Neural substrates of episodic memory dysfunction in behavioural variant frontotemporal dementia with and without C9ORF72 expansions

The recently discovered hexanucleotide repeat expansion, C9ORF72, has been shown to be among the most common cause of familial behavioural variant frontotemporal dementia (bvFTD) and to be present in a significant minority of apparently sporadic cases. While mounting evidence points to prominent episodic memory dysfunction in bvFTD cases, recent reports have also suggested an amnestic profile in C9ORF72 mutation carriers. No study to date, however, has formally characterised the extent to which episodic memory is impaired in C9ORF72 mutation versus sporadic cases, or the underlying neural substrates of such deficits. We conducted a comparison of C9ORF72 (n = 8) and sporadic (n = 15) bvFTD cases using a battery of verbal and visual episodic memory tasks, and contrasted their performance with that of Alzheimer's disease (AD, n = 15) and healthy older control (n = 15) participants. Behaviourally, the two bvFTD groups displayed comparable episodic memory profiles, irrespective of task administered, with prominent impairments evident relative to Controls. Whole-brain voxel-based morphometry analyses revealed distinct neural correlates of episodic memory dysfunction in each patient group. Widespread atrophy in medial prefrontal, medial and lateral temporal cortices correlated robustly with episodic memory dysfunction in sporadic bvFTD cases. In contrast, atrophy in a distributed set of regions in the frontal, temporal, and parietal lobes including the posterior cingulate cortex, was implicated in episodic memory dysfunction in C9ORF72 cases. Our results demonstrate that while episodic memory is disrupted to the same extent irrespective of genetic predisposition in bvFTD, distinct neural changes specific to each patient group are evident. The involvement of medial and lateral parietal regions in episodic memory dysfunction in C9ORF72 cases is of particular significance and represents an avenue of considerable interest for future studies.