Article
Differential DNA Repair Mechanisms in Response to Proton and Photon Irradiation
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Published: | September 24, 2009 |
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Purpose/objective(s): A relative biological effectiveness (RBE) factor for proton irradiation of 1.1–1.2 is presently used in clinical proton radiotherapy. However, the molecular damage pattern and damage response, which are the basis of this RBE value remain largely unknown. We investigated the impact of various DNA repair machineries (non-homologous end joining, homologous recombination, base excision repair) on the treatment response to proton versus photon irradiation using an indirect cellular approach.
Materials/methods: SiRNA-mediated downregulation of specific DNA-repair key enzymes (XRCC1, XRCC3, Rad51, XRCC4) was successfully performed in the human p53-wildtype, osteosarcoma cell line U2OS. SiRNA against HSP70B, which is not expressed in these cells, was used as control. U2OS cells were treated with increasing doses (2–8 Gy) of either proton (PSI scanning beam mid spread-out Bragg peak; energy at target 36.5 MeV; 2keV/µm LET) or photon irradiation (Pantak Therapax 300kV X ray; 0.7Gy/min), followed by clonogenic survival assays and RBE calculation.
Results: An RBE<font size="1">10 at 10 percent cell survival of 1.34±0.06 was determined for the control siRNA-treated cells. Downregulation of XRCC1, involved in single strand break and base excision repair, sensitized cells towards both proton and photon irradiation, resulting in a slight decrease in the RBE10 to 1.21±0.06. Interestingly, targeting the two main DNA double strand break repair pathways resulted in differential cellular hypersensitivity to proton and photon irradiation: By targeting non-homologous end joining (siXRCC4), the observed increase in sensitivity to proton vs. photon irradiation was similar to control cells as the RBE remained unchanged (RBE10 of 1.30±0.11). However, the RBE of proton vs. photon irradiation treatment was different when we silenced Rad51, a key factor involved in the initiation of homologous recombination (RBE10 1.14±0.05). Experiments targeting XRCC3 (RBE10 1.18±0.08), another downstream component of the homologous recombination pathway, also pointed towards the relevance of homologous recombination in a differential response to photon vs. proton irradiation.
Conclusion: We show that cells lacking specific DNA repair proteins display differential sensitivities for the two types of low LET radiation. This might indicate either the formation of different types of DNA damage lesions and/or a different cellular stress response for "traditional" photon compared to proton irradiation. While the underlying mechanism has yet to be defined, an important involvement of homologous recombination can be assumed. A differential cellular response to photon vs. proton irradiation could lead to the development of novel proton-versus-photon specific combined treatment modalities by targeting specific DNA-repair machineries.