Cell cycle dependent DNA break increase in ataxia telangiectasia lymphoblasts after radiation exposure

Abstract

The most striking feature of ataxia telangiectasia (AT) cells is their profound sensitivity to ionising radiation. A deficiency in the rejoining of radiation induced DNA breaks has been suggested to be responsible for AT radiosensitivity; however, the existing literature is controversial. A subpopulation, which is present in irradiated AT lymphoblasts, but rarely in controls, has been reported previously. The cells that make up this subpopulation harbour highly fragmented DNA and are responsible for the overall increase in DNA breaks soon after irradiation in AT lymphoblasts. This study examines the influence of the cell cycle on the highly damaged subpopulation. The frequency of highly damaged cells was highest when AT lymphoblasts were irradiated during the G2/M phase. In contrast, AT lymphoblasts irradiated during the G0/G1 phase displayed a frequency similar to control cells. Thus, only G2/M and to some extent S phase cells contribute to an increased DNA break number in AT lymphoblasts early after irradiation. These findings might explain several inconsistencies reported in the literature.

Figure 1

Cell cycle distribution and corresponding highly damaged comet (HDC) frequencies in ataxia telangiectasia (AT) cells. AT lymphoblasts (GMO9582) were sorted according to their cell cycle phases, exposed to 6 Gy, incubated for two hours, and comet analysis was performed. Their HDC frequencies are shown in the integrated chart. Minus and plus signs indicate unirradiated and irradiated samples, respectively. Error bars refer to the SEM of four independent observations. The radiation induced HDC frequencies (corrected for the corresponding unirradiated samples) of the individual cell cycle phases differed significantly (p < 0.025; Mann-Whitney U test).

Figure 2

Microscopic appearance of highly damaged comets (HDCs). (A) A comet with very low amounts of DNA damage, showing almost no comet tail. (B) A comet typical of the highly damaged subpopulation; the comet head (to the right) is hardly visible, with almost all of the DNA migrated into the comet tail. Both comets were scored in ataxia telangiectasia samples two hours after irradiation.