ATM-Dependent Hyper-Radiosensitivity in Mammalian Cells Irradiated by Heavy Ions

Is activation of the intra-S checkpoint in human fibroblasts an important factor in protection against UV-induced mutagenesis?

“…Velocity sedimentation As previously described, 19,57 the steady-state size distribution of nascent DNA was determined by centrifugation in an alkaline sucrose density gradient 45 min after UV irradiation. Cells were uniformly pre-labeled during logarithmic growth by incubation with 14 C-thymidine (5-10 nCi/mL) for at least 36 h. Medium containing 14 C-thymidine was removed overnight and cells treated the following day. Thirty minutes after UV-irradiation, cells were incubated with 25 µCi/mL 3 H-thymidine for 15 min.…”

“…It is well established that loss of functional ATM increases IR-induced mutation frequency 14 and predisposes mammals to lymphomas and other malignancies. 15 Although ATM is widely recognized as a tumor-suppressor gene, it is unclear whether this function depends primarily on the activation of the ATMdependent intra-S checkpoint or on ATM's roles in other DNA damage responses.…”

Limited Repair of Critical DNA Damage in Cells Exposed to Low Dose Radiation

“…This phenomenon has come to be known as hyperradiosensitivity of cells. Now it has been determined that hyper-radiosensitivity is associated with failures in the ATM-dependent stage of DDR development and is due to the low efficiency of repair of DSB induced in DNA by low IR doses of both low-and high LET (Wykes et al, 2006;Xue et al, 2009). It was established a direct relation between low-dose hyper-radiosensitivity and the lack of ATM-dependent "early" activation of cell cycle checkpoint at the G2/M transition (Marples et al, 2004).…”

“…These relaxed chromatin structures are created through the coupled action of the p400 SWI/SNF ATPase and histone acetylation by the Tip60 acetyltransferase. The resulting destabilization of nucleosomes at the DSB by Tip60 and p400 is required for ubiquitination of the chromatin by the RNF8 ubiquitin ligase, and for the subsequent recruitment of the BRCA1 and others proteins (Xu & Price, 2011). Chromatin dynamics at DSBs can therefore exert a powerful influence on the process of DSB repair.…”

“…Further, there is emerging evidence that the different chromatin structures in the cell, such as heterochromatin and euchromatin, utilize distinct remodeling complexes and pathways to facilitate DSB. The processing and repair of DSB is therefore critically influenced by the nuclear architecture in which the lesion arises (Xu & Price, 2011;Ball & Yokomori, 2011). Phosphorylation of histone H2AX and other proteins by ATM, ATR and DNA-PKcs near DSB also modifies chromatin, which is crucial in the interactions between many proteins after exposure to IR and, above all, for the process of DNA repair.…”