XMRV/MRV Q&A
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Retrovirus
What is a retrovirus?
A retrovirus is a type of virus; a small infectious agent that can replicate only inside the living cells of organisms. It consists of a single strand of RNA, and unlike other viruses, once it gets into a cell it replicates its genetic material by collecting nucleotides and assembles itself as a double strand of DNA that splices itself into the host's genetic material. The virus thereafter replicates as part of the host cell's DNA. Retroviruses are enveloped viruses that belong to the viral family Retroviridae.
What other human retroviruses are there?
There are two other known infectious human retroviruses; HIV, HTLV-1 and 2. HIV causes AIDS and HTLV-1 and 2 causes T-cell leukemia and T-cell lymphoma.
What is a virion?
A complete viral particle, consisting of 2 strands of RNA surrounded by a protein shell and constituting the infective form of a virus.
What is a provirus?
A virus genome integrated into the DNA of a host cell.
What is a genome?
The entirety of an organism's hereditary information.
MRV's
What is the origin of MRV retroviruses?
Is XMRV and PMRV the same virus?
Yes. XMRV is actually a Polytropic/Xenotropic hybrid of MLV class gamma retrovirus.
XMRV
Transmission
Contamination
Why is XMRV, and other MRV's, not a contaminent?
Isolation of the virus
XMRV was first isolated from prostate cancer tissue RNA not DNA. This means that there was either encapsulated RNA present or messenger viral RNA. The genomic RNA is converted to cDNA as soon as it enters into a cell or it will be destroyed by RNAase. Naked nucleic acid cannot enter a cell The same authors used serology tests called FISH and IHC. Both were positive, showing the presence of viral mRNA or cDNA and viral proteins. A contaminant could produce neither result.
Antibody results
The antibody used was a monoclonal antibody, specific to a region shared by all MLV viruses. It will not react with anything else. This antibody test was applied after 42 days of culture to massively amplify the amount of the target protein. A positive test indicates the presence of virions or parts of virions. A contaminate can form neither. More detail on this can be found here
Mouse contamination assays
Clone
What are the limitations of using a clone of XMRV?
A clone, such as VP62, does not represent the genetic diversity of XMRV and other MRV's. Therefore, if a researcher were to calibrate primers very specific for VP62, and not be inclined to use clinical positives to test their assays, they would be ruling out the possibility of significant genetic diversity in XMRV. The only reason a resercher would do this, is if they believed all XMRV strains differed by only a few nucleotides.
Use of a clone for XMRV would of course not account for the other varities of MRV's that have been identified, such as the PMRV in Lo et al.
EXAMPLE:
If this same attitude was been applied to HIV, then if anyone has HIV with any genetic variation compared to the original clone sequenced, then they have nothing to worry about because it is not really HIV. Yet, this is clearly not true, and a different strain is not a different virus.
History
Who discovered XMRV?
XMRV was originally discovered in prostate cancer tumors by Dr. Robert Silverman in 2006.
Who discovered XMRV in patients with ME/CFS?
A group of scientists from the Whittemore Peterson Institute, Cleveland Clinic and the National Cancer Institute. The study was published on the 8th October 2009 in Science magazine.
