HIGH EXPRESSION OF THE MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN (MRP) IN CHRONIC AND PROLYMPHOCYTIC LEUKEMIA

The expression of the multidrug resistance-associated protein (MRP), a new glycoprotein involved in drug resistance, was investigated in tum our samples from 80 patients with chronic B-cell malignancies by a qua ntitative RNase protection assay. In B-cell chronic lymphocytic leukae mia (B-CLL) (n = 32), either treated (n = 18) or untreated (n = 14), a high percentage of patients (20/32: 63%) had relatively high expressi on levels of the MRP gene (25 U or more). In addition, hyperexpression of the MRP gene was demonstrated in 4/10 (40%) untreated patients wit h B-cell prolymphocytic leukaemia (B-PLL). In contrast, low MRP mRNA e xpression levels were detected in hairy cell leukaemia (n = 7), non-Ho dgkin's lymphoma (n = 13) and multiple myeloma (n = 18). Statistical a nalysis of MRP expression in untreated CLL (mean +/-SD 29.2 +/- 18.5 U ) versus treated CLL (mean +/-SD 26.7 +/- 13.7 U) did not show signifi cant differences in MRP expression between the two groups. Southern bl ot analysis did not reveal amplification of the MRP gene in the leukae mia samples with elevated MRP mRNA levels. We conclude that B-PLL and B-CLL frequently display high MRP expression and that this hyperexpres sion is probably due to transcriptional activation and/or increased mR NA stability.