Morbid risk for schizophrenia in first-degree relatives of people with frontotemporal dementia

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Familial co-occurrence of frontotemporal dementia and schizophrenia has never been investigated. To test the hypothesis that frontotemporal dementia and schizophrenia might have a common aetiology in some families in which both syndromes coexist (mixed families). The morbid risk for schizophrenia, calculated in first-degree relatives of 100 frontotemporal dementia probands, was compared with that calculated in first-degree relatives of 100 Alzheimer's disease probands. In mixed families, sequencing analysis of known frontotemporal dementia genes and detailed phenotype characterisation of individuals with frontotemporal dementia and schizophrenia were performed. The morbid risk for schizophrenia was significantly higher in relatives of frontotemporal dementia probands (1.35, s.e. = 0.45) than in relatives of Alzheimer's disease probands (0.32, s.e. = 0.22). Ten mixed families were characterised. In three of them a frontotemporal dementia causal mutation was identified that was present in individuals with schizophrenia. Several specific clinical features were noted in people with schizophrenia and frontotemporal dementia in mixed families. Co-occurrence of schizophrenia and frontotemporal dementia could indicate, in some families, a common aetiology for both conditions.