Traditionally, anti-viral treatment against HIV has focused on drugs that inhibit the activity of proteins or enzymes which HIV uses to manufacture more virus from inside infected cells. Gene therapy targets an earlier stage, interfering with the genetic process by which HIV proteins and enzymes are made in the first place.

How genes work

Our genetic material is present in every cell in the form of two DNA. Particular parts of the DNA strand contain the instructions for making specific enzymes or proteins, and each of these parts is called a gene.

When a cell needs enzymes or proteins to carry out a particular function, the relevant gene which contains the instructions for making them is activated. The activated gene is then converted or 'transcribed' into RNA (ribonucleic acid), a single-stranded copy of that gene's DNA. The RNA molecule then leaves the nucleus of the cell and attracts specific amino acids, so forming the required protein or enzyme. Instead of forming a single protein or enzyme, sometimes the RNA molecule is converted into a large strip of protein, which is then cut up or 'spliced' by enzymes such as protease to form several smaller, functional proteins.

Gene therapy and HIV

Most current research is investigating the potential for gene therapy in treating inherited genetic disorders. Researchers hope that it may be possible to correct genetic abnormalities which can cause diseases previously regarded as untreatable.

HIV infection is not an inherited genetic disorder, but in many ways it can be viewed as an acquired genetic disorder. As it is a retrovirus, HIV inserts the DNA copy of its genetic material into the human cell's DNA. The cell then produces new virus particles by making the viral proteins from the virus's genes that have been incorporated into its DNA, mistaking them for its own genes. Gene therapy approaches may thus be able to target the HIV genes in infected cells in a similar manner to the way in which they target faulty human genes in inherited genetic disorders.

Types of gene therapy for HIV

There are three general ways in which gene therapy could potentially be tested for treating HIV infection:

• As an anti-viral, targeting the HIV genes in infected cells.

• As a protective agent, altering uninfected cells so that HIV is unable to infect them.

• As an immune booster, by increasing the immune system's own ability to detect and attack HIV-infected cells

Some approaches use small molecules called anti-sense oligonucleotides. These bind to the RNA strands before they can attract the necessary amino acids to form the functional proteins.

Another strategy is to use molecules that can detect specific parts of HIV's RNA within infected cells and splice and inactivate it. These molecules are called ribozymes. Ribozymes can be created that splice HIV RNA in several different locations, so that one ribozyme may simultaneously prevent the formation of several different HIV proteins. Even if HIV mutates against one splicing site, the ribozyme will still be effective at others. In addition, ribozymes can attack HIV RNA soon after the virus enters a new cell and before it has incorporated its genetic information into the cell's DNA, theoretically stopping the infection process in its tracks. This approach has been pioneered by Flossie Wong-Staal of the University of California at San Diego.

Another strategy involves creating mutant forms of essential viral proteins. The defective proteins effectively compete with real HIV viral proteins within the body but do not carry out their proper function, thus interfering with the ability of HIV to reproduce. These altered proteins are called transdominant mutant proteins (Lisziewicz 1998). The leading research in this area, by Dr Gary Nabel of the University of Michigan, has employed mutant forms of HIVs rev gene. An early trial has found that CD4 T-cells from HIV-positive people that were treated in this way and then returned to the body persisted in the body for significantly longer than unaltered CD4 cells, without any apparent side-effects (Woffendin 1996).

Immune-based gene therapy

Other researchers are trying to use gene therapy either to boost the immune system's CD8 T-cells, which can kill HIV-infected cells, or to protect CD4 T-cells from being infected. In each of these two approaches, immune cells are taken from people with HIV and a gene is inserted that either protects them from HIV or activates them against HIV. The enhanced cells are then cloned to increase their numbers and reinfused back into the patient. This is called adoptive cell therapy. One such experimental treatment is HGTV43.

Several clinical trials of both these approaches have taken place. To protect participants against unexpected side-effects from this novel treatment, in some trials the cloned cells also had another gene inserted which made them susceptible to ganciclovir (Cymevene), allowing doctors to kill all the altered cells at the first sign of any problems by administering ganciclovir. However, when these cells were re-infused into trial participants no benefits were seen because the cells were rapidly identified as 'foreign' and destroyed by the recipients' immune systems.

Despite this, a company called Enzo Biochem has announced that a gene transfer vector called HGTV43 does not trigger an immune response. This once again raises the possibility that this type of gene therapy may be a viable approach.

Another approach, using genetically modified CD4 T-cells, found that cells persisted after infusion for an average of 3.2 years, with the best persistence seen in those patients who also received infusions of interleukin-2 (IL-2), which would be expected to expand the CD4 cell population (Lu 2002).

However, adoptive cell therapy requires the removal, alteration and cloning of cells individually for each patient and for this reason is unlikely to be a practical or cost-effective way of treating large numbers of HIV-positive people.

Delivering the therapies

The main problem with all these strategies is how to deliver the new genes into cells.

The most common approach is to use genetically altered viruses to carry the gene therapy into the target cells and 'infect' them with it. These altered viruses are called viral vectors. The most commonly used viral vector is a mouse retrovirus that has been genetically manipulated to make it incapable of reproducing itself.

Some researchers think it could prove difficult to get gene therapy into all the parts of the body infected by HIV, such as the lymph nodes and the central nervous system. However, a possible way around this is to use altered HIV itself as a viral vector, although no-one knows the long-term consequences of combining 'therapeutic' HIV with 'normal' HIV within the body.

No-one can be sure that these therapies will not cause unforeseen side-effects. The insertion of the 'foreign' gene therapies into the genetic material of normal cells could affect their normal functions, or lead them to mutate. The body's immune system could also identify protein-based gene therapy as 'foreign' and try to eliminate it from the body.

A virus called SV40 has been used to deliver multiple HIV genes in the test tube, and this approach has been shown to deliver better protection against HIV infection than the delivery of single genes (Strayer 2002).

The gene therapy for HIV that has progressed furthest to date uses a treatment called HIV-IT (V). See HIV-IT (V) in Drugs used by people with HIV: Immune-modulating drugs.

References

Bridges SH et al. Gene therapy and immune restoration for HIV disease. Lancet 345: 427-432, 1995.

Chang HE. Gene therapy strategies for HIV-1 infection. J Physicians Assoc AIDS Care, 1994.

Cotton P. High-tech assault on HIV: gene therapy. JAMA 272: 1235-1236, 1994.

Lisziewicz J et al. Combination gene therapy: synergistic inhibition of HIV-1 Tat and Rev functions by a single RNA molecule. International Conference on the Discovery and Clinical Development of Antiretroviral Therapies, St Thomas, abstract 13, 1998.

Lu A et al. Long-term persistence and safety of gene-modified syngeneic CD4+ T lymphocytes in HIV-infected patients. Ninth Conference on Retroviruses and Opportunistic Infections, Seattle, abstract 525, 2002.

Nabel GJ et al. Gene transfer strategies for AIDS and emerging viruses. International Conference on the Discovery and Clinical Development of Antiretroviral Therapies, St Thomas, abstract 12, 1998.

Strayer D et al. Combining antilentiviral transgenes potentiates inhibition of HIV. Ninth Conference on Retroviruses and Opportunistic Infections, Seattle, abstract 382, 2002.

Woffendin C et al. Expression of a protective gene prolongs survival of T cells in human immunodeficiency virus-infected patients. Proc Natl Acad Sci U S A 93: 2889-2894, 1996.