Testing excision models for responses of mismatch-repair systems to UV photoproducts in DNA.
Mismatch-repair (MMR) systems correct DNA replication errors and respond to a variety of DNA lesions. Previous observations that MMR antagonizes UV mutagenesis, and that the mismatch-recognition protein heterodimer MSH2*MSH6 (MutSalpha) selectively binds DNA containing "mismatched" photoproducts (T[CPD]T/AG, T[6-4]T/AG) but not "matched" photoproducts (T[CPD]T/AA, T[6-4]T/AA), suggested that mismatched photoproducts would provoke MMR excision similar to mismatched bases. Excision of incorrect nucleotides inserted opposite template photoproducts might then prevent UV-induced mutation. We tested T[CPD]T/AG DNA, in a sequence context in which it is bound substantially by hMutSalpha and in three other contexts, for stimulation of 3' MMR excision in mammalian nuclear extracts. T[CPD]T/AG was inactive in HeLa extracts, or in extracts deficient in the photoproduct-binding proteins DDB or XPC* hHR23B, arguing against interference from the nucleotide excision repair pathway. Prior incubation with hMutSalpha and MLH2.PMS2 (hMutLalpha) did not increase excision relative to homoduplex controls. T[6-4]T/AG also failed to provoke excision. T/G, C/A, and T/T substrates, even though bound by hMutSalpha no better than T[CPD]T/AG substrates, efficiently provoked excision. Even a substrate containing three T[CPD]T/AG photoproducts (in different contexts) did not significantly provoke excision. Thus, MMR may suppress UV mutagenesis by non-excisive mechanisms.
Department of Environmental and Molecular Toxicology, Oregon State University, Corvallis, Oregon, USA.
Huxian
H
Wang
Peter D
PD
Hoffman
Christopher
C
Lawrence
John B
JB
Hays
D000818:Animals; D020137:Base Pair Mismatch; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D004247:DNA; D004260:DNA Repair; D006367:HeLa Cells; D006801:Humans; D010957:Plasmids; D011740:Pyrimidine Dimers; D014466:Ultraviolet Rays / Q000009:adverse effects*
D011740:Pyrimidine Dimers; C058406:pyrimidine-pyrimidone dimer; D004247:DNA