HBx function in viral replication
A function related to the cccDNA:
HBV mutants carrying a deletion in the X gene behave differently according to the cell lines used in the experiments. In HuH-7 hepatoma cells, the synthesis of viral proteins, the replicative intermediates and virion export are unaffected (Blum et al. 1992) Rating=2. Instead, HBx is essential for proper viral replication in HepG2 cells (Bouchard et al. 2001) Rating=2 (Leupin et al. 2005) Rating=2. HBx has an augmentation effect on HBV 3.5-kb pre-genomic RNA (pgRNA) and DNA replication intermediate synthesis. As the synthesis of pgRNA is the first regulatory step of HBV replication, transcriptional activation by HBx is linked to viral replication efficiency (Tang et al. 2005) Rating=1. HBx itself is recruited onto the cccDNA HBV minichromosome and modulates pre-genomic RNA (pgRNA) transcription from cccDNA. HBx influences the epigenetic control of nuclear HBV cccDNA function by modulating the recruitment of chromatin modifying enzymes onto the viral minichromosome (Belloni et al. 2009) Rating=1. A study using HepaRG cells confirms that HBx expression is needed for epigenetic modifications of cccDNA
initiating HBV RNA transcription (Lucifora et al.2011) Rating=2.
The interaction of HBx with DDB1 seems essential for HBx role in viral replication (Leupin et al. 2005) Rating=2. Using HBx mutant that cannot bind DDB1 or using DDB1 RNAi, the authors demonstrated the importance of the interaction for the viral replication.
Degradation of host SMC5-6 complex
Adrien Decorsi?re, Henrik Mueller, Pieter C. van Breugel, Fabien Abdul, Laetitia Gerossier, Rudolf K. Beran, Christine M. Livingston, Congrong Niu, Simon P. Fletcher, Olivier Hantz, Michel Strubin
Nature March 17, 2016; 531: 386?389
The results so far strongly suggest a precise function of HBx in relation to the cccDNA minichromosome.
Model 1: HBx directly acts on cccDNA minichromosome accessibility, through epigenetic mechanisms (histone modifications for instance) that would allow the different RNAs to be synthesized. The problems of this model resides in the fact that DHBV does not possess a HBX protein although it replicates its DNA efficiently.
Model 2 (speculative): HBx would be required for the correct cccDNA formation. In that hypothesis, HBx would recruit DDB1 in forming cccDNA to allow correct synthesis. For instance, uracil incorporation is used as weapons against viruses (Sire et al. 2008) and many mammal viruses have develop efficient tools to counteract this uracil incorporation. For example herpes virus simplex 1 (P10186) and Varicella-zoster virus (Q9J3N2) encode a uracil-DNA glycosylase/UNG. In some retroviruses such as mouse mammary virus, Gag-Pro polyprotein possesses a NC-dUTPase (Q9IZT2 ). HIV-1 VPR interacts with DDB1 (Schrofelbauer et al. 2007), and modulates cellular uracil-DNA glycosylase (Chen et al. 2004) but the either positive or negative action of VPR on UNG2 remains controversial (Ahn et al. 2010). In any case, HBx may have a role through DDB1 in avoiding uracil incorporation in the viral genome. Formijn and colleagues generated an in-sillico 3D model of HBx and found that it displays considerable similarity with DNA glycosylases (van Hemert et al. 2011).