Liver-directed gene therapy: Evaluation of liver specific promoter elements

Liver-directed gene therapy could dramatically alter the therapy of many inherited hematologic and metabolic diseases. We have developed a rapid, reliable, low-mortality method for the in vivo delivery in rats of retroviral vectors 24 hr after 70% hepatectomy by intraportal injection during hepatic in-flow occlusion (HIFO). Using the human α₁-antitrypsin (hAAT) reporter gene, we found in vivo that up to 10% of hepatocytes integrated the provirus, and serum hAAT protein levels were sustained for up to 1 year. Despite high in vivo transduction efficiencies, gene expression at the mRNA level is disappointingly low compared to in vitro transduced NIH 3T3 or Hepa A1 tissue culture cells. In this report, LNL-6-derived retroviral vectors (RV) were combined with one of two strong, liver-specific promoters, murine albumin or human α₁-antitrypsin, an upstream insertion of a trimer of hepatocyte nuclear factor-3 (HNF-3) binding sites, and the hAAT reporter gene. HNF-3 has been demonstrated to increase in vitro transcriptional activity [23]. Twenty- four hours after 70% hepatectomy, 10-fold concentrated (by methotrexate- resistant titer) RV-producing cell supernatant was given intraportally during a 3-min HIFO. Serum hAAT levels as quantitated with a human specific ELISA were sustained for over 40 weeks with all of the liver-specific promoter constructions. However, the hAAT protein production with the murine albumin promoter retroviral constructs decreased with time, but was sustained at levels approximately 80% of the initial serum peak levels with the constructs containing the hAAT promoter. We conclude that the retroviral vectors containing liver-specific promoters can lead to sustained, transduced gene expression and the quantitative evaluation of in vivo promoter strengths is essential for the development of high expressing retroviral vectors necessary for hepatic gene therapy in humans.