Using evolution to fight diseases?

Paul Cisek pavel at space.Bu.EDU
Fri May 19 14:55:02 EST 1995

I apologize if this posting exposes my ignorance of virology, or if my
question is a common one.  (My scan of the last 10 days of mail here did
not find it mentioned... so I thought I'd bring it up)

A major difficulty in fighting diseases such as "staph" or Ebola is that
any biochemical weapon only increases the `selective pressure' upon the
disease population.  So if all individuals are not successfully wiped out
then the resistant survivors proliferate and we are faced with a tougher
enemy.  Viruses in particular are extremely quick to evolve resistant strains.
However, is it possible to use their strength against them?

For example, suppose we know the natural host of a particular virus (I realize
we don't in the case of Ebola).  Suppose also that we have identified at
least one of the proteins which is synthesized by transcription from the
viral RNA.  Can we then introduce this protein into the natural environment
of the virus, say, by spraying it on the plants that the host eats?  The
protein is now freely available to the viruses, and the RNA sequence that
codes for it becomes a burden rather than a boon.  Strains which do not
bother coding this protein gain a reproductive advantage against their cousins
by virtue of their shorter length.  Even a small advantage should cause the
trait (absence of part of the RNA code) to become common in the viral
population.  At this point, we have a strain of virus which is more efficient,
and more deadly, but is dependent upon the presence of a compound which
we control.

I recall a study on the E.coli bacteriophage Q-beta (by Spiegelman?).  The
virus invades E.coli, and replicates in the presence of some proteins produced
by the host.  Spiegelman placed the virus in a dish containing the needed
host enzymes and let replication proceed.  After some time, he took a sample
from the population and placed it in a fresh dish again rich in needed enzymes.
Such sampling effectively selected for replication speed.  After about 70 such
transfers, he examined the dominant virus strain.  He found that it was much
shorter, and much more reproductively efficient.  However, it was incapable
of invading E.coli.  Presumably, it pruned away the machinery needed for this
because that machinery was a burden in the environment provided by Spiegelman.

Might a similar procedure be feasible on a large scale?  Can we bias the
evolution of diseases toward strains that are specialized toward an environmental
feature that we control, and thus can withdraw?  We would have to be careful
not to present a new niche and induce speciation - we want the entire population
to drift into our trap.  Even if successful, we might cause irreparable damage
to various species to whom the `more efficient virus' would be more deadly.

Again, my apologies if this is an obvious strategy and/or one that is completely
not feasible...

Paul Cisek
Dept. of Cognitive and Neural Systems
Boston University

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