[Frontiers in Bioscience 3, a11-15, February 1, 1998]
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TARGETED INHIBITION OF HEPATITIS B VIRUS GENE EXPRESSION: A GENE THERAPY APPROACH

Frank Y.-T. Tung and Steven W. Bowen

Department of Infectious Disease and Microbiology, University of Pittsburgh, Pittsburgh PA 15261

Received 1/21/98 Accepted 1/28/98

5. DISCUSSION

In this report, we describe a potential gene therapy approach for persistent HBV infection. Antisense RNAs of the HBV surface antigen gene expressed from a retroviral vector can effectively inhibit HBsAg expression by 50-75%. The advantage of using long antisense RNA (0.8 kilobases in this case) to inhibit the target sequence is that variant viruses should also be inhibited by antisense RNA due to complementation to the target sequence. Many HBV variants have been identified and shown to contribute to differences in severity of disease (33). Recently, Yu et al, (34) reported a ribozyme-mediated inhibition of HBsAg by co-transfection assay which is similar to this paper. In that report, high molar ratio (at least >5) of ribozyme construct to HBV expression construct was used to achieve 80% inhibition by measuring endogenous polymerase activity. Due to a different HBV expression construct being used, the efficiency of inhibition can not be compared directly. However, it would be difficult to deliver high copy number of ribozyme to liver cells in practice.

The unique features of retroviral vectors for gene transfer are that they can provide long-term gene expression and are non-immunogenic to the host immune system. To achieve long lasting anti-HBV effect, the antisense transduced cells have to stably express antisesnse RNA in chronic HBV patients. Using current ex vivo gene transfer protocol, only a small percentage of hepatocytes can be transduced with a retroviral vector. However, this small percentage of antisense transduced liver cells may have a selective advantage over unprotected HBV infected cells as the result of immune-mediated damage. With the regenerating ability of normal hepatocytes, the antisense RNA protected cells may dominate the cell population in the liver.

In our assay system, the multiple copies of HBV genome still can enter the antisense transduced cells by co-transfection. Therefore, the anti-viral effect may be more significant in natural HBV infection. In that case, fewer copies of replicative RNA intermediates and/or viral mRNA are present in the infected cells as the targets of antisense RNA.