Role of H₂O₂ in RET/PTC1 chromosomal rearrangement produced by ionizing radiation in human thyroid cells

During childhood, the thyroid gland is one of the most sensitive organs to the carcinogenetic effects of ionizing radiation that may lead to papillary thyroid carcinoma (PTC) associated with RET/PTC oncogene rearrangement. Exposure to ionizing radiation induces a transient "oxidative burst" through radiolysis of water, which can cause DNA damage and mediates part of the radiation effects. H₂O₂ is a potent DNA-damaging agent that induces DNA double-strand breaks, and consequently, chromosomal aberrations. Irradiation by 5 Gy X-ray increased extracellular H ₂O₂. Therefore, we investigated the implication of H ₂O₂ in the generation of RET/PTC1 rearrangement after X-ray exposure. We developed a highly specific and sensitive nested reverse transcription-PCR method. By using the human thyroid cell line HTori-3, previously found to produce RET/PTC1 after γ-irradiation, we showed that H₂O₂, generated during a 5 Gy X-ray irradiation, causes DNA double-strand breaks and contributes to RET/PTC1 formation. Pretreatment of cells with catalase, a scavenger of H₂O₂, significantly decreased RET/PTC1 rearrangement formation. Finally, RET/PTC chromosomal rearrangement was detected in HTori-3.1 cells after exposure of cells to H ₂O₂ (25 μmol/L), at a dose that did not affect the cell viability. This study shows for the first time that H₂O₂ is able to cause RET/PTC1 rearrangement in thyroid cells and consequently highlights that oxidative stress could be responsible for the occurrence of RET/PTC1 rearrangement found in thyroid lesions even in the absence of radiation exposure.