Functional characterization of coding polymorphisms in the human MDR1 gene using a vaccinia virus expression system

The human MDR1-encoded transporter is a 170-kDa plasma membrane glycoprotein [P-glycoprotein (P-gp)] capable of binding and energy-dependent extrusion of structurally diverse organic compounds and drugs. P-gp seems to play a significant role in uptake, distribution, and excretion of many different drugs. To determine whether common polymorphic forms of P-gp are likely to alter function of P-gp, we characterized five known MDR1 coding polymorphisms (N21D, F103L, S400N, A893S, and A998T) using a vaccinia virus-based transient expression system. Cell surface expression of wild-type P-gp was time-dependent over a time course of 5.5 to 34.5 h; highest expression was obtained by 22 to 26.5 h after infection/transfection, indicating that a semiquantitative assay for P-gp expression levels was possible. HeLa cells stained with the P-gp specific monoclonal antibodies MRK-16 and Western blots probed with C219 revealed similar cell surface expression for the polymorphisms and for wild-type protein. Time-dependent P-gp pump function maximal at 22 h after infection/transfection was demonstrated for the following MDR1 fluorescence substrates: 4,4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacene-3-pentanoic acid, succinimidyl ester (bodipy-FL)-verapamil, bodipy-FL-vinblastine, calcein-AM, bodipy-FL-prazosin, bisantrene, and bodipy-FL-forskolin, but not for daunorubicin. Transport studies of all tested substrates indicated that the substrate specificity of the pump was not substantially affected by any of the tested polymorphisms. Cell surface expression and function of double mutants including the more common polymorphisms (N21D-S400N, N21D-A893S, and S400N-A893S) showed no differences from wild-type. These results demonstrate that the common MDR1 coding polymorphisms result in P-gps with a cell surface distribution and function similar to wild-type P-gp.