Having
a family member who has Alzheimer's increases the risk of developing the
disease. However, only a handful of Alzheimer's patients - less than 5 percent - have the disease as a result of an identifiable defective gene (a gene
mutation).
In families with a
mutation, Alzheimer's disease is carried as a dominant trait (which means that
each offspring has a 50 percent chance of inheriting the abnormal gene) on one
of three chromosomes -- the amyloid precursor protein (APP) gene on chromosome 21,
the presenilin-1 (PS-1) gene on chromosome 14 and the presenilin-2 (PS-2) gene
on chromosome 1.
The prevalence of these
gene mutations is low. As an example, the defective APP gene has been found in
only 25 families worldwide. Although these mutations affect only a small number
of families, a person carrying one of them will inevitably develop Alzheimer's
if he or she lives long enough.
Researchers expect to find
other genetic abnormalities linked to Alzheimer's disease. In fact, scientists
have already pinpointed an area on chromosome 10 that appears to contain genes
that influence Alzheimer's. A possible new gene, sortilin-related receptor L
(SORL1), has also been identified. Scientists believe there may be as many as a
dozen genes that contribute to Alzheimer's risk, but each is likely to
contribute only a little bit to the overall incidence of the disease.
Genetic predisposition to
Alzheimer's. Having a genetic predisposition to Alzheimer's disease is different
from carrying a genetic mutation. A predisposition means that, although
Alzheimer's "runs in the family," it is not associated with one of
the genetic defects discussed above. Some family members develop Alzheimer's
while others do not.
A predisposition suggests
that other risk factors interact with a person's genetic makeup to increase the
chance that he or she will develop Alzheimer's or to cause the disease to begin
earlier in life. For example, in one study of identical twins, who share an
identical genetic makeup, the age of Alzheimer's onset varied by as much as 15
years.
(Source: John Hopkins
Health Alert, 2 December 2013)
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