XMRV as a Human Pathogen?
Mark A. Wainberg1 and Kuan-Teh Jeang2,*
1McGill University AIDS Centre, Jewish General Hospital, 3755 Chemin Cote Ste-Catherine, Montreal, Quebec H3T 1E2, Canada
2Molecular Virology Section, LMM, NIAID, NIH, Building 4, Room 306, Bethesda, MD 20892-0460, USA
*Correspondence: kjeang@nih.gov
DOI 10.1016/j.chom.2011.04.001
Xenotropic murine leukemia virus-related virus (XMRV) has been proposed to be associated with prostate cancer and chronic fatigue syndrome (CFS). This proposition has been controversial because many investigators have failed to replicate the reported associations. Here, we explore whether XMRV is an authentic human pathogen in the light of recent findings that indicate otherwise.
Background
Xenotropic murine leukemia virus-related virus (XMRV) is a recently described gammaretrovirus that was originally detected in and molecularly cloned from a human prostate cancer (Urisman et al., 2006). A later paper reported that XMRV was present in the blood of 67% of patients with chronic fatigue syndrome (CFS) and in 4% of healthy controls (Lombardi et al., 2009). Since these publications, some investigators have detected XMRV in human prostate cancer samples while others have not (Silverman et al., 2010; Aloia et al., 2010). Aside from a single study that reported finding polytropic murine leukemia virus sequences (a virus related to but different from XMRV) in 87% of CFS samples (Lo et al., 2010), all other reports (at least seven) have failed to detect XMRV in CFS patients from Europe, China, and the U.S. (van der Kuyl et al., 2011).
The current state of XMRV as a human pathogen is controversial and remains confounding even to the most knowledgeable retrovirology aficionados. In this context, it is important to frame two separate issues for consideration. First, is XMRV a physiologically prevalent pathogen in humans? Second, based on where the virus has been detected in humans, is XMRV a passenger microbe or is it causal for prostate cancer and/or CFS?
XMRV Can Infect Human Cells
The receptor for XMRV is XPR1, a cell-surface protein ubiquitously expressed in many human and animal cells. Although XMRV was originally described as infecting human prostate stromal cells (Urisman et al., 2006), in vitro assays show that the virus can infect and replicate in various human, feral mouse, mink, monkey, and bovine cell lines (Stieler et al., 2010). A recent intravenous infection study of eight adult Indian rhesus macaques (five infected and three mock-infected controls) used immunohistochemical staining and in situ hybridization-based assays to show that the virus was widely disseminated in vivo. Infected tissues included lymphoid organs (CD4-positive cells) as well as the prostatic epithelium and reproductive tract (Onlamoon et al., 2011). These results show that XMRV has the capability to infect humans. Nevertheless, some of the controversy surrounding XMRV centers on the possibility that detection of this virus in human samples is due to contamination from mouse
cells (Smith, 2010). A significant argument against this stance was the identification of 14 XMRV integrated proviral sequences in 9 human prostate cancers (Kim et al., 2008). However, a recent report showed that 2 of these 14 integrated proviral sequences were contaminants from an experimentally infected cell line that was propagated in the laboratory, raising the question of whether the other 12 reported XMRV prostate cancer integration sequences may not also be erroneously tainted results (Garson et al., 2011).
Unlike other human pathogenic/oncogenic viruses such as human T cell leukemia virus (HTLV-1) or human papilloma virus (HPV) (Table 1), there is currently no conclusively reproducible epidemiological link for an XMRV-related human disease (prostate cancer or CFS) or a natural XMRV reservoir in humans, mice or other animals (Figure 1). In contrast to other ‘‘newly discovered’’ viral infections such as the severe acute respiratory syndrome (SARS) coronavirus or avian influenza virus, the infection source of XMRV for humans has remained elusive. Indeed, some investigators have argued that the high conservation of XMRV genome sequences reported from different geographic locales by multiple laboratories are inconsistent with significant infection or extensive replication of this virus in humans (Hue´ et al., 2010).
The Case of XMRV and Prostate Cancer
In developed economies, prostate cancer is the most common noncutaneous malignancy in men. The prevalence of prostate cancer is associated with age. Thus, approximately 50% of men over the age of 70 have asymptomatic in situ prostate cancer that does not significantly impact longevity. Given the large at-risk population for prostate cancer, a potential association with XMRV reported in two studies from the U.S. (Urisman et al., 2006; Schlaberg et al., 2009) is significant, since it suggests the possible use of antiretroviral drugs (ARVs) as a therapeutic or preventive measure. On the other hand, because ARVs are not devoid of serious side effects, one must not use such drugs if XMRV is not the cause of disease. Therefore, it is vital to unravel mechanistic explanations regarding how XMRV could be oncogenic in order to carefully consider the plausibility of a virus-prostate cancer link.
Although data from two laboratories (R. Silverman and I. Singh) were instrumental in raising a connection between XMRV and prostate cancer, there are critical details on which the two studies differ (Urisman et al., 2006; Schlaberg et al., 2009). For example, Silverman and colleagues (Urisman et al., 2006) reported that XMRV infection was restricted to prostatic stromal fibroblasts that surround malignant prostatic epithelial tissue, while Singh and coworkers (Schlaberg et al., 2009) identified XMRV-positive cells primarily in the cancerous prostatic epithelium. A nonparsimonious interpretation of the two studies is that XMRV causes prostate cancer through more than one mechanism. Another conundrum is that one of the two studies reported an association between XMRV-cancer and a function-attenuating R462Q polymorphism in the interferon-induced RNASEL gene in patients (Urisman et al., 2006), while the other study found no such association (Schlaberg et al., 2009).
A further puzzle is the extremely low copy number of XMRV genomes present in putatively virus-positive prostate cancers. It is well accepted that retroviral transformation of cells occurs in one of two ways: transduction by the virus of an oncogene into the cell (whether a cellular oncogene or a virus-encoded oncogene such as HTLV-1 Tax, Table 1) or insertional mutagenesis by the retrovirus that results in the activation of an endogenous cellular oncogene. Both mechanisms are known to result in virus-induced clonal tumors that contain one or more copies of the proviral DNA sequence per transformed cell. The finding that XMRV sequences in virus-positive prostate cancers exist at fewer than one copy per cancer cell (Danielson et al., 2010) posits that either XMRV employs a transformation mechanism unprecedented for other retroviruses or that virus infection of the cell per se is not the direct cause of cancer. In this context, it is relevant (1) that recent findings have suggested that XMRV may itself have resulted from a recombination event involving murine leukemia virus ancestors (van der Kuyl et al., 2011) (abstracts from CROI 2011 meeting) that produced a virus that infected human prostate tumor cells being experimentally passaged in mice, and (2) that multiple new studies have failed to detect XMRV sequences in prostate cancer biopsies.
The Case of XMRV and CFS
The original study that identified a link between XMRV and CFS (Lombardi et al., 2009) remains unconfirmed and has been questioned in regard to the prevalence of in vivo XMRV infection and its causal disease relationship to CFS. Like the connection between XMRV and prostate cancer, several studies now claim that the identification of XMRV sequences in CFS samples has resulted from laboratory contamination involving either faulty primers that were used in PCR analyses or laboratory tainting of human-tissue samples by mouse cells containing XMRV sequences (Smith, 2010; van der Kuyl et al., 2011). Others have failed to identify antibodies to XMRV in the sera of individuals suffering from CFS (Satterfield et al., 2011). Moreover, there has been no clarification on the causal mechanism(s) by which XMRV is responsible for CFS. The fact that XMRV can infect lymphoid tissues, which is not contested, does not establish causality of CFS. This may, in fact, represent a wide array of discrete conditions of unknown etiologies that have been grouped together for reasons of convenience, much as many variations of human hepatitis were termed non-A, non-B for many years, simply because no other definitive hepatitis viruses had yet been identified.
Unresolved Issues and Future Perspectives
There is significant uncertainty in postulating a link between XMRV and either prostate cancer or CFS. Unless and until more definitive reports of in vivo prevalence and disease causality are published, it is probably judicious to assume that some of the currently reported findings may be erroneous. In the bigger picture of XMRV as a public health concern, the following risk-benefit considerations warrant discussion.
1. There is already a suggestion that blood agencies should immediately develop screening tools to prevent the potential transfusion of XMRV in human blood or the acquisition of XMRV through organ transplantation. However, there is still no consensus on the serological- and nucleic acid-based methodologies that are most suitable for executing this screening. In the absence of conclusive evidence of in vivo prevalence and causality (risk) for diseases, it should be recognized that the potential development and implementation of such screenings would be very expensive and have unquantifiable benefits. Thus, it could be that countless units of blood would not be donated or would be sacrificed needlessly without avail. Decision makers should be scrupulously cautious that some individuals and/or companies might stand to gain financially from the notion that XMRV is a human pathogen. A potential conflict of interest was recently alleged in reports on the controversy surrounding the hypothesis that vaccination with the measles, mumps, rubella virus vaccine might cause autism. Lessons learned from this and other past episodes could inform
coolheaded deliberations on future XMRV policies.
2. In regard to CFS, the possibility that XMRV may be a causal pathogen has led many affected individuals to turn to ARVs as possible remedies for their conditions. Usually, this has involved the use of nucleoside reverse transcriptase inhibitors (NRTIs), since these compounds have been shown to be active against the reverse transcriptase (RT) enzyme of XMRV as well as against those of HIV-1, HIV-2, and other retroviruses. At present, there are no clinical reports of benefit following such anecdotal use of ARVs in CFS. Even if XMRV is linked to some CFS cases, it is almost certain that many and perhaps most CFS cases are not caused by XMRV since CFS is a heterogeneous entity. Scientific and regulatory communities might have a responsibility to endorse the appropriate drug therapy if some cases of XMRV-CFS could be established and to strongly discourage the use of ARVs in cases where it is shown beyond a reasonable doubt that an XMRV-CFS link does not exist.
3. Decisions regarding therapeutic or preventive approaches to the XMRV-prostate cancer question are equally complex. As stated above, it has been argued that approximately 50% of men over the age of 70 have asymptomatic in situ prostate cancers that do not significantly impact longevity. Because 50% of men over the age of 70 are unlikely to acquire new XMRV infection, it is imperative that one asks how many, if any, of these age-associated cases are caused by XMRV. If XMRV does cause some prostate cancers, how many longevity-critical cases versus longevity-neutral cases are associated with this virus? In the long run, resource-consuming interventions require a clear cut risk-benefit justification.
4. An important general issue raised by the current XMRV-disease debate is how to approach future medical questions of this nature. Undoubtedly, real and rumor viruses will continue to be discovered in the coming years and probably at accelerated speed with the popularity of next-generation ultrasensitive deep-sequencing technology. In future episodes, what should suffice as proof of a virus-disease link? How hard should we chase down every one, two, or three reports of a pathogen-disease axis? Even more pertinent, how much counter-evidence do we need to accumulate before we stop and close the file on a virus-disease hypothesis? If the current XMRV controversy helps torefine our future decision thinking, then the considerable energy, resources, and emotions that have been expended in this debate could have benefits beyond CFS and prostate cancer for yet undiscovered infectious diseases.
ACKNOWLEDGMENTS
Work in K.T.J.’s laboratory is supported in part by intramural NIAID funds and by the IATAP program from the office of the Director, NIH. Work in M.A.W.’s laboratory is supported by the Canadian Institutes for Health Research (CIHR) and by NIAID, NIH. We thank Tonja van der Kuyl, Ben Berkhout, and Malcolm Martin for critical readings of the manuscript. The views expressed in this article are the personal opinions of the authors and do not necessarily reflect the views of the authors’ employers, the U.S. National Institutes of Health, and McGill University.
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Controversial is an exaggeration as negative is merely that. An inability to detect. If any of the negative papers had used the methodology from Lombardi et al then it would most certainly be controversial. So far no evidence has been presented that would refute the findings of that study or Lo et al. Although a number of authors have claimed to have used those assays, a quick read of the relevant papers demonstrates this to be incorrect.
The US and UK have in fact both declared that it is a human retrovirus.
“XMRV can infect humans but there is currently no evidence that it causes human disease.”
National Expert Panel on New and Emerging Infections (7 May 2010)
http://www.dh.gov.uk/prod_consum_dh/groups/dh_digitalassets/@dh/@ab/documents/digitalasset/dh_118427.pdf
“XMRV is a newly identified human retrovirus that is similar to a group of mouse retroviruses (called murine leukemia viruses, or MLVs) scientists have known about for years.”
(CDC, 23 Aug 2010)
http://www.cdc.gov/xmrv/index.html
XMRV is a hybrid, part polytropic and part xenotropic. So it is misleading for the authors of the paper to state:
“Aside from a single study that reported finding polytropic murine leukemia virus sequences (a virus related to but different from XMRV) in 87% of CFS samples (Lo et al., 2010)”
and to state that:
“Indeed, some investigators have argued that the high conservation of XMRV genome sequences reported from different geographic locales by multiple laboratories are inconsistent with significant infection or extensive replication of this virus in humans (Hue´ et al., 2010).”
As this excludes those polytropic sequences detected by Lo et al, that show HGRV to have an enormous diversity. Lo et al was a confirmatory paper, and the team at the WPI, NCI and elsewhere are finding these various strains. (Xeno, poly, and modified poly)
Although several papers have attempted to construct a hypothesis for contamination, the data on HGRV does not fit, and consequently they have no option but to alter their hypothesis.
Statements about the virus not having been proven to cause disease are at this stage irrelevant as no paper has been published that has attempted to answer this question directly. It is also highly unlikely that such a virus would not cause disease.
One of the authors of this is Kuan-Teh Jeang, who is also the editor of Retrovirology. The journal that tried to declare HGRV over in December. They like to publish lots of negative papers, that use unvalidated assays.
As those of us who read the full papers will know, Satterfield et al and all the other negative studies so far have failed to replicate the methods used by Lombardi et al.
We have yet to see a full replication paper of the Lombardi study. We await the results of the two NIH HGRV studies, which are ongoing at this time.
There are a handful of people in the United States trying ARVs. The numbers are very low. The real need is for clinical trials of not only ARVs, but also Ampligen and GcMAF, with the unique HGRV positive cytokine profile monitored to judge success.
I hope research by the many scientists who have found HGRVs associated with disease is allowed to go forward, and that those in control of the purse strings will keep an open mind and fund research, especially that of the WPI, until concrete evidence is obtained, one way or the other.
As someone with Canadian Consensus Definition ME, I have always known that I am suffering from inflammation, caused by an infection. I can feel it in my bones.
This is a letter from Dr. Michael Snyderman, the Director of Clinical Oncology for the WPI: I repost it from X Rx, http://treatingxmrv.blogspot.com as it seems apposite to the discussion.
Dear Dr. Deckoff-Jones:
It is a shame that the WPI has lost time from having to be on the defensive. I am a believer in MLRVs causing human disease. The supporting data actually goes back to the 1970s. Three different research labs including Robert Gallo’s the co-discoverer of HIV found MLRVs in many different types of lymphoma and leukemia. I believed in “XMRV” enough that when Dr. Mikovits told me that my blood sample was positive, I put myself on AZT and raltegravir for my chronic lymphocytic leukemia and CFS. At the time I started treatment (5/27/10) both were getting worse quickly. I had no treatment for cancer or CFS prior to ARV’s.
I have been a hematologist/oncologist for 40 years. I am tired and am not cured but I am improved at 11 months on treatment. Both the leukemia and the CFS improved coincident with disappearance of infectious virus from my plasma.
I am plotting total lymphocyte count, total B-cells (CD19) and an unfavorable clone of CLL cells (trisomy12). As you can see, my counts were going up rapidly and now are slowly improving. To my eye it looks convincing that there has been a definite biological change in the leukemia. I also feel somewhat less fatigued and my neuropathic pain is gone. Attached are my lymphocyte count, CD19 (leukemia cells) and trisomy 12 cells (the worst group of cells of my leukemia), as well as my original cytokine profile, vs. 9 months of treatment.
There is no other reasonable explanation than that XMRV, or another retrovirus, was at least making things worse, and subtracting its influence has improved my disorders. This is proof of principle in my opinion. Obviously, an enormous amount of work needs to be done. For example, we don’t have a protease inhibitor, and ARVs are not going to be the only answer. My perspectives are mine, but I believe there is a scientific basis.
I have B-cell CLL with unfavorable prognostic factors: doubling time < 1 year and trisomy 12. I also had the characteristic elevations of cytokines, fatigue and pain seen with CFS. I started AZT and raltegravir May 2010, because I felt things were deteriorating and I had little to lose. More importantly, if I was successful, it could translate into a treatment option for cancer patients that otherwise could not be helped.
The use of ARVs for treatment of cancer is not new; it was used in the 1990s for treatment of HTLV-1 associated lymphoma and did help. I knew then that if I used it on patients I would be vulnerable to censure from my fellow oncologists. I look at the fact that I got CLL as the opportunity of a lifetime to do some good. I am a one person project. I hope sharing my data will encourage people that we will prevail. I will be submitting this data to an online Hematology Oncology journal soon. Some excerpts from the paper are included below.
Michael Snyderman, MD