Integration of retroviral DNA into the cellular genome is essential for the production of new infectious particles. A strong argument that the novel human retrovirus XMRV is not a laboratory contaminant is the finding that viral DNA is integrated in chromosomal DNA of prostate tumors. Nucleotide sequence analyses of 14 integration sites in prostate tumor DNAs from 9 different patients previously revealed the expected viral sequences linked to human DNA. But two of these integration sites are identical to those found in a prostate tumor cell line infected with XMRV.
A search of the nucleotide sequence database with the previously identified XMRV integration site sequences revealed that 2 of the 14 sequences (from 2 patients) were identical to two XMRV integration sites in DU145 cells. This cell line was established in 1978 from the brain metastasis of a human prostate tumor. In early 2010 2007 DU145 cells were infected with XMRV, and sequences of two integration sites were determined (the database entries can be found here and here).
Identical retroviral integration sites have never been reported in independently infected cells. Furthermore, XMRV infection of DU145 cells was done in the same laboratory in which the XMRV integration sites were identified in prostate tumor DNA. The conclusion is that two of the 14 XMRV integration sites in prostate tumor DNA are likely to be the result of contamination. These prostate tumor DNA samples were probably contaminated with DNA from XMRV-infected DU145 cells.
These observations do not directly impugn the veracity of the other 12 XMRV integration sites identified in prostate tumor DNA. However, when DNA contamination occurs it is often ubiquitous. Hence the authors write:
Whilst it is conceivable that the other 12 integration sites apparently derived from prostatic tumor tissues are genuine patient-derived sequences, we suspect that some or all of them may also be the result of contamination with DNA from experimentally infected DU145 cells.
This possibility can and must be addressed experimentally.
Update: While writing this post I received an abstract from the 2011 Conference on Retroviruses and Other Opportunistic Infections (CROI) entitled “XMRV probably originated through recombination between two endogenous murine retroviruses during passage of a human prostate tumor in nude mice”. As usual I will await publication of this story in a peer-reviewed journal before discussing it further.
Garson JA, Kellam P, & Towers GJ (2011). Analysis of XMRV integration sites from human prostate cancer tissues suggests PCR contamination rather than genuine human infection. Retrovirology, 8 (1) PMID: 21352548
Stone, K., Mickey, D., Wunderli, H., Mickey, G., & Paulson, D. (1978). Isolation of a human prostate carcinoma cell line (DU 145) International Journal of Cancer, 21 (3), 274-281 DOI: 10.1002/ijc.2910210305
Dong B, Kim S, Hong S, Das Gupta J, Malathi K, Klein EA, Ganem D, Derisi JL, Chow SA, & Silverman RH (2007). An infectious retrovirus susceptible to an IFN antiviral pathway from human prostate tumors. Proceedings of the National Academy of Sciences of the United States of America, 104 (5), 1655-60 PMID: 17234809