What is Alzheimer’s disease?

Alzheimer disease is a degenerative disease of the brain that causes dementia, which is a gradual loss of memory, judgment, and ability to function. This disorder usually appears in people older than age 65, but less common forms of the disease appear earlier in adulthood.

Memory loss is the most common sign of Alzheimer disease. Forgetfulness may be subtle at first, but the loss of memory worsens over time until it interferes with most aspects of daily living. Even in familiar settings, a person with Alzheimer disease may get lost or become confused. Routine tasks such as preparing meals, doing laundry, and performing other household chores can be challenging. Additionally, it may become difficult to recognize people and name objects. Affected people may increasingly require help with dressing, eating, and personal care.

As the disorder progresses, some people with Alzheimer disease experience personality and behavioral changes and have trouble interacting in a socially appropriate manner. Other common symptoms include agitation, restlessness, withdrawal, and problems with speech. People with this disease usually require comprehensive care during the advanced stages of the disease. After the appearance of symptoms, affected individuals usually survive 8 to 10 years, but the course of the disease can range from 1 to 25 years. Death usually results from pneumonia, malnutrition, or general body wasting (inanition).

Four major types of familial Alzheimer disease have been identified. Types 1, 3, and 4 are classified as early-onset Alzheimer disease because their signs and symptoms appear before age 65. Type 2 is classified as late-onset Alzheimer disease because its signs and symptoms appear after age 65. Other cases of Alzheimer disease are classified as sporadic or nonfamilial, which means they do not appear to run in families.

How common is Alzheimer disease?

Alzheimer disease currently affects more than 5 million Americans. Because more people are living longer, the number of people with this disease is expected to more than triple by 2050.

What genes are related to Alzheimer disease?

  • Mutations in the APP, PSEN1 and PSEN2 genes cause Alzheimer disease.
  • Variations of the APOE gene increase the risk of developing Alzheimer disease.

About 75 percent of Alzheimer disease cases are classified as sporadic, which means they occur in people with no history of the disorder in their family. Although the cause of these cases is unknown, genetic changes are likely to play a role. Virtually all sporadic Alzheimer disease begins after age 65, and the risk of developing this condition increases as a person gets older.

The remaining cases of Alzheimer disease are familial, which means they are found in multiple members of a family. Familial Alzheimer disease can be divided into early-onset disease (symptoms begin before age 65) and late-onset disease (symptoms begin after age 65).

The early-onset forms of Alzheimer disease are caused by gene mutations that can be passed from parent to child. Researchers have identified three genes that cause these forms of the disorder. Mutations in the APP gene cause Alzheimer disease type 1. Changes in the PSEN1 gene are responsible for Alzheimer disease type 3, while PSEN2 mutations lead to Alzheimer disease type 4. As a result of mutations in any of these genes, large amounts of a toxic protein fragment called amyloid beta peptide are produced in the brain. This toxic peptide can build up in the brain to form clumps called amyloid plaques, which are characteristic of Alzheimer disease. Amyloid plaques may lead to the death of nerve cells and the progressive signs and symptoms of this disorder.

Some evidence indicates that people with Down syndrome have an increased risk of developing type 1 Alzheimer disease. Down syndrome, a condition characterized by mental retardation and other health problems, occurs when a person is born with an extra copy of chromosome 21 in each cell. As a result, people with Down syndrome have three copies of many genes in each cell, including the APP gene, instead of the usual two copies. Although the connection between Down syndrome and Alzheimer disease is unclear, the production of more amyloid beta peptide in cells may account for the increased risk. People with Down syndrome account for fewer than 1 percent of all cases of Alzheimer disease.

The genetic causes of late-onset (type 2) familial Alzheimer disease are less clear. This disorder is probably related to mutations in one or more risk factor genes in combination with lifestyle and environmental factors. A gene called APOE has been studied extensively as a risk factor for the disease. In particular, a variant of this gene called the epsilon 4 (e4) allele seems to increase an individual’s risk for developing type 2 Alzheimer disease.

How do people inherit Alzheimer disease?

The early-onset familial forms of Alzheimer disease (types 1, 3, and 4) are inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In most cases, an affected person inherits the altered gene from one affected parent.

The inheritance pattern of late-onset (type 2) familial Alzheimer disease is uncertain. People who inherit one copy of the APOE e4 allele have an increased chance of developing the disease; those who inherit two copies of the allele are at even greater risk. It is important to note that people with the APOE e4 allele inherit an increased risk of developing Alzheimer disease, not the disease itself. Not all people with Alzheimer disease have the e4 allele, and not all people who have the e4 allele will develop the disease.

What other names do people use for Alzheimer disease?

  • AD
  • Alzheimer dementia (AD)
  • Alzheimer sclerosis
  • Alzheimer’s Disease
  • Alzheimer syndrome
  • Alzheimer-type dementia (ATD)

Source: Genetics Home Reference, A Service of the U.S. National Library of Medicine, Reviewed: October 2006, Published: February 2, 2008
Provided by: The Staff at www.RetirementConnection.com
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