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.