Mutant SF3B1 promotes AKT and NF-kB driven mammary tumorigenesis

Mutations in the core RNA splicing factor SF3B1 are prevalent in leukemias and uveal melanoma, but also recurrent in epithelial malignancies such as breast cancer. Whereas hotspot mutations in SF3B1 alter hematopoietic differentiation, whether SF3B1 mutations contribute to epithelial cancer development and progression is unknown. Here, we identify that SF3B1 mutations in mammary epithelial and breast cancer cells induce a recurrent pattern of aberrant splicing leading to activation of AKT and NF-kB, enhanced cell migration, and accelerated tumorigenesis. Transcriptomic analysis of human cancer specimens, MMTV-cre Sf3b1K700E/WT mice, and isogenic mutant cell lines identified hundreds of aberrant 3’ splice sites induced by mutant SF3B1, a portion of which were breast-specific. Across mouse and human tumors, mutant SF3B1 promoted aberrant splicing (dependent on aberrant branchpoints as well as pyrimidines downstream of the aberrant branchpoint) and consequent suppression of PPP2R5A and MAP3K7, critical negative regulators of AKT and NF-kB. Coordinate activation of NF-kB and AKT signaling was observed in the knock-in models, leading to accelerated cell migration and tumor development in combination with mutant PIK3CA but also hypersensitizing cells to AKT kinase inhibitors. These data identify mutations in SF3B1 as drivers of breast tumorigenesis and reveal unique vulnerabilities in cancers harboring them. 23 samples, including 11 mouse and 12 human samples with varying SF3B1 status