H3.1K27M-induced misregulation of the TSK/TONSL-H3.1 pathway causes genomic instability

The oncomutation lysine 27-to-methionine in histone H3 (H3K27M) is frequently identified in tumors of patients with diffuse midline glioma-H3K27 altered (DMG-H3K27a). It inhibits the deposition of the histone mark H3K27me3, leading to consequences for the maintenance of transcriptional programs and cell identity. H3K27M-expressing cells are also characterized by defects in genome integrity, but the mechanisms linking loss of methylation at H3K27 to DNA damage remain mostly unknown. In this study, we demonstrate that expression of H3.1K27M in the model plant Arabidopsis thaliana interferes with post-replicative chromatin maturation mediated by the H3.1K27 methyltransferases ATXR5 and ATXR6. As a result, H3.1 variants on nascent chromatin remain unmethylated at K27 (H3.1K27me0), leading to ectopic activity of TONSOKU (TSK/TONSL) which induces DNA damage and genomic alterations. Elimination of TSK activity suppresses defects associated with H3.1K27M expression, while inactivation of specific DNA repair pathways prevents the survival of H3.1K27M-expressing plants. Overall, these results suggest that misregulation of TSK/TONSL may contribute to the etiology of DMG-H3K27a by driving DNA damage upon H3K27M-mediated loss of H3K27 methylation.