DEVELOPMENT OF AN HSV-1 GENE TRANSFER VECTOR WITH LOW TOXICITY

Simply defined, therapeutic gene transfer involves the transfer of genetic material into the cells with the aim of correcting a disease and/or slowing down the progression of a disease. The public health significance is that therapeutic gene delivery carries the hope of curing currently incurable diseases such as cystic fibrosis and Duchenne muscular dystrophy (DMD), or providing better solutions to current treatments for conditions such as chronic pain. There is high expectation in this new generation of medicine that therapeutic gene transfer would rid the patients of the painful conditions and lead them to better lives.One key component of this procedure is the vehicle that has the capability to deliver the therapeutic gene into different tissues efficiently and safely. A number of vectors based on retroviruses, lentiviruses, adenoviruses and adeno-associated viruses have been developed and their efficacy and safety evaluated in animal models and clinical trials.Possessing several advantages, such as its large genome, the ability to transduce both dividing and non-dividing cells, HSV-1 based vectors represent promising candidates for gene delivery. Three types of HSV-based vectors have been developed: HSV amplicons, replication competent vectors and replication defective vectors. Replication defective vectors based on multiple essential immediate early (IE) gene deletions carrying different transgenes have been constructed.However, HSV vectors have disadvantages. The most prominent one is toxicity. To reduce toxicity, more viral genes are deleted in addition to the essential IE genes. One such gene is the one encoding viral regulatory protein ICP0. However, the deletion of ICP0 gene renders the vector highly inefficient in transgene expression although the toxicity is lowered. Thus, how to restrict the toxicity while retaining the expression is one of the issues that needs to be addressed in HSV vector development.One possible solution is to lower ICP0 level through mutations in the ICP0 promoter region. To test this method, activities of reporter gene driven by mutated ICP0 promoter were assayed and the results showed that deletions in the promoter region did reduce ICP0 expression.However, the optimal level to meet the goal could not be determined by reporter gene assays. Thus, constructs carrying mutant promoters driving ICP0 coding sequence were created. During the construction of these viruses, the positive control virus, later named JDTOZHERO, was found to carry an unexpected deletion in the 5'-UTR. This deletion gave this virus some unique features that fulfilled our goal, i.e., low toxicity and reasonable expression. This virus is characterized in detail in this thesis