Human somatic gene therapy - concepts and applications

Human gene therapy can be defined as the transfer of exogenous genes or nucleotide sequences into somatic cells for the purpose of preventing, correcting or healing various diseases.

Somatic gene therapy is a new type of weapon in the fight against acquired diseases.DNA is considered as a drug providing a framework for curing thousands of genetic diseases. It also satisfies one of the greatest dreams of clinical medicine: ‘molecular surgery’ at the root of the disease.

Cystic fibrosis (CF)

People suffering from cystic fibrosis lack CF gene needed to produce a salt-regulating channel protein, cystic fibrosis transmembrane conductance regulator (CFTR). This protein regulates the flow of chloride in to epithelial cells that cover the air passages of the nose and lungs. Without this regulation, patients with CF disease build up thick mucus that makes them prone to chronic lung disease. The gene therapy technique to correct this abnormality might employ an adenovirus to transfer a normal copy of the CFTR gene. The gene is introduced into the patient by spraying into the nose.

Familial hypercholesterolemia (FH)

The patients with genetic disorder unable to process cholesterol properly by a low density lipoprotein receptor(LDLR), which leads to high levels of fat in the blood stream.  Patients with FH disease suffer from heart attacks and strokes because of blocked arteries. A gene therapy approach is developed with partial removal of patient’s liver. Corrected copy of a gene is inserted into liver sections, which are transplanted back into patients. The healthy gene may reduce the cholesterol build-up in the blood of the patient.

Duchene muscular dystrophy (DMD)

DMD is the most common childhood form of muscular dystrophy because of the failure to express dystrophin in the muscle fibres. It is a lethal, recessive X-linked disorder, more frequent in males. Muscular dystrophy is characterized by progressive muscle weakness, defects in muscle proteins and death of muscle cells and tissue. Viral mediated in vivo gene transfer method was developed to deliver dystrophin gene to patient’s muscle. Simple non-viral gene transfer in vivo was also developed using naked plasmid DNA.

Cancer therapy

In general, cancers have at least one mutation to a proto-oncogene (yielding) an oncogene) and at least one to a tumour suppressor gene allowing the cancer to proliferate. Oncogene inactivation may be targeted at the level of DNA, RNA transcription or protein product. Oligonucleotides are designed in sequence specific manner to target the promoter regions of oncogenes. At the RNA level, antisense techniques prevent transport and translation of the oncogene mRNA by providing a complementary RNA molecule (e.g., C-myc gene). Ribozymes, antisense oligonucleotides with a cleavage action will reduce the stability of oncogene mRNA. Restoration of the tumour suppressor gene such as p53 can be sufficient to cause cellular apoptosis and arrest tumour growth.

Molecular chemotherapy for cancer cells

Herpes simplex virus thymidine kinase (HSV-Tk) converts the prodrug ganciclovir into toxic metabolites. This toxicity is sufficient enough to kill the cancer cells.

DNA repair by gene therapy

Another approach involves the repairing of a gene using chimeric oligonucleotides. Homologous recombination is a natural process that controls the replacement of a defective gene. Highly precise DNA repair is performed by using DNA oligonucleotides to introduce site specific changes in the genome, even a single incorrect base can be corrected.

Transplantation tolerance in organ transplanted patients

In organ transplanted patients, the use of immunosuppressive drug is associated with increased risk of the development of cancer, infectious and ischemic heart disease. Gene therapy can be used to reduce the immunogenicity by introduction of genes to block T-cell activation.

HIV treatment by gene therapy

By introducing an antiviral gene into an infected T-lymphocyte, the HIV virus can be killed or expressing an antiviral gene in the normal T-lymphocyte may protect the patient from future HIV infection.

Replacement of hormones and blood factors by gene therapy

If an erythropoietin gene or factor IX gene is transferred via adeno-associated vector into muscles, it will cause the expression of relevant protein.

Prodrug activation

In herpes simplex virus thymidine kinase method, the thymidine kinase enzyme in every transduced cell converts the prodrug ganciclovir into monophosphate and triphosphates forms. The triphosphate form of ganciclovir interferes with the DNA replication and kill  the cancer cells.

Treatment of hepatitis B and C

The genetic sequence of RNA molecules of the particular virus is cut and destroyed by using ribozyme producing genes.