Parg deficiency confers radio-sensitization through enhanced cell death in mouse ES cells exposed to various forms of ionizing radiation

Poly(ADP-ribose) glycohydrolase (Parg) is the main enzyme involved in poly(ADP-ribose) degradation. Here, the effects of Parg deficiency on sensitivity to low and high linear-energy-transfer (LET) radiation were investigated in mouse embryonic stem (ES) cells. Mouse Parg-/- and poly(ADP-ribose) polymerase-1 deficient (Parp-1-/-) ES cells were used and responses to low and high LET radiation were assessed by clonogenic survival and biochemical and biological analysis methods.Parg-/-cells were more sensitive to gamma-irradiation than Parp-1-/- cells. Transient accumulation of poly(ADP-ribose) was enhanced in Parg-/-cells. Augmented levels of phosphorylated H2AX (gamma-H2AX)from early phase were observed in Parg-/- ES cells. The induction level of p53 phophorylation at ser18 was similar in wild-type and Parp-1-/- cells and apoptotic cell death process was mainly observed in the both genotypes. These results suggested that the enhanced sensitivity of Parg-/- ES cells to gamma-irradiation involved defective repair of DNA double strand breaks. The effects of Parg and Parp-1 deficiency on the ES cell response to carbon-ion irradiation (LET13 and 70 keV/um) and Fe-ion irradiation (200 keV/um) were also examined. Parg-/- cells were more sensitive to LET 70 keV/um carbon-ion irradiation than Parp-1-/- cells. Enhanced apoptotic cell death also accompanied augmented levels of gamma-H2AX in a biphasic manner peaked at 1 and 24 h. The induction level of p53 phophorylation atser18 was not different between wild-type and Parg-/- cells. The augmented level of poly(ADP-ribose) accumulation was noted after carbon-ion irradiation compared to gamma-irradiation even in the wild-type cells. An enhanced poly(ADP-ribose) accumulation was further observed in Parg-/- cells. Both Parg-/- cells and Parp-1-/- cells did not show sensitization to Fe-ion irradiation. Parg deficiency sensitizes mouse ES cells to a wide therapeutic range of LET radiation through the effects on DNA double strand break repair responses and enhanced cell death