Background: Upregulation of multidrug resistance-associated protein (MRP1) gene has been detected in many in vitro systems and could be the basis of the drug resistance phenotype in vivo. Increase in gene dosage and overexpression are two major mechanisms for increasing MRP1 expression level. In many drug resistant cell lines, MRP1 gene amplification has been detected. However, it is not yet known whether gene amplification plays a role in inducing the multidrug resistance phenotype clinically.
Methods: To establish whether MRP1 gene copy number is a common feature of the upregulation of MRP1 expression in cancer patients, we studied the MRP1 gene copy number in leukemia patients by fluorescent in situ hybridization (FISH) and real-time PCR. This involved determination of the MRP1 gene copy number and mRNA level in the peripheral blood of 52 adult leukemic patients and ten healthy volunteers. The leukemic CCRF-CEM cell line (drug sensitive) and its drug-resistant subline CCRF-E1000, which has MRP1 overexpression, were used as controls.
Results: The MRP1 gene copy number in CCRF-CEM was normal but increased significantly in CCRF-E1000 cell line. However, in the presence or absence of MRP1 overexpression, increase in gene dosage was not detected in patients.
Conclusions: Our data suggest that the increase in MRP1 gene dosage observed in resistant cell lines is not responsible for the upregulation of MRP1 expression in leukemic patients.