HNRNPK alleviates RNA toxicity by counteracting DNA damage in C9orf72 ALS

A 'GGGGCC' repeat expansion in the first intron of the C9orf72 gene is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The exact mechanism resulting in these neurodegenerative diseases remains elusive, but RNA toxicity has been implicated as a gain-of-function mechanism. Our aim was to use a zebrafish model for C9orf72 RNA toxicity to identify modifiers of the ALS-linked phenotype. We discovered that the RNA-binding protein heterogeneous nuclear ribonucleoprotein K (HNRNPK) can reverse the toxicity of both sense and antisense repeat RNA, which is dependent on its subcellular localization and on RNA recognition, and not on C9 repeat RNA binding. We observed HNRNPK cytoplasmic mislocalization in C9orf72 ALS patient fibroblasts, induced pluripotent stem cell (iPSC)-derived motor neurons and postmortem central cortex, suggesting a disrupted HNRNPK function in C9orf72 ALS. In C9 ALS/FTD patient tissue, we discovered an increased nuclear translocation, but reduced expression of Ribonucleotide Reductase Regulatory Subunit M2 (RRM2), a downstream target of HNRNPK involved in DNA damage response. Finally, we show that increasing the expression of HNRNPK or RRM2 was sufficient to mitigate DNA damage in our C9 RNA toxicity zebrafish model. Overall, our study strengthens the relevance of RNA toxicity as a pathogenic mechanism in C9 ALS and demonstrates its link with aberrant DNA damage response, opening novel therapeutic strategies for C9 ALS/FTD.