Proton Irradiation Increases the Necessity for Homologous Recombination Repair Along with the Indispensability of Non-Homologous End Joining

Abstract

Technical improvements in clinical radiotherapy for maximizing cytotoxicity to the tumor while limiting negative impact on co-irradiated healthy tissues include the increasing use of particle therapy (e.g., proton therapy) worldwide. Yet potential differences in the biology of DNA damage induction and repair between irradiation with X-ray photons and protons remain elusive. We compared the differences in DNA double strand break (DSB) repair and survival of cells compromised in non-homologous end joining (NHEJ), homologous recombination repair (HRR) or both, after irradiation with an equal dose of X-ray photons, entrance plateau (EP) protons, and mid spread-out Bragg peak (SOBP) protons. We used super-resolution microscopy to investigate potential differences in spatial distribution of DNA damage foci upon irradiation. While DNA damage foci were equally distributed throughout the nucleus after X-ray photon irradiation, we observed more clustered DNA damage foci upon proton irradiation. Furthermore, deficiency in essential NHEJ proteins delayed DNA repair kinetics and sensitized cells to both, X-ray photon and proton irradiation, whereas deficiency in HRR proteins sensitized cells only to proton irradiation. We assume that NHEJ is indispensable for processing DNA DSB independent of the irradiation source, whereas the importance of HRR rises with increasing energy of applied irradiation.

Clinical Progress in Proton Radiotherapy: Biological Unknowns

“…Comparing the distribution of γH2AX foci after 3 Gy of photon and proton irradiation, it was reported that photon irradiation resulted in the formation of small γH2AX foci that were equally distributed within the nucleus. Proton irradiation in the SOBP composed of 6 Bragg peaks of 100–110 MeV, on the other hand, induced the formation of larger γH2AX foci that were more heterogeneously distributed within the nucleus [ 67 ]. Multiple other studies have also observed larger γH2AX foci after proton compared to photon irradiation, which could be due to the clustered nature of the DNA damage induced by proton radiation [ 68 , 69 , 70 , 71 , 72 , 73 ].…”

“…Multiple other studies have also observed larger γH2AX foci after proton compared to photon irradiation, which could be due to the clustered nature of the DNA damage induced by proton radiation [ 68 , 69 , 70 , 71 , 72 , 73 ]. In addition, slower repair kinetics after proton therapy have been observed [ 30 , 67 , 71 , 73 , 74 , 75 , 76 ]. This is indicative of clustered DNA damage, which is thought to result in slower repair [ 77 ].…”

“…Due to their complex nature, clustered DNA damage is thought to be preferentially repaired by HR [ 77 ]. Several papers have indeed shown that cells with an impaired HR machinery are more sensitive to proton irradiation [ 67 , 78 , 90 , 91 , 92 , 93 ]. Grosse et al [ 78 ] showed that Chinese hamster ovary (CHO) cells deficient in HR and NHEJ were more sensitive towards 200 kV photon as well as 138 MeV SOBP proton radiation compared to wild-type cells (without deficiencies in HR and NHEJ).…”

Oncometabolites and the response to radiotherapy

“…Here, the suggested higher relative importance of HRR for the repair of DNA damage induced by particle therapy (e.g. carbon ions [49], and proton beam therapy [50][51][52][53]) may even offer potential future opportunities for the stratification of patients with oncometabolite-rich tumors towards particle therapy, or combining particle therapy approaches with metabolic drugs inducing HRR defects during therapy.…”

Could Protons and Carbon Ions Be the Silver Bullets Against Pancreatic Cancer?

“…Recently, Szymonowicz et al compared BRCA2-proficient BxPC3 and Capan-1 pancreatic cancer cells with BRCA2-deficiency, and showed that both cell lines were more sensitive to proton than to photon irradiation. The sensitising effect was even more noticeable in Capan-1 cells, leading the authors to suggest a predominant role of HR in the repair of clustered DNA damage induced by protons [ 142 ]. This was also demonstrated in lung cancer and glioblastoma cell lines where the impairment of HR led to a higher sensitisation after proton irradiation compared to photons for which the effect of NHEJ pathway inhibition was more pronounced [ 143 ].…”