The effect of human cytomegalovirus (HCMV) infection on the frequency
of mutations at the hypoxanthine-guanine phosphoribosyl transferase (h
prt) locus was studied in Chinese hamster lung V79 cells. When V79 cel
ls were infected with HCMV (strain AD169) at multiplicities of 0.1 to
50 plaque forming units (PFU) per cell the presumptive mutation freque
ncy, as determined by the number of 6-thioguanine-resistant (TG(r)) co
lonies, was increased up to 16.8-fold (P < 0.005), depending on the mu
ltiplicity of infection. Increases in the mutation frequency at the hp
rt locus 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 TG(r) colonies wa
s 3 days and was consistent among the HCMV strains evaluated in this s
tudy. UV-irradiation of HCMV stock up to a dose of 9.6 x 10(4) ergs/mm
(2) increased the mutation frequency, but further exposure to UV light
or to heat (56 degrees for 30 min) significantly decreased the freque
ncy of TG(r)-resistant colonies, suggesting that expression of HCMV ge
nes was involved in the mutation process. HCMV-induced TG(r) cells dem
onstrated substantially reduced (>96%) incorporation of [H-3]hypoxanth
ine. PCR analysis of the hprt locus demonstrated deletions in 9 of 19
HCMV-induced TG(r) colonies randomly selected for further study, while
2 of 17 spontaneously developed TG(r) colonies demonstrated deletions
. Although insertions were not detected in spontaneously developed clo
nes, 3 of 19 HCMV-induced TG(r) clones had insertions in the hprt gene
. Neither HCMV-specific DNA sequences nor HCMV-specific proteins were
detected in the TG(r) clones obtained after HCMV infection. infection
of V79 cells with HCMV also increased their sensitivity to mutation wi
th N-methyl-N'-nitro-N-nitrosoguanidine, giving a synergistic enhancem
ent of the mutation frequency. These results indicate that HCMV infect
ion has the capacity to induce mutations in the cellular genome and in
crease the sensitivity of infected cells to mutation by genotoxic chem
icals. Although inactivated HCMV particles are responsible for a modes
t increase in the mutation frequency, expression of HCMV genes is asso
ciated with a substantial enhancement of the mutation frequency. (C) 1
997 Academic Press.