'Lifeless' Prions Capable of
Evolutionary Change and Adaptation
Scientists from The Scripps Research Institute have determined
for the first time that prions, bits of infectious protein devoid
of DNA or RNA that can cause fatal neurodegenerative disease,
are capable of evolving.
The study from Scripps Florida in Jupiter shows that prions can
develop large numbers of mutations at the protein level causing
such evolutionary adaptations as drug resistance, a phenomenon
previously known to occur only in bacteria and viruses. These
breakthrough findings also suggest that the normal prion protein
-- which occurs naturally in human cells -- may prove to be a
more effective therapeutic target than its abnormal toxic relation.
The study was published in the December 31, 2009 issue of the
journal Science Express, an advance, online edition of the journal
"On the face of it, you have exactly the same process of
mutation and adaptive change in prions as you see in viruses,"
said Charles Weissmann, M.D., Ph.D., the head of Scripps Florida's
Department of Infectology, who led the study. "In viruses,
mutation is linked to changes in nucleic acid sequence that leads
to resistance. Now, this adaptability has moved one level down
-- to prions and protein folding -- and it's clear that you do
not need nucleic acid for the process of evolution."
Infectious prions (short for proteinaceous infectious particles)
are associated with some 20 different diseases in humans and animals,
including mad cow disease and a rare human form, Creutzfeldt-Jakob
disease. All these diseases are untreatable and eventually fatal.
Prions, which are composed solely of protein, are classified by
distinct strains, originally characterized by their incubation
time and the disease they cause. Prions have the ability to reproduce,
despite the fact that they contain no nucleic acid genome.
Mammalian cells normally produce cellular prion protein or PrPC.
During infection, abnormal or misfolded protein -- known as PrPSc
-- converts the normal host prion protein into its toxic form
by changing its conformation or shape. The end-stage consists
of large assemblies (polymers) of these misfolded proteins, which
cause massive tissue and cell damage.
"It was generally thought that once cellular prion protein
was converted into the abnormal form, there was no further change,"
Weissmann said. "But there have been hints that something
was happening. When you transmit prions from sheep to mice, they
become more virulent over time. Now we know that the abnormal
prions replicate, and create variants, perhaps at a low level
initially. But once they are transferred to a new host, natural
selection will eventually choose the more virulent and aggressive
In the first part of the study, Weissmann and his colleagues
transferred prion populations from infected brain cells to culture
cells. When transplanted, cell-adapted prions developed and out-competed
their brain-adapted counterparts, confirming prions' ability to
adapt to new surroundings. When returned to brain, brain-adapted
prions again took over the population.
To confirm the findings and to explore the issue of evolution
of drug resistance, Weissmann and his colleagues used the drug
swainsonine or swa, which is found in plants and fungi, and has
been shown to inhibit certain prion strains. In cultures where
the drug was present, the team found that a drug-resistant sub-strain
of prion evolved to become predominant. When the drug was withdrawn,
the sub-strain that was susceptible to swainsonine again grew
to become the major component of the population.
Weissmann notes that the findings have implications for the development
of therapeutic targets for prion disease. Instead of developing
drugs to target abnormal proteins, it could be more efficient
to try to limit the supply of normally produced prions -- in essence,
reducing the amount of fuel being fed into the fire. Weissmann
and his colleagues have shown some 15 years ago that genetically
engineered mice devoid of the normal prion protein develop and
function quite normally (and are resistant to prion disease!).
"It will likely be very difficult to inhibit the production
of a specific natural protein pharmacologically," Weissmann
said, "You may end up interfering with some other critical
physiological process, but nonetheless, finding a way to inhibit
the production of normal prion protein is a project currently
being pursued in collaboration with Scripps Florida Professor
Corinne Lasmezas in our department."
Another implication of the findings, according to the study,
is that drug-resistant variants either exist in the prion population
at a low level prior to exposure or are generated during exposure
to the drug. Indeed, the researchers found some prions secreted
by infected cells were resistant to the drug before exposure,
but only at levels less than one percent.
The scientists show that prion variants constantly arise in a
particular population. These variants, or "mutants,"
are believed to differ in the way the prion protein is folded.
As a consequence, prion populations are, in fact, comprised of
This, Weissmann noted, is reminiscent of something he helped
define some 30 years ago -- the evolutionary concept of quasi-species.
The idea was first conceived by Manfred Eigen, a German biophysicist
who won the Nobel Prize in Chemistry in 1967. Basically stated,
a quasi-species is a complex, self-perpetuating population of
diverse and related entities that act as a whole. It was Weissmann,
however, who provided the first confirmation of the theory through
the study of a particular bacteriophage -- a virus that infects
bacteria -- while he was director of the Institut für Molekularbiologie
in Zürich, Switzerland.
"The proof of the quasi-species concept is a discovery we
made over 30 years ago," he said. "We found that an
RNA virus population, which was thought to have only one sequence,
was constantly creating mutations and eliminating the unfavorable
ones. In these quasi-populations, much like we have now found
in prions, you begin with a single particle, but it becomes very
heterogeneous as it grows into a larger population."
There are some unknown dynamics at work in the prion population
that leads to this increased heterogeneity, Weissmann added, that
still need to be explored.
"It's amusing that something we did 30 years has come back
to us," he said. "But we know that mutation and natural
selection occur in living organisms and now we know that they
also occur in a non-living organism. I suppose anything that can't
do that wouldn't stand much of a chance of survival."
The joint first authors of the Science study are Jiali Li and
Shawn Browning of The Scripps Research Institute. Other authors
include Sukhvir P. Mahal and Anja M. Oelschlegel also of The Scripps
Research Institute. Weissmann notes that after the manuscript
was accepted by Science, an article by Ghaemmanghami et al. appeared
in PLoS Pathogens that described emergence of prions resistant
to a completely different drug, quinacrine, providing additional
support to the Scripps Research team's conclusions.
The Scripps Research study was supported by a grant from the
National Institutes of Health and by a generous donation to the
Weissmann laboratory from the Alafi Family Foundation.
January 4, 2010