No Nukes: A New Way to Treat Hepatitis B

Of the more than 350 million hepatitis B virus (HBV) carriers worldwide, many are unaware of their infection, and a quarter will die of HBV-related complications. Current HBV medications eliminate virus from the blood but don’t really cure the infection, because virus persists in the nucleus of hepatocytes as a latent, covalently closed circular DNA (cccDNA). The holy grail for HBV is a well-tolerated therapy that eliminates all virus, including cccDNA. Now, Lucifora et al . show that interferon-α (IFN-α) can clear HBV cccDNA and delineate a mechanism that paves the way for well-tolerated HBV cures. Current therapies include a family of “nukes”—nucleoside or nucleotide analogs—that effectively clear HBV DNA from the blood but do not touch HBV cccDNA. Treatment is often life-long, and viral resistance has rendered some of these agents virtually obsolete. IFN-α, a nonspecific antiviral cytokine, can treat HBV without the specter of resistance but has numerous limiting side effects. Using primary human hepatocytes and HBV-infected cell cultures, the authors found that IFN-α, but not “nukes,” induced degradation of HBV cccDNA. Screening for similar agents, they found that activators of lymphotoxin β receptor (LTβR) also degraded nuclear cccDNA and suppressed HBV replication. The LTβR antiviral effects persisted for up to 2 weeks, whereas withdrawing “nukes” resulted in a quick flare of HBV replication. After LTβR activation or IFN-α treatment, HBV cccDNA was deaminated (cytosines → uracil), with subsequent uracil base excision and degradation. Using in vitro DNA repair assays, the authors showed that damaged cccDNA can be repaired instead of degraded, but this does not occur in cells or animals. Deamination was determined by means of differential DNA denaturation polymerase chain reaction (3D-PCR) and confirmed in vivo by using humanized HBV-infected mice. The host genome was unaffected by LTβR activation or IFN-α treatment, as measured with 3D-PCR and deep sequencing of several host genes. Thus, host DNA—but not HBV cccDNA—is effectively repaired in response to deamination. The investigators then used expression arrays to identify genes that are regulated by LTβR activation and code for nucleic acid–binding proteins. The top hit was APOBEC3B, one of several cytidine deaminases that have evolved to protect against viral infections. IFN-α treatment separately induced another family member, APOBEC3A. The authors constructed a compelling model in which either APOBEC3A (induced by IFN-α) or APOBEC3B (induced by LTβR activation) forms a complex with the HBV core protein, which leads to deamination, uracil excision, and degradation of the HBV cccDNA. It will be crucial to determine why the host DNA repair machinery does not fix damaged HBV cccDNA. If HBV can co-opt this machinery, it would presumably become resistant to anti-cccDNA therapies. The observation that cccDNA can be eradicated without harming host cell DNA is a positive one for therapeutics discovery, but it remains unclear whether APOBEC mimics will be clinically tolerable. “No side effects in a cell” is a long way from “no side effects in a person.” Still, the new work represents a refreshing new direction for HBV pharmaceutical development. J. Lucifora et al ., Specific and nonhepatotoxic degradation of nuclear hepatitis B virus cccDNA. Science 343 , 1221–1228 (2014). [[Abstract]][1] [1]: http://www.sciencemag.org/content/343/6176/1221.abstract