Stereospecific Differences in Repair by Human Cell Extracts of Synthesized Oligonucleotides Containing trans-Opened 7,8,9,10-Tetrahydrobenzo[a]pyrene 7,8-Diol 9,10-EpoxideN2-dG Adduct Stereoisomers Located within the Human K-rasCodon 12 Sequence†
The potent environmental carcinogen benzo[a]pyrene (BaP), following enzymatic activation to enantiomeric pairs of bay-region 7,8-diol 9, 10-epoxides (the benzylic 7-hydroxyl group and epoxide oxygen are cis for DE-1 diastereomers and trans for DE-2 diastereomers), reacts with DNA to form covalent adducts predominately at the exocyclic amino groups of purines. Specific adducts, corresponding to the trans opening of each of the four optically active BaP DE isomers at C-10 by the N 2-amino group of dG, were synthesized as appropriately blocked phosphoramidites and were incorporated at either the first or second G of codon 12 within the G-rich sequence of human K-ras codons 11-13: GCT G1G2T GGC. The adducted oligonucleotides were incorporated into plasmids by primer extension, followed by purification of the covalently closed circular constructs. Adducts derived from either (+)- or (-)-DE-2, placed at either G1 or G2, presented strong blocks to in vitro transcription elongation by bacteriophage T3 RNA polymerase, but only moderately blocked transcription elongation by human RNA polymerase II in nuclear extracts. Adducts derived from all four DEs, placed on either G1 or G2, were used as substrates in a DNA repair synthesis assay using human whole cell extracts. Adducts derived from three of the DE stereoisomers exhibited significant amounts of repair synthesis, but the (-)-DE-2 adduct experienced no repair synthesis above that of the control. Constructs containing a pre-existing nick at the sixth phosphodiester bond 3' to either (+)-DE-2 or (-)-DE-2 adducts exhibited increased repair synthesis.