Researchers Create an Artificial Prion
July 29, 2004
By SANDRA BLAKESLEE
Scientists are reporting that, for the first time, they
have made an artificial prion, or misfolded protein, that
can, by itself, produce a deadly infectious disease in mice
and may help explain the roots of mad cow disease.
The findings, being reported on Friday in the journal
Science, are strong evidence for the so-called "protein
only hypothesis," the controversial idea that a protein,
acting alone without the help of DNA or RNA, can cause
certain kinds of infectious diseases.
The concept was introduced in 1982 by Dr. Stanley Prusiner,
a neurology professor at the University of California in
San Francisco, and is still unsettling to many scientists
who have been taught that only bacteria and viruses
containing genetic information can spread infectious
Over the years, the idea that a misfolded protein, because
of its shape alone, could trigger an infectious disease,
has been gaining acceptance but it has never been
In this study, Dr. Prusiner said researchers had
successfully created a man-made prion and injected it into
a mouse brain and produced disease. Then, they took tissue
from the diseased mouse and injected it in another mouse,
which also got the same disease.
Dr. Prusiner said in a telephone interview that he was
"flabbergasted" that it had taken 22 years to prove the
hypothesis but that his lab was able to overcome earlier
technical difficulties with a new set of experiments. "We
have compelling evidence," he said. "We've done it all."
"I am thrilled" by this new study, said Dr. Peter T.
Lansbury, an associate professor of neurology at Harvard
Medical School and an expert on protein folding. "You can
never really prove a hypothesis but only try to disprove
it," he said. But with these findings, "we are about as
close as you can ever get to proving it."
Still, many said the evidence was insufficient.
Chesebro, chief of the Laboratory of Persistent Viral
Diseases at the Rocky Mountain Laboratory, said that the
experiment shows that the prion produced something in the
mice. But he said that it remains unclear whether the prion
is causing the infection or somehow just exposing an
underlying infectious process.
Dr. Laura Maneulidis, a neuropathologist at the Yale
University Medical School, one of Dr. Prusiner's most vocal
critics, said the prion strain that turned up in the
experiment looks like a mouse prion frequently used in Dr.
"Basically I think the data look like contamination," she
said, possibly stemming from "inadequately washed
Dr. Prusiner, who won the 1997 Nobel Prize in Medicine and
Physiology for his prion research, said some of his critics
will never be satisfied. "They'll say we need to do 10 more
years of experiments, using controls we didn't do," he
said. "It's just silly."
Dr. Prusiner predicted that the newly gleaned information
will lead to more effective ways to diagnose and treat a
family of deadly diseases called transmissible spongiform
encephalopathies, or TSEs, believed to be caused by
The stakes are enormous. Last week, a British citizen who
died from other causes was found to have been infected by a
human form of TSE from a routine blood transfusion. The
human form, called variant Creutzfedlt Jakob disease, is
contracted from eating cattle infected with mad cow
disease. At least two people who died from the variant
disease gave blood before falling ill, which means many
more Britons could be infected.
American agriculture officials are testing thousands of
cattle in an effort to determine if mad cow disease is a
problem in this country, but the tests are notoriously
imperfect. A deeper understanding of protein diseases
should lead to tests that can diagnose the disease even in
cattle that show no symptoms.
The biology of protein diseases is new and often difficult
to grasp, Dr. Prusiner said. He said he believed that many
proteins cause disease when they adopt an abnormal shape
and form toxic fibrils that create havoc in the brain or
body. Alzheimer's disease, Parkinson's disease, type two
diabetes and at least two dozen other human disorders may
be caused by misshapen proteins, according to Dr. Pruisner.
But as far as scientists have been able to determine only
one protein, the prion, leads to infectious diseases. That
is, infectious prions can be passed between and among
species through the eating of nervous tissue, exchange of
bodily fluids and by direct injection into the brain.
When Dr. Prusiner introduced the protein-only hypothesis,
he was greeted with skepticism. The test should be simple:
create a prion in a laboratory dish or in a bacterial
system far removed from animal cells that might harbor DNA.
Then add chemicals or heat to fold the artificial prion
into different shapes, including many sheet-like structures
seen in virtually all TSEs. Inject the misfolded man-made
prion into healthy animals. If such prions cause disease in
and of themselves, the animals should get sick.
The hallmark of a TSE is spongy holes and inflammation in
the brain. Some of the diseases also feature clumps of
fibrils called amyloid.
Laboratories around the world have tried countless
variations of this experiment for years, Dr. Prusiner said.
They never worked. The mice never got sick. But this time,
Dr. Prusiner says, the mice did get sick.
In hindsight, the reasons are now clear, Dr. Prusiner said.
In tinkering with artificial prions, researchers never knew
what part of the full length protein was responsible for
passing on the disease. They reasoned that an infectious
core was the culprit. When that core - a fragment of
misfolded protein - comes into contact with a similar
fragment in a healthy prion, the healthy prion is fatally
converted into the disease form. But what fragment? What
shapes cause the conversion? If that core could be
isolated, the experiments might work.
Nature offered a solution, Dr. Prusiner said. A few TSEs
produce huge amounts of amyloid composed of core prion
fragments which are easy to see. Researchers synthesized
healthy protein fragments similar to the amyloid. They
subjected the fragments to agents such as urea and vinegar
that alter protein shape and shook the solutions for 40
hours. Amyloids appeared. The man-made proteins were
cleansed and injected into the brains of transgenic mice
bred with prions that would recognize the fragments.
"We waited," Dr. Prusiner said. "And waited. At 300 days,
none of the animals got sick. We thought the experiment had
But over the next 200 days, every animal developed
spongiform degeneration, the scientist said. A brain
extract from a sick animal was injected into normal mice
with different prion structure. They too got sick.