RETROVIRAL GENE-TRANSFER TO THE LIVER IN-VIVO DURING TRIIODOTHYRONINEINDUCED HYPERPLASIA

The liver is an important target organ for gene therapy but its mitoti c quiescence makes it resistant to integrative gene transfer. Retrovir us-based vectors integrate into liver cells in vivo but require the li ver to be primed before transduction; experimentally a 70% hepatectomy is commonly used to stimulate regeneration, rendering the liver susce ptible to transduction during the resulting wave of cell proliferation . Our aim was to develop a clinically acceptable method of inducing he patocyte replication before in vivo retroviral gene transfer which is both simple and effective. We have used the physiological hormone tri- iodothyronine (T3) to stimulate hepatocyte replication. A single dose of T3 (400 mu g/100 g bw) was given subcutaneously to euthyroid rats. This produced a labelling index of 31.7% in the hepatocyte population without histological or biochemical evidence of preceding liver damage . Following T3 administration the rat livers were transfected in vivo with an amphotropic retrovirus, TELCeB/AF-7 which encodes the beta-gal actosidase reporter gene together with a nuclear localisation signal. Transgene expression was noted only within the liver where 1.3% of hep atocytes expressed the beta-galactosidase enzyme. This compared to 5.2 % of hepatocytes transduced following a 70% hepatectomy, and 0.02% in animals receiving neither T3 nor partial hepatic resection before tran sduction. T3 administration is a simple way to prime the liver before in vivo retroviral vector-based gene transfer.