PRMT5 Regulates DNA Repair By Controlling the Alternative Splicing of Epigenetic Factors

PRMT5 is the major type II arginine methyltransferase that symmetrically dimethylates histone and non-histone substrates, including several components of the RNA splicing machinery. PRMT5 is overexpressed in many cancer types and PRMT5 inhibition has recently been shown to be a promising therapeutic approach for these diseases, including lymphoma and leukemia. PRMT5 plays an important role in adult mouse hematopoiesis, as its conditional deletion in the hematopoietic compartment (using an Mx1-Cre based system) leads to rapid, lethal, bone marrow aplasia (F. Liu et al, JCI 2015). To avoid excess toxicity, we have been investigating the myriad of cellular processes regulated by PRMT5, in order to identify combinatorial approaches, that can exploit potential synthetic vulnerabilities, similar to what has been shown for loss of MTAP and PRMT5 inhibition. Here we report that PRMT5 deletion or inhibition leads to the accumulation of DNA damage, at least in part due to defects in the homologous recombination (HR) DNA repair pathway, which ultimately activates the p53 pathway and leads to cell cycle arrest and cell death. Using a Vav1-Cre based PRMT5 knockout mouse model that we generated, we have found that PRMT5 knockout induces specific dysregulation of RNA splicing, including alternative splicing of the multifunctional epigenetic and DNA repair factor Tip60/KAT5. Aberrant splicing of Tip60 leads to selectively defective acetyl-transferase activity, including histone H4 acetylation, a key event in DNA repair pathway choice. Consistently, we found that ectopic expression of wild-type Tip60 in PRMT5-null cells rescues the homologous recombination phenotype. Thus, we have uncovered a novel interaction between PRMT5-dependent RNA splicing and DNA repair, that functions to regulate genomic stability and cellular homeostasis. Importantly, these findings can be exploited therapeutically as PARP inhibitors have been shown to be effective in HR defective cells, such as BRCA1 or BRCA2 mutant cells. We show here that PRMT5 inhibitors and PARP inhibitors have synergistic effects on leukemia cell lines and mouse models, thus demonstrating the advantages of combining targeted epigenetic and non-epigenetic inhibitors.