RESEARCH & DEVELOPMENT

Designer gene in HIV infection - a practical reality?

-Caroline Perry-

US researchers are moving closer towards using gene therapy to treat patients with mv infection. At the 10th International Conference on AIDS [Yokohama, Japttn; August 1994] Dr Flossie Wong-Staal discussed her group's work on gene therapy. A particular focus was research into the use of a hairpin ribozyme gene.

Originally. gene therapy simply referred to gene-replacement strategies for disorders caused by a ueiective gene. However. in its present expanded repertoire'. gene therapy embraces potential long­term treatments for chronic infectious diseases including HIV infection. says Dr Flossie Wong-Staal. of the University of California. San Diego. US.

The concept of using gene therapy in HIV infection has evolved from the basic principle that host-cell genes are used by the HIV virus for self-replication. Thus. the aim is to create a gene that is defective for HIV viral replication. but at the same time does not affect host-cell function. It is postulated that infused cells containing these HIV resistant genes would gradually replace those cells infected with the virus. and therefore prolong the healthy state of the infected individual. Although the work of Dr Wong-Staal and her group is still at an early stage. this line of research offers a 'potentially exciting therapeutic modality '. Two types of gene therapy are currently under investigation: the first approach uses gene therapy to boost the immunity of the host. while the aim of the second approach is to suppress viral replication as described above. This latter approach was explained by Dr Wong-Staal in her presentation.

Antiviral gene therapy

The ultimate aim of antiviral gene therapy (also known as intracellular immunisation) is to provide target cells with resistance to viral replication. Three steps are involved in this process [see boxed text].

To date. 2 approaches have been approved by the National Institutes of Health for phase I clinical trials. One of these is the hairpin ribozyme approach which has been proposed by Dr Wong-Staal and her group.

The hairpin n'bozyme design The hairpin ribozyme is derived from negative­

strand satellite RNA of the tobacco ringspot virus. and can be engineered to match a target cell sequence. Dr Wong-Staal believes this to be an attractive approach as the ribozyme may be able to act at many steps of the replication cycle (unlike other possible candidates which target a single step).

Moreover. since the ribozyme catalytically cleaves the substrate RNA. many substrate molecules may. potentially. be inactivated by one ribozyme.

In transiently transfected cells. Dr Wong-Staal and her team have demonstrated that a functional ribozyme specifically inhibited HIV-l expression. Both pre- and post-integration events were inhibited by the ribozyme. and it is possible that a synergistic antiviral effect resulted . No rapid emergence of resistant mutants was observed. allaying the initial concerns of these researchers.

The gene delivery system The most appropriate vector for delivery of the

hairpin ribozyme is currently being determined. Murine retrovIrus vectors have been studied in animals and man. so there are extensive safety data available. Furthermore. these vectors have been demonstrated to be efficient and persIstent. and to have a broad host range when used to transduce cells. However. they do not transduce nondividing cells - a possible limitation. since the terminally-differentiated macrophage is one of the major target cells for the HIV virus.

As murine leukaemia virus (MLV) vectors have been approved for gene therapy. this delivery system is often used in research. However. study is continuing into the use of alternative vectors. including an HIV vector. which would represent an attractive means of delivering therapeutic genes to the exact target cells that need protection.

Introduction rI the gene into target ce& Primary T lymphocytes and macrophages are the key

targets of HIV. and Dr Wong-Staal and her group are now faced with the challenge of introducing the gene to these target cells. As the ML V vector cannot transduce macrophages. the researchers have attempted to transfer the antiviral gene into haematopoietic progenitor cells. They have found that CD34+ cells obtained from cord blood were transduced at high efficiency. Targeting of the true stem cell may be a possible strategy to pursue for permanent immune reconstitution.

The next step forward Future research will be directed towards refining

gene constructs and delivery vectors. and conducting in vivo efficacy testing in animals.

In the short-term. a phase I trial will investigate the safety and feasibility of ex vivo T-cell therapy.

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Efficacy will be evaluated by measuring the relative survival of the ribozyme-transduced cells and their resistance to infection by IDV.

To prevent expansion of the IDV viral load in culture. 2 antiviral agents specific for mV-l will be included; nevirapine (to block the spread of the virus). and CD4-pseudomonas exotoxin (to kill those cells expressing the viral envelope protein).

The intention is to follow this with a trial designed to target stem or progenitor cells. In this study. the researchers propose to reinfuse autologous CD34+ cells into infants who have acquired IDV infection in utero. Infants would receive the infusion within days to weeks after birth.

'Gene therapy will only be practical for the world at large if one can dispense with ex vivo manipulations and develop vectors that [can) efficiently holM in [on) the right target cells for in vivo administration', said Dr Wong-Staal.

In conclusion. Dr Wong-Staal said that she hoped that the urgency of the AIDS epidemic would provide the impetus necessary to develop this 'powerful technology'. -

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