EZH2 inhibition as a therapeutic strategy for lymphoma with EZH2-activating mutations

EZH2 is a methyltransferase that is mutated in lymphoma; here a potent small molecule inhibitor of EZH2 is described, which inhibits the proliferation of EZH2 mutant cell lines and growth of EZH2 mutant xenografts in mice, thus providing a potential treatment for EZH2 mutant lymphoma. EZH2 as a target in lymphomas EZH2, the catalytic subunit of the polycomb repressive complex 2 (PRC2), is involved in repressing gene expression through methylation of histone H3 on lysine 27 (H3K27). Overexpression of EZH2 is implicated in tumorigenesis, and mutations within its catalytic domain occur in lymphoma. Here, Caretha Creasy and colleagues describe a potent small-molecule inhibitor of EZH2 methyltransferase activity that decreases levels of methylated H3K27 and reactivates silenced PRC2 target genes. It also inhibits the proliferation of EZH2 mutant cell lines and the growth of EZH2 mutant xenografts in mice. Pharmacological inhibition of EZH2 activity may therefore be a viable strategy for treating EZH2 mutant lymphoma. In eukaryotes, post-translational modification of histones is critical for regulation of chromatin structure and gene expression. EZH2 is the catalytic subunit of the polycomb repressive complex 2 (PRC2) and is involved in repressing gene expression through methylation of histone H3 on lysine 27 (H3K27). EZH2 overexpression is implicated in tumorigenesis and correlates with poor prognosis in several tumour types 1 , 2 , 3 , 4 , 5 . Additionally, somatic heterozygous mutations of Y641 and A677 residues within the catalytic SET domain of EZH2 occur in diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma 6 , 7 , 8 , 9 , 10 . The Y641 residue is the most frequently mutated residue, with up to 22% of germinal centre B-cell DLBCL and follicular lymphoma harbouring mutations at this site. These lymphomas have increased H3K27 tri-methylation (H3K27me3) owing to altered substrate preferences of the mutant enzymes 9 , 11 , 12 , 13 . However, it is unknown whether specific, direct inhibition of EZH2 methyltransferase activity will be effective in treating EZH2 mutant lymphomas. Here we demonstrate that GSK126, a potent, highly selective, S -adenosyl-methionine-competitive, small-molecule inhibitor of EZH2 methyltransferase activity, decreases global H3K27me3 levels and reactivates silenced PRC2 target genes. GSK126 effectively inhibits the proliferation of EZH2 mutant DLBCL cell lines and markedly inhibits the growth of EZH2 mutant DLBCL xenografts i