Human tyrosyl-DNA phosphodiesterase 1 (Tdp1) hydrolyzes the phosphodiester bond at a DNA 3' end linked to a tyrosyl moiety and has been implicated in the repair of Topoisomerase I (TopI)-DNA covalent complexes. Tdp1 can also hydrolyze other 3' end DNA alterations including 3' phosphoglycolate and 3' abasic (AP) sites, and exhibits the 3' nucleosidase activity indicating that it may function as a general 3' end-processing DNA repair enzyme. Recently we have shown a new Tdp1 activity generating DNA strand break with the 3' phosphate termini from the AP site. AP sites are formed spontaneously and are inevitable intermediates during base excision repair of DNA base damages. AP sites are both mutagenic and cytotoxic, and key enzymes for their removal are AP endonucleases. However, AP endonuclease independent repair, initiated by DNA glycosylases performing beta, delta-elimination cleavage of the AP sites, has been described in mammalian cells. Here, we describe another AP endonuclease independent repair pathway for removal of AP sites that is initiated by tyrosyl phosphodiesterase Tdp1. We propose that repair is completed by the action of a polynucleotide kinase, a DNA polymerase and finally a DNA ligase to seal the gap.

Keywords: base excision repair, AP site, tyrosyl-DNA phosphodiesterase 1.