A mouse leukemia retrovirus, xenotropic murine leukemia virus-related virus (XMRVretrovirus), has been under consideration as a possible cause of chronic fatigue syndrome (CFS, and also prostate cancer). In a study published in Science in October 2009, Lombardi et al. found XMRV in 67% of CFS patients and 3.7% of controls. Several subsequent studies in the UK, the Netherlands, and the US — by lead authors Erlwein, van Kuppleveld, Groom , Switzer and Henrich — failed to find XMRV at all.
Now a new study published in Retrovirology by Hue et al. shows that the original positive findings were likely erroneous and due to contamination in the lab. The complete article is available online.
We provide several independent lines of evidence that XMRV detected by sensitive PCR methods in patient samples is the likely result of PCR contamination with mouse DNA and that the described clones of XMRV arose from the tumour cell line 22Rv1, which was probably infected with XMRV during xenografting in mice. We propose that XMRV might not be a genuine human pathogen.
Similarity of the XMRV genome in patients from widely separated geographic areas is inconsistent with the way a retrovirus typically changes as an infection spreads. Human cell lines are commonly contaminated with mouse viruses, and polymerase chain reaction (PCR) contamination has previously been found to underlie erroneous association between retroviruses and human disease. The genetic similarity of the XMRVs detected in patients is more consistent with contamination than with human infection. Primers previously used to detect XMRV were not specific to that virus, and previous attempts to assess contamination by detection of murine DNA alone were inadequate.
Contamination may have caused spurious results in studies of prostate cancer as well. Bryan et al. found XMRV in 22% of patients with prostate cancer. XMRV has been found in prostate cancer and in 1–6% of controls in other studies. Examination of infected prostate tumors showed that not all the tumor cells were infected with XMRV, suggesting that it did not cause those cancers.
The authors of the new study provide suggestions to prevent contamination or identification errors in future studies:
We conclude that future screens for MLV-related sequences use more rigorous PCR containment procedures, such as those used to reliably recover ancient DNA, or manage contamination by controlling for its inevitable frequency, for example by screening equal numbers of controls prepared and stored identically, together with test samples. Positive samples must be sequenced and those that are identical to known endogenous murine sequences, or plasmids present in the host laboratory, should be treated with caution.
When the first press release came out about the original Lombardi study, even before the article itself was available, our own Dr. Wallace Sampson commented on it; he was skeptical even about the basic idea that a virus could cause CFS and prognosticated:
Funny things happen. I’m betting that either the work will not be replicated, or that some other innocent explanation such as an artifact of lab technique accounting for viral presence will be found. I estimate the odds at 80:20 (4 to 1).
He won that bet, on both counts.
Last February I wrote about it after the first two subsequent studies had failed to confirm Lombardi’s results. I cautioned against jumping the gun to offer testing and treatment based on a reported but unconfirmed and questionable correlation that in no way proved causation.
In Sept 2010, Dr. Mark Crislip chimed in with his own speculations about possible viral etiologies for CFS. He said:
The nice thing is that, while it is fun to speculate, eventually the science will sort it out.
That sorting has now advanced another step.
This new study will not be the end of the debate and probably won’t even dissuade desperate CFS patients from prematurely requesting viral testing and anti-viral treatments. But the scientific process is working as it should. While a viral cause of CFS has not been conclusively ruled out, the evidence against XMRV far outweighs the evidence for it.
This article was originally published in the Science-Based Medicine Blog