New Study on the Detection of Murine Leukemia Virus-related Virus Gene Sequences in the Blood of Pat

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New Study on the Detection of Murine Leukemia Virus-related Virus Gene Sequences in the Blood of Patients with Chronic Fatigue Syndrome (CFS) and Healthy Blood Donors - Questions and Answers

Questions about Murine Leukemia Viruses (MLV) and Chronic Fatigue Syndrome (CFS) New Study

What are murine leukemia viruses?
Murine leukemia viruses (MLV) are retroviruses known to cause cancer in certain mice. In 2006, investigators found that a type of MLV, called xenotropic murine leukemia virus-related virus (XMRV), could potentially infect humans. XMRV is one of a number of MLVs that appear to be transmitted to humans.

What is CFS?
Chronic fatigue syndrome (CFS) is a debilitating disorder defined solely by clinical symptoms and the absence of other causes. It’s unknown what causes CFS.

Has MLV or XMRV previously been associated with CFS or other disease?
A previous study, published in the journal [Lombardi et. al. Science October 23, 2009 326: 585], reported finding XMRV in a high percentage of CFS patients and a small percentage of healthy blood donors. However, other studies conducted in the U.S., Netherlands, and UK did not detect evidence of XMRV or other MLV-related viruses in CFS patients.
XMRV was first identified in tissue samples from some prostate cancer patients in 2006. However, one subsequent study failed to find XMRV in prostate cancer tissues, and another study found the virus only rarely in such tissues.

What did the new study evaluate?
Investigators from the Food and Drug Administration’s (FDA) Center for Biologics Evaluation and Research, the National Institutes of Health (NIH) Clinical Center, and Harvard Medical School have published a study in the scientific journal Proceedings of the National Academy of Sciences that examines the presence of MLVs in blood collected from two groups -- patients diagnosed with CFS and healthy blood donors.
This study tested blood samples collected from the New England area in the mid-1990s from 37 patients diagnosed with CFS, as well as samples from 44 healthy blood donors collected in the Clinical Center Blood Bank, NIH, between 2003 and 2006. Investigators performed DNA sequencing on each sample that produced positive product for verification of MLV-like gene sequences. Diverse MLV gene sequences, similar to that of the recently discovered XMRV, were identified in samples from 32 of the 37 patients with CFS (86.5%) and 3 of the 44 (6.8%) healthy blood donors that were tested.
Follow-up samples were collected from 8 of the CFS patients in 2010, and 7 of these again tested positive for MLV-like gene sequences.

What did the new study conclude?
This study supports a previous investigation[Lombardi et al. Science October 23, 2009 326: 585]that showed XMRV, a genetic variant of MLV-like viruses, to be present in the blood of people with CFS. The study demonstrates a strong association between a diagnosis of CFS and the presence of MLV-like virus gene sequences in the blood. The study also showed that MLV-like viral gene sequences were detected in a small fraction of healthy blood donors. Although the statistical association with CFS is strong, this study does NOT prove that these retroviruses are the cause of CFS. Further studies are necessary to determine if XMRV or other MLV-related viruses can cause CFS.
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