Gene therapy is the introduction of foreign genetic material into body cells to compensate for abnormal genes or to synthesize beneficial proteins. The efficient transfer of genetic material into the target cell is essential to achieve the desired therapeutic effect. In order to facilitate gene transfer, either viral-based or non-viral based vectors can be used as the delivery system. Viruses are a type of widely-used vectors to transfer foreign DNA into a cell. The most common types of viral vectors used in gene transfer are retrovirus, adenovirus, and adeno-associated virus. The ability of viruses to infect cells is the key feature that allows viruses to be used as vectors in gene therapy.
Key Areas Covered
Key Terms: Alteration of Gene Expression, Functional Genes, Homologous Recombination, Infectious Life Cycle, Gene Therapy, Viruses, Vectors
What is Gene Therapy
Gene therapy is the insertion of normal DNA directly into a cell in order to correct genetic defects. The two main approaches to gene therapy are either introducing a gene that encodes for a functional protein or transferring of an entity that alters the expression of a gene in the genome.
- During the introduction of a functional gene into a genome, relatively large pieces of genes (>1kb) are introduced into the cell along with promoter sequences that initiate gene expression. Signaling sequences that direct RNA processing must also be introduced along with the protein-coding region.
- In order to alter the gene expression of an endogenous gene in the genome, relatively short parts of genetic material (20-50 bp) that are complementary to the mRNA of the defective gene are introduced. The alteration of gene expression can be achieved by blocking mRNA processing, translational initiation or leading to the destruction of mRNA.
Efficient delivery methods such as vectors facilitate gene transfer into the cells during gene therapy. Two types of vectors are used in gene therapy: viral vectors and non-viral vectors. The nonviral-based delivery systems can be plasmids or chemically-synthesized oligonucleotides. Optimal vector selection is based on several parameters:
- Type of the target cell and its characteristics
- The longevity of expression required
- The size of the genetic material transferred
Why are Viruses Used in Gene Therapy
Due to the infectious lifecycle, viruses are widely-used in gene transfer during gene therapy. Generally, viruses infect host cells and replicate inside the host cell by integrating viral DNA into the genome of the host. Hence, the interested DNA fragments can be introduced into the cell by packing it inside the viral particles. The three main types of viral vectors used in gene therapy are retroviruses, adenoviruses, and adeno-associated viruses (AAV). In addition, herpes simplex virus-1 (HSV-1), baculovirus, vaccine virus are also used as vectors. The use of adenoviruses in gene therapy is shown in figure 1.
In order to use a virus as a vector during gene therapy, its virulent genes are replaced with the gene of interest. The rest of the viral genome remains the same. The infected viral elements govern the homologous recombination of the modified viral genome to the host genome. The location for homologous recombination can be determined by the sequence of the recombining elements. Once integrated into the genome, the introduced DNA is stably expressed and replicates along with the host’s genome.
Gene therapy is a technology to treat defective genes in the genome by introducing new DNA. Both viral and nonviral DNA delivery systems facilitate the transfer of new DNA into cells. Viral delivery systems introduce new DNA into the host cells during infection. New DNA stably integrates into the host’s genome by homologous recombination, expressing and replicating along with the genome.
1. PONDER, KATHERINE PARKER. “Vectors of Gene Therapy.” An Introduction to Molecular Medicine and Gene Therapy., Wiley-Liss, Inc., 2001, pp. 77–112. Available here.
1. “Gene therapy” (Public Domain) via Commons Wikimedia