Genetic polymorphisms of the drug transporter ABCB5 associated with advanced liver cancer

Abstract

Introduction: ATP-Dependent Binding Cassette B5 (ABCB5) is a transmembrane protein which acts as energy-dependent drug efflux transporter to mediate multidrug resistance of human cancer. Genetic polymorphisms in different gene regions of ABC transporters had been reported to associate with different clinical significances. Our lab has reported that ABCB5 mRNA expression levels were associated with recurrence-free survival of hepatocellular carcinoma patients (Cheung et al Gastroenterology 2011). Aims: To examine the gene sequence of ABCB5 in human liver cancer and investigate the correlation of genetic polymorphisms with expressions and clinico-pathological features. Methods: Polymerase chain reaction (PCR) amplification and Sanger sequencing approach was used. PCR was performed for a total of 28 exons and exon-intron boundaries of ABCB5 gene using genomic DNA from liver cancer tissues. Genomic blood DNA from the same patient cohort was used to delineate the genetic polymorphism in the tumor as mutation or germline sequence variation. Results: Twenty liver cancer tissues and blood DNA samples were examined. A total of 40 genetic polymorphisms were observed in human ABCB5 gene, where 4 were novel genetic polymorphisms (G IVS6+17 T in intron 6; T IVS14+50 C in intron 14; T2166 G in exon 18; A4609G in 3’untranslated region of Exon28) and 36 had been reported in NCBI dbSNP database and SNP consortium. The genetic polymorphisms observed in the tumors were identical to their paralleled blood specimens which suggested that ABCB5 polymorphisms were germline sequence variations. An association of the genetic polymorphism (C784A) in Exon8 of ABCB5 gene with expression level of ABCB5 was observed (P=0.025). Variant genotypes (heterozygous C/A and homozygous A/A) were associated with significantly lower mRNA level of ABCB5 compared to wild type genotype (homozygous C/C). Importantly, polymorphism C784A was associated with tumor stage (P=0.002). Variant genotypes (C/A and A/A) were associated with early stage whereas wild type genotype (C/C) was associated with late stage liver cancer. Summary: ABCB5 polymorphisms were germline sequence variations in liver cancer patients, as identical genetic sequences were demonstrated in the tumors and their paralleled blood samples. Genetic polymorphism C784A was significantly associated with the transcript levels and tumor stage. Further investigation would increase HCC sample size, inclusion of healthy population, and elucidate the functional implication of the polymorphisms.