MGMT Hydroxymethylation in glioblastoma

Abstract

O⁶ -methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme which is commonly hypermethylated in glioblas-tomas. Methylation of the MGMT promoter is associated with an improved response to alkylating agent chemotherapy and patient survival. Hence MGMT promoter methylation is often assessed as a prognostic marker for determining the course of treatment. Cytosine hydroxymethylation is another DNA modification whose significance is only now becoming clear, and in mammals it is known to occur at high levels in the brain and pluripotent stem cells. Importantly, 5'hydroxymethylcytosine is indistinguishable from 5'methylcytosine by bisulphite-based techniques. We hypothesised that hydroxymethylation may confound MGMT methylation assessment in a subset of glioblastomas, and may account in part for those glioblastomas that are classified as having MGMT hypermethylation but who fail to respond to chemotherapy. In this study the levels of both hydroxymethylcytosine and methyl-cytosine were assessed in a cohort of gliomas. Hydroxymethylation was found to be present in most samples, and ranged from 0.7% to 80% of the total ‘methylation' signature of this region. These results demonstrate that hydroxymethylation can comprise a substantial amount of the ‘methylation' levels detected at the MGMT promoter in glioblastomas, and may potentially confound the classification of patients for treatment with alkylating agents.