BR Biomedical Reports 2049-9434 2049-9442 D.A. Spandidos 10.3892/br.2014.387 br-03-01-0083 Articles Multidrug resistance gene 1 C1236T polymorphism and susceptibility to leukemia: A meta-analysis MALIMIN 1 LIUHONGCHAO 2 RUANLINHAI 1 YANGXUEWEN 1 YANGHAIPING 1 FENGYANMING 2 Department of Hematology, The First Affiliated Hospital, Luoyang, Henan 471003, P.R. China Medical College, Henan University of Science and Technology, Luoyang, Henan 471003, P.R. China Correspondence to: Dr Hongchao Liu, Medical College, Henan University of Science and Technology, 31 Anhui Road, Jianxi District, Luoyang, Henan 471003, P.R. China E-mail: lhongchao@hotmail.com 012015 14112014 3 1 83 87 02092014 09102014 Copyright © 2015, Spandidos Publications 2015

Several studies have investigated the association between multidrug resistance gene (MDR1) C1236T polymorphism and leukemia risk, however, these published studies have yielded conflicting results. Thus, the present study carried out a meta-analysis to provide a more precise estimate of the effect of this polymorphism on the susceptibility to leukemia. The published case-control studies regarding the association between MDR1 C1236T polymorphism and leukemia risk were included following a computerized search of PubMed, Elsevier, The Cochrane Library, China National Knowledge Infrastructure and Wanfang Database. Either fixed- or random-effects models were applied to calculate the combined odds ratios (ORs) and 95% confidence intervals (CIs) by RevMan 5.2 software. Seven studies, including 846 cases and 1,523 controls, were included in the present meta-analysis. The results indicated that there was no significant association between the MDR1 C1236T polymorphism and leukemia risk in overall comparisons in all four genetic models (CT vs. CC: OR, 1.31, 95% CI, 0.89–1.91, P=0.17; TT vs. CC: OR, 2.16, 95% CI, 0.99–4.70, P=0.05; TT vs. CC+CT: OR, 1.72, 95% CI, 0.91–3.25, P=0.09; and CT+TT vs. CC: OR, 1.57, 95% CI, 0.96–2.56, P=0.07). In the subgroup analysis according to specific ethnicity, age and the type of leukemia, a significant association was found in adult leukemia (CT+TT vs. CC: OR, 2.77, 95% CI, 1.05–7.31, P=0.04) and chronic myeloid leukemia (CT vs. CC: OR, 1.71, 95% CI, 1.05–2.80, P=0.03). No significant publication bias was detected by funnel plot. Therefore, the meta-analysis indicated that the MDR1 C1236T polymorphism may contribute to the susceptibility to adult leukemia and chronic myeloid leukemia. Furthe well-designed studies based on larger sample sizes are required to validate these findings.

 multidrug resistance gene polymorphism leukemia susceptibility meta-analysis Introduction

Leukemia is a malignant clonal disorder of the hematopoietic system with extreme heterogeneity and bad prognosis. In recent years, the incidence of leukemia increased gradually (1), however, the exact biological mechanisms of leukemia have not been fully clarified (2). Epidemiological and case-control studies have found that significant environmental exposures contribute to the pathogenesis of leukemia (3). In addition, previous candidate gene association approaches and genome-wide association studies have identified the presence of inherited genetic susceptibility to this disease (4–7). Therefore, it is generally considered that the causation is multifactorial and that the interaction between exogenous or endogenous exposures and genetic susceptibility plays an important role in the etiology of leukemia (2).

The human multidrug resistance gene (MDR1 or ABCB1) locates at 7q21.1 and its cDNA spans ~4.5 kb. MDR1 encodes a 170-kDa membrane transport protein called P-glycoprotein (P-gp), belonging to the adenosine triphosphate (ATP)-binding cassette (ABC) superfamily of transporters (8). P-gp functions as an ATP-dependent efflux pump and is responsible for the efflux of a variety of lipophilic compounds, including multiple chemotherapeutic agents, naturally occurring xenobiotics, pesticides and cellular metabolites, providing a cellular defense mechanism against potentially harmful compounds (8–10). Recently, ≥50 single-nucleotide polymorphisms (SNPs) have been identified within the MDR1 gene locus (11), of which C1236T (silent), G2677T/A (Ala893Ser/Thr) and C3435T (silent) have been associated with altered P-gp expression and activity (12–14). The alteration of the P-gp transport activity and function due to the different genetic polymorphisms resulted in cumulative cytotoxicity of harmful xenobiotics due to decreased extrusion of these xenobiotics. The SNPs in MDR1 have been associated with the development of various cancers, including acute lymphocytic leukemia and B-cell chronic lymphocytic leukemia (15–17). In addition, the altered activity of P-gp has been shown to affect the absorption and elimination of several drugs, thus, the genetic variants of MDR1 may also be associated with the pharmacokinetics and treatment response to anticancer agents (18–20). Several case-control studies have investigated the association between MDR1 C1236T polymorphism and leukemia risk, however, these studies yielded controversial results. To clarify this, a meta-analysis was performed to provide an improved estimate of the effect of the MDR1 C1236T polymorphism on the susceptibility to leukemia.

 Materials and methods <sec> <title>Publication search

A computer literature search was performed to identify studies regarding the association between the MDR1 C1236T polymorphism and leukemia risk. PubMed, Elsevier, The Cochrane Library, China National Knowledge Infrastructure and Wanfang Database were searched with the following subject terms and keywords: ‘Multidrug resistance gene’ or ‘MDR1’ or ‘ABCB1’, and ‘polymorphism’ or ‘variant’ or ‘mutant’, in combination with ‘leukemia’ or ‘leukaemia’ or ‘leukocythemia’, without any restriction on language. The last search was updated on April 30, 2014. The relevant references cited in the eligible studies were also reviewed to identify additional studies. Two investigators reviewed the retrieved studies independently and any disagreement was resolved by discussion.

 Inclusion criteria

The studies included in the present meta-analysis fulfilled the following criteria: i) Raw materials were published studies; ii) case-control study exploring the association between the MDR1 C1236T polymorphism and leukemia risk; iii) the leukemia group had a pathologically confirmed diagnosis; and iv) genotype frequencies for the case and control groups were available.

 Data extraction

Two investigators extracted the information from each study independently with the standard protocol and disagreements were resolved by discussion. For each eligible study, the following information was collected: First author's name, publication year, country, ethnicity, source of control group, genotyping methods, and genotype distribution of the MDR1 C1236T polymorphism in patients and control populations.

 Statistical analysis

The strength of association between the MDR1 C1236T polymorphism and leukemia risk was assessed by calculating the odds ratios (ORs) with the corresponding 95% confidence intervals (CIs). The significance of the combined OR was determined by the Z-test. The χ2-based Q-test was applied to assess heterogeneity between the included studies. The heterogeneity was considered present when P<0.05, in which case the random-effects model (DerSimonian-Laird) was applied, otherwise, the fixed-effects model (Mantel-Haenszel) was selected to calculate the combined OR. When heterogeneity was observed, subgroup analyses according to specific ethnicity, age and the type of leukemia, were performed to find the source of heterogeneity. Publication bias was evaluated visually through funnel plot and sensitivity analysis was performed by removal of studies not in agreement with the Hardy-Weinberg equilibrium (HWE). HWE in the control group was checked using the χ2 test. All the P-values were two-sided and P<0.05 was considered to indicate a statistically significant difference. The data analyses were performed using the Stata v12.0 software (StataCorp, College Station, TX, USA) and Review Manager v5.2 (The Cochrane Collaboration, Oxford, UK).

 Results <sec> <title>Study characteristics

According to the literature retrieval strategies and inclusion criteria, seven case-control studies regarding the association between the MDR1 C1236T polymorphism and leukemia risk were included in the present meta-analysis, including 846 leukemia cases and 1,523 controls in total (21–27). The countries in which these studies were conducted include China, Singapore, Sweden, Brazil and USA. Among these studies, there were four studies of an Asian population (22,24,25,27), two studies of a Caucasian population (21,23) and one study of a mixed population (26). All the cases had a pathologically confirmed diagnosis of leukemia. The source of controls was based mainly on a healthy population and matched for age and gender. The genotype distribution among the control groups of all the included studies was in agreement with HWE, except one study (27). The detailed characteristics of the included studies are shown in Table I.

 Meta-analysis results

The main results of the meta-analysis and the heterogeneity test are shown in Table II. The results indicated that there was no significant association between the MDR1 C1236T polymorphism and leukemia risk in the overall comparisons in all four genetic models (CT vs. CC: OR, 1.31, 95% CI, 0.89–1.91, P=0.17; TT vs. CC: OR, 2.16, 95% CI, 0.99–4.70, P=0.05; TT vs. CC+CT: OR, 1.72, 95% CI, 0.91–3.25, P=0.09; and CT+TT vs. CC: OR, 1.57, 95% CI, 0.96–2.56, P=0.07) (Table II). Similarly, in the subgroup analysis by specific ethnicity, no significant association was found. In the subgroup analysis according to age and the type of leukemia, however, a significant association was found in adult leukemia (CT+TT vs. CC: OR, 2.77, 95% CI, 1.05–7.31, P=0.04) (Table II and Fig. 1) and chronic myeloid leukemia (CT vs. CC: OR, 1.71, 95% CI, 1.05–2.80, P=0.03) (Table II and Fig. 2).

 Publication bias and sensitivity analysis

Funnel plots were used to assess the publication bias and the results showed that all the points in the funnel plots were symmetrically distributed, which indicated that there was no significant bias. Sensitivity analysis was performed by excluding the study by Zhai et al (27), in which the distribution of genotypes in the control group was not in agreement with HWE. However, no significant changes on the combined ORs were observed (data not shown), suggesting that the results of the present meta-analysis were relatively stable and credible.

 Discussion

Exogenous or endogenous exposures and inherited genetic susceptibility have roles in leukemogenesis. Multiple mutations in the MDR1 gene have been demonstrated in previous studies and ≥50 SNPs have been identified in all 28 exons of the MDR1 gene (11,12,18). The most frequent SNP non-synonymous triallelic G2677T/A in exon 21 results in amino acid substitution from the lipophilic residue Ala to the hydrophilic residue Ser or Thr, and this polymorphism was significantly associated with an increased or decreased plasma concentration of P-gp substrates (28). The C3435T polymorphism is a silent mutation in exon 26 of MDR1 and studies have indicated that this polymorphism was associated with altered MDR1 expression levels and subsequently P-gp activity (12,13,18). The third common polymorphism C1236T of the MDR1 gene is a synonymous variant in exon 12. These three polymorphisms are closely associated with linkage disequilibrium (29–31). Functional polymorphisms in MDR1 may play a role in the elimination of xenobiotics, and the mutation of MDR1 may cause various human malignancies, including leukemia.

Given the potential influence of these SNPs on cancer susceptibility, numerous molecular epidemiological studies were conducted to investigate the association between these SNPs and leukemia risk. However, the results from different studies are inconclusive or controversial, which may be owing to different genetic backgrounds and limitations in individual studies. To clarify this, a meta-analysis was performed from eligible studies focusing on the MDR1 C3435T and G2677T/A polymorphisms and leukemia risk. The results indicated that the MDR1 C3435T polymorphism may modify the susceptibility to leukemia (32). Yan et al (33) found that there was no association between the MDR1 G2677T polymorphism and leukemia risk in the overall populations, but a significant association was found in Asian and African populations, and the G2677T polymorphism may be a protective factor in the susceptibility to myeloid leukemia in these populations.

To synthetically evaluate the effect of the MDR1 C1236T polymorphism on the susceptibility to leukemia, a meta-analysis was performed of seven case-control studies that explored the association between the MDR1 C1236T polymorphism and leukemia risk. In the present meta-analysis, no significant impact was found of the MDR1 C1236T polymorphism on leukemia susceptibility in overall comparisons. A subgroup analysis was also performed by ethnicity, age and the type of leukemia to find the source of heterogeneity. A significant association was found in adult leukemia (CT+TT vs. CC: OR, 2.77, 95% CI, 1.05–7.31, P=0.04) and chronic myeloid leukemia (CT vs. CC: OR, 1.71, 95% CI, 1.05–2.80, P=0.03), indicating that the MDR1 C1236T polymorphism may alter the development of the adult leukemia and chronic myeloid leukemia risk.

Certain limitations of the present meta-analysis should be considered when interpreting the results. Selection bias may exist as the studies without sufficient data were excluded. The analysis largely used unadjusted estimates, as not all the included studies were adjusted by the same potential confounders, such as lifestyles and exposures, which may influence the combined results. Although subgroup analysis was performed by ethnicity, age and the type of leukemia to find the sources of heterogeneity, significant heterogeneity existed in certain subgroups. Owing to the limited original information, potential gene-gene and gene-environment interactions, which have an important impact on cancer risk, were not evaluated in the study.

In conclusion, the present meta-analysis found that the MDR1 C1236T polymorphism may be associated with leukemia susceptibility. Since limited studies were included in the study, larger sample sizes and well-designed case-control studies are required in the future to confirm this association.

 References SiegelRNaishadhamDJemalACancer statistics, 2012CA Cancer J Clin6210292012 InabaHGreavesMMullighanCGAcute lymphoblastic leukaemiaLancet381194319552013 GreavesMInfection, immune responses and the aetiology of childhood leukaemiaNat Rev Cancer61932032006 SherborneALHoskingFJPrasadRBVariation in CDKN2A at 9p21.3 influences childhood acute lymphoblastic leukemia riskNat Genet424924942010 VijayakrishnanJHoulstonRSCandidate gene association studies and risk of childhood acute lymphoblastic leukemia: a systematic review and meta-analysisHaematologica95140514142010 WangYChenJLiJAssociation of three polymorphisms in ARID5B, IKZF1 and CEBPE with the risk of childhood acute lymphoblastic leukemia in a Chinese populationGene5242032072013 SpeedyHEDi BernardoMCSavaGPA genome-wide association study identifies multiple susceptibility loci for chronic lymphocytic leukemiaNat Genet4656602014 SchinkelAHThe physiological function of drug-transporting P-glycoproteinsSemin Cancer Biol81611701997 LinJHYamazakiMRole of P-glycoprotein in pharmacokinetics: clinical implicationsClin Pharmacokinet4259982003 HodgesLMMarkovaSMChinnLWGowJMKroetzDLKleinTEAltmanRBVery important pharmacogene summary: ABCB1 (MDR1, P-glycoprotein)Pharmacogenet Genomics211521612011 BreierABarancíkMSulováZUhríkBP-glycoprotein - implications of metabolism of neoplastic cells and cancer therapyCurr Cancer Drug Targets54574682005 HoffmeyerSBurkOvon RichterOFunctional polymorphisms of the human multidrug-resistance gene: multiple sequence variations and correlation of one allele with P-glycoprotein expression and activity in vivoProc Natl Acad Sci USA97347334782000 KurataYIeiriIKimuraMRole of human MDR1 gene polymorphism in bioavailability and interaction of digoxin, a substrate of P-glycoproteinClin Pharmacol Ther722092192002 LambaJStromSVenkataramananRMDR1 genotype is associated with hepatic cytochrome P450 3A4 basal and induction phenotypeClin Pharmacol Ther793253382006 JamroziakKBalcerczakESmolewskiPMDR1 (ABCB1) gene polymorphism C3435T is associated with P-glycoprotein activity in B-cell chronic lymphocytic leukemiaPharmacol Rep587207282006 HattoriHSuminoeAWadaMRegulatory polymorphisms of multidrug resistance 1 (MDR1) gene are associated with the development of childhood acute lymphoblastic leukemiaLeuk Res31163316402007 RaoDNAnuradhaCVishnupriyaSSailajaKSurekhaDRaghunadharaoDRajappaSAssociation of an MDR1 gene (C3435T) polymorphism with acute leukemia in IndiaAsian Pac J Cancer Prev11106310662010 MarzoliniCPausEBuclinTKimRBPolymorphisms in human MDR1 (P-glycoprotein): recent advances and clinical relevanceClin Pharmacol Ther7513332004 RobertJMorvanVLSmithDPourquierPBonnetJPredicting drug response and toxicity based on gene polymorphismsCrit Rev Oncol Hematol541711962005 Brambila-TapiaAJMDR1 (ABCB1) polymorphisms: functional effects and clinical implicationsRev Invest Clin654454542013 UrayamaKYWienckeJKBufflerPAChokkalingamAPMetayerCWiemelsJLMDR1 gene variants, indoor insecticide exposure, and the risk of childhood acute lymphoblastic leukemiaCancer Epidemiol Biomarkers Prev16117211772007 LiuRRThe role of multidrug resistance gene 1 polymorphism to prognosis of adult acute lymphoblastic leukemia patientsCentral South University51482008(In Chinese) GréenHFalkIJLotfiKAssociation of ABCB1 polymorphisms with survival and in vitro cytotoxicty in de novo acute myeloid leukemia with normal karyotypePharmacogenomics J121111182012 LüHDuZZWangWRelationship between genetic polymorphism of multidrug resistance 1 gene and the risk of childhood acute lymphocytic leukemiaZhonghua Er Ke Za Zhi506926962012(In Chinese) SinghOChanJYLinKHengCCChowbayBSLC22A1-ABCB1 haplotype profiles predict imatinib pharmacokinetics in Asian patients with chronic myeloid leukemiaPLoS One7e517712012 VivonaDBuenoCTLimaLTABCB1 haplotype is associated with major molecular response in chronic myeloid leukemia patients treated with standard-dose of imatinibBlood Cells Mol Dis481321362012 ZhaiXWangHZhuXGene polymorphisms of ABC transporters are associated with clinical outcomes in children with acute lymphoblastic leukemiaArch Med Sci86596712012 ElghannamDMIbrahimLEbrahimMAAzmyEHakemHAssociation of MDR1 gene polymorphism (G2677T) with imatinib response in Egyptian chronic myeloid leukemia patientsHematology191231282014 SaiKKaniwaNItodaMSaitoYHasegawaRKomamuraKHaplotype analysis of ABCB1/MDR1 blocks in a Japanese population reveals genotype-dependent renal clearance of irinotecanPharmacogenetic137417572003 SemseiAFErdélyiDJUngváriIAssociation of some rare haplotypes and genotype combinations in the MDR1 gene with childhood acute lymphoblastic leukaemiaLeuk Res32121412202008 YinJYHuangQYangYZhangJTZhongMZZhouHHLiuZQCharacterization and analyses of multidrug resistance-associated protein 1 (MRP1/ABCC1) polymorphisms in Chinese populationPharmacogenet Genomics192062162009 QianXCaoSYangGDongJJinGShenYHuZVariant genotypes of MDR1 C3435T increase the risk of leukemia: evidence from 10 case-control studiesLeuk Lymphoma53118311872012 YanYLiangHXieLAssociation of MDR1 G2677T polymorphism and leukemia risk: evidence from a meta-analysisTumour Biol35219121972014

Forest plot for the association between multidrug resistance gene C1236T polymorphism and leukemia risk (CT+TT vs. CC). Subgroup analysis was performed by age. CI, confidence interval.

Forest plot for the association between multidrug resistance gene C1236T polymorphism and leukemia risk (CT vs. CC). Subgroup analysis was performed by leukemia subtype. CI, confidence interval; ALL, acute lymphocytic leukemia; ML, myeloid leukemia.

General characteristics of studies included in the meta-analysis.

Sample size Case Control
First author Year Country Ethnicity Subtype Control source Genotyping method Case Control CC CT TT CC CT TT HWE (Refs.)
Gréen et al 2012 Sweden Caucasian Adult AML PB Sequencing 100 400 34 44 22 133 187 80 0.33 (23)
Liu 2008 China Asian Adult ALL HB PCR-RFLP 48 100 2 22 24 44 44 12 0.84 (22)
Lü et al 2012 China Asian Child ALL HB SNaPshot SNP 176 170 23 87 66 20 84 66 0.39 (24)
Singh et al 2012 Singapore Asian Adult CML PB Sequencing 38 275 6 11 21 132 112 31 0.33 (25)
Urayama et al 2007 USA Caucasian Child ALL PB Single base extension 284 365 73 144 67 97 199 69 0.06 (21)
Vivona et al 2012 Brazil Mixed Adult CML PB PCR-RFLP 118 120 39 61 18 51 50 19 0.26 (26)
Zhai et al 2012 China Asian Child ALL PB SNaPshot SNP 82 93 16 37 29 18 32 43 0.01 (27)

HWE, Hardy-Weinberg equilibrium; PB, population-based; AML, acute myeloid leukemia; HB, hospital-based; PCR, polymerase chain reaction; RFLP, restriction fragment length polymorphism; ALL, acute lymphocytic leukemia; SNP, single-nucleotide polymorphism; CML, chronic myeloid leukemia.

Results of meta-analysis for MDR1 C1236T polymorphism and leukemia risk.

CT vs. CC TT vs. CC TT vs. CC+CT CT+TT vs. CC
OR OR OR OR
Variables No. (95% CI) P Pha/I2 (95% CI) P Pha/I2 (95% CI) P Pha/I2 (95% CI) P Pha/I2
Overall 7 1.31(0.89–1.91) 0.17 0.03/57 2.16(0.99–4.70) 0.05 <0.01/87 1.72(0.91–3.25) 0.09 <0.01/89 1.57(0.96–2.56) 0.07 <0.01/77
Ethnicity
  Asian 4 1.91(0.81–4.50) 0.14 0.02/70 4.16(0.71–24.30) 0.11 <0.01/93 2.49(0.69–9.00) 0.16 <0.01/94 2.62(0.79–8.65) 0.11 <0.01/87
  Caucasian 2 0.95(0.70–1.28) 0.72 0.89/0 1.21(0.84–1.74) 0.30 0.64/0 1.26(0.92–1.71) 0.15 0.63/0 1.02(0.77–1.35) 0.91 0.79/0
  Mixed 1 1.60(0.91–2.79) 0.10 1.24(0.57–2.67) 0.58 0.96(0.47–1.93) 0.90 1.50(0.88–2.54) 0.13
Age
  Child 3 0.99(0.73–1.34) 0.94 0.77/0 1.07(0.76–1.50) 0.72 0.43/0 1.03(0.79–1.33) 0.83 0.12/53 1.00(0.75–1.34) 0.98 0.91/0
  Adult 4 1.90(0.90–4.01) 0.09 0.01/73 4.84(1.03–22.79) 0.05 <0.01/92 2.90(0.89–9.43) 0.08 <0.01/91 2.77(1.05–7.31) 0.04 <0.01/86
Subtype
  ALL 4 1.40(0.71–2.75) 0.33 0.02/71 1.92(0.67–5.53) 0.23 <0.01/87 1.46(0.70–3.07) 0.32 <0.01/87 1.52(0.70–3.32) 0.29 <0.01/80
  AML 1 0.92(0.56–1.52) 0.74 1.08(0.59–1.97) 0.81 1.13(0.66–1.92) 0.66 0.97(0.61–1.54) 0.89
  CML 2 1.71(1.05–2.80) 0.03 0.61/0 4.21(0.37–48.19) 0.25 <0.01/93 3.04(0.31–30.01) 0.34 <0.01/95 2.56(0.79–8.26) 0.12 0.02/80

When Ph>0.05, the fixed-effects model was selected to combine the data. Otherwise, the random-effects model was applied. MDR1, multidrug resistance gene; OR, odds ratio; CI, confidence interval; Ph, value used to test the heterogeneity; ALL, acute lymphocytic leukemia; AML, acute myeloid leukemia; CML, chronic myeloid leukemia.