TRANSGENIC INSECT

Abstract: A method for the genetic modification of an insect embryo, comprises first treating an insect egg under conditions which prevent or delay the hardening of the insect egg chorion, and then injecting a transposable element into the egg to permit integration of the element into the genome of the embryo. The method permits modifications to be made to mosquitoes, which may prevent transmission of a host parasite.

Description: TRANSGENIC INSECT Field of the Invention This invention relates to the genetic manipulation of insects. In particular, this invention relates to the genetic manipulation of mosquitos . Background of the Invention Malaria is the most important parasitic disease in the world today and is one of the major health threats in Africa, where 10% of the world's population suffers more than 90% of the world's malaria infections. Malaria is caused by protozoan parasites of the genus Plasmodium . Of the four recognised human parasites (P. falciparum, P . vivax, P. ovale and P . malariae) , P . fal ciparum is the most dangerous and is the major cause of mortality. Human malaria parasites are transmitted by mosquitoes of the genus Anophel es . At least 20 of the almost 500 known types of anopheline mosquitoes have been shown to be implicated in malaria transmission. In sub-Saharan Africa, transmission is mainly caused by three anopheline species, A . gamibiae, A . arabi ensis and A . funestus . These three species represent the most efficient vectorial system in the world for P. falciparum . Their distribution is limited by dry environments, salt water, low temperatures and, in the case of A . gambiae and A . arabiensi s , by the thick vegetation of natural forests and humid savannah areas. These three African mosquitoes are the most efficient as malaria vectors because of their marked preference for humans as hosts as well as for their ability to adapt to human-induced environmental changes. In Asia, the most efficient malarial vector is A . Stephens! . Control measures based on the use of pesticides have not been able to control the extremely high P. falciparum inoculation rates. Furthermore, the common occurrence of insecticide resistance, coupled with the ecological costs associated with their use, has generated the need for alternative methods to control the parasite. Attempts made by the massive distribution of antimalarial drugs have not been successful, due partly to the rapid spread of multiple drug-resistant strains of P. falciparum . Biological control measures have been proposed as an alternative to the use of pesticides to control the spread of malaria. The production of host insects that are resistant (refractory) to the development of the parasite and thus incapable of transmitting the infection is one possible method of controlling malaria. The ability of an insect host to support the development and transmission of a parasite is called vector competence. Mosquitoes of the Culex and Aedes genera contain species that regularly feed on humans but cannot transmit malaria. The mechanisms responsible for this are various and usually species- specific. The physiology and genetic basis is incompletely known. The inability to transmit malaria could be due to the absence of some critical factor m the mosquito required by the parasite for normal development, or it could be the result of the action of some other factor (s) inhibiting parasite development. Rosenberg et al , Insect Mol . Biol . , 1985; 7:1-10, showed that A . freeborm were refractory to the simian parasite P . knowl esi because the sporozoites were unable to recognise and penetrate the mosquito salivary glands. The identification and mapping of the genes responsible for refractoriness of a mosquito to a particular parasite is a ma] or goal for molecular biologists. Once technologies for introducing DNA into the mosquito genome become available, the manipulation of genes determining the susceptibility or refractoriness of a given species could be of tremendous importance for preventing malaria transmission, and means for inducing refractoriness genes into wild population could then be developed (Curtis and Graves, J. Trop . Med. Hyg . , 1988; 91:43-48). Furthermore, insects have various defence mechanisms, including the production of a wide variety of peptides m the body, to protect them against bacterial and fungal infections. Among the antibacterial peptides are insect defensins and cecropms, while drosomycm is the best- studied antifungal peptide. Such peptides have been shown to have the ability to interfere with the development of malaria parasites. The lack of an established technology to transform mosquito DNA has severely hampered the attempts to control vector-borne diseases by genetic manipulation. The release of sterile males as a means of genetic control has been shown to be successful m the eradication of the screw-worm fly from North and Central America and Libya (Krafsur et al . , Parasitology Today, 1987; 3:131-137). However, attempts to use this method to control mosquito populations have so far failed, mainly because of the reproductive strategies of mosquitoes, which include high fecundity, short generation time and the ability to rapidly repopulate an area after destruction of existing populations. The replacement of a mosquito population with one incapable of transmitting parasites could represent a valid alternative to the suppression of the mosquito species. Alternatively, it may be desirable to introduce into insects foreign genes expressing anti-parasitic agents able to interfere with the life-cycle of the parasite. For example, the use of modified Wolbachia symbionts to introduce foreign genes into Anopheles mosquitoes has been suggested (Curtis and Smkms, Parasitology, 1998; 116 Suppl : 111-115 ) . Wolbachia represents a potentially useful gene because it is maternally inherited and causes sterility m matings of infected males to unmfected females. However, so far no data concerning mosquito transformations have been reported, due to the difficulty m introducing exogenous DNA into the mosquito genome. Genetic manipulation of the fruitfly Drosophila has been carried out successfully using the P transposable element Transposable elements can be used to introduce heterologous genes into Drosophila to alter the phenotype of the insect. Other transposable elements have also been successfully introduced into the Drosophila genome, including HoJbo from D . melanogaster, mariner from D . aurifiana and Minos from D. hydei (Blackman et al . , EMBO J, 1989; 8:211-217; Garza et al . , Genetics, 1991; 128:303-310; Loukeris et al . , Proc . Natl . Acad. Sci . USA, 1995; 92:9485-9489) . The possibility of using transposable elements as DNA delivery vectors to achieve germlme transformation m mosquitoes has been supported by the encouraging results obtained with Hertnes, mariner and Minos m Drosophila . However, no transposable element has been shown to be capable of transposition m anopheline mosquitoes. The introduction of exogenous DNA into anopheline embryos represents another important limiting step m the transformation procedure. Insect embryos are surrounded by a rigid structure, the choπon, which hardens quickly after oviposition and makes the injection of DNA into anopheline embryos a very difficult and time-consuming process. A few minutes after they have been laid, eggs are already quite rigid and difficult to penetrate with commonly used needles, without killing the embryo. Survival rates of in ected embryos are usually poor, and consequently large amounts of embryos need to be injected m order to obtain a significant number of survivors. Furthermore, while the choπon of Drosophila eggs is removable by bleaching, anopheline embryos are extremely sensitive to the elimination of their eggshell, which provides structural support and protection and allows gas exchange while minimising water loss The establishment of a reliable technology for introducing foreign genes m the Anopheles genome therefore faces two major problems 1) the development of a DNA delivery vector capable of successful transposition m anopheline mosquitoes, and 2) the establishment of a new technology to overcome the technical difficulty of injecting DNA into mosquito embryos. Summary of the Invention The present invention is based, at least in part, on the realisation that injection of heterologous DNA into insect embryos can be facilitated by first manipulating the chorion to prevent or delay the hardening process. Injecting a suitable transposable element into the insect genome can then be carried ou . According to one aspect of the invention, a method for genetic modification of an insect embryo comprises the steps of: (i) treating an insect egg under conditions which prevent or delay the hardening of the insect egg chorion; and (ii) injecting a transposable element into the egg to permit integration of the element into the genome of the embryo . The insect is preferably a mosquito, and more preferably an anopheline mosquito. According to a further aspect of the invention, chorion hardening is prevented or delayed by inhibiting an enzyme involved in the hardening process. The compound p- nitrophenyl- ' -guanidinobenzoate may be used in the method of the present invention to delay the hardening of the chorion. According to a further aspect of the invention, a genetically modified anopheline mosquito is obtainable by: i. treating the egg of an anopheline mosquito embryo under conditions which prevents or delays the hardening of the mosquito egg chorion; and ii. injecting a transposable element into the egg, the transposable element being capable of integrating into the genome of the mosquito embryo . According to a further aspect of the invention, p- nitrophenyl-p ' -guanidinobenzoate is used to delay the hardening of the chorion of an insect egg. According to a further aspect, the Minos transposable element is used to transfer heterologous DNA into the genome of an anopheline mosquito embryo. The present invention provides an efficient gene transfer technology for transforming the genome of insects, particularly anopheline mosquitoes. This enables insects, particularly anopheline mosquitoes, to be genetically modified to exhibit particular traits or to modify the insect to prevent the spread of disease-causing parasites. The widespread applicability of this technology will be apparent to the skilled person, who may adapt existing genetic manipulations, for example as practiced on Drosophila, for use m other insects, e.g. anopheline mosquitoes. Description of the Drawing Figure 1 illustrates the vector (MinHyg) used for transposition into a mosquito embryo. In the drawing, ActmP represents the actmΞC promoter from D . melongaster; hspP represents the heat-shock promoter hsp70 from D. melongaster; hspT represents the heat-shock terminator sequence; Amp R represents the ampicillm-resistance gene; Hyg R represents the hygromycm-resistance gene; ML and MR represent the left and right arms of the minos transposable element, with inverted repeats represented by the black triangles; and H, E and N represent the restriction enzymes Hmdl l , EcoRI and No ti , respectively. Description of the Invention As stated above, an important aspect of the present invention is the treatment of the insect egg under conditions which prevent or delay the hardening of the insect egg chorion. Hardening of the chorion is mediated by a series of enzyme reactions, the first enzyme being phenol oxidase . Other enzymes include dopa decarboxylase, dopamme N-acetyl transferase and N-acetyl dopamine desaturase . Targeting these enzymes with inhibitors is one useful way of delaying or preventing the chorion hardening process. Inhibitors may be competitive or non-competitive inhibitors. Examples of inhibitors of phenol oxidase useful the present mvention, include glutathione, diethyldithiocarbamic acid, l-phenyl-3 - (2 -thiazolyl) -2- thiourea and p-nitrophenyl-p ' -guanid o-benzoate . Of these, p-nitrophenyl-p ' -guanidinobenzoate is preferred. Other inhibitors may be apparent to the skilled person or may be identified using standard enzyme inhibition assays. Typically, the inhibitors will be dissolved an lsotonic solution to prevent swelling of the embryos. Amounts of inhibitor suitable for use m the invention can be determined easily. With regard to p-nitrophenyl-p ' - guanidinobenzoate, a concentration of 0.1 mM has been found to be acceptable. It may be preferable to delay (slow down) rather than prevent the hardening process. Therefore, it may be preferable to use a competitive inhibitor which can be replaced by addition of excess enzyme substrate . Alternatively, the inhibitor may be utilised over time, thereby permitting the enzyme to function with its natural substrate. Delaying hardening should be for a time sufficient for the introduction of the nucleic acid material into the egg. This may require a delay of only a few hours . Insertion of nucleic acid into the egg may be carried out by micro ection . Methods for carrying this out will be apparent to the skilled person, using conventional apparatus . The nucleic acid molecules may be the form of a vector or plasmid containing a heterologous gene to be expressed m the insect embryo. Regulator sequences, including transcπptional promoters, enhancers and initiation signals, may also be present. The purpose of introducing the nucleic acid molecules may be to produce a transgenic insect, having particular genetic traits. Technology for the production of transgenic animals and insects are known and may be adapted for use the present invention. The nucleic acid is integrated into the insect genome using transposable elements. Integration (transposition) is often facilitated by the enzyme transposase, and the transposable element often comprises inverted repeats which function to direct the transposase to the correct position, to initiate excision. Genetic constructs, comprising a transposable element combined (m a genetic fusion) with a heterologous gene, may be prepared using conventional technology, and inserted into the insect egg to produce a transgenic insect. In addition to the heterologous gene, the transposable element may comprise the regulatory factors that ensure successful expression can occur. Transposable elements useful m the present invention may be identified based on experiments carried out on other organisms, e.g. Drosophila. For example, Hermes from Musca domes tica (Atkinson et al . , Proc . Natl . Acad. Sci . USA, 1993; 90:9693-9697) is able to transpose embryos of Drosophila melongaster. Mariner from D . mauri tania (Haymer and Marsh, Dev. Genet., 1986; 6:281-291) was shown to transpose Bactrocera tryom . A preferred transposable element is Minos , found Drosophila hydei (Franz and Savakis, Nucleic Acids Res., 1991; 19: 6646) . It has now been found that mmos transposase can mediate precise insertions into the genome of Anopheles mosquitoes and permit mterplasmid transposition to occur. Therefore, m a preferred embodiment, the invention may be carried out using a Minos transposable element to integrate a heterologous nucleic acid molecule into the genome of an insect embryo, preferably m the presence of a mmos transposase The transposable element may be m the form of a plasmid vector together with a foreign gene and further comprising regulatory sequences, e.g. a promoter. In a preferred embodiment, the promoter is the act Sc promoter from D . melongaster In a further preferred embodiment, the mmos transposase gene is located on a separate helper plasmid, for separate introduction into the embryo. The transposable element may be used to integrate into the insect embryo a heterologous gene which can be expressed m vivo . Alternatively, integration of the transposable element may be required to integrate a heterologous polynucleotide which can be used to disrupt expression of a particular gene. For example, an RNA molecule may be used for gene silencing. The heterologous gene may be used to control the transmission of a parasite, e.g. plasmodium. For example, the gene may encode a product that protects the insect from infection or which encodes an an I -parasitic agent, able to interfere with the life-cycle of the parasite. Some antibacterial peptides are known, including defens s, which may be of use. Alternatively, the gene may be used to produce sterile males which may be released as a means of genetic control. The use of a sex-specific promoter has been proposed for use m Drosophila (Thomas et al . , Science, 2000; 287(5462): 2474-2476), and may be used in the present invention. The Wolbachi a gene may also be used. Suicide genes may also be introduced which can be activated by exposure to certain chemicals. Other suitable genes will be apparent to the skilled person. The transposable elements may also be of use in assays for identifying compounds or products that have msecticidal activity, or for mapping genes responsible for refractoriness of, for example, mosquitoes, to a particular parasite. The insertion of foreign or heterologous genes into a genome can be used to identify enhancer elements located the genome. Significant levels of the product of the gene will not be detectable unless the transposable element inserts next to a region containing the enhancer element. The transposable elements may also be used to perform in vivo site-directed mutagenesis, as described Banga and Boyd, Proc . Natl . Acad. Sci . USA, 1992; 89:1735- The following Example illustrates the invention. Example In the following experiment, the plasmid vector termed MmHyg (illustrated Fig. 1), was used to achieve integration of a heterologous gene into the genome of an anopheline mosquito. As shown Fig. 1, the green fluorescent protein gene, GFPS65T (GFP) (Heim et al . , Nature, 1995; 373:663-664) was chosen as the reporter gene, to show that successful integration of DNA had been achieved. The actm promoter from the D . melanogaster actm5C gene was chosen to drive the expression of the GFPS65T marker (Fyrberg et al . , Cell, 1983; 33:115-123). The hygromycm gene, under the control of the mducible heat-shock protein 70 (hsp70) promoter, was also incorporated into the vector to act as a selectable marker m the event that selection with GFP could not be achieved. The experiment was performed as follows. Blood fed A . Stephensi mosquitoes were allowed to lay eggs 48-72 hours after a blood meal. Eggs were laid m a petπ dish containing 3mm paper soaked a p-nitrophenyl-p ' - guanidinobenzoate (NPGB) solution (Sigma cat. N 8010) 0. ImM m lsotomc buffer (150 mM NaCl , 5 mM KCl , 10 mM HEPES, 2.5 mM CaCl ^ , pH 7.2) . NPGB is not soluble water; it was first dissolved m DMSO and then lsotomc buffer was added to make the 0. ImM final solution. The use of the isotonic buffer is essential as it prevents the embryos from swelling. The petπ dish was removed from the mosquito cage 30 minutes after the first oviposition had occurred. Eggs were then left NPGB until injection, which was carried out between 90 and 120 minutes after oviposition. A total of around 30 embryos were placed on a glass slide covered with paper wet with isotonic buffer, with their posterior poles aligned and oriented towards the inner part of the glass slide. As soon as the embryos started drying they were transferred, by applying a gentle pressure, onto another slide on which a strip of double-sided tape had been stuck at one end. The embryos were then covered with water- saturated halocarbon oil to prevent further desiccation. Glass needles (Eppendorf Femtotips) were loaded with 2μl of the DNA solution by using microloader tips (Eppendorf) . The embryos were microm ected with a mixture of 100 μg/ml of the helper mtronless plasmid pHSS6hsILMι20 (Klmakis et al . , Insect Mol . Biol . 2000; 9(3) :269-275) and plasmid MmHyg(400 μg/ml) . The helper plasmid provides the transposase activity necessary for Minos transposition, while plasmid MmHyg contains the GFP cloned within the inverted terminal repeats of Minos . Micromj ectious were performed by using an Eppendorf transjector 5246 micromanipulator at lOx magnification. The needle was introduced into the posterior pole of the embryos at a 15° angle. The injected volume was controlled by regulating the injection pressure and time. After injection, the embryos were removed gently from the halocarbon oil with the help of a brush and transferred into a new petπ dish containing a stacked layer of filter paper soaked with isotonic buffer to prevent the eggs from floating. They were then allowed to hatch. Hatched larvae were then analysed under the UV light to detect GFP expression. An average of 29% of injected embryos survived and around 50% of the hatched larvae showed strong transient expression of GFP, as monitored by fluorescence. Survival to adult stage (G 0 ) averaged 10% and was a good predictor of successful transformation. In two experiments that gave 16% adult survival, the progeny of 69 G 0 mosquitoes yielded 92 fluorescent individuals among the 10,539 G 1 larvae analysed. It was subsequently determined that the 92 fluorescent G x individuals were derived from a minimum of five independent G 0 founders, representing a transformation frequency of 7% (5/69 surviving adults) . This frequency is higher than that reported m D . melanogaster and C. capi tata transformation experiments using Mmos marked with the whi te gene marker (Loukeπs et al , Science, 1995; 270: 2002-2005, and Proc. Natl. Acad. Sci. USA, 1995; 92: 9485-9489) . These successful experiments provide, for the first time, compelling evidence that germline transformation of anopheline mosquitoes is feasible and that Minos represents an excellent candidate for its achievement.

Claims: CLAIMS I. A method for the genetic modification of an insect embryo, comprising the steps of: i. treating an insect egg under conditions which prevent or delay the hardening of the insect egg chorion; and ii . injecting a transposable element into the egg to permit integration of the element into the genome of the embryo . 2. A method according to claim 1, wherein the insect is an anopheline mosquito. 3. A method according to claim 2, wherein the mosquito is A . gambiae, A . arabi ensi s or A . Stephens! . 4. A method according to any preceding claim, wherein the transposable element is minos . 5. A method according to any preceding claim, further comprising the injection into the egg of a vector comprising a transposase gene, capable of expression in vivo . 6. A method according to any preceding claim, wherein the transposable element comprises a heterologous gene that is capable of being expressed after integration into the embryo . 7. A method according to claim 6, wherein the heterologous gene encodes a product that prevents transmission of a host parasite. 8. A method according to claim 7, wherein the product is an anti-bacterial agent. 9. A method according to claim 7 or claim 8, wherein the host parasite is Plaεmodium falciparum . 10. A method according to claim 6, wherein the gene is a suicide gene. II. A method according to claim 6, wherein the gene product causes male sterility. 12. A method according to any preceding claim, wherein chorion hardening is prevented or delayed by inhibiting an enzyme involved in chorion hardening. 13. A method according to claim 12, wherein the enzyme is phenol oxidase . 14. A method according to claim 12 or claim 13, wherein the inhibitor is p-nitrophenyl-p ' -guanidinobenzoate . 15. A genetically modified anopheline mosquito, obtainable by: i. treating the egg of an anopheline mosquito embryo under conditions which prevent or delay the hardening of the mosquito egg chorion; and ii. injecting a transposable element into the egg, the transposable element being capable of integrating into the genome of the mosquito embryo . 16. A mosquito according to claim 15, wherein the transposable element is Minos . 17. A mosquito according to claim 15 or claim 16, wherein the transposable element comprises a heterologous gene as defined in any of claims 6 to 11. 18. Use of an inhibitor of an enzyme involved in the process of chorion hardening, to prevent or delay hardening of the chorion. 19. Use according to claim 18, wherein the enzyme is phenol oxidase . 20. Use according to claim 18 or claim 19, wherein the inhibitor is p-nitrophenyl-p ' -guanidinobenzoate . 21. Use of the Minos transposable element to transfer a heterologous gene into the genome of an anopheline mosquito embryo . 22. Use according to claim 21, wherein the heterologous gene is as defined in any of claims 6 to 11.

Inventors: CRISANTI ANDREA, FLAMINIA CATTERUCCIA, NOLAN TONY, CRISANTI, ANDREA, FLAMINIA, CATTERUCCIA, NOLAN, TONY
Applicants: CRISANTI ANDREA, FLAMINIA CATTERUCCIA, IMPLYX LTD, NOLAN TONY, CRISANTI, ANDREA, FLAMINIA, CATTERUCCIA, IMPLYX LTD., NOLAN, TONY
Application year: 2000
Main classification: C12N15
IPC codes: A01K67, C12N5, C12N15