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Gene Transfer and Gene Therapy
Vita - Salute San Raffaele University School of Medicine, Italy
Gene therapy is broadly defined as the introduction, using a vector, of an exogenous gene expression unit (transgene) into the somatic cells of an organism in order to prevent, halt or reverse a pathological condition. The transgene may encode for one or more proteins or for non-coding RNA, such as... read morean RNA interference molecule or other type of small RNA. The expression of the transgene can replace a missing function (gene replacement), alter expression of an endogenous gene (gene knock-down, altered splicing or re-activation), instruct a new function (gene addition) or edit an endogenous gene (gene disruption and gene correction). The vector can be the naked nucleic acid itself (DNA plasmid or RNA molecules), or the nucleic acid complexed with lipids and/or other polymers, or be an engineered virus (most often made replication-defective) which contains the transgene within its genome. Depending on the vector, the transgene can integrate as DNA into the host chromosomes, become a DNA episome in the cell nucleus or exist only as an RNA molecule. The choice of vector impacts the efficiency and persistence of transgene expression, as well as the biosafety of the procedure. The vectors can be administered in vivo into an organ, an anatomical region or systemic circulation, or ex vivo using cells harvested from a patient (autologous) or from a healthy donor (allogeneic). Successful gene transfer requires overcoming several biological barriers, including tissue architecture, innate and adaptive immunity to exogenous nucleic acids and viral particles. Ex vivo gene therapy requires isolating stem, progenitor or differentiated cells, expanding and genetically modifying them ex vivo and administering them to the patient to establish a transient or, more often, a stable graft of the infused cells and their progeny. Again, tissue homeostasis and host responses profoundly affect the outcome of the procedure.
Gene therapy has had a rocky course, from its hyped beginnings, which oversold the prospect of an easy cure for many devastating diseases, through the hardship of the first clinical failures and the occurrence of unpredicted serious adverse events in some trials, which undermined confidence and triggered skepticism from the scientific community. Yet, there have been some remarkable successes as well, with several people today living a normal life thanks to gene therapy, which has provided a cure for their otherwise deadly or untreatable disease. More recently, as new or improved generations of vectors have entered clinical testing, they have shown remarkable efficacy in the treatment of some inherited diseases, such as retinopathies, hemophilia, immune-hematological and storage diseases, as well as some types of cancer. The observed benefits are supported by in-depth molecular follow-up of human patients, which is providing unprecedented insights into complex pathophysiological processes, such as stem cell activity, tumor progression and the deployment of immune response. In the laboratory, scientists have been engineering vectors to modify tissue tropism and target transduction, escape the host immune response, alleviate the risk associated with transgene integration and improve the regulation of transgene expression. Overall, this is an exciting time for gene therapy, which enjoys renewed attention from a growing audience of scientists, clinicians and the biotech and pharmaceutical industry.
Despite these important advances, however, much still needs to be done in order to achieve stringently regulated expression of therapeutic transgenes, correction rather than replacement of malfunctioning genes, targeted delivery and lower toxicity of vector administration, reduced immune activation and induction of tolerance to the transgene product and improved engraftment of transplanted cells and tissue regeneration. These are major goals of the research conducted in many laboratories worldwide.
The translation of gene therapy into new medicines widely available to patients faces several formidable challenges from the scientific, technological and regulatory standpoint, as well as in terms of the resources needed and its economic sustainability. The first-in-human testing of a gene therapy product often raises ethical questions of general societal relevance and calls for careful scrutiny of the predicted as well as unexpected hazards for the patient, its proxy’s as well the general public and the environment. The design of relevant preclinical studies and the selection of diseases providing an appropriate risk-benefit ratio for the first human testing of such new types of treatment are paramount in a successful roadmap to the clinic. Gene therapy scientists and clinicians constantly collaborate with the pharmacological industry and Regulatory Authorities worldwide to revise, update and harmonize the regulatory framework for conducting clinical trials of gene therapy in order to address the specific requirements and challenges posed by the new therapies and facilitate their clinical development.
Besides the remarkable clinical achievements, one should not overlook the constant contribution of the gene transfer field to the advancement of experimental biology and biomedical sciences. The development of powerful technologies for efficient and safe gene transfer with minimal impact on target cell biology, regulated transgene expression and precise genome editing are just some examples of the precious tools made available to the daily work of countless investigators and laboratories. The field of gene therapy continues to be inspired by emerging concepts in virology, stem cell biology, transplantation and tissue regeneration, immunology and pathology, as well as by the deeper understanding of the structure and function of the human genome and epigenome.