PROJECT SUMMARY: Mantle cell lymphoma (MCL) is an incurable B-cell Non-Hodgkin’s lymphoma with a median survival time of 8- 12 years if treated aggressively. The vast majority of MCL patients eventually relapse and those with refractory . Multiple mutations have been characterized in MCL including those resulting in cell cycle dysregulation and inactivation of various DNA damage response proteins. In addition to genetic mutations, epigenetic dysregulation, including aberrant histone and DNA methylation, has been identified in MCL. disease or who relapse on targeted therapies have a particularly poor prognosis with short survival Upregulated protein arginine methyltransferase 5 (PRMT5), an enzyme that results in symmetric dimethylation of histone arginine residues, is associated with silencing tumor suppressor genes and supporting multiple oncogenic drivers including CYCLIND1 and MYC. For this reason, PRMT5 has emerged as an attractive therapeutic target to inhibit lymphoma cell survival and proliferation. Our group in collaboration with Prelude Therapeutics has developed a small molecule PRMT5 inhibitor (PRT-382) that exhibits significant anti-tumor activity in MCL cell lines (low nM range) and primary MCL models (10 mg/kg). While the anti-tumor activity of PRMT5 inhibition is encouraging, we have observed some treated animals to develop drug resistance leading to rapid MCL progression. Multiple MCL cell lines (Maver-1, Mino, UPN1, and Jeko) also show primary resistance to PRMT5 inhibition based on their half maximal inhibitory concentrations (IC50s) for PRT-382. In addition, prolonged culture of MCL lines with drug escalation has produced acquired drug resistance in cell lines that persists even after prolonged culture in the absence of drug. Given the great therapeutic potential of PRMT5 inhibition in the setting of refractory MCL overall, this proposal seeks to evaluate the mechanisms underlying emergence of PRMT5 inhibitor resistant phenotypes. To do this, we will utilize next generation sequencing technologies to characterize the epigenetic and genetic alterations that contribute to these PRMT5 inhibitor resistant phenotypes. A CRISPR loss of function screen will be used to highlight candidate genes that confer resistance and sensitivity to PRMT5 inhibitor therapy in MCL lines and aid in biologic validation studies. Several potential markers of increased sensitivity to PRMT5 inhibitor therapy have and continue to be investigated including MTAP deletion, p53 mutation status, dependency on other dysregulated PRMTs, and mutations in spliceosome genes. We hypothesize that identification of compensatory prosurvival pathways that are amplified with PRMT5 inhibitor resistance will lead to the development of rational strategies to circumvent resistance. Agents that selectively target other key pathways in MCL in combination with PRMT5 inhibitor therapy will be evaluated in vivo using patient derived xenograft and double transgenic immunocompetent mu...