[1930] Immunohistochemistry (IHC) for DNA Mismatch Repair (MMR) Proteins in Colorectal Carcinoma: What Does Scanty Staining for MSH6 Mean?

M Guo, D Klimstra, L Tang, E Vakiani, N Katabi, Z Stadler, M Weiser, J Shia. Memorial Sloan-Kettering Cancer Center, NY

Background: IHC for MMR proteins is being increasingly used for screening colorectal cancer (CRC) patients for Lynch syndrome as well as for treatment decision making. However, IHC may be affected by various biologic and technical factors, and interpreting staining results remains challenging. Anecdotally, we have observed a scanty staining pattern for MSH6 antibody that could potentially lead to erroneous interpretation. This study aimed at analyzing the frequency and clinical/pathological significance of this aberrant pattern of staining.
Design: IHC stains for MLH1, PMS2, MSH2, and MSH6 of 420 CRC resections from patients fulfilling the revised Bethesda guidelines (age <50; age 50-60 and tumor shows suggestive morphology; or positive personal or family history) were reviewed, and the patterns of MSH6 staining analyzed; 120 tumors were rectal that had neoadjuvant therapy.
Results: Of the 420 cases, 92 had complete loss of staining for MLH1 and/or PMS2 and 32 for MSH2 and/or MSH6. Overall, 9 cases (9/420, 2%) exhibited a scanty staining pattern for MSH6. In these 9 cases, the positively stained tumor cells were often short segments of individual tumor glands (in gland forming tumors), or confined to a limited focus (in poorly differentiated tumors), constituting <10% of the tumor (often <5%). The staining intensity was uniformly strong; and the negatively stained portions of the tumor showed convincing positive internal control. All 9 tumors had normal staining for MSH2, but 5 lost PMS2/MLH1 (n=3) or PMS2 alone (n=2), which constituted 5% (5/92) of all MLH1 and/or PMS2 abnormal cases. One PMS2 negative case was tested for germline mutation and was found to have a pathogenic mutation in PMS2, but none in MLH1, MSH2 or MSH6. Four of the 9 tumors had normal staining for all other proteins, and all 4 were treated rectal tumors, which constituted 3% (4/120) of all treated rectal tumors tested.
Conclusions: Rare MLH1 and/or PMS2 deficient CRCs as well as post-treatment MMR proficient rectal cancers can show only scanty staining (i.e., near complete loss of staining) for MSH6. Given that MSH6 contains microsatellites in its coding region, secondary MSH6 mutation is a likely explanation for this phenomenon in MLH1 and/or PMS2 deficient cancers. In post-treatment cancers, possible explanation includes environmental factors, such as treatment induced hypoxia. Awareness of such a pattern can avoid misinterpretation of IHC results and the consequent unnecessary genetic work-up.
Category: Techniques

Monday, March 22, 2010 1:00 PM

Poster Session II # 249, Monday Afternoon


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