CRC is one of the most common malignancies in Western countries, and its incidence is continually increasing in Asian countries (1). Our previous studies showed that the expression of several genes, such as CEA and proliferation marker protein Ki-67 (Ki67), were associated with CRC prognosis (2, 3). Genomic instability is one of the main characteristics of CRC, including MSI, chromosome instability, and CpG island methylation (CIMP). A microsatellite is a simple repetitive sequence with high mutations in the genome. The phenomenon of microsatellite changes during DNA replication is termed MSI. The lack of mismatch repair (MMR) protein (d-MMR) due to the mutation or abnormal expression of the MMR genes (including MLH1, MSH2, MSH6, and PMS2) underlie MSI. According to the number of mutation sites, the MSI can be divided into high MSI (MSI-H), low MSI (MSI-L), and no MSS type. Among these, MSI-L is classified due to its clinical characteristics, which are very similar to MSS, indicating a similar tumor location, differentiation, lymph metastasis, and pathological staining attributed to parallel gene expression (4).

Nonetheless, the prognostic role of MSI in CRC is controversial. Sargent DJ et al. (5) did not find any significant correlation between MSI status and prognosis. Recently, Taieb et al. (6) conducted a summary of seven studies and multivariate analysis in 2,630 patients with CRC recurrence. Patients with the MSI-H/d-MMR phenotype showed improved prognosis in the pre-immunotherapy stage after adjuvant chemotherapy. A meta-analysis of 3,063 patients with metastatic CRC by Venderbosch et al. (7) showed that patients with MSI-H/d-MMR had lower progression-free survival (PFS) and OS, which rendered the evidence of the short survival of phase IV MSI-H sufficient. Another study provided an opposite theory that MSI-H had a good prognosis when the ratio of the sample of early and late patients was 1:1 (8). A recent study (9) has shown a dramatical improvement in survival with immunotherapy (programmed death-ligand 1 [PD-(L)1] cytotoxic T-lymphocyte-associated antigen 4 [CTLA-4] blockage) in metastatic or non-metastatic MSI/dMMR CRC and reported new treatment recommendations for this unique CRC population. Despite their efficacy, primary and secondary resistance to immune checkpoint inhibitors (ICIs) are observed in more than 50% of MSI-H/dMMR CRC patients, and in the future, how to identify these patients and overcome resistance will be an important challenge. In 2017, the FDA approved two ICIs (pembrolizumab and nivolumab) for the treatment of MSI-H/dMMR metastatic CRC (mCRC). In 2018, the CheckMate-142 trial demonstrated the successful treatment of mCRC based on “double immunity” provided by nivolumab with ipilimumab, a regimen that may become a standard first-line treatment for MSI-H mCRC. In 2018, the FDA approved nivolumab alone or with ipilimumab for patients who progressed to MSI-H mCRC after standard chemotherapy (10). Colorectal cancer (CRC) patients, especially those with deficient mismatch repair (dMMR)/microsatellite instability-high (MSI-H) tumors, whose sensitivity to immune checkpoint inhibitors (ICIs) is significantly higher than that of patients with microsatellite-stable (MSS)/microsatellite instability-low (MSI-L) tumors, have derived clinical benefits from immunotherapy (10). Lenz et al. (11) recently showed that nivolumab plus low-dose ipilimumab provide a robust and durable clinical benefit and was well tolerated as a first-line treatment for MSI-H/dMMR mCRC. Based on these promising data, randomized studies are warranted. MSI-H has been speculated to have a better prognosis in patients with stage I and II CRC, but its role in patients with stage III and IV CRC is still controversial (8). Through a recent literature search, we found MSI-H/dMMR could guide treatments of advanced or metastatic CRC, but there is still a lot of controversy about the effect of MSI status on the prognosis of conventional treatment for stage I-IV CRC, which was one purpose of our article. Therefore, the current study aimed to explore the correlation between MSI status and the prognosis of stage I–IV CRC.

MSI is characterized by short sequence repeats (SSRs) or short tandem repeats (STRs) of repeated DNA sequences of various lengths (13). Microsatellites are widely distributed throughout the genome in a non-random manner and are prone to mutations during DNA replication (14). The MMR system consists of a family of enzymes that detect DNA replication errors (such as mismatches between the two strands of DNA). The MMR system includes the MHL1, MSH2, MSH6, and PMS2 genes. MSI has been frequently observed in various malignancies and has become a new research hotspot (15). It is associated with Lynch syndrome, which develops CRC easily (16). Although the prognostic role of MSI-H has been elaborated, many prognostic studies worldwide have been inconclusive (14). Studies have confirmed that MSI tumors have a better prognosis than microsatellite stable CRC, but MSI cancers do not necessarily have the same response to the chemotherapeutic strategies used to treat microsatellite stable tumors. Specifically, stage II MSI tumors might not benefit from 5-fluorouracil-based adjuvant chemotherapy regimens. New data suggested possible advantages of irinotecan-based regimens, but these findings require further clarification (17). Colorectal cancers with MSI have distinctive features, including a tendency to arise in the proximal colon, poor differentiation, lymphocytic infiltration, and mucinous or signet-ring histology. Patients with MSI tumors appear to have a better prognosis than those with microsatellite stable tumors, but curiously the responses to 5-fluorouracil-based chemotherapy regimens are poorer with MSI tumors. Preliminary data suggested possible advantages of irinotecan-based regimens, but these findings need validation in well-designed clinical trials (18).The above conclusions did not classify MSI status and the results were illegible. Zoran Gatalica et al. (19) showed that heterogeneous MSI-H colorectal carcinomas as a group showed some distinct biological characteristics when compared with CRC with stable or low-level microsatellite instability. In their present review, they highlighted therapeutically relevant characteristics of MSI-H tumors that could lead to specific responses to some conventional chemotherapy or novel targeted therapy agents. With a similar viewpoint to E. Vilar’s (17), F. Battaglin, M et al. (20) considered that SI-high (MSI-H) status was associated with a better prognosis in early-stage CRC but a lack of benefit from adjuvant treatment with 5-fluorouracil in stage II disease. Moreover, MSI has emerged as a predictor of sensitivity to immunotherapy-based treatments. Based on the above studies, we believe that the role of MSI status in the prognosis of stage I–IV CRC remains unclear. Therefore, the present study aimed to explore the correlation between MSI expression and CRC prognosis from stages I to IV.

This study revealed that MSI-H usually occurs in the left colon and rectum, which was inconsistent with the previous findings that indicated that MSI-H occurs in the right colon (21). However, the present study suggests that MSI-L&MSS is likely to occur in the left colon and rectum for unknown reasons. The operation methods did not differ significantly between the MSI-H and MSI-L&MSS groups. The results also showed that MSI-H had weak tumor invasion and lymph node metastasis. A. Perrier et al. (22) revealed that MSI status exhibits a high response rate in almost all cancer types, and MSI-L was invasive and had lymph node metastasis, which was consistent with our study. Previous studies (23, 24) have shown that MSI-L&MSS has poor tumor differentiation, an elevated signet ring appearance, poor tumor differentiation, and a large tumor. Smyth et al. (25) showed that MSI-L&MSS had young age (median, 62 years) while MSI-H comprised older elderly (median, 66 years), which were not consistent with our study that did not show a significant difference between the two groups (P=0.725, Supplementary Table 1 ); additionally, bivariate correlation analysis did not establish any correlation between age and MSI status. Older CRC patients had a better prognosis than younger patients because tumor cell division was slower in older patients (26–28). The efficacy of PD-1 blockade compared with chemotherapy as a first-line therapy for MSI-H-dMMR advanced or metastatic colorectal cancer is unknown (29). In our study, MSI-H had a smaller chemotherapy proportion but a better prognosis due to its adequate biological behavior compared with MSI-L&MSS. The bivariate correlation analysis revealed that variations in preoperative S-CEA, positive lymph number, tumor size, tumor invasion (Pt status), N stage, differentiation, chemotherapy, and AJCC-8 were correlated to MSI status but not tumor location.

Several retrospective studies, including a systematic review and a meta-analysis, support the favorable stage-adjusted prognosis of MSI-H compared with MSS CRC patients (30–33). Typically, MSI-H has a good prognosis in early CRC. However, a literature search retrieved a few studies on the prognostic analysis of MSI-H in stage I CRC. Reportedly, MSI-H had a good prognosis in stage I–III CRC through 5-FU chemotherapy (34). Recent studies have shown that immunotherapy for stage IV CRC has a significant positive prognosis for MSI-H expression (35–37). However, only a few comparative studies are available on the prognosis of MSI-H CRC patients compared with MSI-L&MSS patients in stage IV. Toh et al. (38) assessed stage IV CRC without immunotherapy and found that MSI-H was not associated with any survival benefit, suggesting that a survival benefit was evident in both stage II and III CRC, and MSI-H was neither a robust prognostic marker in stage I nor stage IV CRC without immunotherapy. Moreover, in the current study, no significant prognosis was detected in stage I, which was consistent with the report by James et al. Interestingly, MSI-H had a better prognosis than MSI-L&MSS in stage IV accompanied by the therapy according to the National Comprehensive Cancer Network (NCCN) guidelines. The reasons for this might be that MSI-H is related to the biological behavior of the tumors, such as better tumor differentiation, poor tumor infiltration ability, and a low positive rate of lymph node metastasis, which were consistent with previous findings (39). G3 CRC was significantly correlated with high MSI (MSI-H) compared with G1 and G2 (P=0.002; odds ratio, 5.750), which improved the prognosis of MSI-H (40). Multivariate analysis identified MSI-H as an independent prognosis factor for CRC patients, and the results were consistent with the published literature (41, 42).

Nevertheless, the current study has some limitations. This is a single-center clinical study. The selected cases were collected up to January 2016, and hence, the CRC patients experiencing the optimal effect of MSI-H immunotherapy are not included. The MSI status tested by immunohistochemistry may have a small bias compared with other testing methods. Moreover, as KRAS gene testing was carried out relatively late in this unit, the relationship between MIS status and KRAS mutations and their prognosis with colorectal cancer were not studied. Aspects of immune parameters (humoral and cell-mediated), or the mutational statuses of genes such as Ras and Raf at various stages and correlated with MMR statuses, were not investigated in this study due to the immature technology. These are other limitations in the study. Prospective studies with different chemotherapy treatments, MSI statuses, CRC prognoses, and other tumor marks associated with CRC should be performed in future if possible.

MSI-H was associated with a good prognosis for stage II–IV CRC patients, but no significance was detected for stage I. Moreover, MSI expression was an independent factor for CRC patients. Further studies regarding the correlations between MSI status and stage I–IV CRC from multicentral institutions should appear. Prognosis using immunotherapy and chemotherapy based on MSI status guidance for advanced colorectal cancer is the next step in our research.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

The studies involving humans were approved by the ethics committee of Huzhou Central Hospital (No. 202110015-01). The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation in this study was provided by the participants’ legal guardians/next of kin.

GT: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Supervision, Validation, Writing – original draft, Writing – review & editing. GZ: Conceptualization, Methodology, Supervision, Writing – original draft, Writing – review & editing. YH: Writing – original draft, Writing – review & editing. XX: Conceptualization, Methodology, Supervision, Writing – original draft, Writing – review & editing. YW: Data curation, Validation, Writing – original draft, Writing – review & editing.