Two forms of human DNA polymerase δ: Who does what and why?

DNA polymerase δ (Pol δ) plays a central role in lagging strand DNA synthesis in eukaryotic cells, as well as an important role in DNA repair processes. Human Pol δ4 is a heterotetramer of four subunits, the smallest of which is p12. Pol δ3 is a trimeric form that is generated in vivo by the degradation of the p12 subunit in response to DNA damage, and during entry into S-phase. The biochemical properties of the two forms of Pol δ, as well as the changes in their distribution during the cell cycle, are reviewed from the perspective of understanding their respective cellular functions. Biochemical and cellular studies support a role for Pol δ3 in gap filling during DNA repair, and in Okazaki fragment synthesis during DNA replication. Recent studies of cells in which p12 expression is ablated, and are therefore null for Pol δ4, show that Pol δ4 is not required for cell viability. These cells have a defect in homologous recombination, revealing a specific role for Pol δ4 that cannot be performed by Pol δ3. Pol δ4 activity is required for D-loop displacement synthesis in HR. The reasons why Pol δ4 but not Pol δ3 can perform this function are discussed, as well as the question of whether helicase action is needed for efficient D-loop displacement synthesis. Pol δ4 is largely present in the G1 and G2/M phases of the cell cycle and is low in S phase. This is discussed in relation to the availability of Pol δ4 as an additional layer of regulation for HR activity during cell cycle progression.

Loss of the p12 subunit of DNA polymerase delta leads to a defect in HR and sensitization to PARP inhibitors

Human DNA polymerase δ is normally present in unstressed, non-dividing cells as a heterotetramer (Pol δ4). Its smallest subunit, p12, is transiently degraded in response to UV damage, as well as during the entry into S-phase, resulting in the conversion of Pol δ4 to a trimer (Pol δ3). In order to further understand the specific cellular roles of these two forms of Pol δ, the gene (POLD4) encoding p12 was disrupted by CRISPR/Cas9 to produce p12 knockout (p12KO) cells. Thus, Pol δ4 is absent in p12KO cells, leaving Pol δ3 as the sole source of Pol δ activity. GFP reporter assays revealed that the p12KO cells exhibited a defect in homologous recombination (HR) repair, indicating that Pol δ4,  but not Pol δ3, is required for HR. Expression of Flag-tagged p12 in p12KO cells to restore Pol δ4 alleviated the HR defect. These results establish a specific requirement for Pol δ4 in HR repair. This leads to the prediction that p12KO cells should be more sensitive to chemotherapeutic agents, and should exhibit synthetic lethal killing by PARP inhibitors. These predictions were confirmed by clonogenic cell survival assays of p12KO cells treated with cisplatin and mitomycin C, and with the PARP inhibitors Olaparib, Talazoparib, Rucaparib, and Niraparib. The sensitivity to PARP inhibitors in H1299-p12KO cells was alleviated by expression of Flag-p12. These findings have clinical significance, as the expression levels of p12 could be a predictive biomarker of tumor response to PARP inhibitors. In addition, small cell lung cancers (SCLC) are known to exhibit a defect in p12 expression. Analysis of several SCLC cell lines showed that they exhibit hypersensitivity to PARP inhibitors, providing evidence that loss of p12 expression could represent a novel molecular basis for HR deficiency.