Failure to detect the novel retrovirus XMRV in chronic fatigue syndrome.
In October 2009, an American research group published a paper in Science which reported that they had found evidence of a new retrovirus called XMRV (xenotropic murine leukaemia virus-related virus) in a very high percentage (68/101) of people with ME/CFS – whose diagnosis met with both 1994 CDC/Fukuda research criteria and the Canadian clinical criteria. This compared to only 8/218 positive tests in the healthy controls.
The MEA has provided regular website updates on these findings and offered to help fund further research studies which would attempt to replicate these findings. Our latest XMRV update can be found here.
A number of research groups both here and abroad are now carrying out XMRV replication studies using stored blood samples.
The first study to be reported in the medical literature comes from a very reputable virology/infectious disease group at Imperial College in London. The group obtained stored blood samples from patients who have been attending the King's College Hospital ME/CFS service.
The virologists examined 186 blood samples from the KCH patients who met Fukuda/CDC criteria for CFS using sensitive molecular testing techniques. DNA (viral genetic material), which was extracted from the blood samples, was screened for XMRV provirus and for the closely related murine leukaemia virus (MLV) by nested PCR (polymerise chain reaction)using specific oligonucleotide primers. PCR is a highly sensitive method that can locate tiny viral fragments. No molecular evidence of XMRV or MLV sequences was found in any of the ME/CFS samples.
These results clearly represent a major difference in scientific opinion on the possible role of XMRV in ME/CFS.
Among the explanations that could be relevant are:
1 The use of different types of ME/CFS patients in the two studies. The American patients had ‘severe disability', were diagnosed using both CDC/Fukuda and Canadian clinical criteria, and were obtained from a small group of private physicians who take a very biomedical approach to ME/CFS. The UK sample, who had ‘high levels of disability', were diagnosed using only Fukuda/CDC criteria and came from King's College Hospital in London – an NHS tertiary referral centre that specialises in behavioural interventions.
2 There may be different prevalence rates for XMRV in different countries and it is interesting to note that German researchers were unable to replicate the American results in relation to the presence of XMRV in patients with prostate cancer.
3 The UK and USA laboratories used slightly different techniques for investigating the presence of XMRV and there may have been differing levels of risk in relation to the possibility of laboratory XMRV contamination.
Comment from Dr Charles Shepherd, honorary medical adviser to the ME Association:
“The ME Association has taken a cautious and open-minded view about the initial XMRV findings and offered to help fund further research into what could be a very significant finding. Although these UK results are clearly questioning the validity of the American findings, no single study can be regarded as being conclusive. So we believe it is important to wait for the results of further replication studies before drawing any firm conclusions about the possible role or pathogenicity (disease-causing ability) of XMRV in ME/CFS. In the meantime, there seems little point in people with ME/CFS spending large sums of money in arranging private tests for XMRV. And in our current state of uncertainty it would not be appropriate for doctors to start prescribing antiretorviral treatment to people with ME/CFS”.