C9orf72 poly-PR helps p53 escape from the ubiquitin-proteasome system and promotes its stability.

C9orf72-derived dipeptide repeats (DPRs) proteins have been regarded as the pathogenic cause of neurodegeneration in amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD). As the most toxic DPRs in C9-ALS/FTD, poly-proline-arginine (poly-PR) is associated with the stability and accumulation of p53, which consequently induces neurodegeneration. However, the exact molecular mechanism via which C9orf72 poly-PR stabilizes p53 remains unclear. In this study, we showed that C9orf72 poly-PR induces not only neuronal damage but also p53 accumulation and p53 downstream gene activation in primary neurons. C9orf72 (PR) 50 also slows down p53 protein turnover without affecting the p53 transcription level and thus promotes its stability in N2a cells. Interestingly, the ubiquitin-proteasome system but not the autophagy function was impaired in (PR) 50 transfected N2a cells, resulting in defective p53 degradation. Moreover, we found that (PR) 50 induces mdm2 mistranslocation from the nucleus to the cytoplasm and competitively binds to p53, reducing mdm2-p53 interactions in the nucleus in two different (PR) 50 transfected cells. Our data strongly indicate that (PR) 50 reduces mdm2-p53 interactions and causes p53 to escape from the ubiquitin-proteasome system, promoting its stability and accumulation. Inhibiting or at least downregulating (PR) 50 binding with p53 may be therapeutically exploited for the treatment of C9-ALS/FTD.