The role of adenomatous polyposis coli (APC) has been implicated in various cellular functions including cell migration, cell-cell adhesion, cell cycle control, chromosomal segregation and apoptosis. Mutations in APC gene impair these functions and render cells susceptible to tumorigenesis. Recently, we discovered a novel role of APC in DNA base excision repair (BER) and showed that APC interacts with DNA polymerase β (pol-β) and flap endonuclease 1 (Fen-1) and interferes long-patch (LP)-BER by blocking strand-displacement synthesis. Many times, the chemotherapeutic drugs induce DNA-alkylation damage, which is primarily repaired by the BER pathway. Thus, the efficacy of such drugs can be increased by APC resulting in the blockage of LP-BER. In the present study, we tested this hypothesis by using isogenic wild-type and pol-β-knockout mouse embryonic fibroblast cell lines in which the APC gene was knocked down by the SiRNA technique and treated with methylmethane sulfonate (MMS). The MEF-APC+/+/polβ-/- cells were hypersensitive to MMS treatment compared to the MEF-APC+/+/polβ+/+ cells. However, once the APC gene was knocked down, these cells became more resistant to MMS treatment, suggesting that the MMS-induced hypersensitivity was associated with APC. We then determined whether the hypersensitivity of MEF-APC+/+/polβ-/- and MEF-APC+/+/polβ+/+ cell lines were due to decreased pol-β-independent and pol-β-dependent LP-BER pathways, respectively. The results of in vivo and in vitro LP-BER assays supported our findings. Further, APC-mediated hypersensitivity to MMS treatment was correlated with increased apoptosis of APC+/+/polβ-/- and MEF-APC+/+/polβ+/+ as compared to APC-/-/polβ-/- and MEF-APC-/-/polβ+/+ cells. These results suggest that an increased level of APC can increase the efficacy of DNA-alkylating drugs used as a curative therapy. Supported by NCI-NIH (CA-097031 and CA-100247) to SN.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA