Gene Therapy for Chronic HBV—Can We Eliminate cccDNA?

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

Use of gene therapy to disable HBV replication has shown promise and generated considerable interest. Different strategies that have been employed include HBV-specific gene silencing, gene editing, epigenome modification, and nucleic acid-based vaccination. Harnessing the RNA interference (RNAi) pathway is a well-established strategy that has been used extensively to silence genes of HBV. RNAi is an endogenous gene regulatory pathway that can be reprogrammed by exogenous RNA molecules. Feasibility of using RNAi to treat HBV has been established and extensively reviewed elsewhere [24, 25] and will not be covered in detail here. Expressed or synthetic antiviral sequences may mimic primary microRNAs, precursor microRNAs, or mature microRNAs (miRs) [26] [27] [28] [29] . Because of easier large-scale production, dose control and delivery to the cytoplasmic site of action, development of synthetic short interfering RNAs (siRNAs) has advanced rapidly. These simulate mature miRs, and are now in Phase 2 clinical trials. Anti-HBV siRNA formulations with impressive antiviral activity include ARC-520 (NCT02065336), ALN-HBV (NCT02826018), and ARB-001467 (NCT02631096). However, since their effect is transient, repeated administration of siRNAs will be required for long term anti-HBV efficacy. To increase durability of an anti-HBV effect expressed HBV-targeting sequences have been developed. For example, adeno-associated viral vectors (AAVs) have been used to deliver cassettes that express HBV-targeting primary microRNA mimics [30] . Safe and sustained inhibition of HBV replication in HBV transgenic mice indicates that this approach has potential for clinical translation. Strategies targeting host factors have also shown significant promise in reducing cccDNA levels. RNAi-mediated gene silencing of tyrosyl-DNA-phosphodiesterase 2, a DNA repair enzyme thought to mediate polymerase release from the relaxed circular DNA, delays its conversion to cccDNA in HepG2 cells [31] . A recent study has also demonstrated that silencing the expression of pre-mRNA processing factor 31, an HBx-interacting partner, reduces cccDNA levels in HepAD38 cells [32] .