5-Formylcytosine mediated DNA-peptide cross-link induces predominantly semi-targeted mutations in both Escherichia coli and human cells

Histone proteins can become trapped on DNA in the presence of 5-formylcytosine (5fC) to form toxic DNA-protein conjugates. Their repair may involve proteolytic digestion resulting in DNA-peptide cross-links (DpCs). Here, we have investigated replication of a model DpC comprised of an 11-mer peptide (NH2–GGGKGLGK*GGA) containing an oxy-lysine residue (K*) conjugated to 5fC in DNA. Both CXG and CXT (where X = 5fC-DpC) sequence contexts were examined. Replication of both constructs gave low viability (<10%) in Escherichia coli, whereas TLS efficiency was high (72%) in HEK 293T cells. In E. coli, the DpC was bypassed largely error free, inducing only 2-3% mutations, which increased to 4-5% with SOS. For both sequences, semi-targeted base substitutions were dominant, and for CXG, the predominant mutations were G→T and G→C at the 3ʹ-base to the 5fC-DpC. In HEK 293T cells, 7-9% mutations occurred, and the dominant mutations were the semi-targeted G → T for CXG and T → G for CXT. These mutations were reduced drastically in cells deficient in hPol η, hPol ι or hPol ζ, suggesting a possible role of these TLS polymerases in mutagenic lesion bypass. Steady-state kinetics studies using hPol η confirmed the ability of this polymerase to induce G → T and T → G transversions at the base immediately 3ʹ to the DpC. This study reveals a unique replication pattern of 5fC-conjugated DpCs, which are bypassed largely error free in both E. coli and human cells and induce mostly semi-targeted mutations at the 3ʹ position to the lesion.