Gene Therapy for Chronic HBV—Can We Eliminate cccDNA?

Bloom, Kristie; Maepa, Mohube Betty; Ely, Abdullah; Arbuthnot, Patrick

Designer nucleases have dominated the anti-HBV gene editing field, with zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR) with CRISPR-associated (Cas) systems all showing antiviral efficacy (Table 1) . Nucleases act by inducing double stranded breaks at a pre-defined target site within the HBV genome ( Figure 1A ). By exploiting the host cells' error-prone non-homologous end joining repair machinery, targeted mutagenesis is realized. HBV cccDNA is a primary candidate for nuclease gene editing, owing to its episomal minichromosome configuration and limited sequence plasticity. The compact viral genome and overlapping reading frames restrict development of escape mutants, despite the low fidelity of the viral reverse transcriptase [35] . Insertions and deletions (Indels) within the viral genome may give rise to aberrant or truncated proteins, which in turn interfere with viral replication.