Virology: The X-Files of hepatitis B

Hepatitis B virus genotypes A1 and A2 have distinct replication phenotypes due to polymorphisms in the HBx gene

HBV genotype A has two major subtypes, A1 (commonly in Africa) and A2 (commonly in Europe) with only 4% nucleotide differences. Individuals infected with these two subtypes appear to have different clinical manifestations and virologic features. Whether such a difference results from the virus or host has not been established. Using HBV generated from molecule clones of subtypes A1 and A2 in cell culture (HBVcc), we demonstrate that HBVcc of subtypes A1 and A2 can be passaged in vitro and in vivo and respond equally well to human IFN-α treatment. HBVcc passaged in human liver chimeric mice (HBVmp) infected human hepatocytes more efficiently than that of the original HBVcc. Subtype A2 showed a much higher viral replication level than that of subtype A1. Mechanistic investigations using constructs with chimeric A1/A2 sequences and specific mutations indicated that subtype A2 has an inherently higher replication phenotype due to specific polymorphisms in the HBx gene resulting in amino acid variations. Studies of HBx expression demonstrated that A1 HBx is expressed at a much lower level than that of A2 HBx. Mutagenesis studies identified two HBx amino acid variations responsible for the observed phenotypic difference. Using AlphaFold2, we generated structural models of HBx proteins of A1 and A2. Superposition of the two models reveal that the overall structural motifs are similarly aligned, except for the C-terminal peptides diverging between the A1 and A2 models, possibly explaining their functional difference. In conclusion, using various in vitro and in vivo models, here we show that subtype A2 has an inherently higher replication phenotype due to polymorphisms in HBx that result in possible differences in structure and expression level of the two subtype HBx proteins. This genotypic difference potentially explains the reported clinical differences between the two subtypes as well as providing a previously unrecognized association between viral sequence variations and clinical manifestations of HBV infection in humans.

HBxAg promotes HBV replication and EGFR activation in human placental trophoblasts

Hepatitis B virus (HBV) infection is a global epidemic. The main transmission route of chronic HBV infection is from mother to child, yet the mechanisms underlying HBV intrauterine infection remain unclear. In the present study, the effect and the mechanism underlying hepatitis B virus X antigen (HBxAg) on HBV replication and EGFR activation in trophoblasts was investigated. Serum samples from pregnant women with HBV infection were used to infect trophoblasts and HBxAg expression was detected using ELISA. HBV plasmids carrying either full length hepatitis B virus X (HBx) or HBx with a deletion mutation (ΔHBx) were transfected into trophoblasts and expression levels of HBV DNA, hepatitis B e-antigen and pregenomic (pg)RNA, and structural maintenance of chromosomes (Smc) 5/6 were assessed. The association between HBx and EGFR promoters was characterized using a luciferase reporter assay and EGFR/PI3K/phosphorylated (p)-AKT expression and apoptosis rate were also monitored. The results of the present study indicated that HBxAg expression increased with the increasing titre of HBV DNA (P<0.05). Compared with the wild-type group, the amount of HBV DNA in the supernatant and cells was significantly reduced (P<0.05) in the ΔHBx group and the intracellular HBeAg and pgRNA levels were also significantly decreased (P<0.05). In addition, Smc5/6 expression was also significantly decreased (P<0.05) when the intracellular HBx protein was expressed compared with mock-transfected cells. Co-transfection of HBx and EGFR promoter plasmids in JEG-3 and HTR-8 cells significantly elevated EGFR promoter driven luciferase expression relative to the control group (P<0.01). In EGFR overexpressing cells, the expression of PI3K/p-AKT was significantly increased, whereas the apoptosis rate was significantly decreased (P<0.05). These results were reversed in the EGFR-knockdown group. In conclusion, the present study demonstrated that HBx promotes HBV replication in trophoblasts via downregulation of Smc5/6, activates the EGFR promoter and inhibits trophoblast apoptosis via the PI3K/p-AKT downstream signalling pathway, thereby increasing the risk of HBV intrauterine infection.

Hepatitis B virus-induced hepatocarcinogenesis: A virological and oncological perspective

Hepatitis B virus (HBV) is a partially double-stranded DNA virus associated with hepatocellular carcinoma (HCC). The viral integration into the hepatocyte genome, the viral protein-induced oncogenesis, the increased hepatocyte turnover and the chronic inflammatory response towards HBV are all hypothesized mechanisms for the development of HCC. The fact that HBV infection and HCC prevalence show different correlations in various regions of the world indicates that there may be virus-independent phenomena for cancer development in these regions. From this point of view, it is important to review our knowledge and to examine the relationship between HBV and HCC in the light of current data. In this article, we investigate the relationship between HBV and HCC by presenting epidemiological and microbiological data, accompanied by the principles of viral oncogenesis.

Host Transcription Factors in Hepatitis B Virus RNA Synthesis

The hepatitis B virus (HBV) chronically infects over 250 million people worldwide and is one of the leading causes of liver cancer and hepatocellular carcinoma. HBV persistence is due in part to the highly stable HBV minichromosome or HBV covalently closed circular DNA (cccDNA) that resides in the nucleus. As HBV replication requires the help of host transcription factors to replicate, focusing on host protein–HBV genome interactions may reveal insights into new drug targets against cccDNA. The structural details on such complexes, however, remain poorly defined. In this review, the current literature regarding host transcription factors’ interactions with HBV cccDNA is discussed.

Analysis of fitness differences of hepatitis B virus genotypes D and F using a cotransfection assay

Hepatitis B virus (HBV) circulates as a collection of genetically related variants that evolve throughout the chronic infection. Those viral variants that have the greatest fitness are fixed. We recently showed different fitness for HBV variants involved in two epidemiological situations. To understand these fitness differences better, we determined the levels of extracellular HBV DNA, the synthesis of HBV DNA intermediates, and the expression of HBeAg and HBsAg in transfection and cotransfection assays. Our results show that for the subgenotype (sgt) D1, which has an 8-nucleotide deletion (sgtD1del) and exhibits lower fitness, the levels of extracellular DNA and intracellular replicative intermediates were much lower than with sgtD1wt or sgtD1mut (G1896A), which had higher fitness. In addition, in the cotransfection assay, sgtD1del inhibited sgtD1mut but not sgtD1wt replication. We also found that sgtF1b, which exhibits higher fitness, produces significantly higher levels of both extracellular DNA and intracellular replicative intermediates than does the lower-fitness sgtF4. These results demonstrate a relationship between fitness and the replicative ability of the HBV genome in the transfection assay. In addition, the data obtained by cotransfecting cells with sgtD1del and sgtD1mut provide new information about the impact of simultaneous replication of two viral variants in the same cell system on HBV replication.