The quality of the ionizing radiation (IR) can be described in terms of its nature, photons or particles, and their corresponding energies. The energy is classified in terms of High or Low linear energy transfer that will produce a different distribution of DNA damage and other molecules in the cell either by direct action or indirect action. Indirect action leads to the production of reactive oxygen species (ROS) modifying nucleotides in DNA or free dNTPs. 8-oxo-dGTP is formed through ROS endogenously when there is an imbalance between the antioxidants defence systems and the production of ROS levels in favour of ROS, leading to an oxidative stress condition. Organisms, organs, and cell types show different degrees of radiosensitivity, and this thesis aimed to investigate the underlying mechanisms of IR induced oxidative stress and its relation with radiosensitivity.

In previous studies, we identified proteins involved in radiation response with a focus on low dose radiation response. Cell models were established in which the expression of some protein/s was downregulated by knocking down/out using CRISPR/Cas9 or shRNA technology. The knockdown or knockout cells were exposed to different doses at low dose rates (LDR) or high dose rate (HDR) to investigate the role of these genes/proteins for survival (radiosensitivity), mutation induction, stress response, differentiation, etc. and they were subjected to further studies in this thesis.

Publication I, cell lines with hMTH1, and MYH knockdown were established and exposed to 0.5 and 1 Gy administered at different dose rates. We found that LDR induces significantly increased levels of extracellular 8-oxo-dG compared to HDR. We also found that hMTH1 and MYH play together an important role in the protection of cells against ROS-induced mutagenicity.

Publication II, the role of NRF2 was investigated for the radiosensitivity of glioblastoma cancer stem cells (CSCs). The neutrosphere cells from the U87MG cell line were irradiated with three different radiation qualities. The results show that cells exposed to LDR produce significantly higher levels of extracellular 8-oxo-dG compared to HDR and carbon ion irradiated cells. Lower proliferation, self-renewal, and neurosphere formation were observed in both LDR and HDR irradiated NRF2-knockdown cells as compared with the wild type. The results show that NRF2 plays an important role in the radiosensitivity of neurosphere cells isolated from the U87MG cell line.

Publication III, we examined the relation between 8-oxo-dG levels and the outcome of radiotherapy and chemotherapy in gastrointestinal cancer patients. The results showed that patients with improved treatment outcomes (responders), had lower levels of the stress marker extracellular 8-oxo-dG before the start of the treatment and the levels were increased 2 weeks after completing the treatment.

Publication IV, mice were whole-body irradiated with different doses administered at LDR and HDR. Three hours or three weeks after exposure, the immune cell populations in the spleens were phenotyped. The effects of dose, dose rate, and time after exposure and interaction between them were investigated to check which of the factors had the main effect on the change of immune cell populations. The results indicate that there was a pro-inflammatory short-term effect at high doses for both HDR and LDR. The results also indicate a pro-inflammatory effect of low doses of radiation three weeks after exposure.