The H3K4-specific Methyltransferase MLL3 Regulates Cell Proliferation and Epithelial-to-Mesenchymal Transition in Pancreatic Cancer

Pancreatic cancer is the 4th leading cause of cancer-related death in Canada. A major reason for the disease’s extremely high overall mortality-to-incidence ratio (0.98) is that most (60%) pancreatic cancer patients present with aggressive metastatic disease at the time of diagnosis, yet mechanistic understanding of tumourigenesis process is incomplete. Intrigued by the relatively high frequency of MLL3 somatic cancer mutations in pancreatic ductal adenocarcinoma, I undertook a series of experiments to understand the molecular and phenotypic impact of MLL3 knockdown in pancreatic cancer cell lines. My results showed that MLL3 regulates cell proliferation in a bi-directional fashion. I also demonstrated that MLL3 knockdown resulted in precocious epithelial-to-mesenchymal transition in cell culture. Analysis of gene expression profiles indicates that MLL3 knockdown likely acts through up-regulation of TGF-2 and BMP4 signalling pathways, which converge on transcription factors ZEB1 and SNAI2, two master regulators of EMT, associatced with altered morphology and migratory capability. To examine the pathophysiological relevance of my results, I exploited the availability of genomic and transcriptomic sequencing datasets from the International Cancer Genome Consortium. I undertook a systematic survey of genetic alterations in MLL3-associated interacting protein components (ASCOM complex) and further compared the transcriptome patterns of tumour samples with mutant and wild-type ASCOM genes in various cancer types. Strikingly, I found that while genetic alterations were generally infrequent in ASCOM genes, with 14% of 2583 cancer samples carrying non-synonymous mutations and 7.6% bearing nonsense or frameshift mutations, the distribution of function-damaging mutations was highly skewed in terms of both cancer type (ranging from 60% to 0%) and ASCOM genes (over 90% found in MLL3, MLL4 and KDM6A). Notably, immunoglobulin production and complement activation pathways were also down-regulated in pancreatic cancer samples carrying function-damaging mutations in ASCOM genes, implicating TGF- signalling pathway. Taken together, my results establish an unexpected role for MLL3 in the malignant transformation within the pancreatic tumour microenvironment that facilitates tumour progression and metastasis. Given that genetic alterations in MLL3 and other ASCOM genes are frequently found in several other cancers, my results have implications in other tumor contexts, opening up promising new mechanistic avenues for cancer research.Ph.D