DNA–Peptide Crosslinks Placed at Codon 249 of the p53 Tumor Suppressor Gene Induce G to A Transitions in Human Cells

Anticancer nitrogen mustard drugs including cyclophosphamide, mechlorethamine and chlorambucil induce covalent DNA–protein cross‐links (DPCs) by sequentially alkylating the N7 position of guanine in DNA and amino acid residues within proteins. In cells, DPCs are proteolytically processed to yield corresponding peptide‐DNA cross‐links (DpCs), allowing polymerase bypass. In this study, site‐specific, hydrolytically stable DNA lesions resembling nitrogen mustard‐induced DpCs were generated via oxime ligation of an 11‐mer peptide derived from histone H4 (NH 2 –GGGK*GLGKGGA, K*=oxy‐lysine) to 7‐deaza‐7‐(2,3‐dihydroprop‐1‐yl)‐dG (DHP) in DNA. DNA sequence was derived from codon 249 of the p53 tumor suppressor gene, a common mutational hotspot in tumors. Translesion synthesis (TLS) experiments of DpC containing plasmids in human HEK 293T cells revealed that 7‐deaza‐DpC lesion was bypassed most efficiently in wild‐type cells (83%) with lower efficiency in Pol ι KO (53%), Pol η KO (59%), and Pol κ KO (63%) cells, with Pol ζ KO unaffected (76%). Mutation frequency for polymerase bypass of 7‐deaza‐DpC (~9%) was significantly higher than for 7‐deaza‐DHP‐dG control (~5%). Pol η and Pol ι knockouts led to increased error rates, suggesting they play a role in error‐free bypass of DpCs. In contrast Pol ζ deficiency reduced mutation rates, implying that it plays a role in error‐prone bypass.