Shannon Reilly
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Derek Dockter
26 July, 2015
Cluster 7: Biomedical Sciences
Prions: Evolution of Animal Use and Lab Practices to Advance Research
Prion-caused diseases have been a widely known plague of mankind for close
to a century, and research and discoveries continue to this day. The structure
and molecular processes of these infectious agents are still relatively
unknown, although the constant evolution of lab practices and methods, as
well as developments in the animals used for testing have allowed for new
understanding. This paper tracks this progress, from end-point titrations to
study the "scrapie agent" in sheep and goats, to incubation-time assays in
transgenic mice.
In 1997, Stanley B. Prusiner accepted the Nobel Prize in the category of Physiology or
Medicine for his groundbreaking research on prions. The award marked recognition by the
greater scientific community of decades of research by many scientists, and studies and
discoveries continue to this day. A prion is a proteinaceous infectious particle, causing central
nervous system (CNS) degenerative disorders in both humans and several other mammals. These
include Creutzfeldt-Jakob Disease (CJD), scrapie, and kuru. Throughout years of research
projects and experiments, methods used to study prion structures and processes, including both
the tests themselves, and the animals these tests were performed upon, have evolved.
Over a nearly five decade period spanning from 1920 to 1968, CJD posed a medical
mystery to physicians, as an early, widespread prion caused disease, with no clue as to its cause.
Then in 1959, three scientists - Igor Klatzo, Carleton Gajdusek, and Vincient Zigas - made an
important observation, when they suggested that the neuropathology of kuru, a disease of New
Guinean Highlanders, was very similar to that of CJD. Later that same year, William Hadlow
observed that kuru was also similar to scrapie, a disease which mainly affects sheep and goats.
The latter idea was more impactful, because Hadlow also suggested that, like scrapie, kuru is a
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transmissible disease. His idea was to perform a demonstration of infectivity using chimpanzees,
because they are closely related to humans, who suffer from CJD. In 1965, Gajdusek, along with
two other scientists did precisely that, and scrapie became "a subject of intense medical interest."
The string of discoveries came full circle in the following few years, when CJD was shown to be
caused by a transmissible agent as well. (Prusiner 1987, 2004)
As scientists narrowed the focus of their research on the so-called "scrapie agent" that
they believed would unlock CJD and Kuru, it became evident that better methods must be
developed for purifying it from the brain matter of hosts. At first, the common method was to
use bioassays of the original carriers of scrapie - namely sheep and goats. The problem was, after
inoculation, the disease had an "incubation time" (period of health before onset of clinical
symptoms) of one to three years, and an entire herd must be tested to achieve a single sample,
because sheep and goats do not consistently develop scrapie, although the latter proved to be
better subjects in terms of consistency. Under these circumstances, it was expensive and
inefficient to test the many available hypotheses regarding prion structure. In a study reported in
1966, an entire herd of goats was required to measure the concentration of prions in just one
sample. In fact, so few studies were completed that research stood at a general standstill for
many years. (Prusiner 1987, 2004)
Later, an important transition in scientists' ability to perform purification studies was the
transmission of scrapie from the original sheep and goats to mice. They were easier to deal with,
and the incubation time of the disease was shorter, by four to five months. Proving that scrapie
could be successfully inoculated in small laboratory rodents opened up the possibility for many
new experiments that were impractical in previous studies on sheep and goats. However, studies
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still took as long as a year, and there were not many labs gathering bioassays to make much
progress. (Prusiner 1987, 2004)
Then, someone came up with an even better idea - golden Syrian hamsters. The hamster
strain of scrapie produced especially high concentrations of prions in their brains. This was very
beneficial, especially in combination with a incubation-time less than that of mice by a factor of
nearly six - only seventy days were required instead of the roughly three hundred and sixty
necessary when working with mice. A third advantage of working with hamsters was that only 4
animals needed to be used, in comparison to the sixty mice required for the previous end-point
titrations, thus allowing for a greater number of possible parallel experiments. These factors
made them the ideal animal for biochemical studies on the nature of the scrapie agent, although
there were several drawbacks to the use of hamsters in the lab, namely the small number of
inbred hamster strains, that hamsters are more expensive to purchase and tend to, and the lack of
methods for transfer and removal of their genes. (Prusiner 1987, 2004)
Some time later, production of transgenic (Tg) mice created an even more well adapted
animal model, with even shorter incubation times. Tg mice are mice which over express mouse
or Syrian hamster prion-protein (PrP) genes. The expression of mutant and foreign PrP
transgenes in these mice has created a new bank of previously unattainable knowledge regarding
prions and their structures, including a clearer understanding of their molecular mechanisms and
the molecular basis of prion strains. Some key studies involving these mice have demonstrated
how molecular genetics in combination with inbred and congenic strains of mice are able to
change the length of incubation time, which then allows for the selection of Tg mice that offer a
shorter time, in order to increase efficiency of results (sometimes less than twenty days for
bioassays). Tg mice are also especially useful for overcoming what is known as the "species
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barrier" in prion research. When the PrP sequence of the recipient animal differs from that of the
donor prion, the incubation time is lengthened. Therefore, when prions from one species are
given to another, experiments take extra time. Even with their modifications, Tg mice still suffer
from this issue (renamed the "Transmission Barrier," because the genes are technically the same
between donor species and the mice), but a 2002 study on the emergence of new prion strains
has demonstrated no effects on incubation time. (Prusiner 1987, 2004; Prusiner et. al. 2002)
For many prion purification studies, the method used to quantify prion concentrations
was endpoint titration. In this process, sixty mice were inoculated in groups of six with gradually
more diluted prion samples, then were monitored for clinical signs of disease. It was an
expensive and tedious process - it took roughly one year to complete one study - but was
considered an advance over previous methods, allowing for much new knowledge about the
physicochemical properties of prions and the pathogenesis of disease. Despite these benefits,
endpoint titration was prone to problems. Accurate measurements were often difficult to achieve
because ten serial dilutions had to be pipetted for a single experiment. In addition, the year long
gap between study planning and achieving results made it inconvenient to perform sequential
studies. Finally, the large number of animals required to create a single sample in one
experiment discouraged large research projects in which multiple studies were performed sideby-side. Some of the issues regarding the number of animals required, and the length of time
needed to chart results were alleviated with the use of hamsters in later years, but the cost
increases when using them, and thus a limited number of endpoint titrations on hamsters have
been performed. (Prusiner 1987, 2004)
Frustrated by the "cumbersome, slow, and expensive" process that was using endpoint
titrations, scientists - including Prusiner - began to research the correlation between incubation
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times and titers of scrapie prions in a sample. These studies showed that concentrations could be
predicted with equal or greater accuracy than endpoint titration by using measurements of time
intervals, leading to the practice of a method called the incubation-time assay. This newer
practice reduced animal populations required, the length of studies, and eliminated the risk of
pipetting errors. Combined with the use of hamsters, the incubation-time assay markedly
accelerated studies on the structure of the scrapie agent. As few as four animals were required,
and if the prion titer exceeded a particular threshold , only about seventy days were needed for a
positive response. (Prusiner 1987, 2004)
In conclusion, the methods of prion research, and the types of animals used have evolved
over time, along with scientists' knowledge of prion structures and mechanisms. Studies on
transmission connected several prion-caused diseases and allowed discoveries about the socalled scrapie agent to also reveal key points about CJD and kuru. Then, the gradual shift from
sheep and goats to Tg mice through various other rodent species allowed for more accurate and
efficient research, compounded by the development of endpoint titration and then incubationtime assays. (Prusiner 1987, 2004)
Even today, the structure of prions is not entirely understood - meaning that there is no
real treatment or prevention - and prion-caused diseases are considered one hundred percent
fatal. Treatments involving gene therapy in combination with cell therapy, as well as treatments
involving the use of immunization are undergoing research, but any type of effective therapy
appears to be the next horizon scientists are aiming for in their ongoing prion research.
(Ramachandran 2014)
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Bibliography
Prusiner, Stanley B., ed. Prion Biology and Diseases. 2nd ed. Cold Spring Harbor: Cold
Spring Laboratory, 2004. Print.
Prusiner, Stanley B., and M. P. McKinley, eds. Prions: Novel Infectious Pathogens Causing
Scrapie and Creutzfeldt-Jakob Disease. London: Academic, 1987. Print.
Peretz, David, and Prusiner, Stanley B., et. al. "A Change in the Conformation of Prions
Accompanies the Emergence of a New Prion Strain." ScienceDirect. Institute for
Neurodegenerative Diseases, San Francisco, CA 94143 USA, et. al., 13 June 2002.
Web. 23 July 2015.
<http%3A%2F%2Fwwwsciencedirectcom%2Fscience%2Farticle%2Fpii%2FS08966273
02007262>
Ramachandran, Tarakad S. "Prion-Related Diseases Treatment & Management."
Prion-Related Diseases Treatment & Management: Medical Care, Consultations.
Medscape, 27 Oct 2014. Web. 23 July 2015.
<http://emedicinemedscapecom/article/1168941-treatment>