Frontotemporal dementia: Clinical aspects, genetics, and neuropathology of a family with a <i>C9ORF72</i> expansion in Argentina

Frontotemporal dementia (FTD) is the second most common cause of early-onset dementia, typically manifesting before the age of 65, with a mean onset at 58 years. FTD may encompass a spectrum of neurodegenerative disorders resulting from frontotemporal lobar degeneration (FTLD), affecting behavior, language, and motor function. Among its clinical variants, the behavioral variant (bvFTD) is the most frequently inherited, often associated with mutations in MAPT, GRN, and C9ORF72, the latter being the most prevalent genetic cause of FTD and FTD-motor neuron disease (FTD-MND). While bvFTD is classically defined by profound behavioral changes and executive dysfunction, cases linked to C9ORF72 expansions exhibit atypical neuropsychiatric features. This study documents two cases within the same family presenting with bvFTD and atypical parkinsonism, associated with a C9ORF72 expansion. Neurocognitive assessments, genetic testing, and neuroimaging (MRI, SPECT) were performed to characterize the clinical phenotype. A detailed review of the familial aggregation of neurodegenerative and psychiatric disorders provided further insight into the genetic contributions to symptomatology. The findings highlight the phenotypic heterogeneity associated with C9ORF72 expansions, demonstrating a spectrum ranging from bvFTD to atypical parkinsonism, with variable neuropsychiatric involvement. While movement disorders in FTD have historically been underestimated, these cases reinforce the association between parkinsonism and familial bvFTD. Given the limited epidemiological data on genetic FTD in Latin America, this study underscores the importance of genetic testing in cases with prominent behavioral and psychiatric symptoms, supporting early identification and genetic counseling for affected families.