Fidelity and mutational specificity of uracil-initiated base excision DNA repair synthesis in human glioblastoma cell extracts

The fidelity of DNA synthesis associated with uracil-initiated base excision repair was measured in human whole cell extracts. An M13mp2 lacZ alpha DNA-based reversion assay was developed to assess the error frequency of DNA repair synthesis at a site-specific uracil residue. All three possible base substitution errors were detected at the uracil target causing reversion of opal codon 14 in the Escherichia coli lacZ alpha gene. Using human glioblastoma U251 whole cell extracts, approximately 50% of the heteroduplex uracil-containing DNA substrate was completely repaired, as determined by the insensitivity of form I DNA reaction products to cleavage by a combined treatment of E. coli uracil-DNA glycosylase and endonuclease IV. The majority of repair occurred by the uracil-initiated base excision repair pathway, since the addition of the bacteriophage PBS2 uracil-DNA glycosylase inhibitor protein to extracts significantly blocked this process. In addition, the formation of repaired form I DNA molecules occurred concurrently with limited DNA synthesis, which was largely restricted to the HinfI DNA fragment initially containing the uracil residue and specific to the uracil-containing DNA strand. Based on the reversion frequency of repaired M13mp2 DNA, the fidelity of DNA repair synthesis at the target was determined to be about one misincorporated nucleotide per 1900 repaired uracil residues. The major class of base substitutions propagated transversion mutations, which were distributed almost equally between T to G and T to A changes in the template. A similar mutation frequency was also observed using whole cell extracts from human colon adenocarcinoma LoVo cells, suggesting that mismatch repair did not interfere with the fidelity measurements.