Modulation of human placental P-glycoprotein expression and activity by MDR1 gene polymorphisms

Abstract

The ABC transporter P-glycoprotein is a product of the MDR1 gene and its function in human placenta is to extrude xenobiotics from the tissue thus decreasing fetal exposure. The goal of this investigation was to examine the effect of three polymorphisms in the MDR1 gene on the expression and activity of placental P-gp. In 199 term placentas examined, the C1236T variant was associated with 11% lower P-gp protein expression than wild-type, while the C3435T and G2677T/A variants each were associated with a 16% reduction (p < 0.05). Homozygotes for the C1236T and C3435T variant allele (TT) were associated with 42% and 47% increase in placental P-gp transport activity, respectively (p = 0.04 and p = 0.02) of the prototypic substrate, [³H]-paclitaxel. These findings indicate that the C3435T and G2677T/A SNPs in MDR1 are significantly associated with decreased placental P-gp protein expression, while the C1236T and C3245T homozygous variants are significantly associated with an increase in its efflux activity.

Introduction

The placental ABC transporter P-glycoprotein (P-gp) decreases fetal exposure to exogenous or endogenous compounds present in the maternal circulation, by extruding those that are substrates from the fetal-to-maternal direction. A recent investigation in our laboratory utilized inside–out vesicles from human placental brush border membrane to determine P-gp activity as transport of the P-gp substrate, paclitaxel. In 200 human placentas examined, there was no correlation between P-gp protein expression and transport activity of paclitaxel [1]. Consequently, we hypothesized that genetic variation could contribute to discrepancies between observed protein expression and activity of placental P-gp.

The MDR1 gene encoding P-gp displays considerable genetic variability, with specific single nucleotide polymorphisms (SNPs) occurring at high frequencies in certain populations [2]. More than 50 SNPs in the MDR1 gene have been reported [3], and the most commonly found SNPs are C1236T, C3435T, and G2677T/A. The C1236T, C3435T, and G2677T/A SNPs are found in linkage disequilibrium in up to 49% of Chinese, Malay and Indian populations [4], [5]. Additionally, the three SNPs are in linkage disequilibrium with allele frequency of 45–55% in Whites and 5–10% in African Americans [4], [5].

Consensus on the functional consequences of MDR1 genetic variation, specifically the effect of SNPs on P-gp protein expression and transport activity in the placenta, remains unclear. In Japanese women, the G2677T/A polymorphisms were associated with lower placental P-gp expression [6]. In German mothers of Caucasian ethnicity, significantly lower P-gp expression in placentas carrying the G2677T and C3435T polymorphisms was reported [7]. Homozygous carriers of the C3435T variant allele (TT) have both reduced P-gp expression and efflux activity in human intestine and leukocytes [8], [9]. However, a study by Mölsä et al., demonstrated that the presence of C3435T and G2677T/A polymorphism did not alter the transplacental transfer of the P-gp substrate saquinavir [10]. Moreover, a meta-analysis of studies containing 1036 patients did not demonstrate a correlation between the C3435T SNP and altered pharmacokinetics of the P-gp substrate cyclosporine [11]. Several reports have implicated the C3435T and G2677T/A variant alleles with increased P-gp activity in vivo[12], [13], [14]. Studies on the consequences of the C1236T polymorphism in humans are scarce; however the TT genotype is associated with increased plasma concentrations of the P-gp substrate irinotecan in cancer patients [15]. These inconclusive findings indicate that direct measurements of P-gp transport activity and the correlation between genotype and protein expression need further investigations.

Therefore, the aim of this investigation was to determine the relationship between C1236T, C3435T, and G2677T/A polymorphisms and P-gp protein expression and transport activity. The health implication of this investigation is that P-gp protein expression and polymorphisms in MDR1 could constitute significant contributing factors to P-gp transport activity, consequently affecting placental transfer and fetal exposure to xenobiotics that are P-gp substrates.