Novel therapeutic approaches for hepatitis B virus covalently closed circular DNA

Figure 1 Hepatitis B virus genome. Hepatitis B virus is of the Hepadnaviridae family and has a 3.2 kb partially double-stranded relaxed circular DNA (rcDNA) as its genome. The genome consists of four genes: C, P, S and X, which partially overlap. The genes encode core protein, DNA polymerase (reverse transcriptase), surface envelope protein, and X protein.

Figure 2 Life cycle of hepatitis B virus. After hepatitis B virus (HBV) virions infect hepatocytes, relaxed circular DNA (rcDNA) enters the nucleus and is converted to covalently closed circular HBV DNA (cccDNA). HBV cccDNA persists in the nucleus as a mini-chromosome. HBV pregenomic RNA (pgRNA) is transcribed from cccDNA and acts as the viral protein mRNA. HBV pgRNA is co-packaged with reverse transcriptase in capsids and is reverse-transcribed into rcDNA. Nucleocapsids containing rcDNA are released from the host cell as virions.

Figure 3 Clustered regularly interspaced short palindromic repeats/Cas9 system. The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas system consists of a Cas9 nuclease and a single guide RNA (sgRNA), which leads Cas9 to the target site. A 20-nt complementary sequence to the target DNA is positioned at the 5′ end of the sgRNA, which is adjacent to the 5′ part of a proto spacer adjacent motif (PAM) matching the 5′-NGG sequence. The Cas9/sgRNA complex is guided to the target site via RNA-DNA complementary base pairing and induces site-specific double-strand breaks.