National Cohort of Colorectal Cancers With Microsatellite Instability

The three main pathways of colorectal carcinogenesis are chromosomal instability, microsatellite instability (MSI) (15% of colorectal cancers =CRCs) and CpG island methylator phenotype (CIMP). MSI CRCs are associated with a better prognosis after curative surgery than CRCs without microsatellite instability (MSS). In contrast, MSI CRCs do not appear to benefit from adjuvant 5-FU chemotherapy, unlike patients with MSS CRCs. Nevertheless, the benefit of adjuvant chemotherapy with FOLFOX seems to be retained. The identification of prognostic markers in this subgroup of patients is therefore essential to decide on adjuvant chemotherapy, the efficacy of which is currently debated in MSI CRC.
To date, there are very few data concerning metastatic MSI CRC. Metastatic forms are rare (about 5% of metastatic CRCs), but are thought to be associated with chemoresistance and poor prognosis. Nevertheless, data are very sparse and there are no data regarding the use of modern chemotherapies and targeted therapies in metastatic MSI CRC. Thus, it is important to characterize the chemosensitivity of metastatic forms.
Clinical predictors of recurrence after curative CRC surgery are known but have only been studied in MSI CRC retrospectively. Similarly, many molecular and immunohistochemical factors, prognostic or predictive of response to adjuvant chemotherapy, have been recently identified in CRC (KRAS, BRAF, TP53, PI3KCA mutations, CIMP phenotype, SMAD4, immune response...). Most of these markers have been studied in all CRCs, but not specifically in the MSI CRC subgroup. All these prognostic and/or predictive biomarkers need to be better characterized in a large cohort of MSI CRCs.