by XMRV Global Action on Friday, March 11, 2011:
full text for More Negative Data for Link Between Mouse Virus and Human Disease- Science
Science 11 March 2011:
Vol. 331 no. 6022 pp. 1253-1254
DOI: 10.1126/science.331.6022.1253
• NEWS & ANALYSIS
RETROVIROLOGY
More Negative Data for Link Between Mouse Virus and Human Disease
Jon Cohen
BOSTON—A new finding presented at a conference here last week throws
cold water on the impassioned debate about the link between a novel
mouse retrovirus and prostate cancer and chronic fatigue syndrome in
humans. Yet few believe it will end the controversy, which began in
2006.
In an extensive sleuthing expedition that looked back nearly 20 years,
two collaborating research teams contend that they have evidence that
xenotropic murine leukemia virus–related virus (XMRV) resulted from
the chance recombination of pieces of two mouse viruses in lab
experiments and that the connections to human disease are spurious.
“That nails it,” said retrovirologist Nathaniel Landau of New York
University. “Everyone working on this thing has this virus
contaminating their stuff. It's been a tremendous waste of time and
money.” But even if XMRV is not a threat to human health, the fact
that a retrovirus that can readily infect human cells was apparently
generated by chance in the lab raises some interesting and potentially
troubling issues.
Vinay Pathak, a retrovirologist who works at the HIV Drug Resistance
Program run by the U.S. National Cancer Institute (NCI) in Frederick,
Maryland, presented the new data at the 18th Conference on
Retroviruses and Opportunistic Infections, which focuses mainly on
another retrovirus, HIV. The fact that the XMRV work garnered so much
attention here reflects the high stakes. The possibility that XMRV
causes human disease has raised both hope and fear among patients and
public health officials. For people who have prostate cancer or the
baffling chronic fatigue syndrome, XMRV offered not only an
explanation but also a treatment: The virus is susceptible to some
anti-HIV drugs. Blood banks, on the other hand, have worried mightily
that, as happened when the AIDS epidemic began, they were unwittingly
helping to spread a dangerous retrovirus.
The unusual life cycle of retroviruses explains how such a
recombination could occur, as Pathak described. Retroviruses contain
RNA that, in addition to coding for viral proteins, carries
instructions to make the enzyme reverse transcriptase. After a retro
virus infects a cell, reverse transcriptase converts the viral RNA
into DNA, which is necessary for the virus to integrate with the host
chromosomes. This is the stage in which recombination between
retroviral DNA from different genomes can happen.
Pathak explained how skepticism has steadily built about the link
between XMRV and these diseases as several labs examined patient
samples and could not find the virus or antibodies to it. One 2009
study particularly piqued Pathak's interest, as it showed how a human
prostate cancer cell line produced high levels of the virus.
The cell line was established at Case Western Reserve University in
Cleveland, Ohio, from a human tumor called CWR22. Prostate cancer
tumors are difficult to grow in lab experiments, but in 1993,
researchers there reported that they had success by injecting tissue
from CWR22 into mice, growing tumors, injecting tissue from those
“xenografts” into new mice, and repeating that passaging process until
they could reliably grow large enough xenografts for study. In 1999,
the same lab described a permanent cell line, 22Rv1, it had made from
a CWR22 xenograft. Because it was one of very few cell lines available
to study prostate cancer, it was widely used. Pathak's group tracked
down samples from different passages of CWR22, different versions of
both 22Rv1 and a second cell line made later from CWR22. Before 1996,
no CWR22 samples contained XMRV DNA.
Pathak's lab found that some of the early samples of xenografts did
have a stretch of DNA that was nearly identical to about half of the
XMRV genome. A group led by John Coffin, who works at both NCI and
Tufts University here, made a similar discovery with different samples
of xenografts. When the teams compared notes, they saw that the two
sequences perfectly overlapped to form XMRV. “It was an amazing
moment, the kind that happens once or twice in a career,” Coffin says.
“It was like seeing a puzzle come together.”
As Pathak emphasized in his talk, the DNA sequences in what they
dubbed preXMRV-1 and preXMRV-2 are nearly identical to the XMRV
sequences reportedly found in humans but suspected to be a lab
contaminant by some groups. Pathak's and Coffin's teams both also
found preXMRVs in some mice strains used in the experiments. But XMRV
itself cannot infect mouse cells, which means the preXMRVs could have
recombined only after the mice received prostate tumor transplants
that contained human cells. Specifically, RNA from both preXMRV
genomes must have been packaged in a newly formed viral particle, or
virion. When that virion infected a human cell derived from the
prostate tumor, the reverse transcriptase enzyme accidentally mashed
up the preXMRVs and created XMRV. “It's a very elegant study,” says
phylogeneticist Stéphane Hué of University College London. “This is
the birth date of the virus.”
Hammering the nail in further, Oya Cingöz in Coffin's lab looked for
XMRV in dozens of inbred and wild mice and reported that she found no
evidence that the virus naturally exists.
Coffin believed earlier that studies linking XMRV to human disease
deserved serious attention. He co-authored an article in the 23
October 2009 issue of Science, which included the first report of XMRV
in patients who had chronic fatigue syndrome. Led by Vincent Lombardi
of the Whittemore Peterson Institute in Reno, Nevada, and NCI's
Francis Ruscetti, the study provided more evidence that, as Coffin's
piece stated, “transmission happened in the outside world and was not
a laboratory contaminant.”
Now, Coffin has changed his thinking. “It's all contamination,” he
says. At this point, Coffin questions whether any human has been
infected with the virus. “It remains a distant possibility,” he says.
Hué, who works with Greg Towers in London, presented complementary
22Rv1 data at the conference that they published 20 December 2010 in
Retrovirology (Science, 7 January, p. 17). They showed that XMRVs
isolated from different 22Rv1 cell lines were more genetically diverse
than sequences reportedly found in chronic fatigue and prostate cancer
patients. If XMRV infected humans, copied itself, and spread to
others, Hué says he would expect to see more diversity in the patients
as it evolved to escape immune defenses. “I don't think XMRV is a
human pathogen,” Hué says. “It's as simple as that.” Like Coffin, he
doubts that XMRV has even infected a human but adds that “one can
never say that something doesn't exist.”
“The evidence coming out at this meeting is incredibly impressive, and
the weight of evidence is indicating that this is not a major human
virus in terms of pathogenesis,” says Michael Busch, who heads the
Blood Systems Research Institute in San Francisco, California, and is
part of a working group convened by the U.S. Department of Health and
Human Services to examine whether XMRV poses a threat to the country's
blood supply.
But Busch said that before he concludes XMRV is simply a contaminant,
he wants to see the results of studies they are coordinating between
several labs with samples from agreed-upon patient and negative
controls, as well as blood donors. If most of these fail to find the
virus, Busch says, “it's going to eliminate concerns” that XMRV has
caused these diseases.
Even if XMRV has not harmed humans, Busch says we got lucky. This is
the first accidental generation of a retrovirus that can infect human
cells. “It's a warning shot,” Busch says. “We've created a highly
infectious virus that may transmit to humans.”
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