Frontotemporal lobar degeneration. An update on clinical, pathological and genetic findings.

Frontotemporal lobar degeneration is the second most common form of cortical dementia in the presenium after Alzheimer's disease. Clinically, based on consensus guidelines, three distinct disease entities can be distinguished: frontotemporal dementia, semantic dementia and progressive nonfluent aphasia. Dementia of frontal type and motor neuron disease inclusion dementia are the most frequent neuropathological subtypes of frontotemporal lobar degeneration. By using immunohistochemistry, the latter is characterized by the presence of filamentous ubiquitin-reactive but tau-negative inclusions in nerve cell bodies and neurites. In contrast, Pick's disease and familial frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) are both characterized by abundant filamentous nerve cell inclusions made up of the microtubule-associated protein tau. The recent discovery of more than 15 different mutations in the tau gene in FTDP-17 brought the tau protein to the centre stage. These findings had a major impact on our understanding of neurodegenerative disorders characterized by tau filamentous inclusions in neurones and/or glial cells which are grouped under the generic term of tauopathies. However, as exciting these new molecular insights are, it would be inappropriate to lump frontotemporal lobar degeneration as tauopathies. Recent neuropathological and genetic data strongly suggest that there is more than one genetic background for frontotemporal lobar degeneration.