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SCIENCE & TECHNOLOGY
Prions—Still a Mystery!
By Max Sherman
In 1975, when Lewis Thomas, physician, scientist and medical writer, was asked to make a list
of the Seven Wonders of the Modern World, he
chose scrapie disease, now known to be caused
by a proteinaceous infectious particle (prion), as
number four.1 To quote Thomas, “The scrapie
agent seems the strangest thing in all biology.”
His reason was that the disease can propagate
from a few infectious units to billions in just
one year despite the absence of DNA or RNA.
Without DNA or RNA, there was a serious question regarding the mode with which the agent
replicated and survived. Prions, which even
in 1975 showed evidence of being all protein,
appeared to violate the central dogma of molecular biology: genetic information flows from
nucleic acids to proteins.2
Even though 35 years have passed since
Thomas’ list was first published, and he is no
longer living, prions remain one of nature’s wonders. In the past three decades, no hypothesis has
been proven to explain the protein-only composition of prions, their chemical composition and
the mechanism for their formation in the neurons
of infected hosts.3
This article reviews the major prion diseases,
regulatory matters pertaining to animal tissues
and sterilization methods. Despite the progression of science, prions still present a biological
conundrum and the risk to humans remains
uncertain.
History
Prions entered the scientific lexicon in 1982,
courtesy of Stanley Prusiner, a neurologist at the
University of California at San Francisco. He was
working on some esoteric maladies that were
known as “slow virus” diseases. Prusiner told
the scientific community that they were caused
by prions, a term he coined by blending “protein” and “infection.” (Logically, the word should
have been proin, but Prusiner, with melodious
intent, transposed the o and the i to make it more
terrific.4) Prion diseases, however, were noted
much earlier.
The earliest written record of scrapie in
English sheep first appeared in the 1730s, but the
disease was already prevalent in central Europe.
A kuru outbreak occurred in the 1950s, which
was observed only among the highland tribes
in New Guinea. In 1959, William J. Hadlow of
the Rocky Mountain Laboratory of the National
Institute of Allergy and Infectious Diseases suggested that scrapie and kuru might be related.
However, it was not until the 1960s that scientists
experimentally transmitted kuru to chimpanzees.
This demonstrated the transmissible nature of
prion diseases. Crueutzfeldt-Jakob disease (CJD)
was first described in the early 1920s and classified with other degenerative brain diseases,
the infectious nature of CJD was not established
until 1928.
Diseases
As mentioned above, prions are unconventional
infectious agents that cause rare but fatal neurological illnesses such as scrapie, kuru, CJD and
bovine spongiform encephalopathy (BSE).
Scrapie, the disease listed by Thomas, is an
infectious, neurodegenerative disorder affecting the central nervous system (CNS) of sheep.
(The name scrapie comes from the tendency of
afflicted sheep to scrape off much of their wool.)
It is a close cousin to BSE in cattle, chronic wasting disease in cervids (deer family), as well
as kuru, fatal and sporadic familial insomnia,
Gerstmann-Straussler syndrome and CJD and
variant CJD (vCJD) in humans. Each is a brain
disease marked exclusively by a deposition in
the CNS of prions.5
A unique feature of these diseases is that
they can have three different origins: sporadic,
inherited and infectious. The clinical epidemiological and neuropathological features can be
very different, but they are classified together
because the key molecular event appears to be
the same: a misfolding of the prion protein. The
damage is thought to occur when abnormal
prion protein (PrPSc) molecules gain access to the
brain and cause normal prion protein to change
shape to the abnormal form. (Normal prion
protein (PrPc) is encoded by a gene on human
chromosome 20, is expressed in the brain, and
in its normal form is probably a receptor.6) The
misshapen protein molecules clump together and
accumulate in brain tissue, causing a severe loss
of neurons, gliosis (excessive development of
neuroglia tissue) and a spongiform appearance.
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a German medical journal.9 The patient in the
report was a 22-year-old woman being treated
for a progressive dementing illness. Jakob
described four older patients with a clinically
similar presentation and course one year later,
and during the ensuing decades, numerous
cases of the disease were described clinically
and pathologically.10 In 1959, Klatzo et al noted
the neuropathological similarities between CJD
and kuru.11 That same year, Hadlow described
the similarities between kuru and scrapie, and
suggested that kuru might be transmissible to
animals after a long incubation period.12 During
the next decade, both kuru and CJD were transmitted to apes and monkeys.13
New vCJD differs dramatically from classic
CJD. In patients with vCJD, symptoms develop
at a mean age of 26 years—nearly four decades
earlier than patients with the classic sporadic
disease. The new variant was linked to exposure
to BSE, or “mad cow” disease, in British beef.
During the period from November 1986 (when
BSE was first identified as a separate disease
entity) until December 1995, an estimated 155,600
head of cattle in almost 33,000 herds were diagnosed with BSE in the UK. The epidemic peaked
in January 1993 at almost 1,000 new cases per
week. As of 2004, it was estimated that four million cattle were infected over the course of the
BSE epidemic and there were 155 vCJD cases
worldwide, all of whom have died. The only
affected US resident, whose probable vCJD case
was identified in 2002, was a 22-year-old woman
who had moved from the UK to Florida in 1992.14
Because of the long incubation period, cases due
to mad cow beef will likely surface well into the
future.15
All of the diseases have a long incubation period,
and lead to dementia and death. There is no
treatment and, thus, no cure.7
Kuru, the first human malady to be labeled
a slow virus disease, came to the attention of
Western doctors only in the 1950s: it was the
primary cause of death among the Fore, a cannibalistic New Guinea tribe. In 1966, Carleton
Gajdusek and Joe Gibbs proved that like scrapie,
kuru was infectious; chimps inoculated with
the brain tissue of human victims developed
kuru-like symptoms and died. The spread of the
disease was attributed to the endocannabalistic
funeral practices of the tribe in which relatives
prepared and consumed the tissues (including
the brain) of deceased family members.8 Male
members of the Fore tribe participated little, if
at all, in these feasts, with the result that kuru
at its peak predominantly affected women and
children.
Classic CJD is the most prevalent of the
spongiform diseases. It occurs spontaneously
in one out of a million people, 10% of which
are inherited mutations in the prion protein
gene (PRPN). The disease was first described
by Alfons Jakob in 1920 in a paper published in
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February 2011
Regulatory Issues
On 20 March 1996, the British government
announced that new information about BSE
suggested a possible relationship between the
disease and 10 cases of a newly identified form
of CJD in humans. The US Food and Drug
Administration (FDA), on 9 May 1996, issued
a notice to manufacturers of drug/biological/
device products recommending avoidance of
materials from cattle born, raised or slaughtered
in countries where BSE was known to exist. The
agency stated its belief that the rapid spread
in cattle was caused by feeding them certain
infected cattle and sheep tissues.
FDA has published two rules to protect
animals and consumers against BSE. The 1997
final regulation prohibited the use of most mammalian protein in feed given to cows, sheep and
goats, and required process and control systems
to ensure that feed does not contain the prohibited tissue. This rule, Title 21 Part 589.200,
called the Ruminant Feed Ban became effective on 4 August 1997. In 2008, FDA published
a regulation that strengthened the 1997 rule
by prohibiting the use of certain cattle-derived
materials that have the highest risk for carrying
the agent thought to cause BSE. These materials are the brains and spinal cords from cattle
30 months of age and older. The 2008 rule, Title
21 Part 589.2001, called the Enhanced Feed Ban,
became effective on 27 April 2009.
Sterilization
The transmissible agent of vCJD (PRPSc) is not
readily destroyed by conventional sterilization,
and transmissions by neurosurgical equipment including instruments and EEG depth
electrodes, and in recipients of pituitary growth
hormone, dura mater or corneal grafts have been
reported.16 Iatrogenic (resulting from the action
of a physician) transmission of the CJD agent has
been reported in more than 250 patients worldwide.17,18 Unfortunately, the incubation period of
vCJD is unknown but could be several decades,
thus it is unlikely that any iatrogenic cases have
yet emerged.19
Previous work indicated that prion protein
bound to steel wire was resistant to most conventional reprocessing procedures, including
enzymatic cleaning, fixatives, acidic treatments,
and autoclaving (steam at 134C for 18 minutes
combined with enzymatic cleaning). Because of
the unusual resistance of PRPSc, special decontamination procedures are needed to prevent
accidental transmission. Studies have been done
in an in vitro carrier assay using steel wires
contaminated with the disease-associated prion
protein and scrapie brain homogenates from
hamsters.20,21 The wires were implanted into the
brains of hamsters after treatment for decontamination and subsequently monitored for their
potential to trigger clinical disease or subclinical
cerebral PRPSc deposition within an observable
period of 500 days. The most effective treatment
consisted on incubating the wires in 1.0 M NaOH
for one hour at 23 C, 2.5% NaOCl for one hour
at 23C, an alkaline cleanser (used at a concentration of 1%) for 10 minutes at 55C or 0.2% sodium
dodecyl sulfate (SDS) /0.3% NaOH for 10 minutes at 23C led to complete decontamination
in terms of detectable infectivity, as well as to
efficient removal of residual brain material from
carriers.22 The authors mention that similar methods should be validated for human transmissible
spongiform encephalopathy agents on different
types of instrument surfaces. The most stringent
chemical and autoclave sterilization methods
for transmissible spongiform encephalopathies
are currently outlined in Annex III of the WHO
infection control guidelines.23
Final Thoughts
There is still much to be learned about the
mysterious and deadly prion. It was recently
reported that US researchers have discovered a
new form of prion disease that does not act like
related illnesses, such as mad cow disease, but
instead causes brain damage similar to that produced by Alzheimer’s disease. In this study, the
researchers examined mice that were genetically
engineered to process prion proteins in a unique
way.24 Then they exposed them to scrapie. The
treated mice did not develop holes in the brain
like those typically caused by prion diseases.
Instead, they developed plaques that resembled
a form of human Alzheimer’s. If a treatment is
found for this new form of prion disease, it may
also be useful in Alzheimer’s disease.
References
1.
Thomas L. A Long Line of Cells. Book of the Month Club,
New York, New York 1975.
2.
Griffin BE. “Unconventional viruses or prions.” BMJ
1985;290(issue?):1765-6.
3.
Suppattapone S. “What makes a prion infectious.”
Science. 2010;327:1091-2.
4.
Taubes G. “The game of the name is fame, but is it science?” Discover. 1986; 7(12):28-52.
5.
Lemmer K, et al. “Decontamination of surgical instruments from prions. II. In vivo findings with a model
system for testing the removal of scapie infectivity from
steel surfaces.” J Gen Virology. 2008;89(issue?):348-358.
6.
Harrison PJ,Roberts GW. “How now mad cow?” BMJ.
1992;304(issue?):929-30.
7.
Ibid.
8.
Kuru Information Page. NINDS website. http://www.
ninds.nih.gov/disorders/kuru/kuru.htm. Accessed 17
January 2011.
9.
Creutzfeldt HG. “Uber eine eigenartige herdformige
Erkrankung des Zentral-nervensystems.” Z Ges Neurol
Psychiatr. 1920;57(issue):1-18.
10. Bockman JM et al. “Creutzfeldt-Jakob disease prion proteins in human brains.” N Engl J Med. 1985;312(2):73-8.
11. Klatzo I et al. “Pathology of kuru.” Lab Invest
1959;8:799-847.
12. Hadlow WJ. “Scrapie and kuru.” Lancet. 1959;2:289-90.
13. Prusiner SB. “Prions and neurological diseases.” N Engl J
Med. 1987;317(25):1371-81.
14. Donnelly CA. “Bovine spongiform encephalopathy in the
United States—an epidemiologist’s view.” N Engl J Med
2004;350(6):539-42.
15. Prions on the trail of killer proteins. University of Utah
website. http://learn.genetics.utah.edu/content/begin/
dna/prions. Accessed 17 January 2011.
16. Zobeley E et al. “Infectivity of scrapie prions bound
to a stainless steel surface.” Molecular Medicine.
1999;5(issue):240-3.
17. Brown P. “Environmental causes of human spongiform
encephalopathy.” In: Baker H, Ridly RM, eds. Prion
Diseases., Totowa NJ, Human Press 1996:105-118.
18. Questions and Answers: Creutzfeldt-Jakob Disease
Infection-Control Practices. CDC website. http://www.
cdc.gov/ncidod/dvrd/cjd/qa_cjd_infection_control.htm.
Accessed 17 January 2011.
19. “Iatrogenic vCJD from surgical instruments” (editorial).
BMJ. 2001;322 (issue):1558-9.
20. Op cit 5.
21. Lemmer K et al. “Decontamination of surgical instruments
from prion proteins: in vitro studies on the detachment,
destabilization and degradation of PRPSc bound to steel
surfaces.” J Gen Virol. 2004;85(issue):3806-16.
22. Op cit 5.
23. WHO Infection Control Guidelines for Transmissible
Spongiform Encephalopathies. WHO website. http://
www.who.int/csr/resources/publications/bse/whocdscsraph2003.pdf. Accessed 17 January 2011.
24. Prion Disease in Mice May Help Advance Alzheimer’s.
Meridian Health Wellness Center website. http://wellnesscenter.meridianhealth.com/RelatedItems/6,636679.
Accessed 17 January 2011.
Author
Max Sherman is president, Sherman Consulting Services,
Warsaw, IN. He can be reached by email by contacting
[email protected].
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