Differential functions of human DNA polymerase delta with p12 variants during DNA replication and DNA damage response

The function of p12, the smallest subunit of human DNA polymerase delta (Polδ), in DNA replication and DNA damage response (DDR) is poorly understood. Due to the identification of two protein encoding p12 isoforms, the respective Polδ variants may function differently; therefore, here we explored their cellular roles in genome maintenance. p12 isoform-1 level remains unchanged along with the catalytic subunit of Polδ throughout the cell cycle stages and upon DNA damage. Cells with a low level of Polδ or p12-depleted Polδ exhibited a slower cell cycle progression, reduced proliferation rate, and higher susceptibility to genotoxic agents and PARP1 inhibitors. Loss of p12 activated checkpoints and altered DDR proteins expression. DNA fiber assays ascertained higher accumulation of stalled and collapsed replication forks in p12 depleted cells. Similar to the dimerization and PCNA interaction defective p12 isoform-1 mutants, p12 isoform-2, which intrinsically lacks the PIP motif, failed to rescue the p12 genomic loss. p12 isoform-2 overexpression enhanced the cytotoxicity of DNA-damaging agents in cells that express a low amount of isoform-1. Thus, p12 isoform-2 seems to possess a negative dominance phenotype in wild type cells. Altogether, our results revealed distinct roles of p12 isoforms in regulating Polδ's function in genome stability.