Gene therapy is still an investigational technique. Viruses are commonly used as vectors to deliver a needed gene to a host (defective) cell. An ideal viral vector is supposed to be genetically stable, non-toxic, and non-immunogenic, to have a high packaging capacity along with other important features.
There are several types of viral vectors. Adenoviruses seemed to be promising ones. Their genetic information is encoded in double-stranded DNA. To engineer adenovirus vectors, the early genes (responsible for the replication of the viral genome and expression of the late genes encoding structural proteins of the capsid) are removed and replaced with a transgene. Adenovirus vectors can hold inserts as long as ~36 kb. The adenovirus DNA is not incorporated into the host DNA. This peculiarity prevents insertional mutagenesis (an undesirable phenomenon observed in gene retrovirus-using therapy, as retroviruses have a specific enzyme, integrase, that can insert the genetic material of the retrovirus into any site in the host genome, sometimes resulting in malignancies; that was the reason of termination of clinical trials when leukemia resulting from insertional mutagenesis was reported in patients who received such gene therapy).
The DNA molecule of an adenovirus vector remains extrachromosomal in the host cell nucleus, but it can be transcribed like any other gene. The only difference is that this transgene is not replicated when the cell divides so the descendants of that cell will not have the transgene. Therefore, adenovirus vector-based therapy requires re-administration as the cell population grows.
The strong immunogenicity of adenoviruses prevents them from being ideal vectors. Most adult patients have been exposed to adenoviruses, as the latter are common pathogens in humans. Hence, the immune system will attack an adenovirus therapeutic vector just like any other adenovirus. The immune system will recognize proteins (antigens) on the adenovirus vector capsid and destruct them before the therapeutic transgene is delivered to the nucleus. Several strategies are employed to “deceive” pre-developed immunity against adenoviruses, such as using non-human adenovirus vectors (e.g., chimpanzee-derived), shielding the adenovirus surface with polymers, or encapsulating vectors into microspheres. Despite the advances in adenovirus vector engineering, they can still trigger the immune response.
In early studies, a number of attempts were made to deliver normal genes to compensate for dysfunctional or deficient ones, which were responsible for several human genetic diseases. Cystic fibrosis is a genetic disease attributed to a mutation in the gene cystic fibrosis transmembrane conductance regulator (CFTR). An adenoviral vector was used to deliver “healthy” CFTR genes to lung tissues. An adenoviral vector was also used to deliver the gene encoding ornithine transcarbamylase, which is a key enzyme in the urea cycle and is responsible for ornithine-transcarbamylase deficiency (X-linked genetic disease of the liver).
These studies faced several challenges, including immunity to adenoviral vectors upon repeated administration of the vector, cellular cytotoxicity, and even oncogenesis. The most serious concerns about the safety of adenovirus vectors were raised in 1999 after Jesse Gelsinger died while participating in a gene therapy trial. Gelsinger suffered from ornithine transcarbamylase deficiency. His liver was unable to metabolize ammonia – a byproduct of protein cleavage. Gelsinger was enrolled in a clinical trial managed by the University of Pennsylvania. He was injected with an adenoviral vector carrying a normal gene. Gelsinger presented with a massive immune response triggered by the viral vector, which led to multiple organ failure and brain death. After his death, all gene therapy trials in the US were suspended. The Gelsinger case was a severe setback. Since then, work using adenovirus vectors has been focused on genetically crippled versions of viruses. The adenoviral immunogenicity and cytotoxicity were suspected to be due to the expression of several viral proteins. The newer generations of adenoviral vectors had these adenoviral genes removed. Nowadays it is even possible to engineer adenovirus vectors by removing almost all viral genes. These are so-called “gutless” vectors. Different adenoviruses vectors were tested to treat parotid salivary dysfunction, varicose ulcer, macular degeneration, angina pectoris/myocardial infarction, pancreatic carcinoma, neuroendocrine carcinoma, breast carcinoma, glioma.
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