Retroviral Etiology for Chronic Fatigue Syndrome Questioned

The report of an apparent link between a novel retrovirus (xenotropic murine leukemia virus–related virus; XMRV) and chronic fatigue syndrome (CFS) attracted much attention in 2009 (JW Infect Dis Oct 21 2009). However, multiple follow-up studies have yielded conflicting results (JW Infect Dis Sep 1 2010), and discoveries in two new studies challenge the validity of the original findings.

Seeking to confirm an association between XMRV and CFS, Knox and colleagues performed extensive virologic tests on blood samples from 61 CFS patients from the same clinical practice that provided the majority of participants covered in the 2009 report; 43 of them had been reported to be XMRV positive. Polymerase chain reaction (PCR) assays for nucleic acids derived from XMRV or murine leukemia viruses (MLVs) and assays for detecting infectious virus or virus-specific antibodies showed no evidence of XMRV or other MLVs in any of the patients. In addition, the researchers found that unheated human sera from either CFS patients or healthy controls readily inactivated XMRV and another tested MLV, and determined that MLV sequences contaminated 12 of 22 laboratory reagents used to detect such viruses.

Poprotka and colleagues sought to confirm the existence of XMRV by studying the human prostate cell line (CWR22Rv1) from which it was first identified. This cell line was derived from a primary human prostate tumor and underwent serial passage in nude mice. The researchers determined that XMRV was present in later passage xenografts but not in earlier ones. They screened multiple mouse strains and wild mice but did not detect endogenous XMRV in any of them. However, they did identify two murine proviruses (preXMRV-1 and preXMRV-2) that were present in host mice and could have recombined to produce XMRV during serial tumor passaging.