Altered DNA Polymerase ι Expression in Breast Cancer Cells Leads to a Reduction in DNA Replication Fidelity and a Higher Rate of Mutagenesis

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Abstract
The recently discovered human enzyme DNA polymerase ι (pol ι) has been shown to have an exceptionally high error rate on artificial DNA templates. Although there is a considerable body of in vitro evidence for a role for pol ι in DNA lesion bypass, there is no in vivo evidence to confirm this action. We report here that pol ι expression is elevated in breast cancer cells and correlates with a significant decrease in DNA replication fidelity. We also demonstrate that UV treatment of breast cancer cells additionally increases pol ι expression with a peak occurring between 30 min and 2 h after cellular insult. This implies that the change in pol ι expression is an early event after UV-mediated DNA damage. That pol ι may play a role in the higher mutation frequencies observed in breast cancer cells was suggested when a reduction in mutation frequency was found after pol ι was immunodepleted from nuclear extracts of the cells. Analysis of the UV-induced mutation spectra revealed that >90% were point mutations. The analysis also demonstrated a decreased C→T nucleotide transition and an increased C→A transversion rate. Overall, our data strongly suggest that pol ι may be involved in the generation of both increased spontaneous and translesion mutations during DNA replication in breast cancer cells, thereby contributing to the accumulation of genetic damage.