One of the largest problems in the treatment of breast cancer is the development of multidrug resistance (MDR), where tumor cells possess or acquire the ability to circumvent the killing action of a variety of structurally unrelated chemotherapy drugs.  The MDR mechanisms involve increased drug efflux from tumor cells due to induction of ATP-binding cassette (ABC) drug transporters. Higher expression of the ABCB1 drug transporter is often observed in drug-resistant tumor cells, although the precise mechanism remains unclear.

Recently epigenetic alterations including changes in methylation of CpG islands within gene promoters have emerged as a prominent mechanism for regulation of gene expression. CpG island methylation may repress transcription via the downstream promoter, while histone acetylation may play an important role in upstream promoter activation.

Through bisulfite sequencing experiments we found that the ABCB1 downstream promoter became increasingly methylated following the acquisition of drug resistance. This hypermethylation in turn correlated with increased ABCB1 gene amplification, a switch from usage of the ABCB1 downstream promoter to the upstream promoter, increased ABCB1 expression, and increased drug resistance. The cytotoxic activity of docetaxel is exerted by promoting and stabilising microtubule assembly, while preventing physiological microtubule disassembly in the absence of GTP.

This study is the first to examine broad-scale changes in ABCB1 gene methylation associated with the acquisition of docetaxel resistance and the first report of ABCB1 transcription exclusively via the upstream promoter.