Sensitivity of excision repair in normal human, xeroderma pigmentosum variant and Cockayne's syndrome fibroblasts to inhibition by cytosine arabinoside

Abstract

Inhibition of the gap-filling, polymerizing step of excision repair by 1-beta-D-arabinofuranosylcytosine (ara-C) after irradiation with ultraviolet light in human diploid fibroblasts resulted in the formation of persistent DNA strand breaks in G1, G2, and plateau phase cells, but not in S phase cells. Addition of hydroxyurea to ara-C resulted in partial inhibition of repair in S phase cells. These observations can be explained either in terms of changing roles in repair for different DNA polymerases throughout the cell cycle or by the presence of a pool of deoxycytidine nucleotides during S phase equivalent to be an external source of deoxycytidine at 50 microM concentration. A similar concentration dependence on ara-C was observed for inhibition of repair in normal human, xeroderma pigmentosum (XP) variant, and Cockayne's syndrome cells but slightly more in XP variant cells. Exonuclease III and S1 nuclease independently both degraded about 50% of the 3H-thymidine incorporated into repaired regions in the presence of ara-C. Sequential digestion with both enzymes degraded nearly 90% of the repaired regions. These observations can be explained if excision repair proceeds by displacing the damaged strand so that both the 3H-labeled patch and the damaged region are still ligated to high molecular weight DNA and compete for the same complementary strand during in vitro incubation with the nucleases. The amount of 3H-thymidine incorporated in DNA by repair decreased with increasing concentrations of ara-C and hydroxyurea, suggesting that the incomplete patches became shorter under these conditions. Extrapolation of the digestion kinetics with exonuclease III permits an estimate of the normal patch size of about 100 nucleotides, consistent with previous estimates.

γH2Ax: Biomarker of Damage or Functional Participant in DNA Repair “All that Glitters Is not Gold!”

“…3A,B) due to a block in NER at the chain extension stage (42,43). Cytosine arabinoside is a chain terminator that maintains repair patches incomplete increasing the number of short stretches of single‐stranded DNA that activates ATR‐dependent phosphorylation (42,43,45). Smaller relative increases in γH2Ax occur after growth in cytosine arabinoside after ionizing radiation (Fig.…”

A Novel Role of DNA Polymerase η in Modulating Cellular Sensitivity to Chemotherapeutic Agents

“…This is the first demonstration that pol D is involved in the action of nucleoside analogues AraC and gemcitabine. Previous studies have shown that when AraC was used to inhibit the nucleotide excision repair (NER) pathway and thus accumulate single-strand breaks at sites of excision and resynthesis there was always a small but significantly higher break frequency in XP-V than normal cells (29). Similarly, the break frequency during NER in the absence of inhibitors was also slightly higher in XP-V than normal cells (30).…”

DNA single stranded gaps formed during DNA repair synthesis induced by methyl methanesulfonate are filled by sequential action of aphidicolin- and dideoxythymidine sensitive DNA polymerases in HeLa cells

“…An inhibitor of DNA polymerase may fail to block the repair synthesis in S phase cells (Cleaver, 1981), while in HeLa cells it may not have any effect. Accordingly, we suppressed the replicative synthesis by serum-starvation and treatment with a high dose hydroxyurea.…”