNA mismatch repair (MMR) proficiency. Given that thymidylate synthase (TS) is inhibited by 5-FU, we studied the relationship between MSI and TS expression, and the prognostic effect of these and other markers (i.e., p53 and 17p allelic imbalance). EXPERIMENTAL DESIGN: Dukes' stage B2 and C colon carcinomas (n = 320) from participants in 5-FU-based adjuvant therapy trials were analyzed for MSI and 17p allelic imbalance. Expression of MMR (hMLH1, hMSH2), TS, and p53 proteins were analyzed by immunohistochemistry. Correlations between markers and associations with overall survival were determined. RESULTS: Of 320 cancers studied, 60 (19%) were MSI-H. TS expression variables were similar in MSI-H and microsatellite stable/low-frequency MSI (MSS/MSI-L) cancers, and unrelated to MMR proteins. MSI-H tumors had lower stage (P = 0.0007), fewer metastatic lymph nodes (P = 0.004), and improved overall survival (P = 0.01). Loss of MMR proteins was also associated with better overall survival (P = 0.006). None of the TS variables were prognostic. Histologic grade (P = 0.0008) and nodal status (P = 0.0002) were associated with overall survival, in contrast to 17p allelic imbalance or p53. Only MSI status or loss of MMR proteins, histologic grade, and tumor stage were independent markers for overall survival. CONCLUSIONS: MSI-H tumors show earlier stage at presentation and better stage-adjusted survival rates. MSI status and TS expression were unrelated and TS was not prognostic, suggesting that TS levels cannot explain therapeutic resistance to 5-FU reported in MSI-H colon cancers.     

Use of molecular tumor characteristics to prioritize mismatch repair gene testing in early-onset colorectal cancer.

PURPOSE: The relationships between mismatch repair (MMR) protein expression, microsatellite instability (MSI), family history, and germline MMR gene mutation status have not been studied on a population basis. METHODS: We studied 131 unselected patients with colorectal cancer diagnosed younger than age 45 years. For the 105 available tumors, MLH1, MSH2, MSH6, and PMS2 protein expression using immunohistochemistry (IHC) and MSI were measured. Germline DNA was screened for hMLH1, hMSH2, hMSH6, and hPMS2 mutations for the following patients: all from families fulfilling the Amsterdam Criteria for hereditary nonpolyposis colorectal cancer (HNPCC); all with tumors that were high MSI, low MSI, or that lacked expression of any MMR protein; and a random sample of 23 with MS-stable tumors expressing all MMR proteins. RESULTS: Germline mutations were found in 18 patients (nine hMLH1, four hMSH2, four hMSH6, and one hPMS2); all tumors exhibited loss of MMR protein expression, all but one were high MSI or low MSI, and nine were from a family fulfilling Amsterdam Criteria. Sensitivities of IHC testing, MSI (high or low), and Amsterdam Criteria for MMR gene mutation were 100%, 94%, and 50%, respectively. Corresponding positive predictive values were 69%, 50%, and 75%. CONCLUSIONS: Tumor IHC analysis of four MMR proteins and MSI testing provide a highly sensitive strategy for identifying MMR gene mutation-carrying, early-onset colorectal cancer patients, half of whom would have been missed using Amsterdam Criteria alone. Tumor-based approaches for triaging early-onset colorectal cancer patients for MMR gene mutation testing, irrespective of family history, appear to be an efficient screening strategy for HNPCC.     

The role of mismatch repair in small-cell lung cancer cells

The role of mismatch repair (MMR) in small-cell lung cancer (SCLC) is controversial, as the phenotype of a MMR-deficiency, microsatellite instability (MSI), has been reported to range from 0 to 76%. We studied the MMR pathway in a panel of 21 SCLC cell lines and observed a highly heterogeneous pattern of MMR gene expression. A significant correlation between the mRNA and protein levels was found. We demonstrate that low hMLH1 gene expression was not linked to promoter CpG methylation. One cell line (86MI) was found to be deficient in MMR and exhibited resistance to the alkylating agent MNNG. Surprisingly, MSI was not detected in 86MI and it appears to express all the major MMR components hMSH2, hMSH6, hMLH1, hPMS2, hMSH3, hMLH3, MBD4 (MED1) and hExo1. These data are consistent with at least two possibilities: (1) A missense mutation in one of the MMR genes, which dissociates MSI from drug resistance, or (2) inactivation of a second pathway that leads to MMR-deficiency and MNNG resistance, but induces negligible levels of MSI. We conclude that MMR deficiency is largely not associated with the pathogenesis of SCLC.     

Gene expression signatures for colorectal cancer microsatellite status and HNPCC

The majority of microsatellite instable (MSI) colorectal cancers are sporadic, but a subset belongs to the syndrome hereditary non-polyposis colorectal cancer (HNPCC). Microsatellite instability is caused by dysfunction of the mismatch repair (MMR) system that leads to a mutator phenotype, and MSI is correlated to prognosis and response to chemotherapy. Gene expression signatures as predictive markers are being developed for many cancers, and the identification of a signature for MMR deficiency would be of interest both clinically and biologically. To address this issue, we profiled the gene expression of 101 stage II and III colorectal cancers (34 MSI, 67 microsatellite stable (MSS)) using high-density oligonucleotide microarrays. From these data, we constructed a nine-gene signature capable of separating the mismatch repair proficient and deficient tumours. Subsequently, we demonstrated the robustness of the signature by transferring it to a real-time RT-PCR platform. Using this platform, the signature was validated on an independent test set consisting of 47 tumours (10 MSI, 37 MSS), of which 45 were correctly classified. In a second step, we constructed a signature capable of separating MMR-deficient tumours into sporadic MSI and HNPCC cases, and validated this by a mathematical cross-validation approach. The demonstration that this two-step classification approach can identify MSI as well as HNPCC cases merits further gene expression studies to identify prognostic signatures.     

修饰型微卫星不稳定性与散发结直肠癌p53基因点突变的相关性

目的 高频度微卫星不稳定性被认定为DNA错配修复缺陷的标志,但既往研究发现一个显著矛盾,即在高频度微卫星不稳定结直肠癌中,p53突变率较一般结直肠癌低.研究旨在确认该矛盾的存在并试图阐明其机制.方法 对180例散发结直肠癌采用高分辨率荧光标记微卫星分析法检测微卫星位点稳定性,PCR扩增直接测序检测p53突变.结果 微卫星不稳定性呈现修饰型和跳跃型两种变化.低频度微卫星不稳定性均呈现修饰型而无跳跃型变化;高频度微卫星不稳定性均检出了跳跃型变化,一部分也并存修饰型变化.微卫星不稳定与肿瘤部位及分化程度明显相关,p53突变与肿瘤分化明显相关.高频度微卫星不稳定肿瘤未检出p53突变,而低频度微卫星不稳定肿瘤p53突变率较高.结论 低频度微卫星不稳定性呈现的修饰型微卫星位点长度变化可能是DNA错配修复缺陷的表型;此表型与提高的碱基置换突变率有关.单纯DNA错配修复缺陷可能不足以导致微卫星不稳定性的跳跃型变化,高频度微卫星不稳定的真正原因仍有待阐明.     

Microsatellite instability occurs in distinct subtypes of pediatric but not adult central nervous system tumors

Length alterations in microsatellite repeats, termed microsatellite instability (MSI), are found in 10-15% of sporadic colon, endometrial, and gastric cancers harboring defects in DNA mismatch repair (MMR) genes We used the microsatellite markers Big Adenine Tract (BAT) 26 and BAT-25 from the reference panel of five markers recommended by the National Cancer Institute to evaluate the incidence of MSI in 206 central nervous system tumors. We screened 102 pediatric and 104 adult cases representing 165 astrocytic and 41 nonastrocytic tumors. The overall incidence of MSI was 8% (16 of 206). All 16 tumors with MSI were found in pediatric rather than adult patients. MSI was associated with two distinct subtypes of pediatric tumors occurring in 27% (12 of 45) of WHO grade III and grade IV astrocytomas and 24% (4 of 17) of gangliogliomas We evaluated the difference in clinicopathological and genetic features among 45 high-grade pediatric astrocytomas by MSI status. The median survival for pediatric patients with MSI (n = 12) was 8 months compared with 15 months for those patients without MSI (n = 33; P = 0.18). The frequency of p53 gene mutations was 13% for pediatric patients with MSI (n = 8) compared with 47% for those patients without MSI (n = 19; P = 0.19). These results revealed a trend between MSI status and prog nosis and MSI status and frequency of p53 gene mutations. Our data suggest that pediatric high-grade astrocytomas can be attributed to two different genetic pathways: a MMR-deficient pathway and a MMR proficient pathway.     

Optimising methods for determining RER status in colorectal cancers

Approximately 13% of colorectal cancers display microsatellite instability (MSI), a form of replication error repair. Colorectal cancers developing in individuals with constitutional defects in the mismatch repair (MMR) genes hMLH1, hMSH2, hPMS1 and hPMS2 consistently show evidence of this phenomenon. Since MSI is indicative of MMR deficiency, testing colorectal cancers for MSI provides a method of refining the identification of carriers of germline MMR mutations. To assess which microsatellites represent the best reporters of replication error (RER) status we have examined 116 early onset colorectal cancers for MSI. MSI was assessed using eight dinucleotide- and two mononucleotide-repeat fluorescently labelled polymerase chain reaction (PCR) markers. The two mononucleotide repeat markers (BAT25 and BAT26) were highly sensitive and typing of either represents an efficient strategy for defining RER status of colorectal cancers and obviates the requirement of typing numerous microsatellite markers.     

Microsatellite instability markers for identifying early-onset colorectal cancers caused by germ-line mutations in DNA mismatch repair genes.

PURPOSE: Microsatellite instability (MSI) testing of colorectal cancer tumors is used as a screening tool to identify patients most likely to be mismatch repair (MMR) gene mutation carriers. We wanted to examine which microsatellite markers currently used to detect MSI best predict early-onset colorectal cancer caused by germ-line mutations in MMR genes. EXPERIMENTAL DESIGN: Invasive primary tumors from a population-based sample of 107 cases of colorectal cancer diagnosed before age 45 years and tested for germ-line mutations in MLH1, MSH2, MSH6, and PMS2 and MMR protein expression were screened for MSI using the National Cancer Institute panel and an expanded 10-microsatellite marker panel. RESULTS: The National Cancer Institute five-marker panel system scored 31 (29%) as (NCI)MSI-High, 13 (12%) as (NCI)MSI-Low, and 63 (59%) as (NCI)MS-Stable. The 10-marker panel classified 18 (17%) as (10)MSI-High, 17 (16%) as (10)MSI-Low, and 72 (67%) as (10)MS-Stable. Of the 26 cancers that lacked the expression of at least one MMR gene, 24 (92%) were positive for some level of MSI (using either microsatellite panel). The mononucleotide repeats Bat26, Bat40, and Myb were unstable in all (10)MSI-High cancers and all MLH1 and MSH2 mutation carriers (100% sensitive). Bat40 and Bat25 were unstable in all tumors of MSH6 mutation carriers (100% sensitive). Bat40 was unstable in all MMR gene mutation carriers (100% sensitive). By incorporating seven mononucleotide repeats markers into the 10-marker panel, we were able to distinguish the carriers of MSH6 mutations (all scored (10)MSI-Low) from the MLH1 and MSH2 mutation carriers (all scored (10)MSI-High). CONCLUSIONS: In early-onset colorectal cancer, a microsatellite panel containing a high proportion of mononuclear repeats can distinguish between tumors caused by MLH1 and MSH2 mutations from those caused by MSH6 mutations.     

Evidence for an age-related influence of microsatellite instability on colorectal cancer survival

It is well established that microsatellite instability (MSI), the hallmark of defective DNA mismatch repair (MMR), is associated with prolonged survival in colorectal cancer compared with tumours that are microsatellite stable (MSS). MSI in sporadic colorectal tumours is primarily due to epigenetic silencing of MLH1. However, there are no prospective population-based studies of survival in patients with germline MMR gene mutations who develop cancer. Although MSI is almost universal in tumours from HNPCC family members, there is a potential confounding effect of ascertainment and other biases that could explain the apparent survival benefit in HNPCC families. Resolving whether germline MMR gene mutations impact on survival is important because it potentially undermines the rationale for surveillance of mutation carriers. Here, we report an investigation of the influence of MSI on survival in cohorts of cancer patients (aged < 30 years at diagnosis, n = 118; non-age-selected, n = 181) in the context of clinicopathologic variables. There was a substantial age-related influence of tumour MSI status on survival. In young patients with tumour MSI, 65% of patients with MSI tumours had germline MSH2 or MLH1 mutations. Clinicopathologic variables and tumour MSI of the cohort were studied with respect to survival and compared with control groups. Young patients had excess MSI tumours (p < 0.000001), mucinous tumours (p < 0.01), advanced disease (p approximately 0.001) and poorer 5-year survival compared with older cases. Cox proportional hazard analysis identified Dukes' stage, age at diagnosis and calendar year of treatment as independent predictors of survival. There was no detectable association between tumour MSI and survival in young patients, although we confirmed previous observations that MSI is associated with better prognosis in later onset cohorts. These findings underscore the rationale for surveillance and early identification of tumours in MMR gene carriers as well as refining understanding of the influence of MSI on cancer progression.     

肾癌微卫星不稳定性及其机制的研究

目的 探讨微卫星不稳定性（microsatelite instablility,MSI)在肾细胞癌中的表现及与基因突变的关系。方法 用PCR方法分析34例肾细胞癌MSI表现;应用RT－PCR方法检测5种人类错配修复（mismatch repair,MMR)基因在肾细胞癌和肾癌细胞系mRNA转录水平的表达;用PCR－SSCP技术对15例肾细胞癌标本进行MMR基因hMLH1的19个外显子进行筛选,观察基因突变的情况;用PCR方法检查肾癌组织中TGF－βⅡ型受体（TGF－βRⅡ）基因和BAX基因移码突变的情况。结果 34例肾细胞癌中有15例（44．1％）表现为MSI,并多见于晚期肿瘤;15例表达MSI阳性肾癌标本中,3例hMLH1基因mRNA表达缺失,3例表达明显降低,但在正常对照及PCC949细胞系都表达5种MMR基因;PCR－SSCP筛检结果,15例MSI（＋）细胞中3例显示异常电泳带型;40％（6／15）及27％（4／15）分别见TGF－βRⅡ基因及BAX基因移码突变,但不表现在MSI（－）肾肿瘤及正常细胞中。结论 肾细胞癌MSI与MMR基因表达有关,较高的突变率在肿瘤发生中的作用与MSI有关。     

胃癌的微卫星不稳定性与hMLH1基因启动子甲基化的关系

背景与目的:由于细胞错配修复功能缺陷而导致基因组微卫星序列高度不稳定是遗传性非息肉性大肠癌发生的主要原因。以往的研究表明胃癌组织也有错配修复蛋白表达的缺失,但错配修复基因突变频率却很低;而启动子的甲基化是肿瘤抑癌基因失活的主要途径,也可能是错配修复基因功能丧失的主要原因。本研究拟通过对胃癌组织的微卫星不稳定性的分析及对hMLH1基因启动子甲基化和蛋白表达的检测,对胃癌发病的分子机制进行探讨。方法:从52例胃癌患者的癌组织及其周围组织提取DNA,PCR扩增基因组的5个微卫星位点BAT-26、D17S261、D3S1283、D2S123和D3S1611,毛细管电泳后,判定胃癌组织的微卫星不稳定性;免疫组织化学方法检测hMLH1蛋白的表达;酶切法检测hMLH1基因启动子甲基化。结果:52例胃癌标本中微卫星高度不稳定13例,低度不稳定2例,稳定37例。微卫星高度不稳定的13例胃癌组织中,均检测到hMLH1基因启动子甲基化(100.0%);微卫星低度不稳定和微卫星稳定的39例胃癌组织中,hMLH1基因启动子甲基化仅1例(2.6%),前者发生率高于后者,两者之间有显著性差异(P<0.01)。微卫星高度不稳定的13例胃癌的癌旁组织中,hMLH1基因启动子甲基化6例(46.2%),而微卫星低度不稳定和微卫星稳定39例胃癌的癌旁肿瘤组织标本中,hMLH1基因启动子甲基     

Microsatellite instability and hMLH1 and hMSH2 expression analysis in soft tissue sarcomas

Alterations of the size of microsatellite DNA sequences, namely microsatellite instability (MSI), have been demonstrated in some types of malignancies. We analyzed the MSI of five microsatellite markers in 40 cases of soft tissue sarcoma (STS) using high resolution fluorescent microsatellite analysis. In addition, we examined the expression of hMLH1 and hMSH2 proteins of DNA mismatch repair (MMR) genes by immunohistochemistry, and promoter methylation of the hMLH1 gene by methylation-specific PCR (MSP). MSI was recognized in 10 of 40 STS cases (25%), which consisted of 2 MSH-high (MSI-H) tumors and 8 MSI-low (MSI-L) tumors. A loss of hMLH1 expression was recognized in 7 of 40 STS cases (18%), and loss of hMSH2 expression was recognized in 3 of 40 STS cases (8%). One case showed a loss of both hMLH1 and hMSH2 expression. Promoter hypermethylation of the hMLH1 gene was detected in only 3 of 40 STS cases (8%). Of 10 cases with MSI, 5 (50%) showed a loss of hMLH1 and/or hMSH2 expression. There was a statistically significant correlation between MSI-positive tumors and the loss of hMLH1 and/or hMSH2 expression (p=0.0286). Although the frequency of MSI (25%) or a loss of hMLH1 and/or hMSH2 expression (23%) was relatively low in STS cases, a loss of hMLH1 and/or hMSH2 was recognized in 5 out of 10 MSI-positive cases (50%). These findings suggest that the inactivation of MMR gene expression might be the cause of MSI in STS cases.