Targeting Epigenetic Maintenance of Stemness in Pediatric Malignancy

A shift of the focus in cancer research from genetic to epigenetic mechanisms can be observed during the last five years. Chromatin modifications are increasingly recognized as a key mechanism. The histone methyl-transferase Enhancer of Zeste, Drosophila, Homolog 2 (EZH2) is the enzymatic subunit of the polycomb PRC2 complex and methylates histone H3K27, thereby, mediating gene silencing. EZH2 is over-expressed in a variety of tumor tissue including breast and prostate. Ewing Tumors (ET), are highly malignant tumors that are molecularly defined by ews/ets translocations, with fli1 being the ets family member most frequently involved. We found EWS-FLI1 bound to the EZH2 promoter in vivo. Other components of the PRC2 complex, like EED or SUZ12 were not deregulated in ET. Down-regulation of EZH2 by RNA interference suppressed tumor development and metastasis in vivo and revealed an EZH2-maintained undifferentiated, reversible phenotype in ET. EZH2 suppression resulted in a generalized loss of H3K27me3 as well as an increase in H3 acetylation. Gene-specific analysis of H3K27me3 verified such genes that had specifically lost H3K27me3 upon EZH2 silencing, suggesting that stemness features are preserved via epigenetic mechanisms and that the genetic EWS-Fli1 translocation is intimately linked to global and gene specific epigenetic alterations in ET biology.