What is it?
Alzheimer’s Disease (AD) is an irreversible form of dementia characterized by memory loss, a progressive decline in intellectual ability, deteriorating language and speech skills, and by personality and behavioral changes that eventually interfere with daily living. Currently, Alzheimer’s Disease has been estimated to affect as many as 4 million older Americans.
Although AD mimics some changes found in the brain as we age, it is not a normal part of the aging process. It causes nerve cell injury and death and is characterized by the build-up of senile plaques and neurofibrillary tangles (twisted protein filaments within nerve cells) in the brain. The destruction of nerve cells also results in decreased levels of acetylcholine and other neurotransmitters (chemicals necessary for communication between nerve cells) in the brain. Over time, AD results in decreased interaction between different areas of the brain.
Relationship with Aging
The risk of having AD and other dementias increases greatly with age. About 10% to 12% of the population will have dementia by the time they are 65 years old, with the risk increasing to 50% for those who reach the age of 100. With the U.S. population aging and living longer, it has been estimated that as many as 9 million Americans will have Alzheimer’s Disease by 2050.
Most of the time AD is “late onset,” beginning after the age of 65, and sporadic (does not seem to be family-related). “Early onset” AD, starting before the age of 65, is more rare and more likely to be family-related. (It accounts for about 5% to 10% of all AD cases).
Genetic Relationship
Alzheimer’s Disease appears to be caused by a variety of factors; although many of the factors are not yet well understood, some have a genetic component. Three genes have been associated with rare forms of Early Onset Familial Alzheimer’s Disease (EOFAD, also called Alzheimer's disease type 3 or AD3). Certain mutations in each of these genes (known as PSEN1, PSEN2, and APP), cause AD in anyone who carries the mutation. In these cases, there is a 50% chance that the gene mutation will be passed on to each of his or her children. So far, mutations in these three genes have only been found in a very small number of specific families, and the normal functioning and role of these genes has yet to be determined.
Other genes are not directly causative but increase the risk of developing late onset AD. These "susceptibility" genes explain why there is an increased risk of developing late onset AD in individuals with other affected family members. The ApoE gene is the most clearly established susceptiblity gene for late onset AD. This gene directs the production of apolipoprotein E, a protein that forms part of the body’s lipoproteins (such as HDL cholesterol) and is involved in lipid transportation and clearing dietary fats from the body. The ApoE gene normally exists in three forms: e2, e3, and e4. Everyone has two copies of the ApoE gene, in some combination of the three forms. ApoE e4 has been associated with an increased risk of AD.
The risk of AD also varies by ethnicity, with the risk in Americans of African descent as much as four times greater and in Hispanics as much as two times greater, respectively, than that of Caucasians.
Most individuals with Down Syndrome (DS), caused by trisomy of chromosome 21, will eventually show some changes associated with AD, usually by the time they are in their 40’s or 50’s. The pathology found in the brains of these adult DS patients is very similar to what is found in AD because the extra copy of chromosome 21 results in increased production of the protein that accumulates in senile plaques. Relatives of individuals with DS do not have any increased risk of AD since they will not have the extra copy of chromosome 21.
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