CARM1-mediated methylation of ASXL2 impairs tumor suppressor gene activation by MLL3/COMPASS

An imbalance in the activities of the Polycomb and Trithorax complexes underlies numerous human pathologies, including cancer. The BAP1 deubiquitinase (DUB) complex negatively regulates Polycomb activity and also recruits the Trithorax histone H3K4 methyltransferase, MLL3/COMPASS, to the enhancers of tumor suppressor genes. We and others have demonstrated that the BAP1-MLL3 pathway is mutated in several differ cancers. Yet, how BAP1 recruits MLL3 to these loci remains a fundamentally important unanswered question. Here we demonstrate that ASXL2 directly mediates the interaction between the BAP1 complex and MLL3/COMPASS. ASXL2 loss abolishes the MLL3-BAP1 interaction, leading to decreased MLL3 occupancy at enhancers and reduced expression of BAP1-MLL3 target genes. The interaction between ASXL2 and MLL3 is negatively regulated by protein arginine methyltransferase 4 (PRMT4, also known as CARM1), which methylates ASXL2 on R639/R641. Methylation of ASXL2 by CARM1 blocks binding to MLL3 binding and impairs the expression of MLL3 dependent genes. This finding uncovers a novel transcription repression function for CARM1 and provides a unique insight into the BAP1/ASXL2/MLL3 axis, which is controlled by arginine methylation and could serve as a target for potential cancer therapeutics. We evaluted how ASXL2 mediates the interaction between BAP1 and MLL3/COMPASS, and how methylation of ASXL2 by CARM1 blocks binding to MLL3 binding and impairs the expression of MLL3 dependent genes.