In gene therapy, doctors modify a person’s genes to treat or cure disease. Human gene therapy seeks to modify or manipulate the expression of a gene or to alter the biological properties of living cells to prevent disease, reduce further damage and pain, or potentially cure the patient. Gene therapies can work by several mechanisms. Gene therapy can be done by:

• Replacing a mutated (defective) gene with a healthy copy
• Introducing a new gene to the body
• Inactivating or “silencing” a gene that doesn’t function properly

If a mutated gene is causing an important protein to function poorly, gene therapy seeks to restore the function of the protein and therefore restore certain functions of the patient.

If a mutated gene causes an important cell-building protein to function poorly, gene therapy may be able to restore the function of the protein by:

• Replacing a disease-causing gene with a healthy copy of the gene.
• Inactivating a disease-causing gene that is not functioning properly.
• Introducing a new or modified gene into the body to help treat a disease

Researchers select the right approach based on the best current understanding of the CAUSE of the disease. This is an important point.

Gene therapy may be performed in vivo, in which a gene is transferred to cells inside the patient’s body, or ex vivo, in which a gene is delivered to cells in a laboratory setting and the treated cells are then transferred back into the body.

Currently, gene therapy developers develop medicines to introduce new or corrected genes into patient cells using vectors. Vectors are delivery vehicles, or carriers, that encapsulate therapeutic genes for delivery to cells. Currently used vectors include disabled viruses and nonviral vectors, such as lipid particles.

Deactivated or disabled viruses cannot make patients sick, even though they rely on the biology of viruses to operate. Viral vectors are made from parts of virus and act as the vehicle to transfer new genetic material into the cell where it is incorporated into the chromosomes in the nucleus.

Deactivated viruses that have been used for human gene therapy vectors include:

• Lentiviral vectors: A Lenti- vector can integrate its genome into both dividing and non-dividing cells in the body, leading to new gene expression that is designed to be stable and durable. Lentivectors can carry genetic information into the nucleus of cells, potentially allowing for stable and durable expression of the genetic information that it integrated into the cells.
• Adeno-associated virus (AAV): These viruses are small single-stranded DNA viruses grouped with parvoviruses. One parvovirus causes a rash in children known as “fifth disease.” Nevertheless, AAVs are a different class of parvoviruses and they are dependent on a helper virus co-infection to replicate. Viruses that have difficulty replicating make them better candidates to use to create vectors.
• Adenoviruses: Adenoviruses is a group of common viruses that can infect the lining of your eyes, airways and lungs, intestines, urinary tract, and nervous system. They are common causes of fever, coughs, sore throats, diarrhea, and pink eye.
• Retroviruses: Retrovirsuses have genes that are encoded in RNA instead of DNA. They are widely used and well-known in laboratory biology. Because they enable persistent gene expression they might be a good approach for several monogenic diseases. Immunogenicity and insertional mutagenesis are obstacles to a wider clinical use of these vectors.
• Herpes simplex virus can be used to create vectors that can carry a large amount of genetic material, for example, for delivery to neural cells.