Adenoviruses are non-enveloped viruses (no outer lipid bilayer) with an icosahedral (20-sided, geometric) capsid and 26-48kbp double stranded DNA. A genome of this size can theoretically carry 22-40 genes, though it is believed a maximum 7500-800o bp of “foreign” DNA can be introduced. In the first-generation vectors, the E1 region was removed to make room for the therapeutic transgene and to prohibit viral replication. Still, antigen-dependent immune responses impaired gene expression. Second- and third-generation adenoviral vectors introduced deletions of various E1, E2, and E4 genes, because viral proteins encoded by these DNA sequences were shown to induce most of the host immune response.
Adenoviruses are named after the adenoids – tonsils (similar to lymph nodes) at the back of the nasal cavity, (different from classic tonsils visible at the back of the mouth). Adenoviruses can be targeted to specific cell types by engineering viral surface proteins. More than 50 distinct adenoviral serotypes (same “species”, different surface proteins) are known and infection manifests predominantly as respiratory disease, conjunctivitis, gastroenteritis. When adenoviral vectors are given intravenously, most of the virus accumulates in the liver but they show tropism for liver, skeletal muscle, heart, brain, lung, pancreas, and tumor tissue in preclinical models. Adenoviruses cause about 5% of common colds but should not be confused with rhinovirus, the primary cause of common cold. Rhinovirus is a type of picornavirus and not an adenovirus.
Because adenovirus is a double-stranded DNA virus, it usually must enter the host nucleus to replicate. Adenovirus relies on host cell polymerases and is, therefore, highly dependent on the cell cycle because the cell’s polymerases are usually active only during division. The virus may therefore induce the cell to divide, which can lead to cancer.
Entry of adenoviruses into the host cell involves two sets of interactions between the virus and the host cell: 1) knob protein and 2) penton protein. Most of the action occurs at the virus’s vertices (icosahedron corners).
First, the knob domain of the fiber protein binds to a cell receptor. The two established receptors are: CD46 for group B human adenovirus serotypes and coxsackievirus adenovirus receptor (CAR) for all other serotypes.Next, the penton base interacts with αv integrin on the cell surface. Binding of αv integrin stimulates actin polymerization and clathrin-mediated endocytosis. The virus-containing endosome acidifies as it approaches the lysosome. The drop in pH causes some capsid components to disband, including pentons which destroy the endosome and release the virion into the cytoplasm. With the help of cellular microtubules, the virus is transported to the nuclear pore complex where the particle fully disassembles. Viral DNA is released and enters the nucleus through the nuclear pore. Once in the nucleus, the DNA associates with histone molecules and new virus particles can be generated. Adenovirus DNA does not integrate into the host genome, so there is little concern for disruption or oncogenesis. However, the DNA is eventually discarded by the cell, resulting in loss of effect. Non-integration limits use of adenovirus to short-term therapies like treating cancer and prevents use in chronic diseases like cystic fibrosis.
A diagram of the process:
Early progress (circa 2001) for adenovirus:
For a few years, adenovirus (not to be confused with adeno-associated virus, AAV, which serves as the basis for Uniqure’s Glybera) was a popular gene therapy vector because it affects both replicating and non-replicating cells, accommodates large transgenes, and codes for proteins without integrating into the host cell genome. As discussed above, adenovirus efforts largely focused on cancer. The most successful efforts delivered wild type p53 to increase sensitivity to chemotherapy in chemo-resistant tumors having mutated p53. By 2001, this strategy was in Phase III clinical trials for NSCLC (non-small cell lung cancer) and women with advanced ovarian cancer. Gene therapy harnessing adenovirus has yet to be approved.