Tracing mad cow to human infection
Case
research maps how illness makes cross-species jump
By John Mangels
Plain Dealer Science Writer
Monday, April 19, 2004
New research by Cleveland scientists may help explain how the infectious
proteins implicated in illnesses such as mad cow disease are able to
penetrate the natural barriers between species.
The study, by Case Western Reserve University researchers, shows it is
relatively easy for a mammal protein called a prion to morph into a
stealthier strain capable of outwitting the species firewall.
All it takes is exposure to a fragment of an abnormal prion protein from a
third species, a process called "seeding," the Case team reports in the
current edition of the journal "Molecular Cell."
The new prion strain has the same chemical fingerprint as before, but a
different three-dimensional shape. This altered profile is like a fugitive's
disguise.
Apparently, the altered profile is what enables the rogue prion to escape
recognition and slip past the defenses meant to stop species from swapping
illnesses.
Scientists have suspected such a process allowed sheep-tainted cow prions to
infect humans beginning in England in 1995. But they lacked a molecular
model demonstrating how prions might have created new strains, enabling them
to jump the species barrier.
The Case study provides that, several prion experts say. Along with similar
recent research in yeast, it also largely rebuts the long-debated idea that
something other than or in addition to a prion - a virus, perhaps - causes
the family of mad-cow-like illnesses in animals and humans.
The Case experiments were done in test tubes, using prions from mice,
hamsters and humans. The study's authors caution that they have not yet
determined if the outcome would be the same in lab animals, whose biology is
much more complex.
"There might be other components in the body that . . . make it much more
difficult for those [species] barriers to be crossed," said senior author
Witold Surewicz, a professor in Case's department of physiology and
biophysics.
Still, the prion's surprising ability to adapt and exploit a weakness in the
species barrier has major implications for the further spread of disease
that policy-makers shouldn't ignore, concluded the authors and several other
scientists.
"The bottom line is, if we don't tightly control these diseases, we're going
to regret it big time," said Dr. Pierluigi Gambetti, director of the
National Prion Disease Pathology Surveillance Center at Case. They "may come
in through the back door."
Gambetti, who was not involved in the study, is a member of the U.S. Food
and Drug Administration advisory committee on mad cow and related diseases.
He and others strongly advocate expanding the government's cattle-testing
program, which currently is limited to animals showing signs of illness.
A common cause
Researchers have known for decades of a seemingly related group of diseases
that progressively destroy brain tissue, causing dementia and death in
animals and humans.
The maladies strike hamsters, cats, sheep, cattle, deer, elk and people.
Some are inherited, others infectious (an epidemic of the brain-wasting
illness called kuru in the 1900s stemmed from New Guinea cannibals eating
their infected relatives). Still others appear to occur spontaneously.
Only in 1982 did University of California, San Francisco, researcher Stanley
Prusiner propose that the brain-wasting diseases had a common cause: a
misshapen protein he'd found in the brains of diseased hamsters. He called
it a "prion," a rough abbreviation of "proteinaceous infectious particle."
It was a radical notion, this idea that a nonliving speck of chemicals could
malevolently transform other prions, thus spreading within and between
bodies like an invading army.
Proteins, after all, are simpletons, nothing but long chains of amino acid
molecules folded into elaborate three-dimensional shapes that resemble the
bows on birthday presents. Unlike the traditional couriers of disease,
proteins lack the genetic instructions for copying themselves. They can't
split like a bacterium or hijack a cell's replication machinery like a
virus.
So how could a prion protein, which normally perches harmlessly on the
surface of a nerve cell, turn deadly and recruit its colleagues to wreak
havoc in the brain?
Prusiner theorized that something initially caused a normal prion protein in
an animal or human to refold into a harmful configuration. When this rogue
came into contact with a normal prion protein, it latched on and bullied its
impressionable partner into matching its aberrant shape. Gradually, lots of
these individual conversions resulted in dark, gummy clusters of prions,
rendering the brain a spongy mess.
Questions linger
Prusiner won a 1997 Nobel Prize for his discovery, but many questions
remained. Two were especially troubling:
How could prions made of the same amino acid ingredients, arranged in the
same order, form different strains that vary in infectiousness and symptoms?
It would be like being able to brew cups of Earl Grey and orange pekoe from
the same tea bag.
Since different prion strains had the same amino acid sequence, why didn't
species barriers keep them all out? Researchers had thought the barriers
worked primarily by reading prions' amino acid sequences, like a bouncer
checking patrons' driver's licenses. If that were true, none of the prions
should get in, since in effect they all had the same bad ID. But some
clearly sneaked past, as evidenced by the outbreak of mad cow disease in
humans beginning in the mid-1990s.
Scientists suspected the answer to both questions had to do with the
invading proteins' altered shape, and that the shape - and thus the strain -
evolved as prions passed through multiple species.
The Case team devised a test. They took mouse prions - which because of
species barriers shouldn't be able to corrupt hamster prions - and "seeded"
them by exposing them to a highly purified bit of infectious hamster prion.
The seed was so small, it didn't change the mouse prion's overall amino acid
sequence - its molecular ID. But it was enough to cause the mouse protein to
start acting like a renegade hamster prion, capable of bullying other
hamster prions into refolding.
So the seeding must have caused the mouse prion to reconfigure its
three-dimensional shape. The experiment showed prions with the same sequence
could fold in more than one way, with different properties, like a Swiss
army knife that can transform itself into a can opener, scissors or
screwdriver.
By changing the mouse prion's shape, the Case researchers had created a new
strain and enabled it to cross the species barrier.
Public health implications
The barrier, then, must rely more on shape recognition than onthe prion's
amino acid sequence. It can be fooled by shape differences, just as a
bouncer might turn away underage guys but admit an attractive girl even
though her ID shows the same birth date.
"It is reasonable to feel a little less safe," said Dr. Neil Cashman, a
neurologist at the University of Toronto's Center for Research in
Neurodegenerative Diseases. "This study points up the fact that our previous
reassurances are wrong. But it also provides a signal that much more science
is needed. There are many things we don't understand, and the whole science
of how prions propagate and cross species barriers is developing as we
speak."
Assuming the Case findings can be verified in lab animals, several
researchers said they are concerned that new prion strains may appear and
spread in the manner the study demonstrated.
For example, sheep prions aren't known to be infectious to humans. But that
could change if they were seeded by exposure to an abnormal cow prion passed
along in contaminated feed.
"We need to be considering all these events very, very cautiously," said
molecular biologist Giuseppe Legname, who is part of Prusiner's research
group in San Francisco. "It's important that better screening be put into
place so that what ends up on our plates is a safe product. I'd rather be
safe than sorry."
Although the research is sobering, Surewicz - one of the study's authors -
and others say it could also lead to drug strategies that stop the prion
from refolding into a harmful shape.
"We don't understand the process of conversion, but the normal form has to
be reinforced," Cashman said. "It's conceivable there are medicines that may
bolster it, rigidify it."
To reach this Plain Dealer reporter: jmangels@plaind.com, 216-999-4842