Prognostic and clinicopathological value of Ki-67/MIB-1 expression in renal cell carcinoma: a meta-analysis based on 4579 individuals

Introduction

Renal cell carcinoma (RCC) ranks the seventh most prevalent cancer type in men and ninth in women¹. Each year, about three hundred thousand cases of RCC are diagnosed, and about 134 thousand deaths are reported worldwide.^2,3 There are multiple treatment methods that could be applied to treat localized RCC; surgery treatment is the most effective, followed by chemotherapy and radiotherapy. Patients with RCC at an early stage may receive complete surgical resection to achieve the purpose of cure; about half of the patients experience disease recurrence after curative resection, and about 30% of RCC patients have metastases at the time of the initial diagnosis.⁴ Metastatic RCC is a treatment-resistant malignant tumor, which is usually treated with targeted drugs or immunosuppressive points for systemic therapy;⁵ however, it has limited effect. Therefore, reliable prognostic biomarkers are needed to distinguish high-risk patients with RCC and improve clinical outcomes of RCC.

MIB-1, also known as Ki-67, is a marker for cell proliferation and tumor growth, which is present during all active phases of the cell cycle, ie, G1, S, G2, and mitosis, but is absent in resting cells (G0 phase).⁶ High Ki-67/MIB-1 expression is often correlated with the clinical course of the disease, and its coexpression with other well-known markers of proliferation indicates a pivotal role in cell division. It is reported that Ki-67/MIB-1 expression predicts poor prognosis in various multiple solid tumor types, including breast cancer,⁷ prostate cancer,⁸ cervical cancer,⁹ gliomas,¹⁰ and hepatocellular carcinoma.¹¹ Many studies have reported the prognostic value of p53 expression in RCC, but the results were conflicting.^12–34 Therefore, it is necessary to conduct a comprehensive meta-analysis to evaluate the prognostic and clinicopathological value of Ki-67/MIB-1 expression in patients with RCC.

We retrieved relevant literature and extracted data from eligible studies to perform a meta-analysis. We aim to reveal the association between Ki-67/MIB-1 expression and prognosis and clinicopathological features in patients with RCC.

Materials and methods

Search strategy

We did this meta-analysis using a predefined protocol in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).³⁵ We searched PubMed, Embase, Web of Science, and the Cochrane electronic databases for studies published before April 14, 2017. The keywords were searched as follows: “renal cell carcinoma” or “renal cell cancer” or “renal cell adenocarcinoma” or “kidney tumor” and “Ki-67” or “MIB-1” and “prognosis” or “survival” or “outcome” in humans, and the language of publications was restricted to English.

Two reviewers (ZW and HX) independently screened the titles and abstracts of all initially identified studies according to the selection criteria. Full-text articles of studies that met all selection criteria were retrieved.

Inclusion and exclusion criteria

To be eligible for inclusion in this meta-analysis, a study must meet the following criteria: 1) the prognostic value of Ki-67/MIB-1 expression for overall survival (OS) and/or cancer-specific survival (CSS) were reported; 2) all patients were diagnosed with histologically confirmed RCC; 3) hazard ratios (HRs) and their 95% CIs for survival analysis were reported in text or could be computed from given data; and 4) the expression of Ki-67 was measured by immunohistochemistry (IHC).

The exclusion criteria were as follows: 1) non-human studies, non-English articles; 2) abstract, case reports, review articles, or comment letters; 3) duplicate publications; 4) with insufficient data to calculate the HR and its 95% CIs, or the Kaplan–Meier curve in the article could not be extracted; and 5) with no >30 eligible RCC patients.

Data extraction and quality

Data was independently extracted by ZW and Shuanghe Peng (The Second Hospital of Tianjin Medical University), and in case of any inconsistency, a consensus was reached with the involvement of QLC. The quality of the selected articles was assessed according to the Newcastle–Ottawa Scale.³⁶ Study with a score of 6 or higher was considered as a high quality study. We used a predesigned data extraction form to obtain relevant information. The data extracted from the eligible studies including the following items: first author, year of publication, country of origin, the number of patients, histopathological stage, detection method, cut-off value, antibody staining for Ki-67/MIB-1, the number of patients with positive Ki-67/MIB-1 expression, HR for survival (OS and/or CSS), and follow-up time. For articles that only provided survival data in a Kaplan–Meier curve, software designed by Tierney et al was used to digitize and extract the relative risk and its 95% CI.³⁷

Statistical analysis

Data were analyzed by using Stata SE12.0 (Stata Corp LP, College Station, TX, USA). According to the Meta-analysis Of Observational Studies in Epidemiology guidelines,³⁸ the associations between clinical factors and Ki-67/MIB-1 expression were presented by odds ratio (OR) and 95% CI. HR with a 95% CI was computed to reveal the correlation between Ki-67/MIB-1 expression and prognosis (OS and CSS). Inter-study heterogeneity was evaluated using the chi-square test and I² statistic (100% × [(Q-df)/Q]),^39,40 the value of P_{heterogeneity} <0.1 and I²>50% represents the existence of significant heterogeneity. A fixed effects model was used when the value of P_{heterogeneity} was >0.05 and I²<50%; otherwise, a random effects model was applied. Subgroup analysis was performed for OS and CSS analysis. Begg’s funnel plot and Egger’s linear regression test evaluated the potential for publication bias. Two-tailed value of P<0.05 was considered statistically significant.

Results

Features of included studies

The work flowchart for this study is shown in Figure 1. Three hundred and one potentially relevant citations were initially retrieved through initial search of relevant databases. After title and abstract screening, 38 articles remained for full-text assessment. Then 15 articles were excluded (2 articles were duplicate studies, 12 lacked key information, and 1 did not measure Ki-67 expression by IHC). At last, 23 studies^12–34 published from 2000 to 2016 with 4579 patients met our inclusion criteria and were included in the meta-analysis.

Figure 1 Flow diagram of the study selection process.

Summary of major characteristics of these studies are shown in Table 1. All the studies were of retrospective study design and detected Ki-67/MIB-1 expression using IHC. The sample size ranged from 43 to 741. Nineteen studies were conducted in non-Asian countries, including France,¹² Finland,^22,28,33,34 Germany,^18,29,30 Italy,²⁵ Sweden,²⁷ and USA.^{13,15–17,20,21,24,26,31} Four studies were conducted in Asian countries, including China,³² Turkey,¹⁹ and Japan.^14,23 For the prognostic indicator of Ki-67/MIB-1 expression in RCC, 1 article reported both OS and CSS, 6 articles only reported OS, and 16 articles only reported CSS.

Table 1 Main characteristics of included studies Notes: *Median follow-up. **The quality of the included studies was evaluated using the Newcastle-Ottawa scale (NOS). aNumber of patients with Ki-67/MIB-1 positive expression. Abbreviations: ccRCC, clear cell renal cell carcinoma; CSS, cancer specific survival; F, female; IHC, immunohistochemistry; M, male, m-RCC, metastasis RCC; NA, not available; OS, overall survival; RCC, renal cell carcinoma.

Prognostic value of Ki-67/MIB-1 expression for OS and CSS

The association between Ki-67/MIB-1 expression and prognosis for OS and CSS in RCC patients were estimated; pooled HRs and 95% CIs are shown in Table 2 and Figure 2.

Figure 2 Forest plot HR for the correlation between Ki-67/MIB-1 expression and OS (A) or CSS (B) in RCC patients. Abbreviations: CSS, cancer-specific survival; HR, hazard ratio; CI, confidence interval; RCC, renal cell carcinoma; OS, overall survival.

Table 2 Ki-67 pooled HRs and 95% CIs in meta-analysis for OS and CSS Note: Bold values in the table indicate the result of pooled HR from a fixed effect model or a random effect model. Abbreviations: OS, overall survival; CSS, cancer-specific survival; HR, hazard ratio; CI, confidence interval; ccRCC, clear cell renal cell carcinoma; OS, overall survival.

OS values were available from 7 studies.^{13,21,22,25,28,33,34} The Ki-67/MIB-1 expression had a significant association with poor OS (HR=2.06, 95% CI: 1.64–2.57, P<0.001; I²=0.0%, P_{heterogeneity} =0.4.73, Table 2, Figure 2A). Seventeen studies^{12,14–21,23,24,26,27,29–32} evaluated CSS outcome. The pooled results indicated that Ki-67/MIB-1 expression was significantly related to poor CSS (HR=2.01, 95% CI: 1.66–2.44, P<0.001; I²=41%, P_{heterogeneity} = 0.04, Table 2, Figure 2B).

Subgroup analysis

Subgroup analyses were stratified by nation, HR estimate, and pathological types (Table 2). Subgroup analysis according to nation showed that Ki-67/MIB-1 expression predicted worse CSS (n=4, HR=3.13, 95% CI: 1.60–6.11, P=0.001; I²=0.0%, pheterogeneity =0.67) in Asian studies. In subgroup analysis according to HR estimate, all the 3 HR estimate methods suggested that Ki-67/MIB-1 expression was significantly associated with poor OS and CSS (Table 2). With regard to histology, Ki-67/MIB-1 expression was significantly correlated with poor CSS (n=13, HR=2.08, 95% CI: 1.67–2.59, P<0.001; I²=45%, P_{heterogeneity} =0041) and poor OS (n=2, HR=3.86, 95% CI: 0.49–30.66, P<0.001; I²=73%, P_{heterogeneity} =0053) in clear cell renal cell carcinoma patients, although a significant heterogeneity exists.

Evaluation of Ki-67/MIB-1 expression and clinicopathological characteristics

We also estimated the association between Ki-67/MIB-1 expression and clinicopathological characteristics in RCC patients. Ki-67/MIB-1 expression was significantly associated with TNM (III/IV vs I/II, OR=1.92, 95% CI: 1.61–2.28), grade (3/4 vs 1/2, OR=1.94, 95% CI: 1.21–3.10), M (M1 vs M0, OR=1.81, 95% CI: 1.34–2.43), N (N1 vs N0, OR=1.67, 95% CI: 1.33–2.12), and tumor stage (pT3/4 vs pT1/2, OR=1.56, 95% CI: 1.21–2.02) (Figure 3 and Table 3).

Figure 3 Association between Ki-67/MIB-1 expression and (A) TNM stage; (B) primary tumor stage; (C) lymph node metastasis; (D) distant metastasis; (E) grade; and (F) gender. Abbreviations: OR, odds ratio; CI, confidence interval.

Table 3 Meta-analysis of Ki-67 expression and clinicopathological features in renal cell carcinoma Abbreviations: OR, odds ratio; CI, confidence interval; N, lymph node involvement; M, distant metastasis; TNM, TNM stage.

Publication bias

Funnel plots for meta-analysis of Ki-67/MIB-1 expression, OS, and CSS are shown in Figure 4. Both the Begg’s funnel plot test (OS: P=1.000, CSS: P=0.149; Figure 4) and the Egger’s (OS: P=0.494, CSS: P=0.010) test verified that there was no publication bias within the included cohorts. The funnel plots for clinical features also indicated no obvious publication bias (Figure 4, Table 3).

Figure 4 Funnel plots evaluating possible publication bias for (A) OS; (B) CSS; (C) TNM stage; (D) primary tumor stage; (E) lymph node involvement; (F) distant metastasis; (G) grade; (H) gender. Abbreviations: CSS, cancer-specific survival; OR, odds ratio; OS, overall survival.

Sensitivity analysis

Sensitivity analysis was performed to examine the stability of the current meta-analysis. The selected studies were sequentially omitted to investigate whether any single study could have an influence on the pooled OS or CSS. As shown in Figure 5, the stable overall HR was found to be not dominantly influenced by each individual study.

Figure 5 Effect of individual studies on the pooled HRs for Ki-67/MIB-1, OS, and CSS of patients. Abbreviations: CI, confidence interval; CSS, cancer-specific survival; HRs, hazard ratios; OS, overall survival.

Discussion

MIB-1, a nuclear protein, is famous as a marker of cell proliferation and tumor growth. Since Gerdes et al⁴² first suggested that Ki-67 labeling index predicted poor prognosis in non-Hodgkin’s lymphomas, a number of studies have examined the usefulness of Ki-67 expression in various tumor types. In recent years, several reports suggested that high Ki-67 expression can serve as a promising biomarker for prognostication in various tumors.^7–11 Many studies have also reported the prognostic value of Ki-67 expression in RCC, but the results were still conflicting.^12–34,41 Therefore, we performed this meta-analysis to explore the association between Ki-67/MIB-1 expression and prognostic value and clinicopathological features in patients with RCC.

Our analysis mainly reports the prognostic role of Ki-67/MIB-1 expression in RCC. Studies from different countries are included in the meta-analysis. Fixed effects model and random effects model were used for the meta-analysis. In this study, we focused on validating Ki-67/MIB-1 expression and evaluated the prognostic values of Ki-67/MIB-1 expression in RCC. Based on results from 24 studies with 4579 participants, we concluded that Ki-67/MIB-1 expression predicted poor prognostic value for patients with RCC. RCC patients with Ki-67/MIB-1 expression exhibited poor OS and CSS. Subgroup analysis results revealed that the pooled HRs obtained from Kaplan–Meier curves and those directly extracted from studies both demonstrated that Ki-67/MIB-1 expression was significantly associated with poor OS and CSS. Our results showed that Ki-67/MIB-1 expression was an unfavorable predictor for prognosis in RCC, which were in accordance with conclusions from other solid cancer types, such as breast cancer,⁷ prostate cancer,⁸ cervical cancer,⁹ gliomas,¹⁰ and hepatocellular carcinoma.¹¹ In addition, Ki-67/MIB-1 expression was also associated with clinical factors in RCC; Ki-67/MIB-1 expression had positive relationship with higher tumor stage and grade, as well as lymph node involvement and distant metastases, which suggested that Ki-67/MIB-1 had potential to be used as a dichotomous biomarker.

The relationship between Ki-67/MIB-1 expression and clinicopathological features was also evaluated. The result suggested that RCC patients with Ki-67/MIB-1 expression were significantly associated with primary tumor stage, regional lymph node involvement, distant metastases, nuclear grade, and TNM stage. High Ki-67/MIB-1 expression was likely to have a higher primary tumor stage, TNM stage, positive regional lymph node involvement and distant metastasis, and a higher nuclear grade.

There are several limitations in this study that should be acknowledged. First, all included studies in this meta-analysis measured Ki-67/MIB-1 expression via IHC, but the cut-off criteria to determine the positive or negative expression of Ki-67/MIB-1 were inconsistent in different studies, which may potentially contribute to heterogeneity. Therefore, a more unified standard should be defined in the future. Second, the number of patients included in the most eligible studies was relatively small. Therefore, large-scale studies are needed to conceive more reliable results. Third, relatively few studies were extracted in some subgroup analyses, which might render premature results. Finally, research with positive results is potentially more likely to be submitted and published than work with negative results, which could cause publication bias, although this bias was not detected in the present analysis.⁴³

Conclusion

Our meta-analysis suggests that Ki-67/MIB-1 expression predicted a poor OS and CSS in patients with RCC. The results also indicate that Ki-67/MIB-1 expression was associated with more aggressive clinical features in patients with RCC. Hence, the detection of Ki-67/MIB-1 in clinic will be beneficial to the treatment and prognostic evaluation for RCC patients. More prospective and large-scale studies are needed to clarify our results.

Acknowledgments

This meta-analysis has been financially supported by the National Natural Science Foundation of China (No 81472682 and No 81572538). The authors thank Shuanghe Peng for their assistance with data extraction.

Disclosure

The authors report no conflicts of interest in this work.

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