Another study by US researchers has discovered gene sequences of a family of mouse retroviruses in a high proportion of patients with Chronic Fatigue Syndrome (CFS), a serious systemic illness of unknown cause, also known as Myalgic Encephalomyelitis (ME); the researchers also found the same gene sequences in a small proportion of healthy blood donors.

However, the investigators cautioned that although their findings support the idea of a link between murine leukemia viruses (MLV) and CFS, more studies are needed to prove whether they actually cause the illness and also whether their presence in healthy blood donors constitutes a threat to the national blood supply.

The investigators were from the US Food and Drug Administration (FDA), the National Institutes of Health, and Harvard Medical School, and wrote about their findings in the 23 August online issue of the Proceedings of the National Academy of Sciences.

MLV is a type of retrovirus that causes cancer in mice. A retrovirus has a genome coded in RNA. When it invades the host cell its RNA converts to DNA, whereupon it buries itself in the host’s DNA and replicates using the host cell’s resources.

The debate about whether MLV-like retroviruses are strongly linked to CFS is fuelled by conflicting evidence: at least two North American studies have suggested there is a link between XMRV, xenotropic MLV-related virus (including one that found the viral sequences in prostate cancer patients), while separate reports from Europe state there is little if any evidence of XMRV being present in patients with either CFS or prostate cancer.

The authors referred to some of this evidence in their background information:

“A recent study identified DNA from a xenotropic murine leukemia virus-related virus (XMRV) in peripheral blood mononuclear cells (PBMCs) from 68 of 101 patients (67%) by nested PCR, as compared with 8 of 218 (3.7%) healthy controls.”

“However, four subsequent reports failed to detect any murine leukemia virus (MLV)-related virus gene sequences in blood of CFS patients,” they added.

However, in their own investigation, the researchers found several different MLV gene sequences in blood samples taken from 32 out of 37 (87 per cent) patients with CFS, and in only 3 out of 44 (7 per cent) of healthy blood donors.

They confirmed the MLV-like sequences via DNA sequencing on all positively amplified samples.

They also pointed out that unlike previous studies there was wide genetic diversity among the MLV-like viruses they found:

“In contrast to the reported findings of near-genetic identity of all XMRVs, we identified a genetically diverse group of MLV- related viruses,” they wrote.

The researchers concluded that:

“Further studies are needed to determine whether the same strong association with MLV-related viruses is found in other groups of patients with CFS, whether these viruses play a causative role in the development of CFS, and whether they represent a threat to the blood supply.”

In a separate commentary, scientists from the Institute of Molecular Genetics at the University of Montpelier in France, and the Department of Medicine at the University of Alberta in Canada, said the current evidence suggests a variety of MLV can be found in North Americans with and without CFS.

“To add to this bewilderment, it is likely that more than one environmental agent impacts on the development of both CFS and prostate cancer,” they wrote, and said it made sense therefore to push for extensive case-control studies.

And, they added, because we don’t have any robust theories that “prove a causal association with a prevalent agent and a chronic disease with genetic predisposition”, perhaps it may also be appropriate to conduct interventional studies, and they refer to the example of the discovery of Helicobacter pylori as a cause of peptic ulcer which was only established when an intervention study with antibiotics cured the disease.

“Detection of MLV-related virus gene sequences in blood of patients with chronic fatigue syndrome and healthy blood donors.”
Shyh-Ching Lo, Natalia Pripuzova, Bingjie Li, Anthony L. Komaroff, Guo-Chiuan Hung, Richard Wang and Harvey J. Alter.
Proceedings of the National Academy of Sciences, Published online before print 23 August 2010
DOI: 10.1073/pnas.1006901107

Related articles:

— Retrovirus Linked To Chronic Fatigue Syndrome (Oct 2009)
— New Virus Is Not Linked To Chronic Fatigue Syndrome, Suggests UK Research (Jan 2010)

Additional source: FDA.

Written by: Catharine Paddock, PhD