Polycomb repressive complex 2 (PRC2) suppresses gene transcription by methylating
lysine 27 of
histone H3 (H3K27) and plays critical roles in embryonic development. Among the core PRC2 subunits, EZH2 is the catalytic subunit and EED allosterically activates EZH2 upon binding trimethylated H3K27 (H3K27me3). Activating mutations on Y641, A677, and A687 within the enzymatic SET (Su(Var)3 to 9, Enhancer-of-zeste, and Trithorax) domain of EZH2 have been associated with enhanced H3K27me3 and tumorigenicity of many
cancers including
B-cell lymphoma and
melanoma. To tackle the critical residues outside the EZH2 SET domain, we examined EZH2 mutations in
lymphoma from
cancer genome databases and identified a novel gain-of-function mutation W113C, which increases H3K27me3 in vitro and in vivo and promotes CDKN2A silencing to a similar level as EZH2 Y641F. Different from other gain-of-function mutations, this mutation is located in the SET-activation loop at the EZH2 N terminus, which stabilizes the SET domain and facilitates substrate binding. This may explain how the W113C mutation increases PRC2 activity.
Tazemetostat is a Food and Drug Administration-approved EZH2-binding inhibitor for
follicular lymphoma treatment. Intriguingly, the W113C mutation leads to
tazemetostat resistance in both H3K27 methylation and
tumor proliferation. Another class of allosteric PRC2 inhibitor binding EED overcomes the resistance, effectively decreases H3K27me3, and blocks
tumor proliferation in cells expressing EZH2 W113C. As this mutation is originally identified from
lymphoma samples, our results demonstrated its activating characteristic and the deleterious consequence, provide insights on PRC2 regulation, and support the continued exploration of treatment optimization for
lymphoma patients.