The HBV variant CpF97L supports the secretion of pgRNA-containing virions at a level much greater than WT HBV

TL;DR

A specific HBV variant, CpF97L, secretes more pgRNA-containing virions than the wild-type virus, which could impact the accuracy of cccDNA reservoir measurements in patients.

Contribution

The study reveals that the HBV CpF97L variant can secrete pgRNA-containing virions at high levels, challenging assumptions about RNA virion secretion in HBV.

Findings

HBV CpF97L secretes high levels of pgRNA-containing virions when reverse transcription is inhibited.

pgRNA virions were detected in both Huh7 and HepG2 cell lines, showing cell line independence.

pgRNA virions were secreted alongside dsDNA virions, suggesting constitutive capsid maturation in CpF97L.

Abstract

Viruses in the Hepadnaviridae family, including hepatitis B virus (HBV), replicate their double-stranded DNA (dsDNA) genomes through reverse transcription of an RNA intermediate, the pregenomic RNA (pgRNA), in the viral capsid within an infected cell. In the cell, capsids containing pgRNA, single-stranded DNA (ssDNA), and dsDNA are present. However, capsids containing dsDNA (referred to as mature genomes) are preferentially secreted in virions while only small amounts of capsids with pgRNA and ssDNA (referred to as immature genomes) are enveloped and secreted. The naturally occurring HBV core protein variant, CpF97L, is an exception; HBV CpF97L secretes high levels of ssDNA-containing virions in addition to dsDNA virions. We asked whether HBV CpF97L is capable of secreting pgRNA-containing virions as well. We found that HBV CpF97L secretes high levels of pgRNA-containing virions compared to wild-type (WT) HBV when reverse transcription was inhibited by entecavir or by the Y63F change in P protein. We detected pgRNA virions in Huh7 and HepG2 cell lines, indicating that RNA virion secretion was independent of the cell line used in virion propagation. More importantly, pgRNA virions were detected when dsDNA virions were synthesized as well. Our findings suggest that the capsids of CpF97L are constitutively matured, allowing for virions with immature genomes (ssDNA and pgRNA) to be secreted in addition to dsDNA virions.

Finding a cure for hepatitis B is critical, as over 250 million people live with a hepatitis B virus (HBV) infection. HBV replicates through a series of nascent RNA and DNA intermediates in capsids, resulting in the secretion of a DNA virion to propagate the infection. HBV infections have been managed with nucleos(t)ide analogs (NAs), which terminate DNA synthesis during replication. During NA treatment, DNA levels plummet, RNA-containing capsids accumulate in infected cells and are secreted, albeit inefficiently, as virions. RNA virions in serum have therefore been proposed to be used as an indicator for covalently closed circular DNA (cccDNA) (HBV’s minichromosome in hepatocytes) to determine patients who can be withdrawn from NAs without virological rebound. However, it is unknown if RNA virions are efficiently secreted by the frequent HBV variants that secrete high levels of ssDNA-containing virions, as these will lead to an erroneous overestimate of the cccDNA reservoir; hence, the need for our study.

Figures