|Twins Comparison Suggests Genetic Risk for Dementia
On average, twins of people who have been diagnosed with dementia score lower
on cognitive tests than do the twins of people without dementia, new research
has found. The study, which included more than 100 Swedish twins age 65 and older,
also found that, on average, identical twins of people with dementia have poorer
cognitive skills than do fraternal (non-identical) twins of people with dementia.
The researchers suggest that these differences in thinking skills reflect a
genetic risk for dementia. However, they emphasize that cognitive changes and
elevated genetic risk do not always predict that twins or siblings of people
with dementia will eventually develop dementia themselves.
The research, reported in the December 2005 issue of the Journal of Geriatric
Psychiatry and Neurology, was led by Margaret Gatz, Ph.D., of the University
of Southern California and the Karolinska Institute in Sweden. The study was
funded by the National Institute on Aging (NIA), a component of the National
Institutes of Health, U.S. Department of Health and Human Services, and a Zenith
Award from the Alzheimer’s Association. The University of Southern California
Alzheimer’s Disease Center is one of more than 30 Alzheimer’s Disease Centers
nationwide supported by the NIA.
“This research is intriguing because it associates genetic risk for dementia
with twins’ cognitive deficits, even in the absence of dementia,” says Neil Buckholtz,
Ph.D., chief of the Dementias of Aging Branch of NIA’s Neuroscience and Neuropsychology
of Aging Program. “The differences in cognitive deficits between identical and
fraternal twins are also important, suggesting that the twins who were more similar
genetically had the greater risk.”
The study included 112 members of the Swedish Twin Registry who were at least
65 years old in 1998. The registry, established in 1961, includes all twins born
in Sweden. Of the study participants, 23 were identical twins and 62 were fraternal
twins whose co-twins had dementia but who did not have dementia themselves. A
comparison group included 27 non-demented twins whose co-twins did not have dementia.
The comparison group was similar to the other participants in terms of age, gender,
and level of education.
All of the study participants took a series of neuropsychological tests that
assessed their attention, memory, verbal recall, verbal fluency, ability to copy
simple drawings, comprehension, and other cognitive skills. The test results
for twins of people with dementia were weighed against those of the comparison
Twins whose twin siblings had dementia had significantly lower overall scores
on the cognitive-skills tests than those of the comparison group. The twins of
demented co-twins and the comparison group differed most on the tests of memory
and “executive functioning,” such as verbal fluency and remembering patterns
that include symbols and numbers. Gatz and her colleagues say this finding suggests
that the twins of people with dementia are at higher risk for developing dementia
in the future, although they had already lived without dementia for an average
of nearly 8 years beyond their co-twins’ dementia onset.
“Identical twins of dementia cases had a strikingly poorer cognitive performance
profile,” Gatz notes. “It could be that these twins are more likely to progress
to dementia, but we don’t know that. We might be seeing a difference in performance
that could already have persisted for a long time without getting worse or it
could be a signal that the currently non-demented twin is at greater risk for
Gatz and her co-authors point out, however, that the study included only a “modest
number” of twins. They also did not gather long-term data that would show changes
or stability in cognitive performance over time or whether participants would
develop dementia in the future.
“While there may be a genetic risk for dementia, it is important to recognize
that not everyone with a genetic risk factor will develop dementia,” Buckholtz
comments. “More research is needed to help us understand who will and will not
develop dementia, even if they are at risk. Beyond genetics, environmental and
life style factors also play a role.”
The Gatz article is one of several in the December 2005 Journal of Geriatric
Psychiatry and Neurology focusing on children of Alzheimer’s parents (http://jgpn.sagepub.com).
The papers, including a number reporting on studies funded by the NIA, are based
on presentations at a workshop held in conjunction with the American Association
for Geriatric Psychiatry Annual Conference in March 2005.
In other recent NIA-supported research, Gatz and her colleagues focused on lifestyle
prevention factors. They found that twins who are involved in complex work —
particularly complex work with people — are at lower risk of dementia and
disease than are their co-twins who are not involved in complex work, even when
age, gender, and level of education are considered. That study, which included
more than 10,000 members of the Swedish Twin Registry, was published in the September
2005 issue of the Journal of Gerontology: Psychological Sciences.
To contact Dr. Neil Buckholtz: Call Susan Farrer or Vicky Cahan, NIA Office
of Communications and Public Liaison, 301-496-1752.
To contact Dr. Margaret Gatz: Call Carl Marziali, University of Southern California
Media Relations, 213-740-4751 or e-mail email@example.com.
The NIA supports several research projects on the genetic factors that may
influence the risk of developing Alzheimer’s disease or dementia, including
the Alzheimer’s Disease Genetics Study. For information on participating in
that study or in an AD clinical trial, visit http://www.clinicaltrials.gov/ (search for Alzheimer’s disease trials), or the Alzheimer’s Disease Education
and Referral (ADEAR) Center website at http://www.alzheimers.org. ADEAR may
also be contacted toll free at 1-800-438-4380. The ADEAR Center is sponsored
by the NIA to provide information to the public and health professionals about
AD and age-related cognitive change and may be contacted at the website and
phone number above for a variety of publications and fact sheets, as well as
information on clinical trials.
The National Institutes of Health (NIH) — The Nation's Medical Research
Agency — includes 27 Institutes and Centers and is a component of
the U. S. Department of Health and Human Services. It is the primary Federal
agency for conducting and supporting basic, clinical, and translational medical
research, and it investigates the causes, treatments, and cures for both common
and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov.