Sharing essential knowledge with health researchers and policy makers

Communities of practice

Harvard Malaria Initiative opens new avenues for research

9 Oct 2008

Tatum Anderson

Source: TropIKA

Dyann Wirth.

As a brash, recently-appointed post-doc in molecular biology who was working under Walter Gilbert the Nobel prize-winner for his work on DNA sequencing, Dyann Wirth's fate was sealed when she heard a talk by another Nobel Laureate.

Konrad Bloch, who discovered vitamin B12, was asked to talk about malaria at a retreat for Harvard University scholars. 'He had been asked by the World Health Organization to advise them what to do about the disease. He had hand-drawn slides about the malaria parasite, biology and the world health problem,' she says. 'I remember sitting in the back of the auditorium and said to myself this is really fascinating. This is what I want to work on.'

Wirth immediately embarked on a course in parasitology and began a career in developing an understanding of the fundamental biology of the malaria parasite, and applying that knowledge to developing new tools - from vaccines to drugs.

The beginning of her career coincided with the rise of resistance to chloroquine, the hitherto anti-malarial treatment of choice, in the early eighties.

As a result of work by her group and scientists at NIH, it is now known that the mutant parasite emerged in south-east Asia and South America at about the same time and migrated via infected people moving along existing trade routes - from Asia to East Africa, and then towards West Africa. She has also looked into molecular mechanisms that control the way drugs are react with the parasite, molecular genetic tools used to investigate malaria and leishmania parasites and her group claims to be the first to discover multidrug-resistance mechanisms in these organisms. Today Professor Wirth is chair of the Department of Immunology and Infectious Diseases at the Harvard School of Public Health (HSPH) and the Richard Pearson Strong Professor of Infectious Diseases.

As well as directing the Harvard Malaria Initiative at HSPH, she has served as president of the American Society of Tropical Medicine and Hygiene, and on numerous committees and advisory boards, including those for the National Institute of Medicine, the National Institutes of Health and the World Health Organization.

Focus on the malaria genome

Wirth's scientific work, more recently, has focused on the genome of the malaria parasite (the entire package of genes that make up the organism) and how natural selection in humans and mosquitoes changes the way the parasite causes disease.

To this end, the malaria group is taking advantage of technology breakthroughs that have allowed scientists to study the human genome. Here, geneticists have not only decoded genes in an average healthy human body, they are now looking at the genome of humans that are affected by different diseases. By comparing the two genomes they are able to pinpoint the genes that vary and might be associated with particular conditions, from diabetes to heart disease.

Wirth's group is using exactly the same principle to study drug resistance in parasites using similar DNA sequencing and genotyping methods. They compare the genome of drug-resistant parasites and compare them with a reference genome for a drug-sensitive parasite to discover which genes within the parasite might be associated with resistance.

By analysing the genomes of different parasites - taken from multiple locations, with resistance to different drugs, and that cause different levels of disease - the group hopes to establish how the parasite has migrated and which parasite genes cause different types of disease - severe or asymptomatic.

That work could have more profound consequences, especially as the group is now looking at parasites derived directly from patients, rather than tissue-cultured parasite specimens that have been analysed traditionally.

'We are looking at the parasite as it is causing disease today but with very sophisticated tools to understand how the parasite genome changes in response to human immune response, in response to drugs', she says.

Advances in gene analysis technology means her group, with collaboration from the Broad Institute in Boston, can generate the entire sequence of a genome in a matter of days (the first malaria genome, completed in 2002, took five years and the work of several institutions around the world (1) So far, Wirth's group has completed the analyses of 20 different samples of Plasmodium falciparum, the most deadly species of the malaria parasite. In a couple of years the group hopes to have analysed a few hundred.

The group has seen some interesting patterns. The majority of the variations occur in the genes most likely to be targets of the human immune system - the proteins or gene products that are expressed on the surface of the infected red cell, for instance. They have also seen variations in the genes that express molecules on the surface of the parasite itself as it escapes and have worked out where other genes with previously unknown functionality must be located because of the variation they too exhibit.

'It's more complicated than we thought'

Probing the genome has revealed that the parasite uses many more mechanisms to evade human immune system than previously thought. In other words many more genetic variations are appearing in antigens - the substances that cause an immune response in humans - than expected. And only a small proportion of those antigens are being studied by vaccine scientists.

'It is much more complicated than we thought previously. It may be that you actually have to expand the number of antigens,' she says.

But crucially, because antigen candidates in early-stage vaccines show many genetic variations, there are enormous implications for vaccine discovery work says Wirth.

Although vaccine scientists are well aware of the many variants, and are already looking at vaccines that combine multiple antigens, Wirth proposes even more diversity in the types of vaccine that are developed.

'There is a possibility a vaccine made against, say, two closely-related variants is not protective against a third variant. So you might need geographical or temporal-specific vaccines,' she said. 'But although it is much more complicated, I'm much more hopeful of the possibility of vaccines but it's going to require a considerable paradigm shift in our thinking.'

This approach is increasingly similar to that used against another disease, influenza. The influenza virus evolves so fast that new vaccines must be developed regularly to combat the different versions of the virus that emerge.

'I now think of malaria much more in an influenza paradigm rather than smallpox or measles,' says Wirth. 'People are infected with one parasite and the next time they are infected with a new parasite, from an antigenic sense. But it is still malaria.'

Parallels between influenza virus and malaria could have larger ramifications, she says even though malaria is a much more complicated beast - containing 5000 genes.

If fully-drug-resistant parasites enter populations through the movement of infected people - in a similar way that chloroquine and fansidar-resistant species are believed to have done - there might be an argument for controlling malaria in a similar way to influenza and other viruses. For instance, severe acute respiratory syndrome (SARS), caused by a coronavirus, was controlled by restricting the movement of infected people.

'I now believe that the transit and the transmission of parasites from one geographic region to another is much more common than we have previously realised,' she says. 'A lot of a public effort is spent making sure people take their drugs correctly. But if drug resistance is actually caused by the importation and spread of a foreign gene then it is much more important to pay attention to surveillance to see when new genomes are introduced to the population.' But a startling discovery (2) makes parallels between influenza and malaria all the more poignant.

The group had been probing parasites taken from infected people in Senegal, rather than those grown in tissue culture, the traditional laboratory-based method for culturing parasites for over 40 years.

By probing the genome of parasites in these patients, they discovered that parasites act completely differently when cultured in the laboratory, compared with those living in infected blood. In many patients, perhaps unsurprisingly given the poor conditions many of their victims, parasites live in a form of starvation-mode and exhibit a completely different metabolism from those living in the more opulent tissue-culture surroundings.

The discovery caused a bit of storm says Wirth. 'This was quite a shock and no one believed it. But it has now has been repeated by other groups. And in Plasmodium vivax (another another species of malaria parasite) too.'

A new avenue

Dyann Wirth and her colleagues now want to find out more about how this starvation-mode works. For instance, they already know that the parasite feeds on glucose in the tissue culture but not what it feeds on in these particular patients. Crucially, they want to discover how the metabolism influences the severity of the disease in the patients.

That premise has led to some interesting changes in thinking.

Firstly, it may go some way to explaining why some drugs fail when they move from trials in-vitro and animal models to human clinical trials.

'All of our targets for drug development are derived from what we know from in-vitro culture. But i's not just implications for current drugs and vaccines development, it opens a whole new avenue,' says Wirth.

They hope to mimic these surroundings and thus use the metabolism difference to their advantage. If ultimately, scientists can work out how to fool the parasite into thinking that it is in the surroundings that cause less virulent disease they might make significant in-roads in future vaccine research.

A vaccine might be used, merely, to make an attack of malaria less virulent in the particularly susceptible, such as young children and pregnant women. Certainly many of the millions of people infected every year with the malaria parasite suffer from a less severe form of the disease, or show no symptoms at all.

That's quite a change in thinking says Wirth. Killing the parasite may not be the only end-game in vaccine research and again it is very similar to the influenza vaccine approach, which does not necessarily prevent infection but ensures the patient suffers a less severe form of the condition.

Crucially, looking at alternatives to killing the organism may also get around the problem of constantly fighting resistance she says.

‘One of the important things to think about as we move to long-term elimination of severe disease and the potential elimination of the majority of infections and symptomatic malaria is that we are going to need different tools,' she says. 'One of those tools might be to just reduce severe disease by changing the way the parasite grows in the human.'

That may run contrary to the idea behind the eradication roadmap, called the Global Malaria Action Plan, put together by several of the world's multilateral donors.

Eradication will be realistic only if there is both the widespread use of existing tools such as residual insecticide-spraying and artemisinin combination therapy but also novel drugs and vaccines and a sustainable commitment from the highest levels of government, she says.

'With my public health hat on, as an aspirational goal, eradication is something a lay population can understand and get behind. But from a biological standpoint is going to be a real challenge,' she says. 'It's time to both use what we have today and anticipate what we're going to need in five to 10 years from now. What I'm doing, its practical applications will only be felt in a 10 to 15-year period.'

Key Questions

Briefly, what are the priority concerns of your organisation?

To understand the fundamental biology of the malaria parasite and apply that knowledge to the development of new tools to combat malaria
And, more precisely, what goals have you set?

To analyse the plasmodium genome in infected patients and look at gene variations.
What progress has been made so far?

We have analysed the genome of 20 samples of Plasmodium falciparum.
What are the main challenges outstanding?

Among others, to take analyse the link between disease and the metabolism of the parasite.
Which other organisations will you be working with most closely?

The Broad Institute of MIT and Harvard, with Genzyme on new drug targets within the Medicines for Malaria (MMV) project, Cheikh Anta Diop University, Senegal, University of Ibadan, Nigeria, the Blantyre Medical Research Institute, Malawi, the Wellcome Trust Sanger Institute and the Genomics Institute of the Novartis Research Foundation (GNF)

References

Wirth DF (2002). The parasite genome. Biological revelations. Nature; 419(6906):495-496.
Volkman SK, Sabeti PC, DeCaprio D, Neafsey DE, Schaffner SF, Milner DA Jr, Daily JP, Sarr O, Ndiaye D, Ndir O, Mboup S, Duraisingh MT, Lukens A, Derr A, Stange-Thomann N, Waggoner S, Onofrio R, Ziaugra L, Mauceli E, Gnerre S, Jaffe DB, Zainoun J, Wiegand RC, Birren BW, Hartl DL, Galagan JE, Lander ES, Wirth DF (2008). A genome-wide map of diversity in Plasmodium falciparum. Nat Genet; 39(1):5-6. Nature 13;450(7172):1091-5