[551] Only Two Microsatellite Markers (Bat 26 and Bat 34c) Are Needed, with Immunohistochemistry, To Accurately Detect High Microsatelite Instability Cancers

J Beheshti, F Kuo, N Lindeman. Brigham and Women's Hospital, Boston, MA

Background: Microsatellite instability (MSI) is a consequence of dysfunctional mismatch repair proteins MLH1, MSH2, MSH6, or PMS2, which is seen in sporadic colon carcinomas and in cancers associated with the Hereditary Non-Polyposis Colorectal Cancer Syndrome (HNPCC). In addition to immunohistochemistry (IHC) of these proteins, molecular testing for MSI is performed, which consists of PCR and electrophoresis of 10 microsatellites. If >30% of the microsatellites are unstable, the diagnosis is MSI-high, with increased risk for HNPCC, and further genetic testing is performed. Testing 10 markers is costly and time-consuming, so the minimum number of markers that were needed, in conjunction with IHC, for accurate diagnosis of MSI-high, was assessed.
Design: 181 consecutive MSI and IHC results were reviewed. Each of 10 MSI markers (Bat25, Bat 26, Bat 34c, Bat 40, D17S250, D5S346, D18S55, D10S197, MYCL1, ACTC) were independently assessed for association with IHC results and with MSI-high (3+ unstable markers), MSI-low (1-2 unstable markers) or MSS (microsatellite stable; no unstable markers).
Results: 154/181 (83%) of the cancers were colorectal, with the remainder from ovary, endometrium, appendix, omentum, skin, pancreas, and duodenum. 25 cases (8.3%) were MSI-high, and all of these (100%) had loss of expression of at least one IHC marker: 24 negative and 1 weak. 19 cases (10.4%) were MSI-low, of which one had loss of IHC expression. All 137 MSS cases had intact IHC staining. Therefore, the sensitivity of IHC for MSI-high was 100%, with 99.3% specificity. Of the MSI markers, BAT26 and BAT34c alone were sufficient to correctly classify every tumor as either MSI-high or not. All 25 (100%) of MSI-high cases were unstable with at least one of these two markers, and all of the 156 (100%) MSI-low and MSS cases were stable with both of these markers.
Conclusions: In 181 consecutive samples, only two markers, BAT26 and BAT34c, were needed to make this diagnosis with absolute (100%) accuracy. IHC also detected every case of MSI-high, with only one false positive result in a case of MSI-low. While these findings need corroboration from other studies, the implication is that IHC and a more limited MSI panel could be used, reducing time and cost, without impairing diagnostic accuracy. Limiting the MSI panel to BAT26 and BAT34c would have misclassified MSI-low cases as MSS, but this is of no clinical consequence, as MSI-low is not associated with HNPCC and does not require further genetic assessment.
Category: Gastrointestinal

Monday, March 9, 2009 1:00 PM

Poster Session II # 66, Monday Afternoon