Impaired self-other differentiation in frontotemporal dementia due to the C9ORF72 expansion

Introduction: An expanded hexanucleotide repeat in the C9ORF72 gene has recently been identified as an important cause of frontotemporal dementia and motor neuron disease; however, the phenotypic spectrum of this entity and its pathophysiologic basis have yet to be fully defined. Psychiatric features may be early and prominent, although a putative cortico-thalamo-cerebellar network has been implicated in the pathogenesis of the clinical phenotype. Differentiation of self from others is a core cognitive operation that could potentially link network disintegration with neuropsychiatric symptoms in C9ORF72-associated frontotemporal dementia. Methods: We undertook a detailed behavioral analysis of self-other attribution in a 67-year-old male patient with behavioral variant frontotemporal dementia (bvFTD) due to the C9ORF72 expansion by using a novel paradigm requiring differentiation of the effects of self-and non-self-generated actions. The patient's performance was assessed in relation to two older male patients with bvFTD not attributable to the C9ORF72 expansion and four healthy older male subjects. Results: Compared with the healthy control group, the patient with the C9OFR72 mutation showed a deficit of self-other differentiation that was disproportionate to his otherwise relatively indolent clinical phenotype. The performance of the other patients with bvFTD was similar to that of healthy subjects. Conclusion: We propose that impaired self-other differentiation is a candidate mechanism for neuropsychiatric decline in association with the C9ORF72 expansion. We offer this preliminary observation as a stimulus to further work.