Application of CRISPR-Cas9 gene editing to treat HBV

Abstract

An estimated 291 million people worldwide are living with hepatitis B virus (HBV) infection. Patients with chronic HBV infection are at an increased risk of developing liver cirrhosis and hepatocellular carcinoma (HCC), and about 1 million people died of hepatitis B and its related complications every year. Therefore, HBV represents a significant challenge to public health. The current HBV treatment regime mainly relies on nucleoside (acid) analogs (NUCs) and interferons (IFNs). NUCs inhibit the synthesis of viral DNA from genomic RNA by targeting the multifunctional reverse transcriptase. IFNs acts through both direct antiviral and indirect immunomodulatory effects. Due to the presence of covalently closed circular DNA (cccDNA), the template of HBV transcription, which are highly stable in the nucleus of host cells and difficult to be eliminated by licensed therapies, chronic HBV infection can’t be cured virologically. Consistent efforts have to be made to develop a treatment regime that directly targets persistent cccDNA, inactivates or eliminates it to achieve a totally functional cure for chronic hepatitis B. The clustered regularly interspaced palindromic repeats (CRISPR)-Cas system is an acquired immune system found in prokaryotes. With the development of related research, the CRISPR-Cas9 gene editing system is rapidly becoming an efficient and potentially applicable therapeutic tool. The CRISPR-Cas9 system that specifically targets the conserved regions of the HBV genome, not only effectively inhibits viral replication but also is much simpler and more flexible than other gene editing tools, making it a very attractive treatment option. Numerous experimental studies have shown that the CRISPR-Cas9 system is able to disrupt free cccDNA, excise integrated HBV DNA, reduce pre-genomic RNA and a variety of viral proteins in vitro and in vivo . Furthermore, there are no detectable off-target effects and cytotoxic effects in these relevant anti-HBV studies. However, the feasibility and safety of the CRISPR-Cas9 system still face many challenges in clinical applications, including setting up a more ideal animal model, avoiding unintended off-target effects, achieving high efficiency and low immunogenicity in vivo delivery and establishing a platform of verification that adequately assesses possible risks. While many obstacles need to be overcome, the CRISPR-Cas9 system indeed enhances our ability to change the genome and brings new strategies for treating HBV infection. Besides, the CRISPR-Cas9 systems with better specificity continue to be developed, such as well-designed gRNA and new types or versions of Cas. Viral and no-viral vectors based on in vivo delivery methods for treatment continue to be innovated, and their therapeutic potential will continue to increase. In view of the powerful role on virus suppression of CRISPR-Cas9 technology and the lack of effective means of curing HBV, the CRISPR-Cas9 system still has a bright future in clinical research. This article will summarize the existing CRISPR-Cas9 systems in the study of anti-HBV, and try to explore its possible problems and potential solutions.