MGMT promoter methylation and glioblastoma: a comparison of analytical methods and of tumor specimens

It is already well known that hypermethylation of the O-6-methylguanine DNA methyltransferase (MGMT) gene promoter is a predictive biomarker of response to temozolomide treatment and of favorable outcomes in terms of overall survival (OS) and progression-free survival (PFS) in glioblastoma (GBM) patients. Nevertheless, MGMT methylation status has not currently been introduced into routine clinical practice, as the choice of the ideal technique and tissue sample specimen is still controversial. The aim of this study was to compare 2 analytical methods, methylation-specific polymerase chain reaction (MSP) and pyrosequencing (PSQ), and their use on 2 different tissue type samples, snap-frozen and formalin-fixed paraffin-embedded (FFPE), obtained from a single-center and uniformly treated cohort of 46 GBM patients. We obtained methylation data from all frozen tissues, while no results were obtained for 5 FFPE samples. The highest concordance for methylation was found on frozen tissues (88.5%, 23/26 samples), using PSQ (76.7%, 23/30 samples). Moreover, we confirmed that OS and PFS for patients carrying methylation of the MGMT promoter were longer than for patients with an unmethylated promoter. In conclusion, we considered MSP a limited technique for FFPE tissues due to the high risk of false-positive results; in contrast, our data indicated PSQ as the most powerful method to stratify methylated/unmethylated patients as it allows reaching quantitative results with high sensitivity and specificity. Furthermore, frozen tumor tissues were shown to be the best specimens for MGMT methylation analysis, due to the low DNA degradation and homogeneity in methylation throughout the tumor.