Background.  Protein arginine methyltransferases (PRMTs) regulate cellular differentiation and proliferation and are critically involved in tumorigenesis and metastasis. PRMT5 and PRMT7 are the main representatives of type II and III PRMTs catalyzing mono- and symmetric dimethylation of arginine residues and are also highly expressed in the heart. Known targets include histones and non-genomic proteins such as RNA-binding proteins. However, their physiological and pathophysiological relevance in the heart is poorly understood.

Results. The analysis of heart tissue of patients with dilated cardiomyopathy revealed a substantially decreased overall symmetric arginine dimethylation pattern along with a significant downregulation of both PRMT5 and PRMT7. In order to address their pathophysiological role in the heart we used mouse models harboring an inducible cardiomyocyte-specific deletion of Prmt5 and Prmt7 as single knockouts (cKO) and a double KO (cDKO) compared to Cre⁺ wildtypes (Ctrl.). Strikingly, induction of a cDKO in adult mice led to progressive systolic dysfunction within 6 months (ejection fraction (EF), Ctrl.: 85.4±1.0%;n=12 vs cDKO: 40.7±5.0%;n=11; p<0.0001) indicating that the loss of PRMT5 and PRMT7 might be causatively linked to cardiac failure. Consistently, a β-adrenergic stress model revealed an Isoprenaline (ISO)-induced contractile dysfunction in cDKO animals within 10 days (EF, Ctrl.+ISO: 68.9±2.4%;n=15 vs cDKO+ISO: 39.3±2.7%;n=13; p<0.0001) along with massive subendocardial fibrosis. RNA-seq demonstrated a prominent early downregulation of genes whose products are involved in fatty acid metabolism. One of the most altered genes encodes for an acyl-CoA dehydrogenase (ACADVL), the rate-limiting enzyme of the very-long chain fatty acid oxidation pathway in mitochondria, which is of special interest as mutations of ACADVL are associated with severe human cardiomyopathy. To evaluate the pathophysiological role of ACADVL a preventive gene therapy was applied maintaining physiological Acadvl expression levels in cDKO hearts. Interestingly, cDKO mice which received an Acadvl-AAV9 were protected against the ISO-induced dysfunction unmasking ACADVL as a PRMT5/7-regulated, phenotype-critical mediator (EF, cDKO+ISO+Ctrl-AAV: 36.8±5.8%;n=4 vs cDKO+ISO+Acadvl-AAV: 60.2±7.2%;n=6; p=0.003).

Apart from metabolic derangement cDKO mice presented with an early localized pro-inflammatory response leading to excessive fibrotic remodeling in the subendocardium after 10 days of ISO treatment. Spatial transcriptomics revealed a distinct crosstalk between the myocardium expressing cytokines and the subendocardial area enriched for corresponding cytokine receptors indicating a PRMT5/7-regulated potentially cardiocrine-mediated signaling underlying localized fibrosis. 

Conclusion. Conclusively, PRMT5 and PRMT7 are significantly involved in the pathogenesis of cardiac dysfunction. Our data suggest a major role in the regulation of genes related to lipid metabolism in particular Acadvl, which could be unmasked as a critical downstream target, as well as pro-inflammatory pathways upon catecholaminergic stress. Further investigation aims to delineate enzyme-specific mechanistical and functional contributions of PRMT5 and PRMT7 to the phenotype and their eligibility for PRMT-directed therapies.