title: Base excision repair of both uracil and oxidatively damaged bases contribute to thymidine deprivation-induced radiosensitization.
abstract: OBJECTIVE
  Increased cellular sensitivity to ionizing radiation due to thymidine depletion is the basis of radiosensitization with fluoropyrimidine and methotrexate. The mechanism responsible for cytotoxicity has not been fully elucidated but appears to involve both the introduction of uracil into, and its removal from, DNA. The role of base excision repair of uracil and oxidatively damaged bases in creating the increased radiosensitization during thymidine depletion is examined.
  
  
  METHODS
  Isogenic strains of S. cerevisiae differing only at loci involved in DNA repair functions were exposed to aminopterin and sulfanilamide to induce thymidine deprivation. Cultures were irradiated and survival determined by clonogenic survival assay.
  
  
  RESULTS
  Strains lacking uracil base excision repair (BER) activities demonstrated less radiosensitization than the parental strain. Mutant strains continued to show partial radiosensitization with aminopterin treatment. Mutants deficient in BER of both uracil and oxidatively damaged bases did not demonstrate radiosensitization. A recombination deficient rad52 mutant strain was markedly sensitive to radiation; addition of aminopterin increased radiosensitivity only slightly. Radiosensitization observed in rad52 mutants was also abolished by deletion of the APN1, NTG1, and NTG2 genes.
  
  
  CONCLUSIONS
  These data suggest radiosensitization during thymidine depletion is the result of BER activities directed at both uracil and oxidatively damaged bases.
authors.affiliation: Department of Biochemistry, Carver College of Medicine, University of Iowa, Iowa City, IA, USA.
authors.forename: Bryan G
authors.initials: BG
authors.lastname: Allen
authors.forename: Monika
authors.initials: M
authors.lastname: Johnson
authors.forename: Anne E
authors.initials: AE
authors.lastname: Marsh
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Dornfeld
mesh_terms: D000630:Aminopterin / Q000494:pharmacology; D015169:Colony Count, Microbial; D004249:DNA Damage; D004260:DNA Repair / Q000187:drug effects / Q000235:genetics; D045643:DNA Repair Enzymes; D004706:Endodeoxyribonucleases / Q000235:genetics; D005493:Folic Acid Antagonists / Q000494:pharmacology; D006898:Hydroxylamines / Q000494:pharmacology; D009154:Mutation; D051721:Rad52 DNA Repair and Recombination Protein / Q000235:genetics* / Q000378:metabolism; D011836:Radiation Tolerance / Q000235:genetics / Q000502:physiology*; D012441:Saccharomyces cerevisiae / Q000187:drug effects / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics; D000077145:Sulfanilamide; D013424:Sulfanilamides / Q000494:pharmacology; D013936:Thymidine / Q000172:deficiency* / Q000378:metabolism; D013942:Thymine Nucleotides / Q000172:deficiency*; D014498:Uracil / Q000378:metabolism*; D051981:Uracil-DNA Glycosidase / Q000172:deficiency / Q000378:metabolism
chemical_list: D005493:Folic Acid Antagonists; D006898:Hydroxylamines; D051721:Rad52 DNA Repair and Recombination Protein; D029701:Saccharomyces cerevisiae Proteins; D013424:Sulfanilamides; D013942:Thymine Nucleotides; D000077145:Sulfanilamide; D014498:Uracil; C005214:methoxyamine; D004706:Endodeoxyribonucleases; C083274:Apn1 protein, S cerevisiae; D051981:Uracil-DNA Glycosidase; D045643:DNA Repair Enzymes; D000630:Aminopterin; C024157:thymidine 5'-triphosphate; D013936:Thymidine