Effects of Long Term Tissue Fixation on the Immunohistochemical Expression of MSI Makers in Colon Adenocarcinoma
Patrick Adegboyega. LSU Health Sciences Center, Shreveport, LA
Background: Colorectal adenocarcinomas with microsatellite instability (MSI) do not respond well to Fluorouracil-based chemotherapy and do have treatment outcome that differs from that seen in microsatellite stable tumors. In clinical settings, immunohistochemical staining for makers of MSI [Mismatch repair (MMR) gene products] is used to screen for the presence of MSI; and has been shown to have comparable sensitivity and specificity with MSI detection by PCR. It was recently shown that routinely used tissue fixative such as dissect aid negatively impacts MMR proteins immunohistochemistry. 10% Neutral Buffered Formalin (NBF) was shown to be the optimal tissue fixative for MMR protein immunostaining in routine surgical pathology practice. This study explores the effects of long term NBF tissue fixation on the immunohistochemical expression of three MMR gene products (MLH1, MSH2 and MSH6).
Design: Study materials consisted of cases of colonic adenocarcinoma: 7 primary colectomy and 1 secondary hepatectomy specimens received for tumor diagnosis and staging. Samples of normal colon and tumor from each specimen were fixed in NBF and submitted for routine processing with paraffin embedding after fixation for 1 day, 1 month, 3 months, 6 months and 1 year. Immunohistochemistry for MLH1, MSH2 and MSH6 was performed on representative sections of each block. Immunoreactivity scoring was done using a semi-quantitative score of 0, 1, 2, 3 and 4.
Results: MLH1 immunoreactivity scores for all the samples were strong for the samples within the first 3 months of fixation (previous findings) but thereafter became drastically reduced and is completely negative in 5 of 8 cases for MLH1, 3 of 8 cases for MSH2 and 2 of 8 cases for MSH6 after one year of fixation.
Conclusions: Although 10% Neutral Buffered Formalin solution is the preferred fixative for MMR immunohistochemical assay, long term tissue fixation (greater 3 months) results in loss of immunoreactivity of MMR proteins in tissue sections and so may produce spurious results (false MSI status).
Monday, March 19, 2012 1:00 PM
Poster Session II # 306, Monday Afternoon