Background
Hepatitis B virus (HBV) infection remains a major health problem even with a high coverage of effective vaccination meanwhile achieved. The infection can be life-threatening due to its frequent severe complications, consisting of fulminant acute hepatitis, acute or chronic hepatitis (CHB), liver cirrhosis (LC) and hepatocellular carcinoma (HCC) [
1]. The prevalence of HBV infections in Vietnam is high, ranging from 10 to 20% in the general population [
2]. As a foreseeable result, the long-term consequences of HBV infection will constitute a huge burden for Vietnamese economic conditions and the health system in the next decades.
Interactions between host immune responses and HBV determine the clinical manifestations of the infection. Compelling evidence indicates that vitamin D signaling and VDR variants have a significant implication on susceptibility to HBV infection and subsequent liver diseases. In addition to its function in regulating calcium and bone homeostasis, various non-skeletal effects of vitamin D have been identified, including anti-inflammatory, anti-fibrotic and anti-tumor properties. The effects of vitamin D deficiency on various health conditions such as autoimmune diseases, malignancies and a plethora of infectious diseases have extensively been discussed [
3‐
6]. Among communicable diseases, vitamin D deficiency is frequently observed in patients with hepatitis B and C (HCV)-related liver diseases and linked to unfavorable clinical outcomes as well as poor response to antiviral treatment in chronic hepatitis B [
7‐
10] and chronic hepatitis C [
11‐
13].
The active form of vitamin D (calcitriol, 1,25(OH)2D) can modulate both innate and adaptive immune responses through binding to the vitamin D receptor (VDR) [
14‐
17]. The VDR is a member of the nuclear receptor family of transcription factors [
18], which is particularly expressed on macrophages, T cells, and B cells [
14,
15,
19], but also in more than 30 other types of tissues [
20]. It has been suggested that Vitamin D-VDR signaling regulates the expression of more than 900 genes involved in a wide array of physiological functions [
17]. Therefore, VDR is considered a powerful modulator of pathophysiological mechanisms in several human diseases such as cancers, metabolic disorders and infectious diseases, including viral hepatitis [
6,
11,
21‐
25].
The
VDR gene consists of eight exons (exon 2–9) encoding the VDR structural component and six alternatively spliced untranslated exons (1a-1f) [
26]. Among numerous single nucleotide polymorphisms (SNPs) identified in the
VDR gene, the four common variants FokI G/A (rs2228570, exon 2), BsmI C/T (rs1544410, intron 8), ApaI A/C (rs7975232, intron 8) and TaqI T/C (rs731236, exon 9) have been examined in many studies in different ethnic groups in order to identify associations of these variants with HBV infection and liver disease outcomes [
27‐
34]. However, the results were inconsistent. Our study in Vietnamese patients chronically infected with HBV aimed to investigate the association of these
VDR polymorphisms with susceptibility to HBV infection and liver disease outcomes.
Discussion
Vitamin D plays a crucial role in the metabolism of calcium and bone homeostasis, but has also multiple properties in the regulation of immune responses through engagement with the VDR, a member of the nuclear receptor family of transcription factors expressed on many immune cells and other tissues [
14,
15,
19]. Vitamin D-VDR signaling is involved in pathophysiological mechanisms in several human diseases, including viral hepatitis [
6,
11,
21‐
25]. We investigated the functional role of the four common
VDR SNPs TaqI (rs731236) FokI (rs2228570), ApaI (rs7975232) and BsmI (rs1544410) in Vietnamese HBV-infected patients and observed a significant association between ApaI variability and clinical presentation and progression of HBV-related liver disease. The major and minor allele frequencies observed in this study were compared to available data in the 1000 genomes project. The allele frequencies were ApaI [rs7975232: Major allele C = 0.7424; Minor allele A = 0.2576]; FokI [rs2228570: Major allele G = 0.5859; Minor allele A = 0.4141]; Taq1 [rs731236: Major allele T = 0.9596; Minor allele C = 0.0404] and for BsmI [rs1544410 Major allele C = 0.9596; Minor allele T = 0.0404]. The observed minor allele frequencies from this study corroborated well with the available data. Kinh ethnicity is a major ethnic population (> 87% ~ 77 million) in Vietnam. Based on mtDNA data, all Vietnamese carry South East Asian haplotypes, which show a limited geographic and ethnic stratification [
35], therefore no population substructure is foreseen.
Several studies have investigated the biological functions of the four
VDR variants FokI (exon 2), BsmI (intron 8), ApaI (intron 8) and TaqI (exon 9) and their correlation with HBV infection [
27‐
34]. However, the relationship between distinct
VDR variants and HBV infection and liver disease progression has remained controversial. Regarding the association of these polymorphisms with the risk of HBV infection, a recent meta-analysis including 15 studies could not reveal any evidence of correlation between TaqI, ApaI and BsmI polymorphisms with HBV infection; correlations were only observed for the FokI polymorphism [
25].
Five of the 15 studies in the above-mentioned meta-analysis have assessed the association of ApaI polymorphisms with HBV infection; however, no correlation was observed (CC vs. CA and AA). This finding corresponds to the results of our present study (Table
3). On the other hand, there is not much information available on the ApaI polymorphism and progression of HBV-related liver diseases. Two previous studies have shown that ApaI polymorphism did not show positive association with the risk and clinicopathological features of HCC [
33,
36]. To our knowledge, our present study is the first to show an independent association between the
VDR ApaI AA genotype and an increased risk of HBV-related development of HCC with a significant OR of 2.56 when comparing the HCC with the CHB group. An association of the ApaI CC genotype with the development of HCC among chronic hepatitis C patients has previously been demonstrated [
37]. Allele ApaI C appears to contribute to a decreased susceptibility to HCV infection in a high-risk Chinese population [
38]. A recent study has shown that the ApaI polymorphism was associated with the severity of liver cirrhosis caused by various etiologies [
39].
Among numerous polymorphisms in the
VDR gene, the FokI polymorphism is of great importance, as this missense variant causes a change of the start codon ATG to ACG, resulting in a modification of the VDR protein by three amino acids [
40]. The modified protein, encoded by the FokI G allele, binds more efficiently to vitamin D than does the longer version coded by the A allele [
26,
40]. It was suggested that the change in the VDR structure may alter the biological function of VDR-vitamin D transcriptional signaling and may contribute to carcinogenesis, autoimmunity and susceptibility to infectious diseases, including viral hepatitis [
34,
41]. As shown in the above-mentioned meta-analysis, eight studies including a total of 2516 cases, 1088 controls were subjected to the evaluation of the role of the FokI polymorphism in HBV infection. The FokI polymorphism was significantly associated with HBV infection in a codominant model [
25].
Three other studies have investigated possible association of the FokI polymorphism with liver cancer and have suggested that the FokI polymorphisms might be used as a molecular marker to predict the risk and to evaluate the disease severity of HCC in HBV-infected patients [
33,
34,
36]. Triantos et al. reported that the occurrence of the GG genotype was also an independent prognostic marker of survival, providing a possible protective role in LC patients [
39]. Other studies have been conducted to investigate the potential role of the FokI variant in response to treatment with pegylated interferon in chronic hepatitis B and chronic hepatitis C [
42,
43]. These studies have shown that the FokI polymorphism was an independent factor predicting success of PEG-IFN treatment response in HBV- or HCV-infected patients. Although these studies demonstrated the potential role of FokI variability in HBV infection and progression of liver diseases as well as response to antiviral treatment, our study could not confirm any correlation between the FokI polymorphism with either susceptibility to HBV infection or progression of HBV related-liver diseases.
The TaqI polymorphism has widely been studied in several malignant diseases such as breast cancer, prostate cancer, skin cancer, colorectal cancer, liver cancer and other malignancies [
41,
44]. However, any effects of this variant on the development of cancer remain inconclusive. For instance, two meta-analyses have assessed the association of the TaqI polymorphism with the development of colorectal cancer, yielding inconsistent results [
45,
46]. While Serrano et al. reported that the genotype CC increases the risk of colorectal cancer by 43%, but not for other malignancies [
45], Sheng et al. concluded that no association between this genotype and susceptibility to colorectal cancer exists [
46]. With regard to the association of the TaqI polymorphism with liver cancer, our study clearly confirms other studies, showing no significant association between the TaqI polymorphism and liver disease progression as well as development of HCC in both HBV and HCV infections [
33,
37,
47].
Data on BSMI polymorphisms in HBV infection are scanty. Only three studies on associations of the BSMI polymorphism with HBV infection have been published to date [
33,
48,
49]. These studies were included in the meta-analysis [
25] and showed that BsmI was not a reliable indicator for the risk of HBV infection. In line with these studies, we did not find any association between
VDR polymorphisms and HBV infection, as well as progression of liver disease. Unlike the FokI variant, TaqI and BsmI variability does not influence the function of VDR expression [
48]. This may explain why TaqI and BsmI polymorphisms were not associated with either HBV infection or disease progression in most studies conducted so far.
Vitamin D levels and liver disease progression and the response to antiviral therapy in HBV and HCV patients have previously been described [
49]. Our previous study has shown that vitamin D levels decreased according to the progression of HBV-related liver disease [
7]. One limitation of the study is that Vitamin D levels was measured only in fewer healthy controls compared to a large group of HBV patients. In the present study, we did not find any association of VDR polymorphisms with vitamin D levels in HBV-infected patients. Nevertheless, the correlation between VDR polymorphisms and vitamin D serum levels is not yet well established or this association is weak only. The influences of VDR polymorphisms on the course as well as the pathogenesis of HBV-related liver diseases remain still controversial and need to be explored further.
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