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Quasispecies Theory and the Behavior of RNA Viruses. Sumeeta Singh, Steve Bowers, Greg R ice, Tom McCarty BINF 704 02/19/13. What is a virus?. Viruses are obligate intracellular parasites . Small infectious agents bearing nucleic acid instructions.

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quasispecies theory and the behavior of rna viruses

Quasispecies Theory and the Behavior of RNA Viruses

Sumeeta Singh, Steve Bowers, Greg Rice, Tom McCarty

BINF 704

02/19/13

what is a virus

What is a virus?

Viruses are obligate intracellular parasites.

Small infectious agents bearing nucleic acid instructions.

Classified based on the form of nucleic acid, DNA or RNA.

virus types

Virus types

Plus strand RNA virus

Minus strand RNA virus

Double strand RNA virus

Retrovirus

Single strand DNA virus

Double strand DNA virus

quasispecies theory

“Quasispecies Theory”

Mathematical framework describing evolution of macromolecules (Eigen, 1971)

Extends the classic population genetics ideas of mutation-selection into quasispecies (Eigen, Schuster, 1977)

Eventually borrowed to describe RNA virus evolution dynamics

virus replication error

Virus Replication Error

Plus strand RNA virus

Minus strand RNA virus

Double strand RNA virus

Retrovirus

Single strand DNA virus

Double strand DNA virus

RNA dependent-

RNA polymerase

Error rate =10-3 to 10-5

Reverse Transcriptase

Error rate =10-4to 10-5

DNA polymerase

Error rate =10-7to 10-9

requirements and consequences

Requirements and Consequences

Polymerase responsible for high error rates.

Estimated that each single and some double nucleotide sequence changes occur.

Resulting in a collection of related sequences around a “master” sequence.

Variation is related to ability to survive (population genetics) AND probability of occurring based on sequence neighbors (quasispecies).

quasispecies and rna viruses

Quasispecies and RNA viruses

Survival of the “flattest”

Error Catastrophe

Fidelity and Fitness

survival of the fittest or survival of the flattest
Survival of the Fittest or Survival of the Flattest

A flat species is a species which exists in a genetically diverse group. Not dominated by one variant.

A quasispecies must be a flat species.

A fit species (in this part of the presentation) is a species which reproduces very fast.

properties of a flat species
Properties of a flat species

A flat species will have high mutation rates.

A flat species is able to mutate without a major effect on fitness.

advantages of a flat species
Advantages of a flat species

Different mutants in a flat quasispecies can help each other.

Flat species are better able to adapt.

dengue 1 virus
Dengue-1 Virus
  • Lives as a quasispecies
  • One variant of the virus which is found in high concentrations cannot survive on its own.
  • It can only survive because other viruses (infecting the same cell) create the protein which it lacks.
viroid experiment
Viroid Experiment
  • Theory: More genetically diverse (flatter) species are better able to adapt to mutations.
  • Viroid 1 - CSVd – very fit
  • Viroid 2- CChVMd – more flat, but less fit
  • Procedure:

Infect plants with each viroid

Subject the plants to two environment either normal, or UVC light

results
Results

UVC light will cause mutations

Results:

Under normal conditions the fitter viroid did better, then the flatter viroid.

Under the UVC light the two were about equal

Under the UVC light the flatter became more diverse.

The fitter did not become more diverse.

fitter or flatter which is better able to survive
Fitter or Flatter, which is better able to survive?
  • The fit species will grow faster in an ideal environment.
  • The flat species will be able to adapt more quickly.
error threshold
Error Threshold

RNA Viruses have a high mutation rate

The point at which accumulated mutations reduce fitness:

- Too much mutation can lead to loss of vital information

- Too little mutation can lead to host defenses overcoming the virus

Error Threshold: position in informational space where a phase transition occurs such that the genomic sequence information can no longer be perpetuated.

The greatest fitness is when mutation rates approach the error threshold

model of error catastrophe
Model of Error Catastrophe

http://www.pnas.org/content/98/12/6895.full

error catastrophe
Error Catastrophe

Extinction of a virus as a result of excessive RNA mutations – lethal mutagenesis

Decrease viral fitness by increasing the rate at which new mutations appear.

There is an intrinsic limit to the maximum variability of viral genetic information before it loses meaning. If an RNA virus quasispecies goes beyond that mutation limit, the population will no longer be viable.

increasing mutation rate
Increasing Mutation Rate
  • Ionizing radiation(eg X-rays) – cause mutations by damaging DNA.
  • Base Analogs – chemicals that replace one of the usual nucleotides in the DNA. These mutagens cause copying errors.
apobec3g
APOBEC3G
  • Humans have the ability to induce lethal mutagenesis
  • Protein found inside cells that has a very specific antiviral role
  • Cytidinedeaminase enzyme
  • Unfortunately, HIV has the ability to bind to APOBEC3G proteins and cause their degradation
apobec3g1
APOBEC3G

It de-aminates the cytosine base, thus mutating it to a uracil base

fidelity and fitness mutation rate
Fidelity and Fitness: Mutation Rate
  • Evolutionary Theory: viral error rates
  • RNA Virus: low fidelity generates diverse population of variants
  • Homogenous population vs dynamic environment

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fidelity and fitness mutation rate1
Fidelity and Fitness: Mutation Rate
  • Ribavirin and Lethal Mutagenesis
  • Hypothesis: mutant with low mutation rate less sensitive to LM and resistant to ribavirin
  • Poliovirus experimental groups: G64S polymerase mutation

30

fidelity and fitness virulence
Fidelity and Fitness: Virulence
  • G64S species attenuated in transgenic mouse model for poliovirus infection
  • Virulence determined by diversity of coinfecting population
  • Quasispecies diversity, rather than the selection of individual variants, correlates with enhanced virulence

32

fidelity and fitness attenuation
Fidelity and Fitness: Attenuation
  • Vaccine Design
  • Vignuzzi(2008): G64 engineered mutants stimulated high titers of neutralizing antibodies in mice
  • Fidelity modulation as therapeutic strategy

33

future perspectives
Future Perspectives
  • How do models apply to infected hosts?
  • What is best measure of viral fitness in dynamic population?
  • How does population diversity influence pathogenesis (subpopulation cooperation)?
  • Improved assays for characterizing viral populations
  • Modern techniques: Deep sequencing, Molecular barcoding

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