gms | German Medical Science

Objective: Methylation of the gene promoter region of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) is known to predict overall and progression free survival in glioblastoma (GBM) patients treated with temozolomide. A few cases of switching the MGMT methylation status in GBM relapse have been published, however the potential effect on the clinical course of the patients is still poorly understood. In contrast to the usual "methylated" versus "unmethylated" rating, we precisely quantified the degree of MGMT promoter methylation of GBM patients and correlated the changes with the clinical outcome.

Methods: In a retrospective study, data on clinical course, prognostic factors and treatment of 13 patients (male: 8, female: 5) with GBM relapse (local: 10, multifocal: 3) were collected. The degree of MGMT promoter methylation was analyzed by High Resolution Melting PCR after DNA extraction from frozen tumor specimens of the primary tumor and first (n=13) and second (n=2) relapse.

Results: 5 of the patients (38%) displayed a switch of MGMT promoter methylation in relapse. Interestingly, these alterations occurred in both directions with 2 patients previously methylated losing MGMT methylation and 3 patients previously unmethylated gaining it in relapse. Furthermore, quantification of the degree of MGMT promoter methylation revealed that 3 patients with initially positive methylation showed a further increase in the methylation quantity by more than 20%. Therefore, a total of 8 patients (62%) were unstable in their MGMT promoter methylation. The degree of methylation of both patients with second relapse remained unchanged compared to the first relapse. In correspondence to recent publications, we could not find any correlation of the detected changes in MGMT methylation, the applied adjuvant therapy and the outcome.

Conclusion: Although the MGMT-methylation statusremained stable for the majority of GBM patients, there was an increase in the quantity of methylation in more than 60%. It is unlikely that a switch of the methylation status is a response to therapy, since it occurs equally to both directions and does not lead to therapy resistance in all cases. Thus, the molecular mechanisms and potential triggers remain to be further examined.