GENETIC-POLYMORPHISM IN MDR-1 - A TOOL FOR EXAMINING ALLELIC EXPRESSION IN NORMAL-CELLS, UNSELECTED AND DRUG-SELECTED CELL-LINES, AND HUMANTUMORS

By using RNase protection analysis, residues 2677 and 2995 of MDR-1 we re identified as sites of genetic polymorphism. Through use of oligonu cleotide hybridization, the genomic content and expression of individu al MDR-1 alleles were examined in normal tissues, unselected and drug selected cell lines, and malignant lymphomas, In normal tissues, unsel ected cell lines, and untreated malignant lymphoma samples, expression of MDR-1 from both alleles was similar. In contrast, in drug selected cell lines, and in relapsed malignant lymphoma samples, expression of one allele was found in a large percentage of samples. To understand how expression of one allele occurs, two multidrug resistant sublines were isolated by exposing a Burkitt lymphoma cell line to increasing c oncentrations of vincristine. The resistant sublines expressed only on e allele and had a hybrid MDR-1 gene composed of non-MDR-1 sequences p roximal to MDR-1. Previous studies showing hybrid MDR-1 genes after re arrangements provided a potential explanation for activation and expre ssion of one MDR-1 allele. We conclude that oligonucleotide hybridizat ion can be used as a sensitive tool to examine relative allelic expres sion of MDR-1, and can identify abnormal expression from a single alle le. Acquired drug resistance in vitro and in patients is often associa ted with expression of a single MDR-1 allele, and this can be a marker of a hybrid MDR-1 gene. (C) 1998 by The American Society of Hematolog y.