Microsatellite instability in metastatic colorectal cancer: a review of pathology, response to chemotherapy and clinical outcome

Approximately 10% of colorectal carcinomas demonstrate microsatellite instability (MSI). Distinct from the majority of colorectal cancers with chromosomal instability (CIN) which harbour allelic imbalance from chromosomal polyploidy and aneuploidy, MSI tumours retain intact chromosomal numbers but contain microsatellite repeats due to deficiency in mismatch repair which are thought to contribute to the early steps of tumourigenesis in colorectal cancer. While emerging clinical data has highlighted improved prognosis of tumours with MSI in early colorectal cancer and potentially circumventing the need for adjuvant chemotherapy, the implications of MSI deficiency in metastatic colorectal cancer (mCRC) remain uncertain. In order to assess the significance of MSI in mCRC, a broad literature review was carried out through online PubMed search on published articles encompassing pathological and clinical papers. This included pathological studies identifying the correlation with MSI status between primary sites and metastases, and chemotherapeutic studies assessing the impact of fluoropyrimidine-, irinotecan- and oxaliplatin-based regimens on mCRC with MSI.