From in vitro studies, flap endonuclease 1 (FEN1) has been proposed to play a role in the long patch (LP) base excision repair (BER) sub-pathway. Yet, the role of FEN1 in BER in the context of the living vertebrate cell has not been thoroughly explored. In the present study, we cloned a DT40 chicken cell line with a deletion in the FEN1 gene and found that these FEN1-deficient cells exhibited hypersensitivity to H2O2. This oxidant produces genotoxic lesions that are repaired by BER, suggesting that the cells have a deficiency in BER affecting survival. In experiments with extracts from the isogenic FEN1 null and wild type cell lines, the LP-BER activity of FEN1 null cells was deficient, whereas repair by the single-nucleotide BER sub-pathway was normal. Other consequences of the FEN1 deficiency also were evaluated. These results illustrate that FEN1 plays a role in LP-BER in higher eukaryotes, presumably by processing the flap-containing intermediates of BER
