Expanded C90RF72 Hexanucleotide Repeat in Depressive Pseudodementia

IMPORTANCE Expanded hexanucleotide repeats in C90RF72 are a common genetic cause of frontotemporal dementia and amyotrophic lateral sclerosis. Repeat expansions have also been detected infrequently in other disorders, including Alzheimer disease, dementia with Lewy bodies, and parkinsonian disorders. OBSERVATIONS A consecutive series of 31 cases from the brain bank for neurodegenerative disorders at Mayo Clinic was screened to assess the incidence of the expanded C90RF72 repeat in cases of depressive pseudodementia. The presence of the hexanucleotide repeat was established using immunohistochemistry with a highly disease-specific antibody (C9RANT), and was further validated in carriers using repeat-primed polymerase chain reaction and Southern blotting. Two individuals harbored the C90RF72 repeat expansion. Both patients were men with refractory depression. One patient experienced drug-induced parkinsonism and sudden-onset dementia, while the other patient had a more insidious disease course suspected to be Alzheimer disease. CONCLUSIONS AND RELEVANCE This report increases the range of clinicopathologic presentations of C90RF72 expanded hexanucleotide repeat to include psychiatric disorders such as depressive pseudodementia.