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Cancer treatments such as radiotherapy and chemotherapy induce DNA damage, which can be a factor determining therapeutic efficacy. A DNA double strand break (DSB) is considered to be the most critical type of DNA lesion, since DSBs cause cell death when they are unrepaired and generate mutations if they are misrepaired. Ionising radiation (IR) produces a broad spectrum of DNA damage, including DSBs, single strand breaks (SSBs) and base damages. Specific poly(ADP-ribose)polymerase (PARP) inhibitors, currently being tested in clinical trials, compromise SSB repair after IR, resulting in the accumulation of replication-associated DSBs. Since replication-associated DSBs are effectively repaired by homologous recombination, PARP inhibition sensitizes cells that are defective in homologous recombination. In addition, PARP inhibition effectively blocks backup DSB repair in cells defective in non-homologous end joining (NHEJ) following IR. Importantly, the sensitization in NHEJ-defective cells occurs independently of DNA replication. In this chapter, we discuss the multiple effects of PARP inhibition in DSB repair-defective cells in the context of the potential availability of PARP inhibitor in clinical use. We further discuss how a PARP inhibitor influences the type of cell death, which may affect prognosis following cancer treatment. In cancer therapy using PARP inhibitors, a comprehensive understanding of PARP signaling from DNA damage to cell death may be required to augment DNA damage-induced cell death and to direct restrained cell death in order to reduce inflammation responses in surrounding tissues.

A carbon ion categorized as a heavy ion particle has been used for cancer radiotherapy. High linear energy transfer (LET) carbon ion irradiation deposits energy at a high density along a particle track, generating multiple types of DNA damage. Complex DNA lesions, comprising DNA double-strand breaks (DSBs), single-strand breaks, and base damage within 1–2 helical turns (< 3–4 nm), are thought to be difficult to repair and critically influence cell viability. In addition to the effect of lesion complexity, the most recent studies have demonstrated another characteristic of high LET particle radiation-induced DNA damage, clustered DSBs. Clustered DSBs are defined as the formation of multiple DSBs in close proximity where the scale of clustering is approximately 1–2 μm3, i.e., the scale of the event is estimated to be > ∼ 1 Mbp. This chapter reviews the hallmarks of clustered DSBs and how such DNA damage influences genome instability and cell viability in the context of high LET carbon ion radiotherapy.