What Causes Alzheimer's Disease?

Brain Cells Affected by Alzheimer's Disease
JUAN GAERTNER/SCIENCE PHOTO LIBRARY/ Getty Images

Scientists are still trying to fully understand the cause or causes of Alzheimer’s disease. In the meantime, it’s helpful to understand the hallmarks of Alzheimer’s –- plaques and tangles –- and the risk factors that affect a person’s likelihood of developing the disease.

Plaques and Tangles

Alzheimer's disease is characterized by a build-up of proteins in the brain. Though this cannot be measured in a living person, extensive autopsy studies have revealed this phenomenon.

The build-up manifests in two ways:

  • Plaques – deposits of the protein beta-amyloid that accumulate in the spaces between nerve cells
  • Tangles – deposits of the protein tau that accumulate inside of nerve cells

Scientists are still studying how plaques and tangles are related to Alzheimer’s disease. One theory is that they block nerve cells’ ability to communicate with each other, making it difficult for the cells to survive.

Autopsies have shown that most people develop some plaques and tangles as they age, but people with Alzheimer’s develop far more than those who do not develop the disease. Scientists still don’t know why some people develop so many compared to others. However, several risk factors for Alzheimer’s disease have been uncovered.

Alzheimer's Risk Factors

  • Age: Advancing age is the number one risk factor for developing Alzheimer’s disease. One out of eight people over the age of 65 has Alzheimer’s disease, and almost one out of every two people over the age of 85 has Alzheimer’s. The probability of being diagnosed with Alzheimer’s nearly doubles every five years after age 65.
  • Family History: People who have a parent or sibling that developed Alzheimer’s disease are two to three times more likely to develop the disease than those with no family history of Alzheimer’s. If more than one close relative has been affected, the risk increases even more.

    Scientists have identified two kinds of genes that are associated with this familial risk factor. The first is thought to be a “risk gene,” ApoE 4, that increases the likelihood of developing Alzheimer’s, but does not guarantee it. In addition to ApoE 4, scientists think there could be up to a dozen more risk genes yet to be discovered.

    The second kind of gene is a “deterministic gene” and is much rarer than risk genes. Deterministic genes are only found in a few hundred extended families around the world. If a deterministic gene is inherited, the person will undoubtedly develop Alzheimer’s, probably at a much earlier age.

    Sources:

    "Alzheimer's disease: Unraveling the mystery." National Institute on Aging. August 29, 2006.

    "Alzheimer's Research on Causes and Risk Factors." Fisher Center for Alzheimer's Research Foundation. May 1, 2003. http://www.alzinfo.org/research/alzheimers-research-on-causes-and-risk-factors

    "Genes, lifestyles, and crossword puzzles: Can Alzheimer’s disease be prevented?" National Institutes of Health. 2005. 

    -Edited by Esther Heerema, MSW, Alzheimer's Disease Expert

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