Potential for the G2/M arrest assay to predict patient susceptibility to severe reactions following radiotherapy

Background and Purpose: Cell-cycle regulation and checkpoint activation are crucial factors for radiation-induced DNA damage processing. The G2/M phase arrest was assessed in lymphoblastoid cell lines and phytohemagglutinin-stimulated T-lymphocytes of different radiosensitivities to study the relationship of G2/M arrest to radiosensitivity. Material and Methods: G2/M arrest was analyzed after in vitro irradiation by 2 and 5 Gy of ionizing radiation up to 6 days using 17 lymphoblastoid cell lines from healthy individuals, ataxia-telangiectasia (AT) patients, Nijmegen breakage syndrome (NBS) patients and cancer patients with clinically increased radiosensitivity. In a second approach, phytohemagglutinin-stimulated T-lymphocytes from 15 healthy individuals, twelve cancer patients, and five cancer patients hypersensitive to ionizing radiation were studied. Image cytometry was performed to analyze G2/M arrest. Results: Two of the three AT cell lines showed markedly increased G2/M arrest compared to controls. NBS cells were comparable to controls up to day 3, but then demonstrated a slightly increased G2/M arrest. Two of the six radiosensitive lymphoblast cell lines and the five radiosensitive cancer patients' T-lymphocytes assayed showed a reduction in G2/M arrest, while healthy individuals showed no difference from cancer patients. Conclusion: The interrelation between G2/M arrest and radiosensitivity is not readily apparent since a variety of radiosensitive cells from patients with radiosensitive syndromes and patients identified as radiosensitive following radiation treatment showed inconsistent G2/M arrest dynamics. Secondary effects, like loss of clonogenicity, G1/S phase arrest and failure of G2/M arrest may contribute to variation of the G2/M arrest endpoint and obscure assessment of cellular radiosensitivity using this method.