Amyotrophic Lateral Sclerosis associated FUS mutation shortens mitochondria and induces neurotoxicity

Amyotrophic Lateral Sclerosis (ALS) is a lethal neurodegenerative disorder that primarily affects motor neurons. Dominant mutations in the RNA binding protein Fused in Sarcoma (FUS) have been identified as causative factors of ALS. Mutation, R495X, results in a premature stop codon and induces an aggressive disease phenotype by a largely unknown process. Here, we employ CLIP-Seq, RNA-Seq and Ribo-Seq in cultured neurons expressing R495X or wild-type FUS to identify the mutation effects on the FUS targetome and on the neuronal transcriptome at the expression and translation level, simultaneously. We report that, unlike wild-type FUS that binds on precursor mRNAs (pre-mRNAs), R495X binds mature mRNAs in the cytoplasm. R495X has a moderate effect on target mRNA expression and its binding induces only modest expression changes. In contrast, we find that R495X controls the translation of genes that are associated with mitochondria function and results in significant reduction of mitochondrial size. Importantly, we show that introduction of the 4FL mutation that alters binding of R495X to RNA, partially abrogates R495X-induced effects on mRNA translation, mitochondrial size and neurotoxicity. Our findings uncover a novel RNA-mediated pathway of FUS R495X-induced neurotoxicity that affects mitochondria morphology and provide insight to previous studies associating mitochondria dysfunction to ALS.