Characterization of gene rearrangements leading to activation of MDR-1

Expression of the MDR-1/P-glycoprotein gene confers drug resistance both in vitro and in vivo. We previously reported that gene rearrangements resulting in a hybrid MDR-1 transcript represent a common mechanism for acquired activation of MDR-1/P-glycoprotein. We have identified hybrid MDR-1 transcripts in nine MDR-1-overexpressing cell lines and two patients with relapsed ALL. We characterize these rearrangements as follows. 1) Non-MDR-1 sequences in the hybrid MDR-1 transcripts are expressed in unselected cell lines, showing that these sequences are constitutively expressed. 2) The rearrangements occur randomly and involve partner genes (sequences) on chromosome 7 and on chromosomes other than 7. Breakpoints have been characterized in six cell lines. In one, the rearrangement occurred within intron 2 of MDR-1; in the other five, the rearrangement occurred 24 to >96 kb 5' of the normal start of transcription of MDR-1. In one cell line, homologous recombination involving an Alu repeat was observed. However, in the remaining five cell lines, nonhomologous recombination was observed. 3) The rearrangements arise during drug selection. The acquired rearrangements are not detected in parental cells. 4) Five of the six active promoters that captured MDR-1 controlled MDR-1 from a distance of 29 to more than 110 kb 5' to MDR-1. Transcription was initiated in an antegrade or retrograde direction. We conclude that drug selection with natural products targeting DNA or microtubules leads to DNA damage, nonhomologous recombination, and acquired drug resistance, wherein MDR-1 expression is driven by a random but constitutively active promoter.