[689] Routine Mismatch Repair Protein Immunostaining in Colorectal Carcinomas Equivocal MSH6 Staining Is a Challenge

MM Yearsley, AM Bellizzi, CD South, H Hampel, WL Marsh, WL Frankel. Ohio State University, Columbus, OH

Background: Over the last 2 years, we have routinely performed mismatch repair protein (MMR) immunohistochemistry on all colorectal cancer (CRC) resections. While there have been no equivocal MLH1, MSH2 or PMS2 stains, MSH6 has been equivocal in some cases. To determine whether these cases were likely due to Lynch syndrome or another cause, we evaluated all MSH6-equivocal and absent cases.
Design: CRC resections with equivocal or absent MSH6 expression were obtained from our database. Age, family history (FH), tumor site and neoadjuvant therapy (NT) were noted. H&E slides were evaluated for amount of tumor and histologic features of microsatellite instability (histMSI), including mucin and/or Crohn's-like reaction. Initial and repeat MSH6 and MLH1, MSH2 and PMS2 slides were reviewed. MSH6 was scored as present (5% convincing nuclear staining), equivocal (1-5% or faint staining) or absent. Cases were reviewed by 3 GI pathologists and contained appropriate controls. Adjacent stroma and/or normal colon served as an internal control.
Results: Absent (8) or equivocal (5) MSH6 expression was seen in 13 of 306 cases. Six of the 13 (3 rectum, 3 right) with ample tumor showed absent MSH6. No patient (age 48-54) had FH of CRC, one had prior NT, and 5 tumors showed histMSI. Mutational analysis, completed in 1, demonstrated MSH6 mutation. The 7th case occurred in the rectum of a 32-year-old with FH of CRC in a distant relative. The tumor did not show histMSI but was MSH6-absent; however, tumor was limited (prior NT). An additional MSH6-absent rectal tumor occurred in a 78-year-old with no FH of CRC and with little evaluable tumor (prior NT). MSH6 was present in the pre-treatment biopsy; mutational analysis confirmed lack of MSH6 mutation. The final 5 cases (patients age 50-67) had equivocal MSH6; and 1 had FH of CRC while another had histMSI. Four had NT, 3 with little tumor in the resection. All 5 are likely MSH6-present. Mutational analysis has been completed in 2 confirming the lack of MSH6 mutation.
Conclusions: Routine immunohistochemical MMR protein evaluation in CRC is relatively straightfoward in most cases; equivocal results are uncommon, but MSH6 interpretation is difficult in occasional cases. Before molecular workup is undertaken in MSH6 equivocal/negative tumors, age, FH, histMSI, NT and sufficient tumor should be considered. Repeat MSH6 on a prior biopsy may be helpful. Studies are necessary to evaluate potential causes of difficulty in MSH6 interpretation including antibody and fixation issues.
Category: Gastrointestinal

Monday, March 9, 2009 1:00 PM

Poster Session II # 67, Monday Afternoon