Genetic Disease

Gene therapy offers the prospect of long-term and potentially curative benefits to patients with genetic or acquired diseases by directing the expression of a therapeutic protein or restoring the expression of a missing protein through a single administration. 

Genes are the specific areas of DNA that provide the blueprint used by the body’s cellular machinery to make proteins. A defect or mutation in a specific gene can result in the inability or reduced ability to express a protein, or the reduced functionality of a protein. For example, when the gene associated with the production of a protein required for blood clotting is missing or mutated in hemophilia B patients, these patients’ blood cannot clot enough to stop bleeding, even after a minor trauma or surgery. Introducing a copy of the proper gene into the cell could restore the presence and natural function of the blood-clotting factor, which would prevent bleeding.

Simply put, gene therapy is an investigational approach to treat or prevent genetic disease. Gene therapy research is not new. In fact, scientists have been investigating and evolving it for more than 50 years and it has the potential to find ways to treat many diseases.

The goal of gene therapy research is to determine whether a new gene can be used to replace or inactivate a mutated gene to treat a disease or help the body fight a disease. For a gene to be delivered into a cell, a transporter is typically used. A transporter is known as a vector.

The role of a vector

Cells don’t like—or even let in—foreign bodies, so a natural route to the cell nucleus is needed. Vectors are used to penetrate the cell and deliver healthy genes within.

VIRAL VECTOR - Designed to imitate a virus by retaining its ability to penetrate a cell, while removing elements that risk causing disease once inside the body. Some of different types of viruses can be used as gene therapy vectors.

Retroviruses: A class of viruses that can create double-stranded DNA copies of their RNA genomes. These copies of its genome can be integrated into the chromosomes of host cells. An example of a retrovirus is the Human immunodeficiency virus (HIV). One of the problems of gene therapy using retroviruses is that, the integrase enzyme can insert the genetic material of the virus into any arbitrary position in the genome of the host; it may randomly insert the genetic material into a chromosome; thus, if the genetic material happens to be inserted in the middle of one of the original genes of the host cell, this gene will be disrupted (insertion mutagenesis); furthermore, if the gene happens to be one regulating cell division, uncontrolled cell division (i.e. cancer) can occur. This problem has begun to be addressed by utilizing zinc finger nucleases or by including certain sequences such as the beta-globin locus control region to direct the site of integration to specific chromosomal sites.

Adenoviruses: A class of viruses with double stranded DNA genomes that cause respiratory, intestinal, and eye infections in humans. The virus that causes the common cold is an adenovirus.

Herpes simplex viruses: A class of double stranded DNA viruses that can infect a particular cell type, neurons. The herpes simplex virus type 1 is a common human pathogen that causes cold sores.

NON-VIRAL VECTORS - Non-viral vectors are simple in theory but complex in practice. Apart from intracellular and extracellular barriers, number of other challenges also needs to be overcome in order to increase the effectiveness of nonviral gene transfer. These barriers are categorized as production, formulation and storage. No one-size-fits-all solution to gene delivery, which is why in spite of various developments in liposome, polymer formulation and optimization, new compounds are constantly being proposed and investigated.

Injection of Naked DNA: This is the simplest method of non-viral transfection. Clinical trials carried out of intramuscular injection of a naked DNA plasmid have occurred with some success; however, the expression has been very low in comparison to other methods of transfection.

These naked DNA, particle, and chemical-based transporters are used due to their reduced pathogenicity and increased cost-efficiency compared with viral vectors.

DELIVERY SYSTEMS

IN VIVO - Genes are modified within the body after the therapy is administered. Most effective when genes need manipulation.

EX VIVO - Genes are modified outside the body, before being administered back to the patient. Most effective when defective genes need replacement.

• A vector is made from an altered virus
• Before the virus is used as a vector, its viral genes are removed
• Once the virus is modified, it is intended to transport the desired gene to a cell without causing disease
• Once inside, the desired gene should restore the function of the protein

Vectors can be given intravenously, which means they are administered into a vein, or injected into a specific tissue in the body.

Other procedures, such as surgery, can also be used to deliver vectors into specific areas of the body.

The potential of gene therapy research has given hope to millions of people impacted by genetic diseases.