Researchers investigating UK samples have found no association between the controversial xenotropic murine leukaemia virus-related virus (XMRV) and chronic fatigue syndrome (CFS). Their study, published in BioMed Central's open access journal Retrovirology, calls into question a potential link described late last year by an American research team.

Kate Bishop from the MRC National Institute for Medical Research worked with a team of researchers to test blood and serum samples from 170 CFS patients and 395 healthy controls, using quantitative PCR and a virus neutralization assay. She said, "No association between XMRV infection and CFS was observed in the samples tested, either by PCR or serological methodologies. Our findings therefore appear inconsistent with the previous report that isolated XMRV from the blood cells of CFS patients. We are confident that, although we were unable to replicate the detection, our PCR assay is more sensitive than the earlier method and possessed the necessary sensitivity to detect XMRV had it been present."

Bishop and her colleagues point out that CFS likely encompasses a range of diseases, and it is still possible that some of them might be associated with XMRV infection. They say, "There has been much discussion and controversy amongst CFS researchers and patients alike, which highlights the need for additional investigations in this area. Following our findings, it would seem a prudent next step for subsequent studies to compare samples and protocols between different laboratories around the world."

The findings of this Retrovirology study are supported by results from a recently published work from Imperial College London that also found no proof that XMRV infection is associated with CFS.

Comments from Whittemore Peterson Institute

WPI is aware of the recent UK study that was unable to detect the presence of XMRV in any CFS patient samples. Although researchers at the WPI were not involved in this project, our work in XMRV continues with researchers around the world. We look forward to the results of studies which replicate the methods used in the original research described in the journal Science in October, 2009.

Information Regarding XMRV Studies

1. The authors of the Science paper established the existence of XMRV as an infectious human blood borne retrovirus for the first time in blood of patients diagnosed with Chronic Fatigue Syndrome (CFS). Previous studies had established the presence of XMRV sequences and protein in human prostate tissue.

2. In the Science paper, the presence of XMRV in well-characterized patients with CFS was established using multiple technologies:

a) PCR on nucleic acids from un-stimulated and stimulated white blood cells;
b) XMRV protein expression from stimulated white blood cells;
c) Virus isolation on the LNCaP cell line; and
d) A specific antibody response to XMRV.

3. The authors of the two UK studies did not attempt to "replicate" the WPI study. Replication requires that the same technologies be employed. The WPI sent reagents and information to several groups of researchers in an effort to support their replication studies. Neither UK study requested positive control blood, plasma or nucleic acids from the WPI.

4. The collection, preparation and storage of DNA were completely different between the Science and UK papers. The latter studies do not show data on blood harvesting or storage. Nor do the studies disclose the quantity of isolated cells. Insufficient number of cells analyzed may result in failure to detect a low copy virus like XMRV, regardless of the sensitivity of the assay. Neither UK study provides detail to allow interpretation of how many white blood cells were analyzed.

5. Patient population selection may differ between studies.

6. The UK authors were unable to detect XMRV, even though 4% of healthy individuals were found to be infected in the US. Japanese scientists detected XMRV in 1.7% in healthy blood donors in Japan. The two previously identified human retroviruses have distinct geographical distributions.

7. Perhaps the most important issue to focus on is the low level of XMRV in the blood. XMRV is present in such a small percentage of white blood cells that it is highly unlikely that either UK study's PCR method could detect it using the methods described. Careful reading of the Science paper shows that increasing the amount of the virus by growing the white blood cells is usually required rather than using white blood cells directly purified from the body. When using PCR alone, the Science authors found that four samples needed to be taken at different times from the same patient in order for XMRV to be detected by PCR in freshly isolated white blood cells. More importantly, detection methods other than PCR showed that patients whose blood lacks sufficient amount of XMRV detectable by PCR are actually infected. This was proven by the isolation of viral proteins and the finding of infectious XMRV isolated from the indicator cell line LNCaP. The authors of the Retrovirology paper admit that their neutralization assay did not detect bacterially expressed XMRV gag and that positive control sera was needed to validate their assay. The WPI's monoclonal antibodies specifically and sensitively completed the immune response demonstrating the assays sensitivity and specificity for XMRV envelope.

Simply stated the only validated reliable methods for detecting XMRV in CFS patients, to date, are the methods described in Science. Failure to use these methods and validated reagents has resulted in the failure to detect XMRV. A failure to detect XMRV is not the same as absence of this virus in patients with CFS.