Involvement of DNA polymerase β in DNA replication and mutagenic consequences

Overexpression in mammalian cells of the error-prone DNA polymerase β (Pol β) has been found to increase the spontaneous mutagenesis. Here, we investigated a possible mechanism used by Pol β to be a genetic instability enhancer: its interference in replicative DNA synthesis, which is normally catalysed by the DNA polymerases α, δ and ϵ. By taking advantage of the ability to incorporate efficiently into DNA the chain terminator ddCTP as well as the oxidised nucleotide 8-oxo-dGTP, we show here that purified Pol β can compete with the replicative DNA polymerases during replication in vitro of duplex DNA when added to human cell extracts. We found that involvement of Pol β lowers replication fidelity and results in a modified error-specificity. Furthermore, we demonstrated that involvement of Pol β occurred during synthesis of the lagging strand. These in vitro data provide one possible explanation of how overexpression of the enzyme could perturb the genetic instability in mammalian cells. We discuss these findings within the scope of the up-regulation of Pol β in many cancer cells.