A Human ABCG2 SNP Variant, P269S (rs34678167, c.805C>T), Increases Cellular Resistance to Mitoxantrone and SN-38 Without Detectable Differences in ABCG2 Abundance in Total and Biotinylated Protein Fractions

ABCG2 (BCRP/MXR) is an ATP-binding cassette transporter that plays a role in cellular resistance to various anticancer agents. While several non-synonymous single-nucleotide polymorphisms (SNPs) in the ABCG2 gene have been functionally characterized, the effects of many variants remain uncertain. This study examined the human ABCG2 SNP variant P269S (rs34678167, c.805C>T) in cellular resistance to mitoxantrone and SN-38 using a Flp-In-293 controlled expression system. Quantitative real-time PCR, whole-cell Western blotting, and cell-surface biotinylation analysis showed comparable ABCG2 mRNA and total protein levels and ABCG2 abundance in the biotinylated protein fraction between ABCG2(P269S)- and ABCG2(WT)-expressing cells. In MTT assays, ABCG2(P269S)-expressing cells exhibited significantly higher EC50 values than ABCG2(WT)-expressing cells, with mean values approximately 3.0- and 3.5-fold higher for mitoxantrone and SN-38, respectively. Consequently, P269S is associated with increased cellular resistance to mitoxantrone and SN-38 despite no detectable differences in total ABCG2 protein abundance or abundance in the biotinylated protein fraction. Collectively, these findings underscore the utility of the Flp-In-293 system for assessing variant-specific effects of ABCG2 on cellular drug responses within a controlled background. These results support future human pharmacogenomic research on rs34678167 to determine whether this variant contributes to inter-individual differences in responses to ABCG2 substrate drugs.