The involvement of a LINE-1 element in a DNA rearrangement upstream of the mdr1a gene in a taxol multidrug-resistant murine cell line

Two closely related but functionally distinct P-glycoprotein isoforms are encoded by the murine multidrug-resistance genes mdr1a and mdr1b. In a series of independently selected multidrug-resistant (MDR) J774.2 cell lines, mdr gene amplification and/or overexpression and overproduction of either the mdr1a or mdr1b products, or both gene products, correlates with the MDR phenotype. To investigate the possibility that mutations in the promoter regions of the mdr1a or mdr1b genes could influence their differential expression, mdr promoter-specific probes were used to detect and map potential structural alterations. An unusual structural rearrangement was found in the 5′-region of the amplified mdr1a allele in J7.T1, a cell line selected with taxol. To characterize this rearrangement, the regulatory regions of the mdr1a and mdr1b genes were analyzed. Whereas no gross structural alterations were detected by Southern blot hybridization using the mdr1b promoter probe, a novel amplified EcoRI fragment was detected by the mdr1a promoter probe. To determine the precise nature of this mutation, an mdr1a 5′-genomic clone was isolated from J7.T1 cells. Sequence analysis revealed an unusual DNA rearrangement consisting of the mdr1b gene, from its fourth intron toward its 3′-end, upstream of an intact mdr1a promoter on the amplified allele. We propose that this event occurred by an unequal sister chromatid exchange that was mediated by LINE-1 repetitive elements.