Abstract

The DNA polymerase POLQ (pol theta) can bypass abasic sites in DNA and thymine glycol, a common product of reactive oxygen species damage to DNA. POLQ may also have a back-up role in base excision repair. To explore the role of POLQ in tolerance of DNA damage following irradiation, and exposure to agents that induce oxidative stress we established bone marrow stromal cell lines from the PolQ+/+ and PolQ−/− mice. Irradiation survival curves were obtained for PolQ +/+, and 2 clones of PolQ −/− cells. Dose-response experiments using the free radical generators paraquat, hydrogen peroxide and bleomycin were done on 2 clones each of PolQ +/+ and PolQ −/− cells. There was no significant difference over 23 weeks in cumulative granulopoiesis in LTBMCs from PolQ +/+ and PolQ −/− mice. PolQ+/+ cells and 2 independent clones of PolQ−/− cells were exposed to 0 – 8 Gy of gamma radiation, and colony forming ability was measured. PolQ −/− cells were more sensitive to irradiation as shown by decreased Do from 1.38 + 0.06 Gy for PolQ +/+ cells compared to 1.27 + 0.16 and 0.98 + 0.10 (p = 0.0316) Gy for PolQ −/− clones 1 and 3 respectively. Micronucleated reticulocytes were measured in peripheral blood of PolQ+/+, +/− or −/− mice after irradiation with 75 cGy or 700 cGy. Before irradiation, PolQ−/− mice had a higher fraction of MN- reticulocytes compared to PolQ+/+. Forty hr after irradiation, MN reticulocytes in PolQ−/− mice increased to 8.53 ± 1.48%, compared to 2.35 ± 0.12% and 2.55 ± 0.17% in PolQ+/+ mice (p = 0.0032). PolQ +/+ and PolQ −/− cells were exposed to paraquat (0–20 mM), hydrogen peroxide (0–40 uM) and bleomycin (0–5 ug/ml) for 1 hr prior to plating. After 7 days, the cells were stained and colonies of 50 cells or greater were counted. Statistical analysis was done using ANCOVA. PolQ −/− cells were modestly more sensitive to paraquat (p< 0.0001) and bleomycin (p< 0.0001) than PolQ +/+ cells. When the cells were treated with hydrogen peroxide, there was no significant difference in surviving fraction (p=0.7327) between the PolQ +/+ and the −/− cells. The moderate but significant sensitivity of POLQ-defective cells to bleomycin, ionizing radiation and paraquat supports the hypothesis that POLQ normally participates in helping cells tolerate DNA damage. The increased frequencies of spontaneous and radiation-induced micronuclei in peripheral blood red cells indicate that POLQ has a role in maintaining genomic integrity via limiting double-strand DNA breaks.

Author notes

Disclosure: No relevant conflicts of interest to declare.