Prognosis of glioblastoma with faint MGMT methylation-specific PCR product

Methylation-specific polymerase chain reaction (MSP) for the promoter methylation status of O⁶-methylguanine-DNA-methyltranferase (MGMT) gene theoretically provides a positive or negative result. However, the faint MSP product is difficult to interpret. The aim of this study was to evaluate the significance of faint MSP product in glioblastoma (GBM). Critical concentrations of methylated control DNA, i.e., 100, 1, 0.5 and 0 % were run parallel with 116 newly diagnosed GBMs in order to standardize the interpretation and to distinguish positive (+), equivocal (±), and negative (−; unmethylated) results. Cases with the faint MSP product and its intensity between those of 1 and 0.5 % DNA controls were considered equivocal (±). MGMT methylation quantifications were also determined by quantitative real-time MSP (qMSP) and pyrosequencing (PSQ), and protein expression was detected by immunohistochemistry. There were significant correlations between MSP and all the aforementioned studies. The concordance rates between the MSP+ and qMSP+ cases, as well as the MSP− and qMSP− cases were 100 %, and the MSP± cases comprised 76.5 % of qMSP+ cases and 23.5 % of qMSP− cases. PSQ study showed that heterogeneous methylation was more frequently encountered in the MSP± cases. Multivariate analyses disclosed that although the overall survival of the MSP± cases was indistinct from that of the MSP+ cases, its progression free survival was significantly worse and was indistinct from that of the MSP− cases. In conclusion, GBMs with faint MGMT MSP products should be distinguished from MSP+ cases as their behaviors were different.