SMC5/6-Mediated Transcriptional Regulation of Hepatitis B Virus and Its Therapeutic Potential

Abstract

Cells have developed various mechanisms to counteract viral infections. In an evolutionary arms race, cells mobilize cellular restriction factors to fight off viruses, targeted by viral factors to facilitate their own replication. The hepatitis B virus (HBV) is a small dsDNA virus that causes acute and chronic infections of the liver. Its genome persists in the nuclei of infected hepatocytes as a covalently closed circular DNA (cccDNA) minichromosome, thus building up an episomal persistence reservoir. The chromosomal maintenance complex SMC5/6 acts as a restriction factor hindering cccDNA transcription, whereas the viral regulatory protein HBx targets SMC5/6 for proteasomal degradation, thus relieving transcriptional suppression of the HBV minichromosome. To date, no curative therapies are available for chronic HBV carriers. Knowledge of the factors regulating the cccDNA and the development of therapies involving silencing the minichromosome or specifically interfering with the HBx-SMC5/6 axis holds promise in achieving sustained viral control. Here, we summarize the current knowledge of the mechanism of SMC5/6-mediated HBV restriction. We also give an overview of SMC5/6 cellular functions and how this compares to the restriction of other DNA viruses. We further discuss the therapeutic potential of available and investigational drugs interfering with the HBx-SMC5/6 axis.

Keywords: HBx; NSE; SMC5/6; antiviral therapy; cccDNA; chronic hepatitis B; hepatitis B virus; interferon; siRNA; transcription.

Figure 1

Transcriptional status of the cccDNA minichromosome in the presence or absence of HBx. This proposed model combines established and emerging concepts. (A) In the presence of HBx, the cccDNA is actively transcribed and associates with histone-acetyltransferases (PCAF (P300/CBP-associated factor), CBP (CREB-binding-protein), and p300) and active histone marks, including acetylated H3/H4 and H3K4me3. HBx targets SMC5/6 for ubiquitylation by the HBx-DDB1 E3 ligase complex and subsequent degradation by the proteasome. The mechanism of the DDB1-HBx-SMC5/6 interaction awaits further elucidation. (B) In the absence of HBx, either achieved through therapeutic intervention or infection with an HBx-deficient virus, the cccDNA is transcriptionally repressed and associates with transcriptional repressors, such as HP1 (heterochromatin protein 1), HMGB1 (High mobility group box 1), SETDB1 (Histone-lysine N-methyltransferase), HDAC1 (Histone Deacetylase 1), as well as the heterochromatin mark H3K9me3. As SMC5/6 is not degraded, it binds the cccDNA and induces transcriptional silencing. The specific recognition of the cccDNA and the orchestration of the subsequent steps, especially the involvement of epigenetic factors/histone PTMs, still need to be fully understood. Whereas the localization of SMC5/6 to ND10 was shown in several studies, the association with SLF2/SIMC1, which might help locate the host restriction factor to highly SUMOylated regions inside ND10, needs to be confirmed in the setting of HBV.

Figure 2

Schematic presentation of therapeutic targets in the DDB1-HBx-SMC5/6-axis. Hindering HBx production with siRNA (small interfering RNA) or ASOs (antisense oligonucleotides), lowering its stability (Dicoumerol, Rapamycin, Pevonedistat), directly inhibiting its function (HBx-specific antibodies) or interfering with the binding of HBx to DDB1 (Nitazoxanide) inhibits HBx-DDB1 E3 ligase complex-mediated degradation of SMC5/6 via the proteasome. The exact mechanism of the anti-HBV activity of interferons is not fully understood but likely involves transcriptional repression or direct HBx inhibition.