Transfer of genetic material into somatic cells to effect changes in the pathogenetic processes that contribute to a disease.
Gene therapy is the therapeutic delivery of nucleic acid polymers into a patient’s cells as a drug to treat disease.
Gene therapy refers to the use of recombinant DNA or messenger RNA, delivered via a vector, to either provide a missing gene or repair a damaged gene.
The goal of gene therapy is to achieve durable expression of therapeutic genes or a transgene at a level sufficient to ameliorate or cure diseases with minimal adverse events.
Viuses are RNA polymers with their own genetic code that acts upon human cells, thus live vaccines can be considered a primitive form of gene therapy, though not in the sense that is generally implied today.
Not all medical procedures that introduce alterations to a patient’s genetic makeup can be considered gene therapy.
Gene therapy is defined by the precision of the procedure and the intention of direct therapeutic effects.
Gene therapy is a way to fix a genetic problem at its source.
Two basic change strategies are: integrating vector into a precursor or stem cell so that the gene is passed to every daughter cell or the gene is delivered in a non-integrating vector to long lived post mitotic or slowly dividing cell, ensuring the expression of the gene for the life of the cell.
With gene transfer functional genetic material provides compensation for a non-functional gene.
With gene editing genetic repair is used to correct a non-functional gene.
In the second mechanism, integration of the therapeutic DNA into chromosomes of the patients cells is not required, the transfer of DNA is stabilized extrachromosomally.
Both gene transfer and gene editing can be carried out in vivo, in which the genetic material is directly injected into the patient’s body, or ex vivo, in which the patient’s own cells are removed and altered before returning them to the patient.
Bone marrow stem cells are often used as the basis for ex vivo gene therapy.
The transduction of stem cells is usually an X vivo process and requires an integrating vector, whereas delivery to long lived post mitotic cells is usually achieved through in vivo gene delivery.
EX vivo transduction is carried out as cells are extracted from the patient and transducer with the genes of interest, and then the cells are return to the patient in procedures such as hematopoetic stem cell transplantation.
EX vivo transduction approach requires a gene-delivery vehicle, the DNA that makes up the gene itself, and a facility for processing the cells.
In vivo gene delivery resembles the delivery of other types of pharmaceutical agents as the vector-gene construct is stored frozen, then thawed, and administered as an outpatient procedure.
The most common form uses DNA that encodes a functional, therapeutic gene to replace a mutated gene.
The transgene, or its protein product, must be delivered to a relevant target tissue or tissues, must be stably expressed, and must not interfere with functional integrity of those cells.
Genes are delivered to the body using vectors, which can be integrated or non-integrated, as well as viral, or non-viral.
The polymer molecule is packaged within a vector, which carries the molecule inside cells.
DNA must be administered, reach the damaged cells, enter the cell and express/disrupt a protein.
Gene editing has been a potential therapy for many genetic diseases.
Major risks of integrating the vectors arise from the potential for insertional mutagenesis, in which the vector insert into the DNA of a cell and disrupts a functional element of the DNA, such as a gene.
Targeted genome editing using nucleases provides a general method for inducing deletions or insertion.
Other technologies employ antisense, small interfering RNA and other DNA.
Gene therapy may be classified into two types:somatic cell and germline.
In somatic cell gene therapy (SCGT), therapeutic genes are transferred into any cell other than a gamete, germ cell, gametocyte or undifferentiated stem cell.
Such modifications affect the individual patient only.
Somatic gene therapy in which therapeutic DNA is used to treat disease.
Trials utilizing somatic cell gene therapy are underway, mostly on severe genetic disorders including: Immunodeficiencies, hemophilia, thalassaemia and cystic fibrosis.
Such single gene disorders are appropriate candidates for somatic cell therapy, as correction of a genetic disorder or the replacement of multiple genes is not yet possible.
In germline gene therapy (GGT), germ cells (sperm or eggs) are modified by the introduction of functional genes into their genomes.
Modifying a germ cell causes all the organism’s cells to contain the modified gene.
The change is therefore heritable and passed on to later generations.
In order to replicate, viruses introduce their genetic material into the host cell, tricking the host’s cellular machinery into using it as blueprints for viral proteins.
A number of viruses have been used for human gene therapy, including retrovirus, adenovirus, lentivirus, herpes simplex, vaccinia and adeno-associated virus.
Therapeutic DNA can serve as a temporary blueprint to enter the host’s genome, becoming a permanent part of the host’s DNA in infected cells.
Non-viral gene therapy have advantages over viral methods, such as large scale production and low host immunogenicity.
Non-viral gene therapy methodology include: the injection of naked DNA, electroporation, the gene gun, sonoporation,magnetofection, the use of oligonucleotides, lipoplexes, dendrimers, and inorganic nanoparticles.
Therapeutic DNA introduced into target cells must remain functional to be effective.
Treated cells containing the therapeutic DNA must be stable.
Problems with integrating therapeutic DNA into the genome and the rapidly dividing nature of many cells prevent it from achieving long-term benefits.
Patients require multiple treatments.
Gene therapy stimulates the immune system in a way that can reduce effectiveness, especially to repeated treatments.
Viral vectors carry the risks of toxicity, inflammatory responses, and gene control and targeting issues.
Some disorders, are affected by variations in multiple genes, which complicate gene therapy.
If DNA is integrated in a tumor suppressor gene, the therapy could induce a tumor, as has occurred in clinical trials for X-linked severe combined immunodeficiency(X-SCID) patients, in which hematopoietic stem cells were transduced with a corrective transgene using a retrovirus, and this led to the development of T cell leukemia.
Gendicine is a cancer gene therapy that delivers the tumor suppressor gene p53 using an engineered adenovirus.
Speculated uses for gene therapy include:
Human genetic engineering.
Gene therapy has been approved for the treatment of adenosine delaminates-severe combined immunodeficiency, a disease that is usually fatal in early childhood.
Gene therapy for SCID consists of infusion of hematopoetic stem cells genetically modified with the use of a gamma of gamma retroviral vctot to insert a functional copy of the gene adenosine delaminates (ADA).
Gene therapy with AAV5-hFVIII-SQ vector in patients with hemophilia A resulted in sustained benefit with substantial reduction in annualized rates of bleeding events a complete cessation of prophylactic factor III use in all participants who received such therapy. (Pasi KJ).
In patients with severe hemophilia A Valoctocogene roxaparvovec treatment provided endogenous factor VIII production and significantly reduced bleeding and factor VIII concentrate use relative to factor VIII prophylaxis (GENEr8-1 Trial Group).
Etranacogene dezaparvovec approved for use an adult with hemophilia B.
Etranacogene dezaparvovec gene therapy is superior to prophylactic factor IX with respect to annualized bleeding rate and has a favorable safety profile.
Current gene therapy for factor IX hemophilia can increase the level effective of factor IX to greater than 30% with clinical improvement at more than 5% of the normal level.
Voretigene neparvovec approved for treating retinal dystrophy.
Onasemnogene abeparvovec approved for pediatric, spinal muscular atrophy,
Stem cells genetically engineered to produce healthy hemoglobin have demonstrated remarkable results in 3 adult patients with sickle cell disease (SCD) in the open-label, nonrandomized, single-center Gene Transfer for Sickle Cell Disease trial.
Lentviral vector (LentiGlobin) infused engineered cells modifying them so they can produce more fetal hemoglobin or express fetal like hemoglobin changes containing a glutamine residue in place of threonine at position 87 creating hemoglobin A t87q.
BCL11A protein, which research from shows regulates the switch of HbF to adult hemoglobin.
The autologous CD34+ cells using BCH-BB694, a lentiviral vector, the delivery method that introduces new genetic information to host cells.
The patients’ CD34+ cells were modified to target and repress BCL11A by encoding a short hairpin RNA.
Following busulfan conditioning, intense chemotherapy meant to prepare a patient for SCT, on days –5 through –2,on d1 the CD34+ cells were reintroduced to the patients.
Results now show the patients are producing normal or near normal hemoglobin levels, with enough HbF to prevent sickling.
The transcription factor BCL11A is a strong repressor of gamma-globin, making it an appealing target for fetal hemoglobin induction.
The major risks of integrating vectors arise from the potential for insertional mutagenesis, in which the vector inserts into the DNA of the cell and disrupts functional elements of the DNA, such as a gene.
For vectors administered in vivo, risks from immune responses to the vectors can occur.