Fibroblasts carrying intermediate C9orf72 hexanucleotide repeat expansions from iNPH patients show changes in energy metabolism but no cell pathologies.

Long C9orf72 hexanucleotide repeat expansions (C9-HRE) are the most common genetic cause of frontotemporal dementia (FTD), a group of neurodegenerative syndromes leading to cognitive dysfunction and frontal and temporal atrophy. FTD is a potential comorbidity of idiopathic normal pressure hydrocephalus (iNPH) and carrying the C9-HRE can modify the age-of-onset in iNPH patients. While intermediate-length C9-HRE (<30 repeats) are often considered non-pathogenic, the exact pathological cutoff is unclear. In this study, we assessed whether skin fibroblasts from iNPH patients carrying intermediate C9-HRE display C9-HRE-associated pathological hallmarks and changes in cellular function. C9-HRE-associated RNA foci, present in the long (>60 repeats) C9-HRE carrier fibroblasts, were not detected in those of the intermediate carriers. The number of p62-positive puncta was significantly increased in long but not intermediate C9-HRE carrier fibroblasts, in line with p62-positive intracellular inclusions observed in a brain biopsy from the patient. Induction of autophagy did not suggest any defects in the intermediate carrier fibroblasts. Fibroblasts from the intermediate C9-HRE carriers showed upregulated glycolytic activity, possibly to counteract the slightly reduced mitochondrial respiration. This could not be observed in the long C9-HRE carrier fibroblasts. In conclusion, these data suggest that while the long C9-HRE leads to more severe cellular pathologies than intermediate C9-HRE, the latter might predispose cells to deficits in specific cellular functions, such as energy metabolism.