Replication-induced DNA damage after PARP inhibition causes G(2) delay, and cell line-dependent apoptosis, necrosis and multinucleation

PARP inhibitors have been approved for treatment of tumors with mutations in or loss of BRCA1/2. The molecular mechanisms and particularly the cellular phenotypes resulting in synthetic lethality are not well understood and varying clinical responses have been observed. We have investigated the dose- and time-dependency of cell growth, cell death and cell cycle traverse of 4 malignant lymphocyte cell lines treated with the PARP inhibitor Olaparib. PARP inhibition induced a severe growth inhibition in this cell line panel and increased the levels of phosphorylated H2AX-associated DNA damage in S phase. Repair of the remaining replication related damage caused a G(2) phase delay before entry into mitosis. The G(2) delay, and the growth inhibition, was more pronounced in the absence of functional ATM. Further, Olaparib treated Reh and Granta-519 cells died by apoptosis, while U698 and JVM-2 cells proceeded through mitosis with aberrant chromosomes, skipped cytokinesis, and eventually died by necrosis. The TP53-deficient U698 cells went through several rounds of DNA replication and mitosis without cytokinesis, ending up as multinucleated cells with DNA contents of up to 16c before dying. In summary, we report here for the first time cell cycle-resolved DNA damage induction, and cell line-dependent differences in the mode of cell death caused by PARP inhibition.