[<sup>11</sup> C]PBB3-PET can capture four-repeat tau pathologies characteristic of N279K mutant frontotemporal dementia and parkinsonism linked to chromosome 17/MAPT.
Aggregation of microtubule-associated protein tau into insoluble intracellular neurofibrillary tangles is a characteristic hallmark of Alzheimer's disease (AD) and other neurodegenerative diseases, including progressive supranuclear palsy, argyrophilic grain disease, corticobasal degeneration, frontotemporal dementias with Parkinsonism linked to chromosome 17, and Pick's disease.
The aggregation of NFTs, the abnormal hyperphosphorylation of tau protein, and the interaction between tau and alpha-synuclein may all contribute to the cell death and poor axonal transport observed in PD and Parkinsonism.
Hyperphosphorylation and accumulation of tau in neurons (and glial cells) is one the main pathologic hallmarks in Alzheimer's disease (AD) and other tauopathies, including Pick's disease (PiD), progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease and familial frontotemporal dementia and parkinsonism linked to chromosome 17 due to mutations in the tau gene (FTDP-17-tau).
Variants of the MAPT gene have been suggested to be associated with Parkinson's disease (PD) and to modify the risk for leucine-rich repeat kinase 2 (LRRK2) Parkinsonism.
These mice contain a tau gene with a mutation of the frontotemporal dementia parkinsonism (FTDP-17) type, in which valine is substituted with methionine residue 337.
Several distinct clinical syndromes presenting with parkinsonism have been associated with subcortical neurofibrillary degeneration and the abnormal accumulation of hyperphosphorylated tau protein in the brain.
However, since 1998, the identification of more than 25 mutations in the tau gene, associated with frontotemporal dementia and parkinsonism linked to chromosome 17, has demonstrated that tau dysfunction can lead to neurodegeneration and the development of clinical symptoms.
Parkinsonism in frontotemporal dementia (FTD) was first described in families with mutations in the microtubule-associated protein tau (MAPT) and progranulin (PRGN) genes.
Mutations in the microtubule-associated tau (MAPT) gene are associated clinically with frontotemporal dementia with or without supranuclear palsy, corticobasal syndrome or parkinsonism.
Analysing MAPT alternative splicing, the expression of 1N/4R isoform was inversely associated with global parkinsonism (p = 0.008) and bradykinesia (p = 0.008).
Tau is not only a basic component of neurofibrillary degeneration, but is also an aetiological factor, as demonstrated by mutations on the tau gene responsible for frontotemporal dementias with parkinsonism linked to chromosome 17.
The analysis of tau gene and the study of familial cases of tauopathies have led to the discovery of tau gene mutations that cause inherited dementia designated as Frontotemporal dementia (FTD) with parkinsonism linked to chromosome 17 (FTDP-17).
It is unclear how tau gene mutations cause frontotemporal dementia (FTD) with parkinsonism linked to chromosome 17 (FTDP-17), but those in exon 10 (E10) or the following intron may be pathogenic by altering E10 splicing, perturbing the normal 1:1 ratio of four versus three microtubule-binding repeat tau (4R:3R tau ratio) and forming tau inclusions.
Mutations in the gene encoding the microtubule-associated protein tau are associated with frontotemporal dementia and parkinsonism linked to chromosome 17.
Mutations in MAPT were previously identified in familial FTD with parkinsonism (FTDP-17); however, in FTDU-17 patients, no pathogenic mutations were found in exonic regions consistent with the lack of tauopathy in FTDU-17 brains.
Frontotemporal dementia is commonly associated with parkinsonism in several sporadic (i.e., progressive supranuclear palsy, corticobasal degeneration) and familial neurodegenerative disorders (i.e., frontotemporal dementia associated with parkinsonism and MAPT or progranulin mutations in chromosome 17).
These findings suggest that tau proteins are not always assembled in abnormal filaments such as twisted ribbons, paired helical filaments and straight tubules in neurons and glial cells, which have been shown in previous cases with frontotemporal dementia and parkinsonism linked to chromosome 17.
In addition, disease-causing mutations in the tau gene on chromosome 17 have been detected in some families with autosomal dominant FTD and parkinsonism.
However, exome sequencing identified a missense mutation, N279K, in exon 10 of MAPT gene, verifying that the early parkinsonian symptoms in this family are caused by the genetic mutation for hereditary frontotemporal lobar dementia.
In addition, the recent identification of mutations in the tau gene associated with a similar neurodegenerative condition (frontotemporal dementia and parkinsonism linked to chromosome 17) has further strengthened the argument that tau dysfunction is somehow involved in the pathogenesis of PSP.