The MDR1 C3435T polymorphism: Effects on P-glycoprotein expression/function and clinical significance

C3435T, have been investigated. 1-3 Originally identified in a German Caucasian population by Hoffmeyer et al., C3435T was found to correlate with P-gp expression in the duodenum as determined by Western blots and quantitative immunohistology (P=0.056). 1 Individuals with the CC genotype (n=6) had higher levels of P-gp expression, approximately 2-fold, compared with individuals with the TT genotype (n=5); heterozygotes had intermediate expression levels (n=10). The mechanism by which the T allele results in lower duodenal P-gp expression is unknown, because C3435T is a silent mutation and does not result in changes in the P-gp sequence. However, Hoffmeyer et al. hypothesize that C3435T may be linked to other variants in the MDR1 gene. 1 The MDR1 gene product P-glycoprotein (P-gp) is a member of the ATP-binding cassette transporter family. P-gp utilizes the energy derived from ATP hydrolysis to pump a wide range of compounds, including numerous clinically used drugs, out of cells; this activity has important pharmacokinetic and pharmacodynamic consequences. For example, P-gp is expressed within the apical membranes of intestinal, renal, and hepatic epithelial cells, where it affects the absorption and elimination of its substrates. P-gp is also located within the apical membranes of capillary endothelial cells of the brain, where it can limit the penetration of drugs to the CNS. In addition to the roles of P-gp in absorption, distribution, and elimination, the overexpression of P-gp is implicated in the development of the multi-drug resistance (MDR) phenotype of some tumor cells. Consequently, P-gp inhibitors are now being developed as MDR reversal agents. Hoffmeyer et al. only examined the effects of the MDR1 C3435T polymorphism on P-gp expression in the duodenum. However, because the MDR1 gene is expressed in many normal tissues and cell types, it is important to establish whether the mutation alters P-gp expression exclusively in the duodenum, thereby affecting only drug absorption, or whether expression is altered in other tissues as well, leading to changes in distribution, elimination, or both of these processes. Using a rhodamine efflux assay as a measure of P-gp activity, Hitzl et al. examined P-gp activity in CD56+ natural killer cells from healthy subjects with the different genotypes at the 3435 locus. 2 Rhodamine is a P-gp substrate, thus CD56+ cells with higher P-gp activity would be predicted to have lower intracellular rhodamine fluorescence. Hitzl et al. found that CD56+ cells from individuals with the CC genotype (n=10) had lower rhodamine fluorescence (51.1 ± 11.4%) compared with CD56+ cells from individuals with the TT genotype (n=11) (67.5 ± 9.5%), indicating that cells from CC carriers have higher P-gp activity compared with cells isolated from TT carriers. 2 Although this difference was statistically significant, the consequences of a functional difference of this magnitude are debatable. In addition to these functional studies, Hitzl et al. quantified MDR1 RNA transcript levels in leukocytes. 2 They did not find a correlation between RNA levels and genotype at position 3435. Hitzl et al. hypothesize that the lack of a correlation was due to their use of leukocytes as the RNA source; leukocytes are a heterogeneous pool of cells that include CD56+ cells, but also other cell types. Although the results of the RNA expression experiments of Hitzl et al. do not necessarily invalidate the results of their functional studies, further experiments examining P-gp transcript levels, and ideally P-gp protein levels, in CD56+ cells are needed to resolve this issue. Interestingly, a number of P-gp substrates, including digoxin and cyclosporin A, exhibit substantial interindividual variability in their pharmacokinetics. Some of this variability could be attributed to environmental factors, but it is also reasonable to predict that some of this variability arises due to genetic factors, including mutations in genes involved in drug metabolism and transport, such as MDR1. Understanding the functional and clinical consequences of MDR1 variants is important—if this variability could be assigned to a mutation in the MDR1 gene, patients could be screened and appropriate dose adjustments could be made on the basis of their MDR1 genotype. Furthermore, MDR1 variants could have important pharmacodynamic consequences: patients carrying null MDR1 alleles, if such alleles exist, might not respond to P-gp inhibitors used as MDR reversal agents in cancer treatment.