[588] Base Pair Changes in Assessing Microsatellite Instability and Correlation to Mismatch Repair Status by Immunohistochemistry

Kajsa Affolter, Andrew Wilson, Wade Samowitz, Katherine Geiersbach. University of Utah, Salt Lake City, UT; ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT

Background: Microsatellite instability (MSI) testing by PCR amplification and capillary electrophoresis is a standard method of assessing mismatch repair status in patients with colorectal and other Lynch associated cancers. Many laboratories use a commercial kit (MSI Analysis System, Promega Corp.) containing five mononucleotide microsatellite markers (BAT-25, BAT-26, MONO-27, NR-21, and NR-24). The Promega kit defines MSI-High tumors as being unstable for at least 2 of the 5 markers and instability is defined as an allele size shift of at least 3 base pairs (bp) between the tumor and non-tumor tissue. The current available data to support the threshold of 3 bp or more for instability is limited.
Design: We retrospectively identified 175 adenocarcinomas (mostly colorectal and endometrial) which had been evaluated by both the Promega kit for microsatellite instability and immunohistochemistry for mismatch repair genes. MSI results were re-classified into groups according to the specific number of base pair changes. MSI data were then compared to immunohistochemistry results for mismatch repair proteins (MLH1, MSH2, MSH6, PMS2) on the same tumor.
Results: One of 72 tumors with no change in repeat size and one of 73 tumors with a one base pair change in repeat size as compared to normal were associated with abnormal IHC results (loss of MSH2/MSH6 and isolated loss of PMS2, respectively). Two of six tumors with a two base pair change in at least one repeat were associated with abnormal IHC results (MLH1/PMS2 loss and isolated PMS2 loss). Twenty-three of 24 tumors with a three or more base pair change in at least one repeat were associated with abnormal IHC results. The proportion of tumors with a two (p=0.0001) or three base pair change (p<0.0001) and abnormal IHC results was significantly greater than that seen with either no change or a one base pair change.
Conclusions: This study confirms the notion that a three base pair change in a mononucleotide repeat is an excellent marker for mismatch repair deficiency. Although further study is necessary, these data also suggest that a 2 base pair change in a mononucleotide repeat may be indicative of mismatch repair deficiency, and that IHC testing may be helpful in such cases. Finally, mismatch repair deficiency as indicated by abnormal IHC testing may occur in tumors without detectable microsatellite instability; optimal detection of mismatch repair deficiency probably requires both testing modalities.
Category: Gastrointestinal

Wednesday, March 6, 2013 9:30 AM

Poster Session V # 90, Wednesday Morning

 

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