Restoration of vitamin C synthesis in transgenic Gulo^-/- mice by helper-dependent adenovirus-based expression of gulonolactone oxidase

Inability to synthesize vitamin C, because of a deficiency in gulonolactone oxidase (GULO) expression, is a genetic deficiency shared by a small number of animals including humans. Although the most overt symptom of vitamin C deficiency, scurvy, can be readily corrected by modest consumption of vitamin C, there is increasing interest in the effect of high-level administration in treating human disease. Using a previously derived Gulo-expressing vector, which produces murine GULO under the control of the murine cytomegalovirus (mCMV) promoter, we constructed and validated a recombinant helper-dependent adenovirus (HDAd-mCMV-Gulo) that can be used to correct this genetic defect. A human liver cell line (Hep G2) infected with the HDAd-mCMV-Gulo vector expressed GULO in a time- and gene dose-dependent manner. These cells also produced ascorbic acid when exogenous gulonolactone was supplemented in the medium. Likewise, Gulo ^-/- mice treated with HDAd-mCMV-Gulo at 2 × 10¹¹ VP expressed GULO in the liver and produced ascorbic acid. Serum ascorbic acid concentrations in Gulo^-/- mice injected with GULO-expressing HDAd were elevated to levels comparable to those of wild-type mice (62 ± 15 μM) after 4 days of infection and were maintained at significantly higher levels compared with those in untreated Gulo^-/- mice for at least 23 days. A similar elevation was observed in urine and tissue ascorbic acid concentrations in vector-treated animals. In conclusion, we demonstrate here that gene therapeutic HDAd-mCMV-Gulo vectors can mediate the expression of GULO and endogenous production of ascorbic acid in human cells and in Gulo ^-/- transgenic mice. Taken together, these data show that a gene therapy approach can be successfully employed in the treatment and further study of vitamin C deficiency in scurvy-prone mammals.