Increased Frequency of Specific Locus Mutation Following Human Cytomegalovirus Infection

The effect of human cytomegalovirus (HCMV) infection on the frequency of mutations at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus was studied in Chinese hamster lung V79 cells. When V79 cells were infected with HCMV (strain AD169) at multiplicities of 0.1 to 50 plaque forming units (PFU) per cell the presumptive mutation frequency, as determined by the number of 6-thioguanine-resistant (TGr) colonies, was increased up to 16.8-fold (P< 0.005), depending on the multiplicity of infection. Increases in the mutation frequency at thehprtlocus were also observed for other laboratory-adapted HCMV strains (C-87, Davis) and for low passage clinical isolates (82-1, 84-2). The expression time required for the maximum increase in TGrcolonies was 3 days and was consistent among the HCMV strains evaluated in this study. UV-irradiation of HCMV stock up to a dose of 9.6 × 104ergs/mm2increased the mutation frequency, but further exposure to UV light or to heat (56° for 30 min) significantly decreased the frequency of TGr-resistant colonies, suggesting that expression of HCMV genes was involved in the mutation process. HCMV-induced TGrcells demonstrated substantially reduced (>96%) incorporation of [3H]hypoxanthine. PCR analysis of thehprtlocus demonstrated deletions in 9 of 19 HCMV-induced TGrcolonies randomly selected for further study, while 2 of 17 spontaneously developed TGrcolonies demonstrated deletions. Although insertions were not detected in spontaneously developed clones, 3 of 19 HCMV-induced TGrclones had insertions in thehprtgene. Neither HCMV-specific DNA sequences nor HCMV-specific proteins were detected in the TGrclones obtained after HCMV infection. Infection of V79 cells with HCMV also increased their sensitivity to mutation withN-methyl-N′-nitro-N-nitrosoguanidine, giving a synergistic enhancement of the mutation frequency. These results indicate that HCMV infection has the capacity to induce mutations in the cellular genome and increase the sensitivity of infected cells to mutation by genotoxic chemicals. Although inactivated HCMV particles are responsible for a modest increase in the mutation frequency, expression of HCMV genes is associated with a substantial enhancement of the mutation frequency.