Lethal DNA Lesions Caused by Direct and Indirect Actions of X rays are Repaired via Different DSB Repair Pathways...

Area of Science:

• Molecular Biology
• Radiation Biology
• Genetics

Background:

• X-rays induce DNA damage through direct and indirect mechanisms.
• DNA double-strand breaks (DSBs) are critical lesions repaired by homologous recombination (HR) and non-homologous end joining (NHEJ) pathways.
• Understanding the differential roles of these repair pathways in response to direct vs. indirect radiation damage is crucial.

Purpose of the Study:

• To investigate the distinct contributions of direct and indirect X-ray actions to cell killing.
• To assess the role of homologous recombination (HR) and non-homologous end joining (NHEJ) DNA repair pathways in mitigating radiation-induced lethal damage.
• To determine how oxygen levels influence the balance between direct and indirect radiation effects on cell survival.

Main Methods:

• Utilized two Chinese hamster ovary (CHO) cell lines deficient in either HR (51D1) or NHEJ (xrs6) repair.
• Irradiated cells under aerobic and anoxic conditions to modulate indirect damage contribution.
• Employed dimethyl sulfoxide (DMSO) to scavenge hydroxyl radicals, thereby quantifying indirect action effects.

Main Results:

• Indirect action contributed significantly to cell killing in HR-defective cells (76% aerobic, 57% anoxic).
• Indirect action's contribution to cell killing in NHEJ-defective cells was lower (52% aerobic, 33% anoxic) and varied with oxygen levels.
• Cell killing by indirect action was dependent on repair pathway and oxygen, but not radiosensitivity.

Conclusions:

• The NHEJ pathway is critical for repairing DNA double-strand breaks induced by direct X-ray action.
• The HR pathway appears less effective in repairing DSBs caused by direct radiation action compared to NHEJ.
• Direct and indirect radiation actions generate distinct types of DNA double-strand breaks, differentially processed by cellular repair machinery.