How hexanucleotide GGGGCC (G 4 C 2 ) repeat expansions in C9orf72 cause frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) is not understood. We developed a mouse model engineered to express poly(PR), a proline-arginine (PR) dipeptide repeat protein synthesized from expanded G 4 C 2 repeats. The expression of green fluorescent protein–conjugated (PR) 50 (a 50-repeat PR protein) throughout the mouse brain yielded progressive brain atrophy, neuron loss, loss of poly(PR)-positive cells, and gliosis, culminating in motor and memory impairments. We found that poly(PR) bound DNA, localized to heterochromatin, and caused heterochromatin protein 1a (HP1a) liquid-phase disruptions, decreases in HP1a expression, abnormal histone methylation, and nuclear lamina invaginations. These aberrations of histone methylation, lamins, and HP1a, which regulate heterochromatin structure and gene expression, were accompanied by repetitive element expression and double-stranded RNA accumulation. Thus, we uncovered mechanisms by which poly(PR) may contribute to the pathogenesis of C9orf72-associated FTD and ALS.
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