Who Gets Alzheimer's? Genes Hold Key
But Genes Aren't Destiny, Study of Identical Twins Shows
Will you ever get Alzheimer's disease? Genetics may have the answer. The
genes you've inherited carry most of the risk, an identical-twin study
It's surprising news. While people clearly inherit Alzheimer's disease risk,
most researchers have given equal blame to "environmental factors."
These factors may include things we encounter, things we do or don't do, or
diseases we develop.
But an international study of nearly 12,000 Swedish twin pairs -- a fourth
of them identical twins -- now finds that some 80% of Alzheimer's risk is
genetic. University of Southern California psychologist Margaret Gatz, PhD, and
colleagues report the findings in the February issue of Archives of General
"It appears that genetic influences outweigh environmental influences in
relative importance for Alzheimer's risk," Gatz tells WebMD.
What this means is that close relatives of people with Alzheimer's disease
are at much higher risk of getting the disease than people without such a
relative. What it does not mean is that if you have such a relative,
you're doomed to get Alzheimer's.
"'Genetic' does not mean cast in stone," Gatz tells WebMD. "In
no way is having a relative with Alzheimer's disease -- even a genetically
identical twin -- a guarantee that a person is going to get Alzheimer's
When 1 Twin Has Alzheimer's
Gatz's team screened for dementia among twins over age 65 in the Swedish
Twin Registry. They then tested each twin with dementia for Alzheimer's
disease. Extensive medical and lifestyle information was available for each
The researchers found that genetic inheritance explained about 80% of
Alzheimer's risk. Environmental factors not shared among twins explained the
other 20%, as might be expected.
But even among identical twins -- who share the same genetic makeup --
Alzheimer's disease in one twin did not mean the other twin inevitably got the
Among male identical twins, when one brother had Alzheimer's disease, the
other developed the disease 45% of the time.
Among female identical twins, when one sister had Alzheimer's disease, the
other developed the disease 60% of the time.
The difference between men and women, Gatz says, is simply that women live
longer and thus have a better chance of surviving long enough to get
If Alzheimer's disease was strictly genetic, one identical twin should get
the disease about the same time as the other. But when both identical twins had
Alzheimer's, there was as much as a 16-year difference in age of onset.
That, Gatz says, clearly shows that there must be a strong interplay between
a person's genes and a person's environment.
"For instance, if twins share a gene that is a more risk-promoting gene
with regard to how they process fats, then eating more fats would be more
dangerous for them," she says. "If one twin ate very few fats, her risk
would be much lower. We are sure that kind of thing is going on. That is why it
is hard to talk about genetic risks independently of environmental