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(PP329) Low-dose hyper-radiosensitivity does not result from a failure to recognize DNA double-strand breaks.
Wykes, Susan1, Wilson, George1, Joiner, Michael1, Piasentin, Evano1, Wright, Jennifer1, Marples, Brian*,1, 1 Department of Radiation Oncology, Detroit, Michigan, USA
ABSTRACT- One of the earliest cellular responses to radiation-induced DNA damage is the phosphorylation of the histone variant H2AX (-H2AX). -H2AX facilitates the local concentration and foci formation of numerous repair-related proteins within the vicinity of DNA double-strand breaks (DSBs). Previously, we have shown that low-dose hyper-radiosensitivity, the excessive sensitivity of mammalian cells to very low doses of ionizing radiation, is a response specific to G2 phase cells and is likely attributed to evasion of an ATM dependent early G2 cell cycle checkpoint. To further define the mechanism of low-dose hyper-radiosensitivity, we investigated the relationship between the recognition of radiation-induced DNA double strand break damage as defined by -H2AX staining and the incidence of low-dose hyper-radiosensitivity in three isogenic-paired cell lines with known differences in radiosensitivity and DNA repair functionality. Marked differences in cell survival were observed between the six cell lines after high dose exposures (> 1 Gy) reflective of DNA repair capabilities of the individual six cell lines. In contrast, the absence of functional ATM or DNA-PK activity did not affect cell survival outcome after very low doses exposures below 0.2 Gy. These data are supportive of the concept that low-dose hyper-radiosensitivity is a measure of radiation sensitivity of cells in the absence of fully functional cellular repair processes. Moreover, the pattern of -H2AX dose response was generally conserved between the six cell lines at both 0.2 Gy and 2 Gy, differing only in magnitude with dose. No relationship was evident between the initial numbers of DNA DSBs scored immediately after radiation exposure with either low or high dose cell survival. Therefore, we conclude that the initial recognition of DNA DSBs is not the prime determinant of cellular radiosensitivity. However, residual DNA DSB damage as indicated by the persistence of -H2AX foci 4hrs after radiation exposure was significantly correlated with cell survival after high dose exposures. This observation suggests that the persistence of -H2AX foci could be adapted as a surrogate assay of cellular radiosensitivity in order to predict clinical radiation responsiveness.
Key words: low-dose hyper-radiosensitivity, -H2AX, DNA repair, low doses
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