Multiple integrated copies and high-level production of the human retrovirus XMRV (xenotropic murine leukemia virus-related virus) from 22Rv1 prostate carcinoma cells

Abstract

The human retrovirus XMRV (xenotropic murine leukemia virus-related virus) is associated with prostate cancer, most frequently in humans with a defect in the antiviral defense protein RNase L, suggesting a role for XMRV in prostate carcinogenesis. However, XMRV has not been found in prostate carcinoma cells. Here we show that 22Rv1 prostate carcinoma cells produce high-titer virus that is nearly identical in properties and sequence to XMRV isolated by others and consist primarily of a single clone of cells with at least 10 integrated copies of XMRV, warranting further study of a possible role for XMRV integration in carcinogenesis.

FIG. 1.

Presence of retrovirus-like particles in culture medium from 22Rv1 cells. Medium was harvested from 22Rv1 cells, clarified by centrifugation at 16,500 × g for 20 min, filtered through 0.22-μm-pore-size filters, and centrifuged at 120,000 × g for 70 min, and the pelleted material was analyzed by transmission electron microscopy. Arrows indicate retrovirus-like particles with dark cores surrounded by a membrane.

FIG. 2.

Phylogenetic tree of 22Rv1 gag sequences (clones EK1 and EK2), most closely related GenBank sequences, and one more distant sequence (polytropic mouse endogenous retrovirus [GenBank accession number FJ544576.1]). Sequences were aligned using ClustalX, and an unrooted neighbor-joining tree was generated using this alignment and MEGA version 4 software. Bootstrap values (500 replicates) are shown as percentages, and the scale bar indicates evolutionary distance in base substitutions per site. mERV, mouse endogenous retrovirus; MRV, mouse AIDS-related virus; chr.12, chromosome 12.

FIG. 3.

Southern analysis of genomic DNA from 22Rv1 and HTX cells using an XMRV gag probe. Both panels shown are from the same exposure of the same blot with some internal lanes removed. At the bottom of the figure is a scale diagram of XMRV based on the sequence of the VP42 XMRV clone (GenBank accession number DQ241302) with sizes indicated for the internal restriction fragments that hybridize with the gag probe (2.81 and 2.13 kb). Bands in the blot corresponding to these fragments are marked with asterisks. Copy number standards were made using a plasmid containing the gag probe sequences cut with a restriction enzyme that cuts once in the bacterial plasmid sequences and using an estimate of 6 × 10⁹ bp for the genome size of 22Rv1 cells. LTR, long terminal repeat.