Alzheimer's disease is inherited to a substantial degree, a twin study
believed to be the largest conducted on this topic has revealed.
The investigation was headed by Margaret Gatz, Ph.D., a professor of
psychology at the University of Southern California. Results were published in
the February Archives of General Psychiatry.
Some 12,000 twin pairs enrolled in the Swedish twin registry, of which 392
had one or both members with Alzheimer's, served as a data source. When
identical twins are more concordant for a disease than fraternal twins,
genetic influences are suggested. So Gatz and her colleagues looked to see
whether identical-twin pairs were more similar in risk for developing
Alzheimer's than fraternal twin pairs were. The answer was yes.
The concordance rate among identical twins was 83 percent, whereas it was
only 46 percent for fraternal twins. So, genes play a role in Alzheimer's
susceptibility, Gatz and her group concluded.
Furthermore, when they took the genes, shared environment, and nonshared
environments of the twins into consideration, they found that 58 percent of
individual differences in Alzheimer's could be explained by genes, 19 percent
by a shared environment, and 23 percent by a nonshared environment.
"Previous twin studies have presented estimates [for Alzheimer's
genetic susceptibility] that vary quite a bit from each other," Gatz
told Psychiatric News. "[Thus] I was surprised by how
consistent we were with the heritability estimates found before by a smaller
sample of Swedish twins and by the Norwegian twin registry [and] also how
consistent we were in concordance rates for identical twins with the New York
State Psychiatric Institute results for `senile dementia' from decades
Indeed, one of the scientists who conducted this latter study—Lissy
Jarvik, M.D., Ph.D., a professor emeritus of psychiatry at the University of
California at Los Angeles—said that she was pleased that Gatz and
colleagues had obtained a "dementia concordance rate for monozygotic
pairs very similar to that reported by us.. .back in 1956."
Another salient result to emerge from the study was that Alzheimer's was
more prevalent among women than men; yet once women's greater longevity was
taken into account, women were no more susceptible than men were.
Also of note, when the investigators focused on twin pairs concordant for
Alzheimer's, they found that the intra-pair difference in age at onset was
significantly greater in fraternal twins than in identical twins—an
average of eight years versus four years. Therefore, because the age of onset
was more similar among identical twins, one can conclude that genes not only
influence whether people will develop Alzheimer's, but
Nonetheless, as the study results make clear, Alzheimer's risk and timing
are also influenced by nongenetic factors, and scientists may have identified
a few of those factors, Gatz said.
For example, brain injury and high cholesterol appear to predispose people
to Alzheimer's, whereas higher education levels seem to help shield them from
it. In fact, "higher education has been found in many, many studies to
be related to lower risk of Alzheimer's disease," she pointed out."
We have found this to be true even within identical-twin pairs [in
which] one has developed Alzheimer's disease and the twin sibling has
Yet undoubtedly, a "multitude of factors... protect against (or
promote) the development of Alzheimer's," Jarvik asserted, and"
twin studies could contribute significantly to the assessment of the
effectiveness of changes in diet, exercise... and the like in preventing or
And the people who stand to profit most from the identification of
prevention measures are the millions of Americans whose parents had
Alzheimer's. "They are deeply concerned about their risk of developing
Alzheimer's disease, but there are as yet no data to determine specific risk
of Alzheimer's in children of Alzheimer's parents."
The study was funded by the National Institute on Aging and the Alzheimer's
An abstract of "Role of Genes and Environments for Explaining
Alzheimer Disease" is posted at<http://archpsyc.ama-assn.org/cgi/content/abstract/63/2/168>.▪
Arch Gen Psychiatry