Gene therapy at the simplest level is still a diverse and highly researched topic in the field of medicine. With years of research and many more years to come, unpacking the unknown within gene therapy holds wonders for the future. With such praise of gene therapy, the topic must be unpacking and explaining for conventional use. A simple overview of gene therapy encompasses the experimental nature of the technique coupled with the use of genes for possible treatment or prevention of disease. This method is non-surgical and drug free, as it allows the doctor to insert genes into patient’s cells, ultimately treating or prevent sickness [1]. While this technique is still experimental, researchers have taken a plethora of complex and multidimensional approaches in hopes of further advancing the knowledge of gene therapy.

With all the research behind gene therapy, scientists continue to progress. New concepts, experimental methods, and clinical trials of gene therapy show many advancements and data collection for future reference. One experimental method of using gene therapy constitutes the use of a healthy copy of a gene to replace a mutated gene that may currently or in the future cause harm to the body. This method would consist of essentially replacing or substituting the mutated gene for a new, healthy one. Other experimental approaches introduce the concept of the addition or removal of genes. One of the approaches takes a mutated gene, which is functioning improperly, and inactivates it, thus eliminating the function completely. This technique follows the basis of suppression, which can be used for cancer scenarios. Another approach introduces a new gene into the environment of the body to counteract the mutated gene’s function, thus contributing to the prevention and active immunity against disease [1]. The use of another gene to make the disease more relevant to the body is such that the gene stimulates the body. All the methods stated above are experimental methods and concepts of gene therapy that can target multiple locations. Below, Figure 1.1 shows the percentage of diseases that are addressed by gene therapy clinical trials.

From Figure 1.1, it is shown that gene therapy is mostly used for cancer diseases, while a small percent is for experimental purposes with the use of healthy volunteers.

Gene therapy has an overall aim that depends on the approach used. The main goal of gene therapy is to use genes to correct mutations that arise from mechanisms involving DNA within the human body. It also counts for the mutations that occur from pathogenic substances, such as virions and bacterium [2]. From the approaches stated in the previous sections, gene therapy can be divided into three distinct categories based on technique. Each technique has the main purpose of correcting a mutation, but the method of correction yields a variety of mechanisms. In one of the techniques, the mutated gene is inactivated or suppressed. One major cancer treatment is cancer suppressor gene therapy, which refers to when genes function to inhibit cell proliferation and have regulation of development, growth, and differentiation of cells [3]. Suppressing this gene, for this instance, might be better suited to being suppressed rather than replaced by a healthy copy. Just like this technique, other techniques work in different mechanisms. Another example would be substituting mutated genes for healthy genes. This would cause the body to slowly correct the expression of the abnormal gene to expression normal function.

With different treatments incorporating gene therapy, multiple mechanisms have been tested for a variety of diseases the human body may experience. The most notable use of gene therapy is to use a delivery system that delivers gene therapy to a targeted cell. Delivery systems are mechanisms of carriers for a controlled release of a specific therapy that is targeted for a specific cell. As more research is conducted on gene therapy, biomedical engineers are making progress on different delivery systems for different types of gene therapy. Delivery systems essentially control the rate at which the therapy is released and where it is released in the body. Two major categories make up the components of delivery systems. These categories can be classified by the route of delivery, which refers to the mode of medication delivery, and the vehicle with its cargo, which refers to the carrier and the treatment in simple terms [4]. Modes of medication delivery are not complicated. The vehicle is sent into the body by several modes of transport. Many people can relate to swallowing, inhaling, injecting, or rubbing medication on. Currently, there are several vehicles or vectors of delivery systems used in gene therapy. Some of the delivery systems used by gene therapy include retroviral, lentiviral, adenoviral, and non-viral vectors [3].

The vectors of gene therapy are sorted into two categories: viral and nonviral. Both vectors are used in gene therapy, but the each has its own unique purpose. Viral vectors are comprised of altered viruses that have been specifically modified to become carrying vessels. These vectors are modified to carry treatment and release a specific amount of treatment at a certain location. While viral vectors use DNA or RNA as their genetic makeup, modification are generally made to the genetic makeup, due to the possibility of the virus infecting the host cell. Helper DNA from the virus contains genes crucial for viral replication of the DNA that is delivered by a plasmid into the host chromosomal DNA. From there, the DNA is replicated and translated into the final vector for use in gene therapy [5]. Below is a visual representation of how the virus DNA is modified and how the new vector is created.

Figure 1.2 refers to a generic method of engineering a virus into a carrying vector. As more specific viral vectors are used, differences are often presented. In the previous section, the different viral vectors of gene therapy were mentioned. These vectors can now be covered in more depth after discussing the overall concept of viral vectors.

Gene therapy usually has two main ways of delivery: in vivo and ex vivo. Another name for these delivery systems would be “direct delivery” for in vivo and “cell-based delivery” for ex-vivo delivery. In in vivo, the work performed, in this instant the delivery of gene therapy, is performed within the natural condition of the organism or quite literally within the organism. Ex-vivo refers to the opposite, which would be outside the living organism [6]. Ex-vivo can almost be compared to that of invitro. Invitro, refers to the work within a set environment, such as a test tube. Ex-vivo gene therapy would use the concept of invitro, as ex-vivo refers to cells being taken out of the body and then transduced with the gene in a test tube, invitro, and then simply readministered to the body, where the gene expression can start to occur. Below in the figure is a summary of direct and cell-based delivery (Figure 1.3).

Retroviruses are a subsection of viruses, which can place a copy of its own single stranded RNA makeup into the DNA of the virion’s host cell. These viruses can retro transcribe their single-stranded RNA by retro-transcribing it into linear double-stranded DNA, which causes a seamless insertion of the virus and its contents with little immunological repercussions, if any. An advantage of retroviral vectors in gene therapy is the ability to retro-transcribe their RNA into DNA, which when integrated into the host cell’s ...