XMRV Not Laboratory Contaminant -Strong Argument

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>>>> 4 January 2011 <<<<
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Many thanks to Dr Speedy


Professor Racaniello: Why XMRV is not a
laboratory contaminant at: http://bit.ly/dRhPZu


~jvr



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http://bit.ly/hh2KQn


virology blog
ABOUT VIRUSES AND VIRAL DISEASE




Retroviral integration
and the XMRV provirus


4 January 2011


By Vincent Racaniello
A virology Professor unravels viruses
Questions? virology@virology.ws




A strong argument that the novel human retrovirus
XMRV is not a laboratory contaminant is the the finding
that viral DNA is integrated [http://bit.ly/eTDb5T] in
chromosomal DNA of prostate tumors.

Why does this result constitute
such strong proof of viral infection?



Establishment of an integrated copy of the viral genome –
the provirus – is a critical step in the life cycle of retroviruses.

Proviral DNA is transcribed by cellular RNA polymerase II to
produce the viral RNA genome and the mRNAs required to
complete the replication cycle. Without proviral DNA, retroviral
replication cannot proceed.



To produce proviral DNA, the retroviral RNA genome is
converted to a double-stranded DNA by the viral enzyme
reverse transcriptase.

This step occurs in the cytoplasm.

Specific and efficient insertion of the viral DNA into the host
cell DNA is catalyzed by a viral enzyme called integrase.

This enzyme recognizes and nicks the two ends of viral DNA,
and the new 3'ends are then joined covalently to the host
DNA at staggered nicks made by integrase.


The image below shows some of the characteristic features
of retroviral integration.

A the top is the unintegrated linear DNA of avian
sarcoma/leukosis virus produced by reverse transcription.
Upon completion of integration, two base pairs (AA•TT) are
lost from both termini, and a 6-bp target site in host DNA
(pink) is duplicated on either side of the proviral DNA.

This target site varies in length from 4 to 6 bp among
different retroviruses. The proviral DNA (middle) ends with
the conserved 5'-T G…C A-3' sequence.

The provirus serves as a template for the production of the
viral RNA genome (bottom).



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Photo: http://bit.ly/hh2KQn

a bigger version can be found at: http://bit.ly/ePtaUU

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To identify XMRV proviral DNA, genomic DNA was isolated from
prostate tumors, and DNA was amplified using a primer that
annealed in the viral env gene, near the right-hand LTR.

Nucleotide sequence analyses of amplified DNAs from 14
different patients showed the expected viral CA sequence
followed by human DNA.

However, the other cardinal sign of retroviral integration –
duplication of host DNA sequences flanking the integration
site – could not be confirmed, because only the right-hand
integration site was studied.


The isolation of the entire proviral DNA, including both flanking
integration sites, from patients with prostate cancer or chronic
fatigue syndrome would be additional evidence that XMRV is a
virus that infects humans.




Kim, S., Kim, N., Dong, B., Boren, D., Lee, S., Das Gupta, J.,
Gaughan, C., Klein, E., Lee, C., Silverman, R., & Chow, S.
(2008). Integration Site Preference of Xenotropic Murine
Leukemia Virus-Related Virus, a New Human Retrovirus
Associated with Prostate Cancer Journal of Virology, 82 (20),
9964-9977 DOI: 10.1128/JVI.01299-08 [http://bit.ly/ffRhsS]




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