What is this Technology
In general, gene therapy reverses diseases cause by damage of the genes. This can be done by delivering the functional version of the mutated gene by using carriers called vectors [16]. There are two types of vectors, viral vectors and non-viral vectors, the most common type being viral vectors [16].
Viral vectors are viruses that have been genetically altered to carry normal human DNA [16]. Viruses have evolved to be able to transfer parts of their DNA to human cells [19]. The DNA from the virus instructs the cell infected to produce more copies of the virus which leads to the infection of more and more cells [19]. Some viruses insert their gene into the genome of a cell [19]. This way, the gene from the virus stays in the cell causing viral diseases and infections [16]. In gene therapy, the disease causing gene is removed from the virus and replaced with a normal functional version of the mutated gene (therapeutic gene) [16]. The virus is then injected into the body and locates target cells, such as lung or liver cells, and infects the cells with the therapeutic genes [16].
There are many different types of viruses used at viral vectors [7]. One of the viruses used are retroviruses which are viruses that are able to create double stranded DNA copies of their RNA genomes. The DNA copies can be integrated into the chromosome of the body cell it has infected [7]. One example of a retrovirus is HIV (human immunodeficiency virus [7]. Another type of virus that scientists use as a vector is adenoviruses. Adenoviruses are viruses with double stranded genomes that can cause respiratory infections [7]. Another type of viral vectors is Adeno-associated viruses which have single stranded DNA [7]. The single stranded DNA can be inserted at specific sites on chromosome 19 [7]. Another virus used is the Herpes simplex virus [7]. These viruses have double stranded DNA that causes infections in the human body [7].
Non viral vectors are also used for gene therapy, although not as commonly used have some advantages over viral vectors [14]. Two of the advantages are, one, non-viral vectors can be easily produced in a large scale, and two, there are less risks since the body’s immune system is less likely to activate and attack these non-viral vectors [14]. One form of the non viral vector is naked DNA [14]. Naked DNA is a form of non viral transfection [14]. Transfection is the technique scientist use by deliberately introducing nucleic acids to cells [.
This is not commonly used because it is not as effective as other types of vectors [14]. Another non-viral vector is an Oligonucleotide [14]. The Oligonucleotide is able to inactivate genes that are causing disease [14]. This can be achieved by disrupting the transcription of a faulty gene by using a process called antisense specific [14]. Another method to achieve inactivity of the mutated gene is by using small molecules of RNA called siRNA to disrupt the translation of the faulty gene [14].
There are also several approaches to gene therapy that are being researched and tested. The most common approach is replacing the mutated gene that causes disease by inserting a healthy copy of the gene into a nonspecific location within the genome [7]. Another approach is swapping the dysfunctional gene with a functional gene through homologous recombination where nucleotide sequence exchanges between two similar or identical DNA molecules [7]. Another approach is by deactivating, “turning off” the mutated gene [16]. The abnormal gene can also be repaired through a process known as selective reverse mutation which causes the gene to relapse back to its original sequence [7]. There is also another method to gene therapy in which a new gene is introduced to the body to help fight the disease [16].
Picture shows how an adenovirus inserts its DNA into the host cell [9].
Viral vectors are viruses that have been genetically altered to carry normal human DNA [16]. Viruses have evolved to be able to transfer parts of their DNA to human cells [19]. The DNA from the virus instructs the cell infected to produce more copies of the virus which leads to the infection of more and more cells [19]. Some viruses insert their gene into the genome of a cell [19]. This way, the gene from the virus stays in the cell causing viral diseases and infections [16]. In gene therapy, the disease causing gene is removed from the virus and replaced with a normal functional version of the mutated gene (therapeutic gene) [16]. The virus is then injected into the body and locates target cells, such as lung or liver cells, and infects the cells with the therapeutic genes [16].
There are many different types of viruses used at viral vectors [7]. One of the viruses used are retroviruses which are viruses that are able to create double stranded DNA copies of their RNA genomes. The DNA copies can be integrated into the chromosome of the body cell it has infected [7]. One example of a retrovirus is HIV (human immunodeficiency virus [7]. Another type of virus that scientists use as a vector is adenoviruses. Adenoviruses are viruses with double stranded genomes that can cause respiratory infections [7]. Another type of viral vectors is Adeno-associated viruses which have single stranded DNA [7]. The single stranded DNA can be inserted at specific sites on chromosome 19 [7]. Another virus used is the Herpes simplex virus [7]. These viruses have double stranded DNA that causes infections in the human body [7].
Non viral vectors are also used for gene therapy, although not as commonly used have some advantages over viral vectors [14]. Two of the advantages are, one, non-viral vectors can be easily produced in a large scale, and two, there are less risks since the body’s immune system is less likely to activate and attack these non-viral vectors [14]. One form of the non viral vector is naked DNA [14]. Naked DNA is a form of non viral transfection [14]. Transfection is the technique scientist use by deliberately introducing nucleic acids to cells [.
This is not commonly used because it is not as effective as other types of vectors [14]. Another non-viral vector is an Oligonucleotide [14]. The Oligonucleotide is able to inactivate genes that are causing disease [14]. This can be achieved by disrupting the transcription of a faulty gene by using a process called antisense specific [14]. Another method to achieve inactivity of the mutated gene is by using small molecules of RNA called siRNA to disrupt the translation of the faulty gene [14].
There are also several approaches to gene therapy that are being researched and tested. The most common approach is replacing the mutated gene that causes disease by inserting a healthy copy of the gene into a nonspecific location within the genome [7]. Another approach is swapping the dysfunctional gene with a functional gene through homologous recombination where nucleotide sequence exchanges between two similar or identical DNA molecules [7]. Another approach is by deactivating, “turning off” the mutated gene [16]. The abnormal gene can also be repaired through a process known as selective reverse mutation which causes the gene to relapse back to its original sequence [7]. There is also another method to gene therapy in which a new gene is introduced to the body to help fight the disease [16].
Picture shows how an adenovirus inserts its DNA into the host cell [9].
DNA
The main purpose of gene therapy is to alter the mutated DNA or insert functional DNA in to the cell so that the cell can function normally [16]. There are many forms of gene therapy and they all impact a cell’s DNA in different ways. DNA is inserted into cells in the body by vectors; the impact on DNA varies with each vector.
One of the vectors used in gene therapy is the Adenovirus, a virus that causes the common cold [18]. This virus is the vector used in gene therapy for cystic fibrosis [4]. First of all, to ensure that the virus does not replicate and infect the patient with its own DNA, the E1 region of its DNA is deleted so that it is safe to be used in the body [9]. The adenovirus carries genetic material in the double stranded DNA form [9]. When the adenovirus injects the carrying DNA into the host cell, it does not have an effect on the DNA of the host cell [9]. The DNA that is released becomes free in the nucleus [9]. And just like the DNA of the host cell, the information in the DNA inserted by the Adenovirus is transcribed and translated to produce proteins [9]. In patients with cystic fibrosis, the CFTR gene is mutated and normal protein channels cannot be produced. To treat cystic fibrosis the adenovirus containing normal CFTR gene is inhaled into the body and the cells are injected with the normal DNA [4]. The normal DNA is then transcribed and produces proteins that the mutated gene can no longer create [4]. The only thing about this type of gene therapy is that the new genes that are inserted does not replicate when the cell undergoes cell division [9]. Therefore, the patient receiving this therapy needs to be treated multiple times so that the new cells can also be injected with the new DNA [9].
Another type of vector used is the retrovirus. The genetic material in retroviruses is made up of RNA molecules [15]. When a retrovirus infects a host cell, it injects the RNA molecules as well as the enzymes reverse transcriptase and integrase in to the cell [15]. When the RNA molecules are injected into the cell, it needs to produce a DNA copy of itself 15]. The reverse transcriptase enzyme is responsible for the production of the DNA copy; this process is called reverse transcription [15]. When the DNA copy it produced, the enzyme integrase incorporates the copy into the genome of the host cell [15]. The molecules of DNA from the retrovirus are embedded into the DNA of the host cell, replacing mutations [15]. When the cell undergoes cell division, the new daughter cells will contain the new DNA created by the insertion of DNA by the retrovirus [15]. Basically what happens is the mutated gene may have molecules of DNA that is missing causing the production of proteins to be defective, when the retrovirus injects its RNA molecules, they make DNA copies of themselves and are inserted into the DNA of the host cell to fix the mutation. This way, normal proteins can be produced during translation. One of the problems with this type of gene therapy is that the integrase enzyme is able to insert the DNA molecules into any part of the genome of the host [15]. Therefore if the DNA molecules are inserted into the wrong place, such as a normal strand in the DNA of the host cell, the functional gene would be disrupted, which may cause new genetic mutations to occur within the body [15].
Picture shows the process of gene therapy, a vector inserting it's DNA into the host cell [6].
One of the vectors used in gene therapy is the Adenovirus, a virus that causes the common cold [18]. This virus is the vector used in gene therapy for cystic fibrosis [4]. First of all, to ensure that the virus does not replicate and infect the patient with its own DNA, the E1 region of its DNA is deleted so that it is safe to be used in the body [9]. The adenovirus carries genetic material in the double stranded DNA form [9]. When the adenovirus injects the carrying DNA into the host cell, it does not have an effect on the DNA of the host cell [9]. The DNA that is released becomes free in the nucleus [9]. And just like the DNA of the host cell, the information in the DNA inserted by the Adenovirus is transcribed and translated to produce proteins [9]. In patients with cystic fibrosis, the CFTR gene is mutated and normal protein channels cannot be produced. To treat cystic fibrosis the adenovirus containing normal CFTR gene is inhaled into the body and the cells are injected with the normal DNA [4]. The normal DNA is then transcribed and produces proteins that the mutated gene can no longer create [4]. The only thing about this type of gene therapy is that the new genes that are inserted does not replicate when the cell undergoes cell division [9]. Therefore, the patient receiving this therapy needs to be treated multiple times so that the new cells can also be injected with the new DNA [9].
Another type of vector used is the retrovirus. The genetic material in retroviruses is made up of RNA molecules [15]. When a retrovirus infects a host cell, it injects the RNA molecules as well as the enzymes reverse transcriptase and integrase in to the cell [15]. When the RNA molecules are injected into the cell, it needs to produce a DNA copy of itself 15]. The reverse transcriptase enzyme is responsible for the production of the DNA copy; this process is called reverse transcription [15]. When the DNA copy it produced, the enzyme integrase incorporates the copy into the genome of the host cell [15]. The molecules of DNA from the retrovirus are embedded into the DNA of the host cell, replacing mutations [15]. When the cell undergoes cell division, the new daughter cells will contain the new DNA created by the insertion of DNA by the retrovirus [15]. Basically what happens is the mutated gene may have molecules of DNA that is missing causing the production of proteins to be defective, when the retrovirus injects its RNA molecules, they make DNA copies of themselves and are inserted into the DNA of the host cell to fix the mutation. This way, normal proteins can be produced during translation. One of the problems with this type of gene therapy is that the integrase enzyme is able to insert the DNA molecules into any part of the genome of the host [15]. Therefore if the DNA molecules are inserted into the wrong place, such as a normal strand in the DNA of the host cell, the functional gene would be disrupted, which may cause new genetic mutations to occur within the body [15].
Picture shows the process of gene therapy, a vector inserting it's DNA into the host cell [6].