Aberrant CDK4/6-driven cell-cycle reentry drives neuronal loss and defines a therapeutic target in C9orf72 ALS/FTD

The C9orf72 hexanucleotide repeat expansion (G4C2) is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), yet targeted therapies remain unavailable. Here, we show that induced pluripotent stem cell (iPSC)-derived post-mitotic neurons from C9orf72 carriers exhibit age-dependent cell-cycle reentry, increased S-phase entry, and elevated cyclin and CDK expression. Mechanistically, arginine-containing dipeptide repeat proteins (poly-GR and poly-PR) translated from G4C2 repeats drive this aberrant activation through stimulation of the CDK4/6 pathway, whereas poly-GP and C9orf72 loss-of-function show no effect. Importantly, the FDA-approved CDK4/6 inhibitor palbociclib normalizes cell-cycle progression, reduces S-phase entry, decreases motor neuron death, and restores synaptic proteins PSD95 and synapsin-1. Single-nucleus RNA sequencing from C9orf72 patient cortex reveals cell-cycle activation within excitatory neuron subclusters and alterations in DNA repair and cell-cycle regulation pathways, supporting our <i>in vitro</i> findings. These findings establish cell-cycle dysregulation as a central pathogenic mechanism in C9orf72 ALS/FTD and highlight CDK4/6 signaling as a promising therapeutic target.