Prognostic and Clinicopathological Significance of Survivin Expression in Renal Cell Carcinoma: A Systematic Review and Meta-Analysis

Previous studies have elevated the prognostic value of survivin in renal cell carcinoma (RCC). To increase statistical power and improve translation, we systematically searched PubMed, Web of Science, and Embase to identify relevant studies until December 2015 and conducted a standard meta-analysis. Based on the inclusion and exclusion criteria, a total of 12 studies, including 2051 patients, were eligible for further analysis. Results showed that high survivin expression in RCC was associated with poor OS (HR = 2.84, 95% CI 1.68–4.79), CSS (HR = 2.36, 95% CI 1.41–3.95), and PFS (HR = 2.20, 95% CI 1.58–3.08). Survivin expression was also correlated with TNM stage (RR = 2.75, 95% CI 2.21–3.44), pathological T stage (RR = 2.19, 95% CI 1.75–2.75), lymph node metastasis (RR = 2.28, 95% CI 1.61–3.25), distant metastasis (RR = 1.56, 95% CI 1.16–2.08), Fuhrman grade (RR = 2.81, 95% CI 2.29–3.45), tumor size (RR = 1.49, 95% CI 1.24–1.78). Our study suggested that survivin was a prognostic marker in RCC. High survivin expression was correlated with poor prognosis and more advanced clinicopathological features, and it could serve as a biomarker for disease management.

Renal cell carcinoma (RCC) accounts for 3% of all human malignancies and is the third most prevalent genitourinary cancers 1 . RCC is highly aggressive. Approximately 30% of patients have metastases at first diagnosis, and another 20% of RCC patients with clinically localized disease will develop metastasis even after curative nephrectomy 2 . Most metastatic RCCs still eventually cause death in spite of the application of targeted therapy 3 . In this regard, prediction models identifying patients with poor prognosis, who may benefit from early systematic therapy, are greatly needed. To date, the tumor, node, and metastasis (TNM) staging system is a widely used RCC prognostic predictor. However, such classic clinical and pathological factors fail to address the inherent biological heterogeneity of RCC 4 . Therefore, novel biomarkers that can stratify patients with poor prognosis of RCC are required to guide clinical decisions precisely.
Survivin is a member of the inhibitor of apoptosis protein family and is usually present in embryonic tissues 5 . Survivin plays a role in cell cycle regulation, inhibition of apoptosis, angiogenesis, and other biological effects 6 . Intriguingly, survivin is barely detectable in most normal adult tissues but overexpressed in many cancers, including RCC 7 . With further understanding of the molecular mechanisms of RCC, numerous studies focusing on survivin have been conducted in the fields of outcome prediction and potential therapeutic targets. To obtain a more precise evaluation of the prognostic and clinicopathological value of survivin expression in RCC, we conducted a systematic review and meta-analysis to evaluate the prognostic value of survivin quantitatively and explore the associations of survivin with the clinicopathological features of RCC.

Meta-Analysis.
The results showed that high survivin expression in RCC was associated with poor OS (a fixed-effect model, HR = 2.84; 95% CI: 1.68-4.79; p < 0.001; I 2 = 6.9%, p = 0.359; Fig. 2A  In the comprehensive analyses of the role of survivin expression in RCC as a biomarker, we investigated the association of high survivin expression and clinicopathological characteristics. As reported in Table 3

Sensitivity Analyses.
To validate the reliability of our results and investigate the source of significant heterogeneity, sensitivity analysis was performed. Sensitivity analyses showed that the pooled HR was not significantly influenced after omitting any single study for the effect of survivin expression on CSS and revealed that the study 15 was the source of statistical heterogeneity (Table 4). When this study was deleted, no significant heterogeneity was observed in the remaining studies (I 2 = 47%, p = 0.079) and pooled HR was 1.39 (a fixed-effect model, 95% CI: 1.27-1.53; p < 0.001) or 1.84 (a random-effect model, 95% CI: 1.37-2.47; p < 0.001). No significant heterogeneity was detected in either OS or PFS; hence, we did not conduct sensitivity analysis.
Publication Bias. In the present meta-analysis, we introduced Begg's and Egger's tests, as well as funnel plots, to assess publication bias. As presented in Fig. 3, the funnel plots revealed that the included studies had no evident asymmetry. Further, the results from Begg's test (P value) and Egger's test (intercept with corresponding 95% CI, P value) for the included studies evaluating the survival outcomes were P Begg's = 0.308, intercept 1.97 with 95% CI − 2.43 to 6.36, P Egger's = 0.194 (OS); P Begg's = 0.711, intercept 2.27 with 95% CI − 1.22 to 5.76, P Egger's = 0.163 (CSS); P Begg's = 0.734, intercept 1.13 with 95% CI − 3.11 to 5.38, P Egger's = 0.369 (PFS), respectively. Therefore, the aforementioned evidences suggested significant publication bias did not exist in our meta-analysis.

Discussion
High expression of survivin is observed in many malignancies, but it is barely detectable in most normal adult tissues; thus, survivin is an attractive prognostic prediction marker and potential therapeutic target for several cancer types [21][22][23][24] . The new paper by Gulati et al. validated survivin gene expression is adverse implications in the prognosis of RCC 25 . To increase statistical power and improve translation, we conducted a meta-analysis to determine a pooled conclusion and provide evidence on the correlation.
In the current study, we focused exclusively on validating survivin IHC expression and evaluated the prognostic values of survivin IHC expression in RCC. We concluded that high survivin expression predicted poor prognosis in RCC patients. In particular, RCC patients with high survivin expression had shorter OS, CSS, and PFS. Subgroup analysis revealed that high survivin expression was significantly associated with poor OS and CSS, regardless of ethnicity, subcellular localizations, extent of tumor, histopathological subtype, and staining cutoff. Among patients with staining cutoff ≥ 10%, no significant association was identified between high survivin expression and poor PFS, even though patients with high survivin expression presented a relatively unfavorable PFS. The absence of a significant association was possibly due to the relatively limited studies in the subgroups. The reasons why high survivin expression may be of prognostic relevance in patients suffering from RCC remain speculative. Survivin is an anti-apoptotic protein that has been associated with cellular apoptosis inhibition function through preferential blocking of mitochondrial-dependent apoptosis by targeting caspase 9 and second mitochondria-derived activator of caspases/direct inhibitory apoptotic protein-binding protein with a low isoelectric point. Moreover, survivin presents a mitosis-regulated pattern of expression during the G2/M phase of the cell cycle. In addition to the widely accepted apoptosis inhibition function, survivin also plays a critical role in mitosis and microtubule stability 26,27 . Abnormal inhibition of apoptosis and cell division during cellular homeostasis is a critical process for the development and progression of RCC.  Consistent with previous reports, our results also suggested that RCC patients with high survivin expression were likely to have a higher TNM stage, pathological T stage, positive lymph nodes, distant metastasis, a higher Fuhrman grade, and a larger tumor size. Currently, survivin is believed to be related to angiogenesis by interacting with vascular endothelial growth factor, angiopoietin, and basic fibroblast growth factor. In addition, survivin is associated with metastatic behavior by activating various signaling pathways 28 . These mechanisms can explain the association between high survivin expression and clinicopathological characteristics in RCC patients.
This study is comprehensive analysis on the effect of survivin on prognostic and clinicopathological significance in patients with RCC, but several limitations should be pointed out. First, although all included studies measured survivin expression via IHC, the criteria to determine the positive or negative expression of survivin were inconsistent in different studies, which may generate heterogeneity of the overall results. Thus, a more standard cutoff value should be defined in the future. Second, marked heterogeneity of studies was observed in CSS analysis. The heterogeneity of CSS analysis was probably caused by differences in factors such as the patients' characteristics (ethnicity, nationality, gender, age, and tumor stage and grade), variation of cut-off values for survivin expression, and different duration of follow-up. Third, in evaluating associations between survivin expression and clinicopathological characteristics of RCC, some studies lacked complete data, which may cause heterogeneity and contribute to the low reliability of the results. Furthermore, positive results were more likely to be published in most studies, whereas studies with negative results were often rejected or less assessable, which could lead to publication bias 29 , although this bias was not detected in the current analysis.
In conclusion, despite the abovementioned limitations, our meta-analysis suggested the prognostic and clinicopathological values of survivin expression in RCC. High survivin expression was correlated with poor prognosis and more advanced clinicopathological features, which may potentially serve as risk stratification markers and even therapeutic targets in RCC. However, more multicenter prospective studies with standardized methods and long-term follow up are needed to verify our results.

Methods
Search Strategy. This meta-analysis followed the guidelines of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 30 .
A systematic literature search was performed in the electronic databases PubMed, Web of Science, and Embase on 31 December 2015 using the following search strategy: ("survivin" or "BIRC5" or "baculoviral inhibitor of apoptosis repeat-containing 5") and ("carcinoma" or "neoplasm" or "tumor" or "cancer" or "malignancy") and ("kidney" or "renal") and ("prognosis" or "prognostic" or "survival" or "outcome" or "mortality"). Furthermore, we manually searched the reference lists of relevant literature.   Quality Assessment. The quality of the included studies was evaluated using the Newcastle-Ottawa scale, which was recommended by the Cochrane Non-Randomized Studies Methods Working Group 8 . Each study can be assessed by eight methodology items with a score ranging from 0 to 9. High scores indicated high quality, and we considered studies with scores of 6 or more as high quality for the meta-analysis.
Statistical Analysis. Pooled HR and risk ratio (RR) with 95% CI were used to evaluate the association of survivin expression with RCC prognosis and clinicopathological characteristics, respectively. An observed HR > 1 implied worse prognosis for patients with high survivin expression. An observed RR > 1 implied more advanced clinicopathological characteristics for the group of high survivin expression. A heterogeneity test of pooled HR and RR was conducted using Cochran's Q test and Higgins I-squared statistic. I 2 values > 50% indicated heterogeneity among studies 32 . When heterogeneity was observed (I 2 > 50%), a random-effect model was used; otherwise, a fixed-effect model was used. For additional analyses, meta-analyses were subgrouped based on ethnicity, subcellular localizations, extent of tumor, histopathological subtype, and staining cutoff. For the investigation of heterogeneous studies, we also conducted sensitivity analysis to evaluate the influence of individual studies on the robustness of pooled results. Publication bias was assessed by funnel plot visual inspection and statistically evaluated by Begg's 33 and Egger's tests 34 . All statistical analyses were performed using Stata 12.0 software (StatCorp, College Station, TX, USA) and p < 0.05 was considered statistically significant.