31 MAY 2011
The novel human retrovirus XMRV has been associated with prostate
cancer and chronic fatigue syndrome. The nucleotide sequence of XMRV
isolated from humans indicates that the virus is nearly identical with
XMRV produced from a human prostate tumor cell line called 22Rv1. This
cell line was derived by passage of human prostate tumor tissue in
nude mice. Sequence analyses reveal that the genomes of these mouse
strains contain two different proviral DNAs related to XMRV. These
viral genomes recombined to produce XMRV that has been isolated from
XMRV was originally isolated from a human prostate cancer in 2006, and
subsequently associated with ME/CFS. The human cell line 22Rv1, which
was established from a human prostate tumor (CWR22), produces
infectious XMRV. An important question is whether XMRV was present in
the original prostate tumor, or was obtained by passage through nude
mice. To answer this question, DNA from various passages of the
prostate tumor in nude mice (called xenografts), and the mouse strains
used to passage the tumor, were analyzed for the presence of XMRV
Early-passage xenografts did not contain XMRV, but mouse cells found
in them did contain two related proviruses called PreXMRV-1 and
PreXMRV-2. The 3'-3211 nucleotides of PreXMRV-1, and both LTRs, are
identical to XMRV save for two nucleotide differences. The genomic
5'-half of XMRV and PreXMRV-1 differs by 9-10%. PreXMRV-1 is defective
for replication due to mutations in genes encoding the gag and pol
proteins. PreXMRV-2 does not contain obvious mutations that would
prevent the production of infectious viruses. The gag-pro-pol and a
part of the env region of this viral genome is identical to that of
XMRV save for two base differences; the LTRs and the remainder of the
genome differ by 6-12% from XMRV.
Comparison of the sequences of PreXMRV-1 and PreXMRV-2 indicates that
recombination between the two viral genomes led to the formation of
XMRV. When the sequences of PreXMRV-1 and -2 are used to construct the
recombinant XMRV, the resulting virus differs by only 4 nucleotides
from the consensus XMRV sequence derived from all human isolates
reported to date.
The nude mice used for passage of the original prostate tumor were
likely the NU/NU and Hsd strains. Neither mouse strain contains XMRV
proviral DNA, but both contain PreXMRV-1 and PreXMRV-2 proviral DNA.
These data demonstrate that XMRV was not present in the original CWR22
prostate tumor, but arose by recombination of PreXMRV-1 and PreXMRV-2
between 1993-1996. When the original prostate tumor was implanted into
nude mice, some of the mice harbored both pre-XMRV-1 and -2 endogenous
proviruses, which recombined to form XMRV. The authors believe that
XMRV originating from the CRWR22 xenografts, the22Rv1 cell line, or
other related cell lines has contaminated all human samples positive
for the virus. In addition, they suggest that PCR assays for XMRV may
actually detect PreXMRV-1 and -2 or other endogenous viral DNA from
contaminating mouse DNA.
Another possibility to explain the origin of XMRV is that it arose in
mice and can infect humans. If this is true, then XMRV would have to
be present in the nude mice used to passage the CWR22 human prostate
tumor. No evidence for an XMRV provirus was found in 12 different nude
mouse strains, including two used to passage the CWR22 tumor.
Furthermore, a screen of 89 inbred and wild mice failed to reveal the
presence of proviral XMRV DNA. Hence the authors conclude:
...that XMRV arose from a recombination event between two endogenous
MLVs that took place around 1993-1996 in a nude mouse carrying the
CWR22 PC xenograft, and that all of the XMRV isolates reported to date
are descended from this one event.
It is possible that XMRV produced during passage of CWR22 in nude mice
subsequently infected humans. Because XMRV arose between 1993-1996, this scenario could not explain cases of prostate cancer and chronic fatigue syndrome that arose prior to that date.
How can these findings be reconciled with the published evidence that sera of ME/CFS patients from the 1980s contain antibodies to XMRV?
Those antibodies were not shown to be directed specifically against
XMRV, and therefore cannot be used to prove that XMRV circulated in
humans prior to 1993-96. Furthermore, in the absence of clear
isolation of an infectious virus, antibody tests alone have proven
highly unreliable for identification of new viruses.
Where do these findings leave the hypothesis that XMRV is the
etiologic agent of prostate cancer and ME/CFS? All published sequences
of human XMRV isolates are clearly derived by recombination of
PreXMRV-1 and -2. The finding of human XMRV isolates that are not
derived from PreXMRV-1 and -2 would leave a role for XMRV in human
disease. As of this writing, no such XMRV isolates have been reported
in the scientific literature.
Update: A second paper has also been published in Science Express
today entitled "No evidence of murine-like gammaretroviruses in CFS
patients previously identified as XMRV-infected". Editors of the
journal Science have asked the authors to retract their 2009 paper
linking XMRV infection with chronic fatigue syndrome. The authors have
T. Paprotka, K. A. Delviks-Frankenberry, O. Cingoz, A. Martinez, H.-J.
Kung, C.G. Tepper, W-S Hu, M. J. Fivash, J.M. Coffin, & V.K. Pathak
(2011). Recombinant origin of the retrovirus XMRV. Science Express