A Polymorphism In the 3'UTR Region of ABCB1 Is Associated with Increased Allele Activity with Corresponding Increases In P-Glycoprotein Expression and Function

Abstract 3990

Multidrug resistance is one of the major prognostic factors in acute myeloid leukemia (AML). Expression of p-glycoprotein (p-gp), a member of the ABC transporter family and encoded by the ABCB1 gene, is associated with failure to respond to chemotherapy. Control of MDR1 expression is multifactorial and to-date emphasis has been on the 5'-promoter region of the gene. Evidence is now accumulating regarding a role for the 3'-untranslated region (3' UTR) of genes in controlling expression. We therefore studied a single nucleotide polymorphism (SNP) in the 3'UTR of ABCB1 (rs3842) and determined its effect on p-gp expression.

Samples were available on 450 AML patients who had been entered into the UK LRF AML14 and NCRN AML15 trials. The distribution of the rs3482 A/G SNP was determined using the SNaPshot method; 10% of samples had their genotype confirmed by sequencing. P-gp protein expression and function were analysed by flow cytometry of CD45-gated cells; expression using MRK-16 and function by assessing the modulation of R123 accumulation by PSC833. To determine the activity of the two polymorphic alleles the 3'UTR of ABCB1 was cloned into a modified pGL3 luciferase vector and the normalised luciferase activity was assessed for both the A and G allele.

The distribution of the rs3482 SNP in our AML patient cohort was 71% AA, 27% AG and 1.5% GG. These results are comparable with the published prevalence of the SNP in European populations. Approximately 60% of AML patients do not express p-gp so we therefore focused upon the 40% of patients who expressed p-gp. We found that in the p-gp-expressing patients a higher p-gp protein and function was associated with patients with the wild–type homozygous AA genotype when compared to the GA+GG genotypes. Furthermore when the ABCB1 3'UTR was cloned downstream of the firefly luciferase gene and transfected into HL-60 cells the ‘A’ construct demonstrated significantly increased activity when compared to the G construct (normalized ratio (firefly luciferase/renilla luciferase): A 7.7, G 1.9; n=4, p<0.05).

Outcome data was available on 414 of the intensively treated trial patients. Whilst the ABCB1 3'UTR polymorphism had no impact on overall survival or relapse risk there was a trend towards an increased relapse free survival in patients with the wildtype AA genotype (RFS AA 37%; AG+GG 28%, p=0.08). This may correspond to higher p-gp expression in immunocompetent cells post-remission.

In conclusion this work demonstrates that the ABCB1 rs3482 3'UTR SNP is functionally significant with the A allele associated with higher activity and a concomitant increase in p-gp expression and function in p-gp expressing patient samples.