Psychiatric News September 17, 2004
Volume 39 Number 18
© 2004 American Psychiatric Association
p. 32
Alzheimer's, Parkinson's Don't Show Familial Relationship
Joan Arehart-Treichel
Some people with Parkinson's disease develop Alzheimer's disease, and
vice versa. The reason, however, does not seem to be because the illnesses
share common genes.
Could it be that the two most common neurodegenerative
disorders—Parkinson's disease and Alzheimer's disease—share common
origins?
Some evidence suggests that they might. For example, as many as one-fourth
of Alzheimer's patients develop Parkinsonian symptoms, and patients with
rapidly progressing Parkinson's have more than an eightfold increased risk of
developing Alzheimer's compared with patients with slowly progressing
Parkinson's (Psychiatric News, October. 19, 2001; June 20, 2003).
A new, large study, however, argues that the two illnesses are not
related—or at least, not genetically related. It found no evidence of an
increased risk of Alzheimer's in some 2,500 relatives of patients with
Parkinson's compared with some 2,300 relatives of similar persons without
Parkinson's, thus arguing against a common genetic cause for the two
diseases.
One of the primary investigators was Karen Marder, M.D., a professor of
neurology at Columbia University. Results appeared in the July Archives of
Neurology.
Some 500 persons with Parkinson's disease were recruited from the Center
for Parkinson's Disease and Other Movement Disorders of the Columbia
Presbyterian Medical Center in New York City and also from a neurology clinic
serving persons in northern Manhattan. About 400 other individuals were
recruited by random-digit dialing and matched with the Parkinson's subjects on
age, gender, and ethnicity.
Each subject was given the Mini-Mental State Examination. Anyone screening
positive for dementia on this exam was excluded from the study. The reason the
researchers excluded anyone who screened positive for dementia was to guard
against subjects providing inaccurate information about whether their
relatives had Alzheimer's disease. The researchers admitted in their study
report, however, that by eliminating some subjects who had both Parkinson's
and Alzheimer's, they could not "explore the incidence of Alzheimer's
disease in relatives of patients with Parkinson's disease with
dementia."
The subjects with Parkinson's disease were then given a structured family
history interview to learn whether some 2,500 of their first-degree relatives
had Alzheimer's. The researchers also tried to obtain a second interview by
telephone with each first-degree relative of a subject to confirm what the
subject had reported, regardless of whether the subject said that the relative
had Alzheimer's. And if the first-degree relative had died or was unable to
provide information, the researchers tried to contact another informant to
verify what the subject had said.
A similar protocol was followed to learn if any of the approximately 2,300
first-degree relatives of control subjects had Alzheimer's disease.
A total of 61 first-degree relatives (2.4 percent) of patients with
Parkinson's were found to have Alzheimer's, while 52 first-degree relatives of
control subjects (2.3 percent) were found to have the illness.
The risk of Alzheimer's was thus not increased in relatives of Parkinson's
subjects compared with relatives of control subjects.
The researchers also compared the incidence of Alzheimer's in the relatives
of subjects who had developed Parkinson's at age 50 or younger with the
incidence of Alzheimer's in relatives of matching control subjects. The
investigators' rationale was that early onset of a disorder often signals a
genetic origin. But again, they found no significantly increased risk of
Alzheimer's in the relatives of Parkinson's subjects.
Thus, it looks as though Parkinson's disease and Alzheimer's disease do not
share a common genetic origin, Marder and her team concluded. "This is
also consistent with the lack of overlap in currently known genetic causes of
Parkinson's disease and Alzheimer's disease," they pointed out.
"The evidence appears quite convincing," Walter Kukull, Ph.D.,
of the National Alzheimer's Coordinating Center at the University of
Washington, wrote in an accompanying editorial. Yet there are several
limitations to the study, he cautioned. For instance, as mentioned above,
relatives of persons who had both Parkinson's and Alzheimer's were not
included in the study, which may have skewed results. In other words, if such
relatives had been included, then perhaps the researchers might have found
that Parkinson's and Alzheimer's are genetically related. Also, the family
history interview method that was used in the study is rather low in
sensitivity. (However, as the researchers pointed out in their report, they
"tried to maximize sensitivity by collecting a second informant
report....")
The study was supported by federal grants and by the Parkinson's Disease
Foundation.
An abstract of the report, "Lack of Familial Aggregation of
Parkinson Disease and Alzheimer Disease," is posted online at
<http://archneur.ama-assn.org/cgi/content/abstract/61/7/1033>.
Arch Neurol 2004 61 1033[Abstract/Free Full Text]
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