Microsatellite Instability and Mistmatch Repair protein analysis [article]

Salvador J. Diaz-Cano
2012 Figshare  
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
more » ... on 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 e [...]
doi:10.6084/m9.figshare.103687.v1 fatcat:m7smi3mo5baynagjoewb6kzm2i