The Radiobiological Effects of Proton Beam Therapy: Impact on DNA Damage and Repair

Abstract

Proton beam therapy (PBT) offers significant benefit over conventional (photon) radiotherapy for the treatment of a number of different human cancers, largely due to the physical characteristics. In particular, the low entrance dose and maximum energy deposition in depth at a well-defined region, the Bragg peak, can spare irradiation of proximal healthy tissues and organs at risk when compared to conventional radiotherapy using high-energy photons. However, there are still biological uncertainties reflected in the relative biological effectiveness that varies along the track of the proton beam as a consequence of the increases in linear energy transfer (LET). Furthermore, the spectrum of DNA damage induced by protons, particularly the generation of complex DNA damage (CDD) at high-LET regions of the distal edge of the Bragg peak, and the specific DNA repair pathways dependent on their repair are not entirely understood. This knowledge is essential in understanding the biological impact of protons on tumor cells, and ultimately in devising optimal therapeutic strategies employing PBT for greater clinical impact and patient benefit. Here, we provide an up-to-date review on the radiobiological effects of PBT versus photon radiotherapy in cells, particularly in the context of DNA damage. We also review the DNA repair pathways that are essential in the cellular response to PBT, with a specific focus on the signaling and processing of CDD induced by high-LET protons.

Keywords: DNA damage; DNA repair; proton beam therapy; radiobiology.

Figure 1

Depth–dose distribution of protons and relationship to energy and linear energy transfer (LET). (A) An unmodulated (pristine) Bragg peak produced by a proton beam. (B) Spread-out Bragg peak (SOBP) from several modulated proton beams.

Figure 2

The response to ionising radiation (IR)-induced DNA damage. Proton beam therapy (PBT), similar to other radiotherapy techniques, targets DNA and can generate an abundance of DNA lesions, where oxidative DNA base damage, abasic sites, and single-strand breaks (SSBs) predominate, and which are repaired via (A) the base excision repair (BER) pathway. This involves recognition of the damaged base by a damage specific DNA glycosylase, incision of the abasic site by AP-endonuclease 1 (APE1) and SSB binding by poly(ADP-ribose) polymerase-1 (PARP-1), 5’-deoxyribosephosphate (dRP) removal and gap filling by DNA polymerase β (Pol β), and finally ligation by X-ray repair cross-complementing protein 1-DNA ligase IIIα (XRCC1–Lig IIIα) complex. Double-strand breaks (DSBs) are repaired by different pathways dependent on cell-cycle phase. In the G₀/G₁ phases, DSBs are repaired by either (B) classical non-homologous end-joining (NHEJ) involving Ku70/80 that binds to the DNA ends, followed by DNA-dependent protein kinase catalytic subunit (DNA-Pkcs) and XRCC4–Lig IV that promote DNA ligation, or via (C) alternative NHEJ which involves DSB end resection by the MRE11–RAD50–NBS1 (MRN) complex, PARP-1 binding to the DSB ends, and subsequent repair by Lig I or XRCC1–Lig IIIα. In the S/G₂ phases of the cell cycle, DSB repair is achieved by (D) homologous recombination (HR) which uses a sister chromatid for repair. Therefore, following DNA end resection by the MRN complex, replication protein A (RPA) and RAD51 bind to the single-stranded DNA overhangs that promote strand invasion and subsequent DNA synthesis in the presence of RAD52/RAD54, as well as formation and resolving of Holliday junctions. The induction of complex DNA damage (CDD), consisting of several DNA lesions in close proximity, particularly by high-LET protons at the distal edge of the SOBP, likely require multiple pathways for repair.

Figure 3

Proposed model for the cellular response to complex DNA damage (CDD) induced by proton beam therapy (PBT) in chromatin. On induction of CDD, this triggers monoubiquitylation of histone H2B on lysine 120 (Ub) by the E3 ubiquitin ligases ring finger 20/40 complex (RNF20/40) and male-specific lethal 2 homolog (MSL2). This stimulates recruitment of the necessary DNA repair proteins and/or chromatin remodeling factors that promote CDD accessibility. Poly(ADP-ribose) polymerase-1 (PARP-1) in particular is essential for efficient CDD repair. Our evidence also suggests the involvement of histone trimethylation (Me) and predictably a deubiquitylation enzyme (DUB) that is able to regulate access to CDD. Repair then proceeds through the respective DNA repair pathway dependent on the nature of the damage, although we suggest a particular dependence on the base excision repair (BER) pathway in the cellular response to high-LET protons, prior to subsequent chromatin assembly.