The PRMT5-splicing axis is a critical oncogenic vulnerability that regulates detained intron splicing
Protein arginine methyltransferase 5 (PRMT5) is a promising cancer target, yet it is unclear which PRMT5 roles underlie this vulnerability. Here, we establish that PRMT5 inhibition induces a special class of unspliced introns, called detained introns (DIs). We used depletion of  CLNS1A, a PRMT5 cofactor that specifically enables Sm protein methylation to interrogate the impact of DIs. We found that disruption of Sm protein methylation is sufficient to induce DI upregulation, cell cycle defects, and loss of viability. Finally, we discovered that PRMT5-regulated DIs, and the impacted genes, are highly conserved across human, and also mouse, cell lines but display little interspecies conservation. Despite this, human and mouse DIs have convergent impacts on proliferation by affecting essential components of proliferation-regulating complexes. Together, these data argue that the PRMT5-splicing axis, and particularly appropriate DI splicing, underlies cancer's vulnerability to PRMT5 inhibitors. Overall design: RNA-seq profiling of established human or mouse cell lines, either WT or dTag-CLNS1A knock-in,  after treatment with PRMT5 inhibitors, dTag-13, or vehicle control.