A literature search was conducted to looking for eligible studies that explored the association between TLR4 polymorphisms and AP risk using Pubmed, Embase, Web of Science, CBM (China Biological Medicine Database) on September 27, 2013 with combinations of the following key words including (“toll like receptor” or “TLR”) and (“polymorphism” or “genotype” or “variant” or “mutation”) and “pancreatitis”. There was no language restriction in the literature search. All reference lists from relevant studies and reviews were hand searched for additional eligible studies. The studies with the latest sample size were included when there were republished studies.

Eligible studies had to meet all of the following criteria: (1) evaluating the association of TLR4 Asp299Gly or Thr399Ile polymorphisms and AP risk; (2) a case-control study; (3) it is of Hardy Weinberg equilibrium (HWE) in the control group; and (4) sufficient genotyping information to evaluating an odds ratio (OR) and 95% confidence interval (CI).

The following information of each study was extracted independently by two reviewers: the name of first author, year of publication, country, ethnicity, genotypes distribution in both AP and controls, P values for HWE evaluation, source of controls, sample size (case/control), and genotyping methods.

The pooled OR with its 95%CI was calculated to evaluate the strength of association between TLR4 polymorphisms and AP susceptibility in five different genetic models, and the Z test was used to determine the significance of the pooled OR. Cochran’s χ²-based Q statistic test was performed to assess possible heterogeneity between the individual studies[3]. The fixed-effects model was applied to calculate the pooled OR with its 95%CI when there was no obvious between-study heterogeneity, otherwise, the random-effects model was used[4,5]. In the case of zero cells, an appropriate continuity correction (addition of 0.5) was implemented[6]. Publication bias analysis was performed by the funnel plot and Egger’s test[4]. All P values are two-sided, and P < 0.05 was considered statistically significant. Statistical analyses were done with Stata software (version 12.0).

We collected 24 studies after database searches. After evaluation of title and abstract for the association of TLR4 polymorphisms and AP susceptibility, nine relevant studies were identified and retrieved for further investigation. Finally, six studies were identified according to the selection criteria. A total of 6 studies[7-12] with 1255 cases and 998 controls were included in this meta-analysis. In these studies, six studies[7-12] with 1255 cases and 998 controls were about the association of TLR4 Asp299Gly polymorphism and AP risk; three studies[7,8,11] with 815 cases and 744 controls were about the association of TLR4 Thr399Ile polymorphism and AP risk. Among these studies, five studies were published in English[7-11], and one study was in Chinese[12]. There were two studies of subjects of Caucasian descent[7,11], and four studies of subjects of Asian descent[8-10,12]. A classic polymerase chain reaction restriction fragment length polymorphism assay (PCR-RFLP) was used in four studies[8-10,12], a Taqman assay was conducted in two studies[7,11]. Table 1 listed the main characteristics of these six studies for two SNPs of TLR4 and AP. All five studies were consistent with HWE in the controls except for one study[11] (Table 1). Moreover, TLR4 Asp299Gly polymorphism and the severity of AP susceptibility from 4 studies[7,9,11,12] are also summarized in Table 2.

When those six studies were included in the meta-analysis, there was no obvious heterogeneity between the individual studies using five genetic models (P > 0.05). In overall analysis, TLR4 Asp299Gly polymorphism was not associated with AP risk when all studies were pooled into the meta-analysis using five genetic models (for A vs G: OR = 1.022, 95%CI: 0.748-1.397, P = 0.891; for AA vs GG: OR = 1.537, 95%CI: 0.466-5.075, P = 0.480; for AG vs GG: OR = 1.828, 95%CI: 0.454-7.368, P = 0.396; for AA + AG vs GG:OR = 1.576, 95%CI: 0.477-5.205, P = 0.456; for AA vs AG + GG:OR = 0.764, 95%CI: 0.458-1.277, P = 0.305, Table 3, Figure 1). Moreover, the five studies possessed highly homogeneity in all four genetic models (Pheterogeneity > 0.05, Table 3, Figure 1). Additionally, the further subgroup analysis by ethnicity or test assay showed that TLR4 Asp299Gly polymorphism was not a risk for in Asian or Caucasian populations or using PCR-RFLP assay or Taqman assay (P > 0.05, Table 3) with highly homogeneity.

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Figure 1 Forest plot of TLR4 Asp299Gly polymorphism and acute pancreatitis risk for A vs G genotype.

Considering the two different types of AP (MAP and SAP), we then assessed the association of TLR4 Asp299Gly polymorphism and two different types of AP risk using an allele genetic model. Our meta-analysis showed that the TLR4 Asp299Gly polymorphism has no association with MAP risk or SAP risk or the severity of AP using a fixed-effects model (P > 0.05, Table 4).

For TLR4 Thr399Ile polymorphism, overall no association was found between TLR4 Thr399Ile polymorphism and AP risk using four genetic models (for C vs T: OR = 1.090, 95%CI: 0.736-1.614, P = 0.667; for CC vs TT: OR = 1.523, 95%CI: 0.258-9.012, P = 0.643; for CT vs TT: OR = 1.455, 95%CI: 0.166-12.756, P = 0.735; for CC + CT vs TT: OR = 1.494, 95%CI: 0.253-8.844, P = 0.658; for CC vs CT + TT: OR = 1.530, 95%CI: 0.580-4.041, P = 0.390, Table 5) with highly homogeneity.

A funnel plot of these six included studies was symmetrical and didn’t suggest a possibility of publication bias (Figure 2). The statistical results from Egger’s test still did not show publication bias for TLR4 Asp299Gly polymorphism (for A vs G P_egger = 0.659; for AA vs GG P_egger = 0.204; for AG vs GG P_egger = 0.051; for AA + AG vs GG P_egger = 0.250; for AA vs AG + GG P_egger = 0.594) in all five genetic models (Figure 2).

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Figure 2 Funnel plot of TLR4 Asp299Gly polymorphism and acute pancreatitis risk for A vs G genotype.

Acute pancreatitis (AP) is a potentially lethal disease and many proinflammatory factors play important roles in the pathogenesis of AP.

Toll-like receptor 4 (TLR4) Asp299Gly and Thr399Ile polymorphisms have been found to interrupt the binding of TLR4 to lipopolysaccharide pathway, however, the results remains unclear.

In this paper, the authors for the first time conducted a systematic meta-analysis to evaluate the association between TLR4 polymorphisms and AP risk, and the results suggest that TLR4 Asp299Gly and Thr399Ile polymorphisms play little role in the pathogenesis of AP.

This study helped people to further understand the relationship between TLR4 Asp299Gly and Thr399Ile polymorphisms and AP susceptibility.

This paper deals with a hot topic of association between gene single nucleotide polymorphisms and AP risk.