Carcinogenesis, Vol. 20, No. 5, 799-804,
May 1999
© 1999 Oxford University Press
Combined mismatch and nucleotide excision repair defects in a human cell line: mismatch repair processes methylation but not UV- or ionizing radiation-induced DNA damage
Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, Hertfordshire EN6 3LD, UK and
1 Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy
Interaction between long patch mismatch repair (MMR) and persistent DNA O6-methylguanine or 6-thioguanine (6-TG) is implicated in the cytotoxicity of methylating agents and 6-TG, respectively. Human cells with defective MMR tolerate DNA methylation damage and are cross-resistant to 6-TG. To determine whether MMR contributes to the lethal effects of persistent UV-induced DNA lesions, MMR deficiency was introduced into nucleotide excision repair (NER)-defective XP12RO cells. The doubly repair-defective cells, designated XP12ROB4, did not express detectable hMSH2 protein. They had the mutator phenotype, N-methyl-N-nitrosourea and 6-TG resistance typical of MMR-defective cells. Active MMR was not required for the cytotoxicity of UV light, and the hMSH2 defect did not detectably alter the survival of XP12ROB4. The level of spontaneous or UV-induced SCE was also similar in XP12RO and XP12ROB4, indicating that hMSH2 is not required for this recombination process. The combined deficiency in MMR and NER did not confer a significant degree of tolerance to ionizing radiation, and the survival of XP12RO and XP12ROB4 after 
-radiation was similar. Although it recognizes and processes some persistent damaged or modified DNA base pairs, MMR is unlikely to serve as a general sensor of DNA damage.
Abbreviations: 6-meTG, 6-methylthioguanine; 6-TG, 6-thioguanine; 8-AzaG, 8-azaguanine; DTT, dithiothreitol; Mex–, MGMT-deficient; MGMT, O6-methylguanine-DNA methyltransferase; MMR, mismatch repair; MNNG, N-methyl-N'-nitro-N-nitrosoguanidine; MNU, N-methyl-N-nitrosourea; NER, nucleotide excision repair; O6-meGua, O6-methylguanine; SCE, sister chromatid exchange; TCR, transcription-coupled repair; XP, xeroderma pigmentosum.
2 To whom correspondence should be addressed Email: karran{at}icrf.icnet.uk
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