NEK1 haploinsufficiency worsens DNA damage but not defective ciliogenesis in C9ORF72 patient-derived iPSC-motoneurons

A hexanucleotide G ₄ C ₂ repeat expansion (HRE) in C9ORF72 gene is the major cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), leading to both loss- and gain-of-function pathomechanisms. The wide clinical heterogeneity among C9ORF72 patients suggests potential modifying genetic factors. Notably, C9ORF72 mutations often co-occur with other variants in ALS/FTD-associated genes, such as NEK1 , which encodes for a kinase involved in multiple pathways including DNA damage response and ciliogenesis. In this study, we generated induced pluripotent stem cells (iPSCs) and differentiated motoneurons (iPSC-MNs) derived from an ALS patient carrying both C9ORF72 HRE and a NEK1 loss-of-function mutation to study the effect of the NEK1 haploinsufficiency on C9ORF72 pathology. Double mutant C9ORF72 / NEK1 cells showed increased pathological C9ORF72 RNA foci in iPSCs and higher DNA damage levels in iPSC-MNs compared to single mutant C9ORF72 cells. In contrast, ciliogenesis was similarly impaired in both C9ORF72 and C9ORF72/NEK1 iPSC-MNs showing shorter cilia. Altogether, our study supports the use of patient-derived iPSCs to functionally explore the contribution of genetic modifiers in C9ORF72 -associated pathology.