In Vitro Chromosomal Radiosensitivity and Cell Cycle Progression in Cancer Survivors

Abstract

The in vitro 02 chromosomal radiosensitivity assay is a technique used to investigate variation in the cellular response to radiation. In brief, lymphocytes are irradiated in the 02 phase of the cell cycle to induce DNA damage, which is exhibited at the subsequent metaphase as chromatid gaps and breaks. Radiation-induced arrest at the end of (32 is believed to allow time for adequate DNA repair before the onset of mitosis. Therefore, variation in the level of aberrations observed at metaphase is likely to be driven in part by 02 checkpoint control. This led to an investigation into whether variation in in vitro G2 chromosomal radiosensitivity is related to 02 checkpoint efficacy.

A modified version of the 02 chromosomal radiosensitivity assay was validated with samples from staff at Westlakes Research Institute. The standard 02 assay protocol was
altered by the addition of the chemical calyculin A which induces Premature Chromosome Condensation (PCC) in interphase cells enabling visualisation and classification of all cell cycle stages (G1, S, G2 and metaphase). Initial attempts at assessing G2 to metaphase transition by visualising and scoring damage directly in G2 cells failed. However, by measuring changes in the ratio of PCC-G2 and metaphase cells before and after irradiation, it was possible to measure 02 checkpoint delay. Following validation of the PCC technique, both the G2 assay and the modified assay were applied to a group of 29 cancer survivors and the extent of any individual G2 checkpoint delay was compared to the radiation-induced chromatid aberration frequency.

No significant relationship between chromatid aberration frequency and G2 checkpoint delay was observed. Providing that the PCC technique is accurately assessing G2 delay,
Ill the results suggest that variation in G2 chromosomal radiosensitivity is more likely to be driven by variation in DNA repair pathways than variation in G2 checkpoint delay.