Despite intense research efforts, our pharmaceutical repertoire against high-grade brain tumours has not been able to increase patient survival for a decade and life expectancy remains at less than 16 months after diagnosis, on average. Inhibitors of protein arginine methyltransferases (PRMTs) have been developed and investigated over the past 15 years and have now entered oncology clinical trials, including for brain tumours. This review collates recent advances in the understanding of the role of PRMTs and arginine methylation in brain tumours. We provide an up-to-date literature review on the mechanisms for PRMT regulation. These include endogenous modulators such as alternative splicing, miRNA, post-translational modifications and PRMT–protein interactions, and synthetic inhibitors. We discuss the relevance of PRMTs in brain tumours with a particular focus on PRMT1, -2, -5 and -8. Finally, we include a future perspective where we discuss possible routes for further research on arginine methylation and on the use of PRMT inhibitors in the context of brain tumours.

Glioblastomas (GBM) are the most common grade 4 brain tumours; patients have very poor prognosis with an average survival of 15 months after diagnosis. Novel research lines have begun to explore aberrant protein arginine methylation (ArgMe) as a possible therapeutic target in GBM and ArgMe inhibitors are currently in clinical trials. Enzymes known as protein arginine methyltransferases (PRMT1-9) can lead to mono- or di-ArgMe, and in the latter case symmetric or asymmetric dimethylation (SDMA and ADMA, respectively). Using the most common GBM cell line, we have profiled the expression of PRMTs, used ArgMe inhibitors as tools to investigate post-translational modifications cross-talk and measured the effect of ArgMe inhibitors on cell viability. We have identified novel SDMA events upon inhibition of ADMA in GBM cells and spheroids. We have observed cross-talk between ADMA and lysine acetylation in GBM cells and platelets. Treatment of GBM cells with furamidine, a PRMT1 inhibitor, reduces cell viability in 2D and 3D models. These data provide new molecular understanding of a disease with unmet clinical needs.

BackgroundThe inhibitory subunit of cardiac troponin (cTnI) is a gold standard cardiac biomarker and also an essential protein in cardiomyocyte excitation-contraction coupling. The interactions of cTnI with other proteins are fine-tuned by post-translational modification of cTnI. Mutations in cTnI can lead to hypertrophic cardiomyopathy. Methods and ResultsHere we report, for the first time, that cTnI is modified by arginine methylation in human myocardium. Using Western blot, we observed reduced levels of cTnI arginine methylation in human hypertrophic cardiomyopathy compared to dilated cardiomyopathy biopsies. Similarly, using a rat model of cardiac hypertrophy we observed reduced levels of cTnI arginine methylation compared to sham controls. Using mass spectrometry, we identified cTnI methylation sites at R74/R79 and R146/R148 in human cardiac samples. R146 and R148 lie at the boundary between the critical cTnI inhibitory and switch peptides; PRMT1 methylated an extended inhibitory peptide at R146 and R148 in vitro. Mutations at R145 that have been associated with hypertrophic cardiomyopathy hampered R146/R148 methylation by PRMT1 in vitro. H9c2 cardiac-like cells transfected with plasmids encoding for a methylation-deficient R146A/R148A cTnI protein developed cell hypertrophy, with a 32% increase in cell size after 72 h, compared to control cells. DiscussionOur results provide evidence for a novel and significant cTnI post-translational modification. Our work opens the door to translational investigations of cTnI arginine methylation as a biomarker of disease, which can include e.g. cardiomyopathies, myocardial infarction and heart failure, and offers a novel way to investigate the effect of cTnI mutations in the inhibitory/switch peptides. Abbreviations: Arginine methylation (ArgMe), collision induced dissociation (CID), dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), matrix-assisted laser desorption ionisation -time of flight (MALDI-TOF), protein arginine methyltransferases (PRMTs), Sadenosyl-L-methionine (SAM).

and is available as an interactive resource at: www.cassandra-genes.org. A network cytoscape session with the vertebrate network as well as the ClueGo results is available at doi:10.6084/m9.figshare.15010872. Code is provided in the Supplementary Materials as Supplementary Appendix 1.

AIMS Assess/evaluate apoptosis in GBM samples maintained on a microfluidics system in response to GSK3368715 and other PRMT inhibitors, currently in clinical trials, with the ultimate goal of synergising with personalised patient care and precision medicine. Investigate the effect of treating GBM biopsies on-chip with PRMT inhibitors at the molecular level, including RNA and protein modifications. METHOD GBM biopsies are received from Hull Royal Infirmary and maintained on-chip for 8-days. They are perfused with media, at a rate of 3 μl/min, mimicking the in vivo environment and allowing real-time analysis of tumour behaviour. PRMT inhibitors, such as GSK3368715, are added to the media, in conjunction with TMZ, to determine their efficacy ex vivo using a range of techniques, such as: immunohistochemistry, cell viability assays, protein analysis and RNA-sequencing. RESULTS We show that PRMT inhibition increases apoptosis five-fold above the control, untreated GBM-on-chip samples. This is compounded by cell viability assays, which have indicated that cell viability in these post-chip tissues is reduced by 30% upon treatment with 1μM GSK3368715. Additionally, western blot analysis has indicated that PRMT inhibition with GSK3368715 appears to switch the methylation status of fused-in-sarcoma (FUS) protein in GBM biopsies. CONCLUSION These results indicate that PRMT inhibition may not only be a viable target for GBM therapy, but could also highlight a mechanism for re-sensitising MGMT-negative GBM to TMZ. This data produces an exciting argument for further research into the use of this novel inhibitor for improving prognosis for patients diagnosed with this devastating disease.