Etiology of Alzheimer’s Disease

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Dr. Dennis Selkoe, professor of neurology and neuroscience at Harvard Medical School, provided journalists with a detailed overview of the etiology and mechanisms of the disease. He described how the abnormal protein structures of amyloid plaques and neurofibrillary tangles are recognized as the hallmarks of the disease. The science of Alzheimer’s disease, building on this characterization, has focused on three major research questions–the causes of the disease, the mechanisms of brain cell death, and the types of brain cells that are affected and subsequently die.

A great deal of recent research into the cause of AD has focused on how the abnormal plaques and tangles are formed in the brain in the first place, Dr. Selkoe said. He explained how the plaques consist of fragments (beta-amyloid) of amyloid precursor protein, or APP. Over the years, research has revealed how APP is made, metabolized, and broken down by enzymes that cut it in certain places. Specifically, some of these enzymes destroy beta-amyloid while others clip out either a short form of beta-amyloid (A-beta 1-40) or a longer, more “sticky” form (A-beta 1-42). Dr. Selkoe then described to the group the link between genetic mutations and formation of beta-amyloid fragments, showing the journalists how mutations found in the APP gene in early-onset families, for example, appear to increase the amount of total beta-amyloid or of A-beta 1-42. He also discussed the role of presenilins and APOE in helping form more amyloid.

This body of research suggests that the deposition of beta-amyloid into plaques is the primary event in AD, triggering a “cascade” of downstream changes in the brain, Dr. Selkoe said. These include the development of neurofibrillary tangles, inflammation, generation of free radicals, the death of brain cells and loss of their ability to communicate. Scientists are actively investigating each part of this cascade. Laboratories such as Selkoe’s and others “went to work to find out how these genes did their dirty work,” he noted, detailing his group’s findings that presenilin mutations, for example, “crank up” the production of A-beta 1-42.

Investigations of these mechanisms have led to a number of “exciting, therapeutic possibilities,” according to Dr. Selkoe. He and his colleagues, for example, are seeking ways to interfere in the processes by which APP is clipped to form beta-amyloid and how beta-amyloid aggregates into plaques, as well as examining how beta-amyloid in plaques can be harmlessly broken down. One very new finding, he reported, involves the effectiveness of insulin degrading enzyme, or IDE, in breaking down A-beta 1-40 and A-beta 1-42.

Scientists are zeroing in as well on the protein tau, which is the protein constituent of neurofibrillary tangles, he said. Additional research is examining therapies that might address some of the other downstream events associated with AD–testing anti-inflammatory drugs to reduce inflammation, antioxidants to mop up free radicals and prevent their damage, and protective agents such as estrogen and growth factors to fight neuron dysfunction and death.

Another currently exciting development in this field is the use of transgenic mice as bioassay tools. These mice, bred to carry various combinations of human genes linked to AD, are being used to test theories about the etiology of AD. Dr. Selkoe suggested that in addition to enabling scientists to explore, for example, whether amyloid plaques themselves are a cause of neurodegeneration or are a byproduct of the disease process, these mouse models are a powerful tool for testing the effectiveness of drugs to slow the progress of the disease. Mouse models for AD are already helping to unravel how the disease can be triggered and modulated by a complex interplay of different genes, Dr. Selkoe said. He expressed optimism that analyzing the different mouse strains could lead to the identification of new genes that may modify the AD process.

Source: Connections Magazine [Volume 8(1), Spring 1999]

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