Single cell RNA sequencing in isogenic FUS and TARDBP mutant ALS lines reveals early mitochondrial dysfunction as a common pathway in motor neurons

Mutations in the RNA/DNA-binding proteins FUS and TDP-43 cause amyotrophic lateral sclerosis (ALS) with distinct neuropathological features. It is currently unclear how these gene mutations lead to selective motor neuron death and if there are common mechanisms across disease causations. Using single cell RNA sequencing of neurons derived from isogenic induced pluripotent stem cell lines, we show that motor neurons harbouring FUS P525L or FUS R495X mutations show a 4.9-to 15.5-fold larger transcriptional response than interneurons. About 20% (737 DEGs) of transcripts were coregulated across FUS R495X and P525L motor neurons and by comparing to a FUS knockout line we could discern that 48% (355 DEGs) were part of gain-of-function of FUS. Cross-comparing with isogenic TDP-43 M337V motor neurons, identified common mitochondrial dysfunction across FUS gain-of-function and TARDBP gene mutations, as did comparison with published RNA-Seq data from C9orf72-ALS motor neurons. Metabolic assessment confirmed a decrease in mitochondrial respiration and ATP turnover in mutant FUS and TARDBP lines and live cell microscopy showed a decrease in mitochondrial motility across ALS motor axons. Thus, we have identified early mitochondrial dysfunction in motor neurons shared across ALS-causative mutations, that could have major implications for their survival and which could be targeted therapeutically. ### Competing Interest Statement The authors have declared no competing interest.