http://www.sciencemag.org/cgi/content/full/293/5537/1974
Volume 293, Number 5537, Issue of
14 Sep 2001, pp. 1974-1977.
A wealth of evidence suggests that
pathogens may play a role--perhaps even a causal one--in chronic diseases
like Alzheimer's or MS. Proving that theory, however, is another matter.
In the 1970s, epidemiologists documented
remarkably high rates of multiple sclerosis (MS) on the isolated Faeroe
Islands in the North Atlantic. MS is nothing new to medicine, but it was
new to the Faeroe Islands: There was no sign of it there before the 1940s.
The epidemiologists found that the disease got its start during an outbreak
coinciding with the arrival of British soldiers during World War II.
That an influx of visitors can trigger
an outbreak is not unusual. But for it to trigger this specific disease
was, because MS was generally thought to be a chronic condition brought
on by genetic factors and perhaps a defective immune system.
But if MS is caused by a germ, what
germ is it? That's a question that scientists have been asking about a
growing list of chronic diseases that were once thought to be mainly a
matter of genes or lifestyle. Since the 1970s, epidemiological clues have
emerged for other diseases, and in some cases, scientists have been able
to make a persuasive case for specific bugs.
In the 1980s, for example, researchers
discovered that bacteria cause ulcers and that certain viruses trigger
cancer. And in the past 2 decades, dozens of pathogens have been implicated
in a range of diseases, from Alzheimer's to arthritis. Recently, some biologists
have argued that evolutionary theory predicts that all but the rarest chronic
diseases must be caused by infections.
But despite many exciting hints,
researchers are a long way from clinching the argument. Alzheimer's disease,
MS, and schizophrenia offer three cautionary lessons. Every step of the
research is fraught with controversy, from isolating the pathogens to determining
how they might cause the disease to sorting out how a host's genetic profile
influences the course of disease. And because these illnesses are chronic,
scientists have to confront frustrating questions about cause and effect
that don't come up with acute illnesses like Ebola or the mumps. Is the
pathogen the cause of a particular disease or just a late-coming bystander?
And what if two or more pathogens are implicated in the same chronic disease?
Are both the cause, or neither? "There is a real chicken-and-egg problem
here," says Stephen Reingold, vice president of research at the National
Multiple Sclerosis Society.
A humbling lesson
For researchers who suspect that
pathogens lie behind many chronic diseases, ulcers are the great inspiration.
In 1981, a young Australian gastroenterologist named Barry Marshall learned
of a mysterious bacterium lurking in the stomachs of patients. Over the
next few years, he discovered that people suffering from ulcers often carried
the microbe, which came to be known as Helicobacter pylori. Defying decades
of conventional wisdom, Marshall speculated that the bacterium--and not
acid or stress--might actually cause ulcers.
To test his idea, Marshall swallowed
a broth full of H. pylori, and sure enough, he soon developed gastritis,
the prelude to ulcers. Marshall cured himself with antibiotics, and subsequently,
he and his co-workers successfully treated a number of people suffering
from ulcers, clearly pinning the bacterium as the culprit. Other researchers
have since shown that H. pylori infects perhaps one-third of all people,
causing not only ulcers but gastric cancers as well.
"All of us have been humbled by the
Helicobacter story," says Subramaniam Sriram of Vanderbilt University in
Nashville, Tennessee. "It's made us look at infectious agents once again."
Robert Yolken of John Hopkins University agrees: "The Helicobacter model
is the big success story." But it was not the only one. At about the same
time that Marshall was swigging H. pylori, other researchers were finding
some of the first compelling evidence that cancers could also be triggered
by viruses. Hepatitis B was associated with liver cancer, for example,
while human papillomaviruses were linked to cervical cancer.
For other chronic diseases, however,
the evidence is little more than circumstantial. Multiple sclerosis, for
example, sometimes strikes its victims more like an epidemic than a genetic
disorder, as it did in the Faeroes. Similarly, schizophrenia has signs
of being triggered by infections during pregnancy. It is more likely to
strike people born in cities than on farms and to strike people born in
winter (when the flu and other diseases are common) than other times of
the year.
Even without decisive evidence, some
biologists argue that pathogens must cause most common chronic diseases.
Foremost among these advocates is Paul Ewald, an evolutionary biologist
at Amherst College in Massachusetts. Ewald suggests that people with chronic
diseases ought to leave fewer children and grandchildren behind to propagate
their genes than do healthy individuals. And so genetic disorders should
gradually reduce themselves to minuscule levels. (The only exceptions would
be disorders that are balanced by some benefit provided by the same genes,
as in the case of sickle cell anemia, which is linked to protection from
malaria.)
Purely genetic disorders, Ewald contends,
can't cause more than about 1 death in 10,000. "That's the point at which
you'd be able to barely maintain a genetic disease," he says. "When you
get above that, you know that something must be maintaining it."
To Ewald, that something is most
likely a pathogen, because it can cause a chronic disease without paying
this evolutionary penalty. As long as the pathogen can escape to new hosts
before its own dies, it can continue to create sickness. Humans may evolve
better defenses against a parasite, but the parasite can respond in kind,
evolving new tricks for getting around them.
Ewald has captured public attention,
with features on his work appearing in magazines like Newsweek and Atlantic
Monthly. But he has had a mixed reception among specialists in chronic
diseases. "Infection may be important at a broad level, but so is starvation,"
says Paul Ridker of Brigham and Women's Hospital in Boston. Although he
finds the theory intellectually stimulating, Ridker argues that it is no
substitute for detailed research. Perhaps not surprisingly, scientists
who are investigating possible infectious causes tend to be positive. "Generally,
I think Ewald's right," says Alan Hudson of Wayne State University in Detroit,
Michigan. But even champions of pathogens like Barry Marshall, now at the
University of Western Australia in Crawley, concede that "so far nothing
looks as good as H. pylori--and most of the leads have been rather weak."
Mysterious MS
Take MS, a disease in which immune
system cells attack the insulating sheath of myelin that surrounds neurons
in the brain and spine. As the disease progresses, its victims typically
lose their muscle coordination, speech control, and eyesight. More than
300,000 people in the United States alone suffer from the disease--many
more than evolutionary theory would predict if it were strictly a genetic
disorder. Epidemiological studies also hint at a pathogen. People who migrate
before age 15 from MS hot spots (such as Australia and Ukraine) to places
with low rates are less likely to contract the disease than are those who
stay behind. That decline is consistent with a scenario in which MS is
caused by an infection that strikes in adolescence.
Experimental studies likewise hint
that a pathogen could cause MS if its own proteins resembled myelin. Once
the immune system became primed to attack the invader, it might inadvertently
ravage the myelin as well. Indeed, in the July 2001 issue of the Journal
of Clinical Investigation, a team of immunologists at Northwestern University
Medical School in Chicago described creating symptoms resembling MS in
a mouse by using an infectious agent. They injected the mouse with a normally
harmless virus--but to which they had added a gene from a bacterium, Haemophilus
influenzae, that makes a myelinlike protein. The mouse's immune system
quickly became primed to attack the engineered virus; within 2 weeks its
myelin was under assault as well.
But this sort of research enables
scientists to create animal models of MS--not to identify the actual culprit
in humans. Over the years, scientists have prowled for pathogens that could
cause this sort of mimicry in association with MS, and they've found no
shortage of candidates, 17 microorganisms in all. But today researchers
are focusing the hunt for an MS agent on two ubiquitous pathogens: a virus
and a bacterium that were both discovered just 15 years ago.
Human herpesvirus 6 (HHV-6) usually
infects people when they are just a few months old, causing a sizable fraction
of the fevers experienced by babies. After causing a brief illness in its
host, the virus goes into hiding and may lie dormant for the rest of the
host's life. But researchers have found that it sometimes becomes active
again. Patients who receive bone marrow or organ transplants are often
plagued by reactivated HHV-6, possibly because their immune systems are
compromised.
Researchers studied this connection
in 1995 at the Pathogenesis Corp. in Seattle, Washington (now part of Chiron),
finding evidence of HHV-6 in the brains of several dozen people with MS.
In these patients, the viruses were producing proteins, and they lurked
close to the myelin of their hosts. In people who did not suffer from MS,
the virus was there, but researchers found little evidence that it was
active.
Since then, several groups have pursued
this lead with distinctly mixed results. Donald Carrigan and Konstance
Knox of the Institute for Viral Pathogenesis in Milwaukee, Wisconsin, published
results last year showing that 56% of patients with MS had active HHV-6
in their brains, whereas healthy subjects had none. But several other teams
have failed to find the virus in people with MS, while a paper this January
in the Journal of Medical Virology found HHV-6 in one-third of healthy
people's brains.
Carrigan and Knox dispute those negative
findings, arguing that other researchers did not check carefully enough
to see whether the virus was active or dormant in the brains. That hasn't
been enough to sway many herpes experts, however. "I am frankly rather
skeptical about the possible link between HHV-6 and MS," says Steven Dewhurst
of the University of Rochester in New York. Just this May, he had more
reason for doubt: At the annual meeting of the American Academy of Neurology,
a Swedish team reported treating people with MS with valacyclovir, an antiviral
medication. It produced no change in the symptoms.
Another suspect in MS is the bacterial
scourge Chlamydia pneumoniae (see table). Its cousin, C. trachomatis, is
notorious as a sexually transmitted disease and this year was implicated
in cervical cancer. C. pneumoniae invades the lungs, where it sometimes
causes respiratory diseases. It can then settle into a host's body for
decades, living quietly in white blood cells. Like HHV-6, C. pneumoniae
is practically universal. Just about everyone becomes its victim at some
point.
C. pneumoniae was first suspected
to play a role in heart disease. Shortly after its discovery in 1986, researchers
encountered it lurking in the coronary blood vessels. Several teams also
found that people with heart disease were more likely to have antibodies
to C. pneumoniae than were healthy individuals. Later research has shown
that the bacteria actually live in the lesions associated with atherosclerosis.
In 1999, scientists reported that a protein made by C. pneumoniae closely
resembles one found in heart muscle. As it tries to attack the bacteria,
the immune system may attack the heart as well, creating the inflammation
that may cause atherosclerosis (Science, 26 February 1999, p. 1335). Two
clinical trials are now under way to see whether antibiotics can lessen
further damage in people with heart disease. But many researchers still
doubt that the connection is real. "The data are actually weaker than people
think," says Ridker.
In 1998, Vanderbilt's Sriram reported
that he had found C. pneumoniae in yet another part of the body: in the
cerebrospinal fluid of a man with MS. Sriram subsequently looked at other
people with MS and reported that genetic profiling revealed that 97% of
them had the DNA of C. pneumoniae in the fluid, while only 18% of the controls
did.
Sriram's results raised the possibility
that C. pneumoniae could trick the immune system into attacking myelin
just as it attacked heart tissue, or at least make a bad situation worse
by aggravating the inflammation. Animal experiments suggest there might
be something to this idea. In the August 2001 Journal of Immunology, Hudson
of Wayne State and his colleagues reported that when they injected a protein
from C. pneumoniae into the brains of rats, it produced remarkably MS-like
symptoms. "His is a very seminal paper," says Sriram.
But when other researchers tried
to confirm Sriram's results, many of them failed. Skepticism has been running
high, and in April 2001, the title of a review in Trends in Microbiology
bluntly summed up the feeling of many researchers: "Chlamydia pneumoniae
and multiple sclerosis; no significant association."
Sriram disputes this finding, noting
that other labs used different methods than his and might have missed the
bacteria. To resolve the issue, he and the other researchers agreed to
conduct a blind test of cerebrospinal fluid from the same set of MS patients
and controls. Sriram found Chlamydia in 73% of the people with MS and 23%
of those without. The other three labs found no evidence of Chlamydia at
all.
Ewald is among Sriram's defenders,
arguing that his methods are more sensitive than those of other labs. "A
scientific response to the test would be, 'Well, it looks like the Vanderbilt
group was right after all!' " he claims. But Sriram concedes that the debate
is open: "I hope that physicians will view this as a debate that's ongoing
and not an observation that's being finalized." If the association is real,
he notes, it's possible that the bacteria only arrive in the brain after
MS has already begun. "Having this infection on top of the preexisting
damage may be harmful," he suggests. "The ultimate answer would be a successful
clinical trial showing that when you eliminate the agent, you eliminate
the disease." As a small step in that direction, Sriram is running a trial
on MS patients with antibiotics.
Analyzing Alzheimer's
C. pneumoniae plays an equally controversial
role in the debate over Alzheimer's disease. Theoretically, Chlamydia is
a compelling candidate for a causal agent. In Alzheimer's patients, protein
clumps appear in the brain and neurons become tangled; researchers suspect
that inflammation is a key ingredient in this recipe. "One of the things
Chlamydia does better than anything is elicit inflammation," says Hudson.
"If they are in the brain, they are causing inflammation."
In 1998, Hudson and his colleagues
reported genetic evidence of C. pneumoniae in the brains of 17 out of 19
Alzheimer's patients. Meanwhile, 18 out of 19 healthy people tested negative.
When the researchers examined the diseased brain tissue, they found evidence
of the bacteria in the very regions of the brain that had been damaged.
Once again, other labs tried to confirm
Hudson's results. Two failed to find any bacteria, and a third obtained
results that were ambiguous at best. "The interest in pursuing associations
between Chlamydia and Alzheimer's has lost a great deal of steam," claims
Robert Ring of Wyeth-Ayerst Neurosciences in Princeton, New Jersey, one
of the researchers who failed to find a link.
Hudson, however, is suspicious of
the methods used in the studies. In two out of three cases, the scientists
tried to find Chlamydia in brains preserved in paraffin instead of fresh
tissue. "It's pretty erratic getting stuff from paraffin-fixed samples,"
says Hudson. And he also maintains that the bacteria exist at low levels
that can be missed if researchers don't run enough tests. "If you come
up negative, how do you know you didn't just miss the DNA?" he asks.
New research bolsters his case. At
an international Chlamydia meeting last August in Helsinki, two teams reported
finding Chlamydia in fresh tissue of numerous Alzheimer brains and in almost
none of the healthy brains.
One of the complicating factors in
the search for chronic bugs is that many of the best candidates, like Chlamydia
and HHV-6, are widespread. Far more people carry them than develop the
diseases in question, which suggests that other factors, such as the genes
of their hosts, must play a role. Ruth Itzhaki of the University of Manchester
Institute of Science and Technology in the U.K. is exploring the pathogen-gene
relationship in her work on Alzheimer's.
Itzhaki has found preliminary evidence
linking another herpesvirus, herpes simplex virus type 1 (HSV1), to Alzheimer's.
As with HHV-6 and C. pneumoniae, most people become infected with HSV1
at some point. The virus lurks primarily in the nerves surrounding the
mouth, and in 20% to 40% of its hosts, it causes occasional cold sores.
Among young people, HSV1 is entirely absent from the brain, Itzhaki's team
has found. But it is often present in the brains of elderly people. Itzhaki
suspects that the virus sneaks into the brain as the immune system declines
with age. Itzhaki's team has found that 63% of elderly people carry the
virus, while 74% of elderly people with Alzheimer's do.
This small difference between the
two groups might suggest that HSV1 is a minor risk factor for Alzheimer's.
But the genetic evidence suggests otherwise, says Itzhaki. Those of her
subjects who carried a gene variant called ApoE4, a known risk factor for
Alzheimer's, as well as the herpesvirus, were much more likely to have
Alzheimer's than were people with either the gene or the virus alone (Science,
15 May 1998, p. 1002). She concluded that the combined risk accounts for
the disease in 60% of the 61 cases her team has examined.
Itzhaki speculates that people with
ApoE4 may not be able to repair cell damage caused as the virus triggers
inflammation in the brain. Stopping the virus from getting into the brain
might be one way to fight the disease. "Vaccines against HSV1 might prevent
Alzheimer's, at least in some cases," says Itzhaki. But so far, no one
has tested that proposition. Still, Itzhaki has earned some admirers. "I
think there's significant merit in her work," says Keith Crutcher of the
University of Cincinnati. "This type of work is notoriously difficult,
and I think her studies have been carefully conducted."
Genetic interplay
Hopkins's Yolken and his colleagues
have been exploring whether reawakened retroviruses might somehow be involved
in schizophrenia, as some are known to cause brain damage. They searched
for retroviral DNA in the cerebrospinal fluid of 35 people who had recently
developed schizophrenia. As they reported in the 10 April Proceedings of
the National Academy of Sciences, genetic material from one type of retrovirus,
HERV-W, turned up in 29% of schizophrenics, whereas none was found in the
cerebrospinal fluid of healthy people or even people with other neurological
disorders. One hypothesis that Yolken and his colleagues are now pursuing
is that these retroviruses are unleashed in certain individuals before
they are born, altering the development of their brains in ways that don't
become clear until adulthood.
In a report to appear in the November
Archives of General Psychiatry, Yolken and his colleagues report on a potential
trigger for these retroviruses. The researchers sifted through the records
of a study known as the Collaborative Perinatal Project, in which thousands
of pregnancies were monitored between 1959 and 1966. During the project,
blood samples were taken from the mothers, and the health of their children
was followed for 7 years. The group tracked down 27 subjects who had developed
schizophrenia and other psychotic illnesses as adults. They revisited their
mothers' blood samples, measuring levels of antibodies to various pathogens.
They then measured the same antibodies from mothers of healthy subjects,
using two controls for each psychotic subject who were born during the
same time of the year and were of the same race and gender.
For five out of six pathogens, the
researchers found no significant association with psychosis. But one did
pass the test: HSV2--the sexually transmitted form of HSV, which causes
genital sores. Women with signs of infection with the virus when they were
pregnant were more likely to give birth to children who would later develop
schizophrenia and other forms of psychosis.
Yolken points out that HSV2 is a
compelling candidate for triggering schizophrenia--and a treatable one
at that: "We know they're capable of activating retroviruses, we know they're
capable of replicating in the brain, and we know that there are treatments
that are available."
Yolken concedes that even with a
study that spans 40 years in molecular detail, he is far from proving that
a particular pathogen causes schizophrenia. As with other chronic diseases,
it defies the classic standards for recognizing infectious diseases articulated
by Robert Koch in 1882: showing that the pathogen is present in all victims
suffering from a specific disease but not in healthy people, for example,
and that an isolated pathogen can cause the same disease in a new host.
"We've solved the easy problems"--identifying the agents that cause many
acute infectious diseases--says Ewald. He argues that for uncovering the
pathogens that may lie at the root of chronic diseases, Koch's postulates
should not be the guiding factor. "We're just not going to get the kind
of evidence for causation as we do for acute infections. There's just no
way. If you're dealing with a disease where the symptoms take 5 decades
to develop, how are you going to get an animal model of that?"
But Yolken and other researchers
trying to show a link generally believe that they have to come as close
to the classical methods as possible if they are to convince their medical
colleagues. Says Yolken: "In this day and age, when we have good treatments,
you have to show that when you remove the agent, you get a change in the
disease to have people to believe it."
Carl Zimmer is the author of Evolution:
Triumph of an Idea.
Copyright © 2001 by The American
Association for the Advancement of Science
Carl Zimmer*
In some diseases, the line between
pathogens and the genes of their hosts may be nearly indistinguishable.
Certain kinds of viruses, known as endogenous retroviruses, paste their
DNA into their host cells. If one of them should infect a cell destined
to become a sperm or egg, the virus will infect every cell in the body
of the person it gives rise to. It will also be handed down to subsequent
generations. Endogenous retroviruses make up an estimated 1% of the human
genome, but most of their sequences have mutated into harmless nonsense.
Still, some endogenous retroviruses may be able to come back to life (often
during fetal development), harnessing their host's genes to make new viruses
that can invade new cells.