The identification and confirmation of a viral cause for CFS would have major implications for those afflicted by this condition, which affects approximately 0.2% of the adult population (Jason, et al., 1995
). If a viral cause were identified, especially if treatable with current antivirals (as a retrovirus such XMRV presumably might be), this would provide new treatment options directed at the root cause of this condition, potentially benefiting millions of affected individuals.
Given the critical clinical need and the compelling nature of the original report, multiple laboratories, including our own, have sought to confirm the association of XMRV with CFS. To the surprise and disappointment of many, the majority of such studies reported to date have failed to confirm an association of XMRV with CFS. Most of these studies have relied heavily on PCR for detection of virus (Erlwein, et al., 2010
; Groom, et al., 2010
; Henrich, et al., 2010
; Hong, et al., 2010
; Switzer, et al., 2010
; van Kuppeveld, et al., 2010
). One study also used a neutralization assay to evaluate serologic status (Groom, et al., 2010
), and also failed to confirm an association of XMRV with CFS. A particularly thorough study was performed by investigators at the US Centers for Disease Control and Prevention (Switzer, et al., 2010
), who performed Western blot assays on sera from 121 US blood donors and 26 retrovirus-infected individuals (HTLV 1/2, HIV-1, or dual HIV-1/HIV-2 infection), and on plasma from 51 patients with CFS and 53 controls, all of which were found to be negative for XMRV. Similarly, PCR testing of PMBC from 50 CFS patients, 56 controls, and 41 US blood donors was also negative. The negative status of the PBMC from these CFS patients and controls was also confirmed by an independent laboratory.
In contrast to the preponderance of negative studies, a recent paper from a group at the US National Institutes of Health found evidence, in a cohort of patients diagnosed with CFS, for another genetically diverse set of murine viruses, collectively referred to as murine leukemia virus (MLV)-related viruses (Lo, et al., 2010
). MLV-related viral sequences were found in 32 of 37 patients with CFS, compared to only 3 of 44 healthy volunteer blood donors. The sequences of these viruses were more closely related to polytropic mouse endogenous retroviruses than to XMRV. Thus, although not directly confirming the initial report (Lombardi, et al., 2009
), these findings suggest the possibility of a retroviral association with XMRV.
The reasons for the disparities between the published studies are unclear, but several possibilities present themselves. First, these studies have been performed on a wide variety of populations, and genetic or geographical variations could explain the differences observed. This is especially true if the findings of Lo, et al. (Lo, et al., 2010
) prove correct, and CFS is associated with a genetically diverse set of murine-related viruses. In this scenario, XMRV might represent only one of many potential causative viruses, and it might have been overrepresented in the cohort of the original study (Lombardi, et al., 2009
). Thus, PCR assays designed specifically to amplify XMRV might not detect other potentially causative viruses.
Second, the published reports have used a variety of molecular and serologic assays in their searches for XMRV. It is possible that methodological differences in primers, probes, or assay conditions could explain the disparate PCR results. Arguing against this explanation, the published papers come from groups experienced in the detection of human viruses, including retroviruses, and one would be hard-pressed to provide an example of another virus for which a wide variety of sensitive and specific assays can not be designed. A recent study carefully attempted to replicate the methodology of the original report by Lombardi et al (Lombardi, et al., 2009
). Using this methodology, Shin et al were unable to detect XMRV in a cohort of 100 CFS-affected patients, or in 14 patients who were evaluated in the original Lombardi study (Shin, et al., 2011
). Although some authors have called for the standardization of assays, we would argue that this is premature because no gold standards yet exist; the pressing need at this time is for open sharing of specimens and testing protocols between laboratories to determine the basis for lack of agreement. A final and unfortunate possibility is that the exquisite sensitivity of PCR may be allowing the unintended amplification of murine sequences contaminating the human samples (Hue, et al., 2010
; Oakes, et al., 2010
; Robinson, et al., 2010
; Sato, Furuta, & Miyazawa, 2010
; Shin, et al., 2011
; Smith, 2010
). Because mouse tissues are common in scientific laboratories, this possibility cannot be ignored, and rigorous phylogenetic sequence analysis is imperative.
The main strength of our study was the use of monozygotic twins discordant for CFS status, which we reasoned would allow us to evaluate an association between XMRV and CFS, while minimizing confounding from host genetic factors. In addition, our cohort was recruited from throughout the United States, which could minimize the effect of any hypothesized geographic differences in XMRV prevalence. However, our study was limited by a small sample size (6 CFS-affected twins), and we did not detect XMRV in any samples from the CFS-affected twins. While it is possible that investigation of a larger cohort might have identified XMRV in some CFS-affected twins, our results do not appear to be consistent with the original report of XMRV being present in 67% of patients with CFS (Lombardi, et al., 2009
Clearly, the question of a role for XMRV in CFS needs to be resolved quickly and definitively. The clinical need for answers for CFS has motivated many laboratories to investigate this issue. Substantial resources have already been expended in this effort, as witnessed by the large number of publications on this topic. It is critical that all interested and involved groups share their results in the scientific literature, which will speed the search for answers. Persons suffering from CFS will benefit when the role of XMRV is resolved, either through the identification and treatment of a causative virus, or though a renewed focus of the scientific community on the search for the underlying cause of this devastating condition.