Human topoisomerase IB role in repairing of genomic damage induced by ionizing radiation

Ionizing radiation (IR) can cause damage to DNA molecules, both through direct ionization and induction of oxidative stress. Exposure to IR may induce single strand breaks (SSB), double strand breaks (DSB) in the genomic DNA as well as trigger oxidative stress (increasing production of reactive free oxygen species, ROS) that ultimately mediates DNA damage. Previous studies have shown that human topoisomerase IB (hTop IB) is proposed to be involved in the SSB and DSB repair as a response to various DNA damages but its role still remains unclear. To study the role of topoisomerases (Top IB) in repair of DNA damage induced by both IR and ROS, we used three model system such as Saccharomyces cerevisiae and human epithelium cell lines (both normal and cancer). In this context, we evaluated the effect of different doses of X-rays and different concentrations of H2O2 on the enzyme. For all cases, it was confirmed that following IR exposure and ROS, Top IB activity is increased as well as the activity of Top IB to bind with genomic DNA is also increased. On the other hand, Top IB has shown no effect on the survival rate of S. cerevisiae after radiation exposure. Whereas, from H2O2 treatment, cancer cell lines showed increased survivability than normal one. Following exposure to IR S. cerevisiae cells bearing Top IB showed a higher initial damage comparing to defective cells, but repair was more efficient and, 20 h from radiation exposure, DNA damage was higher in the defective cells. Whereas, for both normal and cancer cells, number of DNA damage and micronuclei formation shown to be more following IR but cancer cells showed faster repair rate in respect to normal cell. Even for Top IB activity assay normal and cancer cells behaved in a different way that demands for further study for cancer cells. Another fact is that, for both IR and H2O2 stress condition Top IB expression level changes very little but not noticibly, opposite to its activity that drives us to find about post translational modification of this enzyme. Based on all these outcomes, in future, we hope to carry on with different cancer cells and to focus more on posttranslational modifications of this enzyme and also to find out correlation of Top IB with other repair proteins such as tyrosyl DNA phosphodiesterase (TDP1) and poly (ADP-ribose) polymerase 1 (PARP 1) to make our data pharmacologically more significant. We hope that the results of our research could be used to improve the use of camptotecin in radiotherapy or to better understand the early phases of DNA breaks repair and also the role of oxidative stress in reducing repair which ultimately may provide a new insight into the mechanisms underlying the maintenance of genomic integrity and treatment of cancer.