A Blood Test for Alzheimer's disease?
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disease management. Could a blood test do the same for Alzheimer
disease? In yesterday's Nature Medicine online, researchers report
that plasma levels of a panel of 18 proteins can identify Alzheimer
patients with roughly 90 percent accuracy. The test performed
similarly well in predicting which patients with mild cognitive
impairment eventually went on to develop AD. "Our study is consistent
with the concept that there is a systemic change [in AD] and that this
change begins quite early," said Tony Wyss-Coray, the study's
principle investigator, in an interview with ARF. The study could lead
to the development of a diagnostic test and to new insights into the
biology of the disease. Alzforum reported preliminary findings from
this study, which were presented at a satellite workshop on AD
biomarkers at the 2005 Society for Neuroscience meeting (see ARF
related news story).
The findings come from a collaboration among Wyss-Coray and colleagues
at Stanford University; Sandip Ray and colleagues at Satoris Inc., San
Francisco (Wyss-Coray co-founded the company); and numerous
collaborating researchers in the U.S. and Europe. "When we started
this study it seemed, to some extent, a bit of a crazy idea, that you
could study AD in the blood, but there have been several papers since
that suggest there is active recruitment of immune cells into the
brain and that they may modulate disease," said Wyss-Coray. Just this
year, for example, Joseph El Khoury and colleagues at Harvard Medical
School reported that migrating microglia can modulate pathology in
mouse models of the disease and that blocking chemokine signals
exacerbates pathology (see ARF related news story). Wyss-Coray and
colleagues had also opted to focus on chemokines, cytokines, growth
factors, and other signaling molecules that might mediate crosstalk
between the diseased brain and the periphery. "We decided to focus on
a group of factors that have the highest density of information," said
Wyss-Coray.
Joint first authors Markus Britschgi at Stanford and Ray measured
plasma concentrations of 120 well-characterized signaling proteins and
looked for differences in levels between Alzheimer patients and non-
demented controls. "This is one of the strengths of the study. There
are advantages in looking for proteins for which there are sensitive
assays already up and running, compared to trying to look at the
plasma proteome which is a much less well-defined entity," said co-
author Douglas Galasko, University of California, San Diego, in an
interview with ARF.
The researchers used a type of computer algorithm called predictive
analysis of microarrays (PAM) to narrow the 120 to the smallest number
of proteins that could distinguish AD patients from controls. The
algorithm compares protein levels between the two groups to find which
have the highest significance. Then it compresses the data to find
only the markers necessary to discriminate between AD and controls.
Using PAM, the researchers found a panel of 18 markers that classify
AD and non-demented controls with 95 percent positive agreement and 83
percent negative agreement, respectively.
One might question how specific this test is for AD, since other
diseases of the brain might elicit similar changes to plasma signaling
molecules. "One of the problems I've always perceived is that if one
looks at plasma from Alzheimer patients and controls, there may well
be differences that don't have to do with the neurobiology of
Alzheimer's disease but have to do with patients simply having a
chronic illness, losing weight, having an inflammatory process, or a
number of other questions along those lines, but the study goes a
number of ways to address those issues," said Galasko.
One way was to apply the test to a blinded set of samples to see if it
could correctly distinguish AD patients from those with no, or other
forms of dementia. The protein panel correctly identified eight of
nine postmortem-confirmed AD patients, and also 10 of 11 patients with
other dementias, who were classified as non-Alzheimer's. The test also
correctly identified patients with other neurologic diseases
(Parkinson disease, ALS, multiple sclerosis, and peripheral
neuropathy) and the autoimmune disorder, rheumatoid arthritis, as not
having AD. These findings suggest that the panel of proteins is
reporting on more than just inflammatory responses or
neurodegeneration in general.
The researchers next asked if the test could predict which patients
with mild cognitive impairment (MCI) might go on to develop AD; MCI is
often, but not always, the precursor to Alzheimer's. The researchers
applied their protocol to 47 plasma samples taken from MCI patients at
their time of diagnosis. Of 22 MCI patients who went on to develop AD
2-5 years later, 20 were correctly identified as AD-positive. The test
also correctly classified as non-AD all eight MCI patients who
subsequently went on to develop other dementias. Of the 17 patients
still with an MCI diagnosis, the test predicts that seven will go on
to develop AD. "The reason we don't believe this is an artifact is,
first of all, that we have a blinded test for Alzheimer's and control,
and then we use a completely independent MCI data set. It would be
unlikely that the data would all fall into place like this," suggested
Wyss-Coray.
Nevertheless, he does acknowledge caveats. The main one concerns the
sample sets, since the researchers were not able to always match AD
samples perfectly with controls. One reason for this is that many
clinics do not collect blood for plasma, but instead freeze it or use
it for serum, explained Wyss-Coray. He said that plasma must be
immediately prepared from the serum and then frozen. "The ideal study
would be to have five centers with each center having 20 cases and
control matches. Like every scientific study, this work needs to be
reproduced," he said.
That task will fall to Satoris Inc., which plans to develop an AD
diagnostic. Wyss-Coray, for his part, plans to focus on basic biology.
"What I'm interested in most, now, is looking at normal aging," he
said. He will compare plasma patterns from normal humans and mice to
see if there are any similarities, and plans to study AD mouse models.
He also wants to expand the scope to investigate Down syndrome and
other dementias and increase the panel of 120 proteins to around 500.
Galasko also thinks expanding the test will be worthwhile. "Tony has
done a reasonably good job of coming up with the 120, based on assays
that were available at the time, but there may be more signaling
proteins that can be measured since then, and it may certainly be
worth trying to replicate or extending these studies with a broader
network," he said.
One of the most interesting questions coming from this work might be
what the panel of 18 proteins is saying about the disease process. "I
was surprised that there were quite a few colony stimulating factors
in the panel," said Wyss-Coray. Levels of granulocyte colony
stimulating factor (G-CSF), macrophage-colony stimulating factor (M-
CSF), and interleukin 3 were all reduced in AD plasma. Wyss-Coray said
that there are reports that all of these are neuroprotective and that
some of them may promote neurogenesis. In fact, just this year a group
from Taiwan reported that G-CSF can rescue cognitive decline in a
mouse model of AD (see Tsai et al., 2007). "I find it so interesting
that traditional immune molecules may have unexpected functions in the
brain," said Wyss-Coray. G-CSF is currently undergoing clinical trials
in Germany for stroke.
Could this panel of 18 proteins serve as the basis for a diagnostic
test? Satoris Inc. is planning to reproduce the study. "We will put
this on a real clinical platform and do it again with additional
hundreds of samples and come out with a researcher test that can be
used by pharma, research institutions etc.," said Patrick Lynn,
president and CEO of the company. Lynn said Satoris is currently
working with the Memory and Aging Center at UCSF and other
institutions to develop prospective sample collections, and also wants
to look at retrospective collections. He said after the "research use
only" test, they are planning to develop a CLIA- approved test that
could be used by clinicians. (The CLIA, or Clinical Laboratory
Improvement Amendments, program, run by the Center for Medicare and
Medicaid Services, regulates laboratory testing done on humans in the
U.S.; see http://www.cms.hhs.gov/clia/). Ultimately, the company wants
to develop a diagnostic test that would be approved by the FDA. "It is
hard to predict when that might get approval," said Lynn, "but our
hope is that sometime in 2009 we would like to have FDA approval." The
CLIA test could come as early as next year.
A blood test for AD would have advantages over more complex tests,
such as measuring cerebrospinal fluid markers or running magnetic
resonance imaging or PET scans. "One could envision plasma being used
as a screening test to trigger PIB, or MRI or CSF analysis," suggested
Galasko. Wyss-Coray agreed that it would not be a stand-alone test for
AD but could trigger further testing, much like prostate-specific
antigen measurements can trigger biopsies or other testing for
prostate cancer.
Some question the value of a diagnostic test when a treatment is yet
to be found that slows or halts the progression of Alzheimer disease.
It might be worth keeping in mind that plasma cholesterol measurements
were around long before statins. A blood diagnostic for AD could help
drug development.-Tom Fagan.
Reference:
Ray S, Britschgi M, Herbert C, Takeda-Uchimura Y, Boxer A, Blennow K,
Friedman LF, Galasko DR, Jutel M, Karydas A, Kaye JA, Leszek J, Miller
BL, Minthon L, Quinn JF, Rabinovici GD, Robinson WH, Sabbagh MN, So
YT, Sparks DL, Tabaton M, Tinklenberg J, Yesavage JA, Tibshirani R,
Wyss-Coray T. Classification and prediction of clinical Alzheimer
diagnosis based on plasma signaling proteins.