THE ADENOVIRUS AND ITS USE AS A VECTOR

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Structure

In general, viruses are classified by two main features: Enveloped or Non-enveloped. Adenoviruses are a common cause of upper respiratory tract infections and are considered one of the larger virions of the non-enveloped icosahedral viruses measuring at 70-90nm.

The adenovirus family is characterized by its non-enveloped icosahedral shape containing twelve vertices and seven surface proteins (Fields et al.). In reference to Figure 1 below, the adenovirus structure can be examined. The surface coat proteins: fibers, pentons, and hexons make up the protective layer of the virus known as the capsid. The capsid not only protects the virus' genome, but also plays a primary role in host cell attachment (Tortora et al.).


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In addition to the importance of the viral surface coat, the virus' genome is its most sacred component. The adenoviruses are linear double stranded DNA viruses (Fields et al.). Therefore, during multiplication the virus is not dependent upon a reverse transcriptase to create its own DNA. The virus' genome codes for approximately thirty proteins and begins with a two stage transcription: Early and Late Transcription (Russell et al.). Due to the adenovirus' extremely virulent nature it can be transmitted between individuals through direct and indirect contact. These transmissions include hand-to-eye contact, fecal/oral contact, venereal contact, and respiratory droplet transmittance. The virus is also capable of airborne and waterborne transmission (Russell et al.). Person-to-person contact is the most common mechanism of transmission. The virus is especially common in young children through fecal-oral contact. Once viral transmission is successful, the virus will incubate within the individual between 2 and 9 days before clinical symptoms are expressed (Russell et al.).

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Adenovirus Basic Replication Cycle

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As seen in the diagram below, there are four stages to the adenovirus replication cycle. First the cell binds to surface receptors on the target cell, and enter the cell via endocytosis. The viron then sheds its endosome coat and enters into the nucleus. In the nucleus the viron releases its genes, which are then read by the machinery inside of the host cell. The adenovirus has both early and late genes. The early genes code for non-structural, regulatory proteins, while the late genes code for replication substrates and machinery. After transcription, these genes are translated, the new progeny are assembled, and then they exit through cell lysis. (Wagner et al.)

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Transmission of the Adenovirus

Due to the adenovirus' extremely virulent nature it can be transmitted between individuals through direct and indirect contact. These transmissions include hand-to-eye contact, fecal/oral contact, venereal contact, and respiratory droplet transmittance. The virus is also capable of airborne and waterborne transmission (Russell et al.). Person-to-person contact is the most common mechanism of transmission. The virus is especially common in young children through fecal-oral contact. Once viral transmission is successful, the virus will incubate within the individual between 2 and 9 days before clinical symptoms are expressed (Russell et al.).

Currently, over 100 serotypes of the adenovirus family have been identified. Across the United States the Ad4 viral strain is one of the latest emerging infectious strains affecting a broad age range of individuals. The Adenovirus is most commonly found in children and in medical settings. Symptoms diagnosed with the Ad4 strain appear similar to severe flu-like and cold-like symptoms.

Diseases Caused by the Adenovirus

When Adenoviruses infect humans, they normally result in an upper respiratory infection. These infections are often seen as conjunctivitis, tonsillitis (strep throat), an ear infection or croup. Sometimes in young children these infections can escalate into pneumonia or bronchiolitis. Additionally, the adenovirus can occasionally cause gastroenteritis (stomach flu). Vary rarely, the adenovirus can cause cystitis (a form of urinary tract infection) (Wagner et al.)



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The majority of people who are infected by the adenovirus are able to recover by themselves. Those at increased risk, such as the young, old, and those with a compromised immune system can die from this virus (Wagner et al.).

Adenoviruses Used As Vectors For Gene Therapy in the Fight against Cancer

By Nicole Luke

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Abstract

The Adenovirus is a virus that infects humans, and normally causes upper respiratory tract infections. This is a non-enveloped icosahedral virus that contains many sought after characteristics for its use as a gene therapy vector. This virus does not incorporate its genes into the host genome, it can infect a large range of human cells, it can hold large segments of DNA, and its DNA can be easily manipulated using recombinant techniques. Gene therapy works by manipulating viruses to contain “good genes” in which they can transport to the cells to code for needed proteins, hormones, or enzymes. This technique of gene therapy has found to be beneficial in the fight against cancer. Researchers are currently in the process of developing this technique to target, suppress, and hopefully cure cancer.

Inroduction

The fight against cancer has been an ongoing uphill battle for decades. The concept of gene therapy offers many options in terms of how to fight this increasingly prevalent illness. In 2008, there were 1,437,180 cases of cancer reported in the United States. Additionally, in 2008 there were 565,650 deaths reported due to cancer in the United States. This is an area of concern for all citizens of the world, no matter your ethnicity, sex, or age (American Cancer Society).

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Discussion

The process of gene therapy begins with the removal of the viral genes. These genes are then replaced with the genes that the host needs. These genes can code for a variety of different proteins, enzymes, hormones, etc. The virus is then introduced into the host, where they will bind to surface receptor proteins. The virus then enters into the cell, and inserts its DNA into the nucleus. This DNA is then transcripted using the host machinery, before being translated. These translated genes can then provide the host with what it needs (Wagner et al.).

The adenovirus has numerous characteristics that make it a good vector for gene therapy. First, the adenovirus does not incorporate its genes into the host genome. Other prospective virus vectors, such as the retrovirus, will insert their genes into the host genome. While incorporating the genes is a favorable characteristic because the genes will be passed on when the cell replicates, they are randomly incorporated and can disrupt vital genes, that code for processes such as replication. This can result in cell death, or uncontrolled replication (Vorburger et al.).

Secondly, the adenovirus is ubiquitous, and has been isolated from a large number of different species. There are over 100 known serotypes. Additionally, there are a very large range of cells in the human body in which this virus can infect (Vorburger et al.).

Structurally, the adenovirus is great for gene therapy due to the fact that it has a fairly large capsid and can hold large segments of DNA. Additionally, the genome does not undergo rearrangement at high rate. Therefore, the foreign gene segments are generally kept intact after many rounds of viral replication. And lastly, the DNA is easy to manipulate by scientist, using current recombinant DNA techniques.

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Researchers have uncovered numerous means in which the Adenovirus can be used in gene therapy. These methods include mutagen compensation, molecular chemotherapy, genetic immunopotentiation, the genetic modulation of resistance, and tumor suppression (Baurschmitz et al.).

Mutagen compensation works by replacing or inactivating oncogenes. Molecular chemotherapy is used to cause the suicide of the target tumor cell. Genetic immunopotentiation, also known as immunotherapy, is used to modify the tumor or immune cells in order to amplify the immunological recognition of the neoplastic cells. Immunotherapy is the most used form of this vector in gene therapy as seen in the figure below. Genetic modulation of resistance does as the name sounds, and modifies the sensitivity or resistance of the target cells to chemotherapy treatment. Lastly, tumor suppressors (Antiangiogenic gene therapy), target the development of new vessels in tumor tissues in order to inhibit its growth (Baurschmitz et al.).

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Percentage of the Use of the Adenovirus in Gene Therapy (McCormick)

The majority of clinical trials done using the adenovirus in gene therapy have been phase 1. Phase 1 trials are used to determine safety, feasibility, and toxicity of the process. These trials have set the stage for future work with the next generation of Adenovectors that will show less stimulation of the host immune system and can be selectively targeted to specific tissues. Hopefully, with advancements in this research, we will soon be able to either improve the lives of cancer patients, or even cure cancer (McCormick).

Adenoviruses use as a Vector for Vaccines

By Sara Specht

Abstract

Viruses are termed as obligatory intracellular parasites in that they require a host cell to become active and multiply (Tortora et. al.). The adenovirus was first identified by scientists in tonsils in 1955, and through the years has mutated into more virulent strains now leaving many individuals severely infected (Akusjärvi et al.). There are multiple strains of the Adenovirus, but the most common include the Human Adenovirus 4 and 7 (HA-dv4 and HA-dv7), and the Canine Adenovirus 2 (CA-dv2). Fortunately the virus is not zoonotic, therefore it the canine strain cannot infect a human. In research, the adenovirus serves as a prime vector for recombinant techniques in vaccine development. The Human Adenovirus offers many avenues for vaccine use. The adenovirus as a vector can be used potentially to vaccinate against hepatitis B, HIV and herpes simplex virus. In canines, the live recombinant adenovirus vaccine has already proven positive results in reducing contractility of Canine Infectious Tracehobronchitis.

Introduction

One of the many debates in medical health is the amount of boosters administered throughout one's lifespan. Currently, this issue is more so directed toward our pet's vaccines. Many people fear that we are over-vaccinating ourselves and our pets with each booster given. In additionto this, many vaccines are incredibly expensive to manufacture and administer in foreign countries. As a result, researchers are looking toward a long-lasting vaccine that will be beneficial both economically and medically. Opportunities such as these give scientists the lead toward a vaccine that works against deadly viral pathogens

Discussion

In addition to the innovative use of adenoviruses in gene therapy, they have also been implemented in the vaccine field of therapy. In vitro experiments have shown valuable results for adenoviruses as vectors. Current research has tested the virus in subunit vaccines against infectious agents such as malaria and HIV-1 (Tatsis et al.). The results show promise for the adenovirus vector in vaccines, however due to ongoing research the vaccine is only available for human testing in the military until further notice.

In development of a vaccine, techniques have come a long way since Jenner’s first attempt to vaccinate against smallpox using the scrapings of a cowpox virus lesion (Tortora et al.). Today, a live virus is weakened by altering its growth conditions. Then, a segment of human DNA that codes for antigenic immune responses is extracted and inserted into an adenovirus genome. Following this, the altered viral genome can be inserted into a host cell. Developing a modified live virus vaccine does propose risks of the virus mutating back to its viral form. However, mutation rates are low and the benefits of a life long lasting immunity outweighs the potential disease effects (Tatsis et al.). Figure 2 illustrates a simple schematic of vaccine development.

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Human adenovirus 4 (HAdv-4) was the first serotype to be isolated from man. Studies have shown that HAdv-4 is one of the more prevalent serotypes infecting humans. However, HAdv-4 has responded beautifully to an oral enteric-coated capsule adenovirus vaccine. Now, the military is testing a live recombinant adenovirus vaccine in hopes to develop more economical vaccines that provide a longer lasting immunity (Gray).

In addition to this, the adenovirus has also been found to infect the canine species. The canine adenovirus 2 is thought to be one of the contributors to Infectious Tracheobronchitis “Kennel Cough.” This disease is characterized as an upper respiratory infection that is highly contagious among dogs. Clinical symptoms of Kennel Cough include a dry hacking cough and nasal discharge, in severe cases death may even follow. Vaccination against CAdv-2 is done through a 5-way or 7-way vaccine in which other infectious viral strains are vaccinated against. These vaccines are manufactured in two forms: intranasal or subcutaneous injection.

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References for Gene Therapy

American Cancer Society. Cancer Facts & Figures 2008. Atlanta: American Cancer Society; 2008.

Baurschmitz G.J., Barker S.D., Hemminki A. Adenoviral Gene Therapy for Cancer-from Vectors to Targeted and Replication Competent Agents. International Journal of Oncology. 2002.

McCormick F. Cancer Gene Therapy: Fringe or Cutting Edge? 2001. Nature Reviews Cancer. 1:1:2: 130-142 pp.

Vorburger S.A., Hunt K.K. Adenoviral Gene Therapy. Oncologist. 2002. 7: 46-59 pp.

Wagner E., Hewlett M.J., Bloom D.C., Camerini D. 2008. Basic Virology. Blackwell Publishing., M.A. 452 pp.

References for Adenoviruses and Vaccine Development

Akusjärvi G, Pettersson U, Roberts RJ: Structure and function of the adenovirus-2 genome. In Doerfler W (ed): Adenovirus DNA. Martinus Nijhoff, Boston, 1986.

Fields. Virology, 3rd edition. . Lippincott-Raven: Philadelphia, 1996. pp.2111-2171.

Gray, G.C. Adenovirus Transmission-Worthy of our Attention. J Infect Dis. 194: 871-873, 2006-editorial.

Human Adenoviruses. National Surveillance for Emerging Adenovirus Infections. www.publichealth.uiowa.com.

Johns Hopkins Bloomberg School of Public Health (2005, March 29). Live Recombinant Adenovirus Vaccine Technique Explored. ScienceDaily. Retrieved May 11, 2009, from http://www.sciencedaily.com­ /releases/2005/03/050325150229.htm

Russell KL, Broderic MP, Franklin SE, et al. Transmission dynamics and prospective environmental sampling of adenovirus in a military recruit setting. J Infect Dis 2006; 194: 877–85.

Tatsis, N., Hildegund, C.J. Adenoviruses as Vaccine Vectors. Molecular Therapy (2004) 10, 616–629.

Tortora,G.J., Funke, B.R., Case, C.L.. 2008. Microbiology an introduction. Pearson Education. India. 11, 391, 425 pp