Gene Delivery Using Viral Vectors - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 254

5.6.5 Hepatitis B Virus
Hepatitis B virus (HBV) is the causative agent of chronic hepatitis in humans. It is
the prototype member of the hepadnavirid. Other related members of the hepadnavi-
rus family include woodchuck hepatitis virus (WHV), ground squirrel hepatitis virus
(GSHV), and duck hepatitis B virus (DHBV). Structure of HBV genome is a circu-
lar DNA of only 3.2 kbp in length, and the viral genome is a partially duplex circu-
lar DNA possessing a single-stranded gap region in plus-strand DNA. Even though
HBV has a DNA genome, it can replicate through reverse transcription of an RNA
intermediate and the pgRNA, within the subviral core particle. There are four major
ORFs and all are encoded in the same strand. Inspection of the sequence led to the
recognition of preserved repeat elements that play important roles in the genome
replication. These direct repeats (DR1 and DR2) are located near the 5 end of the
minus and plus DNA strand.
The hepadnaviruses are thought to enter the hepatocytes through receptor-mediated
endocytosis, and upon entry a partial duplex genome is repaired to a covalently closed
circular (CCC) DNA that acts as the template for transcription. Generally, four viral
transcripts are synthesized and transported to cytoplasm, and also the 3.5 kbp RNA,
called pgRNA, serves as a template for reverse transcription as well as for translation
of the core (C) and polymerase (P).
Various HBV vectors are constructed for the gene therapy, for example:
1. Replication-competent plasmid for wild-type HBV
2. R402 plasmid (pCMV-HBV/164)
3. R015 plasmid (pCMV-HBV/30)
4. R063 (pCMV-CPS) helper plasmid
One report shows a novel recombinant hepatitis B viral vector for liver-targeting
by gene therapy. These recombinant HBV particles will also specifically target hepa-
tocytes of liver tissue, and it is thought that the HBV vector will be particularly use-
ful in gene transfer to liver tissue. It is also believed that the tissue specificity of the
HBV vector will enable the HBV vector to be suitable even for in vivo therapy as
well as ex vivo therapy. Novel HBV vectors may be used to deliver genes to the liver
in vivo by a different way that includes infection via circulation or direct injection of
DNA into liver tissue [227] .
5.7 Immune Response to Viral Vectors
Viral vectors are potential platforms used for the treatment of genetic and acquired
diseases. However, just as viruses have evolved to infect cells efficiently, the immune
system has evolved to defend against what it perceives as invading pathogens.
Thus, innate immunity and antigen-specific adaptive immune responses against
vector-derived antigens reduce the efficacy and stability of in vivo gene transfer. The
details of innate and adaptive immune responses of various viral vectors are shown in
Table 5.7 [228] .