Dissecting the contribution of dipeptide repeats to the toxicity in C9orf72 iPSC-derived neurons from ALS/FTD patients

A large (GGGGCC) repeat expansion in C9ORF72 gene is the commonest genetic cause of amyotrophic lateral sclerosis (ALS). It causes both loss- and gain-of-function, but the relative contribution of the different mechanisms to the development of ALS remains uncertain. One of the pathomechanisms is the production of dipeptide repeat proteins (PR, PA, GR, GP and GA) via repeat-associated non-ATG translation (DPRs). Animal and cellular models have suggested that the arginine-rich DPRs are toxic, but staining in fixed brains of C9orf72 patients show that the same DPRs are not very abundant compared to GA, GP and PA. Moreover, the DPR-related phenotypes exhibited in in vitro models are not always verified in C9orf72 patients. For these reasons I developed a doxycycline-inducible lentiviral system to regulate DPR expression in induced pluripotent stem cell (iPSC)-derived motor neuron (MN) cultures. The effects of GA and PR expression in CRISPR/Cas9 corrected C9orf72 iPSC-derived MNs were compared to the phenotypes of C9orf72 mutant lines. Both DPRs were partially accountable for altered ER and mitochondrial calcium homeostasis, and stress granule formation. Moreover, PR expression may reduce maximal mitochondrial respiration, while GA induced lysosomal activation. The activation of chaperone-mediated autophagy cleared CRISPR/Cas9 corrected iPSC-derived MNs from GA and PR, but it did not rescue C9orf72 mutant lines. Moreover, I studied the effects of a peptide designed to reduce RNA foci, and I showed that it may improve mitochondrial respiration in C9orf72 mutant iPSC-derived MNs.