Genomics and Malaria Control
http://www.100md.com
《新英格兰医药杂志》
Two thirds of the global population is at risk for malaria. However, about 90 percent of deaths from this disease occur in sub-Saharan Africa, where 1.5 million to 2.5 million of those who die of malaria each year are children. What is more, the incidence of malaria is on the rise — owing in part to the resistance of parasites and mosquitoes to drugs and insecticides and in part to social factors such as migration and political instability. We now know the genomic sequences of the most important malaria parasite of humans (Plasmodium falciparum), the mosquito vector (Anopheles gambiae), and the human: three participants in a complex yet durable system of disease transmission. Such knowledge offers new opportunities for intervention, but the use of existing low-technology tools, such as bed nets, surveillance, and health care, including the strategic application of drugs, can also be effective. Thus, we are not obliged to wait idly for new forms of technology, and the failure to implement existing tools consistently lies at the heart of the current situation. Malaria is a disease of economic underdevelopment, and the success of new measures will depend on their ability to surmount this obstacle.
It is against this background that the genomic sequences must be exploited. The challenge is to provide affordable, deliverable strategies and products that will protect and cure people in areas in which the disease is endemic. Study of the annotated genomes, fleshed out with global gene-expression and protein profiles, has begun to yield detailed insights into the basic biology of the system. Control strategies can be grouped into one of two categories — vector-targeted and human-targeted approaches — but in reality, malaria results from three intimately interacting organisms (Figure 1). Fortunately, genomics-based approaches to malaria offer the strategic advantage of considering all three participants through integrative data sets, allowing us to identify the molecules from the human host, vector, and parasite that are involved in the interaction at the organismal interfaces.
Figure 1. Targets for Interventions to Prevent the Spread of Malaria.
The female mosquito bites the host and injects sporozoites from its salivary glands into the host's bloodstream. The sporozoite enters a hepatocyte, where it differentiates and divides; a single sporozoite can yield 10,000 infectious merozoites. Merozoites are released into the bloodstream and invade erythrocytes by a receptor–ligand–mediated mechanism, where they differentiate, grow, and multiply in a vacuole to yield more merozoites. This cycle of erythrocyte consumption, with its parasite-induced sequestration of infected erythrocytes in capillaries deep within many organs, causes many of the pathological hallmarks of infection. Merozoites cannot be transmitted to another human host by a mosquito bite, but a small proportion of merozoites follow an alternative developmental pathway that yields a transmissible form, the gametocyte. These long-lived, nondividing cells circulate in the bloodstream, awaiting uptake by the mosquito during a blood meal. Once in the insect's stomach, gametocytes produce gametes that undergo sexual fertilization to generate first a zygote and then the motile ookinete, which invades the midgut epithelium, thereby infecting the mosquito. Ookinetes are transformed into oocysts; during a period of apparent latency lasting approximately 10 days, approximately 10,000 sporozoites are formed. The oocysts then rupture and release the sporozoites into the mosquito's body cavity, where they migrate to invade the salivary glands.
Vector-Targeted Strategies
To maintain the chain of human-to-human transmission of malaria, a vector must strongly prefer human-blood meals to animal-blood meals, and A. gambiae will choose the human at least 90 percent of the time. Mosquitoes discriminate human hosts in part by olfactory cues from volatile compounds. Families of odorant receptor genes have been identified in the A. gambiae genome, and one receptor responds to volatile compounds in human sweat. Identification of mosquito genes responsible for homing could lead to the design of repellants, aerosol "confusants," or lures to reduce the insect's biting of people.1 Conceivably, A. gambiae could be diverted from feeding on humans to feeding on animals by genetically altering or replacing a critical target gene, thus denying the parasite its most efficient vector (Figure 1). The transgenic technique for such a change does not yet exist but is under active investigation.
The vector is not a passive carrier of malaria. It, too, is infected and mounts its own complex immune response that could be exploited in several ways. Host-defense and innate immune functions are a major theme of the A. gambiae genome, and there are many families of relevant genes — the proteins encoded by some of these genes drastically limit malaria in A. gambiae.2 So one proposal is to use transgenic techniques to introduce antimalaria genes into A. gambiae populations, creating malaria-blocking mosquitoes that could not serve as disease vectors (Figure 1). Alternatively, new drugs could emerge through the identification of parasiticidal effectors in the A. gambiae genome that are used by the insect's immune system.
Insecticides have historically been a pillar of malaria-control efforts. Their chief benefit lies not in reducing the number of vectors but in reducing their longevity. Infective parasites develop near the end of a mosquito's life span in nature, so even a small decrease in survival can have a large epidemiologic effect. However, the use of insecticides has resulted in resistance. Knowledge of the A. gambiae genome allows us to be better informed about the families of genes responsible for resistance and therefore to monitor and perhaps manage resistance — for example, by adding synergists to current formulations of insecticides. More beneficial in the long run would be the development of new generations of insecticides with high specificity and low mammalian toxicity. In this regard, study of the A. gambiae genome has begun to reveal potential new targets in developmental, hormonal, and nervous system pathways.
Strategies Targeting Human Infection
No malaria vaccine is clinically available. Some antigens are protective, particularly in animal models of malaria, but their effectiveness varies greatly, depending on the adjuvants and delivery methods used. Study of the P. falciparum genome will most likely provide new candidate antigens, some of which may be more effective or easier to deliver than known ones.3 For example, first-pass annotation suggests that 911 proteins have a signal peptide and may therefore be exposed on the surface of the parasite. Antigenic diversity and variation remain a potential problem, because what is effective in one geographic area may not be effective in another, and parasite populations can evolve as a result of immune selective pressure.
Live vaccines are also a possibility. Irradiated sporozoites are proven inducers of protective immunity, and the use of these attenuated parasites remains the proof of principle and the gold standard of successful malaria immunization. Unfortunately, standardization is problematic — too little irradiation and the parasite remains infectious, too much and the treatment is not protective. Genomics and associated molecular tools may provide new ways to develop a source of standardized attenuated parasites that could elicit the same level of protection. Technical hurdles would need to be overcome to ensure the safety and purity of the resulting product.
With respect to drugs, new candidate compounds (such as the antifungals triclosan and fosmidomycin) were immediately suggested by the discovery of the curious evolutionary history of malaria parasites. It turns out that a critical organelle, the apicoplast, organizes some metabolic pathways that are completely different from those in humans. Examination of the genes encoding metabolic pathways housed by the apicoplast led to the suggestion of their protein products, and those of some others, as new targets. Genomics also allows a more detailed description of known target pathways and even the identification of ancillary targets of existing drugs such as artemisinin, thus providing new leads (Figure 2).4
Figure 2. Ways to Attack the Malaria Parasite.
Knowledge of the genomic sequence of P. falciparum provides details of the proteins in known and presumptive metabolic pathways and suggests targets for existing and potential drugs.
Study of the human genome should help us to identify malaria-parasite receptors and molecules related to pathological processes or resistance. The genome contains an archive of information about genetically induced protection against malaria. The clearest examples of such defensive mutations are those involving the hemoglobin gene and alleles of the Duffy chemokine receptor. Both protective alleles may be costly in other ways, as exemplified by the fact that the former mutation may result in sickle cell anemia. Important but still anonymous human genetic loci influencing malaria infection have been identified in linkage or population association studies, and loci influencing the mouse's interaction with rodent malaria have also been identified. Haplotype maps are currently under development and should facilitate the identification of genomic regions that influence the host's response to malaria.
Regardless of the approach used, strategic problems must be overcome. For example, the gulf between animal and human testing may be surmountable with respect to drugs, but the complete absence of any prospect of a reliable, high-throughput animal model for vaccine testing means that new drugs for the prophylaxis or treatment of active disease will almost certainly be the first fruits of the malaria genomics initiative. Similarly, the lack of a transgenic means of introducing antimalaria genes into vector populations is likely to limit the immediate vector-based prospects to chemical control.
Technology aside, a public framework must be devised to involve affected communities in the consideration of ethical (including biosafety), legal, and social issues in calculating new control strategies. This is particularly true for the more daring approaches involving mosquito or parasite transgenesis, in which valid concerns related to ethical, legal, and social issues may prevent the implementation of some proposals, although the daily cost of inaction must also be remembered. Resistance to new-generation drugs and insecticides must be anticipated and sidestepped. Lastly, the affected populations are among the world's poorest, and it will take multilateral initiatives and partnerships between industry and nongovernmental organizations to ensure that the bright promise of malaria genomics is translated into effective applications that truly relieve the misery imposed by this pernicious disease.
Source Information
From the Center for Microbial and Plant Genomics and the Department of Microbiology, University of Minnesota, St. Paul (K.D.V.); and the Department of Parasitology, Centre of Infectious Diseases, Leiden University Medical Centre, Leiden, the Netherlands (A.P.W.).
References
Justice RW, Biessmann H, Walter MF, Dimitratos SD, Woods DF. Genomics spawns novel approaches to mosquito control. Bioessays 2003;25(10):1011-20
Osta MA, Christophides GK, Vlachou D, Kafatos FC. Innate immunity in the malaria vector Anopheles gambiae: comparative and functional genomics. J Exp Biol 2004;207:2551-2563.
Doolan DL, Aguiar JC, Weiss WR, et al. Utilization of genomic sequence information to develop malaria vaccines. J Exp Biol 2003;206:3789-3802.
Rosenthal PJ. Antimalarial drug discovery: old and new approaches. J Exp Biol 2003;206:3735-3744.(Kenneth D. Vernick, Ph.D.)
It is against this background that the genomic sequences must be exploited. The challenge is to provide affordable, deliverable strategies and products that will protect and cure people in areas in which the disease is endemic. Study of the annotated genomes, fleshed out with global gene-expression and protein profiles, has begun to yield detailed insights into the basic biology of the system. Control strategies can be grouped into one of two categories — vector-targeted and human-targeted approaches — but in reality, malaria results from three intimately interacting organisms (Figure 1). Fortunately, genomics-based approaches to malaria offer the strategic advantage of considering all three participants through integrative data sets, allowing us to identify the molecules from the human host, vector, and parasite that are involved in the interaction at the organismal interfaces.
Figure 1. Targets for Interventions to Prevent the Spread of Malaria.
The female mosquito bites the host and injects sporozoites from its salivary glands into the host's bloodstream. The sporozoite enters a hepatocyte, where it differentiates and divides; a single sporozoite can yield 10,000 infectious merozoites. Merozoites are released into the bloodstream and invade erythrocytes by a receptor–ligand–mediated mechanism, where they differentiate, grow, and multiply in a vacuole to yield more merozoites. This cycle of erythrocyte consumption, with its parasite-induced sequestration of infected erythrocytes in capillaries deep within many organs, causes many of the pathological hallmarks of infection. Merozoites cannot be transmitted to another human host by a mosquito bite, but a small proportion of merozoites follow an alternative developmental pathway that yields a transmissible form, the gametocyte. These long-lived, nondividing cells circulate in the bloodstream, awaiting uptake by the mosquito during a blood meal. Once in the insect's stomach, gametocytes produce gametes that undergo sexual fertilization to generate first a zygote and then the motile ookinete, which invades the midgut epithelium, thereby infecting the mosquito. Ookinetes are transformed into oocysts; during a period of apparent latency lasting approximately 10 days, approximately 10,000 sporozoites are formed. The oocysts then rupture and release the sporozoites into the mosquito's body cavity, where they migrate to invade the salivary glands.
Vector-Targeted Strategies
To maintain the chain of human-to-human transmission of malaria, a vector must strongly prefer human-blood meals to animal-blood meals, and A. gambiae will choose the human at least 90 percent of the time. Mosquitoes discriminate human hosts in part by olfactory cues from volatile compounds. Families of odorant receptor genes have been identified in the A. gambiae genome, and one receptor responds to volatile compounds in human sweat. Identification of mosquito genes responsible for homing could lead to the design of repellants, aerosol "confusants," or lures to reduce the insect's biting of people.1 Conceivably, A. gambiae could be diverted from feeding on humans to feeding on animals by genetically altering or replacing a critical target gene, thus denying the parasite its most efficient vector (Figure 1). The transgenic technique for such a change does not yet exist but is under active investigation.
The vector is not a passive carrier of malaria. It, too, is infected and mounts its own complex immune response that could be exploited in several ways. Host-defense and innate immune functions are a major theme of the A. gambiae genome, and there are many families of relevant genes — the proteins encoded by some of these genes drastically limit malaria in A. gambiae.2 So one proposal is to use transgenic techniques to introduce antimalaria genes into A. gambiae populations, creating malaria-blocking mosquitoes that could not serve as disease vectors (Figure 1). Alternatively, new drugs could emerge through the identification of parasiticidal effectors in the A. gambiae genome that are used by the insect's immune system.
Insecticides have historically been a pillar of malaria-control efforts. Their chief benefit lies not in reducing the number of vectors but in reducing their longevity. Infective parasites develop near the end of a mosquito's life span in nature, so even a small decrease in survival can have a large epidemiologic effect. However, the use of insecticides has resulted in resistance. Knowledge of the A. gambiae genome allows us to be better informed about the families of genes responsible for resistance and therefore to monitor and perhaps manage resistance — for example, by adding synergists to current formulations of insecticides. More beneficial in the long run would be the development of new generations of insecticides with high specificity and low mammalian toxicity. In this regard, study of the A. gambiae genome has begun to reveal potential new targets in developmental, hormonal, and nervous system pathways.
Strategies Targeting Human Infection
No malaria vaccine is clinically available. Some antigens are protective, particularly in animal models of malaria, but their effectiveness varies greatly, depending on the adjuvants and delivery methods used. Study of the P. falciparum genome will most likely provide new candidate antigens, some of which may be more effective or easier to deliver than known ones.3 For example, first-pass annotation suggests that 911 proteins have a signal peptide and may therefore be exposed on the surface of the parasite. Antigenic diversity and variation remain a potential problem, because what is effective in one geographic area may not be effective in another, and parasite populations can evolve as a result of immune selective pressure.
Live vaccines are also a possibility. Irradiated sporozoites are proven inducers of protective immunity, and the use of these attenuated parasites remains the proof of principle and the gold standard of successful malaria immunization. Unfortunately, standardization is problematic — too little irradiation and the parasite remains infectious, too much and the treatment is not protective. Genomics and associated molecular tools may provide new ways to develop a source of standardized attenuated parasites that could elicit the same level of protection. Technical hurdles would need to be overcome to ensure the safety and purity of the resulting product.
With respect to drugs, new candidate compounds (such as the antifungals triclosan and fosmidomycin) were immediately suggested by the discovery of the curious evolutionary history of malaria parasites. It turns out that a critical organelle, the apicoplast, organizes some metabolic pathways that are completely different from those in humans. Examination of the genes encoding metabolic pathways housed by the apicoplast led to the suggestion of their protein products, and those of some others, as new targets. Genomics also allows a more detailed description of known target pathways and even the identification of ancillary targets of existing drugs such as artemisinin, thus providing new leads (Figure 2).4
Figure 2. Ways to Attack the Malaria Parasite.
Knowledge of the genomic sequence of P. falciparum provides details of the proteins in known and presumptive metabolic pathways and suggests targets for existing and potential drugs.
Study of the human genome should help us to identify malaria-parasite receptors and molecules related to pathological processes or resistance. The genome contains an archive of information about genetically induced protection against malaria. The clearest examples of such defensive mutations are those involving the hemoglobin gene and alleles of the Duffy chemokine receptor. Both protective alleles may be costly in other ways, as exemplified by the fact that the former mutation may result in sickle cell anemia. Important but still anonymous human genetic loci influencing malaria infection have been identified in linkage or population association studies, and loci influencing the mouse's interaction with rodent malaria have also been identified. Haplotype maps are currently under development and should facilitate the identification of genomic regions that influence the host's response to malaria.
Regardless of the approach used, strategic problems must be overcome. For example, the gulf between animal and human testing may be surmountable with respect to drugs, but the complete absence of any prospect of a reliable, high-throughput animal model for vaccine testing means that new drugs for the prophylaxis or treatment of active disease will almost certainly be the first fruits of the malaria genomics initiative. Similarly, the lack of a transgenic means of introducing antimalaria genes into vector populations is likely to limit the immediate vector-based prospects to chemical control.
Technology aside, a public framework must be devised to involve affected communities in the consideration of ethical (including biosafety), legal, and social issues in calculating new control strategies. This is particularly true for the more daring approaches involving mosquito or parasite transgenesis, in which valid concerns related to ethical, legal, and social issues may prevent the implementation of some proposals, although the daily cost of inaction must also be remembered. Resistance to new-generation drugs and insecticides must be anticipated and sidestepped. Lastly, the affected populations are among the world's poorest, and it will take multilateral initiatives and partnerships between industry and nongovernmental organizations to ensure that the bright promise of malaria genomics is translated into effective applications that truly relieve the misery imposed by this pernicious disease.
Source Information
From the Center for Microbial and Plant Genomics and the Department of Microbiology, University of Minnesota, St. Paul (K.D.V.); and the Department of Parasitology, Centre of Infectious Diseases, Leiden University Medical Centre, Leiden, the Netherlands (A.P.W.).
References
Justice RW, Biessmann H, Walter MF, Dimitratos SD, Woods DF. Genomics spawns novel approaches to mosquito control. Bioessays 2003;25(10):1011-20
Osta MA, Christophides GK, Vlachou D, Kafatos FC. Innate immunity in the malaria vector Anopheles gambiae: comparative and functional genomics. J Exp Biol 2004;207:2551-2563.
Doolan DL, Aguiar JC, Weiss WR, et al. Utilization of genomic sequence information to develop malaria vaccines. J Exp Biol 2003;206:3789-3802.
Rosenthal PJ. Antimalarial drug discovery: old and new approaches. J Exp Biol 2003;206:3735-3744.(Kenneth D. Vernick, Ph.D.)