P04.16 * THE BIOMARKER POTENTIAL OF MGMT PROTEIN EXPRESSION IN GLIOBLASTOMA IS IMPROVED BY EXCLUSION OF NON-TUMOR CELLS

Abstract

INTRODUCTION: Methylation of the DNA repair protein O6 methylguanine-DNAmethyltransferase(MGMT) gene promoter is predictive of benefit from temozolomide (TMZ) in glioblastoma (GBM). Non-tumor cells are numerous in GBMs and a potential source of bias in attempts to determine MGMT status by immunohistochemistry. The aim of the present studywasto explore the prognostic potential ofMGMTprotein expression excluding these cells. METHOD: Immunofluorescence multiplexing and automated analysis were combined to quantify MGMT (green fluorescence) in tumor cell nuclei using image analysis algorithms. Microglia/macrophages, lymfocytes and vascular structures (red fluorescence) known to express MGMTwere excluded from the analysis. The study included paraffin sections from 173 primary GBM's. RESULTS: High area fraction of tumor cell nuclei expressing MGMT was associated with poor prognosis (HR = 1.59, p = 0.005). Prognostic value of MGMTexpression was particularly strong in patients treated with TMZ as first line therapy (HR = 2.31, p = 0.0003). The 2-year survival in this group was 44% versus 7 %for patients with low and high expression respectively. Non-tumor cell nuclei often showed higher MGMT expression than adjacent tumor cells. Exclusion of these cells was shown to improve the prognostic value of MGMT expression. CONCLUSION: MGMT protein expression in tumor cell nuclei holds prognostic potential in GBM. Exclusion of non-tumor cells from the analysis is crucial to avoid false positives. The path to possible clinical implementation will require further testing in independent patient cohorts as well as development of a highly robust multiplexing assay with not yet defined cut-off levels.