Inhibition of ATM Induces Hypersensitivity to Proton Irradiation by Upregulating Toxic End Joining

Abstract

Proton Bragg peak irradiation has a higher ionizing density than conventional photon irradiation or the entrance of the proton beam profile. Whether targeting the DNA damage response could enhance vulnerability to the distinct pattern of damage induced by proton Bragg peak irradiation is currently unknown. Here we performed genetic or pharmacologic manipulation of key DNA damage response elements and evaluated DNA damage signaling, DNA repair, and tumor control in cell lines and xenografts treated with the same physical dose across a radiotherapy linear energy transfer spectrum. Radiotherapy consisted of 6 MV photons and the entrance beam or Bragg peak of a 76.8 MeV spot scanning proton beam. More complex DNA double strand breaks induced by Bragg peak proton irradiation preferentially underwent resection and engaged homologous recombination (HR) machinery. Unexpectedly, the ATM inhibitor AZD0156 but not an inhibitor of ATR rendered cells hypersensitive to more densely ionizing proton Bragg peak irradiation. ATM inhibition blocked resection and shunted more double strand breaks to processing by toxic ligation through nonhomologous end-joining, whereas loss of DNA ligation via XRCC4 or Lig4 knockdown rescued resection and abolished the enhanced Bragg peak cell killing. Proton Bragg peak monotherapy selectively sensitized cell lines and tumorxenografts with inherent HR defects, and the repair defect induced by ATM inhibitor coadministration showed enhanced efficacy in HR proficient models. In summary, inherent defects in HR or administration of an ATM inhibitor in HR proficient tumors selectively enhance the relative biological effectiveness of proton Bragg peak irradiation.

Does the uncertainty in relative biological effectiveness affect patient treatment in proton therapy?

“…Studies in vitro have demonstrated proton radiation-induced DNA double strand breaks (DSBs) to be preferentially repaired by homologous recombination (HR), one of the major DNA repair pathways in DNA-damage response [40][41][42][43]. In a recent in vivo study, tumours with HR deficiency due to BRCA1 mutations display an increased radiosensitivity in the Bragg peak as compared to photon radiation, and thereby an increased RBE compared to tumours with a functional HR pathway is expected [44]. These findings point towards the possibility of selecting specifically proton radiosensitive tumours for proton therapy.…”

Factors affecting the radiation response in glioblastoma

“…In terms of PBT, it has been shown that U2OS and BT549 cells irradiated at the Bragg peak with a higher RBE upregulate utilization of HR, but interestingly that inhibiting ATM redirected cells to the NHEJ pathway leading to the formation of toxic DSB repair end products. 67 This evidence was replicated in an in vivo xenograft model, in which inhibiting ATM in combination with PBT was shown to significantly delay tumor growth, and that Bragg peak protons were furthermore highly effective in the killing of HR-defective (BRCA-1 deficient) cells and tumors. 67 Such findings suggest that cells may switch between different repair pathways when repairing more complex DSBs induced by high-LET radiations.…”

T cell-mediated additive cytotoxicity – death by multiple bullets

“…Overall, 40% of antigenic CTL-tumor cell interactions cause DNA damage-associated repair foci in the nucleus, which are associated with death at ~30% probability; alternatively, DNA damage-associated foci are reversible (within hours) followed by tumor cell survival [13]. Due to its central role in securing tumor cell survival and emerging resistance, the DNA damage response (DDR) is considered an important target for improving combinations of chemo-, radio-and molecular therapy, for example, by ATM/ATR and poly(ADP-ribose) polymerase (PARP) inhibitors [30][31][32][33].…”