NCI’s Vinay K. Pathak on the “De-Discovery” of a Retrovirus-Disease Link
Fast Breaking Paper Commentary, 2012
According to a recent analysis of Thomson Reuters Web of Science, the 2011 report “Recombinant origin of the retrovirus XMRV,” (T. Paprotka, et al., Science 333(6038): 97-101, 1 July 2011) has been identified as a Fast Breaking Paper in Microbiology. This designation is based on the paper’s increase in citations as tracked over two recent, successive bimonthly periods. The report, cited 63 times in the Web of Science as of August 3, 2012, displayed a higher bimonthly citation increase than any other paper of comparable age and type in its field.
Corresponding author Vinay K. Pathak is head of the Viral Mutation Section, HIV Drug Resistance Program, National Cancer Institute, Frederick National Laboratory for Cancer Research, Maryland.
Why do you think your paper is highly cited?
Two studies in 2009 reported that xenotropic murine leukemia virus-related virus (XMRV), a gammaretrovirus, is associated with a high percentage of human prostate cancers and chronic fatigue syndrome. These publications were greeted with much interest from scientists and patients. For the scientists, the papers suggested that XMRV may be circulating in the human population and that it might be contributing to many human diseases. As a result, many groups began their own studies to elucidate XMRV’s replication, its interactions with the human host, and its presence in their patient cohorts. Because of the intense interest among researchers in XMRV and its role in human disease, our paper on the recombinant origin of XMRV has been highly cited.
Does it describe a new discovery or new synthesis of knowledge?
Our paper showed that XMRV is most likely a laboratory-derived mouse virus that was generated through recombination between two endogenous murine retroviruses during propagation of a prostate cancer xenograft in the mid-1990’s. Since prostate cancer has been around for decades, and chronic fatigue syndrome has been described at least since the mid-1980’s, our results strongly imply that XMRV is not associated with these human diseases. Instead, our results suggest that all positive reports of XMRV in human tissues and samples are due to contamination with mouse DNA, viral plasmid DNA, or XMRV produced from a prostate cancer cell line named 22Rv1. Thus, our paper describes a “de-discovery” and new synthesis of knowledge.
“The major significance of our paper is that scientists interested in prostate cancer and chronic fatigue syndrome can now redirect their efforts towards identifying the real causes of these diseases.”
Would you summarize the significance of your paper in layman's terms?
XMRV was generated during a laboratory experiment, and the formation of XMRV is almost certainly a unique event. Consequently, it is very unlikely that XMRV is replicating or circulating in the human population and contributing to human disease. The major significance of our paper is that scientists interested in prostate cancer and chronic fatigue syndrome can now redirect their efforts towards identifying the real causes of these diseases.
How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes you've encountered along the way?
Our laboratory has been working on human antiviral APOBEC3 proteins that potently inhibit replication of retroviruses. We knew from previous studies that human APOBEC3 proteins inhibit replication of HIV and murine leukemia viruses, which are closely related to XMRV. When we saw the report indicating that XMRV was apparently replicating at high levels in human peripheral blood cells, we wanted to know whether and how XMRV was overcoming these host defenses to replicate in humans. Later, we and others demonstrated that XMRV was highly sensitive to inhibition by APOBEC3 proteins and was unable to establish a spreading infection in human peripheral blood cells. These basic questions about the replication potential of XMRV in human cells led us to investigate the origin of XMRV.
This project involved a lot of detective work with regard to finding archived tissues and developing sensitive assays to detect small quantities of nucleic acids. The success and impact of these studies was in large part due to collaboration with all of my colleagues who are coauthors on this paper.
Where do you see your research leading in the future?
Recombinant viruses are often generated during xenograft passaging of human tumor tissues in immunodeficient mice. We are interested in the processes involved in generating such recombinants. The generation of XMRV provides an opportunity to gain insight into these processes, and we are currently elucidating the recombination potential of the two parental viruses that recombined to form XMRV.
Do you foresee any broader social implications or impact for your research?
Chronic fatigue syndrome is a disease of unknown etiology, and its association with XMRV generated a lot of hope and optimism for patients. Many patients believed that they might be successfully treated with antiviral drugs that are used to treat HIV-1 infection and AIDS. Unfortunately, our results indicate that XMRV cannot be the cause of their disease, and that treatment with antiviral drugs is not going to help chronic fatigue syndrome patients.
While the results of our studies have been disappointing to many scientists and patients, I believe they provide valuable lessons for future microbe hunters. First, it’s important to rigorously rule out contamination, which is likely to become an ever-greater issue as assays for detection of viral DNAs become increasingly more sensitive. Second, any claim of a novel virus must be substantiated with fully sequencing the entire viral genome, not just PCR fragments, and demonstrating that the virus is capable of replication in the species it is presumably infecting. Third, it is important to collect and process patient samples and negative controls in parallel to insure that contamination with the patient samples, but not the negative controls, does not lead to a false association of a microbe with disease. Finally, routine assays akin to mycoplasma testing should be developed to make it feasible for scientists to easily monitor their cell lines for virus contamination.
Vinay K. Pathak
Head, Viral Mutation Section
HIV Drug Resistance Program
National Cancer Institute
Frederick National Library for Cancer Research
KEYWORDS: RETROVIRUS, XMRV, GAMMARETROVIRUS, APOBEC3 PROTEINS, PROSTATE CANCER, CHRONIC FATIGUE SYNDROME