Abstract 4696: Expression of the DNA repair protein UNG predicts lung cancer sensitivity to pemetrexed

Abstract

Abstract Uracil misincorporation is a direct consequence of TS inhibition by the multi-target antifolate, pemetrexed. The DNA base excision repair (BER) enzyme Uracil DNA glycosylase (UNG) is the major glycosylase responsible for the removal of misincorporated genomic uracil. We have previously illustrated that human colon cancer cells (DLD1) with engineered stable somatic knockout of UNG expression are hypersensitive to pemetrexed owing to the cytotoxicity of genomic uracil accumulation. Here, we examined the relationship between the expression of UNG and the pemetrexed sensitivity in a small panel of lung cancer cell lines (n∼10). The levels of UNG were measured by western blot (for protein) and RT-PCR (for mRNA). Results revealed that UNG mRNA expression is well correlated with protein expression in all cell lines. Moreover, there was a wide range of distribution of UNG levels in the lung cancer cell lines evaluated. UNG cutting assay and AP site measurements confirm that these differences in UNG expression result in varied potential for BER activation and subsequent AP site formation. To determine the role of this variation in UNG expression in pemetrexed cytotoxicity, colony survival studies were performed. In general, cell lines with higher UNG expression were more resistant to pemetrexed. H460 cells having 7-fold lower expression of UNG mRNA than H1975 cells are >5 fold more sensitive to pemetrexed than H1975 cells (H460 IC50 = 200nM; H1975 IC50 = >1000nM). Analysis of tissue microarray data in Oncomine confirms variations in UNG expression in primary human lung cancer samples. Moreover, small cell lung cancer and squamous cell lung cancer, both notoriously resistant to pemetrexed, have significantly increased median UNG expression compared to all other lung cancer subtypes (1.5-fold greater UNG expression p<0.005). These data motivate future evaluation of UNG as a clinical predictor of pemetrexed response using lung cancer biopsy specimens. Together, our results clearly indicate that the BER protein, UNG is differentially expressed in lung cancer cells. The inherent variation in UNG expression may have particular impact on our ability to predict the therapeutic efficacy of pemetrexed in lung cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4696. doi:1538-7445.AM2012-4696