Microsatellite instability in colorectal cancer

Abstract

Microsatellite instability (MSI) is a hypermutable phenotype caused by the loss of DNA mismatch repair activity. MSI is detected in about 15% of all colorectal cancers; 3% are of these are associated with Lynch syndrome and the other 12% are caused by sporadic, acquired hypermethylation of the promoter of the MLH1 gene, which occurs in tumors with the CpG island methylator phenotype. Colorectal tumors with MSI have distinctive features, including a tendency to arise in the proximal colon, lymphocytic infiltrate, and a poorly differentiated, mucinous or signet ring appearance. They have a slightly better prognosis than colorectal tumors without MSI and do not have the same response to chemotherapeutics. Discovery of MSI in colorectal tumors has increased awareness of the diversity of colorectal cancers and implications for specialized management of patients.

Figure 1

MSI research associated with colorectal cancer from 1990 to 2010. EpCAM, epithelial cell adhesion molecule.

Figure 2

The DNA MMR system functions through a series of steps. (A) MSH2–MSH6 (MutSα) recognizes single base-pair mismatches, in which the DNA polymerase has matched the wrong base (G) with the T on the template (shown on left), and creates a sliding clamp around the DNA. This step that requires the exchange of adenosine triphoshpate (ATP) for adenosine diphosphate (ADP) (by MSH2, but not MSH6 or MSH3). The complex diffuses away from the mismatch site, which is then bound by the MLH1-PMS2 (MutLα) complex (right). This “matchmaker” complex moves along the new DNA chain until it encounters the DNA polymerase complex. (B) The DNA MMR protein sliding clamp interacts with exonuclease-1, proliferating cell nuclear antigen (PCNA), and DNA polymerase. This complex excises the daughter strand back to the site of the mismatch (shown on left). Eventually, the complex falls off the DNA and resynthesis occurs, correcting the error. (C) Variations on the DNA MMR theme. Whereas MSH2–MSH6 recognizes single pair mismatches and small IDLs, MSH2–MSH3 (MutSβ) complements this by also recognizing larger IDLs (shown on left). The right side shows the possible interactions with different MutL dimers, as MLH1 can dimerize with PMS2, PMS1, or MLH3. The preferred interaction with MSH2–MSH3 is MLH1–MLH3 (MutLγ), but the precise roles of the other MutL heterodimers in this reaction are not entirely understood.

Figure 3

MSI was initially discovered by autoradiography analyses of the PCR products after separation by gel electrophoresis (upper panel). N refers to DNA from the normal colon, and T from the tumor. The DNA polymerase used in PCR also has difficulty with the accurate amplification of templates, which is thought to be the explanation for the “ladder” of DNA bands that can be seen in the lanes for normal and tumor DNA. The upper panel illustrates the use of the 5 markers recommended by the National Cancer Institute consensus group; these consist of 3 dinucleotide repeats and 2 mononucleotide repeats (BAT25 and BAT26). In each instance, the DNA in the tumor has undergone somatic mutations (frequently, but not always, deletions), and the PCR product migrates to a different position on the gel, as indicated by the arrowheads. The lower panel shows the PCR products as they are analyzed by most laboratories using automated DNA sequencing with fluorescent primers. In this instance, 5 mononucleotide repeats have been analyzed, and in each instance, the mutations consist of deletions with different electrophoretic mobility (mutant alleles indicated by the arrows).

Figure 4

The regions of protein–protein interactions among the members of the MMR system. Mutations in the regions of protein interactions are particularly likely to disrupt MMR function. Key interactions are illustrated among the MutS proteins, among the MutL proteins, and between these proteins and Exo1. ATP, adenosine triphosphate; PMS, post-meiotic segregation.

Figure 5

There are 2 molecular pathways to the development of CRC with MSI. Approximately 20%–25% of colorectal tumors with MSI arise in individuals with Lynch syndrome. These tumors contain a germline mutation in one of the MMR genes, followed by a second hit to the wild-type copy (inherited from the unaffected parent); this could occur via LOH, methylation, or point mutation. Defects in MMR result in MSI and rapid accumulation of somatic mutations. It has been proposed that tumors arise via mutations in a few critical genes, but that large numbers of microsatellite mutations also occur—most of which are simply passengers that provide the mutational signature used to identify tumors with MSI. Colorectal tumors that arise in patients with Lynch syndrome often have mutations in KRAS. Most cases of CRC associated with MSI are not inherited (familial), but arise through sporadic methylation-induced silencing of MLH1. These sporadic tumors have the CIMP signature, resulting in methylation of many gene promoters. When the MLH1 promoter is methylated, MMR activity fails and MSI ensues. Thus, the mutational signature of sporadic tumors includes CIMP and MSI. BRAF mutations are also observed in most sporadic colorectal tumors, but do not occur in tumors of patients with Lynch syndrome.