XRCC1 participates in DNA single strand break and base excision repair (BER) to preserve genetic stability in mammalian cells. XRCC1 participation in these pathways is mediated by its interactions with several of the acting enzymes. Here, we report that XRCC1 interacts physically and functionally with hOGG1, the human DNA glycosylase that initiates the repair by BER of the mutagenic oxidized base 8-oxoguanine. This interaction leads to a 2-to 3-fold stimulation of the DNA glycosylase activity of hOGG1. XRCC1 stimulates the formation of the hOGG1 Schiff-base DNA intermediate without interfering with the endonuclease activity of APE1, the second enzyme in the pathway. On the contrary, the stimulation in the appearance of the incision product seems to reflect the addition of the effects of XRCC1 on the two first enzymes of the pathway. The data presented support a model by which XRCC1 will pass on the DNA intermediate from hOGG1 to the endonuclease APE1. This results in an acceleration of the overall repair process of oxidized purines to yield an APE1-cleaved abasic site, which can be used as a substrate by DNA polymerase ␤. More importantly, the results unveil a highly coordinated mechanism by which XRCC1, through its multiple protein-protein interactions, extends its orchestrating role from the base excision step to the resealing of the repaired DNA strand.A major threat to genetic stability is the damaging of DNA by either endogenous or exogenous compounds. This is underscored by the cancer-prone phenotype of human cells defective in DNA repair processes. Exposure of the cellular DNA to reactive oxygen species (ROS), 1 generated either by the normal metabolism of the cell or by chemical and physical exogenous agents, is at the origin of lesions that can have genotoxic or mutagenic consequences. To avoid the effects of ROS and, therefore, to maintain the integrity of their genetic information, organisms have evolved multiple DNA repair mechanisms (1). Because of its capacity to pair with an adenine during replication, 7,8-dihydro-8-oxoguanine (8-oxoG), an oxidized derivative of guanine, is arguably the major mutagenic lesion in DNA. Indeed, in Escherichia coli, the inactivation of the genes involved in the repair of this oxidized base leads to one of the strongest spontaneous mutator phenotypes, characterized by the exclusive increase in G to T transversions. Like for other ROS-induced modifications of DNA, 8-oxoG is mainly repaired by the base excision repair (BER) pathway. This pathway is initiated by the recognition and excision of the oxidized guanine by a DNA glycosylase, OGG1 being the major one in yeast and mammalian cells (2). In human cells, the resulting abasic (apurinic/apyrimidinic (AP)) site can be cleaved by a second enzymatic activity of the hOGG1 polypeptide, namely an AP lyase activity. If such a reaction takes place, the nick produced has a 3Ј-open aldehyde residue that is supposed to be removed by the 3Ј-deoxyribose phosphatase activity of APE1, the major AP endonuclease. However, recent data s...

XRCC1 protein is essential for mammalian viability and is required for the efficient repair of single strand breaks (SSBs) and damaged bases in DNA. XRCC1-deficient cells are genetically unstable and sensitive to DNA damaging agents. XRCC1 has no known enzymatic activity and is thought to act as a scaffold protein for both SSB and base excision repair activities. To further define the defects leading to genetic instability in XRCC1-deficient cells, we overexpressed the AP endonuclease APE1, shown previously to interact with and be stimulated by XRCC1. Here, we report that the overexpression of APE1 can compensate for the impaired capability of XRCC1-deficient cells to repair SSBs induced by oxidative DNA damage, both in vivo and in whole-cell extracts. We show that, for this kind of damage, the repair of blocked DNA ends is rate limiting and can be performed by APE1. Conversely, APE1 overproduction resulted in a 3-fold increase in the sensitivity of XRCC1-deficient cells to an alkylating agent, most probably due to the accumulation of SSBs. Finally, the overproduction of APE1 results in increases of 40% in the frequency of micronuclei and 33% in sister chromatid exchanges of XRCC1− cells. These data suggest that the spontaneous generation of AP sites could be at the origin of the SSBs responsible for the spontaneous genetic instability characteristic of XRCC1-deficient cells.