Gene Delivery Using Viral Vectors - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 210

Figure 5.2 Adenovirus morphology.

Knob protein

Hexon

Core
protein

Linear genomic DNA

Terminal protein

Fiber

Penton base

The hexon, penton base, and knobbed fiber are the most important capsid pro-
teins with regards to gene delivery. Hexon is the major protein forming the 20 tri-
angular faces of the viral capsid. The knobbed fiber protrudes from the fiber base
( Fig. 5.2 ).

5.2.4.1 Adenovirus Life Cycle
The adenovirus infection cycle can be clearly divided into two phases. The first, or
“Early,” phase and the “Late” phase. Both the phases are separated by viral DNA
replication. The first, or Early, phase covers the entry of the virus into the host cell
and the entry of the virus genome to the nucleus. The Early phase is defined by the
synthesis of viral RNA from the input template before the onset of viral DNA rep-
lication. Selective transcription and translation of the early genes sets the stage for
replication of the virus and transcription/translation of the “Late” genes. The entire
Early phase duration is 6-8 h. The Late phase is involved in making gene products
that are related to production and assembly of capsid proteins. This phase, in which
the late gene products are generated in the permissive cell, is normally much more
rapid, yielding the virus in another 4-6 h. Transcription of late genes occurs from the
major late promoter. The life cycle of the adenovirus vector is shown in Fig. 5.3 .
Events:

1. Adsorption, penetration, and nuclear localization of genome
2. Early transcription
3. DNA replication
4. Late transcription
5. Virus assembly
6. Induction of S phase, apoptosis blocked, cellular mRNA transport blocked, host cell trans-
lation blocked
7. Disruption of cytoskeleton
8. Interferon antagonism
9. Major histocompatibility complex (MHC) downregulation

5.2.4.2 Adsorption and Entry into the Cell
The adsorption of the virus to target cell receptors involves high-affinity binding
via the knob portion of the fiber. The prime receptor for human adenovirus serotype 5