Colorectal cancer hot spot mutations attenuate the ASXL-MLL4 interaction.

Human additional sex combs like (ASXL) proteins are involved in the maintenance of both transcriptional activation and repression through their ability to bind multiple chromatin regulators, including two tumor suppressors: deubiquitinase BAP1 and methyltransferase MLL4 (KMT2D). The ASXL genes are often altered in colorectal cancer (CRC), and ASXL1 is one of the four hub genes related to the pathogenesis of CRC. Here, we show that MLL4 and BAP1 interdependently target specific genomic regions and positively or negatively regulate expression of a subset of genes in the human colon carcinoma HCT116 cells. MLL4 and BAP1 colocalize on a subset of enhancers and promoters in an interdependent manner. Genomic distribution of BAP1 in CRC cells differs from that in ESCs, with substantially more BAP1 binding sites identified on enhancers and promoters in HCT116 cells. MLL4 occupancy on MLL4(+) BAP1(+) genomic regions depends on functional ASXLs that interact with both MLL4 and BAP1, and CRC-relevant mutations attenuate the formation of the MLL4-ASXL complex. Mutational analysis and binding assays identified CRC hot spot mutations in ASXLs. Our findings suggest that alterations in the genomic distribution of the MLL4-ASXL-BAP1 axis and CRC hot spot mutations in ASXLs perturb normal transcriptional programs and may trigger pathogenic events in colon cancer.