LKB1 inactivation elicits an NNMT-mediated methyl sink and confers dependence on PRMT5 in lung cancer

The protein arginine methyl transferase 5 (PRMT5) emerges as a therapeutic target in S-methyl-5'-thioadenosine phosphorylase (MTAP)-deleted cancers, where 5'-methylthioadenosine (MTA) accumulation partially inhibits its activity. However, it remains unclear whether other genetic alterations can dictate PRMT5 activity in cancer. Here, we identify liver kinase B1 (LKB1) as an alternative predictor of PRMT5 inhibition in lung cancer independent of MTAP. Mechanistically, LKB1 loss activates salt-inducible kinase 1/2 (SIK1/2)-cAMP response element-binding protein-regulated transcription coactivator 2 (CRTC2) signaling to upregulate nicotinamide N-methyltransferase (NNMT), creating a "methyl sink" that lowers the S-adenosylmethionine/S-adenosylhomocysteine (SAM/SAH) ratio and attenuates PRMT5 activity. NNMT overexpression is sufficient to induce this hypomorphic PRMT5 state and heighten sensitivity to PRMT5 inhibitors. Functionally, PRMT5 inhibition induces senescence in LKB1-deficient cells and confers vulnerability to navitoclax, synergistically blunting tumor growth in vivo. Collectively, we identify PRMT5 as an actionable therapeutic vulnerability in LKB1-deficient lung cancer, and propose LKB1 status/NNMT expression as potential biomarkers for PRMT5 inhibition. These findings may expand the clinical utility of PRMT5-targeted therapies beyond MTAP-deleted cancers.