The cellular mismatch repair system is able to repair mismatches within MLV retroviral double-stranded DNA at a low frequency

Eukaryotic cells possess several distinct mismatch repair pathways. A misma tch can be introduced in retroviral double-stranded DNA by a pre-existing m utation within the primer binding site (PBS) of the viral RNA genome. In or der to evaluate mismatch repair of retroviral double-stranded DNA, Moloney leukemia virus (MLV)-based vectors with a mutation in their PBS were used t o infect mismatch repair-competent as well as mismatch repair-deficient cel l lines. If the target cells were capable of repairing the mismatch before an infected cell divided, the mismatch within the PBS could be repaired to the wild-type or mutant PBS, If the target cells were unable to repair the mismatch, half the cells in the colony should contain the mutant PBS while the other halt should contain the wild-type PBS. To evaluate these predicti ons, individual colonies were isolated and analyzed by PCR. Almost all mism atch-deficient cell colonies analyzed (cell lines HCT 116 and PMS2-/-) cont ained both the wild-type and mutated PBS, therefore, mismatches within retr oviral double-strand DNA could not be repaired by the mismatch-deficient ce lls. in contrast, mismatches in similar to 25% of the mismatch repair-compe tent cell clones analyzed (cell lines HeLa and PMS2+/+) were repaired, whil e 75% were not. Therefore, the cellular mismatch repair system is able to r epair mismatches within viral double-stranded DNA, but at a low frequency.