Expression of DNA methyltransferases 1 and 3B correlates with EZH2 and this 3-marker epigenetic signature predicts outcome in glioblastomas.

Abstract

This study aims to analyze expression of EZH2 and DNA-methyltransferases (DNMT1, 3A and 3B) in astrocytic tumors and investigate their link as well as their correlation with survival, especially in GBMs. Expression of EZH2 and DNMTs (DNMT1, DNMT3A and DNMT3B) in different grades of astrocytomas (n=93) was assessed by qRT-PCR and immunohistochemistry. GBM-U87MG cell line was used for functional studies. Strong immunopositivity (LI≥25%) for EZH2, DNMT1 and DNMT3B was detected in 52%, 56% and 64% cases of GBMs respectively, which was significantly higher as compared to Grade II/III cases. Similarly, their median fold change of mRNA expression was also significantly higher in GBMs. There was also a significant positive correlation between DNMT1/DNMT3B and EZH2 mRNA and protein expression, which was in concordance with TCGA data set. Inhibition of DNMTs in cell line by Azacytidine resulted in down-regulation of EZH2, while knock-down of EZH2 by siRNA was not associated with any significant alteration of DNMTs, indicating that EZH2 expression in GBMs is possibly regulated by DNMTs, but not the reverse. Strong immunopositivity for EZH2, DNMT1 and DNMT3B were individually associated with significantly shorter survival and showed no correlation with IDH1 mutation status. In addition, the combination of these 3 markers represented an independent prognostic signature with cases having weak/negative expression of all 3 markers being associated with best prognosis. For the first time, the present study describes an epigenetic prognostic signature in GBMs based on immunohistochemical expression of EZH2, DNMT1 and 3B which can be used easily in routine neuropathology practice.