Exploiting the AIDS virus to fight cancer

Centre National de la Recherche Scientifique (CNRS) researchers at the Architecture et Réactivité de l'ARN (ARN) laboratory have taken an important step towards transforming the human immunodeficiency virus (HIV) into a biotechnological tool capable of facilitating cancer treatments. Using cell cultures, the team has succeeded in developing a mutant protein that improves the effectiveness of an anticancer drug, thus allowing it to be administered in lower concentrations.

The researchers, whose findings have been published in the journal PLoS Genetics, set about exploiting certain characteristics of HIV – the virus which causes AIDS – for therapeutic purposes. By taking advantage of HIV’s replication machinery, the team has succeeded in developing an agent that activates anticancer drugs more effectively than proteins found naturally within the human body. Beforehand, however, the scientists modify the virus to make it completely benign.

I spoke to Dr Matteo Negroni, Research Director at ARN, to find out more about this novel approach to cancer treatment. Dr Negroni began by explaining why an activating agent is necessary for anticancer drugs.

"Our procedure is concerned with mutagenesis," he began. "A gene encodes for a protein and it is the protein that influences the behaviour of a cell. There is a whole branch of research orientated towards generating new proteins that do not actually exist in nature. Proteins capable of performing reactions that natural proteins cannot perform can be very useful indeed.

"The best example that I can give is that of the gene that we targeted as part of our research – deoxycytidine kinase (dCK). An active anticancer drug is not capable of passing through cell membranes. Essentially, an active drug cannot get into a cell. We therefore deliver the drug in the form of a ‘predrug’. Once the predrug reaches its destination, it needs cellular genes to activate it. Natural enzymes are capable of activating compounds that are not found in nature, but they cannot do so efficiently. This is because there has been no evolution to provide optimal compatibility between the enzyme and this new, synthetic substrate. It is therefore advantageous to produce proteins that are better equipped to activate anticancer drugs. This is a completely artificial method of drug activation."

By inserting its genetic material into the host cell’s genome, HIV is able to use human cell material to proliferate. This ability – a characteristic that makes AIDS so difficult to treat – was something that Dr Negroni and his colleagues set about exploiting to fight cancer.

"We wished to enlist two features of HIV’s replication strategy: its ability to mutate and its potential to deliver our gene in an effective manner," said Dr Negroni. "Firstly, HIV is highly mutagenic. This characteristic allows the virus to escape the human immune system. The virus is never the same; it constantly mutates whilst replicating within a patient, so why not try to exploit its ability to generate mutants? We began by emptying the virus of its genomic material and we replaced this with our own gene. The virus was therefore unable to replicate because it contained no viral proteins. The proteins were provided by us – by hand – and it was those proteins that were replicated instead.

"The second feature that we wished to exploit was the virus’s ability to enter the cell and insert its genetic material within the cell’s chromosomes. This characteristic enabled us to automatically insert our gene within the cell. Essentially, we began by making mutations and then, all of the variants were inserted within the target cell. We directly influenced the cell and caused it to behave in exactly the way that we desired. In this case, we wanted it to die when it encountered low concentrations of the anticancer drug."

The CNRS team conducted their studies using cell cultures. This method is preferable to using test tubes as it allows scientists to see the more complex interactions that take place within a cellular environment. The next stage of the research will be to perform preclinical tests using laboratory animals.

"Our approach allowed us to identify mutants capable of making target cells die at drug concentrations lower than those required when relying upon naturally occurring enzymes," explained Dr Negroni. "Of course, a whole series of intermediate experiments will be required before we can know whether this treatment is effective in humans, but if our results hold true, we will be able to achieve the same efficiencies at concentrations much lower than those used currently. The toxicity of anticancer drugs – which is often the cause of problems during treatment – will be much, much lower. Adverse side effects will be dramatically reduced."

If the researchers succeed in targeting and delivering their transgenes to specific tumour cells, they will be able to exploit another phenomenon to maximise the efficacy of their method. As Dr Negroni explained, whilst the mechanisms involved in the process are not fully understood, the ‘bystander effect’ could prove to be a useful tool.

"The bystander effect is a phenomenon that triggers the death of surrounding cells," he said. "This could be due to drugs passing into neighbouring cells. Alternatively, the immune system might notice that cells are dying, assume that there is a tumour, and act on this information. Nobody knows exactly why this happens, but it means that you don’t have to saturate the entirety of a tumour’s mass with your transgene. You just need to deliver it to a few cells and then expose the tumour to the anticancer drug.

"There is another way in which we might usefully deploy our mutants," continued Dr Negroni. "The genes of some patients are unable to perform certain metabolic reactions that are required for this technique. However, it is possible to remove the cells, modify them by inserting an extra gene with a correct sequence, and reintroduce these cells into the patient. Once they have been returned, the modified cells can proliferate and regenerate a whole subset of cells that contain the desired sequence. This method has been used in the past to correct certain immunological disorders.

"There is a danger, however, that when you reintroduce modified cells into a patient, you might trigger a process of uncontrolled proliferation. In some previous cases, patients developed leukaemia as a result of such proliferation. To avoid this situation, we have found that you can simultaneously introduce a suicide gene. In the event that the cells undergo uncontrolled proliferation, you can expose the patient to low concentrations of a particular drug and it will selectively remove these cells."

Whilst the technique is still at a fairly early stage of development, it is certainly promising news for the field of oncology.

"The next step is to move onto preclinical animal tests," concluded Dr Negroni. "Once we have seen whether our method is successful in more complex and physiologically relevant contexts, we can begin to think about applying the treatment in humans."