Several neurodegenerative diseases, including AD, are characterized by the aggregation of tau into insoluble filaments in neurons and glia, leading to dysfunction and death. Until now, there was no evidence that changes in the tau protein itself could actually initiate neuronal degeneration in humans with any forms of dementia. Rather, the tangles of Alzheimer’s disease were seen as secondary to some other insult or genetic change, for example, the buildup of beta-amyloid. Determining exactly what relationship exists between plaques and tangles is a major goal of AD research.
Very recently, several groups of NIA-supported researchers at the University of Pennsylvania, Mayo Clinic, University of Washington, and Cambridge University, who were studying families with a variety of hereditary dementias other than AD, found the first mutations in the tau gene on chromosome 17 (Clark et al., 1998; Hutton et al., 1998; Poorkaj et al., 1998; Spillantini et al., 1998). In these families, mutations in the tau gene cause neuronal cell death and dementia. These disorders, collectively called “frontotemporal dementia and parkinsonism linked to chromosome 17” (FTDP-17), share some characteristics with AD but differ in several important respects. Patients with FTDP-17 differ from AD patients in the cognitive and behavioral problems they exhibit, for example. They also differ in the area of the brain most affected (frontotemporal dementia disorders primarily affect the frontal cortex). In addition, only neurofibrillary tangles made up of abnormal tau proteins form–there are usually no amyloid plaques in these diseases. However, a recent scientific article describes a 55-year-old individual with FTDP-17 who did have plaques in his brain, suggesting the possibility that tau mutations could sometimes result in beta-amyloid deposition (D’Souza et al., 1999).
These studies describing tau mutations are important because they support the idea that tau in and of itself can be an important contributor to neurodegeneration. Researchers speculate that these families have slightly different mutations, but that they all work in one of two ways to kill neurons–either by preventing tau from binding to the microtubules, which provide support for cellular structures such as axons (Dayanandan et al., 1999; Hong et al., 1998) or by causing tau to aggregate to form neurofibrillary tangles (Nacharaju et al., 1999). These disruptions to the cell’s basic function and structure could lead to its death.
The benefits of this research lead far beyond FTDP-17, for a better understanding of the impact of tau mutations in this disease may well provide clues to other age-dependent neurologic diseases that also are characterized by abnormal aggregation of tau protein, such as AD, Parkinson’s disease, some forms of amyotrophic lateral sclerosis, corticobasal degeneration, progressive supranuclear palsy, and Pick’s disease.
National Institutes of Health
National Institute on Aging
1999 PROGRESS REPORT ON ALZHEIMER’S DISEASE