Gene therapy.pdf

Gene therapy chapter 14 What is gene therapy Gene therapy is the introduction of a biologically active gene into a cell to achieve a therapeutic benefit. In 2012, the first gene therapy product was licensed in the US and Europe for the treatment of lipoprotein lipase deficiency, and...

Gene therapy chapter 14 What is gene therapy Gene therapy is the introduction of a biologically active gene into a cell to achieve a therapeutic benefit. In 2012, the first gene therapy product was licensed in the US and Europe for the treatment of lipoprotein lipase deficiency, and gene therapy has now been approved for the treatment of several more disorders. The goal of gene therapy is to transfer the therapeutic gene early enough in the life of the patient to prevent the pathogenetic events that damage cells. Essential Requirements of Gene Therapy for an Inherited Disorder Identity of the molecular defect The identity of the affected gene must be known. Essential Requirements of Gene Therapy for an Inherited Disorder A functional copy of the gene A complementary DNA (cDNA) clone of the gene or the gene itself must be available. If the gene or cDNA is too large for the current generation of vectors, a functional version of the gene from which nonessential components have been removed to reduce its size may suffice. Essential Requirements of Gene Therapy for an Inherited Disorder An appropriate vector The ideal vector is safe, readily made, and easily introduced into the appropriate target tissue, and it would express the gene of interest for life. The most commonly used vectors at present are derived from the adeno-associated viruses (AAVs) or retroviruses, including lentivirus.  Essential Requirements of Gene Therapy for an Inherited Disorder An appropriate vector Retrovirus Adeno-Associated Adeno Virus Characteristics Virus - Nontoxic to the cell. - Do not elicit - Infects a wide variety of - Low number of copies of the strong dividing or nondividing viral DNA integrate into the immunologic cell types. host genome. responses. - Accommodates insertion of - The integrated DNA is - They infect longer genes. stable dividing or - Lentiviruses do not show nondividing preferential integration into cells. Limitations any specific gene locus, thus reducing the chances of activating an oncogene The target cell must undergo - Accommodate - Elicits strong immune division for integration of insertion of small response (restricted to Essential Requirements of Gene Therapy for an Inherited Disorder Knowledge of the pathophysiologic mechanism Knowledge of the pathophysiologic mechanism of the disease must be sufficient to suggest that the gene transfer will ameliorate or correct the pathologic process and prevent, slow, or reverse critical phenotypic abnormalities. Loss-of-function variants require replacement with a functional gene; for diseases due to dominant negative alleles, inactivation of the mutant gene or its products will be necessary. Essential Requirements of Gene Therapy for an Inherited Disorder Favorable risk-to-benefit ratio A substantial disease burden and a favorable risk-to-benefit ratio, in comparison with alternative therapies, must be present. Essential Requirements of Gene Therapy for an Inherited Disorder Appropriate regulatory components for the transferred gene Tight regulation of the level of gene expression is relatively unimportant in some diseases and critical in others. In thalassemia, for example, overexpression of the transferred gene would cause a new imbalance of globin chains in red blood cells, whereas low levels of expression would be ineffective. In some enzymopathies, a few percent of normal expression may be therapeutic, and abnormally high levels of expression may have no adverse Essential Requirements of Gene Therapy for an Inherited Disorder An appropriate target cell Ideally, the target cell must have a long half-life or good replicative potential in vivo. It must be accessible for direct introduction of the gene or, alternatively, it must be possible to deliver sufficient copies of the gene to it (e.g., through the bloodstream) to attain a therapeutic benefit. The feasibility of gene therapy is often enhanced if the target cell can be cultured in vitro to facilitate gene transfer into it; in this case, it must be possible to introduce a sufficient number of the recipient cells into the patient and have them Essential Requirements of Gene Therapy for an Inherited Disorder An appropriate target cell Stem cells are the idea; targets cells; they are self replicating and eliminate the risk of graft- versus- host disease. The number of cells into which the gene must be introduced have a significant therapeutic effect (The organ plays a role: liver vs muscle). Essential Requirements of Gene Therapy for an Inherited Disorder Strong evidence of efficacy and safety Cultured cell and animal studies must indicate that the vector and gene construct are both effective and safe. The ideal precedent is to show that the gene therapy is effective, benign, and enduring in a large animal genetic model of the disease in question. At present, however, large animal models exist for only a few monogenic diseases. Genetically engineered or spontaneous mutant mouse models are much more widely available. Essential Requirements of Gene Therapy for an Inherited Disorder Regulatory approval Protocol review and approval by an institutional review board are essential. In most countries, human gene therapy trials are also subject to oversight by a governmental agency. Risks of Gene Therapy 1- Adverse response to the vector or vector- disease combination. 2- Insertional mutagenesis causing malignancy. Activating a protooncogene or disrupting a tumor suppressor gene, leading possibly to cancer. 3- Insertional inactivation of an essential gene. Disrupt a gene essential for viability Insertion into a gene in the germline could create a dominant disease-causing mutation that might manifest in the treated patient’s