SUZ12-Nucleic Acid Interactions Constrain PRC2 Activity to Maintain Targeted Gene Silencing Essential to Diffuse Midline Glioma.

Polycomb Repressive Complex 2 (PRC2) mediates transcriptional silencing through trimethylation of histone H3 at lysine 27 (H3K27me3), an epigenetic modification critical for development and frequently altered in cancer. Pediatric diffuse midline gliomas (DMGs) bearing the histone H3 K27M mutation exhibit global loss of H3K27me3 due to dominant inhibition of PRC2 by the mutant histone. Despite widespread hypomethylation, focal retention of H3K27me3 persists, and tumor cells maintain dependency on residual PRC2 activity for proliferation. The molecular basis underlying this residual enzymatic function and its regulation remain poorly defined. To address this mechanism, we investigated the role of SUZ12, the architectural core of PRC2 that facilitates interactions with accessory subunits. We identified the SUZ12 N-terminal region as a regulatory domain that constrains PRC2 catalytic activity through transient interactions with nucleic acids, thereby limiting non-specific chromatin engagement. Expression of a truncated SUZ12 variant retaining the catalytic VEFS domain, but lacking the nucleic acid-binding regulatory elements, led to widespread H3K27 hypermethylation, displacement of canonical PRC1 complexes, disruption of chromatin architecture, and impaired H3 K27M glioma cell growth in vitro and in vivo . Biochemical analyses revealed a SUZ12 N-terminal domain that modulates PRC2 activity by promoting non-productive binding to nucleic acids, thus establishing a kinetic equilibrium essential for precise chromatin targeting. These findings redefine Polycomb specificity as a dynamic equilibrium between productive nucleosomal engagement and non-productive nucleic acid interactions, providing critical insights into PRC2 regulation and highlighting potential therapeutic vulnerabilities in PRC2-dependent cancers.