Oxygen-effect on Strand Breaks and Specific End-groups in DNA of Irradiated Thymocytes

Abstract

Thymocytes were irradiated with fast electrons up to 6 Mrad in the presence and absence of oxygen. The cells were treated before irradiation with a cold shock to prevent any repair rejection during irradiation. The DNA isolated subsequently was analysed for double-strand breaks (dsb), actual single-strand breaks (ssb) and alkali-induced strand breaks (alisb). We observed a linear increase of all types of lesions with dose and an o.e.r. for dsb of 3-6, for ssb of 4-9 and for alisb of 2-1. The data do not deviate significantly from those, measured on thymocytes irradiated without cold shock. In DNA of irradiated thymocytes, the frequency of 3' and 5' hydroxyl and 5' phosphate end-groups was analysed enzymatically. In both the ssb and alisb, about 11 per cent of the terminals carry 5'OH end-groups and 20-40 per cent 5' phosphate groups. On the 3' terminals, 60-80 per cent of the ssb are identified as 3'OH end-groups, whereas on the alisb only a small amount of 3'OH end-groups if found. The frequency of characterized end-groups shows the same oxygen effect as the corresponding strand breaks. Therefore, in the presence and absence of oxygen, the same mechanism may be responsible for formation of DNA strand breaks in vivo.

Human Polynucleotide Kinase Participates in Repair of DNA Double-Strand Breaks by Nonhomologous End Joining but not Homologous Recombination

“…An alternative, equally plausible, explanation is that not all DSB acted on by DNA-PKcs require hPNK processing. Irradiation under aerated conditions generates termini with 3 ¶-phosphate and 3 ¶-phosphoglycolate end groups in approximately similar proportions and only f15% of 5 ¶-termini lack a 5 ¶-phosphate group (12)(13)(14)28). Although the 3 ¶-phosphate termini are extremely poor substrates for Ape1 (6,29), Ape1 can remove a significant proportion of phosphoglycolate groups, especially from 3 ¶-recessed termini (30)(31)(32), although phosphoglycolates at 3 ¶-overhanging termini require another processing pathway involving hPNK (33).…”

“…Deletion of PNK in fission yeast resulted in cellular hypersensitivity to ionizing radiation and camptothecin, a topoisomerase I inhibitor. Both of these agents are capable of generating DNA SSB and DSB with termini requiring either or both PNK activities (12)(13)(14)(15). RNA interference (RNAi) silencing of hPNK in A549 human lung carcinoma cells yielded a similar response to that seen in yeast (16).…”

Purification and Characterization of an Endonuclease from Micrococcus luteus that Acts on Depurinated and Carcinogen‐Modified DNA

“…MeSOZOMe,methyl methanesulfonate;ethyl methanesulfonate;MeNOUr,EtNOUr,BuNOUr,PeNOUr,MezS04, dimethyl sulfate; Et2SO4, dielhyl sulfate; nitrogen mustard, 2,2'-dichloro-Nmethyldiethylamine hydrochloride; (Ac)lONFln, N-acetoxy-2-acet y laminofluorene. double-strand breaks [2], whereas mutagens and carcinogens modify DNA by transferring electrophilic chemical residues [3 -51 which might cause steric alterations [6] and/or loss of bases [7,8].…”

Repair of Radiation-induced DNA Strand Breaks in Thymocytes

“…The preparation of thymocytes in Hank's balanced solution has been described previously (Lennartz, Coquerelle, Bopp and Hagen 1975 a) . Two millilitres of the cell suspension were irradiated in a 60 Co y-source (Gammacell 220, Atomic Energy of Canada Ltd .)…”