Spliceosomal disruption of the non-canonical BAF complex in cancer

SF3B1 is the most commonly mutated RNA splicing factor in cancer, but the mechanisms by which SF3B1 mutations promote malignancy are poorly understood. Here, we integrated pan-cancer RNA sequencing to identify mutant SF3B1-dependent aberrant splicing with a positive enrichment CRISPR screen to prioritize splicing alterations that functionally promote tumorigenesis. We identify that diverse, recurrent SF3B1 mutations converge on repression of BRD9, a core component of the recently described non-canonical BAF (ncBAF) complex. Mutant SF3B1 recognizes an aberrant deep intronic branchpoint within BRD9, thereby inducing inclusion of an endogenous retrovirus-derived poison exon and BRD9 mRNA degradation. BRD9 depletion causes loss of ncBAF at CTCF-bound loci and promotes melanomagenesis. We demonstrate that BRD9 is a potent tumor suppressor in uveal melanoma, such that correcting BRD9 mis-splicing in SF3B1-mutant cell lines and patient-derived melanoma xenografts with antisense oligonucleotides (ASOs) or by directly targeting its poison exon with CRISPR-directed mutagenesis profoundly suppresses tumor growth. Our results implicate disruption of ncBAF in the diverse malignancies characterized by SF3B1 mutations, identify a single aberrant splicing event which functionally contributes to the pathogenesis of SF3B1-mutant cancers, and suggest a mechanism-based therapeutic for these malignancies. Gene expression and chromatin association by components of the non-canonical BAF complex in different cancer cell lines (K562, MEL270) under genetic and chemical perturbations