We now have the ability to precisely alter and replace the DNA of living cells - the targeted engineering of genomes. Today the ever growing toolset enables genetic modifications ranging from the alteration of single base pairs to the generation of entirely synthetic chromosomes. Reprogrammable enzymes such as the modular TAL or RNA guided CRISPR/Cas endonucleases have been used to modify a plethora of genes in model and non-model organisms.
Such nucleases are also being developed to modify and correct genes patient cells; the beginning of genome engineering as therapy. Other novel tools such as synthetic programmable transcriptional activators as well as transcriptional repressors can be used to rewire genetic pathways. Form here one can move to create new genetic functions, new gene networks, new organs and even new species. Such a synthetically-oriented approach to life and biological species that goes beyond the natural is a prominent and recurring theme in the history of biology over the last century. It ranges from Luther Burbank’s famous “New Creations” in 1893 to Hugo de Vries claim that evolution had to become an experimental science with the help of which he wished to create new species rather than crossing existing ones.This power is beginning to transform genetics and genomics and will ultimately transform agriculture and medicine and society.
Our research is based on the exploitation of some these new tools and possibilities; not only for growing the toolkit itself and to move forward with the construction of ever more complex human-designed biological systems but also for applying this new powers to tackle real problems.
One application we are interested in the control of insect pests or insect vectors of disease. So while we utilize model systems such as Drosophila and yeast our current focus is on Anopheles gambiae the malaria mosquito. Mosquito species of the Anopheles gambiae complex represent the major vectors of human malaria and they pose an enormous burden on global health and economies. Every year around 500 million people are infected by malaria and half a million people die as a consequence of Plasmodium parasite infections.
How can genome engineering help to control an insect vector population?
The Sterile Insect Technique (SIT) is a tried-and-tested cost-effective method for the suppression or elimination of insect pest populations. Conventional SIT developed in the 1950s involves releasing millions of sterilized insects (e.g. by radiation) over a wide areas to mate with the native female insects present. Unproductive mating of the native females leads to a decrease in the population’s reproductive potential because no viable offspring are produced. Ultimately, if males are released in sufficient numbers over a prolonged period the local pest population is suppressed or eliminated. We have expressed the endonuclease I-PpoI derived from a slime mold in male transgenic mosquitoes. I-PpoI destroys the essential ribosomal genes of the mosquito. This caused dominant male sterility without the need for irradiation. We found that I-PpoI males were capable of introducing high levels of infertility in target populations in indoor cage trials. This technology is currently moving to the field.
Then we took this a step further. The ribosomal genes of the mosquito are located only on the X chromosome. When we restricted the activity of the potentially toxic endonuclease I-PpoI to spermatogenesis during male meiosis this prevented the X chromosome from being transmitted to the next generation. However, Y bearing sperm were unaffected by I-PpoI. As a result our transgenic mosquitoes produced close to a 100% male offspring. Again this is a potential avenue for control: We showed that transgenic distorter males, when released into caged wild-type population, could efficiently eliminate these populations by shifting the sex ratio towards males until the population crashed.
What if we go a step further?
The next step is to move from the genetic engineering of individuals to the genetic engineering of populations: The target and controlled genetic modification of all the individual genomes that together constitute a single species; the modification of the pan-genome.
Together with a number of other research groups we have engaged in a long-term effort to assess the potential of re-engineered homing endonuclease genes (HEGs) another class of programmable and highly specific endonucleases as new tools for mosquito population genetic engineering and control. Homing endonuclease genes are 'selfish' genetic elements that combine the capability to selectively disrupt specific gene sequences with the ability to rapidly spread from a few individuals to an entire population through homologous recombination events which the endonuclease itself induces.
Because of these properties, HEGs are regarded as promising candidates to transfer genetic modifications from engineered laboratory mosquitoes to wild-type populations. For examples mosquitoes might be engineered to resist pathogen transmission, which would be considerably cheaper and more ecologically friendly than the heavy use of insecticides. These pathogen-refractory genes would be spread through a population with the help of HEGs.
We are currently measuring the homing frequency of HEGs in mosquitos in the lab. We are also engineering HEGs to target mosquito genes sequences and test their capability to spread in cage mosquito populations.