Search results
The study selection procedure is shown in Figure 1. In the initial search, a total of 439 publications retrieved from PubMed, EMBASE, Web of Science, SCOPUS, and Chinese National Knowledge Infrastructure (CNKI) databases seemed to meet the inclusion criteria. Of these, 303 publications were identified as duplicates and were eliminated. After reading article titles and abstracts, 120 records were eliminated as no association between circRNA expression and CRC was described or the articles were reviews. In the full-text verification, 16 articles were excluded as the studies were out of topic or lacked sufficient data. Finally, 13 studies were included in the quantitative meta-analysis.
Study characteristics and study quality
The included 13 studies comprised eight studies on clinicopathologic parameters [9-11, 13, 14, 16, 19, 20], nine on diagnosis [9-14, 16, 19, 20], and seven on prognosis [10, 12, 14, 15, 17, 18, 21]. The baseline characteristics of all included studies are summarized in Tables 1 and 2. All 13 studies were carried out in Asia. A total of 1430 CRC cases were included, and the sample size ranged from 32 to 318. All CRC cases were diagnosed by histological and pathological examinations. The tissue samples were obtained prior to clinical treatment. circRNA expression level was determined using quantitative real-time polymerase chain reaction (qRT-PCR) or RNA sequencing, and the reference genes included GAPDH [10-18, 20, 21], 18S rRNA [9], and U6 [19]. Six types of circRNAs were recognized as tumor promoters[12, 15, 16, 18, 19, 21], and seven were tumor suppressors [9-11, 13, 14, 17, 20]. For survival analysis, the follow-up period was available in two studies, and three articles contained data on HR and 95% CI, whereas HR values in the remaining four articles were unclear and were calculated indirectly.
Study bias and quality assessed by QUADAS II and NOS checklists are shown in Tables 3 and 4. The rating scores of all eligible studies for diagnosis ranged from 4 to 6, and for prognosis ranged from 6 to 8, indicating high methodological quality in all included studies.
Meta-analysis of clinical parameters
The association between circRNAs and clinicopathologic features in patients with CRC is shown in Table 5. Altered expression of circRNAs was markedly associated with poor clinicopathologic parameters (tumor diameter: pooled P = 0.0350; differentiation: pooled P = 0.0038; lymphatic metastasis: pooled P = 0.0119; distant metastasis: pooled P<0.0001; TNM stage: pooled P = 0.0002; depth of invasion: pooled P = 0.0016). In contrast, no significant correlations were observed for age (pooled P = 0.3141), gender (pooled P = 0.5696), tumor location (pooled P = 0.8627), as well as levels of carcinoembryonic antigen (CEA) (pooled P = 0.2047), and carbohydrate antigen (CA) 19-9 (pooled P = 0.7954).
Diagnostic performance
The weighted diagnostic parameters of circRNAs in distinguishing CRC from non-tumor controls were as follows: sensitivity of 0.77 (95%CI: 0.70–0.82), specificity of 0.81 (95%CI 0.73–0.86), PLR of 4.00 (95%CI 2.80–5.60), NLR of 0.29 (95%CI 0.22–0.38), DOR of 14.0 (95%CI 8.0–24.0), and AUC of 0.86. Forest plots of the pooled sensitivity, specificity, DOR and summary receiver operating characteristic (ROC) curve of circRNAs in diagnosing CRC are shown in Figure 2.
Stratified analysis showed that the performance of up-regulated circRNAs (function as tumor promoters) for CRC detection was significantly superior to that of down-regulated circRNAs (function as tumor suppressors) (AUC: 0.86 vs. 0.75; DOR: 10.63 vs. 6.55). When analyzed based on test matrix, these results showed that tissue-based circRNA testing achieved higher diagnostic efficacy than plasma-based analysis (AUC: 0.79 vs. 0.50; DOR: 7.68 vs. 7.13).
Overall survival
Survival analysis showed that oncogenic circRNAs predict worse prognosis in terms of OS in patients with CRC (HR = 2.66, 95%CI: 2.03-3.50, P = 0.000; Chi2 = 4.34, P = 0.740, I2 = 0.0%) (Figure 3). We identified one outlier study in the combined effect of decreased circRNAs by sensitivity analysis (Figure 4), and the outlier data were eliminated. The weighted effect showed that decreased circRNAs expression (function as tumor suppressors) in patients with CRC was associated with favorable OS (weighted HR=0.30, 95%CI: 0.17-0.53, P = 0.000; X2 = 1.34, P = 0.909, I2 = 0.0%) (Figure 3)
Sensitivity analysis and meta-regression
Results of the sensitivity analysis showed that the effect did not alter when omitting studies one by one in relation to the combined diagnostic effect and prognostic effect of oncogenic circRNAs (Figure 4).
To identify the causes of heterogeneity, meta-regression of the pooled diagnostic effect in terms of the specified covariates such as sample size, text matrix, circRNA signature, expression status, reference gene, and quality score was conducted. The results showed that circRNA expression status (pooled DOR = 3.67, 95%CI: 1.10-12.28, P = 0.0386), and reference gene (pooled DOR = 0.29, 95%CI: 0.09-0.91, P = 0.0383) were likely to be the sources of heterogeneity (detailed data not shown).
Publication bias
Deeks’ funnel plot asymmetry test showed that no evidence of publication bias (P = 0.37) existed for diagnostic analyses (Figure 5A and 5B). Begg's and Egger's tests were also performed to assess publication bias among the eligible articles. There was no obvious publication bias in the prognostic effects according to Begg's test (P = 0.129, or 0.266) (Figure 5C and D), and Egger's test (detailed data not shown). Therefore, we excluded the possibility of publication bias.
The included 13 studies comprised eight studies on clinicopathologic parameters [9-11,
13, 14, 16, 19, 20], nine on diagnosis [9-14, 16, 19, 20], and seven on prognosis
[10, 12, 14, 15, 17, 18, 21]. The baseline characteristics of all included studies
are summarized in Tables 1 and 2. All 13 studies were carried out in Asia. A total
of 1430 CRC cases were included, and the sample size ranged from 32 to 318. All CRC
cases were diagnosed by histological and pathological examinations. The tissue samples
were obtained prior to clinical treatment. circRNA expression level was determined
using quantitative real-time polymerase chain reaction (qRT-PCR) or RNA sequencing,
and the reference genes included GAPDH [10-18, 20, 21], 18S rRNA [9], and U6 [19]. Six types of circRNAs were recognized as tumor promoters[12, 15, 16, 18, 19,
21], and seven were tumor suppressors [9-11, 13, 14, 17, 20]. For survival analysis,
the follow-up period was available in two studies, and three articles contained data
on HR and 95% CI, whereas HR values in the remaining four articles were unclear and
were calculated indirectly.