DNA repair pathways involved in repair of lesions induced by 5-fluorouracil and its active metabolite FdUMP.

5-Fluorouracil (5-FU) is an antitumor antimetabolite that can be converted into fluoronucleotides and FdUMP. Fluoronucleotides are incorporated into DNA and RNA, while FdUMP results in nucleotide pool imbalance. Saccharomyces cerevisiae is unable to convert 5-FU into FdUMP, making yeast a unique model system to study the cellular effects of 5-FU and FdUMP independently. A panel of repair-deficient yeast strains was used to identify the DNA repair pathways needed for repair of lesions generated by 5-FU or FdUMP. This included yeast deficient in base excision repair (BER), nucleotide excision repair (NER), translesion synthesis (TLS), mismatch repair (MMR), post-replication repair (PRR), homologous recombination (HR) and non-homologous end-joining (NHEJ). The results revealed an important role of BER, since BER-mutants (ntg1, ntg2, apn1, apn2) showed pronounced sensitivity to both 5-FU and FdUMP. MMR mutants also showed high sensitivity to both compounds. In contrast, deficiencies in NER, NHEJ and TLS repair had only minor influence on the sensitivity to FU and FdUMP. Interestingly, deficiencies in HR (rad52) and PPR (rad6, rad18) were associated with increased sensitivity to 5-FU, but not to FdUMP. Taken together, our study reveals an important contribution of DNA repair pathways on the sensitivity to 5-FU and its active metabolite FdUMP. Importantly, the repair mechanisms differed for the 2 antimetabolites since lesions induced by 5-FU were repaired by BER, MMR, HR and PRR, while only BER and MMR were required for repair of FdUMP-induced lesions.