Current developments and future prospects for HIV gene therapy using interfering RNA-based strategies

Acquired immunodeficiency syndrome (AIDS) is a slow, progressive, degenerat ive disease of the human immune system, ultimately leading to premature dea th of the patient. This disease is primarily caused by human immunodeficien cy virus type-1 (HIV-1). The major targets of HIV infection are blood cells , namely lymphocytes and macrophages. While the immune response fails to el iminate the infected cells, the virus continues to spread. The purpose of H IV gene therapy is to provide "anti-HIV" genes to cells that are susceptibl e to HIV infection. Anti-HIV genes may be designed to express RNAs or prote ins that interfere with the function of viral or cellular RNA(s)/protein(s) , thereby inhibiting virus replication. Whereas interfering proteins may be cytotoxic and/or immunogenic, interfering RNAs are not. Interfering protei n-based strategies requiring inducible gene expression (under the control o f HIV regulatory proteins) can only be designed to block steps subsequent t o the viral regulatory protein production. In contrast, interfering RNAs ca n be produced in a constitutive manner, which further enhances their antivi ral activity and allows one to design strategies to inhibit virus replicati on before viral regulatory protein production occurs. Thus, interfering RNA s are of particular interest and are the focus of this review. Genes expres sing interfering RNAs were designed to inhibit syncytium formation to preve nt the death of the gene-modified cells. Strategies may also be developed t o prevent gene-modified cells from becoming infected by HIV or from support ing HIV replication. Genes expressing interfering RNAs have been designed t o inhibit HIV-1 entry and to cleave the incoming virion RNA, thus blocking virus replication before provirus DNA synthesis can be completed. A number of genes were also designed to express interfering RNAs that inhibit HIV re plication at a post-integration step, by inhibiting the function of HIV RNA s or proteins produced in the infected cell. Also in development are anti-H IV genes that produce RNAs that would not only inhibit HIV replication in t he gene-modified cell, but also prevent HIV RNA packaging and/or reverse tr anscription such that the progeny virus produced would be non-infectious. F urther refinements to these strategies may lead to the development of "self -propagating" anti-HIV genes. These genes would express interfering RNAs th at not only inhibit virus replication in the cell and prevent HIV RNA packa ging and/or reverse transcription in the progeny virus, but also make use o f the HIV itself to deliver the anti-HIV gene(s) to other cells. Thus, more and more cells susceptible to HIV infection would become resistant. Such " self-propagation" of anti-HIV-1 genes would only occur in cells that are su sceptible to HIV infection, and would continue to take place for as long as HIV exists in the body.