Predictive value of two polymorphisms in ABCB1, rs1045642 and rs1128503, in prognosis following taxane-containing chemotherapy: A meta-analysis


 Background: Although taxane-containing chemotherapy is widely used to treat solid tumors, genetic polymorphisms can influence the chemotherapeutic response. This meta-analysis was conducted to determine the correlation between two polymorphisms in ABCB1 , rs1045642 and rs1128503, and survival of patients administered taxane-containing chemotherapy. Methods: PubMed, Web of Science, Embase, Wanfang database, VIP database, and China National Knowledge Infrastructure database were used to obtain articles published up to August 2019 describing the association between the ABCB1 rs1045642 and rs1128503 polymorphisms and survival. A meta-analysis was conducted using R 3.6.1 software to determine the pooled hazard ratio (HR) and 95% confidence intervals (95% CI). Furthermore, publication bias was assessed, and sensitivity analysis was performed to validate the analysis. Results: Fifteen studies involving 3320 patients were included in the meta-analysis. The summary results showed that the effect of the C1236T polymorphism on progression-free survival remained significant in the heterozygote model (HR 0.81; 95% CI: 0.67–0.98) and homozygote model (HR 0.71; 95% CI: 0.58–0.88). Compared to the C1236 TT phenotype, the CC genotype was associated with a poor overall survival (HR 0.72; 95% CI: 0.53–0.97). Finally, subgroup analysis suggested that different areas, tumor types, and treatment regimens influence patient survival. Conclusions: Patients who are ABCB1 rs1045642 and rs1128503 T gene carriers show a survival benefit with taxane-containing chemotherapy.


Abstract
Background: Although taxane-containing chemotherapy is widely used to treat solid tumors, genetic polymorphisms can influence the chemotherapeutic response. This metaanalysis was conducted to determine the correlation between two polymorphisms in ABCB1 , rs1045642 and rs1128503, and survival of patients administered taxanecontaining chemotherapy. Methods: PubMed, Web of Science, Embase, Wanfang database, VIP database, and China National Knowledge Infrastructure database were used to obtain articles published up to August 2019 describing the association between the ABCB1 rs1045642 and rs1128503 polymorphisms and survival. A meta-analysis was conducted using R 3.6.1 software to determine the pooled hazard ratio (HR) and 95% confidence intervals (95% CI). Furthermore, publication bias was assessed, and sensitivity analysis was performed to validate the analysis. Results: Fifteen studies involving 3320 patients were included in the meta-analysis. The summary results showed that the effect of the C1236T polymorphism on progression-free survival remained significant in the heterozygote model (HR 0.81; 95% CI: 0.67-0.98) and homozygote model (HR 0.71; 95% CI: 0.58-0.88). Compared to the C1236 TT phenotype, the CC genotype was associated with a poor overall survival (HR 0.72; 95% CI: 0.53-0.97). Finally, subgroup analysis suggested that different areas, tumor types, and treatment regimens influence patient survival. Conclusions: Patients who are ABCB1 rs1045642 and rs1128503 T gene carriers show a survival benefit with taxane-containing chemotherapy.

Background
Taxane is widely used to treat different types of solid tumors, such as breast cancer, ovarian cancer, some head and neck cancers, and lung cancer. Taxane can disrupt the dynamic balance between microtubules and tubulin dimers to promote tumor cell apoptosis and has anti-tumor effects. Clinical studies have shown that the toxicity and efficacy of taxane vary widely and exhibit inter-individual differences, leading to large variations in individual prognosis. An increasing number of studies have indicated that genetic factors such as single-nucleotide polymorphisms (SNPs) play an important role in individual differences in the prognosis of patients administered taxane-containing chemotherapy [1][2][3][4].
Several gene polymorphisms have been shown to affect the activity of taxane. The most common is the adenosine triphosphate-binding cassette transporter B subfamily member 1 (ABCB1) gene, also known as multidrug resistance gene 1, which is located in humans on the long arm of chromosome 7. This gene encodes P-glycoprotein (P-gp), which can transport a variety of chemical drugs and function as a transmembrane osmotic pump [5].
When P-gp activity increases, antitumor drugs that enter the cell can be actively pumped out of the cell [6][7][8]. Because of this increased outflow and/or decreased inflow, antitumor drugs in tumor cells show insufficient accumulation, reducing their efficacy, which is the key mechanism of chemotherapy resistance [9,10]. P-gp is involved in the transport of multiple antitumor drugs including paclitaxel and docetaxel [11]. SNPs may affect drug sensitivity and drug resistance through various mechanisms [12,13], such as altering the expression levels, stability, degradation, substrate specificity, activity, and/or role of transporters [14][15][16][17], which can affect prognosis [18].
The association between ABCB1 polymorphisms and the prognosis of patients following taxane-containing chemotherapy has been explored in several studies. However, the results are inconclusive and studies showed contrasting results and weak data [19][20][21][22][23][24].
Given the importance of ABCB1 polymorphisms, information obtained by doctors, pharmacists, and statisticians was used to perform this meta-analysis, systematically integrating the current pharmacogenetic literature to obtain more credible evidence. This study was conducted to assess the association between rs1045642 (C3435T) and rs1128503 (C1236T) and the prognosis of patients administered taxane-containing chemotherapy as well as to provide a basis for clinical personalized medicine. outcomes: all studies reported the OS and PFS of patients with different genotypes, and studies with enough genotype data to estimate the hazard ratio (HR) and 95% confidence interval (CI) in at least one genetic comparison model. The study selection process was performed by 2 authors independently; a third author determined the final criteria for any inconsistencies.

Data collection and quality assessment
Two authors were responsible for extracting the data from eligible studies using a standardized data extraction table. Disagreements were resolved by group discussion or a third author if a consensus could not be reached. Information on the first author's name, publication year, country, patient mean age, sample size, tumor type, regimen, detection, Hardy-Weinberg equilibrium, PFS, and OS for each category of genotypes was collected.
Bias caused by individual studies was examined by 2 authors independently according to the Newcastle-Ottawa Scale score, which is useful for comprehensively evaluating the quality of observational studies in meta-analysis. The bias of selection (4 items), comparability (1 item), and outcome (3 items) were assessed during this process.

Data synthesis and analysis
Three genetic models were analyzed for each gene site in this meta-analysis: homozygote model (TT vs. CC), heterozygote model (CT vs. CC), and dominant model (CT + TT vs. CC).
For time to event survival analysis, we assessed the effect of ABCB1 status on patient prognosis by calculating the hazard ratio (HR). For each study, the HR and its 95% confidence interval (CI) were retrieved. If these parameters were not available in the studies, we used the software Engauge Digitizer 4.1 to extract specific survival rates according to Kaplan-Meier curves to calculate the HR as described by Tierney et al. [26].
Heterogeneity in the pooled analyses was determined by statistical analyses using the Q statistic for homogeneity and the I 2 statistic. A P value of < 0.10 or I 2 > 50% was considered to indicate significant heterogeneity [27]. When statistical heterogeneity existed, the random-effects model was used; otherwise the fixed effects model was applied. Sensitivity analyses were performed to evaluate the influence of single studies on overall analysis. Subgroup analyses, including area (Asia, Europe, and others), tumor types (ovary, breast, lung, and others), and regimen (paclitaxel plus cisplatin (TP) and others) were conducted. Publication biases were estimated by using Egger tests [28]. A two-sided P value of less than 0.05 was regarded as statistically significant. All statistical analyses were performed using R version 3.6.1 software.

Study characteristics
The characteristics of all included studies are listed in Table 1. Fifteen studies reporting data from 3320 patients who were administered taxane-containing chemotherapy were included in this study. These articles were all published in English, and the years of publication ranged from 2008 to 2018. The sample size ranged from 43 to 511 patients and their mean age ranged from 46 to 65 years. Further, Table 1 shows information on the country, sample sizes, tumor types, regimens, detection, and Newcastle-Ottawa Scale score. Study quality was assessed using the Newcastle-Ottawa Scale score: 1 study had a score 6, 11 studies had a score of 8, and the remaining 5 studies had a score of 9.

PFS
Eleven studies involving 2670 patients were included in the meta-analysis of PFS (Fig. 2)

OS
Nine studies involving 2255 patients were included in the meta-analysis of OS (Fig. 3). The

PFS
Seven studies involving 1943 patients were included in the meta-analysis of PFS (Fig. 4).
The summary results showed that the effect of the C1236T polymorphism on PFS remained

OS
Six studies with 1217 patients were included in the meta-analysis of OS (Fig. 4). The CC genotype showed a predictive effect on OS. Compared to the TT phenotype, the CC genotype was associated with a poor OS (HR 0.72; 95% CI: 0.53-0.97). There was heterogeneity among studies.

Publication bias
No significant publication bias was observed.

Discussion
This is the first meta-analysis to investigate the role of ABCB1 in patients administered taxane-containing chemotherapy. This comprehensive quantitative study included 3320 patients from 15 studies with a broad range of characteristics. The rs1045642 and rs1128503 polymorphisms are the most extensively studied SNPs in the ABCB1 gene. The summary results from this study indicated that C3435T polymorphisms did not affect PFS and OS; in contrast, subgroup analysis showed that the geographical area, tumor type, and treatment regimen were associated with the PFS and OS. This is consistent with the results of two previous multicenter studies in the United States and Japan. A meta-analysis reported by Jiang revealed that the rs1045642 and rs1128503 polymorphisms were not related to the response to chemotherapy [37]. Although heterogeneities were found in the overall analyses of PFS and OS in the C3435T homozygote and dominant models, we conducted subgroup analysis and identified the geographical area, tumor type, and chemotherapy regimen as the three main sources of heterogeneity between studies.
There are differences in the genetic and biological characteristics, clinical progression pattern, therapeutic responses, and prognoses among different tumor types; the distribution of paclitaxel in different tumors is also different [38]. Sensitivity analysis revealed that the study by Li et al. [21] was a source of statistical heterogeneity. When this outlier study was removed, there was no evidence of heterogeneity in the four remaining studies in the C3435T homozygote model. This may be because Li et al.
enrolled patients with breast cancer, which differs from the patient groups in other studies. Therefore, it is important to separately analyze the relationship between ABCB1 and prognosis for different tumor types. Our study showed that in patients with ovarian cancer, being a C3435T TT carrier had predictive value, as these patients had better OS and PFS. Wang et al. showed that the rs1045642 wild-type allele translates P-gp mRNA better, producing significantly higher mRNA levels [39], whereas patients with homozygous variants of C3435T exhibited reduced P-gp expression [16]. Penson et al.
reported that patients with ovarian cancer with high expression of P-gp had a short survival time and poor prognosis. ABCB1 gene polymorphisms have different effects on the activities of different chemotherapeutic drugs, and the chemotherapy response and prognosis following administration of different regimens vary widely [40]. Studies have shown that ABCB1 polymorphisms are related to the response to platinum-based chemotherapy [41,42]. Considering TP as a first-line chemotherapy regimen and platinum as a substrate of ABCB1, inconsistent effects may be observed when the chemotherapy regimen involves TP. Our study showed that TT carriers exhibited conflicting outcomes among patients treated with different regimens. Additionally, the level of medical care and medical resources in different regions is inconsistent, which affects the prognosis of patients [43].
Our study demonstrated the predictive value of ABCB1 C1236T for the PFS and OS in patients who were administered taxane-contained chemotherapy, which is consistent with the results reported by Bjorn et al. [1] and Zhou et al. [44]. The effect of genetic variation on prognosis may be related to mutations that cause changes in the encoded amino acids, affecting the normal function of certain drug transporters, such as decreased expression of P-gp. This can lead to a change in the pharmacokinetics of a drug, decreased levels of drug pumped out of the cell, and then accumulation of the drug in the cell, which may adversely affect therapeutic efficacy [18]. Bosch et al. reported that the C1236T homozygous mutant decreased the clearance of docetaxel, leading to an increase in the area under the curve [45]. The C1236T polymorphism may indirectly affect the stability of the mRNA [46]. Shen et al. [47] suggested that allele-specific differences in RNA folding influence downstream mRNA splicing, processing, or translational control and regulation.
Reduced translational activity may also occur.
There are some limitations to this study. The adjusted factors of the extracted data on survival time differed among the included studies, which may have affected the results for disease progression and death. Additionally, subgroup analyses based on other baseline characteristics of patients were not conducted because they were not available in the included studies. The composite effect with other clinical factors and gene variants, such as G2677T/A, was not evaluated because of the present data status. Despite these limitations, our meta-analysis showed that the precision of the estimation was improved by integrating multiple data sets and enlarging the sample size. Additionally, we found no publication bias, supporting the results of our meta-analysis.

Conclusions
In conclusion, our study suggests a predictive role of

Availability of data and materials
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Authors' contributions
CQY and CY made substantial contributions to the study design. CQY, YJH, LM, and LXX screened papers and conducted the quality rating and meta-analysis. Statistical analyses were conducted by CQY and HJF. CQY and LWL were involved in drafting the manuscript.
WYY and GJJ were involved in critically revising the manuscript. All authors read and approved the final manuscript.

Ethics approval and consent to participate
Not applicable.

Consent for publication
Not applicable.

Figure 1
Flow diagram of the literature search and trial selection process.

Figure 2
Forest plots of ABCB1 rs1045642 polymorphisms and subgroups on progressionfree survival of patients administered taxane-based chemotherapy.

Figure 3
Forest plots of ABCB1 rs1045642 polymorphisms and subgroups for overall survival of patients administered taxane-based chemotherapy.

Figure 4
Forest plots of ABCB1 rs1128503 polymorphisms and subgroups for progressionfree survival and overall survival of patients administered taxane-based chemotherapy.

Supplementary Files
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