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A neurologist has presented some novel ideas on the cause of some
neurological disorders. These thoughts were published in the August 1996
edition of Scientific American Magazine.



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The Prion Diseases

 Prions, once dismissed as an impossibility, have now gained wide
 recognition as extraordinary agents that cause a number of
 infectious, genetic and spontaneous disorders

           by Stanley B. Prusiner


  Fifteen years ago I evoked a good deal of skepticism
  when I proposed that the infectious agents causing
  certain degenerative disorders of the central nervous
  system in animals and, more rarely, in humans might
  consist of protein and nothing else. At the time, the
  notion was heretical. Dogma held that the conveyers of
  transmissible diseases required genetic material,
  composed of nucleic acid (DNA or RNA), in order to
  establish an infection in a host. Even viruses, among the
  simplest microbes, rely on such material to direct
  synthesis of the proteins needed for survival and
  replication.

  Later, many scientists were similarly dubious when my
  colleagues and I suggested that these "proteinaceous
  infectious particles"-or "prions," as I called the
  disease-causing agents-could underlie inherited, as well
  as communicable, diseases. Such dual behavior was
  then unknown to medical science. And we met
  resistance again when we concluded that prions
  (pronounced "pree-ons") multiply in an incredible way;
  they convert normal protein molecules into dangerous
  ones simply by inducing the benign molecules to change
  their shape.

  Today, however, a wealth of experimental and clinical
  data has made a convincing case that we are correct on
  all three counts. Prions are indeed responsible for
  transmissible and inherited disorders of protein
  conformation. They can also cause sporadic disease, in
  which neither transmission between individuals nor
  inheritance is evident. Moreover, there are hints that the
  prions causing the diseases explored thus far may not be
  the only ones. Prions made of rather different proteins
  may contribute to other neurodegenerative diseases that
  are quite prevalent in humans. They might even
  participate in illnesses that attack muscles.

  The known prion diseases, all fatal, are sometimes
  referred to as spongiform encephalopathies. They are so
  named because they frequently cause the brain to
  become riddled with holes. These ills, which can brew
  for years (or even for decades in humans) are
  widespread in animals.

  The most common form is scrapie, found in sheep and
  goats. AfBicted animals lose coordination and eventually
  become so incapacitated that they cannot stand. They
  also become irritable and, in some cases, develop an
  intense itch that leads them to scrape off their wool or
  hair (hence the name "scrapie"). The other prion
  diseases of animals go by such names as transmissible
  mink encephalopathy, chronic wasting disease of mule
  deer and elk, feline spongiform encephalopathy and
  bovine spongiform encephalopathy. The last, often
  called mad cow disease, is the most worrisome.

  Gerald A. H. Wells and John W. Wilesmith of the
  Central Veterinary Laboratory in Weybridge, England,
  identified the condition in 1986, after it began striking
  cows in Great Britain, causing them to became
  uncoordinated and unusually apprehensive. The source
  of the emerging epidemic was soon traced to a food
  supplement that included meat and bone meal from dead
  sheep. The methods for processing sheep carcasses had
  been changed in the late 1970s. Where once they would
  have eliminated the scrapie agent in the supplement, now
  they apparently did not. The British government banned
  the use of animal-derived feed supplements in 1988, and
  the epidemic has probably peaked. Nevertheless, many
  people continue to worry that they will eventually fall ill
  as a result of having consumed tainted meat.

  The human prion diseases are more obscure. Kuru has
  been seen only among the Fore highlanders of Papua
  New Guinea. They call it the "laughing death." Vincent
  Zigas of the Australian Public Health Service and D.
  Carleton Gajdusek of the U.S. National Institutes of
  Health described it in 1957, noting that many highlanders
  became aUeicted with a strange, fatal disease marked
  by loss of coordination (ataxia) and often later by
  dementia. The affected individuals probably acquired
  kuru through ritual cannibalism: the Fore tribe reportedly
  honored the dead by eating their brains. The practice
  has since stopped, and kuru has virtually disappeared.

  Creutzfeldt-Jakob disease, in contrast, occurs
  worldwide and usually becomes evident as dementia.
  Most of the time it appears sporadically, striking one
  person in a million, typically around age 60. About 10 to
  15 percent of cases are inherited, and a small number
  are, sadly, iatrogenic-spread inadvertently by the
  attempt to treat some other medical problem. Iatrogenic
  Creutzfeldt-Jakob disease has apparently been
  transmitted by corneal transplantation, implantation of
  dura mater or electrodes in the brain, use of
  contaminated surgical instruments, and injection of
  growth hormone derived from human pituitaries (before
  recombinant growth hormone became available).

  The two remaining human disorders are
  Gerstmann-Str...ussler-Scheinker disease (which is
  manifest as ataxia and other signs of damage to the
  cerebellum) and fatal familial insomnia (in which
  dementia follows diUculty sleeping). Both these
  conditions are usually inherited and typically appear in
  midlife. Fatal familial insomnia was discovered only
  recently, by Elio Lugaresi and Rossella Medori of the
  University of Bologna and Pierluigi Gambetti of Case
  Western Reserve University.

  In Search of the Cause

  I first became intrigued by the prion diseases in 1972,
  when as a resident in neurology at the University of
  California School of Medicine at San Francisco, I lost a
  patient to Creutzfeldt-Jakob disease. As I reviewed the
  scientific literature on that and related conditions, I
  learned that scrapie, Creutzfeldt-Jakob disease and kuru
  had all been shown to be transmissible by injecting
  extracts of diseased brains into the brains of healthy
  animals. The infections were thought to be caused by a
  slow-acting virus, yet no one had managed to isolate the
  culprit.

  In the course of reading, I came across an astonishing
  report in which Tikvah Alper and her colleagues at the
  Hammersmith Hospital in London suggested that the
  scrapie agent might lack nucleic acid, which usually can
  be degraded by ultraviolet or ionizing radiation. When
  the nucleic acid in extracts of scrapie-infected brains
  was presumably destroyed by those treatments, the
  extracts retained their ability to transmit scrapie. If the
  organism did lack DNA and RNA, the finding would
  mean that it was not a virus or any other known type of
  infectious agent, all of which contain genetic material.
  What, then, was it? Investigators had many
  ideas-including, jokingly, linoleum and kryptonite-but no
  hard answers.

  I immediately began trying to solve this mystery when I
  set up a laboratory at U.C.S.F. in 1974. The first step
  had to be a mechanical one-purifying the infectious
  material in scrapie-infected brains so that its composition
  could be analyzed. The task was daunting; many
  investigators had tried and failed in the past. But with the
  optimism of youth, I forged ahead [see "Prions," by
  Stanley B. Prusiner; SCIENTIFIC AMERICAN,
  October 1984]. By 1982 my colleagues and I had made
  good progress, producing extracts of hamster brains
  consisting almost exclusively of infectious material. We
  had, furthermore, subjected the extracts to a range of
  tests designed to reveal the composition of the
  disease-causing component.

  Amazing Discovery

  All our results pointed toward one startling conclusion:
  the infectious agent in scrapie (and presumably in the
  related diseases) did indeed lack nucleic acid and
  consisted mainly, if not exclusively, of protein. We
  deduced that DNA and RNA were absent because, like
  Alper, we saw that procedures known to damage
  nucleic acid did not reduce infectivity. And we knew
  protein was an essential component because procedures
  that denature (unfold) or degrade protein reduced
  infectivity. I thus introduced the term "prion" to
  distinguish this class of disease conveyer from viruses,
  bacteria, fungi and other known pathogens. Not long
  afterward, we determined that scrapie prions contained
  a single protein that we called PrP, for "prion protein."

  Now the major question became, Where did the
  instructions specifying the sequence of amino acids in
  PrP reside? Were they carried by an undetected piece
  of DNA that traveled with PrP, or were they, perhaps,
  contained in a gene housed in the chromosomes of cells?
  The key to this riddle was the identification in 1984 of
  some 15 amino acids at one end of the PrP protein. My
  group identified this short amino acid sequence in
  collaboration with Leroy E. Hood and his co-workers at
  the California Institute of Technology.

  Knowledge of the sequence allowed us and others to
  construct molecular probes, or detectors, able to
  indicate whether mammalian cells carried the PrP gene.
  With probes produced by Hood's team, Bruno Oesch,
  working in the laboratory of Charles Weissmann at the
  University of Zurich, showed that hamster cells do
  contain a gene for PrP. At about the same time, Bruce
  Cheseboro of the NIH Rocky Mountain Laboratories
  made his own probes and established that mouse cells
  harbor the gene as well. That work made it possible to
  isolate the gene and to establish that it resides not in
  prions but in the chromosomes of hamsters, mice,
  humans and all other mammals that have been examined.
  What is more, most of the time, these animals make PrP
  without getting sick.

  One interpretation of such findings was that we had
  made a terrible mistake: PrP had nothing to do with
  prion diseases. Another possibility was that PrP could
  be produced in two forms, one that generated disease
  and one that did not. We soon showed the latter
  interpretation to be correct.

  The critical clue was the fact that the PrP found in
  infected brains resisted breakdown by cellular enzymes
  called proteases. Most proteins in cells are degraded
  fairly easily. I therefore suspected that if a normal,
  nonthreatening form of PrP existed, it too would be
  susceptible to degradation. Ronald A. Barry in my
  laboratory then identified this hypothetical
  protease-sensitive form. It thus became clear that
  scrapie-causing PrP is a variant of a normal protein. We
  therefore called the normal protein "cellular PrP" and the
  infectious (protease-resistant) form "scrapie PrP." The
  latter term is now used to refer to the protein molecules
  that constitute the prions causing all scrapielike diseases
  of animals and humans.

  Prion Diseases Can Be Inherited

  Early on we had hoped to use the PrP gene to generate
  pure copies of PrP. Next, we would inject the protein
  molecules into animals, secure in the knowledge that no
  elusive virus was clinging to them. If the injections
  caused scrapie in the animals, we would have shown
  that protein molecules could, as we had proposed,
  transmit disease. By 1986, however, we knew the plan
  would not work. For one thing, it proved very difficult to
  induce the gene to make the high levels of PrP needed
  for conducting studies. For another thing, the protein that
  was produced was the normal, cellular form.
  Fortunately, work on a different problem led us to an
  alternative approach for demonstrating that prions could
  transmit scrapie without the help of any accompanying
  nucleic acid.

  In many cases, the scrapielike illnesses of humans
  seemed to occur without having been spread from one
  host to another, and in some families they appeared to
  be inherited. (Today researchers know that about 10
  percent of human prion diseases are familial, felling half
  of the members of the affected families.) It was this last
  pattern that drew our attention. Could it be that prions
  were more unusual than we originally thought? Were
  they responsible for the appearance of both hereditary
  and transmissible illnesses?

  In 1988 Karen Hsiao in my laboratory and I uncovered
  some of the earliest data showing that human prion
  diseases can certainly be inherited. We acquired clones
  of a PrP gene obtained from a man who had
  Gerstmann-Str...ussler-Scheinker disease in his family
  and was dying of it himself. Then we compared his gene
  with PrP genes obtained from a healthy population and
  found a tiny abnormality known as a point mutation.

  To grasp the nature of this mutation, it helps to know
  something about the organization of genes. Genes
  consist of two strands of the DNA building blocks called
  nucleotides, which differ from one another in the bases
  they carry. The bases on one strand combine with the
  bases on the other strand to form base pairs: the "rungs"
  on the familiar DNA "ladder." In addition to holding the
  DNA ladder together, these pairs spell out the sequence
  of amino acids that must be strung together to make a
  particular protein. Three base pairs together-a unit called
  a codon-specify a single amino acid. In our dying
  patient, just one base pair (out of more than 750) had
  been exchanged for a different pair. The change, in turn,
  had altered the information carried by codon 102,
  causing the amino acid leucine to be substituted for the
  amino acid proline in the man's PrP protein.

  With the help of Tim J. Crow of Northwick Park
  Hospital in London and Jurg Ott of Columbia University
  and their colleagues, we discovered the same mutation in
  genes from a large number of patients with
  Gerstmann-Str...ussler-Scheinker disease, and we
  showed that the high incidence in the affected families
  was statistically significant. In other words, we
  established genetic linkage between the mutation and the
  disease-a finding that strongly implies the mutation is the
  cause. Over the past six years work by many
  investigators has uncovered 18 mutations in families with
  inherited prion diseases; for five of these mutations,
  enough cases have now been collected to demonstrate
  genetic linkage.

  The discovery of mutations gave us a way to eliminate
  the possibility that a nucleic acid was traveling with prion
  proteins and directing their multiplication. We could now
  create genetically altered mice carrying a mutated PrP
  gene. If the presence of the altered gene in these
  "transgenic" animals led by itself to scrapie, and if the
  brain tissue of the transgenic animals then caused scrapie
  in healthy animals, we would have solid evidence that the
  protein encoded by the mutated gene had been solely
  responsible for the transfer of disease. Studies I
  conducted with Hsiao, Darlene Groth in my group and
  Stephen J. DeArmond, head of a separate laboratory at
  U.C.S.F., have now shown that scrapie can be
  generated and transmitted in this way [see box on pages
  56 and 57].

  These results in animals resemble those obtained in
  1981, when Gajdusek, Colin L. Masters and Clarence
  J. Gibbs, Jr., all at the National Institutes of Health,
  transmitted apparently inherited
  Gerstmann-Str...ussler-Scheinker disease to monkeys.
  They also resemble the findings of Jun Tateishi and
  Tetsuyuki Kitamoto of Kyushu University in Japan, who
  transmitted inherited Creutzfeldt-Jakob disease to mice.
  Together the collected transmission studies persuasively
  argue that prions do, after all, represent an
  unprecedented class of infectious agents, composed only
  of a modified mammalian protein. And the conclusion is
  strengthened by the fact that assiduous searching for a
  scrapie-specific nucleic acid (especially by Detlev H.
  Riesner of Heinrich Heine University in DYesseldorf)
  has produced no evidence that such genetic material is
  attached to prions.

  Scientists who continue to favor the virus theory might
  say that we still have not proved our case. If the PrP
  gene coded for a protein that, when mutated, facilitated
  infection by a ubiquitous virus, the mutation would lead
  to viral infection of the brain. Then injection of brain
  extracts from the mutant animal would spread the
  infection to another host. Yet in the absence of any
  evidence of a virus, this hypothesis looks to be
  untenable.

  In addition to showing that a protein can multiply and
  cause disease without help from nucleic acids, we have
  gained insight into how scrapie PrP propagates in cells.
  Many details remain to be worked out, but one aspect
  appears quite clear: the main difference between normal
  PrP and scrapie PrP is conformational. Evidently, the
  scrapie protein propagates itself by contacting normal
  PrP molecules and somehow causing them to unfold and
  Bip from their usual conformation to the scrapie shape.
  This change initiates a cascade in which newly converted
  molecules change the shape of other normal PrP
  molecules, and so on. These events apparently occur on
  a membrane in the cell interior.

  We started to think that the differences between cellular
  and scrapie forms of PrP must be conformational after
  other possibilities began to seem unlikely. For instance, it
  has long been known that the infectious form often has
  the same amino acid sequence as the normal type. Of
  course, molecules that start off being identical can later
  be chemically modified in ways that alter their activity.
  But intensive investigations by Neil Stahl and Michael A.
  Baldwin in my laboratory have turned up no differences
  of this kind.

  One Protein, Two Shapes

  How, exactly, do the structures of normal and scrapie
  forms of PrP differ? Studies by Keh-Ming Pan in our
  group indicate that the normal protein consists primarily
  of alpha helices, regions in which the protein backbone
  twists into a specific kind of spiral; the scrapie form,
  however, contains beta strands, regions in which the
  backbone is fully extended. Collections of these strands
  form beta sheets. Fred E. Cohen, who directs another
  laboratory at U.C.S.F., has used molecular modeling to
  try to predict the structure of the normal protein based
  on its amino acid sequence. His calculations imply that
  the protein probably folds into a compact structure
  having four helices in its core. Less is known about the
  structure, or structures, adopted by scrapie PrP.

  The evidence supporting the proposition that scrapie
  PrP can induce an alpha-helical PrP molecule to switch
  to a beta-sheet form comes primarily from two
  important studies by investigators in my group. Mar'a
  Gasset learned that synthetic peptides (short strings of
  amino acids) corresponding to three of the four putative
  alpha-helical regions of PrP can fold into beta sheets.
  And Jack Nguyen has shown that in their beta-sheet
  conformation, such peptides can impose a beta-sheet
  structure on helical PrP peptides. More recently Byron
  W. Caughey of the Rocky Mountain Laboratories and
  Peter T. Lansbury of the Massachusetts Institute of
  Technology have reported that cellular PrP can be
  converted into scrapie PrP in a test tube by mixing the
  two proteins together.

  PrP molecules arising from mutated genes probably do
  not adopt the scrapie conformation as soon as they are
  synthesized. Otherwise, people carrying mutant genes
  would become sick in early childhood. We suspect that
  mutations in the PrP gene render the resulting proteins
  susceptible to Bipping from an alpha-helical to a
  beta-sheet shape. Presumably, it takes time until one of
  the molecules spontaneously Bips over and still more
  time for scrapie PrP to accumulate and damage the brain
  enough to cause symptoms.

  Fred Cohen and I think we might be able to explain why
  the various mutations that have been noted in PrP genes
  could facilitate folding into the beta-sheet form. Many of
  the human mutations give rise to the substitution of one
  amino acid for another within the four putative helices or
  at their borders. Insertion of incorrect amino acids at
  those positions might destabilize a helix, thus increasing
  the likelihood that the affected helix and its neighbors will
  refold into a beta-sheet conformation. Conversely,
  Hermann Sch...tzel in my laboratory finds that the
  harmless differences distinguishing the PrP gene of
  humans from those of apes and monkeys affect amino
  acids lying outside of the proposed helical
  domains-where the divergent amino acids probably
  would not profoundly inBuence the stability of the helical
  regions.

  Treatment Ideas Emerge

  No one knows exactly how propagation of scrapie PrP
  damages cells. In cell cultures, the conversion of normal
  PrP to the scrapie form occurs inside neurons, after
  which scrapie PrP accumulates in intracellular vesicles
  known as lysosomes. In the brain, filled lysosomes could
  conceivably burst and damage cells. As the diseased
  cells died, creating holes in the brain, their prions would
  be released to attack other cells.

  We do know with certainty that cleavage of scrapie PrP
  is what produces PrP fragments that accumulate as
  plaques in the brains of some patients. Those aggregates
  resemble plaques seen in Alzheimer's disease, although
  the Alzheimer's clumps consist of a different protein. The
  PrP plaques are a useful sign of prion infection, but they
  seem not to be a major cause of impairment. In many
  people and animals with prion disease, the plaques do
  not arise at all.

  Even though we do not yet know much about how PrP
  scrapie harms brain tissue, we can foresee that an
  understanding of the three-dimensional structure of the
  PrP protein will lead to therapies. If, for example, the
  four-helix-bundle model of PrP is correct, drug
  developers might be able to design a compound that
  would bind to a central pocket that could be formed by
  the four helices. So bound, the drug would stabilize
  these helices and prevent their conversion into beta
  sheets.

  Another idea for therapy is inspired by research in which
  Weissmann and his colleagues applied gene-targeting
  technology to create mice that lacked the PrP gene and
  so could not make PrP. By knocking out a gene and
  noting the consequences of its loss, one can often
  deduce the usual functions of the gene's protein product.
  In this case, however, the animals missing PrP displayed
  no detectable abnormalities. If it turns out that PrP is
  truly inessential, then physicians might one day consider
  delivering so-called antisense or antigene therapies to the
  brains of patients with prion diseases. Such therapies
  aim to block genes from giving rise to unwanted proteins
  and could potentially shut down production of cellular
  PrP [see "The New Genetic Medicines," by Jack S.
  Cohen and Michael E. Hogan; SCIENTIFIC
  AMERICAN, December 1994]. They would thereby
  block PrP from propagating itself.

  It is worth noting that the knockout mice provided a
  welcomed opportunity to challenge the prion hypothesis.
  If the animals became ill after inoculation with prions,
  their sickness would have indicated that prions could
  multiply even in the absence of a preexisting pool of PrP
  molecules. As I expected, inoculation with prions did not
  produce scrapie, and no evidence of prion replication
  could be detected.

  The enigma of how scrapie PrP multiplies and causes
  disease is not the only puzzle starting to be solved.
  Another long-standing question-the mystery of how
  prions consisting of a single kind of protein can vary
  markedly in their effects-is beginning to be answered as
  well. Iain H. Pattison of the Agriculture Research
  Council in Compton, England, initially called attention to
  this phenomenon. Years ago he obtained prions from
  two separate sets of goats. One isolate made inoculated
  animals drowsy, whereas the second made them
  hyperactive. Similarly, it is now evident that some prions
  cause disease quickly, whereas others do so slowly.

  The Mystery of "Strains"

  Alan G. Dickinson, Hugh Fraser and Moira E. Bruce of
  the Institute for Animal Health in Edinburgh, who have
  examined the differential effects of varied isolates in
  mice, are among those who note that only pathogens
  containing nucleic acids are known to occur in multiple
  strains. Hence, they and others assert, the existence of
  prion "strains" indicates the prion hypothesis must be
  incorrect; viruses must be at the root of scrapie and its
  relatives. Yet because efforts to find viral nucleic acids
  have been unrewarding, the explanation for the
  differences must lie elsewhere.

  One possibility is that prions can adopt multiple
  conformations. Folded in one way, a prion might convert
  normal PrP to the scrapie form highly eUciently, giving
  rise to short incubation times. Folded another way, it
  might work less eUciently. Similarly, one "conformer"
  might be attracted to neuronal populations in one part of
  the brain, whereas another might be attracted to neurons
  elsewhere, thus producing different symptoms.
  Considering that PrP can fold in at least two ways, it
  would not be surprising to find it can collapse into other
  structures as well.

  Since the mid-1980s we have also sought insight into a
  phenomenon known as the species barrier. This concept
  refers to the fact that something makes it diUcult for
  prions made by one species to cause disease in animals
  of another species. The cause of this diUculty is of
  considerable interest today because of the epidemic of
  mad cow disease in Britain. We and others have been
  trying to find out whether the species barrier is strong
  enough to prevent the spread of prion disease from
  cows to humans.

  Breaking the Barrier

  The barrier was discovered by Pattison, who in the
  1960s found it hard to transmit scrapie between sheep
  and rodents. To determine the cause of the trouble, my
  colleague Michael R. Scott and I later generated
  transgenic mice expressing the PrP gene of the Syrian
  hamster-that is, making the hamster PrP protein. The
  mouse gene differs from that of the hamster gene at 16
  codons out of 254. Normal mice inoculated with
  hamster prions rarely acquire scrapie, but the transgenic
  mice became ill within about two months.

  We thus concluded that we had broken the species
  barrier by inserting the hamster genes into the mice.
  Moreover, on the basis of this and other experiments,
  we realized that the barrier resides in the amino acid
  sequence of PrP: the more the sequence of a scrapie
  PrP molecule resembles the PrP sequence of its host,
  the more likely it is that the host will acquire prion
  disease. In one of those other experiments, for example,
  we examined transgenic mice carrying the Syrian
  hamster PrP gene in addition to their own mouse gene.
  Those mice make normal forms of both hamster and
  mouse PrP. When we inoculated the animals with mouse
  prions, they made more mouse prions. When we
  inoculated them with hamster prions, they made hamster
  prions. From this behavior, we learned that prions
  preferentially interact with cellular PrP of homologous,
  or like, composition.

  The attraction of scrapie PrP for cellular PrP having the
  same sequence probably explains why scrapie managed
  to spread to cows in England from food consisting of
  sheep tissue: sheep and bovine PrP differ only at seven
  positions. In contrast, the sequence difference between
  human and bovine PrP is large: the molecules diverge at
  more than 30 positions. Because the variance is great,
  the likelihood of transmission from cows to people
  would seem to be low. Consistent with this assessment
  are epidemiological studies by W. Bryan Matthews, a
  professor emeritus at the University of Oxford.
  Matthews found no link between scrapie in sheep and
  the occurrence of Creutzfeldt-Jakob disease in
  sheep-farming countries.

  On the other hand, two farmers who had "mad cows" in
  their herds have recently died of Creutzfeldt-Jakob
  disease. Their deaths may have nothing to do with the
  bovine epidemic, but the situation bears watching. It may
  turn out that certain parts of the PrP molecule are more
  important than others for breaking the species barrier. If
  that is the case, and if cow PrP closely resembles human
  PrP in the critical regions, then the likelihood of danger
  might turn out to be higher than a simple comparison of
  the complete amino acid sequences would suggest.

  We began to consider the possibility that some parts of
  the PrP molecule might be particularly important to the
  species barrier after a study related to this blockade
  took an odd turn. My colleague Glenn C. Telling had
  created transgenic mice carrying a hybrid PrP gene that
  consisted of human codes Banked on either side by
  mouse codes; this gene gave rise to a hybrid protein.
  Then he introduced brain tissue from patients who had
  died of Creutzfeldt-Jakob disease or
  Gerstmann-Str...ussler-Scheinker disease into the
  transgenic animals. Oddly enough, the animals became ill
  much more frequently and faster than did mice carrying a
  full human PrP gene, which diverges from mouse PrP at
  28 positions. This outcome implied that similarity in the
  central region of the PrP molecule may be more critical
  than it is in the other segments.

  The result also lent support to earlier
  indications-uncovered by Shu-Lian Yang in
  DeArmond's laboratory and Albert Taraboulos in my
  group-that molecules made by the host can inBuence the
  behavior of scrapie PrP. We speculate that in the
  hybrid-gene study, a mouse protein, possibly a
  "chaperone" normally involved in folding nascent protein
  chains, recognized one of the two mouse-derived
  regions of the hybrid PrP protein. This chaperone bound
  to that region and helped to refold the hybrid molecule
  into the scrapie conformation. The chaperone did not
  provide similar help in mice making a totally human PrP
  protein, presumably because the human protein lacked a
  binding site for the mouse factor.

  The List May Grow

  An unforeseen story has recently emerged from studies
  of transgenic mice making unusually high amounts of
  normal PrP proteins. DeArmond, David Westaway in
  our group and George A. Carlson of the McLaughlin
  Laboratory in Great Falls, Mont., became perplexed
  when they noted that some older transgenic mice
  developed an illness characterized by rigidity and
  diminished grooming. When we pursued the cause, we
  found that making excessive amounts of PrP can
  eventually lead to neurodegeneration and, surprisingly, to
  destruction of both muscles and peripheral nerves.
  These discoveries widen the spectrum of prion diseases
  and are prompting a search for human prion diseases
  that affect the peripheral nervous system and muscles.

  Investigations of animals that overproduce PrP have
  yielded another benefit as well. They offer a clue as to
  how the sporadic form of Creutzfeldt-Jakob disease
  might arise. For a time I suspected that sporadic disease
  might begin when the wear and tear of living led to a
  mutation of the PrP gene in at least one cell in the body.
  Eventually, the mutated protein might switch to the
  scrapie form and gradually propagate itself, until the
  buildup of scrapie PrP crossed the threshold to overt
  disease. The mouse studies suggest that at some point in
  the lives of the one in a million individuals who acquire
  sporadic Creutzfeldt-Jakob disease, cellular PrP may
  spontaneously convert to the scrapie form. The
  experiments also raise the possibility that people who
  become aUeicted with sporadic Creutzfeldt-Jakob
  disease overproduce PrP, but we do not yet know if, in
  fact, they do.

  All the known prion diseases in humans have now been
  modeled in mice. With our most recent work we have
  inadvertently developed an animal model for sporadic
  prion disease. Mice inoculated with brain extracts from
  scrapie-infected animals and from humans afflicted with
  Creutzfeldt-Jakob disease have long provided a model
  for the infectious forms of prion disorders. And the
  inherited prion diseases have been modeled in transgenic
  mice carrying mutant PrP genes. These murine
  representations of the human prion aUeictions should not
  only extend understanding of how prions cause brain
  degeneration, they should also create opportunities to
  evaluate therapies for these devastating maladies.

  Striking Similarities

  Ongoing research may also help determine whether
  prions consisting of other proteins play a part in more
  common neurodegenerative conditions, including
  Alzheimer's disease, Parkinson's disease and
  amyotrophic lateral sclerosis. There are some marked
  similarities in all these disorders. As is true of the known
  prion diseases, the more widespread ills mostly occur
  sporadically but sometimes "run" in families. All are also
  usually diseases of middle to later life and are marked by
  similar pathology: neurons degenerate, protein deposits
  can accumulate as plaques, and glial cells (which support
  and nourish nerve cells) grow larger in reaction to
  damage to neurons. Strikingly, in none of these disorders
  do white blood cells-those ever present warriors of the
  immune system-infiltrate the brain. If a virus were
  involved in these illnesses, white cells would be expected
  to appear.

  Recent findings in yeast encourage speculation that
  prions unrelated in amino acid sequence to the PrP
  protein could exist. Reed B. Wickner of the NIH
  reports that a protein called Ure2p might sometimes
  change its conformation, thereby affecting its activity in
  the cell. In one shape, the protein is active; in the other,
  it is silent.

  The collected studies described here argue persuasively
  that the prion is an entirely new class of infectious
  pathogen and that prion diseases result from aberrations
  of protein conformation. Whether changes in protein
  shape are responsible for common neurodegenerative
  diseases, such as Alzheimer's, remains unknown, but it is
  a possibility that should not be ignored.

  FURTHER READING

  Scrapie Disease In Sheep. Herbert B. Parry. Edited by
  D. R. Oppenheimer. Academic Press, 1983.

  Molecular Biology Of Prion Diseases. S. B. Prusiner
  in "Science," Vol. 252, pages 1515-1522; June 14,
  1991.

  Prion Diseases Of Humans And Animals. Edited by
  S. B. Prusiner, J. Collinge, J. Powell and B. Anderton.
  Ellis Horwood, 1992.

  Fatal Familial Insomnia: Inherited Prion Diseases,
  Sleep, And The Thalamus. Edited by C. Guilleminault
  et al. Raven Press, 1994.

  Molecular Biology Of Prion Diseases. Special issue of
  "Philosophical Transactions of the Royal Society of
  London, Series B," Vol. 343, No. 1306; March 29,
  1994.

  Structural Clues To Prion Replication. F. E. Cohen,
  K.-M. Pan, Z. Huang, M. Baldwin, R. J. Fletterick and
  S. B. Prusiner in "Science," Vol. 264, pages 530-531;
  April 22, 1994.

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