Background & aims: In the quest for a recombinant viral vector for liver-directed gene therapy that would permit both prolonged and efficient transgene expression in quiescent hepatocytes in vivo and repeated administration, we evaluated a recombinant simian virus 40 (rSV40).
Methods: The rSV40 was generated through replacement of the DNA encoding for the T antigens (Tag) by the coding region of human bilirubin-uridine 5'-diphosphate-glucuronosyl-transferase (BUGT) complementary DNA (SV-hBUGT). Helper-free rSV40 units were generated at infectious titers of 5 x 10(9) to 1 x 10(10) infectious units (IU)/mL in a Tag-producing packaging cell line (COS-7 cells).
Results: After 1, 3, or 7 daily infusions of 3 x 10(9) IU of SV-hBUGT through an indwelling portal vein catheter in bilirubin-UGT-deficient jaundiced Gunn rats, mean serum bilirubin concentrations decreased by 40%, 60% and 70%, respectively, in 3 weeks and remained at those levels throughout the duration of the study (40 days). Results of liver biopsies from SV-hBUGT-treated Gunn rats, but not from controls, were positive for human BUGT DNA, messenger RNA, and protein. Bilirubin-UGT activity in liver homogenates was 8%-12% of normal, and bilirubin glucuronides were excreted in bile. Immunostaining showed that >50%-60% of hepatocytes stably expressed the transgene. Portal vein infusion of an rSV40 expressing hepatitis B surface antigen (HBsAg) in a naive Gunn rat and a Gunn rat that had received 7 injections of SV-BUGT resulted in approximately equal levels of hepatic expression of HBsAg, indicating that multiple inoculations of SV-BUGT did not elicit neutralizing antibodies. Plasma alanine aminotransferase levels and liver histology remained normal despite repeated injections of rSV40.
Conclusions: rSV40 vectors may represent a significant advance toward gene therapy for metabolic diseases.