MLL3 is a de novo cause of endocrine therapy resistance

Background Cancer resequencing studies have revealed epigenetic enzymes as common targets for recurrent mutations. The monomethyltransferase MLL3 is among the most recurrently mutated enzymes in ER+ breast cancer. The H3K4me1 marks created by MLL3 can define enhancers. In ER+ breast cancer, ER alpha genome-binding sites are primarily distal enhancers. Thus, we hypothesize that mutation of MLL3 will alter the genomic binding and transcriptional regulatory activity of ER alpha. Methods We investigated the genomic consequences of knocking down MLL3 in an MLL3/PIK3CA WT ER+ breast cancer cell line. Results Loss of MLL3 led to a large loss of H3K4me1 across the genome, and a shift in genomic location of ER alpha-binding sites, which was accompanied by a re-organization of the breast cancer transcriptome. Gene set enrichment analyses of ER alpha-binding sites in MLL3 KD identified endocrine therapy resistance terms, and we showed that MLL3 KD cells are resistant to tamoxifen and fulvestrant. Many differentially expressed genes are controlled by the small collection of new locations of H3K4me1 deposition and ER alpha binding, suggesting that loss of functional MLL3 leads to new transcriptional regulation of essential genes. Motif analysis of RNA-seq and ChIP-seq data highlighted SP1 as a critical transcription factor in the MLL3 KD cells. Differentially expressed genes that display a loss of ER alpha binding upon MLL3 KD also harbor increased SP1 binding. Conclusions Our data show that a decrease in functional MLL3 leads to endocrine therapy resistance. This highlights the importance of genotyping patient tumor samples for MLL3 mutation upon initial resection, prior to deciding upon treatment plans.