The etiology and pathogenesis of HCC is complex, due to many related risk factors, such as hepatitis B virus (HBV) infection, aflatoxin exposure, as well as physical and chemical factors, especially alcoholism and other unhealthy lifestyle habits. HBV can promote carcinogenesis though chromosomal instability, numerous mutations, and the interaction between HBx protein and host proteins [12]. Besides, the indirect effects of HBV infection include chronic inflammation and oxidative stress, which can subsequently lead to varying degrees of hepatic injury [13]. Chronic hepatitis infection (CHB) is a strong risk factor for the development of HCC mostly due to HBV nucleotide level, HBV genetic mutations, positivity for the hepatitis B e antigen (HBeAg), HBV genotypes, and co-infection with hepatitis C virus [14].
Integration of the HBV genome is currently believed to be an early event in HBV chronic infection. Notably, mutations and deletions to the HBV genome are associated with an increased risk for the development of HCC and the clinical severity of other hepatic diseases [15]. Most mutations to the HBV genome are generated due to the lack of proofreading capacity of HBV polymerase or host immune pressure [16].
In the present study, HBV complete genome sequences were obtained from samples of 51 HCC patients and 76 CHB patients. To determine whether there was any difference in HBV sequences between the HCC and CHB groups, the nucleotide substitutions of HBV whole genome sequences were compared in four overlapping ORF.
The substitution rates of the nucleotide sites located in the pre-C and C regions were mostly higher in the HCC group than the CHB group. Most pre-C/C mutations are generated during HBeAg seroconversion. Several types of HBV pre-C/C mutations, such as G1896A, A1762T, and G1764A, were reportedly related to disease severity[5, 16]. Also, the HBeAg positive rate was significantly lower in the HCC group than the CHB group. However, it remains unclear whether this phenomenon is associated with the nucleotide substitution to C gene, thus further studies are warranted.
It has been reported that the nucleotide substitution rate in HCC tends to be greater in the X and pre-C/C regions. In the present study, the substitution rates of many sites were proved greater in the CHB group than the HCC group. Though there were lots of studies about nucleotide of HCC[10, 16]. However, substitutions identified in this study of 29 sites have been rarely reported previously. Nonetheless, the rates of substitutions to 13 of these sites were significantly greater in the HCC group than the CHB group (Fig. 1). Among them, nt1470 and other six sites were located at the B cell epitope. The sites nt1726 and nt1730 were located at the T cell epitope. Previous study reported almost 40% of the integrated HBV genomes were cleaved at approximately nt1800 [10]. Therefore, the sites (nt1799, nt1802, nt1803, and nt1804) may play a potential role in HBV genome integration for HCC development.
In this study, the genotype-specific amino acid substitution rates, as deduced from the nucleotide sites in Fig. 1, were compared between the HCC and CHB groups. The mutation rates of F22I/L/P in the pre-S2 region, as well as P33S/T and S144A/T/V in the X region, were significantly higher in HCC group. F22I/L/P is reportedly associated with immune nonreactivity[17]. P33S/T and S144A/T/V occurring in the X region have not been previously reported and, thus, are novel mutations possibly associated with a greater risk for the development of HCC. The amino acid at position aa33 was located in the negative regulation domain of HBx (aa 1–50) which formed a B cell epitope (aa 29–48). The HBx region partially overlaps with the RNase H part of HBV polymerase at the C-terminus, and also contains several critical cis-elements. Genetic alterations in this region may not only affect the reading frame of HBx, but also the overlapping cis-elements and the possible binding affinities of this protein to its targets [18]. The amino acid at position aa144 is located in the core promoter of the C-terminus of HBx, which plays a key role in controlling cell proliferation, viability, and transformation[19].
The pre-S1 and pre-S2 regions contain several epitopes of T or B cells and play essential roles in the immune response[20]. Pre-S deletion decreases the expression of the surface proteins of HBV, resulting in intracellular accumulation of HBV envelope proteins and viral particles, which induce endoplasmic reticulum stress and oxidative DNA damage, eventually leading to the development of HCC[21].Truncated pre-S2/S sequences are often found in HBV DNA integration sites of HCC patients, and truncated pre-S2/S proteins could specifically activate the MAPK signaling pathway to activate transcription factors such as AP-1 and NF-κB, and thus promote abnormal proliferation of liver cells [22]. In the present study, there was no significant difference in deletions pre-S1 and pre-S2 between HCC and CHB groups, while the frequency of pre-S2 deletions was higher than pre-S1 deletions among HCC patients, which was consistent with the findings of other studies[23].
Deletions or insertions to the C-terminus of HBx reportedly impair transactivation activity, thereby inhibiting cell proliferation, which may contribute to the development of HCC [24]. In the present study, all 11 deletions (ten for genotype C and one for genotype CD) were located in the C-terminus of HBx. In the CHB patients, there were deletions to codons 125 to 136 of HBx, but involved more codons in HCC patients especially truncations to the C-terminus of HBx. Previous studies have frequently reported deletions to the 3'-end of the X gene, which leads to truncations of the HBx C-terminus [25]. Reportedly, truncations of the HBx C-terminus occur in nearly 80% of HCC tissues, which may contribute to hepatocarcinogenesis via loss of pro-apoptotic capability of full genes, activation of cell transformation, and subsequent tumor promotion [26]. In the present study, the HBx deletion rate was 13.73% (7/51) in HCC patients, which is lower than previous reports, but higher than in CHB patients 5.26% (4/76).
There were some limitations in this study should be acknowledged. First, there was no significant difference in most of the substitution rates between the HCC and CHB groups. Second, as large fragments of deletion in the genome of HBV is the characteristic of HCC samples,the chance of obtaining full sequencing maybe insufficient. To better understand the clinical relevance of HBV gene substitutions, further prospective investigations of HBV-infected patients are required.