Hepatitis D

Background

Hepatitis D is caused by the hepatitis delta virus (HDV), a small, spherical, enveloped particle with a 36 nm diameter. Its viral envelope contains host phospholipids and three proteins from hepatitis B virus: the large, medium, and small hepatitis B surface antigens. The genome is negative-sense, single-stranded, closed circular RNA of approximately 1700 nucleotides, making HDV the smallest 'virus' known to infect animals. It has been proposed that HDV may have originated from a class of plant pathogens called viroids. Its nucleotide sequence is about 70% self-complementary, allowing the genome to form a partially double-stranded, rod-like RNA structure.

A Virus That Cannot Stand Alone

Hepatitis delta virus occupies a strange position among the five recognized hepatitis viruses. Rather than being a fully autonomous pathogen, HDV is classified as a satellite — a subviral agent that can only replicate when hepatitis B virus is already present in the host. This dependency shapes every aspect of how the disease spreads. A person can acquire HDV through simultaneous infection alongside HBV, a scenario called coinfection, or by contracting it later while already living with chronic hepatitis B or a carrier state, known as superinfection.

It is this second route that carries the gravest consequences. Superinfection is regarded as the most severe form of viral hepatitis, driving a heightened risk of acute liver failure, accelerating the march toward cirrhosis, and elevating the probability of liver cancer in chronic cases. When HDV and HBV act together, the combined fatality rate reaches roughly twenty percent, the highest among all hepatitis infections. A 2020 estimate places the global burden at approximately forty-eight million people living with the virus, underscoring how widespread this satellite pathogen truly is.

The Smallest Animal Pathogen

At roughly 1,700 nucleotides, the HDV genome holds the distinction of being the smallest known viral genome to infect animals. It exists as a negative-sense, single-stranded, closed circular RNA molecule, a configuration that sets it apart from virtually every other animal virus. The genome carries an unusually high proportion of guanine and cytosine bases, and about seventy percent of its nucleotide sequence is self-complementary. This self-complementarity allows the strand to fold into a partially double-stranded, rod-like structure — a shape that later plays a critical role in how host enzymes process the RNA.

Researchers have speculated that HDV may have descended from viroids, a class of even smaller plant pathogens that carry no protein-coding capacity at all. The mature virion itself is a compact, spherical, enveloped particle measuring about 36 nanometers across. Its outer envelope is studded with three hepatitis B surface proteins — large, medium, and small — along with host-derived phospholipids. Inside, the genome is wrapped by approximately two hundred molecules of hepatitis D antigen, whose central region has been shown to bind RNA directly. Eight distinct species of HDV are recognized within the genus Deltavirus, placed in the broader realm Ribozyviria.

Hijacking the Cell's Replication Machinery

HDV enters liver cells through the same NTCP bile-transporter receptor used by hepatitis B, but it identifies that receptor through the N-terminal domain of the large HBsAg protein, with amino acid residues 9 through 15 forming the actual binding site. Once inside the hepatocyte, the viral nucleocapsid is directed to the nucleus by a signal embedded in HDAg. Here the virus faces a fundamental problem: it encodes no RNA polymerase of its own. Instead, it co-opts the host's cellular polymerases.

Although RNA polymerase II was initially thought to be the sole player, evidence now implicates polymerases I and III as well. The situation with polymerase II is particularly remarkable. Under normal conditions this enzyme reads a DNA template to produce messenger RNA. If HDV truly recruits it, the virus would be the only known animal pathogen to repurpose a DNA-dependent polymerase as an RNA-dependent one.

The polymerases appear to treat the folded, rod-like RNA genome as though it were double-stranded DNA. Three RNA products emerge: circular genomic RNA, circular complementary antigenomic RNA, and a linear polyadenylated antigenomic RNA carrying the HDAg open reading frame. The linear precursors contain an 85-nucleotide ribozyme sequence that self-cleaves the concatemer into individual genome-length monomers, which are then ligated into closed circles.

Two Proteins, One Elegant Editing Trick

HDV produces a single protein, hepatitis D antigen, but in two distinct isoforms that play opposing roles across the infection timeline. The small form, HDAg-S, weighs about 24 kilodaltons and appears early in infection, migrating into the nucleus to support viral replication. The large form, HDAg-L, at roughly 27 kilodaltons, shows up later, suppresses further replication, and is essential for assembling new viral particles. Despite sharing ninety percent identical amino acid sequences, the two differ by nineteen additional residues at the C-terminus of the large variant.

Both are translated from the same open reading frame, which contains a UAG stop codon at position 196. Under default translation this codon halts the ribosome, yielding only the small protein. However, a cellular enzyme called adenosine deaminase acting on RNA, or ADAR, edits that stop codon into a UGG tryptophan codon, allowing translation to continue and producing the large isoform. This single nucleotide-level edit, performed by the host's own machinery, is therefore the switch that governs the balance between viral replication and virion assembly — a regulatory mechanism without which the HDV life cycle could not progress.