Association of TGF-ß1 polymorphisms and chronic hepatitis C infection: a Meta-analysis

Hepatitis C virus (HCV) infection which is a widely prevalent infectious disease has presented in about 170 million people of the world [1]. There are 60–80% of patients with the acute infection developing into chronic hepatitis C (CHC). In the long run, one out of three CHC patients will progress to hepatic complications such as hepatic fibrosis, liver cirrhosis (LC), eventually hepatocellular carcinoma (HCC), which leads to high mortality [2]. For a long time, scientists have revealed the factors which regulate the responses to HCV infection and affect disease progression. Some studies reported that the viral genotypes, environmental factors and behavioral factors (excessive alcohol intake) were implicated in the development of HCV infection [3, 4]. However, these factors cannot fully explain the large variability in susceptibility or outcomes observed within different populations. Recently, several genetic association studies concluded that the mutations of certain cytokine genes may play an important role in the susceptibility and progression of HCV infection, due to the insufficient or imbalance responses in the cytokine network [5‐7].

Transforming growth factor-beta 1 (TGF-β1), which is an crucial immuno-regulatory cytokine secreted by hepatic stellate cells, fibroblasts, and Kupffer cells, is participated in the regulation of cellular growth, differentiation and proliferation [8]. During the acute infectious stage of HCV, natural killer (NK) cells produce interferon-gamma (IFN-γ), and the proliferation and cytotoxicity of NK cells are pivotal in clearing HCV infection. As a renowned suppressor of NK cells, TGF-β1 inhibits the secretion of IFN-γ and interleukin (IL) -12, leading to the persistence of HCV infection [6]. In different infection states, the frequent mutations and expression of TGF-β1 were various implying a possible role of TGF-β1 in HCV infection [6, 9]. The TGF-β1 gene which is located in chromosome 19q13.1 is constituted by 7 exons and 6 introns. To date, several functional single-nucleotide polymorphisms (SNPs) of TGF-β1 have been reported. Particularly, the −509C/T (rs1800469), codon 25G/C (rs1800471), and codon 10 T/C (rs1982073) SNPs are the most widely evaluated polymorphisms [10, 11]. It has been demonstrated that these functional SNPs are associated with the interindividual differences of TGF-β1 expression [6, 7]. The above facts suggest that −509C/T, codon 25G/C, and codon 10 T/C SNPs may contribute to TGF-β1-mediated immune response in HCV infection.

Recently, great attention has paid to investigate whether the −509C/T, codon 25G/C, and codon 10 T/C SNPs of TGF-β1 gene were associated with the chronic HCV infection. Pooling the related research’s data, we found that the conclusions of these studies were not always consistent. Taking the TGF-β1–509C/T polymorphism as an example, most studies suggested that people with the −509TT genotype and T allele have a higher risk of chronic HCV infection [6, 7, 12, 13]; and Kimura et al. described −509CC genotype and C allele may contribute to HCV clearance rates in Japanese populations [14]; however, no association has been shown in another study [15]. These reported discrepancies may result from the differences of individual studies in sample size, as well as geographical region and ethnicity of the subjects.

To further discriminate the effect of infection or clearance, we compared the chronic HCV infected patients with healthy controls or SR controls, respectively. Specifically, the summary results of our meta-analysis revealed that subjects with the genotype of -509TT and T allele have a about 2 and 3 fold higher stake of the susceptibility to chronic HCV infection in total and Egyptian populations, respectively. Moreover, the related summary effects (ORs) rarely changed after we excluded the studies that didn’t follow the HWE. A series of statistical tests which included the HWE test, sensitivity analysis and publication bias test guaranteed the related pooling ORs were stable, definite and statistically significant.

Several studies reported the polymorphism of codon 10 T/C was located at position + 29, relative the translational start site of TGF-ß1 gene. The transition of T to C of the codon 10 may impact the export productivity of the newly synthesized protein [10, 11]. Wang et al suggested the codon 10C allele was likely to develop more severe fibrosis during chronic HCV infection [19]. And Vidigal et al reported the codon 10CC genotype was associated with the resistance to combined antiviral therapy in HCV infection [26]. However, in most other studies, the polymorphism of codon 10 T/C was not related to the chronic HCV infection [15, 16, 21‐23, 30]. And our meta-analysis summary results of this SNP were consistent with the majority opinion. The deterministic correlation of codon 10 T/C polymorphism and chronic HCV infection needs more case-controls studies to confirm.

Another crucial SNP locus is codon 25G/C which may impact the production of TGF-β1. The transition of G to C may be correlated with the reduced level of TGF-β1 in vitro [27]. Pereira et al reported that the CHC patients have the higher frequency of codon 25G allele than healthy subjects [21]. Theoretically, it is plausible that subjects with the high TGF-β1 producer phenotype which is associated with the codon 25G allele present over suppression in the human immune response. This mechanism may result in the correlation of the polymorphism of codon 25G/C and chronic HCV infection. However, one previous meta-analysis demonstrated that the polymorphism of TGF-β1 codon 25G/C wasn’t correlated with the chronic HCV infection [31]. In our proposed meta-analysis, the summary results were obtained by pooling the data from the latest and most complete studies and conducting the total analysis, subgroup analyses and the different comparison of controls analyses. Eventually, we didn’t find the connection between the polymorphism of TGF-β1 codon 25G/C and the chronic HCV infection. Several factors may contribute to the discrepancy. First, the small size samples of included studies were insufficient to mining the deep connections of the certain genotype and certain clinical disease. Second, some impacted heterogeneity still existed, although we conducted subgroup analyses by the ethnicity and control groups as well as dropped the studies which did not satisfy the HWE rule to explore the sources of heterogeneity. Moreover, other factors such as other polymorphisms or viral factors of the chronic HCV infection may be involved. Notably, the codon 25 polymorphism has been revealed to be connected with the stage/degree of liver fibrosis in Caucasian HCV infected patients [32, 33]. A large-scale study is thereby required to confirm the genetic contribution to HCV infection or live fibrosis.

Due to the unique SR controls of related studies, we weren’t able to use meta-analysis to access the connection of the -509C/T or codon 10 T/C polymorphisms with the clearance of HCV. Moreover, the proposed meta-analysis results of two studies in the gene model of C vs. G showed that no significant association was found between the polymorphism of -25G/C and the spontaneous clearance of HCV. So deep researchs are warranted to accurately clarify the correlation of these SNPs and the spontaneous clearance of HCV.

Previous study reported high differences of these allelic frequencies from different ancestries [22]. Therefore, we conducted subgroup analyses by the ethnicity. The populations from Iran, Germany, American-Caucasian, and Ireland were classified as the Caucasians; the populations from Japan, Pakistan, and China were classified as the Asians; and the populations from Brazil, Morocco, and Mexico were classified as the mixed races populations. For the race of Egyptian, we searched for the relevant articles and consulted the geographers, but the results were inconsistent. In order to reduce the substantial genetic heterogeneity, we categorized the Egyptian as a separate subgroup.