Gene therapy – Treatment and applications  

Gene therapy treatment – Introduction:  

  • Gene therapy is used to treat large number of diseases.
  • Research in the field of human gene therapy is directed towards correcting genetic defects of somatic cells i.e., cells that do not contribute to the next generation.
  • Human germ line therapy is not tried due to ethical, safety and technical reasons.
  • In human somatic cell gene therapy the fully functional and expressible gene is inserted into a target cell, by which that a specific genetic disease can be corrected permanently.
  • The development of a therapeutic agent for human passes through four levels of study and testing:

1.      Pre clinical trials that include extensive invitro experiments and research on laboratory animals.

2.      Phase I trials with a small number (6 to 10) of human subjects.

3.      Phase II trials with an increased number of human subjects to examine the effect of the drug or therapy.

4.      Phase III trial includes a large human sample and fully comphrensive analysis of the safety and efficacy of the products. 

Characteristic diseases caused by Gene mutations that code these products 

Gene product

Disease & symptoms

Current therapy

Adenosine deaminase

Severe combined immunodeficiency.

Loss of T & B cells

Bone marrow transplant, adenosine deaminases replacement.

LDL-receptor

Familial hyper cholesterolemia, elevated blood serum cholesterol level, coronary artery disease

Diet, drugs, liver transplant

Glucocerebrosidase

Gaucher disease, Accumulation of glucocerebroside in macrophages causing liver, spleen and bone damage.

Symptomatic treatment, removal of spleen. Antibiotics, repairing bone damage, bone marrow transplantation, enzyme replacement.

Blood clotting Factor VIII

Hemophilia A, altered plasma protein that causes defective blood clotting, chronic internal bleeding into joints, excessive bleeding after wounding.

Concentration of factor VIII by transfusion. Requires continuous treatment. Risk of viral infections from transfusions.

Phenylalanine hydroxylase

Phenylketonuria. Excess phenylalanine in the blood stream of newborns causes mental retardation

Restriction of dietary phenylalanine

α-Antitrypsin

Emphysema. Deficiency of serum protein protease inhibitor, damage to lungs, cirrhosis of the liver

Replacement therapy, lowering environmental risks.

Cystic fibrosis transmembrane regulator

Cystic fibrosis multi-system disease, pancreatic insufficiency in some cases, intestinal blockage, blocked airways of the lung.

Antibiotics, physical clearing of the lungs, upgrading diet.

Ornithine transcarbamylase

Hyperammonemia urea cycle defects, ammonia accumulation, arginine deficiency.

Restricted protein diet, arginine supplemented diet, drugs, liver transplant.

Dystrophin

Duchenne muscular dystrophy (DME). Progressive muscle wasting.

Only supportive treatments, good nutrition.

β-globin

Sickle cell disease chronic anemia, multi-system disease, damage to spleen, heart, kidneys, liver and brain.

Blood transfusions, drugs, analgesics, bone marrow transplant.

                                          Human Gene therapy trials conducted 

Gene therapy

Condition

Target cells

Adenosine deaminase

ADA defeciency

Lymphocytes, bone marrow cells.

Tumor necrosis factor

Melanoma

Tumor infiltrating lymphocytes, autologous tumor cells.

Interleukin - 2

Melanoma, glioblastoma, renal cell cancer

Autologous tumor cells, tumor cells

Factor IX

Hemophilia B

Autologous skin fibroblasts

LDL receptor

Hypercholesterolemia

Autologous liver cells

Histocompatibility locus antigen class –I B7 plus β2-microglobulin

Melanoma, colorectal cancer, renal cell cancer

Tumor cells

 

HSV thymidine kinase

Glioblastoma, AIDS, ovarian cancer

Tumor cells – T cells

Cystic fibrosis transmembrane conductance regulator

Cystic fibrosis

Nasal and airway epithelium

Multi drug resistance -I

 

Breast cancer

Blood CD 34+ cells

Granulocyte-macrophage colony stimulating factor

Melanoma

Tumor cells

Interleukin- 1 receptor antagonist

Arthritis

Autologous fibroblasts

Human CNTF

Amyotrophic lateral sclerosis

Encapsulated transduced and exogenic cells.

p53

Head and neck squamous carcinoma

Tumor cells

Fanconi anemia

Fanconi anemia

Bone marrow cells.

  

Gene Augmentation Therapy (GAT): 

  • Gene augmentation therapy (GAT), where DNA is added to the genome inorder to replace a missing gene product.
  • Gene targeting to correct mutant alleles.
  • Gene inhibition therapy, using antisense RNA expression or expression of intracellular antibodies to treat diseases.
  • Targeted ablation of specific cells.
  • Somatic cells can only be used as Therapeutic gene transfer generates transgenic human cell clones.
  • In transient gene therapy oligonucleotides can be used which can distrupt gene expression at many levels but does not cause permanent genetic changes.

Gene augmentation therapy for recessive diseases:

  • Gene marking is the first human genetic engineering experiment to demonstrate that an exogenous gene could be safely transferred into a patient and that this gene could subsequently be detected in cells removed from the patients.
  • Tumor infiltrating lymphocytes (kills tumor cells) were isolated from patients with advanced cancer. The cells were then genetically marked with a neomycin resistance gene and injected back into the same patient.
  • Gene augmentation therapies are being undergone for a small number of recessive single gene diseases.
  • Cystic fibrosis (disorder that affects lungs, liver and pancreas). The disease is caused by loss of cAMP –regulated membrane spanning chloride channel, which results in an electrolyte imbalance and accumulation of mucus leading respiratory failure. Cystic fibrosis is a recessive disorder which can be altered by introducing a functional copy of the gene.

Gene augmentation therapy for cancer:

  • Different Gene therapy strategies are under study for cancer treatment.
  • Tumor infiltrating leucocytes were transformed with a gene for TNF in addition to neomycin resistance gene with the aim of improving efficiency of these cells to kill tumors by increasing the amount of TNF they secrete.
  • Transform the tumor cells themselves, making them more susceptible to immune system through expression of cytomines or foreign antigen.
  • Transform Fibroblasts, which are easier to grow in culture and co-inject these together with tumor cells to provoke an immune response against the tumor.
  • Oncogenes (cancer genes) can be targeted using antisense technology either with antisense transgenes, oligonucleotides or ribozyme.
  • Cancers occurring due to loss of Tumor suppressor genes function can be altered by delivering a functional copy of the appropriate gene delivered into affected cells.
  • Pro-drug activation therapy involves activation of a particular enzyme specifically in cancer cells, which converts a non-toxic pro-drug into toxic product, so killing the cancer cells, which can be achieved by driving the expression of so called “suicide gene” selectively in cancer cells.
  • Use of Transcriptional regulatory elements that are active only in cancer cells.

     Gene Therapy and Cancer: 

  • Cancer is caused by the somatic mutation of cellular genes. 
  • These genes include oncogenes, tumor suppressor genes, and DNA repair genes. 
  • The generation of cancer is currently thought to be multi-step process of genetic alterations that vary according to the type and stage of cancer. 
  • Since cancer is a genetic disease, gene therapy could be applied in treatment of tumors.   
  • The most advanced applications of gene transfer technology in medicine are in gene therapy for cancer.   

Immunomodulation – Gene therapy:

  • Cells of the immune system have been found to recognize specific antigens of tumors and mount both humoral and cellular responses. 
  • During cancer development, this response is often limited in intensity and duration.  Strategies are being developed to reconstitute an effective antitumor immune response.  Advancements in immunology have made Immunomodulation (immunological approaches to treatment) the most dominant strategy in gene therapy for cancer. 
  • These approaches can be categorized in to three types according to target cells, mode of delivery, and transferred transgenes. 
  • Target cells include tumor cells, T cells, host cells, APCs (antigen presenting cells), and others.  Mode of delivery refers to choice of vector, in vitro, ex vivo, or in vivo).  Transferred transgenes refer to cytokines, co-stimulatory molecules, and tumor-associated antigens.

 Suicide Strategy:

  • The basic concept of using prodrug-converting enzymes is to limit the action of a known cytotoxic drug to local tumor areas. 
  • Targeted prodrug therapy includes the delivery of a gene that activates a nontoxic prodrug to a cytotoxic product by using viral vectors. 
  • This method maximizes toxicity at site of vector delivery while minimizing toxic effects on distant cells. 
  • The cDNA of the enzyme is delivered in to the tumor by a vector. 
  • The corresponding nontoxic prodrug is applied and is taken up by tumor cells. 
  • Since these cells have incorporated the cDNA in to their genome, they express the prodrug-converting enzyme. 
  • Therefore, when the cells take up the drug, it is converted in to a cytotoxic drug that kills the tumor.   

Tumor Suppressor Genes and anti-oncogenes:

  • Oncogenes, or genes that promote proliferation, become activated while tumor suppressor genes, or genes that terminate proliferation, become inactivated. 
  • Approaches in this method include the inactivation of oncogenes or the re-constitution of tumor suppressor genes that have been inactivated by gene deletion or other mechanisms.  The most effective procedure has been to target genes known to be dysfunctional. 
  • The most targeted tumor suppressor gene has been p53 because it is the most commonly mutated gene. 

Tumor Lysis by Recombinant Viruses:

  • Cancer therapy by viral oncolysis is one of the oldest concepts in gene therapy, and it has been one of the most challenging strategies.
  • The basic concept is to inject a virus in to the tumor, allowing transduction of cells. 
  • The virus causes lysis (oncolysis) of the cancer cells after some method of external disruption. 
  • Since the viruses have replicated in the cells, forming new viruses, the infection can be spread to other cancer cells, which will be lysed at a certain time in the lytic cycle. 
  • The adenovirus is a common virus used in these applications.   

Antiangiogenic - Gene Therapy:

  • Cancer cells require nutrients from the blood stream for proliferation.  Also for cancer to spread, the cells of a tumor must cross the cellular layers of blood vessel walls and get in to the bloodstream. 
  • This method by which tumors spread is called metastasis. 
  • Antiangiogenic Gene Therapy targets endothelial cells of blood vessels.  Several angiogenesis inhibitors can inhibit proliferation of intra-tumoral endothelial cells, therefore inhibiting tumor growth.   

Drug Resistant Genes:

  • This method is targeted at preventing side effects of chemotherapy. 
  • It involves making normal cells resistant to the toxic effects of chemicals used in chemotherapy. 
  • Anti-mitotic drugs are currently used to treat cancer. 
  • These drugs disrupt the formation of the mitotic spindle during mitosis, thereby eventually killing dividing cells. 
  • Since cancer cells are continuously dividing (uncontrollably), these drugs kill cancer. However, these drugs can also kill normal cells that are dividing rapidly, such as skin cells, cells of the intestine, and cells surrounding hair follicles. 

Gene therapy and cardiovascular disease:

  • Gene therapy to help increase blood flow to ischemic tissue.
  • Ischemia is a condition in which the flow of blood, and thus oxygen, is restricted to a part of the body. 
  • Limb ischemia and myocardial ischemia refer to lack of blood flow and oxygen to the limb and heart muscle. 
  • Some proteins in the body can help trigger new blood vessel growth and so increase the oxygen supply to the ischemic tissue. 
  • Such angiogenic proteins include the endothelial growth factors, vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) and hepatocyte growth factor (HGF).  
  • In gene therapy trials, different ways to deliver the genes for VEGF-1, VEGF-2 and FGF4 into the hearts of patients with advanced myocardial ischemia. 
  • After gene therapy, there is less severe angina (chest pain) and their hearts worked better. 
  • Gene therapy has also been successful in preventing re-occlusion, or re-blockage, of coronary artery bypass grafts and in keeping arteries open after angioplasty surgery.

Gene therapy and Adenosine Deaminase Deficiency:

  • Adenosine deaminase (ADA) deficiency is inherited as an autosomal recessive disorder. Defects in the ADA gene can leads to absent or diminished ADA enzyme activity in all tissues of the body.
  • The reversing of this genetic defect by replacing the defective gene may be possible by means of gene therapy.
  • The newly isolated gene may then be used for treatment by inserting it into the cells whose function is dependent on that gene.
  • The gene transfer method used must be highly efficient and able to deliver the corrective gene to the proper cells to restore its normal function.

 Gene therapy and Central Nervous system: 

  • Gene therapy is used for direct gene transfer to the affected brain region will carry out a local therapeutic gene product synthesis.
  • Recessive metabolic diseases of the nervous system caused by defects in single gene potentially could be cured by replacement of the defective gene with a correct gene.
  • Multifactorial neurodegenerative conditions might be improved by the production of therapeutic products from transferred genes.

 Factors hindering gene therapy:

  • Long lasting therapy is not achieved by gene therapy; the transferred DNA must remain functional and stable. But due to rapid dividing of cells long term benefits of gene therapy is short lived.
  •   Immune response to the transferred gene stimulates a potential risk to gene therapy.

  •   Viruses used as vectors for gene transfer may cause toxicity, immune responses, and inflammatory reactions in the host. Virus may also become pathogenic to the host.

  •   Disorders caused by defects in multiple genes cannot be treated effectively using gene therapy.

Recent developments in gene therapy:  

  • Genes are transferred into the brain using liposome coated in a polymer call polyethylene glycol (PEG). The transfer of genes into the brain is a significant achievement because viral vectors are too big to get across the "blood-brain barrier." This technique is used to treat Parkinson’s disease.
            
  • RNA interference or gene silencing may be a new way to treat Huntington's, in which short pieces of double-stranded RNA (short, interfering RNAs or siRNAs) are used by cells to degrade RNA of a particular sequence
  • New gene therapy approach repairs errors in messenger RNA derived from defective genes. Technique has potential to treat the blood disorder thalassaemia, cystic fibrosis, and some cancers.
  • Gene therapy for treating children with X-SCID (sever combined immunodeficiency) or the "bubble boy"        disease.
            
  • Tiny liposomes 25 nanometers are created to carry therapeutic DNA through pores in the nuclear membrane.
  • Sickle cell is successfully treated in mice.

 Future of Gene therapy: 

  • To study methods to learn how to turn gene on and off.
  • To insert a gene and regulate its expression through combination of one vector.
  • Gene therapy for Parkinson's disease.
  • Taming of AIDS virus and using it for gene transfer.
  • Using modified HIV for treatment of AIDS.

Frequently asked questions:

What are the different types of gene therapy?
Somatic cell gene therapy is conducted on the somatic cells (a non-reproductive cell). Germ line gene therapy is performed on reproductive genes (germ line cells) and the goal is to correct the individual’s, and their offspring’s, genetic defect.

What is genetic engineering?
Genetic engineering is making an alteration in genetic material.

It is used for large scale use and changing the genetic make up of the entire organism.

It refers to transgenic organisms (humans, animals, plants).  

Does gene therapy include genetic enhancement?
Genetic enhancement does not provide therapeutic benefits.

It is used for overall enhancement of the genetic make up, like changing the physical characteristics.

Pre-implantation genetic diagnosis, to the select the gender of an embryo.
 
Is there any genetic enhancement currently done?

Human genetic hormone is a genetically engineered hormone given to children’s who are deficient in their natural growth hormones.
 
Does gene therapy transfer genetic changes to next generation?

  • In somatic cell gene therapy it does not pass on to next generation, as it is inserted into somatic cells and genetic changes occurs only in the patient.
  • In Germ cell therapy, the genetic changes will be passed on to the next generation.

What was the first disease treated by gene therapy?

Adenosine deaminase (ADA) deficiency.

What are the risks involved in gene therapy?

  • During gene transfer by virus vector, the virus can infect more than one cell type and the virus may alter more than the intended cell.
  • During gene transfer, the new gene may be inserted in the wrong place and may lead to cancer or other damage.
  • Over expression of the inserted gene producing too much of the proteins, inflammation or an immune response. 
  • Transmission of the virus from the patient to other individuals or into the environment.

 Gene therapy related links and information:

 Gene Therapy Ethics

  • Ethical Issues in Human Gene Therapy - A Human Genome News article.
  • Special Report: Ethics of Genetics - From Guardian Unlimited.
  • Ethical Issues in Human Gene Therapy - A Human Genome News article.

Gene Therapy Clinical Trials

  • University of Pittsburgh Molecular Medicine Institute - Contains information about ongoing and completed clinical trials.
  • Gene therapy studies in ClinicalTrials.gov - The U.S. National Institutes of Health resource for public access to information on clinical research studies.
  • Gene Therapy Clinical Trials - Access to a worldwide database of gene therapy clinical trials at this Web.

Professional Associations

  • American Society of Gene Therapy
  • Australasian Gene Therapy Society (AGTS)
  • European Society of Gene Therapy (ESGT)

Gene Therapy Journals
      ·    Cancer Gene Therapy 

·     Current Gene Therapy.

  • Gene Therapy
  • Human Gene Therapy
  • The Journal of Gene Medicine
  • Molecular Therapy
  • Vector.