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Transformation in parasitic nematodes at last?
News & Comment
TRENDS in Parasitology Vol.18 No.6 June 2002
243
Journal Club
Transformation in parasitic nematodes at last? It is somewhat ironic that nematodes, represented by Caenorhabditis elegans, have been at the forefront of genetic and genomic research for the past 20 years, culminating with the completion of the genomic sequence in 1998. This was the first complete multicellular eukaryote sequence and, of greater general significance perhaps, served as the test-bed for the development of large-scale sequencing technology. However, despite this tremendous basic resource and a rapidly expanding parasitic nematode expressed sequence tag (EST) dataset (www.nematode.net), parasitic nematodes have lagged far behind other parasite genera in recent years owing to a lack of molecular tools for functional genomic analysis. The report by Lok and Massey, the expression of a green fluorescent reporter (GFP) transgene in developing eggs of Strongyloides stercoralis females microinjected with a GFP reporter construct, is a sign of considerable progress. This provides us with several reasons for optimism. First, the use of S. stercoralis is an important development because this
parasite provides access to at least one free-living generation and the potential of recovering the first putatively transgenic progeny without the need to infect a host. This removes one of the most challenging barriers to functional genomics in parasitic nematodes of mammals: the near absolute necessity to infect a host to produce the next generation. It is no coincidence that the parasitic protozoa yielded to transgenesis after in vitro culture was made possible. Unfortunately, the apparently transgenic eggs that showed GFP expression during early in utero development failed to hatch, presumably as a result of a lethality associated with the observed high levels of GFP expression. Therefore, if truly heritable transformation is to be achieved (as it is clear from Caenorhabditis elegans that the majority of transformation events following microinjection are not inherited), obtaining a transgenic line or strain will be a numbers game. The high frequency of apparently transgenic eggs observed suggests that the necessary numbers will also be available if the reporter-associated lethality can be overcome, and the ability to culture the injected free-living or
heterogonic females in vitro means that it will be possible to recover and screen every viable offspring. Second, the authors used endogenous promoters from highly expressed S. stercoralis genes to drive reporter gene expression. The promoter sequences were isolated using complementary DNA (cDNA) sequence as a starting point. The availability of many thousands of parasite ESTs suggests that obtaining other promoters will not be a limiting factor in the development of the technology Finally, the promising results reported here will, hopefully, encourage other labs (perhaps using other species with in vitro culture potential) and the relevant funding agencies to take up the challenge of developing a system for generating truly heritable transgenesis. It is clearly a feasible goal and one that is worth pursuing. 1 Lok, J.B. and Massey, H.C. (2002) Transgene expression in Strongyloides stercoralis following gonadal microinjection of DNA constructs. Mol. Biochem. Parasitol. 119, 279–284
Warwick Grant [email protected]
Climate change and malaria: the debate is heating up The logic is straightforward. The distribution of malaria is restricted at altitude because of low temperatures. Climate change makes the world warmer. Therefore, climate change should explain recent upsurges in the incidence of malaria in highland regions. No less a body than the International Panel on Climate Change accepts this notion but a recent paper by S. Hay et al. suggests that the argument is too simplistic [1]. Hay et al. report a careful statistical analysis which shows that in four highland locations in East Africa, where malaria has been increasing in recent decades, there was no evidence that warming has actually occurred, either during the study period or, indeed, during the whole of the last century. Instead, the authors suggest that the causes of malaria resurgence should be sought elsewhere such as changes in land use or human demography, or increased resistance to antimalarial drugs. http://parasites.trends.com
This study is the latest of several which have explored the relationship between infectious diseases and climate variation. Baylis et al. found that the outbreaks of African horse sickness in South Africa were closely correlated with El Niño events [2], whereas Linthicum et al. failed to demonstrate that outbreaks of Rift Valley fever in Kenya were related to climate and vegetation data [3]. These are challenging analyses to attempt for several reasons: infectious disease surveillance data are often of poor quality; disease outbreaks have multiple causes and it is not always clear which climate variables are the right ones to measure. As a consequence, it is often difficult to detect any signal among the noise. But, as Hay et al. point out, climate change remains both a scientifically and politically controversial subject and it is important that its conjectured implications
are tested as rigorously as possible. However, their analysis perhaps does not entirely decide the issue, even for malaria. For all four of the sites studied by Hay et al. there was an upward trend in the Garnham index of the number of ‘suitable’ months for malaria, although the trend was statistically significant only in southwest Uganda. We might wonder what a follow up analysis in a few decades time will reveal. 1 Hay, S.I. et al. (2002) Climate change and the resurgence of malaria in the East African highlands. Nature 415, 905–909 2 Baylis, M. et al. (1999) Horse sickness and ENSO in southern Africa. Nature 397, 574 3 Linthicum, K.J. et al. (1999) Climate and satellite indicators to forecast Rift Valley fever epidemics in Kenya. Science 285, 397–400
Mark Woolhouse [email protected]
1471-4922/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.
TRENDS in Parasitology Vol.18 No.6 June 2002
243
Journal Club
Transformation in parasitic nematodes at last? It is somewhat ironic that nematodes, represented by Caenorhabditis elegans, have been at the forefront of genetic and genomic research for the past 20 years, culminating with the completion of the genomic sequence in 1998. This was the first complete multicellular eukaryote sequence and, of greater general significance perhaps, served as the test-bed for the development of large-scale sequencing technology. However, despite this tremendous basic resource and a rapidly expanding parasitic nematode expressed sequence tag (EST) dataset (www.nematode.net), parasitic nematodes have lagged far behind other parasite genera in recent years owing to a lack of molecular tools for functional genomic analysis. The report by Lok and Massey, the expression of a green fluorescent reporter (GFP) transgene in developing eggs of Strongyloides stercoralis females microinjected with a GFP reporter construct, is a sign of considerable progress. This provides us with several reasons for optimism. First, the use of S. stercoralis is an important development because this
parasite provides access to at least one free-living generation and the potential of recovering the first putatively transgenic progeny without the need to infect a host. This removes one of the most challenging barriers to functional genomics in parasitic nematodes of mammals: the near absolute necessity to infect a host to produce the next generation. It is no coincidence that the parasitic protozoa yielded to transgenesis after in vitro culture was made possible. Unfortunately, the apparently transgenic eggs that showed GFP expression during early in utero development failed to hatch, presumably as a result of a lethality associated with the observed high levels of GFP expression. Therefore, if truly heritable transformation is to be achieved (as it is clear from Caenorhabditis elegans that the majority of transformation events following microinjection are not inherited), obtaining a transgenic line or strain will be a numbers game. The high frequency of apparently transgenic eggs observed suggests that the necessary numbers will also be available if the reporter-associated lethality can be overcome, and the ability to culture the injected free-living or
heterogonic females in vitro means that it will be possible to recover and screen every viable offspring. Second, the authors used endogenous promoters from highly expressed S. stercoralis genes to drive reporter gene expression. The promoter sequences were isolated using complementary DNA (cDNA) sequence as a starting point. The availability of many thousands of parasite ESTs suggests that obtaining other promoters will not be a limiting factor in the development of the technology Finally, the promising results reported here will, hopefully, encourage other labs (perhaps using other species with in vitro culture potential) and the relevant funding agencies to take up the challenge of developing a system for generating truly heritable transgenesis. It is clearly a feasible goal and one that is worth pursuing. 1 Lok, J.B. and Massey, H.C. (2002) Transgene expression in Strongyloides stercoralis following gonadal microinjection of DNA constructs. Mol. Biochem. Parasitol. 119, 279–284
Warwick Grant [email protected]
Climate change and malaria: the debate is heating up The logic is straightforward. The distribution of malaria is restricted at altitude because of low temperatures. Climate change makes the world warmer. Therefore, climate change should explain recent upsurges in the incidence of malaria in highland regions. No less a body than the International Panel on Climate Change accepts this notion but a recent paper by S. Hay et al. suggests that the argument is too simplistic [1]. Hay et al. report a careful statistical analysis which shows that in four highland locations in East Africa, where malaria has been increasing in recent decades, there was no evidence that warming has actually occurred, either during the study period or, indeed, during the whole of the last century. Instead, the authors suggest that the causes of malaria resurgence should be sought elsewhere such as changes in land use or human demography, or increased resistance to antimalarial drugs. http://parasites.trends.com
This study is the latest of several which have explored the relationship between infectious diseases and climate variation. Baylis et al. found that the outbreaks of African horse sickness in South Africa were closely correlated with El Niño events [2], whereas Linthicum et al. failed to demonstrate that outbreaks of Rift Valley fever in Kenya were related to climate and vegetation data [3]. These are challenging analyses to attempt for several reasons: infectious disease surveillance data are often of poor quality; disease outbreaks have multiple causes and it is not always clear which climate variables are the right ones to measure. As a consequence, it is often difficult to detect any signal among the noise. But, as Hay et al. point out, climate change remains both a scientifically and politically controversial subject and it is important that its conjectured implications
are tested as rigorously as possible. However, their analysis perhaps does not entirely decide the issue, even for malaria. For all four of the sites studied by Hay et al. there was an upward trend in the Garnham index of the number of ‘suitable’ months for malaria, although the trend was statistically significant only in southwest Uganda. We might wonder what a follow up analysis in a few decades time will reveal. 1 Hay, S.I. et al. (2002) Climate change and the resurgence of malaria in the East African highlands. Nature 415, 905–909 2 Baylis, M. et al. (1999) Horse sickness and ENSO in southern Africa. Nature 397, 574 3 Linthicum, K.J. et al. (1999) Climate and satellite indicators to forecast Rift Valley fever epidemics in Kenya. Science 285, 397–400
Mark Woolhouse [email protected]
1471-4922/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.