Microsatellite Instability and Mistmatch Repair protein analysis
Microsatellite Instability Testing in CRC and Interpretation
Microsatellite instability is a type of mutation that occurs in microsatellite regions (short polymorphic DNA segments) by nucleotide addition (increased size) or nucleotide loss (decreased size) in the repeats. These changes in size of the microsatellite repeat are known as microsatellite instability. Microsatellite instability occurs because of deficient/loss of DNA mismatch repair (MMR), which requires the function of several DNA mismatch repair proteins (hMLH1, hMSH2, hMSH6, hPMS2, hMSH3 and hMLH3). Loss of expression of these proteins in tumors can be detected by IHC performed on paraffin sections.
Approximately 15% sporadic colorectal cancers and other sporadic adenocarcinomas of the spectrum of HNPCC cancers can have MSI-positive status. Therefore, confirmation of HNPCC (hereditary non-polyposis colorectal cancer) requires identification of germline mutations (detected in peripheral blood DNA) in one of the DNA mismatch repair genes, whereas somatic hypermethylation of the hMLH1 promoter leading to loss of hMLH1 expression is the underlying abnormality causing MSI in sporadic tumor tissues.
For the MSI test DNA is extracted from unstained sections from formalin fixed and paraffin embedded tissue specimens. PCR amplification is performed with sets of primers that amplify five microsatellite markers (BAT25, BAT26, D2S123, D5S346 and D17S250), classifying the tumors as: 1) MSI-High level (MSI-H, >30% markers positive for MSI); 2) MSI-Low (MSI-L, <30% markers positive for MSI); 3) Microsatellite stable (MSS, no marker shows MSI).
If a tumor is MSI-H the patient might have HNPCC, and it is important that the MSI test report states that there are potential genetic implications of the test results and genetic counseling should be recommended. If HNPCC is ruled out, MSI-H identifies a sub-group of sporadic adenocarcinomas that have distinct clinical pathological features namely better survival and resistance to 5-fluorouracil (5FU). If no loss of expression of hMSH2 or hMLH1 is seen in MSI-H tumors by IHC or if the tumor is MSI-L or MSS but there is clinical suspicion of HNPCC, evaluation of other MMR genes, in particular hMSH6 and hPMS2 may be performed, by immunohistochemical stains and/or germline mutational analyses.
Correlation of MSI Test Results and DNA Mismatch Repair Protein IHC
Note that when hMLH1 expression is lost hPMS2 is also lost because hPMS2 requires hMLH1 for stability through heterodimerization; in contrast, if hPMS2 is lost hMLH1 expression is preserved because hMLH1 also forms heterodimers with other proteins, thus being protected from degradation. Similarly, when hMSH2 expression is lost, hMSH6 is also lost because hMSH6 requires hMSH2 for stabilization through heterodimerization; in contrast, if hMSH6 is lost hMSH2 expression is preserved because hMSH2 also forms heterodimers and is stabilized by other proteins.
Clinical Indications for Microsatellite Instability Testing in Colorectal Cancer
The most recent guidelines to help decide whether a patient should undergo molecular testing to rule out HNPCC (revised Bethesda criteria). The pathologist can easily identify patients with criteria 1 and 2 (tumors before the age of 50, or synchronic-metachronic tumors in specific locations), and criterium 3 specifically depends on the identification by the pathologist of tumors that demonstrate MSI-suggestive histology (Tumor infiltrating lymphocytes, Crohn’s-like lymphocytic reaction, mucinous or signet-ring cell differentiation, poorly differentiated carcinomas with medullary growth pattern).
MSI and Prediction of Tumor Prognosis, Progression and Chemotherapy Response
Several studies have reported unique clinicopathological features of tumors related to their MSI status, namely a relationship with response to chemotherapy (5-FU) and an improved prognosis in tumors that are MSI-H.
Studies have shown that MSI-H colorectal cancers show less lymph node metastases burden and have better survival, independent of stage, site, tumor grade, and age. MSI and MMR downregulation also play a role in tumor progression is bladder and adrenal medullary neoplasms. Chemotherapy of stage II and III CRC with 5-fluorouracil did not improve survival if the tumor was MSI-High. In contrast, patients with microsatellite stable tumors treated with 5-FU had better survival compared with patients who were not treated.
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