Dominant negative effects of Weaver syndrome-associated EZH2 variants

Heterozygous missense mutations in EZH2 cause Weaver syndrome (WS), a developmental disorder characterized by intellectual disability and overgrowth. EZH2encodes the enzymatic subunit of Polycomb Repressive Complex 2 (PRC2), which mediates mono-, di-, and tri-methylation of histone H3 lysine 27 (H3K27me1/2/3). Most WS-associated EZH2 variants lack functional characterization but are presumed loss of function. However, the lack of early truncating mutations in EZH2 led us to hypothesise a dominant-negative mechanism for WS, which was supported by our structural analysis of all known WS-associated EZH2 variants. We isogenically modelled 10 representative variants in embryonic stem cells and showed they reduce global H3K27me2/3 with concomitant increases in H3K27ac and chromatin decompaction. Notably, the pattern of H3K27me2/3 reductions indicated dominant-negative interference on PRC2 activity, even when WS-variants were expressed atlow levels. RNA-seq identified weakly Polycomb-bound genes that lose canonical PRC1 (cPRC1) occupancy and become derepressed, including several phenotypically relevant growth control genes. Comparative analysis of a gain-of-function EZH2 variant causing growth restriction revealed reciprocal chromatin and transcriptional changes compared to WS-associated variants. Taken together, our findings support a model where EZH2 variants associated with opposing developmental growth syndromes affect not only H3K27me3, but also intergenic H3K27me2, chromatin architecture, and cPRC1 recruitment. We generated Ezh2 heterozygous knockout mESCs and then exogenously expressed FLAG/HA-tagged human wild-type EZH2 (EZH2-WT) or 5 Weaver syndrome associated EZH2 variants. For comparison, we also expressed growth restriction-associated EZH2-A733T variant. We performed exogenous reference genome-normalised ChIP-seq (ChIP-Rx) for H3K27me2, H3K27me3, H3K27ac, H3K36me2, SUZ12, CBX7, and FLAG in these cell lines.