BACKGROUND: Mutations in C9ORF72 are an important cause of frontotemporal 
dementia (FTD) and motor neuron disease. Accumulating evidence suggests that FTD 
associated with C9ORF72 mutations (C9ORF72-FTD) is distinguished clinically by 
early prominent neuropsychiatric features that might collectively reflect 
deranged body schema processing. However, the pathophysiology of C9ORF72-FTD has 
not been elucidated.
METHODS: We undertook a detailed neurophysiological investigation of five 
patients with C9ORF72-FTD, in relation to patients with FTD occurring 
sporadically and on the basis of mutations in the microtubule-associated protein 
tau gene and healthy older individuals. We designed or adapted behavioural tasks 
systematically to assess aspects of somatosensory body schema processing 
(tactile discrimination, proprioceptive and body part illusions and 
self/non-self differentiation).
RESULTS: Patients with C9ORF72-FTD selectively exhibited deficits at these 
levels of body schema processing in relation to healthy individuals and other 
patients with FTD.
CONCLUSIONS: Altered body schema processing is a novel, generic 
pathophysiological mechanism that may link the distributed cortico-subcortical 
network previously implicated in C9ORF72-FTD with a wide range of 
neuropsychiatric and behavioural symptoms, and constitute a physiological marker 
of this neurodegenerative proteinopathy.