(Fwd) Re: (Fwd) Re: Ok, so the CDC has confirmed..and other th

[DANDERSON at PRL.PULMONARY.UBC.CA]

In article <566E892421 at prl.pulmonary.ubc.ca>,
DANDERSON at PRL.PULMONARY.UBC.CA wrote:

> bhjelle at unm.edu wrote:
> 
> "This alleged instability of RNA viruses is one of the
> most misunderstood phenomena, even among some virologists."
> 
> An example of just how stable RNA viruses can be is Rubella.  Strains 
> of this virus have been passaged in different laboratories for years 
> with no genetic changes, unless forced.
> 
> Dan

najital at rockvax.rockefeller.edu (Lyle Najita) replyed:

Dan,

>propagated under are not amenable to selecting out differences.
>Poliovirus, in the six years I worked with it, also showed a consistent
>sequence after repeated passage, and yet under conditions - for example,
>upon selecting mutants for conditional-lethal growth one can always find
>revertants. Now if this is your operational definition for "forced" then I
>have no argument. However, given the relatively high error rate of RNA
>polymerases, I would argue that there are always genetic changes being
>made during viral RNA replication and what we "see" when we sequence the
>genome is merely a reflection of the predominant species. Alter the growth
>conditions (ie - change the host cell, growth conditions, etc.) and you
>may very well change the sequence of the predominant species.

>Just my $0.02,


I agree.  Somewhat simular to children recieving Polio vaccine and the 
non-immune parents developing poliomyelitis.  However, a laboratory 
stock should be maintained with very low DI particles, by amplifying 
at a low moi (0.01)!  Thus, the mutations of virus may not be as evident in 
laboratory preparations.

Our favourite virus is coxsackievirus B3 (CVB3), also an 
enterovirus.  I agree that RNA polymerases are constantly making 
mistakes, most of them lethal.  However there is evidence that virus 
may be adapted for particular cell lines or tissues.  For example: if 
we isolate CVB3 from the heart or liver after primary infection, the 
secondary infection in the respective organ with this organ isolate 
is more severe!  Others have shown this as well!  

Furthermore, our stock virus, is virus from a cDNA clone sythesized in 
another laboratory.  Repeatedly, this stock (as well as another stock 
virus from yet another laboratory) does not replicate as fast (48-72 
hours) upon first amplification and will not form plaques in a plaque 
assay, even though all of the cells demonstrate CPE!  However, the 
second passage virus replicates and lyses the cells in 18-24 hours at 
the same moi and is plaqueable.  With cells from the laboratory which 
initially amplified the stock this strain of CVB3 produces plaques as 
expected!  This stock virus and propagated  virus produces the same 
degree of animal pathology as passaged virus. 

There are also "avirulent" CVB3 strains (CVB30), which vary is genomic 
sequences compared to the wild-type, that when injected into SCID mice 
revert to the virulent genotype, and subsequently produce severe 
pathology in  immune competent mice.

With rubella virus the sequence of the genome is the same in any of the 
isolates from different tissues, patients, and laboratories.  
However, other togaviruses, such as semlike forest virus (which for 
you ebola virus lovers was first isolated in the Semlike Forest in 
the African rain forest) have higher rates of genomic mutations.

These data, suggest the ability to mutate, increasing or decreasing 
virulence and tissue tropism, is virus AND host sensitive, and is 
probably dependent upon host and virus replication proteins.  What 
lends fidelity to the viral replication complex?

Dan