Evaluation of O6-methylguanine-DNA methyltransferase by immunohistochemistry: Best clinical and research practices

O6-Methyguanine-DNA methyltransferase (MGMT) repairs DNA damage and acts as a tumor suppressor in normal cells by preventing DNA mutations. Several antibodies against MGMT are used for immunohistochemical assessment of this marker and no universal standard is adopted. We evaluated the immunohistochemical expression of MGMT with 5 commercially available primary antibodies in 59 invasive breast carcinomas. A tissue microarray was constructed using 59 invasive breast carcinoma samples. Five primary antibodies against MGMT were used for immunohistochemistry, including clones MT3.1, SPM287, and MT23.2. Heat-induced antigen retrieval and polymer-based immunohistochemistry were performed. Stains were analyzed by microscopy and automated digital slide technology. qRT-PCR was performed for all tumors. Clone SPM287 had the highest sensitivity (p0.001), and clone MT3.1 had the lowest sensitivity (p0.001). Clone MT23.2 generated higher levels of cytoplasmic staining, which was not observed with the other antibodies (p0.001). Fifty-six samples (94.9%) showed hypoexpression of MGMT compared with normal breast, as measured by qRT-PCR (p0.001). Only clone SPM287 correlated significantly with the qRT-PCR results (p=0.027). Antibody clone SPM287 is the most sensitive and specific antibody for the immunohistochemical evaluation of MGMT, rendering it a reliable and effective reagent for research and clinical practice in breast cancer.