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Clear Genetic Link Found for Alzheimer's in Study of
Twins
Genetics also play a role in the timing of the
disease
Feb. 6, 2006 - In a study of twins - the largest
ever - Alzheimer's disease appears to be highly heritable and genetic
factors may also influence timing of the disease, according to an
article in the February issue of the Archives of General Psychiatry, one
of the JAMA/Archives journals. The researchers estimated heritability
for AD to be between 58 and 79 percent.
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Two-thirds of senior citizens, aged 65 years and
older, with dementia have Alzheimer's disease (AD), and the number of AD
cases is predicted to increase with the growing older adult population.
Although genetic risk factors for AD have been
identified, only a fraction of AD cases can be explained by specific
gene mutations.
Studies of twins have been helpful in investigating
the relative importance of genetic and environmental influences in
disease development. For example, if identical (monozygotic) twins do
not both have a disease, environmental factors may be implicated for its
occurrence.
Margaret Gatz, Ph.D., University of Southern
California, Los Angeles, and colleagues evaluated 11,884 pairs of twins
from the Swedish Twin Registry to examine the association between
genetic and environmental influences and AD.
The study included patients from the registry who
tested positive for dementia and their twins, plus a sample of twins
without dementia. The researchers identified 392 pairs of twins in which
one or both had AD.
They estimate heritability for AD is
between 58 and 79 percent.
Among pairs where both twins had AD, there was an
average of 3.66 years difference in age at onset between 25 sets of
identical twins, and 8.12 years difference between 20 sets of fraternal
(dizygotic) twins.
This lead the researchers to conclude that genes
had a role in disease timing "...because age at onset of AD is
significantly more similar for concordant [occurring in both twins]
monozygotic pairs compared with concordant dizygotic pairs."
Concordance rates were higher in women, reflecting
their greater longevity.
"In the largest twin study to date, we confirmed
that heritability for AD is high and that the same genetic factors are
influential for both men and women," the authors write.
"However, nongenetic risk factors also play an
important role and might be the focus for interventions to reduce
disease risk or delay disease onset."
(Arch
Gen Psychiatry. 2006;63:168-174.
Editor's Note: This work was supported by a grant
from the National Institute on Aging, Bethesda, Md. Dr. Gatz held a
Zenith Award from the Alzheimer's Association, Chicago.
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