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Throughout its year history, the field of gene therapy has been marked by many transitions. It has seen great strides in combating human disease, has given hope to patients and families with limited treatment options, but has also been subject to many setbacks. Treatment of patients with this class of investigational drugs has resulted in severe adverse effects and, even in rare cases, death. At the heart of this dichotomous field are the viral-based vectors, the delivery vehicles that have allowed researchers and clinicians to develop powerful drug platforms, and have radically changed the face of medicine.

Within the past 5 years, the gene therapy field has seen a wave of drugs based on viral vectors that have gained regulatory approval that come in a variety of designs and purposes.

These modalities range from vector-based cancer therapies, to treating monogenic diseases with life-altering outcomes. At present, the three key vector strategies are based on adenoviruses, adeno-associated viruses, and lentiviruses.

They have led the way in preclinical and clinical successes in the past two decades. However, despite these successes, many challenges still limit these approaches from attaining their full potential. To review the viral vector-based gene therapy landscape, we focus on these three highly regarded vector platforms and describe mechanisms of action and their roles in treating human disease.

Gene therapy is the treatment of a genetic disease by the introduction of specific cell function-altering genetic material into a patient. There are two types of vectors: viral and non-viral. Non-viral vectors will not be discussed in this review.

Contemporary viral vector-based gene therapy is achieved by in vivo delivery of the therapeutic gene into the patient by vectors based on retroviruses, adenoviruses Ads or adeno-associated viruses AAVs Fig. Alternatively, a therapeutic transgene can be delivered ex vivo, whereby cells of a patient are extracted and cultured outside of the body.

Cells are then genetically modified by introduction of a therapeutic transgene and are then re-introduced back into the patient. In vivo gene therapy entails the direct administration of vector carrying a therapeutic transgene into the patient.

The first gene therapy trial, at least conceptually, was performed by Dr. Stanfield Rogers, who treated two sisters who had hyperargininemia. Unfortunately, the trial failed to reverse the disease, as the Shope papilloma genome did not encode for arginase production.

In a study that was not formally published, Dr. Although groundbreaking at the time, this first attempt at an ex vivo gene therapy was ultimately a failure. Nonetheless, these two efforts were among several others that endeavored to deliver genetic material to patients, in the hopes of treating disease.

It was not until the early 90s that viral vector gene therapies found clinical success. Several infusions of T cells transformed by a recombinant retrovirus carrying the ADA gene were administered, resulting in what is regarded as the first successful gene therapy in humans. The use of viruses for therapy has long been practiced and actually belongs to a class of viral-based treatments known as virotherapies.

Perhaps, the main reason why earlier viral-based therapies failed to achieve efficacy was due to a lack of full understanding of the viral biology. Now with 40 years of accumulated knowledge on viruses, promising viral vector-based strategies to treat genetic diseases are numerous. Some human diseases even have several effective treatment options to choose from.

Design aspects for these three components are different for each viral vector platform, have unique considerations, and harbor their own strengths and weaknesses. In this review, we will describe three viral vector platforms that have gained wide use for efficacious gene therapy and regulatory approval.

These three strategies are based on Ads, AAVs, and lentiviruses retroviruses , the viruses that a majority of gene therapy vectors are based upon Fig. For each of these vector platforms, we will review their general compositions and their mode of infection, highlight key vector platforms and their biological properties, describe current production strategies in use, feature their uses as commercialized drugs and in clinical trials, and finally discuss challenges and future outlooks.

Ad is a non-enveloped virus that is known to mostly cause infections of the upper respiratory tract but can also infect other organs such as the brain and bladder. It possesses an icosahedral protein capsid that accommodates a to kb linear, double-stranded DNA genome. A packaging signal located at the left arm of the genome is required for viral genome packaging.

The E1A red arrow gene contains four conserved domains CR , each of which interacts with special cellular proteins. The E4 gene encodes 1—7 open reading frames. The major late messenger RNAs L1—L5 mainly encode for virion structural proteins and are derived from a pre-mRNA, which is driven by a major late promoter MLP via alternative splicing and polyadenylation.

L1 encodes for IIIa and 52K. L5 encodes for the fiber gene capsid protein IV. The E1A and E1B regions are deleted and replaced with an expression cassette containing an exogenous promoter and a transgene of interest indicated by the solid red X and yellow arrow. The E3 and E4 regions are usually deleted to accommodate larger insertions and eliminates leaky expression of other viral genes. The E1BK region is deleted solid red X and dashed blue arrow , whereas in some vectors, the E3 region is deleted and replaced with an expression cassette dashed red X and dashed blue arrow.

In some vector designs, the E3 region is deleted and replaced with an expression cassette dashed red X and dashed blue arrow.

These vectors are propagated in the presence of an Ad helper vector. There are 12 penton proteins located at the apex of the icosahedral vertices, giving rise to the protruding fibers.

The V, VII, and X proteins mainly associate with the viral genome and play critical roles in genome replication, condensation, and assembly processes. Two to 3 days after entering the cell nucleus, new virions are assembled and cells lyse to release virions.

Knowledge on the Ad infection pathway is largely based on human Ad5 HAd5. Infection initiates with interaction between the cell surface-localized coxsackievirus-Ad receptor CAR and the distal domain of the virus capsid fiber. Most people carry neutralizing antibodies NAbs against one or more of the prevalent human Ad serotypes, as a result of exposure to Ads from past infections. Some serotypes in species D cause epidemic keratoconjunctivitis, whereas HAd4 from species E causes acute respiratory disease.

Ad vectors have the following advantages: 1 high transduction efficiency, both in quiescent and dividing cells; 2 epichromosomal persistence in the host cell; 3 broad tropism for different tissue targets; and 4 and the availability of scalable production systems.

The major objectives in Ad vector development are to overcome the challenges associated with its widely pre-existing viral immunity among the general population, life-threatening strong innate immune responses to its capsid proteins, and robust adaptive immune responses to de novo synthesized viral and transgene products.

First generation. Removal of the E1A gene results in the inability of recombinant Ad rAd to replicate in the host cell. The first generation of Ad vectors has two main disadvantages as follows: 1 de novo expression of Ad proteins can still activate the host immune response, causing clearance of vector-transduced cells; 38 and 2 possible spontaneous homologous recombination between the vector and engineered E1 region from HEK during genome amplification can generate replication-competent adenovirus.

Second generation. Due to issues with first-generation Ad vectors, researchers developed improved versions by further deleting the other early gene regions E2a, E2b, or E4 , permitting additional space for larger transgene cassettes These new vector designs include temperature-sensitive rAd vectors, generated by ablation of E2A-encoded DNA-binding protein, 39 , 40 deletion of the E2b-encoded DNA polymerase Pol protein, 41 , 42 and deletion of the E4 region.

As a result, transgene expression was substantially prolonged in mice compared to first-generation vectors. Despite the changes, the native Ad late genes that are still retained within the vector genome can trigger host immunogenicity and cellular toxicity.

Third generation. Production of HCAds in cell culture requires an additional adenoviral helper virus HV that is similar in composition to first-generation vectors, but with the distinction that they contain loxP sites inserted to flank the packaging signal. Replication and packaging are permitted by the viral proteins provided by the helper genome.

The engineered Cre in producer cells ensures that only the HCAd genome can be packaged, as the helper-virus genome-packaging signal is excised by Cre-mediated recombination at the loxP sites. However, the main challenge in HCAd production is ensuring that the helper adenovirus is eliminated from vector preparations, which may alter efficacy and safety of HCAd vectors in vivo. Conditionally replicating Ad vectors.

The engineering of tumor-specific gene promoters into the Ad genome can be used to control the initiation of viral replication to create conditionally replicative adenoviral vectors CRAds. The resulting dl and AdD24 vectors both showed high replication potency and selectivity in tumor cells.

It was discovered that innate response to the capsid protein triggered cytokine storm. It is now well-accepted that Ad can trigger strong immune responses in humans, reinforcing safety concerns for their application. The major reason for the lowered efficacies is that deletion of E1BK also causes attenuated viral replication, even in tumor cells in vivo. Ad vectors have seen a rebirth in human gene therapy research. They have been mainly applied towards novel vaccines and cancer therapies.

Ad-mediated genetic vaccines. Immunogenicity is a critical hurdle for viral vector efficacy, but has been exploited in the development of Ad-based vaccines. For example, Ebola vaccines based on Ad vectors showed induction of specific antibody and T-cell responses in clinical trial subjects. At the time of this review, no Ad-based HIV vaccine has been proven successful. At the time that this review was written, SARS-CoV-2 has resulted in more than million infection cases with more than 2 million deaths worldwide.

In response, multiple vaccine strategies have been under development. Thus far, outcomes showed rapid humoral and T-cell responses after 14 days post vaccination in most participants, with no serious adverse events. Anticancer therapy. To date, various approaches using Ad vectors to specifically kill tumor cells have been developed and tested in clinical trials Table 1.

The early generation of Ad vector-based anticancer therapies mainly relied on replication-defective vectors for their immunogenic properties to deliver tumor repressor, cytotoxic, or immune-regulating genes to induce tumor cell death and antitumor immune responses. Delivery of suicide genes. Based on the well-established fact that many tumor types display dysfunction in the p53 tumor repressor pathway, 83 Ad vectors have been engineered to induce p53 expression to cause cell-cycle arrest and apoptosis in tumor cells.

Other applications for Ad vectors in anticancer therapy have been tested in the targeted expression of pro-drug-converting enzymes to achieve tumor cell killing. For example, the enzyme purine nucleoside phosphorylase PNP converts the pro-drug fludarabine monophosphate F-ara-AMP into fluoroadenine, which confers cytotoxicity to proliferating cells. The herpes simplex virus thymidine kinase HSV-TK can convert the pro-drug ganciclovir GCV to a cytotoxic nucleotide to selectively kill dividing cells.

These products cause a blockage of thymidylate synthase and subsequent damage to DNA. Delivery of immuno-regulatory genes.

Ad vectors can also be armed with immune-regulating genes to stimulate antitumor immune responses in the patient. Granulocyte-macrophage colony-stimulating factor GM-CSF is known to induce activation of immune cells to trigger an antitumor response.

 

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