Somatic mutations on glycine 34 of histone H3 (H3G34) cause pediatric cancers, but the underlying oncogenic mechanism remains unknown. We demonstrate that substituting H3G34 with arginine, valine, or aspartate (H3G34R/V/D), which converts the non-side chain glycine to a large side chain-containing residue, blocks H3 lysine 36 (H3K36) dimethylation and trimethylation by histone methyltransferases, including SETD2, an H3K36-specific trimethyltransferase. Our structural analysis reveals that the H3 “G33-G34” motif is recognized by a narrow substrate channel, and that H3G34/R/V/D mutations impair the catalytic activity of SETD2 due to steric clashes that impede optimal SETD2–H3K36 interaction. H3G34R/V/D mutations also block H3K36me3 from interacting with mismatch repair (MMR) protein MutSα, preventing the recruitment of the MMR machinery to chromatin. Cells harboring H3G34R/V/D mutations display a mutator phenotype similar to that observed in MMR-defective cells. Therefore, H3G34R/V/D mutations promote genome instability and tumorigenesis by inhibiting MMR activity.

组蛋白H3（H3G34）的甘氨酸34上的体细胞突变引起儿童癌症，但潜在的致癌机制仍然未知。我们证明用精氨酸，缬氨酸或天冬氨酸（H3G34R / V / D）取代H3G34，可将非侧链甘氨酸转化为含侧链的较大残基，阻止H3赖氨酸36（H3K36）二甲基化和三甲基化的组蛋白甲基转移酶，包括H3K36特异性三甲基转移酶SETD2。我们的结构分析表明，H3“ G33-G34”基序可被狭窄的底物通道识别，并且由于空间碰撞而阻碍了SETD2-H3K36的最佳相互作用，H3G34 / R / V / D突变会损害SETD2的催化活性。H3G34R / V / D突变也阻止H3K36me3与错配修复（MMR）蛋白MutSα相互作用，从而阻止MMR机制募集到染色质。携带H3G34R / V / D突变的细胞表现出与在MMR缺陷细胞中观察到的相似的突变体表型。因此，H3G34R / V / D突变通过抑制MMR活性促进基因组不稳定性和肿瘤发生。