Type I PRMT Inhibition Protects Against C9ORF72 Arginine-Rich Dipeptide Repeat Toxicity

Abstract

Repeat expansion mutations in the C9ORF72 gene are the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Repeatassociated non-AUG translation of this expansion produces dipeptide repeat proteins (DRPs). The arginine containing DRPs, polyGR and polyPR, are consistently reported to be the most toxic. Here we demonstrated that small molecule inhibition of type I protein arginine methyltransferases (PRMT) protects against polyGR and polyPR toxicity. Furthermore, our findings suggest that asymmetric dimethylation of polyGR and polyPR by Type I PRMTs plays important roles in their cytotoxicity.

Hypothesis and Theory: Roles of Arginine Methylation in C9orf72-Mediated ALS and FTD

“…This body of research, illustrating the interaction of PRMTs with key spliceosome components, in conjunction with that illustrating pathological interaction of polyGR and polyPR DRPs ( Figure 5B) with these same spliceosome components to decrease splicing fidelity, raises important questions about the interaction of these two phenomena in C9-ALS. As previously mentioned, in our study, the application of small molecule type I PRMT inhibitors were capable of dose-dependently abrogating cytotoxicity associated with 24 h polyGR or polyPR challenge in NSC-34 cells (Premasiri et al, 2020). One possible explanation for the observed rescue effect could be that inhibiting type I PRMTs decreases asymmetric dimethylation of arginine residues on RBPs and other splicing proteins, in a way that is beneficial to the cell in the presence of polyGR and polyPR.…”

“…Regarding non-LLPS protein:protein interactions, we suspect asymmetrically dimethylated polyGR and polyPR could have enhanced interactions with other proteins, such as Tudor domain-containing molecules ( Figure 2E ). Our results demonstrated a complete abrogation of polyGR toxicity with Type I PRMT inhibitors, and near-complete abrogation of polyPR toxicity (Premasiri et al, 2020 ). This small difference may be attributable to additional conformational flexibility conferred on methylated polyGR, leading to a different cellular interactome.…”

“…Of particular relevance is a recent study demonstrating that either ADMe or SDMe of GR20 reduced its ability to phase separate (Gittings et al, 2020). Given our findings demonstrating abrogated toxicity of polyGR and polyGR when Type I PRMTs are inhibited (Premasiri et al, 2020), it is plausible that reduced ADMe of polyGR and polyPR allows for a greater degree of phase separation. However, when these DRPs are asymmetrically dimethylated by Type I PRMTs, their propensity to phase separate decreases, and the scope of possible aberrant interactions increases ( Figure 2D).…”

“…that small molecule type I PRMT inhibitors dose-dependently abrogate cytotoxicity associated with 24 h polyGR or polyPR challenge in mouse spinal cord neuroblastoma hybrid cells (NSC-34) cells (Premasiri et al, 2020). The rescue effect we observe may be because type I PRMT inhibition alters the arginine methylation associated with stress granule formation and that the resulting changes in stress granule assembly, composition, and dynamics are beneficial to the cell when in the presence of polyGR and polyPR-induced stress (Figure 3D).…”

“…We recently reported that the toxicity caused by exposure to polyGR/PR can be completely abrogated using inhibitors of Type I protein arginine methyltransferases (PRMTs; Premasiri et al, 2020 ). PRMTs are a family of enzymes responsible for the methylation of arginine residues in proteins.…”