Frontotemporal dementias: update on recent developments in molecular genetics and neuropathology.

Frontotemporal dementias (FTD) are the second most common type of presenile dementias, considered to be clinically and pathologically different from Alzheimer's dementia (AD). FTD differs clinically from AD because memory loss is rarely an early symptom. Instead, FTD is usually denoted by behavioural and language difficulties, and may co-occur with motor neuron disease (MND). Frontotemporal lobar degeneration (FTLD) with ubiquitin-positive, tau-negative inclusions (FTLD-U) is the most common underlying pathology with and without MND. TAR DNA-binding protein 43 (TDP-43), encoded by the TARDBP gene, has been identified as the major pathological protein of FTLD-U with or without MND, demonstrating that abnormal TDP-43 alone is sufficient to cause neurodegeneration. FTLD is a genetically complex disorder. A proportion of cases of FTLD-U have various pathogenic mutations in the progranulin (GRN) gene. Other FTLD-U entities with TDP-43 proteinopathy include FTLD-U with valosin-containing protein (VCP) gene mutation and FTLD with MND linked to chromosome 9p. In contrast, chromosome 3-linked dementia, a FTLD-U with chromatic modifying protein 2B (CHMP2B) mutation, has TDP-43 negative inclusions. Thus, TDP-43 defines a novel class of neurodegenerative diseases called TDP-43 proteinopathies. These recent discoveries will contribute to an accurate diagnosis, and facilitate the development of diagnosis and therapy.