Arginine Methylation Mediated By PRMT5 Has Significant Biological and Clinical Impact in Multiple Myeloma

Arginine-specific methyltransferases critically regulate cellular homeostasis by dictating the biological outcome of target proteins. Among them, Protein Arginine Methyltransferase 5 (PRMT5) has attracted growing interest due to its role as an enzyme mediating epigenetic regulation of anti-cancer target genes, as well as in methylation of non-histone proteins involved in growth-regulating and survival pathways including p53. However, little is known about its biologic function in multiple myeloma (MM).

To first evaluate the clinical significance of PRMT5 in MM pathogenesis, we analyzed RNA-seq data from newly-diagnosed MM patients and identified highly upregulated PRMT5 in 320 patients' CD138+ cells compared to 16 samples of normal bone marrow (BM) plasma cells. Additional analysis of PRMT5 expression in two independent datasets also showed further PRMT5 mRNA upregulation during progression of MM. Immunohistochemical staining also confirmed elevated expression of PRMT5 in BM biopsies from MM patients as compared to healthy individuals and monoclonal gammopathy of undetermined significance (MGUS). Moreover, analysis of the prognostic significance of PRMT5 expression in MM patients enrolled on IFM/DFCI 2009 clinical study showed that high PRMT5 expression was associated with poor prognosis in terms of both event free (p= 0.016) and overall survival (p=0.018). Consistently, we also found upregulated PRMT5 expression at both mRNA and protein levels in MM cell lines (N=11) and patients CD138+ MM cells (N=3) as compared to PBMCs from healthy volunteers, associated with a parallel increase of cellular symmetric arginine di-methylation (SDMA) substrates. Interestingly, genetic depletion of PRMT5 in H929 (p53wt) and KMS11 (p53null) MM cell lines by shRNA decreased SDMA levels, associated with cell growth inhibition in a p53-independent manner. Likewise, pharmacological inhibition of PRMT5 with the small molecule inhibitor EPZ015666 triggered decreased SDMA levels, cell growth, survival, and clonogenicity, as well as induction of caspase-dependent apoptosis in MM cell lines. Moreover, although PRMT5 and SDMA levels were increased in MM cells cultured in the presence of BM stromal cell supernatant, cytotoxic activity of EPZ015666 was maintained. Notably, drug treatment significantly impaired cell proliferation of patient MM cells (n=2) even in the presence of BM mononuclear or stromal cells, without toxicity on normal PBMCs. At the level of gene expression modulation, PRMT5 inhibition was associated with downregulation of NF-kB-dependent transcription, evidenced by both gene set enrichment analysis (GSEA) and Ingenuity Upstream Regulator Analysis. Moreover, analysis of protein levels confirmed reduction of both canonical and non-canonical NF-kB pathways, evidenced by significantly decreased NF-kB DNA binding activity by ELISA. Importantly, Mass Spectrometry analysis identified TRIM21 as a new PRMT5 interactor; and EPZ015666-treated cells showed that PRMT5 methylates TRIM21 evidenced by WB analysis. Since TRIM21 mediates monoubiquitination of IKKbeta, thereby triggering its selective autophagy-mediated degradation, we next analyzed EPZ015666 effects on IKKbeta. Treatment increased both monoubiquitination of IKKbeta and the formation of IKKbeta-TRIM21-pBECLIN1-pULK1 autophagic complexes. Conversely, inhibition of autophagosome formation by 3-methyladenine abrogated the anti-MM activity of EPZ015666 and IKKbeta degradation, indicating that selective autophagic degradation of IKKbeta and inhibition of NF-kB signaling mediates EPZ015666-triggered anti-MM activity. Consistent with this view, confocal microscopy analysis also confirmed co-localization of IKKbeta in the autophagosome after EPZ015666 treatment. Finally, stable silencing of TRIM21 in MM cell lines significantly abrogated the anti-proliferative effect of EPZ015666. Collectively, these data delineate arginine methylation as a new control mechanism of MM cell growth, and demonstrate that inhibiting PRMT5 decreases tumor cell survival via blockade of NF-kB signaling, even in the context of the BM milieu. These data demonstrate the biologic and prognostic significance of PRMT5 in MM pathogenesis, and provide the rationale for novel therapies targeting PRMT5 to improve patient outcome in MM.