Abstract
Ionizing radiation (IR) induces highly cytotoxic double-strand breaks (DSBs) and also clustered oxidized bases in mammalian genomes. Base excision repair (BER) of bi-stranded oxidized bases could generate additional DSBs as repair intermediates in the vicinity of direct DSBs, leading to loss of DNA fragments. This could be avoided if DSB repair via DNA-PK-mediated nonhomologous end joining (NHEJ) precedes BER initiated by NEIL1 and other DNA glycosylases (DGs). Here we show that DNA-PK subunit Ku inhibits DGs via direct interaction. The scaffold attachment factor (SAF)-A, (also called hnRNP-U), phosphorylated at Ser59 by DNA-PK early after IR treatment, is linked to transient release of chromatin-bound NEIL1, thus preventing BER. SAF-A is subsequently dephosphorylated. Ku inhibition of DGs in vitro is relieved by unphosphorylated SAF-A, but not by the phosphomimetic Asp59 mutant. We thus propose that SAF-A, in concert with Ku, temporally regulates base damage repair in irradiated cell genome.
Highlights
Ionizing radiation (IR) as well as radiomimetic drugs induce clusters of damage in the genome, including most cytotoxic double-strand breaks (DSBs), as well as single-strand breaks (SSBs) with nonligatable termini, bi-stranded clusters of abasic (AP) sites, and oxidized bases [1, 2]
We demonstrate that Ku negatively regulates Base excision repair (BER) by inhibiting base excision/strand-scission activity of all oxidized base-specific DNA glycosylases (DGs) at IR-induced clustered damage sites to allow the completion of DSB repair via nonhomologous end joining (NHEJ)
NHEJ proteins interact with BER enzymes in irradiated cells
Summary
Ionizing radiation (IR) as well as radiomimetic drugs induce clusters of damage in the genome, including most cytotoxic double-strand breaks (DSBs), as well as single-strand breaks (SSBs) with nonligatable termini, bi-stranded clusters of abasic (AP) sites, and oxidized bases [1, 2]. NHEJ, the predominant repair process in all cells, is initiated by the binding of Ku (Ku70/Ku80 heterodimer) subunit of DNA-dependent protein kinase (DNA-PK) to the DSB site, which recruits the catalytic subunit (DNA-PKcs). DNA-PK in addition to phosphorylating www.impactjournals.com/oncotarget many downstream targets enables formation of a large NHEJ complex comprising DNA ligase4/XRCC4/XLF that joins the DSBs after their end processing [6]. The base lesions and AP sites are repaired via the base excision/SSB repair (BER/SSBR) pathway and involves formation of SSB intermediates, which like the IR-induced direct SSBs bind PARP-1. While NHEJ is the major pathway for DSB repair [10], alternative NHEJ (Alt-EJ) involving PARP-1 and the BER machinery may repair DSBs, including those generated during the repair of bi-stranded oxidized lesion clusters; its contribution to DSB repair is not very clear [11]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
