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Volume 11, Issue 5: November 2004
Infection
by Protein: The Prion Theory of Disease
Sravisht Iyer, Science Journalist
Biomedical Eningeering, Johns Hopkins University
iyer@jyi.org
Take
anything you learned about genetics in high school and throw it out the
door. Incredibly, we may have to do so as we come to understand the prion theory of disease. Suggested to explain Mad Cow and
other related diseases, prions are an earth-shaking
idea, casting doubt on the status of DNA and RNA as the molecules of life. Not
a bacteria, not a fungi, not a virus, but a protein, the prion is a novel infectious agent. The theory that a
protein, a non-living object, can propagate disease has become a lightning-rod
for controversy and has made the prion hypothesis
one of the most hotly contested issues in molecular biology.
Mad cows, shedding sheep, and the birth of the prion theory
The prion theory of disease was created to explain Transmissible
Spongiform Encephalopathies (TSEs),
a group of fatal diseases that, as they progress, overwhelm neural cells
and riddle the brain with sponge-like holes.
TSEs have been found in various species, including sheep,
cows, elk and human beings. The disease was first observed by English shepherds
in the 18th century when their sheep acted abnormally, rubbing
up against any object they could find – scraping off most of their
skin and wool in the process. The disease, appropriately named scrapies, was devastating, often requiring the
sacrifice of entire herds.
Human
variants of TSEs are extremely rare, limited mainly
to a neurodegenerative disease called Kuru found
in cannibalistic Pacific Islanders, and Creutzfeldt-Jakob Disease (CJD), a
disorder diagnosed in one person per million. Both these diseases share
common symptoms, including gait disorders, jerky movements, and dementia that
lead to death months after the first appearance of symptoms.
Throughout
the 1970s and much of the 1980s, the prevailing view was that TSEs were caused by a “slow virus,” a virus
that had a long incubation period. No such virus, however, had ever been
purified from the brains of animals that had died from the disease.
When
Stanley B. Prusiner, a researcher at the University of California at San Francisco, obtained a pure sample of
infectious material, he began working to identify the disease causing
agent. When Prusiner added enzymes that destroyed
DNA and RNA, he found no change in infectivity. Adding protein-neutralizing
enzymes to this cocktail, however, caused a sharp drop in infectivity. This
observation set the stage for a theory of disease transmission that Prusiner admitted was “heretical” when he
suggested it. In a 1982 Science paper,
Prusiner introduced the world to the idea of a proteinaceous infectious particle – or “prion” as he dubbed it.
Transformers! Robots
in disguise: how prions infect the body
The
idea that a protein alone could be an infectious agent remains a
contentious issue.
“It
has been an extremely difficult scientific question to nail down,”
said Byron Caughey, a senior investigator
National Institutes of Health's Rocky Mountain Laboratories in Hamilton, Montana. “The complete nature
of the theory remains unknown and some of the fundamental issues are
unresolved.”
Researchers
believe that at the heart of TSEs is the conversion
of a particular protein (known as the Prion-related
Protein, or PrP) from a normal (PrPC) to an abnormal, “scrapie-cell” (PrPSC),
shape. Both forms of the protein consist of the same building blocks but
different final products – just like the Transformers toys that could
assume two shapes, one benign and the other aggressive. In the prion theory of disease, a PrPSC
recruits and re-shapes a PrPC to match
its own form, overwhelming neural cells that eventually explode and expose
neighboring cells to more PrPSC. As more
cells die of infection, they leave behind the spongy, holey brain that is a
hallmark of TSEs.
Dr. Jekyll to Mr. Hyde: How PrPs
transform
Many
lines of evidence support the belief that proteins alone are responsible
for the transformation of PrPC to PrPSC. Among the most important are the facts
that the gene encoding PrP can be found in many
species (including humans) and that mice lacking this gene fail to succumb
to the disease. Two significant observations can be made based on this data.
Firstly, the fact that the body can produce PrP naturally
means that no virus is required to supply the PrPSC
that triggers the cascade leading to TSE. Secondly, the fact that PrPSC fails to infect animals lacking normal
PrP suggests that the key to the disease
is the interaction between the two forms of the protein.
Critics
of the prion theory, however, were not satisfied
by these experiments, claiming that samples of pure protein had never been
shown to cause infection in live animals. The lack of such a finding left
the door open for the theory that TSEs were
caused by bacteria or viruses. In research published in the July 30, 2004 issue of Science,
however, Prusiner’s group showed, for the
first time, that synthetically created proteins can infect mice.
“A
great deal of evidence indicates that prions are
composed only of protein,” says the lead author of the study,
Giuseppe Legname, a researcher working in the Prusiner lab, “but this is the first time that
this has been directly shown in mammals. The challenge in the last few
years has been to figure out exactly how to demonstrate that prions are made entirely of protein.”
When
they infected mice with synthetic protein, the researchers observed no
signs of sickness for 300 days. They were ready to throw in the towel when,
on the 380th day, one of the mice showed signs of sickness. By
the 660th day, all infected mice had become ill.
While
admitting these results represent a key step, Caughey
cautioned that they must be taken with a grain of salt. Noting that the
results could be attributed to low-level contamination, Caughey
added, “In order to settle the question, they need to have all the
controls and generate robust, highly-tittered infectious proteins.”
Critics
of prion diseases have also proposed their own
theory to refute the protein-only view supported by Prusiner’s
lab. Among the most vociferous critics of prion
disease has been Dr. Laura Manuelidis, a neurophysiologist
at Yale School of Medicine. She has published several papers indicating an
immune response in subjects suffering from CJD. This response, she argues,
would not be present if the infectious agent was the body’s own
protein.
Another
researcher who has presented dissenting views is Dr. Frank Bastian of Tulane University in New Orleans. Dr. Bastian found traces of a bacteria called spiroplasma
in animals and humans suffering from TSE, and he believes that these
bacteria, not aberrant proteins, are causing the disease.
Are we there yet?
Questions yet to be answered
Recent
work has found some key evidence to support the prion
hypothesis. There remain, however, several crucial
questions about the prion theory that must be
answered before statements about it can be made with any certainty.
How
is PrPC converted to PrPSC? How do prions
reach the brain after entering the body? How does PrPSC
lead to neurodegeneration? What is the function
of PrP in the body?
With
so many key questions yet to be addressed, there is still a long way to go
for the prion hypothesis, and controversy is sure
to follow every step of the way.
References
and Suggested Reading
Guyer RL. Prions: Puzzling Infectious Proteins.
NIH - Research in the News. <http://science-education.nih.gov/nihHTML/ose/snapshots/multimedia/ritn/prions/prions1.html>
Taubes G. The Game of the Name is Fame. But is it Science? Discover. December,
1986. Reprinted at: http://www.slate.com/id/2096/sidebar/42786/.
Prusiner SB. (1995).The Prion Diseases. Scientific American. (http://www.sciam.com/article.cfm?articleID=0009FD80-C3C6-1C5A-B882809EC588ED9F&pageNumber=1&catID=2)
Prusiner SB. (1982). Novel proteinaceous infectious
particles cause scrapie. Science. 9;216(4542):136-44.
Legname G, Baskakov IV, Nguyen HB, et al. (2004). Synthetic Mammalian Prions. Science.
7;305:673-676.
Johnston N. (2004). Clearing Hurdles: Prions Know How to Do It. The Scientist. 18(11):18.
(http://www.the-scientist.com/yr2004/jun/feature_040607.html)
Bastian FO, Foster JW. Spiroplasma SP. (2001).
16S rDNA in Creutzfeldt-Jakob disease and scrapie as shown by PCR AND DNA sequence analysis. J Neuropathol
Exp Neurol. 60:613-620.
Aguzzi A, Polymenidou M. (2004). Mammalian Prion Biology: One Century of Evolving Concepts. Cell. 116(2):313-327.
(http://www.sciencedirect.com/science/article/B6WSN-4BJK582-H/2/70143387edac888b362f04571b6a7988)
Journal of Young
Investigators. 2004. Volume Eleven.
Copyright © 2004 by Sravisht Iyer
and JYI. All rights reserved.
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