The year 1953 saw the isolation of adenovirus, which was soon recognized as an invaluable tool for investigating mammalian molecular biology. Several of the distinguishing features of adenovirus have made it the preferred vehicle for gene transfer and transgene expression in mammalian cells. The following presents a short overview covering the biology of adenovirus.
Adenoviruses are associated with a number of disorders, most of which are mild. The pathology is primarily from inflammation and loss of infected epithelial cells. Viruses of subgroup C (serotype 2, 5) cause various respiration infections in confined groups (elderly, military recruits and children).
Adenovirus is a non-enveloped, 80-110 nm diameter virus presenting icosahedral symmetry. Human adenoviruses contain a linear, double stranded DNA genome, with a terminal protein (TP) attached covalently to the 5 termini. The DNA, which has a length of approximately 36,000 bp, is wrapped in a histone-like protein and has inverted terminal repeats (ITRs) of 50-200 bp, which act as origins of replication.
The hexon, penton base, and knobbed fiber, are the most important capsid proteins for gene delivery. Hexon is the major protein forming the 20 triangular faces of the viral capsid. The 240 hexon capsomers in the capsid are trimers, each interacting with six other trimers. The 12 vertices are formed by the penton capsomere, a complex of five copies of the penton base, and three copies of fiber. Each penton capsomere interacts with five hexon capsomeres, one from each of the five faces that converge at the vertex. The knobbed fiber protrudes from the fiber base.
Figure 1: Adenovirus Morphology
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 fibre. The prime receptor for human adenovirus is identical to that for coxsackie B virus and has been named the coxsackie/adenovirus receptor (CAR). After the attachment step, interaction between the penton base and v integrins on the cell surface leads to internalisation of the virus through endocytosis. Once inside the cell, and with help from the penton base, the virus escapes the endosome and translocates to the nuclear pore complex, where the viral DNA is released into the nucleus and transcription begins. Transcription, replication and viral packaging take place in the nucleus of the infected cell.
A complex series of splicing accompanies transcription, and genes are transcribed from both strands. Adenovirus transcription is a two-phase event, early and late, occurring before and after viral DNA replication, respectively. The early transcribed regions are E1, E2, E3 and E4. The E1 gene products can be further subdivided into E1A and E1B. E1 gene products are involved in the replication of the virus. To insure replication deficiency of the virus used as gene delivery tool, and therefore to prevent cell lysis, first generation recombinant adenoviruses are E1 deleted. Once packaged into a complementing cell line, i.e. a cell line that provides the E1 products in trans (e.g. QBI-HEK 293A Cells), viral replication will be enabled.
The E2 region is subdivided into E2A and E2B. These proteins provide the machinery for viral DNA replication and the ensuing transcription of late genes.
Most of the E3 proteins are involved in modulating the immune response of infected cells, a function not essential for viral growth in vitro. Therefore, in addition to being E1 deleted, the first generation adenoviruses are most often E3 deleted (E1/E3).
The gene products encoded by the E4 region (called ORFs 1-6/7) are involved in the metabolism of virus messenger RNA and provide functions that promote virus DNA replication and shut-off of host protein synthesis. Furthermore, E4 products prevent viral DNA concatenation.