The human multidrug resistance (mdr1) gene. cDNA cloning and transcription initiation.

Multidrug resistance in human KB carcinoma cells selected for resistance to colchicine, vinblastine, or adriamycin results from overexpression, and frequently amplification, of a specific gene (mdr1). Overlapping cDNA clones representing a complete 4.7-kilobase mdr1 transcript have been obtained from multidrug-resistant KB cells. Primer extension and S1 nuclease protection experiments show that two transcripts initiate 136 and 140 bases upstream from the first ATG codon in all human multidrug-resistant cell lines. The mdr1 gene is expressed in human normal kidney cells and HepG2 liver cells as a poly(A)+ RNA which starts from the same sites. Less prominent transcripts were found to initiate 155-180 bases upstream from the first ATG codon in vinblastine- or adriamycin-selected cell lines and 480-630 bases upstream in colchicine-selected cell lines. Southern hybridization analyses with different portions of a full-length cDNA indicate that the human mdr1 gene encompasses at least 70 kilobases of DNA amplified in all highly multidrug-resistant cell lines.