The overall goal of this study is to validate our hypothesis that lentivirus vectors can serve as an efficient and safe platform for therapeutic gene delivery to the liver tissue. The ability of HIV-1 and other lentiviruses to transduce non-dividing cells prompt the development of an HIV-l based gene delivery system. The novel lentivirus vectors proved efficient at transducing various tissues in vivo (brain, liver, muscle, retina, and hematopoietic stem cells) without any detectable pathology. Recently, we showed that a single intraperitoneal injection of hemophilic mice with lentivirus vectors resulted in long term expression of therapeutic levels of canine factor IX. The treated mice demonstrated aPTT values equivalent to those obtained from heterozygous littermates. In addition our preliminary results indicated that cis-regulatory sequences in the lentivirus down-regulate transgene expression. These studies are most encouraging, however we believe that further improvements in: vector production, trsansgene expression, and regulation, and better characterization of the mechanism responsible for the development of inhibitory antibodies are required before we can consider the use of the lentiviral system as a safe and efficient viral vector for liver gene therapy. To facilitate safe vector production we propose to generate a novel third generation packaging cell line, which will be devoid of the Tat and all HIV-l accessory proteins. As an additional measurement of safety, we will separate the new packaging system into four stably integrated plasmids (vector, envelope, packaging, and rev). To improve transgene expression from the lentivirus vector cassette we will attempt to identify and delete inhibitory sequences from the lentivirus vector genome. To improve regulation of transgene expression we will generate an improved new inducible lentivirus vector which will exhibit minimal basal inducible promoter activity. Testing the proposed improvements in hemophilic mouse and canine animal models will allow us to characterize potential immune response against the newly synthesized factor IX. We believe that the ability to maintain therapeutic levels of factor IX in these animal models will determine the feasibility of using lentivirus vector based gene therapy to cure hemophilia B and other hepatic metabolic diseases.
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