Who Needs DNA? Prions Evolve Without It

By Andrew Moseman | January 5, 2010 1:41 pm

prionsFor evolution to take place, you need DNA or RNA to change through mutation, providing the variations for natural selection to select. Right? Well, it may be more complicated than that. A new study suggests an exception: prions, the infectious protein bits that can cause degenerative brain diseases like mad cow disease. In a paper in Science, researchers document these lifeless structures evolving, despite the fact that they lack any DNA or RNA.

Study leader Charles Weissmann and his team transferred prions from brain cells to other kinds of cells and watched as certain members of the prion population adapted to the new environment and took over, out-competing their brethren. When he transferred the prions back to brain cells, the ones most adapted to brain living got the upper hand and increased in number as they out-competed the prions that had adapted to other cells. Weissmann argues that this shows Darwinian evolution can go even further than we thought: “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 (DNA or RNA) for the process of evolution” [BBC News].

In diseases like mad cow and the human version of it, Creutzfeldt-Jakob disease, proteins in the brain go awry. Mammals produce the normal protein cousins of infectious prions as part of normal cell development, but during infection, misfolded or warped proteins can convert normal host prion protein into its own toxic, misfolded form. When this happens enough times, massive tissue and cell damage can occur [Popular Science].

This pattern of warping proteins could be the key to how this DNA-less evolution occurs, the researchers propose. When a prion converts a normal protein, it typically forces it into the same structure as itself, but at a low probability, other variant structures result. The population of these variants can then expand or contract based on selective pressures [Ars Technica]. This appears to happen more quickly when the prions are transferred to a new host, as happened in previous research when scientists moved prions from sheep to mice—the more virulent variants got a chance to take hold.

Given the problems today with bacteria and viruses evolving drug resistance, the idea of evolving prions seems like more bad news. But, taking a note from DISCOVER blogger Carl Zimmer’s “Evolving Viruses to Death,” perhaps doctors could use prions’ strength against them: Since infectious prions need their normal cousins to feed their own replication and evolution, therapies that limit the supply of normal prion proteins could essentially starve the degenerative cycle that makes those illnesses so effective at killing their hosts [Popular Science].

Related Content:
80beats: Ripped From the Journals, including a Nature study fingering prions from spreading chronic wasting disease in deer.
The Loom: Return of Mad Cow Memories
DISCOVER: When Bad Prions Go Good
DISCOVER: Prionlike Protein Help Form Memories
DISCOVER: Picturing Prions

Image: Eye of Science / Science Photo Library

CATEGORIZED UNDER: Health & Medicine, Living World
  • andrew

    But is this Natural Selection?

  • Gil

    What else is it if not natural selection? The selective pressure is different reproductive rate in different environments, pretty standard fare.

  • andrew

    Natural Selection:

    “Natural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations.” Wiki

    Do the terms ‘heritable traits’, ‘organism’ and ‘reproduce’ even apply to prions?

  • http://themagi.tumblr.com Kalev

    How can this be darwinan evolution when that still is a theory? You have a very interesting fact here I see how this can apply to change over time but not darwinism. Keep up the great work! Just remember there is a huge world of difference between evolution and darwinian evolution! :)

  • andrew

    Difference between evolution and Darwinian evolution?

    Do tell…

  • Matt

    Yup…I want to hear the difference there too: what’s the difference, Kalev (you’re probably working up the response now anyways).

    And andrew, I would suggest that since it was lab researchers doing the selection, then it was decidedly NOT natural

  • http://www.armedangryatheist.com ArmedAngryAtheist

    Darwinian Evolution is a theory in the same sense of the Heliocentric Theory of the Solar system. Creationist that argue otherwise are ignorant to the point of delusion.

    Theory: A scheme or system of ideas or statements held as an explanation or account of a group of facts or phenomena; a hypothesis that has been confirmed or established by observation or experiment, and is propounded or accepted as accounting for the known facts; a statement of what are held to be the general laws, principles, or causes of something known or observed. – from the Oxford English Dictionary

  • lynn

    Heliocentric theory is still a theory because although it does make the true and proven statement that the sun is the center of the solar system, it also claims that our sun is the center of the universe and is a stationary object, which is not proven, thus it remains a theory(http://science.jrank.org/pages/3278/Heliocentric-Theory.html).

    Similarly, evolution remains a theory because while many of its elements contain facts, there are still several holes. It cannot be proven that there is no force that moves its design, this is because, scientifically, you can neither prove there is or is not a God. Although there are even more holes this is the one big one that will make evolution difficult to be called a law. Although I will admit that I have also met several ignorant creationists . Mainly because (for the same reason as evolution theory) you can not prove the existence of God scientifically. Thus the debate between the atheist and the believer are out of place in the context of science.

    God can not be measured scientifically, though this is true (as a student of science) I still believe because while he can not be measured he can be detected, I feel God in my life. Just like our ancient ancestors believed in the wind, the sun and the earth, despite the fact that at the time they could not be measured “scientifically”. Does this mean that until they “could” be measured scientifically” they did not exist?

  • jraustin

    @lynn – excellent points. I’d also like to point out that as these prions adapt to survive in new cells no additional data is being added to them. a million generations and it will still be a prion not a single celled organism.

  • JJ

    This is a very interesting finding – evolution via natural selection does not by itself require nucleic acids as information storage – it simply requires variation and heritable changes that are beneficial in certain environments. Organic information storage without nucleic acids would seem to be a “new” finding, though researchers studying epigenetic changes above the level of DNA have found an analogous system – no changes in the nucleic acid sequence are necessary to effect changes that selection can act upon. As of yet, it is unclear how long epigenetic changes can persist, and whether you could get changes at the epigenetic level that essentially have escaped reliance on nucleic acids (unlikely in my mind, since genes are still being transcribed / translated or not) As to the question, is this “natural” selection? Well, no, but if you can produce changes through artificial selection, the principle is the same, and it would be foolish to dismiss the possibility of natural selection producing similar changes, if over a different time scale. This might provide an answer to why people never appear to get scrapie from sheep, but that we do appear to get BSE from cows, which cows most likely acquired from scrapie-infected sheep. The prions were evolving / adapting to their new host.

    @lynn – like many, you misunderstand what a scientific theory is. The statement that the sun is the center of the solar system is not a “true and proven statement.” A properly constructed scientific theory cannot be proven true, it can only be proven false. Evidence can be collected in support of a scientific theory, which strengthens it, but it is never “proven true”. Evolution is a theory, and will always be “just a theory” since this is the nature of science. There are no data that falsify the theory of evolution, just as there are no data that falsify heliocentrism, but in both cases you could discover evidence to falsify those two theories. This is why you cannot prove the existence of God – and why positing the existence of God, while a perfectly valid theological statement, is not a valid scientific hypothesis, for while you conceviably could find evidence to support the existence of God, you can never disprove the existence of God – and hence it lies outside the realm of science.

  • amphiox

    @lynn – even if one day a super-human designer were discovered, it would mean absolutely zilch pertaining to the validity of evolutionary theory. The mechanisms of evolution will still proceed exactly as they always have, irrespective of what the designer may or may not choose to do. Humans have been acting as small-scale intelligent designers of organisms ever since we started domesticating animals and plants, and our designed organisms immediately continue to evolve independent of our control.

  • amphiox

    If one uses the definition of life as “a self-replicating system capable of darwinian evolution” which some have argued for, this would make prions qualify as life.

    It should be noted that there is nothing that says that evolution by natural selection requires DNA/RNA. Indeed, with respect to abiogenesis theories, evolution has to precede DNA, because the DNA/RNA/protein system used by extant organisms must have itself arisen through a process of evolution.

    In short, any system, regardless of what it is made of, that self-replicates with imperfection (ie heredity with variation), is capable of darwinian evolution if subjected to a survival-limiting environment.

    It is intriguing to wonder also if the mechanism of prion evolution (reproduction of a template) might turn out to have any relevance to abiogenesis research (though of course the prions themselves, specifically, can’t).

  • Brian Too

    With all due respect to Mr. Weissmann, is this really news? I mean, I had understood that evolution governed all sorts of systems, both living and otherwise.

    Furthermore, since we don’t even have a bulletproof, tested, universally accepted definition of life, why would we be surprised to find evolution operative in molecules we don’t generally consider to be living? Some don’t consider viruses to be alive, yet they clearly evolve. What’s the difference? And no, I don’t consider it a strong answer to say “DNA”. DNA is just a molecule, and proteins are just molecules too. Big molecules, fancy ones, molecules used extensively in biological systems, but ultimately just molecules.

    Or maybe I’m missing something.

  • jraustin

    @amphiox
    “with respect to abiogenesis theories, evolution has to precede DNA, because the DNA/RNA/protein system used by extant organisms must have itself arisen through a process of evolution.”

    This is engaging in more than a little circular logic to say the least, to suppose that “evolution HAS to precede DNA, because the DNA…must have itself arisen through a process of evolution.” does not provide any supporting evidence.

    @JJ – Hmm, I have to disagree. What you are describing is trying to prove a negative (which is impossible) and not the scientific method. We don’t have the “Theory of Gravity” we have the “Law of Gravity”. This is because one can postulate a hypothesis and develop an experiment or make predictions that will prove or disprove the hypothesis. However, trying to make the case that prion “adaption” is what abiogenesis would predict thereby “proving” evolution precedes DNA is a huge and unsupported jump.

  • JJ

    Darn tootin we have a Theory of Gravity, and it has less empirical support than the Theory of Evolution. The hypothesis – theory – law is probably what you learned in 4th grade 40 years ago. The difference between a scientific theory and a “law” is nonsense and simply semantics. The law of gravity states what? . . .that when you drop your shoe it will fall to the ground? Unless you’re in a black hole, or outer space??
    http://www.physorg.com/news85310822.html
    I’d send you to an Onion link, but that might be interpreted as snide.
    Scientific theories are testable and refutable. Musings are not.

  • jraustin

    @JJ – Haha, wow, that’s pretty good. almost creepy. Anyway, this article reminded me a little of your logic. Enjoy…

    New Lack of Evidence Boosts Certainty of Darwinism

    Posted By Scott Ott On September 18, 2007 @ 12:29 pm In Science | 75 Comments

    (2007-09-18) — Recent discoveries indicating no direct line of descent from ape-like creatures to modern man have further bolstered anthropologists’ belief that Darwin’s theory of descent-with-modification by natural selection must certainly account for the rise of Homo sapiens.

    New research on a pair of recently-unearthed African skulls shows that Homo habilis and Homo erectus most likely did not descend one from another, as scientists have believed for years, but that the two diminutive hominids lived as neighbors during the same epoch. Other recent research indicates that Homo sapiens lived at the same time as Neanderthals.

    Far from casting doubt on Darwin’s theory, experts say that the lack of evidence and contradictory discoveries have helped to build “a consensus of certainty in the field.”

    “Finding little physical evidence to substantiate the theory only means there must still be a great deal of supportive evidence out there to be found,” said an unnamed editor of the journal Nature, which plans to publish a paper on the African skulls this week. “The more we realize how little we know, the more certain we are that we’re right. As I once read in a scholarly paper somewhere, ‘faith is the assurance of things hoped for, the conviction of things not seen’.”

    http://www.scrappleface.com/?p=2688

  • jraustin

    Sorry, is that what you meant by snide?

  • Fatkid

    I read in Fast Food Nation that prions can be found in cows bone matter. That matter is rendered into the calcium bleach that makes our table sugar white :)

    Many of us are lucky to have inherited antibodies to prions. Thousands of years ago our head hunter forebears became immune to madcow disease after generations of canibalism. Where were you churchy types then, eh? You used baby toes as toothpicks like the rest of us.

  • JJ

    @jraustin – Yes, congratulations! That is indeed snide, and, like the Onion, made up. You have falsified my previous hypothesis that anyone who could idenify snide could also understand the concept of falsifiability. Ok – I’m done with internet sniping. Sorry for indulging in it.

    As to abiogenesis – the current RNA world hypothesis has some good support that RNA preceded DNA and that self-replicating, information-storing, variable RNA might have been the original evolving biotic molecule. Ultimately, it comes down to how you define life – the textbook standard of cell-based life does omit things like viruses and prions, but the argument is that these things, while they have a huge effect on cell based life, are still dependent on cells. Whether cells should be our broad definition of “life” is another question.

  • lynn

    @amphiox- agree to what you said the theory of evolution would be unharmed by the exsistence of God, just the same as God is unharmed by the theory of Evolution. They don’t even really relate to eachother and the argument is out of place here as anywere. That was the point I was trying to make. Cant we just enjoy some great research without somebody trying to turn it into a religouse debate. also agree when you say that theories are not just meant to be disproven the scietific method works both ways and if we are not working toward either Law or dimisal then whats the point.

    @jj- I was not alive 40 years ago and my education is ongoing. Besides I read your link and did not see anything that disproves gravity as a law or theory, just dark matter, which relates to phenomenon in space and gravity but is not the bases for all of gravity. It is more like an explanation for exceptions to understanding of the Law or theory. Sounds to me like the researchers were just rying to call attention to there theory by playing up its importance, which has been common practice in scientific papers for as long as I have been reading them and probably longer. Besides who wants to right a paper no one will read? Questioning dark mater would have grabbed my attention much less than questioning gravity. I have no dought though that dark matter does need further explanation but dought if it qualifies as much attention as to demote Gravity from law to theory. But I could be wrong, thats not really my area of expertise. Agree with you about leaving God out of science and that he can niether be proven or disproven through it. RNA seems likely to come first, it is a much simpler structure but shares many simular features it is very concievble. Plus I remember when this theory first came out there was some good research behind it but heck if I can remember where I read it. What is life is a question that again is circular and difficult to nail down. Life in the past has been based on what makes us work but now we see viruses that seem wierdly life like in there habits and prions capable of changing themselves to fit enviromental conditions. I love microbiology!

    By the way to staff enjoyed your artical, Prions have always facinated me. It is good to see they are getting the attention they deserve from research.

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