title: Lung cancer cell line screen links fanconi anemia/BRCA pathway defects to increased relative biological effectiveness of proton radiation.
abstract: OBJECTIVE
  Growing knowledge of genomic heterogeneity in cancer, especially when it results in altered DNA damage responses, requires re-examination of the generic relative biological effectiveness (RBE) of 1.1 of protons.
  
  
  METHODS
  For determination of cellular radiosensitivity, we irradiated 17 lung cancer cell lines at the mid-spread-out Bragg peak of a clinical proton beam (linear energy transfer, 2.5 keV/μm). For comparison, 250-kVp X rays and (137)Cs γ-rays were used. To estimate the RBE of protons relative to (60)Co (Co60eq), we assigned an RBE(Co60Eq) of 1.1 to X rays to correct the physical dose measured. Standard DNA repair foci assays were used to monitor damage responses. FANCD2 was depleted using RNA interference.
  
  
  RESULTS
  Five lung cancer cell lines (29.4%) exhibited reduced clonogenic survival after proton irradiation compared with X-irradiation with the same physical doses. This was confirmed in a 3-dimensional sphere assay. Corresponding proton RBE(Co60Eq) estimates were statistically significantly different from 1.1 (P≤.05): 1.31 to 1.77 (for a survival fraction of 0.5). In 3 of these lines, increased RBE was correlated with alterations in the Fanconi anemia (FA)/BRCA pathway of DNA repair. In Calu-6 cells, the data pointed toward an FA pathway defect, leading to a previously unreported persistence of proton-induced RAD51 foci. The FA/BRCA-defective cells displayed a 25% increase in the size of subnuclear 53BP1 foci 18 hours after proton irradiation.
  
  
  CONCLUSIONS
  Our cell line screen has revealed variations in proton RBE that are partly due to FA/BRCA pathway defects, suggesting that the use of a generic RBE for cancers should be revisited. We propose that functional biomarkers, such as size of residual 53BP1 foci, may be used to identify cancers with increased sensitivity to proton radiation.
authors.affiliation: Laboratory of Cellular and Molecular Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Qi
authors.initials: Q
authors.lastname: Liu
authors.affiliation: Laboratory of Cellular and Molecular Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Priyanjali
authors.initials: P
authors.lastname: Ghosh
authors.affiliation: Laboratory of Cellular and Molecular Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Nicole
authors.initials: N
authors.lastname: Magpayo
authors.affiliation: Division of Radiation Physics, Department of Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Mauro
authors.initials: M
authors.lastname: Testa
authors.affiliation: Division of Radiation Physics, Department of Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Shikui
authors.initials: S
authors.lastname: Tang
authors.affiliation: Laboratory of Cellular and Molecular Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Liliana
authors.initials: L
authors.lastname: Gheorghiu
authors.affiliation: Division of Radiation Physics, Department of Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Peter
authors.initials: P
authors.lastname: Biggs
authors.affiliation: Division of Radiation Physics, Department of Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Harald
authors.initials: H
authors.lastname: Paganetti
authors.affiliation: Laboratory of Cellular and Molecular Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Jason A
authors.initials: JA
authors.lastname: Efstathiou
authors.affiliation: Division of Radiation Physics, Department of Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Hsiao-Ming
authors.initials: HM
authors.lastname: Lu
authors.affiliation: Laboratory of Cellular and Molecular Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Kathryn D
authors.initials: KD
authors.lastname: Held
authors.affiliation: Laboratory of Cellular and Molecular Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts. Electronic address: hwillers@mgh.harvard.edu.
authors.forename: Henning
authors.initials: H
authors.lastname: Willers
mesh_terms: D019313:BRCA1 Protein / Q000235:genetics*; D002289:Carcinoma, Non-Small-Cell Lung / Q000235:genetics* / Q000532:radiotherapy*; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000235:genetics / Q000528:radiation effects; D003037:Cobalt Radioisotopes; D004260:DNA Repair / Q000235:genetics*; D005199:Fanconi Anemia / Q000235:genetics; D052236:Fanconi Anemia Complementation Group D2 Protein / Q000235:genetics*; D006801:Humans; D018499:Linear Energy Transfer; D008175:Lung Neoplasms / Q000235:genetics* / Q000532:radiotherapy*; D061766:Proton Therapy; D051135:Rad51 Recombinase / Q000378:metabolism; D011836:Radiation Tolerance / Q000235:genetics*; D012016:Reference Values; D012062:Relative Biological Effectiveness
chemical_list: D019313:BRCA1 Protein; D003037:Cobalt Radioisotopes; D052236:Fanconi Anemia Complementation Group D2 Protein; D051135:Rad51 Recombinase