Gene Therapy: Present and Future

Gene therapy is a technique using vectors, either viral or non viral, for introducing therapeutic genetic material into target cells to solve a specific problem.

The main events that led to the development of gene therapy include: 

• Discovery of DNA as the keeper of genetic information.
• Development of PCR.
• Sequentiation of the human genome.
• Finding of restriction enzymes.

In September 1999, 18-year-old Jesse Gelsinger took part in a gene-therapy clinical trial. The adverse patient reaction to an adenovirus vector during the clinical safety trial led to the realization that the failure to understand the biology of vector interactions with the human immune system could have fatal consequences. 

In April 2000, a paper published in Science marked the highest point in the history of gene therapy. It described the first gene therapy success in which three children were cured of a fatal immunodeficiency disorder, but this therapy has subsequently caused a leukaemia-like disease in 2 of the 11 patients who have been treated.

The efficiency of gene therapy mainly resides in vectors. Intense efforts have been pointed to understand the molecular basis of how viruses and viral vectors interact with the host. In this sense, an ideal vector should have the following features:

• Do not trigger immune response.
• Easy to prepare.   
• Enough capacity to harbour large genes.
• Act both in proliferating and in resting cells.
• Easily dirigible to the target cell.
• Survival during large periods of time in the infected/transfected cells.
• Have regulatory elements to correct gene expression.

Vectors are classified in viral and non-viral:

1. Viral vectors: Viral vectors were the first to be used, given its high efficacy as vehicles for nucleic acids. Viruses are composed of DNA or RNA surrounded by a protein capsid, and in some cases, a lipoprotein envelope. There are five main classes of clinically applicable viral vectors: retrovirus, lentivirus, herpes simplex virus-1, adenovirus and adeno-associated virus.

Taken from Thomas, Ehrhardt and Kay. Nat Rev Genet 2003.

 Steps in gene therapy using an adenovirus vector:

2. Non-viral vectors: This type of vectors represents an attempt to mimic the virus functions as transferring vehicles using synthetic systems, but reducing the adverse characteristics of viruses. Efficacy of infection is much lower as it has to overcome both external and internal barriers. Liposomes and naked DNA are the main useful types with the advantage of unlimited length of the genomic material that can be introduced. However, they are still less efficient than viral vectors.

A summary of the extra- and intracellular barriers faced by non-viral gene therapies following systematic delivery can be observed below:

Taken from Miyata et al. Chem Soc Rev 2012 and McCrudden and McCarthy. Cancer Gene Therapy – Key Biological Concepts in the Design of Multifunctional Non-Viral Delivery  Systems. Gene Therapy - Tools and Potential Applications. ISBN 978-953-51-1014-9.

Nowadays, the discovery of interference RNA has opened a new field concerning the control of gene expression.

Target cells are selected depending on the tissue in which the gene should be expressed. They must also be cells with a long half-life being important to consider the rate of division of the cells. Best target cells are hematopoietic stem cells but lymphocytes, respiratory epithelium, hepatocytes, fibroblasts and muscle cells are highly targeted in gene therapy.

In the following videos, the gene therapy procedures to Parkinson and tumoural diseases are explained:

1. Definition of gene therapy and main developments of the field.
2. Know the difference between in vivo and ex vivo.
3. Study the characteristics of an ideal vector.
4. Know the classification of vectors and types of genetic information inserted.
5. Familiarize yourself with the main applications used nowadays.