Somatic mutations of MLL4/COMPASS induce cytoplasmic localization providing insight into cancer prognosis and treatment

Cancer genome sequencing consortiums have recently catalogued an abundance of somatic mutations in the chromatin-modifying enzymes that regulate enhancer chromatin function across a wide range of human cancers. Defining the molecular mechanisms underlying the potential oncogenic function for these epigenetic mutations could serve as the basis for precision medicine approaches for cancer treatment. MLL4 (KMT2D) is a key histone lysine mono-methyltransferase within the COMplex of Proteins ASsociated with Set1 (COMPASS) family that regulates chromatin at enhancers, potentially functions as a tumor suppressor and is highly mutated in a large number of human cancers. We report that the mutations which cause MLL4 protein truncation also alter its subcellular localization, resulting in loss-of-function in the nucleus and gain-of-function in the cytoplasm. We demonstrate that isogenic correction of MLL4 truncation mutation rescues the aberrant localization phenotype and restores multiple MLL4 regulatory functions, including COMPASS integrity and stabilization, histone H3K4 mono-methylation, enhancer activation, and gene regulation. Moreover, isogenic correction diminishes the sensitivity of MLL4-mutated cancer cells to targeted metabolic inhibition. Using immunohistochemistry (IHC), we identified cytoplasmic MLL4 unique to the tissue of bladder cancer patients with MLL4 mutations. Using a preclinical carcinogen bladder cancer mouse model, we demonstrate that truncated, cytoplasmic MLL4 can be a biomarker for MLL4-mutated bladder cancers and predicts response to targeted metabolic inhibition therapy. We propose using the cytoplasmic MLL4 truncation phenotype as a prognostic marker in bladder cancer. We also highlight the broader potential for patient stratification based on MLL4 mutation status in MLL4 truncation diseases, including human cancers and Kabuki Syndrome. We generated multiple isogenic HCT116 cell lines with allelic knockout, complete knockout, or knockin mutation correction. Then, we conducted RNA-seq, ChIP-seq and PRO-seq experiments. To study the gene expression regulation, we performed PC analysis in different clones versus parental cell line, and determined the specific MLL4 effects. To understand how mutation correction impacts enhancer function, we integrate ChIP-seq, PRO-seq with RNA-seq to determine the enhancer functions directed regulated by MLL4/COMPASS.