Gene therapy is defined as the specific genetic manipulation and modification of an organism’s genome or genes through the delivery of therapeutic DNA or genes into host cells with little or no toxicity as a way of treating an inherited genetic disease or correcting a disorderly gene. Gene therapy techniques are basically used to correct the genetic defects of somatic cells i.e. cells of the body that do not in any way contribute to hereditary (the transfer of genetic information from parents to their offspring’s via the genes). It is a promising field of clinical research that is still an experimental discipline but it has the potential to revolutionize medicine in terms of the way genetic disorders are managed (i.e. using therapeutic DNA to repair defective gene copy in genetic disorder patients). In gene therapy techniques, mutant genes of a cell are replaced with normal or wild type gene that carries therapeutic proteins or DNA expected to treat a particular acquired genetic disease (e.g. cancer and severe combined immune deficiency, SCID disease) in the affected human host. Faulty genes in people with genetic disorder or disease can be replaced with functional genes through a series of molecular/genetic engineering techniques in which the nucleic acid molecules are specifically modified to correct the anomaly. Gene therapy techniques as shall be seen later in this section are usually carried out in two main approaches.
- In vivo gene therapy (in which therapeutic DNA are transferred directly into the body). This method or technique of gene therapy is generally known as the direct delivery technique.
- Ex vivo gene therapy (in which some defective genes or cells are extracted from the body and treated in vitro, selected and then transferred back into the body). This method is known as the cell-based delivery technique.
Therapeutic DNA or protein is to gene therapy what antibiotics are to conventional medicine. Instead of using the traditional antimicrobial agents (e.g. antibiotic) and chemotherapy known to medicine for treating some diseases, gene therapy employs “functional genes” as its medicine to treat defects in the gene of an organism. With therapeutic DNA, cellular dysfunction or genetic disease in a cell can be corrected by inserting functioning genes into the cell; and as the genetic disorder is being corrected by the functional gene, a normal or new cellular function is restored in the affected individual. Genetic disorder is a disease that is caused by a mutation (i.e. a change) in the gene of an organism; and such molecular diseases can be passed on from parents to their offspring’s who inherit the defective gene.
They usually arise when the genes or genetic compositions of an organism are altered so that the proteins they encode are incompetent to carry out their normal physiological function in the cell. In order words, such defective genes begin to express itself in an abnormal way that affects the organism’s phenotype. The completion of the “human genome project” is a landmark for advancing gene therapy applications in clinical medicine because the practice will help to identify the genes responsible for a wide variety of human diseases with a view to correcting and treating them effectively. Information from the human genome project will help molecular biology scientists to compare the genetic makeup of normal and unhealthy individuals with a view to deciphering whether or not mutation in the gene of an organism could be responsible for some genetic disorders; and this can be improved upon by the techniques of gene therapy via targeted delivery of normal and therapeutic genes or DNA to treat genetic disorders.
The fact that we carry two (2) copies of practically all genes (with each derived from our mother and father) may be answerable to the question why most people (with defective genes) do not suffer any significant genetic disease. It is possible that virtually every human being on earth harbour one defective gene or the other, but these abnormal genes are apparent in some individuals (who show clinical signs of genetic disorders) and obscure or in unapparent in others who are unaware of any defective gene in them.
BRIEF HISTORY OF GENE THERAPY
The history of gene therapy techniques dates back to the early 1970s and 1980s when scientists (molecular biologists in particular) proposed the idea of using functional and therapeutic gene as medicines to treat genetic disorders in man. Though the idea received little support as at the time, gene therapy techniques is still in its experimental phase and research is still on going to make this innovative field of molecular medicine widely acceptable for the management of some molecular diseases. The Mendelian “Garden pea experiment” which showed that traits can be transferred from parents to their offspring’s in a defined and predictable manner, a process known as hereditary gave impetus to this experimental field of molecular medicine which is used to rectify or correct defective genes in individuals with genetic disorders.
Gregor Mendel’s experiment on garden pea plant which showed that traits could be inherited by offspring’s from their parents as genes laid the solid foundation that ushered in the field of genetics; which today have allowed scientists to attempt gene therapy techniques. Though still at its infancy and at an experimental stage, the techniques of gene therapy owes its growth to the discovery of the DNA (as the genetic material). The identification of the actual carrier of genetic information in a cell (i.e. the DNA) by Watson and Crick in 1953 coupled with the discovery of restriction enzymes and the genetic code amongst other genetic engineering techniques including the emergence of the recombinant DNA technology boosted the study and application of gene therapy to manage some genetic disorders in human.
These advances in recombinant DNA technology and/or molecular biology gave molecular scientists (in particular gene therapists) the impetus to effectively manipulate and alter the genetic makeup of an individual with a view to correcting a faulty gene. Restriction enzymes allows biologists to cut DNA at specific sites; and with the knowledge of the genetic code, the actual sequence and flow of informational molecules in the cell of an organism (i.e. DNA – RNA – Protein) can be decoded by molecular biologists and the information used to repair faulty genes or cells. The concept of transferring genes to tissues for clinical applications has been discussed for a long time, but man’s ability to manipulate the genetic material (i.e. the DNA) through the principles of recombinant DNA technology has only brought this goal to reality via gene therapy.
The first genetic disorder to be treated with gene therapy techniques was adenosine deaminase (ADA) deficiency otherwise known as severe combined immune deficiency (SCID), a genetic disorder that leaves its victims with weakened immune system. This first treatment using gene therapy techniques occurred in the United States of America and in 1990 in a 4 year old girl who suffered from SCID (a rare congenital genetic disease). White blood cells or lymphocytes (i.e. T cells) were medically extracted from the lad and genetically engineered with a retroviral vector bearing a normal ADA gene for the synthesis of adenosine deaminase, required for a functional immunity. Adenosine deaminase is an important enzyme required for the synthesis of functional B and T cells; and a lack of this enzyme (especially congenitally) causes a marked reduction in the numbers of functional lymphocytes essential for fighting infectious diseases in the body.
The genetically modified ADA gene was then reinjected into the child, and normal adenosine deaminase synthesis was restored but the patient later died from complications that ensued from the treatment. However, a similar genetic disorder in children known as X-Linked Severe Combined Immunodeficiency (X-SCID) or bubble boy syndrome (a genetic disease that only affect the male child) was successfully treated and cured in Paris, France in 2000; and this marked the first genetic disease that was successfully treated with gene therapy techniques. Gene therapy has experienced some series of setbacks due to the death of some genetic disorder patient’s who passed through clinical trials of the gene therapy techniques. These setbacks have limited gene therapy to only an experimental discipline but the approach is being considered as possible cure to some genetic disorders in some developed countries especially in Europe and USA.
Gene therapy may in the near future replace the customary way of treating diseases or infections which usually involves the use of drugs and surgery; and this is because gene therapy techniques when fully developed can afford physicians the opportunity of treating diseases by the insertion of functional therapeutic DNA or proteins that specifically target a particular disease condition. Some of the diseases being investigated for gene therapy techniques include sickle cell disease, various types of cancer, cystic fibrosis, Huntington’s disease, Down’s syndrome and haemophilia amongst other genetic disorders. Several controversies still surround the application of gene therapy techniques in clinical medicine due to some ethical issues and safety of the practice. Nevertheless, the future prospects of gene therapy applications are bright; and this experimental discipline holds the potential to revolutionize the practice of medicine worldwide.
PREREQUISITES OR STEPS FOR GENE THERAPY
Gene therapy is an experimental discipline or research that uses functional gene (i.e. therapeutic DNA) to repair defective genes in genetic disease patients; and this growing field of molecular medicine is usually undertaken using modified nucleic acid molecules through a series of steps. Conventional medicine utilizes oral and parenteral drug administration as well as surgery and chemotherapy to treat infectious diseases, but this is not the case for gene therapy which normally makes use of gene delivery techniques that delivers specific and functional gene copies into the body of individuals with genetic disorders. The transfer of genetically modified genes or DNA into the body especially for curative purposes requires several critical steps in order to ensure the success of the process. Most gene therapy applications are mainly based on manipulation of the somatic cells with little or no attempt made on altering the individual’s germ-line (i.e. the gamete cells). While somatic gene therapy (in which functional genes are inserted into the body) may be permitted in some quarters, germ line gene therapy (that envisages the manipulation of gamete cells) is not yet allowed anywhere in the world due to some ethical issues surrounding the later. Genetic modifications in somatic gene therapy techniques cannot be passed on to the next generation (i.e. from parents to offspring’s) but any alteration in the gamete cells (i.e. the egg and sperm) as is being proposed by the germ line gene therapy can be passed on to the next generation. Some of the basic steps to be taken when gene therapy is anticipated are discussed in this section.
- Identification of candidate genetic disease: The genetic disorder especially those that cannot be successfully treated with available conventional medical practices and the faulty cells must be identified and accessible.
- Gene cloning: Gene cloning is a molecular biology procedure that is used to obtain many copies of a particular gene or piece of DNA. The genetic basis of the molecular disease must be understood and determined through the identification of the genes that encodes for the defective genes so that a mutant functional gene can be generated via rDNA technology (e.g. gene cloning).
- Determining disease pathophysiology: The pathophysiology of the molecular disease should be determined beforehand so that the cellular sites for the insertion of the functional gene or DNA required to correct the faulty genes can be ascertained and well targeted in the process.
- Identification of gene expression: Gene expression in the cell is often identified via the production or synthesis of particular protein molecules which the gene encodes. It is critical that the tools required for identifying the expression of the correct genes in vivo be available and used to determine the production of specific protein molecules in the host receiving therapeutic DNA or genes.
- Pre-clinical in vitro and vivo tests: Gene therapy is still an experimental discipline, and thus it is being undertaken mainly at clinical trial levels but with utmost caution. It is critical to conduct series of in vitro and in vivo non-clinical and clinical trials whenever gene therapy techniques are anticipated so that optimum result will be obtained. The mode of delivery of the therapeutic DNA or gene into the desired cells or tissues in the body (which is still a challenge in gene therapy techniques) must also be worked out to be efficient and available.
GENE THERAPY TARGETING AND DELIVERY
The efficient delivery of therapeutic proteins or DNA into specific cells or tissues of an organism to correct a mutant gene is paramount to the success of any gene therapy procedure; and the inserted therapeutic DNA or desired gene must be continuously expressed in vivo at appropriate physiological level in order to correct the mutated gene and thus restore the normal genetic composition of the affected individual. Gene therapy generally involves the delivery of one or more desired genes and/or proteins into the body so that the inserted gene will spur the synthesis of missing protein or enzymes (whose lack caused the genetic disorder) in the affected individual. But this process of gene delivery is often the most difficult aspect of gene therapy because some of the available methods of delivering desired genes into the body have some deficiencies.
For example, some of the vectors used for gene delivery are viral particles; and these viruses can become pathogenic to the host receiving the gene insert as well as cause several other adverse immunological and inflammatory reactions in the body. Direct delivery (i.e. in vivo gene therapy) and cell-based delivery (i.e. ex vivo gene therapy) are usually the two main approaches used for delivering therapeutic gene or DNA meant to repair a faulty gene in the body (Figure 1). In direct delivery or in vivo gene therapy, desired genes are delivered into the body by means of viral vectors. But non-viral vectors as shall be seen later in this section are required for the delivery of desired genes or DNA in cell-based delivery or ex vivo gene therapy techniques.
Fig. 1. Illustration of approaches or techniques used to achieve gene therapy. Gene therapy techniques is usually achieved via in vivo techniques (in which the therapeutic gene or DNA is directly injected into the body) and ex vivo techniques (in which cells are genetically altered in vitro and then injected back into the body). When cells are extracted or removed from the body of genetic disorder patients and genetically modified in vitro and then transferred back into the same body, the process is known as ex vivo gene therapy. But in the in vivo gene therapy techniques, therapeutic DNA or genes are inserted directly into the body of genetic disorder patients using viral vectors.
IN VIVO DELIVERY
In vivo delivery is the gene therapy technique that delivers DNA, RNA or therapeutic protein directly into the cell or tissue of an organism. Desired or therapeutic genes are delivered in this technique through the genetic transfer method known as transduction. Transduction is the genetic process of transferring genes of interest from one cell to another using viruses or bacteriophages. It is usually achieved using viral vectors; and the delivery of desired gene into the cell of the recipient host using this technique is specific. In vivo gene therapy is the most feasible strategy for delivering desired genes or DNA into the cell or tissues. The cells of the body targeted by in vivo gene therapy include those of the brain, lungs, blood vessels, liver and muscle cells.
In vivo gene delivery to a particular organ of the body is usually achieved through the catheterization of that organ by surgical approaches that employ invasive medical devices and other advanced medical techniques that ensure that the desired gene is delivered properly. Some in vivo gene therapy techniques may also be guided by computerized techniques that ensure the delivery of the desired genes or therapeutic DNA to target host cells where they are expected to efficiently repair a mutant DNA or gene. Though finding an efficient delivery system for gene therapy may still not be possible; viruses are the best vectors for in vivo gene therapy because viral particles or viruses are efficient in transducing (i.e. transforming) host cells. And this is because viruses have a history of penetrating host cells and transforming same by inserting their own genome into target host cells, a phenomenon that gives them an edge over other means of delivering therapeutic genes.
Most of the viruses used for in vivo gene therapy are “harmless viruses” that have been attenuated and made to lose their virulence so that they do not become pathogenic when used as vectors to deliver therapeutic genes to host cells. Examples of viruses considered for in vivo gene therapy include adenoviruses (adenoviral vectors), retroviruses (retroviral vectors) and adeno-associated viral vectors amongst others. The virulence genes of these viruses have been altered and made apathogenic, and they are used to deliver therapeutic DNA or genes into a target host cell. The viral vector can be given intravenously or injected directly into a specific tissue in the host body, where it is then taken up by individual target cells that will be transformed and made to start secreting the correct type of proteins or enzymes they encode. However, the use of viruses for in vivo gene therapy has some safety concerns that limit their usage for gene delivery. The viral particles used for in vivo gene therapy (though apathogenic or non-virulent) may become pathogenic on entering the host’s body, and the viruses may cause other adverse reactions in the individual taken the therapy.
EX VIVO DELIVERY
Ex vivo delivery is the gene therapy technique in which cells extracted from a patient are genetically engineered in vitro and then re-introduced into the host’s body. Such extracted cells must be capable of survival outside the host’s body and re-implantation into the body (after they must have been transformed) before they can qualify to be used for ex vivo gene therapy. Desired genes or proteins known as transgenes (i.e. genetically modified genes) are inserted into the body where they are expected to bring out the desired response after being transformed in vitro. The cells of the body usually targeted by the ex vivo gene therapy technique include cells of the bone marrow, muscles, liver and fibroblasts amongst others. Ex vivo gene therapy employ non-viral vectors or techniques to deliver therapeutic genes or DNA into the cells of a host; and they are normally easy to use.
Unlike the viral vectors that is normally associated with adverse host cell response; non-viral vectors used for ex vivo gene techniques are not capable of stimulating the host immune response. Ex vivo gene therapy delivers desired genes that have been extracted and modified from outside the body; and they are usually not as specific as the in vivo delivery method that uses viruses as their vector. Ex vivo gene therapy is generally achieved through the process of transfection which is the in vitro introduction of proteins or DNA into cells. Non-viral techniques used for ex vivo gene therapy include lipofection, microinjection, electroporation, the use of naked DNA or plasmids and the use of calcium phosphate precipitate and liposome’s amongst other techniques. Though much safer than the use of viruses, non-viral vectors at usually inefficient at transforming genes and some of the techniques or approaches used are not specific in action.
Non-viral vectors or delivery systems as exemplified in this section are fast becoming the alternatives to the use of viral vectors in delivering therapeutic genes or DNA into hosts cells due to the safety and health risks or concerns of the later (i.e. the viral vectors). Understanding the in vivo cellular barriers and other cellular restrictions in the host cell that impede the efficient delivery of therapeutic genes or DNA will help in developing efficient gene delivery systems that will ensure proficient expression of the delivered gene within the limits of the targeted hosts cells. After their successful delivery, it is critical that the inserted therapeutic DNA or genes continue to express their encoded gene products (e.g. enzymes or proteins) within the cells or tissues of the host because this will help to guarantee a thriving repair of the mutated gene responsible for the genetic disorder.
SYNOPSIS OF HOW GENE THERAPY WORKS
Genes are made of deoxyribonucleic acid (DNA) molecules – which are the main genetic material of living cells. It is in the gene that the genetic information that directs the activities of a cell is located; and this genetic information also direct the production of malfunctioned proteins or activities that result in genetic disorder in a living host. Successful gene delivery requires an efficient way to get the DNA into living host cells; and to make it happen, a vector (usually a virus) is used to deliver the gene of interest into the host receiving the gene therapy. These DNA delivery vehicles or systems are known generally as vectors. Vectors are cloning agents that act as self-replicating DNA molecules, and which is used in molecular biology techniques to carry cloned genes or other segments of DNA into another recipient host.
It is noteworthy to note that in parasitology, a vector may mean an agent which is usually an insect or animal that is able to carry infectious disease agents or pathogens from one host to another. To be successful in gene therapy techniques, a vector must be able to target the right type of cells in the recipient host. It must be able to integrate the gene or DNA it is carrying into the host cell. Vectors must leave no untoward effect on the recipient host and the vector must also be able to activate the gene once it has entered the host cell. The inserted desired gene must go to the nucleus of the target host cell and be “turned on. For a gene to be turned on, it means that the gene is transcribed and translated (according to the central dogma of molecular biology as aforesaid) in order to make the desired protein product that the inserted gene encodes.
And for the gene delivery process to be successful, the protein must function properly in the recipient host cell. Gene therapy as aforementioned is the addition of new genes to a patient’s cells to replace missing or malfunctioning genes – as a panacea to correcting a particular molecular/genetic disease or disorder. It is usually done using a virus to carry the genetic cargo into cells, because that’s what viruses evolved to do with their own genetic material. A vector which can be viral or nonviral vector carrying the desired gene or DNA is used to deliver the functional/normal gene into the recipient patient’s target cell or tissues. The vector carrying the therapeutic gene (viral vector in particular) infects and invades the non-functional tissue or cell, and transforms it. Particularly, the viral vector (harbouring the therapeutic DNA) uncoats and releases the therapeutic gene into the host cell genome. Taking control of the infected host cell’s genome, the therapeutic gene starts to synthesize functional or normal genes which bring about the desired effect by replacing the non-functional gene product or repairing the defective gene; and this returns the diseased target cell to its normal physiological function.
The major differences between viral and non-vectors used for gene therapy applications are as shown in Table 1.
Table 1: DIFFERENCE BETWEEN VIRAL AND NON-VIRAL VECTORS
|Viral vectors||Non-viral vectors|
|1.||Highly efficient in transferring desired genes.||Less or fairly efficient in transferring desired genes.|
|2.||Viral vectors pose some health risk to the recipient
|Non-viral vectors are usually safer to use, and they do not pose any health risk to the recipient patient.|
|3.||Viral vectors are used for in vivo gene therapy to
deliver desired genes through the process of
|Non-viral vectors are used for ex vivo gene therapy to deliver desired genes through the process known as transfection.|
|4.||Viruses (e.g. adeno-associated viruses and adenoviruses)are used as vectors.||Non-viral vectors such as gene gun or microinjection and calcium phosphate precipitate e.t.c. are used as vehicles to convey the gene of interest into the body of the host.|
POTENTIAL USES OF GENE THERAPY
The medical and general therapeutic effects of gene therapy technologies as it relates to the delivery of desired genes or DNA to the tissues/cells of genetic disease patients may well be a new frontier for the management of other related diseases as well as some other infectious diseases. Gene therapy may be used to restore the health of a sick patient undergoing particular clinical condition especially genetic diseases since it sees to the delivery of desired genes which are supposed to repair defective genes in the recipient host. Thus gene therapy has some specific functions, and these shall be listed in this section.
- Gene therapy techniques replace defective or missing genes in genetic disorder patients through the delivery of therapeutic DNA or genes.
- The desired therapeutic DNA delivered or inserted into the tissue/cells of the host stimulates the development of new tissues that replaces the defective ones.
- Gene therapy technologies deliver therapeutic genes that destroy cancerous cells and/or tumours in the body of the recipient host.
- Therapeutic DNA inserted into host cells also help to spur the healing process of damaged tissues; and they can also cause cancer cells to regress back to normal cells.
- Of most importance is the fact that gene therapy could be used in the near future to improve the process of immunization/vaccination through the specific and targeted delivery microbial genes (especially those of viruses, bacteria and fungi) that will help to ameliorate some of the infectious diseases of man.
APPLICATIONS OF GENE THERAPY IN TREATING MOLECULAR DISEASES
Molecular diseases (genetic disorders) are non-infectious inheritable diseases which are usually caused by mutations that alter the normal function of a particular gene. They are different from infectious diseases such as malaria and tuberculosis which are mainly caused by pathogenic microorganisms in that genetic disorders can be passed on from affected parents to their offspring’s or other family members who inherit the defective gene responsible for the disease. Mutations (i.e. genetic alterations in gene sequence) lead to a variety of genetic diseases in humans. Some mutations can cause single gene disorders in which there is an error in a particular gene; and typical example of the disease condition that can occur when there is a mistake in a single gene is sickle cell disease (in which an abnormal form of haemoglobin is produced) and cystic fibrosis (in which there is a malfunctioning of the exocrine glands especially the pancreas, and respiratory disorders usually occur in victims of CF).
Chromosomal disorders arise when there is a deficiency in a particular gene or when a given gene is in excess amount. Down syndrome is a typical example of chromosomal gene disorder; and it is a medical condition due to the existence of an extra copy of chromosome 21 in newborns. Children or newborns with Down’s syndrome usually experience learning difficulties and they usually have speech difficulties and broad faces. When mutation occurs in several genes in addition to some environmental factors (e.g. exposure to mutagens or harmful chemicals), a multifactorial gene disorder such as Alzheimer’s disease and Parkinson’s disease amongst other central nervous system (CNS) malfunctions may ensue. While some genetic disorders are evident at birth, others are obscure and may only appear during the upbringing of the child or later in their lifetime. The alteration of the normal functioning of a gene (i.e. gene mutation) will cause the proteins that they encode to lose their normal physiological function in vivo; and this may result to a wide variety of genetic disease or disorder.
Some notable molecular or genetic disorders in man include haemophilia, sickle cell anaemia, Alzheimer’s disease, Down syndrome, cystic fibrosis (CF), Tay Sachs disease, SCID and various types of cancers including those of the lungs, heart and skin. Gene therapy techniques could be used to treat these genetic disorders; and they are mainly targeted at correcting or repairing the defective genes that is responsible for the disease condition. In some cases, faulty or defective genes are replaced with the functional or normal genes through the insertion or delivery of the correct genes into the host tissues or cells via gene delivery techniques as earlier explained in this section. Irrespective of some of the drawbacks associated with the use of gene therapy to manage genetic disorder patients, the approach still holds potential of not only providing cure for sufferers of molecular diseases, but it also has the chances of revolutionizing the practice of medicine in the near future.
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