Microsatellite instability in sporadic colorectal cancer is not an independent prognostic factor.

Hereditary non-polyposis colorectal cancer (HNPCC) is linked to an inherited defect in the DNA mismatch repair system. DNA from HNPCC tumours shows microsatellite instability (MSI). It has been reported that HNPCC patients have a better prognosis than patients with sporadic colorectal cancer. We examined whether the presence of MSI in a series of unselected colorectal tumours carries prognostic information. In a series of 181 unselected colorectal tumours, 22 tumours (12%) showed MSI. Survival analysis at 5-10 years follow-up showed no statistically significant difference in prognosis between MSI-positive and -negative tumours. Our results suggest that the MSI phenotype as such is not an independent prognostic factor.     

CHFR silencing or microsatellite instability is associated with increased antitumor activity of docetaxel or gemcitabine in colorectal cancer

Phenotypic differences among cancers with the same origin may be associated with chemotherapy response. CHFR silencing associated with DNA methylation has been suggested to be predictive of taxane sensitivity in diverse tumor types. However, the use of microsatellite instability (MSI:unstable–MSS:stable) as a predictive marker for therapeutic effect has had conflicting results. We examined these molecular alterations as predictors of chemotherapy sensitivity in colorectal cancer (CRC). Differential sensitivity to docetaxel and gemcitabine was compared to potential predictive biomarkers CHFR methylation and MSI status. Cell lines that were MSI‐H/CHFR‐methylated, MSS/CHFR‐methylated and MSS/CHFR‐unmethylated were assessed for in vivo sensitivity of CRC cell line xenografts to docetaxel and/or gemcitabine. We observed increased sensitivity in vitro to gemcitabine in cell lines with MSI and docetaxel in cell lines with CHFR inactivation via DNA methylation. In vivo treatment of human xenografts confirmed differential sensitivity, with the MSI‐H/CHFR‐methylated line RKO having tumor growth inhibition to each agent, and at least additive tumor growth inhibition with combination therapy. The MSS‐CHFR‐unmethylated line, CACO₂, was resistant to single and combination therapy, while COLO205, the MSS/CHFR‐methylated line, showed tumor growth inhibition with docetaxel, but not gemcitabine, therapy. CHFR methylation in CRC cell lines predicted for sensitivity in vitro and in vivo to docetaxel, while MSI‐H cell lines were more sensitive to gemcitabine. These data suggest that a subset of CRC patients would be selectively sensitive to a novel combination of gemcitabine and docetaxel, and are the basis for an ongoing clinical trial of this combination in a biomarker‐selected patient population.     

Clinicopathological significance of microsatellite instability and mutated RIZ in colorectal cancer.

BACKGROUND: Several studies have shown that microsatellite instability (MSI) is related to favourable survival in colorectal cancer patients but there are controversial results. Tumour suppressor gene RIZ is a susceptible mutational target of MSI. However, its clinicopathological significance has not been investigated. We investigated the prognostic significance of MSI in Swedish colorectal cancer patients and the clinicopathological significance of RIZ mutations. PATIENTS AND METHODS: We analysed 438 colorectal adenocarcinomas for MSI by microsatellite analysis. Among them, 29 MSI and 28 microsatellite stable (MSS) tumours were examined for RIZ mutations by DNA sequencing. RESULTS: MSI (13% of 438 cases) was not associated with survival (rate ratio = 0.97, 95% confidence interval = 0.57-1.64, P = 0.90), although it was related to proximal tumour (P <0.001), poor differentiation and mucinous carcinomas (P <0.001), multiple tumours (P = 0.01) and negative/weak expression of hMLH1 (P = 0.03). RIZ mutations were detected in 31% of 29 MSI tumours but in none of the 28 MSS tumours. The mutations were related to female (P = 0.01), proximal tumour (P = 0.01), stage B (P = 0.01) and poor differentiation (P = 0.047). CONCLUSIONS: MSI was not a prognostic factor in the Swedish patients included in this study. Clinicopathological variables associated with RIZ mutations might be a consequence of the MSI characteristics.     

Mitochondrial microsatellite instability of colorectal cancer stroma

Mitochondrial microsatellite instability (mtMSI) and mutations of mitochondrial DNA has been reported in cancer epithelia of carcinomas. However, mtMSI in cancer stroma has not yet been identified in human cancers. In this study, we attempted to determine if mtMSI occurs in the cancer stroma of sporadic colorectal cancers, and if the stromal mtMSI has any correlations with stromal nuclear MSI (nMSI) and cancer epithelial mtMSI. Nine microsatellite sequences within the D-loop and 5 coding genes for mtMSI, and 9 microsatellites for nMSI were analyzed in the microdissected cancer epithelia and adjacent stromas of 48 sporadic colorectal cancers. Overall, 23 somatic mitochondrial DNA alterations were detected in 15 cancer epithelia (31.2%) and 5 stromas (10.4%). The mutations consisted of 19 D-loop mtMSI alterations, and 1 missense and 3 framshift mutations of repeat sequences within the coding genes. All of the 5 stromal genetic alterations showed D-loop mtMSI. In regards to other MSI status, the stromal mtMSI had no association with stromal nMSI or epithelial mtMSI, either. These findings indicate that in addition to the cancer epithelia the cancer stroma harbor mtMSI, and suggest a possible role of stromal mtMSI in the pathogenesis of colorectal cancers. Furthermore, the data suggest that stromal mtMSI may occur independently of stromal nMSI and epithelial mtMSI in sporadic colorectal cancers.     

微卫星不稳定结直肠癌临床病理特征及生存预后

目的 国内外已有学者提出微卫星不稳定(microsatellite instability,MSI)状态可能是影响结直肠癌(colorectal cancer,CRC)患者预后的因素,同时提出微卫星不稳定结直肠癌患者存在较为特殊的临床病理特征,本研究旨在探讨微卫星不稳定CRC的临床病理特征及生存预后.方法 应用免疫组织化学方法检测2010-03-24-2015-12-24济南市第四人民医院60例CRC组织中人MutL蛋白同系物1(human mutl homologue 1,hMLH1)、人MutS蛋白同系物2(human muts homologue 2,hMSH2)及人MutS蛋白同系物6(human muts homologue 6,hMSH6)3种DNA错配修复蛋白表达缺失情况,判断肿瘤微卫星不稳定状态,并分析高度微卫星不稳定(microsatellite instability-high,MSI-H)和低度微卫星不稳定(microsatellite instability-low,MSI-L)/微卫星稳定(microsatellite stable,MSS)不同组别间的临床病理特征及生存预后情况;应用Cox风险比例模型对可能影响CRC患者预后的因素进行多因素分析.结果 60例CRC患者的肿瘤组织中MSI-H为40.0%(24/60),MSI-L为31.7%(19/60),MSS为28.3%(17/60).MSI-H的CRC患者,与MSS和MSI-L患者相比,好发于右半结肠(χ2=6.279,P=0.043),黏液腺癌多见(χ2=6.025,P=0.049);3组在性别、年龄、分期、肿瘤浸润深度、淋巴结转移和分化程度差异无统计学意义.MSI-H患者的中位无病生存期(disease-free survival,DFS)为21个月,明显长于MSS的11个月及MSI-L的13个月,χ2=7.994,P=0.018.多因素Cox分析结果显示,淋巴结转移(P=0.013)和MSI(P=0.018)为CRC患者DFS的独立预后因素.结论 MSI-H的CRC患者与MSI-L及MSS相比,具有独特的临床病理特征且预后相对较好.检测MSI状态对提高CRC治疗水平,及改善预后有重要的临床意义.     

Promoter methylation of Wnt5a is associated with microsatellite instability and BRAF V600E mutation in two large populations of colorectal cancer patients

Background:

In colorectal cancer (CRC), tumour microsatellite instability (MSI) status and CpG island methylator phenotype (CIMP) status are indicators of patient outcome, but the molecular events that give rise to these outcomes remain largely unknown. Wnt5a is a critical regulator of non-canonical Wnt activity and promoter hypermethylation of this gene has emerging prognostic roles in CRC; however the frequency and prognostic significance of this epigenetic event have not been explored in the context of colorectal tumour subtype. Consequently, we investigated the frequency and prognostic significance of Wnt5a methylation in a large cohort of MSI-stratified CRCs.

Methods:

Methylation was quantified in a large cohort of 1232 colorectal carcinomas from two clinically distinct populations from Canada. Associations were examined between methylation status and clinicopathlogical features, including tumour MSI status, BRAF V600E mutation, and patient survival.

Results:

In Ontario, Wnt5a methylation was strongly associated with MSI tumours after adjustment for age, sex, and tumour location (odds ratio (OR)=4.2, 95% confidence interval (CI)=2.4–7.4, P<10⁻⁶) and with BRAF V600E mutation, a marker of CIMP (OR=12.3, 95% CI=6.9–21.7, P<10⁻¹⁷), but was not associated with patient survival. Concordant results were obtained in Newfoundland.

Conclusion:

Methylation of Wnt5a is associated with distinct tumour subtypes, strengthening the evidence of an epigenetic-mediated Wnt bias in CRC.     

Possible alternative carcinogenesis pathway featuring microsatellite instability in colorectal cancer stroma

Differential microsatellite instability (MSI) in tumour epithelial and stromal compartments has not been well examined for colorectal cancers. Using laser-captured microdissection, separate specimens of these compartments of 40 sporadic colorectal cancers were sampled and MSI was tested with four markers. To examine the relation between the MSI phenotype in the stroma and other genetic events and histopathological features, p53 and K-ras gene mutations were analysed, and the expression of p53, hMLH1, and hMSH2 protein was determined by immunohistochemistry. Microsatellite instability positive results were obtained for both epithelium (34%) and stromal tissue (41%). While MSI in epithelium correlated with differentiation and Dukes' stage, that in stroma demonstrated an inverse relation, being particularly frequent in well-differentiated adenocarcinomas (54%) and Dukes' A lesions (55%). Further, a significant inverse correlation between p53 protein overexpression in the epithelium and MSI in the stroma was found (P=0.02475). The results suggest an alternative pathway of carcinogenesis involving stromal genetic instability in the development of colorectal cancers.     

Mutational profiling of colorectal cancers with microsatellite instability

Microsatellite instability (MSI) is caused by defective mismatch repair in 15–20% of colorectal cancers (CRCs). Higher mutation loads in tumors with mismatch repair deficiency can predict response to pembrolizumab, an anti-programmed death 1 (PD-1) immune checkpoint inhibitor. We analyzed the mutations in 113 CRCs without MSI (MSS) and 29 CRCs with MSI-High (MSI-H) using the 50-gene AmpliSeq cancer panel. Overall, MSI-H CRCs showed significantly higher mutations than MSS CRCs, including insertion/deletion mutations at repeat regions. MSI-H CRCs showed higher incidences of mutations in the BRAF, PIK3CA, and PTEN genes as well as mutations in the receptor tyrosine kinase families. While the increased mutations in BRAF and PTEN in MSI-H CRCs are well accepted, we also support findings of mutations in the mTOR pathway and receptor tyrosine kinase family genes. MSS CRCs showed higher incidences of mutations in the APC, KRAS and TP53 genes, confirming previous findings. NGS assays may be designed to detect driver mutations for targeted therapeutics and to identify tumors with high mutation loads for potential treatment with immune checkpoint blockade therapies. Further studies may be warranted to elucidate potential targeted therapeutics against mutations in the mTOR pathway and the receptor tyrosine kinase family in MSI-H CRCs as well as the benefit of anti-PD-1 immunotherapy in hypermutated MSS CRCs or other cancers.     

检测微卫星不稳在中国遗传性非息肉病性结直肠癌患者诊断中的应用

背景与目的：遗传性非息肉病性结直肠癌（hereditary nonpolyposis colorectal cancer，HNPCC）是常染色体显性遗传综合征，国外报道90％的HNPCC肿瘤表现为微卫星不稳（microsatellite instability，MSI），可作为筛查HNPCC的金标准。本研究旨在了解中国HNPCC肿瘤MSI发生率以及可疑HNPCC患者大肠癌肿瘤中的MSI发生率，由此探讨大肠癌患者家族中的胃癌等HNPCC相关肿瘤对发现HNPCC患者的意义。方法：选择符合Amsterdam标准的HNPCC组大肠癌标本18例，和不符合Amsterdam标准、但高度怀疑为HNPCC的可疑HNPCC组大肠癌标本16例，检测BAT26、D2S123、BAX、IGFIIR、hMSH3和hMSH66个做卫星位点的微卫星不稳在两组中的发生率，比较两组微卫星不稳频率的差异。结果：上述各傲卫星位点在HNPCC组和可疑HNPCC组标本中均显示较高的突变率。高度微p星不稳肿瘤在两组标本中的检出率分别为94．4％和93．7％．差异无显著性。BAT26对高度微卫星不稳肿瘤敏感度高。结论：MSI在中国HNPCC患者中的发生频率与国外类同。仅用BAT26可发现大部分高度微卫星不稳肿瘤。将胃癌等HNPCC相关肿瘤纳入临床诊断标准，可能有助于避免在中国大肠癌人群中漏诊HNPCC患者。     

Incidence and detection of high microsatellite instability in colorectal cancer in a Chinese population: a meta-analysis

BackgroundThe 4 most common types of DNA mutations in tumors are single-nucleotide variations, insertion-deletion, fusion, and copy number variations. This is followed by microsatellite instability (MSI), which is known to trigger the development of MSI-high (MSI-H) cancer and is responsible for 300,000 new cases of cancer per year in China. We aim to conduct a meta-analysis based on a comparison between the positive rates of the National Cancer Institute (NCI) panel (also known as 2B3D NCI panel) and mononucleotide panels for the diagnosis of MSI in the Chinese population.MethodsIn the present meta-analysis, we searched the PubMed, Embase, Web of Science, CNKI, Wanfang, CQVIP, and CBM databases. MSI diagnosis studies by PCR and capillary electrophoresis were included to compare the incidence of MSI-H in colorectal cancer obtained from panels with different microsatellite markers. Egger’s bias test was used to assess risk of bias.ResultsSeventeen articles were included, which used the Newcastle-Ottawa Scale (NOS) scale for quality evaluation. The NOS scores of the included documents were ≥7 points, and the quality of the documents met the requirements. The incidence of MSI-H detected by the 2B3D NCI panel was 13.5% [95% confidence interval (CI): 10.8–16.4, I²=52.321%, P=0.026, n=10 studies including 2,681 participants], the incidence of MSI-H detected by the mononucleotide panels was 10.6% (95% CI: 7.1–14.7, I²=81.147%, P=0.000, n=7 studies including 3,249 participants). This indicates that, in the Chinese population, the 2B3D NCI panel can detect 27.4% more MSI-H cancers than the mononucleotide panels, 54.7% more MSI-H cancers than the panel of 6 mononucleotides, and its sensitivity is comparable to that of Promega.ConclusionsThe findings of the meta-analysis demonstrated that, using the 2B3D NCI panel for MSI detection can avoid the underestimation of the incidence MSI-H in colorectal cancer and can be considered the most suitable panel for MSI detection in the Chinese population. The inclusion of only published data might be a potential source of publication bias.     

微卫星不稳定散发性大肠癌的临床病理特征和DNA倍体研究

目的:探讨微卫星不稳定的散发性大肠癌的临床病理特征及微卫星不稳定表型和DNA倍体类型的关系。方法:对71例散发性大肠癌行BAT25和BAT26两个位点的微卫星不稳定检测和流式细胞术倍体分析,探讨微卫星不稳定状态和临床病理特征及DNA倍体类型的关系。结果:微卫星不稳定的阳性率为9.86%(7/71),微卫星不稳定表型和发病部位、组织学类型及分化程度相关(P<0.05),而与性别、年龄、淋巴结转移和分期无关。微卫星不稳定的散发性大肠癌中右半结肠癌和低分化腺癌的比例高于微卫星稳定者。68例患者检出二倍体和异倍体分别为18和50例,微卫星不稳定表型者5例为二倍体,因此和DNA倍体类型显著相关(P=0.012)。结论:微卫星不稳定的散发性大肠癌好发于右半结肠,具有低分化腺癌的倾向,多为二倍体。     

The role of MYH and microsatellite instability in the development of sporadic colorectal cancer

Biallelic germline mutations in MYH are associated with colorectal neoplasms, which develop through a pathway involving somatic inactivation of APC. In this study, we investigated the incidence of the common MYH mutations in an Australian cohort of sporadic colorectal cancers, the clinicopathological features of MYH cancers, and determined whether inactivation of mismatch repair and base excision repair (BER) were mutually exclusive. The MYH gene was sequenced from lymphocyte DNA of 872 colorectal cancer patients and 478 controls. Two compound heterozygotes were identified in the cancer population and all three cancers from these individuals displayed a prominent infiltration of intraepithelial lymphocytes. In total, 11 heterozygotes were found in the cancer group and five in the control group. One tumour from an individual with biallelic germline mutation of MYH also demonstrated microsatellite instability (MSI) as a result of biallelic hypermethylation of the MLH1 promoter. Although MYH-associated cancers are rare in a sporadic colorectal population, this study shows that these tumours can develop through either a chromosomal or MSI pathway. Tumours arising in the setting of BER or mismatch repair deficiency may share a biological characteristic, which promotes lymphocytic infiltration.     

Proficiency of DNA repair genes and microsatellite instability in operated colorectal cancer patients with clinical suspicion of lynch syndrome

Background

Lynch syndrome (LS) diagnosis is underestimated, and most of the patients remain undetected after colorectal resections. The study aims to assess the frequency of LS in patients undergoing surgical treatment for colorectal cancer (CRC).

Methods

A total of 458 CRC patients were operated from January 2005 to December 2008. Positive CRC family history (FH) was present in 118 (25.8%) patients. Histologic sections were reviewed for microsatellite instability (MSI) criteria (Bethesda guidelines), immunohistochemical (IHC) analysis for MLH1, MSH2, MSH6, PMS2 proteins, through the avidin-biotin-peroxidase complex, MSI (BAT-25, BAT-26, NR-21, NR-24 and MONO-27) and BRAF somatic mutation.

Results

Of the 118 patients with FH, 61 (51.69%) met at least one of the revised Bethesda criteria. IHC was abnormal in 8 (13.1%) and MSI in 12 patients (20%). BRAF was negative in all cases. MSI histopathological included: intratumoral lymphocytes (47.5%), expansive tumors (29.5%) mucinous component (27.8%) and Crohn’s like reaction in (14.7%). There was an association between the revised Bethesda criteria with: sex, mucinous histology and Crohn’s like reaction; MSI and IHC with PMS2 and MLH1. Revised Bethesda criteria 4 had 10.6 increased chances to display positive MSI. We have proposed a score to contribute as a practical tool in the diagnosis of LS.

Conclusions

The frequence of LS in resected CRC patients was 2.6%. The criterion 4 Revised Bethesda was associated more strongly with the presence of MSI.     

Impact of BRAF mutation and microsatellite instability on the pattern of metastatic spread and prognosis in metastatic colorectal cancer

BACKGROUND:

It is hypothesized that BRAF mutant cancers represent a discrete subset of metastatic colorectal cancer (CRC) defined by poorer survival. This study investigates whether BRAF mutant CRC is further defined by a distinct pattern of metastatic spread and explores the impact of BRAF mutation and microsatellite instability (MSI) on prognosis in metastatic CRC.

METHODS:

By using prospective clinical data and molecular analyses from 2 major centers (Royal Melbourne Hospital and The University of Texas MD Anderson Cancer Center), patients with known BRAF mutation status were analyzed for clinical characteristics, survival, and metastatic sites.

RESULTS:

The authors identified 524 metastatic CRC patients where BRAF mutation status was known; 57 (11%) were BRAF mutant tumors. BRAF mutant tumors were significantly associated with right‐sided primary tumor, MSI, and poorer survival (median, 10.4 months vs 34.7 months, P < .001). A distinct pattern of metastatic spread was observed in BRAF mutant tumors, namely higher rates of peritoneal metastases (46% vs 24%, P = .001), distant lymph node metastases (53% vs 38%, P = .008), and lower rates of lung metastases (35% vs 49%, P = .049). In additional survival analyses, MSI tumors had significantly poorer survival compared with microsatellite stable tumors (22.1 months vs 11.1 months, P = .017), but this difference was not evident in the BRAF mutant population.

CONCLUSIONS:

The pattern of metastatic spread observed in this study further defines BRAF mutant CRC as a discrete disease subset. The authors demonstrated that, unlikely early stage disease, MSI is associated with poorer survival in metastatic CRC, although this is driven by its association with BRAF mutation. Cancer 2011;. © 2011 American Cancer Society.     

A distinct DNA methylation profile associated with microsatellite and chromosomal stable sporadic colorectal cancers

Aberrant DNA methylation, microsatellite instability (MSI) and chromosomal instability (CIN) are well-characterised molecular features of sporadic colorectal cancers (CRCs). In addition to CpG island methylator phenotype (CIMP) associated with MSI, an intermediate methylation subgroup is also a feature of non-MSI cancers. A large proportion of CRCs have no evidence of either MSI or CIN, here called Microsatellite and Chromosomal Stable (MACS), and require their methylation profile to be established. The clinical and molecular features of 170 sporadic CRC patients were investigated and stratified into MSI, CIN and MACS groups. MACS were most often found in the left colon and had a significantly lower BRAF mutation frequency (p < 0.001) compared with MSI. MACS had better survival [hazard ratio (HR) = 0.244, p = 0.017] compared with CIN, but were similar to MSI. The methylation status of 1,505 CpG loci from cancer-related genes was analysed in a subset of CRCs (n = 44 normal-tumour pairs) and compared with CIN, MSI and MACS status. Using two-way hierarchical clustering, three subgroups were identified, which associated with CIN, MSI and MACS status. Using significance analysis of microarray, 16 CpG loci demonstrating methylation changes associated with MACS were identified. A combination of six loci identified MACS with 81% sensitivity and 93% specificity. This result now requires independent validation. Hypomethylation of a CpG locus within the sonic hedgehog (SHH) promoter correlated with increased gene expression and was associated significantly with MACS cancers. In conclusion, we propose that MACS have distinct clinicopathological features and can be distinguished from other CRCs by a specific set of methylation loci.     

错配修复基因变异与中国人散发性大肠癌发病年龄之间的关系

目的 :探讨错配修复基因变异与中国人散发性大肠癌发病年龄之间的关系。方法 :随机选取 1 0 0例临床初诊的散发性大肠癌患者 ,配对提取手术切除的癌组织和同源正常组织的基因组DNA ,检测并分析癌细胞微卫星DNA不稳定性 (MSI)。结果 :46/1 0 0 (46 % )的癌组织MSI阳性 (MSI+ ) ,其中 1 8%为MSI+ H ,2 8%MSI+ L。按年龄分组分析表明 ,发病年龄 <45岁的大肠癌患者中MSI+ 检出率明显高于≥ 65岁的大肠癌患者 (P <0 .0 5) ,且MSI+ 的检出率与患者的发病年龄呈负相关 (r =- 0 .95 ,P <0 .0 5) ,但与癌细胞的分化及患者的临床分期无关。结论 :中国人散发性大肠癌中错配修复基因功能的丧失可能出现在癌症发生的早期 ,可能参与构成部分散发性大肠癌发病的遗传背景因素     

Population‐based detection of Lynch syndrome in young colorectal cancer patients using microsatellite instability as the initial test

Approximately 1–2% of colorectal cancers (CRC) arise because of germline mutations in DNA mismatch repair genes, referred to as Lynch syndrome. These tumours show microsatellite instability (MSI) and loss of expression of mismatch repair proteins. Pre‐symptomatic identification of mutation carriers has been demonstrated to improve survival; however, there is concern that many are not being identified using current practices. We evaluated population‐based MSI screening of CRC in young patients as a means of ascertaining mutation carriers. CRC diagnosed in patients aged <60 years were identified from pathology records. No prior information was available on family history of cancer. PCR techniques were used to determine MSI in the BAT‐26 mononucleotide repeat and mutation in the BRAF oncogene. Loss of MLH1, MSH2, MSH6 and PMS2 protein expression was evaluated in MSI+ tumours by immunohistochemistry. MSI+ tumours were found in 105/1,344 (7.8%) patients, of which 7 were excluded as possible Lynch syndrome because of BRAF mutation. Of the 98 “red flag” cases that were followed up, 25 were already known as mutation carriers or members of mutation carrier families. Germline test results were obtained for 35 patients and revealed that 22 showed no apparent mutation, 11 showed likely pathogenic mutations and 2 had unclassified variants. The proportion of MSI+ cases in different age groups that were estimated to be mutation carriers was 89% (<30 years), 83% (30–39), 68% (40–49) and 17% (50–59). We recommend MSI as the initial test for population‐based screening of Lynch syndrome in younger CRC patients, regardless of family history. © 2008 Wiley‐Liss, Inc.     

Microsatellite instability in colorectal cancer

Approximately 20 percent of right-sided colon cancers and 5 percent of left-sided colon and rectal cancers have a deficient DNA mismatch repair system. This results in the widespread accumulation of mutations to nucleotide repeats, some of which occur within the coding regions of cancer-related genes such as TGFβRII and BAX. A standardized definition for microsatellite instability (MSI) based on the presence of deletions to mononucleotide repeats is gaining widespread acceptance in both research and the clinic. Colorectal cancer (CRC) with MSI are characterized histologically by an abundance of tumor-infiltrating lymphocytes, poor differentiation and a signet ring or mucinous phenotype. In younger patients these tumors usually develop along the chromosomal instability pathway, in which case the mismatch repair genes are inactivated by germline mutation, somatic mutation and loss of heterozygosity. In older patients MSI CRC usually develops against a background of widespread hypermethylation that includes methylation-induced silencing of the mismatch repair gene MLH1. The overall biological and clinical phenotype of MSI CRC that arise in these two pathways is likely to be different and may account for some of the discordant results reported in the literature relating to the clinical properties of these tumors. The available evidence indicates that MSI is unlikely to be a clinically useful marker for the prognostic stratification of early-stage CRC. The predictive value of MSI for response to 5-fluorouracil-based chemotherapy remains controversial, while for other agents the predictive value is difficult to assess because they are used in combination regimens. The MSI phenotype is being actively investigated for novel therapeutic approaches based on the principle of synthetic lethality. Finally, the MSI status of CRC is an extremely useful marker for population-based screening programs that aim to identify individuals and families with the hereditary cancer condition known as Lynch syndrome.