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Gene Therapy and Modification as a Therapeutic Strategy for Cancer
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Abstract: 210 words
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Text body: 3070 words
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Abstract
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Genetic therapy and modification is an exciting new paradigm of personalized medicine, allowing
for medical procedures that can target diseases such as cancer in novel ways. Technologies which
involve gene transfer treatments allow for the insertion of foreign DNA to affect or restore protein
expression, and can alter the function of tumor cells. Gene therapy can also be used as a form of
immunotherapy, either by modifying cancer cells to make them better targeted by the immune
system, or by modifying the body’s immune cells to make them more aggressive towards tumours.
Additionally, oncolytic virotherapy uses classes of genetically modified viruses that can
specifically target and interfere with tumor cells. The ongoing development of the CRISPR-Cas9
gene editing tool may also have promise in future therapeutic applications, with the tool being
capable of destroying cancer-causing latent viral infections like HPV from afflicted cells.
Nonetheless, there are still many questions of safety, efficacy, and commercial viability which
remain to be resolved with many gene therapy procedures, as well as the emerging controversy
over the ethical, legal, and moral implications that modifying the genetic content of human beings
will have on society. These concerns must also be confronted and addressed if the benefits
promised by gene therapy are to be properly realized.
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Introduction
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Cancer is one of the most chronic and pressing health issues in the world today, causing
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over 8 million deaths per year worldwide [1]. While existing surgical, radiation, and
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chemotherapeutical procedures have improved outcomes in some cancer types, many cancers still
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do not respond well to treatment [2]. Moreover, the overall prevalence and incidence rates of
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cancer are expected to continue to rise [2,3]. Consequently, there is a great impetus to explore
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novel approaches to treat the different cancers in ways that current standard approaches cannot. In
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the past decade, the emerging field of gene therapy has arisen as a possible avenue to produce
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innovative treatment options against cancer [4].
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Gene therapy focuses on using targeted delivery of genetic material and methods of genetic
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modification to modify cells or tissues in order produce a positive therapeutic outcome [5]. The
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advantage of gene therapy lies in its ability to enable new kinds of medical procedures and a wide
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range of treatment options that were not previously possible. These include the insertion of foreign
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DNA into target cells to affect or restore protein expression, targeting viruses to abnormal cells for
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lysis and death, and even repairing deleterious genetic mutations [5]. Consequently, gene therapy
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may prove to be an essential tool in personalized medicine, as it can design treatments that are
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specific to a patient’s genetic composition. Although many of these techniques are largely
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experimental and/or still in clinical trials, the progress in the field in the last decade has been quite
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strong, with some gene therapy treatments starting to become commercially available. In the
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context of cancer research, recent developments such as cancer vaccines, oncolytic virotherapy,
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and gene transfer treatments are emerging as promising technologies to treat certain cancers with
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a better efficacy and safety profile than current standard treatment modalities [5].
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This review aims to describe the current state of gene therapy research into applications for
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cancer, to highlight the relative advantage and disadvantage they have over existing therapeutic
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options, and to address the limitations currently being faced by the field and future directions
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needed to overcome them.
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Gene Transfer Treatments
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Currently, one of the most widespread and well-established methods of gene therapy is the
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insertion of foreign genes into target cells through a number of different transfer methods,
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collectively called ‘gene transfer treatments’. One common approach is through viral vectors,
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usually belonging to a class of viruses called adenoviruses, which carry and release the therapeutic
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gene into a target tissue [5]. These adenoviruses are advantageous due to the ease in which foreign
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genes can be inserted into their viral DNA, the relatively mild immune response it provokes from
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the host, an exceedingly low rate of integration into the host’s chromosome that decreases the
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possibility of unwanted mutations, and an inability to replicate (replication-incompetent) so that it
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will not continue its lytic cycle spread to other cells [6].
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Indeed, many adenoviral vectors have been developed for use in the treatment of cancer,
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and have generated a remarkable deal of excitement over their therapeutic impact. The drug
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Gendicine, which became the first commercially approved gene therapy treatment in the world in
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2003, is a recombinant adenovirus that contains the gene for the tumour suppressor p53 [7].
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Delivery of Gendicine will overexpress the p53 gene in tumour cells, or restore its activity in
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tumour cells with a mutated and dysfunctional p53 gene [7]. Gendicine was a landmark in the
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history of gene therapy for cancer, particularly squamous cell carcinoma, as it stimulated apoptosis
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in tumours, increased the expression of other tumour suppressor genes, decreased the prevalence
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of multi-drug resistance factors, and reduced vascular growth towards the cancerous tissues—all
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while producing less side effects than conventional chemotherapy and radiotherapy [7]. There are
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other types of viral classes that are available as well, such as retroviruses or adeno-associated
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viruses, and each has their own strengths and effectiveness for different cell types/purposes. For
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example, the drug Rexin-G is a specially designed retrovirus that integrates into the genome of
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pancreatic tumour cells [8]. It carries a modified gene encoding a construct that interferes with
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cyclin G1, thereby causing cell cycle arrest or death [8]. Rexin-G is showing promise for advanced
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pancreatic cancer, including an increased mean survival time by almost 10 months compared to
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standard treatment, and is currently in Phase III trials [8,9].
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However, there are many limitations and hurdles that remain with gene transfer treatments.
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In 2003, attempts to treat the rare disorder X-linked severe combined immune-deficiency (SCID-
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X1) using a retrovirus based agent unfortunately led to the development of T-cell leukemia in one
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patient due to integration of the virus with the patient’s genome that caused genetic instability
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leading to cancer [10]. This highlighted the need to develop vectors which took extraordinary
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precautions to avoid integrating with host DNA in undesired regions, which is why most vectors
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used today are adenoviruses or adeno-associated viruses. Nevertheless, the transition to these kind
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of viruses has brought about its own unique share of problems. It has proven difficult to achieve
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viral infection of a sufficient number of cells in target tissues to produce a clinically meaningful
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response for many types of tumours. It has been observed that many viruses oftentimes end up
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collecting in the liver for unknown reasons, thus reducing efficiency and causing a potentially
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harmful immune response [5]. Further complicating this, since the viruses do not replicate,
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additional viral loads must be injected periodically to sustain the expression of the therapeutic
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gene. However, by the second time that the virus is introduced, the body has already developed an
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acquired immunity to it and neutralizes much of the virus, reducing its efficiency even further [11].
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Currently, the delivery method is the most crucial factor underpinning the success of gene
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transfer treatments. Yet, a remarkable degree of progress is being made in this area lately, with the
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engineering of viral vectors becoming more sophisticated, as well as the emergence of non-viral
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means of transfer, such as the insertion of naked DNA directly into cells [12,13]. Further advances
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in this will significantly improve the ability to safely and precisely alter cellular function in order
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to achieve the desired clinical outcome.
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Immunotherapy
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Gene therapy can also be used to modify a patient’s immune system in order to strengthen
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the response against cancer cells. Those treatments which boost the immune system’s ability to
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better target and destroy cancer cells are referred to as immunotherapy, and have been an aim of
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cancer treatment for more than a century [14]. However, traditional immunotherapy based on
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conventional technologies has proven to be limited, as cancer cells tend to evolve mechanisms
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which help them evade immune detection. A number of different gene therapy techniques are being
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explored as methods to overcome this limitation [5].
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A particularly interesting approach in this area is the development of genetically modified
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‘cancer vaccines’. Unlike conventional vaccines for infectious diseases which utilize agents that
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help the body prevent the illness, cancer vaccines aim to cure or contain cancerous growth by
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delivering material which trains the immune system to better recognize and attack present cancer
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cells [4,15,16]. This material is created by harvesting cancer cells from the patient’s body, and
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genetically modifying them through the addition of genes which produce antigenic and
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immunostimulatory proteins; these are usually genes which encode for cytokines and pro-
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inflammatory molecules [5]. The idea of this technique is essentially to produce modified cellular
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debris from the cancer cells that contains more potent antigenic factors than the endogenous
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tumour antigens. When this is delivered back into the patient, the co-stimulatory effects expressed
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with the modified tumour antigens increases the activity of antigen-presenting cells and cytotoxic
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T lymphocytes, thereby creating a stronger and more aggressive immune response against the
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tumours [16]. Alternatively, a variation of this approach entails delivering the immunostimulatory
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and antigenic protein genes directly to the cancer cells in the body, which increases immune
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recognition of tumour cells and a more localized immune response [5].
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There are a number of genetically modified cancer vaccines being tested in clinical trials,
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and many are showing a great deal of promise. A prominent example is GVAX, a vaccine that
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targets advanced pancreatic cancer and consists of tumour cells modified to express granulocyte-
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macrophage colony-stimulating factor (GM-SCF) [17,18]. The presence of GM-CSF on injected
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cells containing tumour antigens stimulates the release of cytokines at the injection site which
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activate antigen-presenting cells, CD4+, and CD8+ T cells to better recognize the circulating
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tumor-associated antigens and strengthen the tumor-specific immune response [18]. The vaccine
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is currently in Phase II trials, and recent studies have demonstrated a significant increase on patient
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survival for cases of metastatic pancreatic adenocarcinoma as compared to standard
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chemotherapeutic treatments [19]. Most of the side effects observed have been limited to minor
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injection site reactions or flu-like symptoms [17,19]. Moreover, another advantage of the vaccine
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is that it can be specifically designed for the patient using their tumour cells (called an ‘autologous’
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vaccine), thereby increasing its specificity for the patient’s unique immunological environment.
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While the previously described concept focused on modifying the cancer cells to induce a
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more potent immune response, another (and more unique) strategy is to modify the body’s immune
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cells directly to make them more aggressive towards tumour cells [20]. This is done by extracting
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and culturing lymphocytes from a patient’s body, and then genetically engineering them to
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overexpress potent cytokines like Interleukin-2 (IL-2), or to produce T-cell receptors (TCRs) that
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are specific for antigens on certain tumours [15,20]. A successful example of this is the use of T-
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cells engineered to express TCRs against the NY-ESO-1 antigen in patients with metastatic
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melanoma and metastatic synovial cell sarcoma [21]. Trials have shown that 50-80% of patients
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demonstrate objectively better regression of the cancer compared to conventional chemotherapy,
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with no reported toxic side effects against other tissues [21]. One caveat of this technology however
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is that it is currently incredibly expensive; the average cost for a patient to have their T-cells
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genetically engineered is about USD 75,000.
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Overall, immunotherapy for cancer treatment is rapidly developing, and may be a source
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of a large deal of breakthroughs against cancer in the near future. The main hurdles currently are
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the immense cost and time needed to produce autologous vaccines and genetically engineered T-
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cells. This is improving as more pharmaceutical companies are investing into the area and
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developing increasingly efficient methods of production, as well as through the use of allogenic
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cancer cells (derived from cultured cell lines) to reduce the amount of autologous cells needed [5].
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Oncolytic Virotherapy
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A related concept to immunotherapy is the use of genetically engineered viral particles
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which specifically recognize and attack cancerous tumours, called oncolytic virotherapy [15,22].
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These viruses are unable to replicate in healthy cells, thus providing for a treatment option that can
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attack cancerous cells to the exclusion of normal cells—an advantage over existing chemotherapy
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or radiation therapy [22,23]. This is a relatively new area of research, and largely experimental.
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Nevertheless, many potent viruses have been designed which have performed very well in clinical
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trials, and some have been approved onto the market. The oncolytic virus Oncorine used in
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nasopharyngeal cancer, for example, is a type of adenovirus that has been engineered to lack the
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E1B protein, which is responsible for deactivating the p53 protein in the host cell [23]. The tumour
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suppressor p53 plays a crucial role in the host cell’s ability to destroy the virus, and without viral
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E1B to deactivate the host’s p53 defense, the host’s normal cells will be able to clear the Oncorine
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infection. However, many cancerous cells have mutations in their p53 gene which renders the
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protein dysfunctional (a main cause of neoplastic proliferation), and so the Oncorine virus can
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replicate in them and cause toxicity/death to the exclusion of normal cells [23]. Phase III trials of
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the drug showed a response rate of 80% for head and neck tumours, which was double the rate of
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40% in patients given standard chemotherapy, and other studies have consistently demonstrated a
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good safety profile with only flu-like side effects [24].
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There are a number of important limitations currently facing this field. Firstly, the classes
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of viruses from which these oncolytic agents are derived from are fairly common in nature, and
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many humans have been exposed to and developed acquired immunity against them. Some
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possible solutions include the use of immunosuppressants to temporarily block the immune
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reaction, or using carrier cells to deliver the viruses directly to the tumour [23]. Moreover, for the
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virus to be able to effectively reduce a tumour, the rate of viral replication in infected cells must
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be greater than the growth rate of the uninfected cancerous cells. As such, this approach may not
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be suitable for very large or fast-growing tumours. Nevertheless, the maturation of this tool will
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allow for highly-specific treatments against many other forms of cancer without compromising
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healthy tissue, and so is a promising cancer treatment of the future [15].
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CRISPR/Cas9
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There is major leap forward currently underway brought by the advent of the CRISPR-
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Cas9 tool that promises to revolutionize the entire field of gene therapy. This technology allows
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for precise modification of DNA sequences in an efficient and simple manner, and may offer
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several notable therapeutic uses. It is one of the fastest growing areas of gene therapy research,
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generating a fair deal of excitement—and controversy [25].
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In the context of cancer research, the tool is also showing remarkable success against viral
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infections which are implicated with the development of cancer. A recent study programmed a
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Cas9 construct to specifically target and cleave the E6 oncogene of human papilloma virus (HPV)
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in cervical cancer cells, and found a substantial reduction in HPV viral-load and restoration of
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normal apoptotic genes [26]. Administration of a similar construct in HPV infected mice with
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cervical cancer found a significant reduction in tumour size [27]. This is not without limitations
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however, as viruses have the capacity to quickly evolve resistance against CRISPR-Cas9
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constructs, indicating the need for more research to minimize this resistance [28]. More generally,
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the biggest barrier to success currently is the difficulty in creating delivery methods which can
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modify a sufficient number of cells to produce clinical benefits. This is a decidedly active area of
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CRISPR research, and advances here are required before the tool can ever be used clinically [25].
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The ease and accuracy of DNA modification allowed by the tool can also be used to repair
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deleterious mutations or replace sections of DNA with any desired sequence [25,29,30].
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Accordingly, the possibilities of restoring mutations accumulated in cancerous cells or fixing
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alleles associated with increased cancer risk have been suggested as ways to control the
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development of disease [25,29,30]. As a result, ethical, moral, and legal questions result from the
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ability to modify the genetic content of human beings [29–32]. Current issues include the extent
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to which traits and parts of the genome should be allowed for modification, the consequences of
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socioeconomic access disparities on creating biological inequalities, and the question of whether
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human embryos can be modified as a means to cure genetic components of disease [29–32]. The
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capacity to “improve” humanity has also raised the specter of eugenics, which has provoked
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considerable controversy as well as opposition from several religious, philosophical, and legal
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perspectives [31,32]. The developers of CRISPR-Cas9 have even requested a ban on all attempts
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at human germline modification until society can have a discussion about its consequences [30].
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Conclusion
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Genetic therapy and modification is an exciting new paradigm of personalized medicine,
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allowing for medical procedures that can target diseases such as cancer in novel ways. The
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development of gene transfer treatments, immunotherapy, oncolytic virotherapy, and direct gene
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editing are emerging as strong therapeutic applications (Table 1). Nonetheless, there are still many
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questions of safety, efficacy, and commercial viability which remain to be resolved, as well as the
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emerging controversy over the ethical, legal, and moral implications that human genetic
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modification will have on society. These issues must be confronted and addressed through prudent
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solutions and regulatory/legal frameworks before gene therapy and genome modification can
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become widely available treatment modalities for human diseases [32].
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Table 1. Summary outlining the mechanism, advantages, and limitations of each of the classes of gene
therapy modalities for cancer treatment.
Class
Mechanism
Advantages
Limitations
Gene Transfer
Treatments
Insertion of foreign genes
into target cells, mainly
through viral based
vectors. Other non-viral
delivery methods are
being developed as well.
Can alter tumor cell function,
restore apoptotic or tumour
suppression pathways, and
enable targeted disruption of
specific types of cancer cells.
Immunotherapy
Modifies cancer cells to
produce antigenic and
immunostimulatory
molecules, or to modify
immune cells to express
cytokines and T-Cell
Receptors against specific
tumor antigens.
Produces genetically
modified viruses which
target and attack tumour
cells preferentially over
normal cells.
Creates a more aggressive and
targeted immune response
toward tumours, Can create
‘cancer vaccines’ that are
personalized for the patients
tumour type and immunological
environment.
There are difficulties with
achieving efficient transfer
methods, host immune
responses to many viral
vectors, and concerns about
safety due to increased risks of
mutations.
Very time-intensive and
expensive; some cost
estimates exceed USD 75,000.
Oncolytic
Virotherapy
Genome
Modification
Tools like CRISPR/Cas9
can be used to edit or
replace sections of the
patient’s genome in a
highly accurate and
practical manner.
Allows for treatment directed
specifically against cancer cells
with minimal side effects. Also
enables targeting of certain
cancers that do not respond to
well to standard therapy.
Can remove latent cancercausing viral infections such as
HPV. Also, it is able to repair
deleterious mutations, restore
tumour suppression and
apoptosis, and change alleles
which increase cancer risk.
Host can develop immune
response against the viruses.
Also not efficacious against
fast-growing tumours due to
growth rates that exceeds viral
replication rate.
Current delivery methods of
Cas9 constructs have
difficulties with efficiency,
and oncogenic viruses can
develop resistance against the
constructs. There are also
concerns over the ethics and
effects of permanent genome
modification.
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