Single-nucleotide polymorphisms in a short basic motif in the ABC transporter ABCG2 disable its trafficking out of endoplasmic reticulum and reduce cell resistance to anticancer drugs

Abstract

ATP-binding cassette (ABC) subfamily G member 2 (ABCG2) belongs to the ABC transporter superfamily and has been implicated in multidrug resistance of cancers. Although the structure and function of ABCG2 have been extensively studied, little is known about its biogenesis and the regulation thereof. In this study, using mutagenesis and several biochemical analyses, we show that the positive charges in the vicinity of the RKR motif downstream of the ABC signature drive trafficking of nascent ABCG2 out of the endoplasmic reticulum (ER) onto plasma membranes. Substitutions of and naturally occurring single-nucleotide polymorphisms within these positively charged residues disabled the trafficking of ABCG2 out of the ER. A representative ABCG2 variant in which the RKR motif had been altered underwent increased ER stress-associated degradation. We also found that unlike WT ABCG2, genetic ABCG2 RKR variants have disrupted normal maturation and do not reduce accumulation of the anticancer drug mitoxantrone and no longer confer resistance to the drug. We conclude that the positive charges downstream of the ABC signature motif critically regulate ABCG2 trafficking and maturation. We propose that single-nucleotide polymorphisms of these residues reduce ABCG2 expression via ER stress-associated degradation pathway and may contribute to reduced cancer drug resistance, improving the success of cancer chemotherapy.