Title

Involvement of p53 in ionizing-radiation-induced DNA repair

Date of Completion

January 1998

Keywords

Health Sciences, Radiology|Biophysics, Medical

Degree

Ph.D.

Abstract

Exposure of mammalian cells to ionizing radiation (IR) evokes physiological responses including cell-cycle arrest, DNA-repair induction, and if severely damaged apoptosis. This dissertation reports the DNA-repair processes induced by IR, and the involvement of the tumor suppressor protein p53 in these processes. Shuttle vector pZ189 containing IR-caused base damage plus single-strand break (ocDNA) or ultraviolet radiation damage (uvDNA), and pZ189 containing a double-strand break (linDNA) were used as probes to measure excision and recombination repair activities, respectively, of unirradiated or $\gamma$-irradiated human lymphoblasts, expressing wild-type (p53$\sp{\rm wt}$) or mutant (p53$\sp{\rm mut}$) or no p53 (p53$\sp{\rm null}$).^ Excision repair fidelity of p53$\sp{\rm wt}$, p53$\sp{\rm mut}$ and p53$\sp{\rm null}$ cells were similar. However, exposure of lymphoblast hosts to low-dose radiation altered both excision repair in a p53-dependent manner. Irradiation enhanced the fidelity of excision repair in p53$\sp{\rm wt}$ hosts, but in p53$\sp{\rm mut}$ or p53$\sp{\rm null}$ hosts. Irradiation altered recombination repair in time-dependent manner: repair fidelity increased two-fold compared with unirradiated control hosts if plasmid transfection occurred immediately after host irradiation, but decreased two-fold if transfection occurred 2hrs post-host-irradiation. However, unlike its effects on excision repair, IR-modulation of recombination repair fidelity was similar in p53$\sp{\rm wt}$, p53$\sp{\rm mut}$ and p53$\sp{\rm null}$ cells.^ DSB-rejoining capacity differed between irradiated p53$\sp{\rm wt}$ and p53$\sp{\rm mut}$ or p53$\sp{\rm null}$ hosts. Immediately after host irradiation, DSB-rejoining capacity in p53$\sp{\rm wt}$ hosts was similar to that in unirradiated p53$\sp{\rm wt}$ hosts, but increased significantly in irradiated p53$\sp{\rm mut}$ and p53$\sp{\rm null}$ hosts. These data show that low-dose IR modulates DNA excision- and recombination-repair in mammalian cells and that p53 is required for both these processes. ^