The human gamma retrovirus, xenotropic murine leukemia virus-related virus (XMRV), has been found in the tumor tissue of prostate cancer patients with the RNASEL genotype . XMRV sequences have also been identified in peripheral blood cell DNA from 68 of 101 (67%) of patients with chronic fatigue syndrome and eight of 218 (3.7%) of healthy controls regardless of the presence of the RNASEL genotype . Of note, there is more than 99% sequence identity between the XMRV genomes sequenced to date, suggesting that the virus descended from a common ancestor . Here we have investigated the prevalence of XMRV sequences in blood cells of men enrolled in the Chicago component of the Multicenter AIDS Cohort Study, a natural history study of men who have sex with men.
We isolated nucleic acids from peripheral blood cells collected in ethylene diaminetetraacetic acid from 996 men (562 men infected with HIV and 434 men at risk for infection) and tested the samples for XMRV gag sequences by the real-time quantitative PCR (qPCR) using oligonucleotide primers specific for a section of the DNA . Men infected with HIV were treated with antiretroviral therapy appropriate to the time of collection that is, from 1994 to 2003. Amplification of DNA (250 ng per reaction) included an internal control, CC chemokine receptor 5 (CCR5). Cell DNA was previously analyzed for genetic variants of host proteins related to HIV disease . Detection of XMRV was confirmed at five copies by qPCR – amplifying a synthetic DNA fragment inserted into vector pCR 2.1 TOPO. The qPCR assay was validated with DNA from the 22Rv1 human prostate carcinoma epithelial cell line (American Type Tissue Collection catalog number CRL-2505D). Of the 996 peripheral blood cell DNA specimens analyzed by real-time qPCR for gag sequences, none was positive. DNA was tested in three independent experiments. All DNA specimens were positive for the CCR5 reference sequence.
Our results indicate that the same XMRV sequences derived from a subset of prostate cancer patients and patients with chronic fatigue syndrome are absent in blood cells of men at risk for and infected with HIV. The prevalence of XMRV sequences was significantly higher in the previously described healthy controls [exact 95% binomial confidence interval (CI), 1.6–7.1%] than in our population as a whole (95% CI, less than 0.37%) or in the HIV-uninfected men in particular (95% CI, less than 0.85%), suggesting that infection may be population-specific.
This observation does not support the ubiquity of XMRV infection in populations at risk for or infected with HIV. Furthermore, this evidence argues against a high likelihood for sexual transmission of the virus . A more recent study has failed to detect XMRV sequences in prostate tumor tissue , suggesting that there is sufficient ambiguity about the pathogenic potential of this infectious retrovirus, which does not encode accessory genes or any host-derived oncogenes.
The finding that none of the blood cells of men in our study tested positive for XMRV does not necessarily mean that the original finding of a relatively high frequency of infection in healthy controls was false; rather, it could signify a population-specific risk for infection. XMRV may not replicate as efficiently in blood cells as in cells of prostate tissue origin . Nevertheless, both the sensitivity and specificity of the real-time qPCR assay used to detect XMRV sequences in peripheral blood cell DNA were equivalent . The fact that we could not replicate this finding would indicate that XMRV infection is not as common worldwide as has been suggested [1,3]. Further study is needed to determine whether the prevalence and distribution of XMRV in different geographical regions is indeed different [3,8].
This work was supported by National Institute of Allergy and Infectious Diseases Multicenter AIDS Cohort Study grant AI35039. All men provided written informed consent according to the guidelines of the human subjects protection committee of Northwestern University.
K.K. carried out the molecular biology studies. T.B. carried out the data analysis and helped to draft the manuscript. J.F. and J.P.P. played significant roles in recruitment and follow up of the study participants. S.M.W. conceived of the study and participated in its design and coordination. All authors read and approved the final manuscript.
The authors have no conflicts of interest to declare.
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