|Year : 2022 | Volume
| Issue : 1 | Page : 3-6
Gene therapy – Principles and applications
Department of Transfusion Medicine, Max Super Specialty Hospital, New Delhi, India
|Date of Submission||21-Sep-2021|
|Date of Decision||02-Feb-2022|
|Date of Acceptance||02-Feb-2022|
|Date of Web Publication||29-Apr-2022|
Dr. Sangeeta Pathak
Department of Transfusion Medicine, Max Super Specialty Hospital, New Delhi
Source of Support: None, Conflict of Interest: None
The concept of gene therapy dates back to the 1960s and 1970s and it is yet to come of age, as there is a paucity of reliable, long-term data on the safety and efficacy of this novel therapy. It is fast emerging as a promising approach to treat, cure or prevent genetic based diseases by correction of defective genes that are responsible for disease development. Gene therapy is a key treatment strategy for disorders caused by a missing or faulty gene. It involves introduction of functional genes into appropriate cells so that they can produce sufficient amounts of proteins, encoded by the transferred gene or turn off genes that are causing problems and eventually resulting in permanent correction of the disorder. While the concept of gene replacement therapy is mostly suitable for recessive diseases, novel strategies have been suggested that are capable of also treating conditions with a dominant pattern of inheritance. Gene therapy treatments are of broadly two types, Germline gene therapy, & Somatic Germline gene therapy. There are two methods for administering gene therapy -Ex vivo & in-vivo. There are two main parts to gene replacement: Genes and Vectors. Gene therapy is useful for hematologic conditions as Hemoglobinopathies, Hemophilia and Inherited bone marrow failure. Future Prospective of Gene Therapy in Transfusion Medicine ,work is being done to explore ex vivo generation of RBCs utilizing various cell fractions, including hematopeitic progenitor cells from cord blood and adult sources. Cost of therapy and ethical issues are the limiting factors for the use of Gene therapy.
Keywords: Applications, gene therapy, principles, steps in gene therapy Gene alterations
|How to cite this article:|
Pathak S. Gene therapy – Principles and applications. Glob J Transfus Med 2022;7:3-6
| Introduction|| |
Gene therapy is the correction of defective genes that are responsible for disease development. It is a medical field that focuses on the genetic modification of cells to produce a therapeutic effect or the treatment of disease by repairing or reconstructing defective genetic material. In gene therapy, scientists can do one of several things depending on the problem that is present. They can replace a gene that causes a medical problem with one that does not, add genes to help the body to fight or treat disease, or turn off genes that are causing problems.
The concept of gene therapy is to fix a genetic problem at its source. If, for instance, in an (usually recessively) inherited disease a mutation in a certain gene results in the production of a dysfunctional protein, gene therapy could be used to deliver a copy of this gene that does not contain the deleterious mutation and thereby produces a functional protein. This strategy was referred to as gene replacement therapy and is employed to treat inherited retinal diseases.,
Gene therapies are designed to be one-time treatments that target the genetic root cause of diseases.
While the concept of gene replacement therapy is mostly suitable for recessive diseases, novel strategies have been suggested that are capable of also treating conditions with a dominant pattern of inheritance.
| Methodology of Review|| |
The study methodology was by review of available information in Google and PubMed databases by using key words.
| History|| |
The concept of gene therapy arose during the 1960s and 1970s and it is still in its infancy, meaning that there is a paucity of reliable, long-term data on the safety and efficacy of this therapy. The first attempt at modifying human DNA was performed in 1980 by Martin Cline, but the first successful nuclear gene transfer in humans, approved by the National Institutes of Health, was performed in May 1989. The first therapeutic use of gene transfer as well as the first direct insertion of human DNA into the nuclear genome was performed by French Anderson in a trial starting in September 1990. It was thought to be able to cure many genetic disorders or treat them over time. Gene therapy holds the promise to transform medicine and create options for patients who are living with difficult, and even incurable, diseases such as Sickle Cell Diseases, Thalassemia, and Hemophilia.
After extensive research on animals throughout the 1980s and a 1989 bacterial gene tagging trial on humans, the first gene therapy widely accepted as a success was demonstrated in a trial that started on 14 September 1990, when Ashanti DeSilva was treated for severe combined immunodeficiency. The defective gene of the patient's blood cells was replaced by the functional variant. Production of the missing enzyme was temporarily stimulated, but the new cells with functional genes were not generated. The effects were successful, but temporary.
| Types of Gene Alterations|| |
Gene therapy is a key treatment strategy for disorders caused by a missing or faulty gene and may involve three stages, namely, addition, inhibition, editing (functional replacement of a gene).
It involves the introduction of a new gene into the body to target a specific aspect of what causes the disease. Gene addition delivers a new gene into the body to target a specific aspect of what causes disease and can supplement another medication that targets that same aspect to help it work better at treating that disease. Initially, retroviral vectors (ɤ-RVs), such as those based on the Moloney murine leukemia virus, owing to their ability to stably integrate exogenous DNA (gene addition) into target-cell chromosomes were used. Another LV-based experimental gene therapy aiming at enhancing ɤ-globin expression.
Gene inhibition involves deactivating or “silencing” the expression of a mutated or faulty gene that codes for a toxic protein or too much protein.
The mutant gene that is causing disease is edited in order to correct the mutation. This technique aims to repair the altered gene by inserting, removing, or changing specific pieces of a person's existing DNA. Programmable nucleases can modify specific genomic sequences in eukaryotic cells in a highly precise and efficient manner to, for instance, study the function of genes or correct genes associated with human disorders, both acquired and inborn. In the recent past, feeding the progression of safer and more targeted innovative genome editing techniques are under development, specifically those directed at treating hemoglobinopathies. The tailoring of genome editing strategies for the genetic correction of hemoglobinopathies is progressing rapidly and holds a lot of promise.
Gene replacement is a way to treat genetic diseases. It replaces the function of a missing, faulty, or nonworking gene with a new, working copy of the malfunctioning gene. The new gene sits inside the nucleus, or control center, of cells and allows the cells to produce the missing proteins that are critical for the body to function. Viruses are used in gene replacement because of their natural ability to enter into the cells of the body. In gene replacement, scientists alter or reengineer a virus so it can be used as a vector, or delivery vehicle, without causing disease in humans. The vector acts like an envelope in that it holds the new, working copy of the gene and delivers it to where it needs to go in the body. A monogenic disease can be caused by a mutation on one gene. Because gene replacement delivers a new, working gene to the body, it has the potential to help people with monogenic diseases.
The difference between gene addition and gene replacement:
What may be confusing about gene replacement and gene addition (because they sound similar) is that, in gene replacement, a gene that is normally found in the body is added and with gene addition, a gene that is novel to the body is added.
Key components of gene replacement
There are two main parts to gene replacement: Genes and Vectors.
A new, working copy of the human gene is deliver to the cell, allowing the body to make the protein that is missing or in short supply. This new gene is create in a laboratory and is specific to the disease being treated. That is, scientists work to discover which gene needs to be replaced and figure out how to create the new, working gene. This is one reason why a single gene replacement therapy can take many years – even decades – to research and produce.
Vectors are the delivery vehicles used to carry a new, working copy of the missing or nonworking gene into the right cells inside the body. Viruses are used because they are very good at getting inside of cells and carry the new working genes into the nucleus of the cell. Viruses commonly studied for use as viral vectors in gene therapy include retroviruses, adenoviruses, adeno-associated viruses (AAVs), and lentiviruses. AAVs have been approved for use in gene therapy. AAVs are not known to cause illness in people and have demonstrated safety in clinical trials. Other viruses are also being researched as possible vectors for use in gene replacement. They have a reduced risk of genotoxicity and could allow the expression of the introduced gene for a long period.
Steps involved in gene therapy
Creating a working gene
The gene transfer therapy involves creating a working (or functional) gene in the laboratory. The working gene contains the instructions for making a needed protein. Scientists design working genes to meet specific needs. For example, in patients with hemophilia A, the F8 gene is needed to code for the factor VIII protein, which is essential for clotting, and in hemophilia B, an F9 gene is needed to code for the factor IX protein.
Building a therapeutic vector
The working gene now has to be delivered into the body. The shell of the virus is created without the viral DNA and working gene is put inside the empty shell. No longer is a virus, the therapeutic vector designed to deliver the working gene to the cells in the body where it is needed.
As part of gene therapy research, a health-care provider must determine whether a patient is eligible.
Delivering the working gene
Once a patient is determined to be eligible, the gene therapy is ready for administration to evaluate its safety and impact.
A single, one time infusion in an appropriate clinical sting delivers large number of therapeutic vectors into the body. The therapeutic vectors are designed to both protect and guide the working gene toward preferred cells where it can be used to make the needed protein. Once in the body, the new gene is designed to do the work of the gene that is missing or is not functioning properly. The goal is provide instructions for the body to make the protein it needs on its own, and ongoing research is evaluating the risk and impact of introducing the new gene.
Monitoring safety and efficacy
Regular monitoring after gene therapy is important because it allows researchers to understand any risk and what impact the gene transfer is having. As with all medications, responses to gene therapy may vary. How long gene therapy might keep working is being evaluated in ongoing clinical trials with researchers aiming to create a long-lasting therapy.
Researchers are testing several approaches to gene therapy, including:
- Replacing a mutated gene that causes disease with a healthy copy of the gene
- Inactivating, or “knocking out,” a mutated gene that is functioning improperly
- Introducing a new gene into the body to help fight a disease.
Gene therapy products are being studied to treat diseases including cancer, genetic diseases, and infectious diseases.
There are a variety of gene therapy products, including:
- Plasmid DNA: Circular DNA molecules can be genetically engineered to carry therapeutic genes into human cells
- Viral vectors: Viruses have a natural ability to deliver genetic material into cells, and therefore some gene therapy products are derived from viruses. Once viruses have been modified to remove their ability to cause infectious disease, these modified viruses can be used as vectors (vehicles) to carry therapeutic genes into human cells
- Bacterial vectors: Bacteria can be modified to prevent them from causing infectious disease and then used as vectors (vehicles) to carry therapeutic genes into human tissues
- Human gene editing technology: The goals of gene editing are to disrupt harmful genes or to repair mutated genes
- Patient-derived cellular gene therapy products: Cells are removed from the patient, genetically modified (often using a viral vector) and then returned to the patient.
Current gene therapies are the cumulative result of nearly a century of genetics research. There are two types of gene therapy treatment:
| Germline Therapy and Somatic Cell Gene Therapy|| |
Germline therapy involves the modification of the genes inside germ or gamete cells, which include sperm or ova. Once fused together, the zygote divides and passes on the modified gene to all other cells of the body during the development of offspring. In this way, germline therapy alters the genome of future generations to come. Although theoretically, germline therapy could counteract hereditary diseases, jurisdictions in various countries such as Switzerland, Australia, and Germany prohibit the use of germline therapy due to fears on the unknown risks of this therapy and whether it causes any long-term effects in future generations. Germline therapy is also extremely expensive, which further limits its practical use.
Somatic gene therapy
Unlike germline therapy, somatic gene therapy involves the insertion of therapeutic DNA into body cells, rather than germ cells or gametes. The field of somatic gene therapy is surrounded by fewer ethical issues as compared to germline gene therapy. While this may be true, this therapeutic approach remains in the early stages of development.
The first hurdle in somatic gene therapy is the successful incorporation of the new gene into the genome. In fact, integrating the modified gene into the wrong part of the DNA could induce rather than prevent disease. In addition to requiring the desired gene needs to be expressed, the gene expression of the new gene needs to be regulated in order to prevent over-expression that could also trigger disease.,,,,
Gene therapy for hemoglobinopathies
Sometimes, the whole or part of a gene is defective or missing from birth or a gene can change or mutate during adult life. At present, there are two main categories of genetic therapies being developed for treating hemoglobinopathies, i.e., gene therapy and gene editing involving exogenous gene addition and direct modification of endogenous DNA, respectively. Backed by decades of fundamental and preclinical research, gene therapy is the first modality of genetic therapy entering clinical trials targeting diseases of the hematopoietic system. Efforts are being directed to treatments based on auto-HSCT in which the patient's own stem cells are harvested, genetically modified ex vivo and reinfused back to the patient.
Hemoglobinopathies are the world's most common group of monogenic disorders with an estimated 7% of the global population carrying these diseases.
| Conclusion|| |
Gene therapy is arguably the most exciting area of biotechnology at this moment-both due to recent progress and because of the possibilities on the horizon. Unprecedented levels of control over nucleic acid delivery, modulation of the immune system, and precise manipulation of the human genome – technologies not imaginable 10 years ago – will certainly unlock new areas of medicine over the next 10 years. At the same time, this nascent glimpse of a new world of technical capabilities has inspired whole new areas of research, such as synthetic biology, cell reprogramming, and high-throughput functional genomics, which will undoubtedly continue to reshape the face of biomedical research.
Financial support and sponsorship
Conflicts of interest
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