by Christoph Weigel
Back in 2003, the British geneticist Austin Burt suggested the development of "Site-specific selfish genes as tools for the control and genetic engineering of natural populations". He wanted to stimulate discussions on the desirability ‒ and feasibility ‒ of eradicating or genetically modifying particular species, namely the Anopheles mosquitoes, which are responsible for the transmission of malaria-causing Plasmodium parasites to humans, with an estimated 600,000 ‒ 800,000 human casualties worldwide in 2013.
So, what are selfish genes ? Whenever E. coli geneticists in the last century wanted to 'tag' a gene in one of their lab strains they introduced a temperature-sensitive plasmid carrying Tn10, a transposon that encodes a transposase ‒ a copy&paste-type of enzyme ‒ and resistance to tetracyclin. After several generations of growth at elevated temperature, the plasmid was lost from the cells. Nevertheless, after plating, a number of resistant colonies remained, now carrying a copy of the transposon on their chromosome: the selfish transposon had refused to perish together with its carrier plasmid and had hopped to the chromosome for rescue (whether the transposon would be located anywhere close to the gene to be 'tagged' was another question, usually taking a keen PhD student a week to find out). Transposons, or 'transposable elements', come in (almost) countless varieties (STC featured particularly cute ones here and here) and are widespread in all types of genomes; up to an estimated 45% of our human genomic DNA is made up of transposons
A site specific selfish gene, as envisioned by Burt, could be one of the so-called homing endonucleases (HEG). These are restriction enzyme-type of proteins that cut DNA at a specific sequence and thus alarm the double-strand break repair squads of the cell. Cells loathe double-strand breaks in their DNA and deal with them pronto! by either non-homologous end joining (NHEJ) or homologous recombination (HR). During the latter process, an intact copy of the disrupted DNA stretch anywhere in the genome serves as template for precision-repair of the lesion. Since in diploid eukaryotes this 'intact copy' is the endonuclease gene on the sister chromatid ‒ and actually the culprit for having the endonuclease synthesized in the first place ‒ it gets copied during the repair process to the new location (Figure 2). In sexually propagating species, such genes easily convert heterozygotes into homozygotes because they 'drive' through a population at a rate that largely exceeds the Mendelian 50% chance of inheritance in the offspring. Thus the term 'gene drive'. In the lab, this seemingly straightforward approach has met with limited success so far. Mostly because HEGs turned out to be hard to modify genetically to recognize different 'target sequences', and to cut them with high-enough efficiency. Also engineering TALENs was not a solution to these latter two problems yet.
Enter CRISPR/Cas9. This elegant ‒ and also much hyped ‒ molecular 'toolkit' will probably make the design of gene drives much easier (STC's emerita Merry Youle considered CRISPRs earlier here, here, here, and here). In contrast to the HEG-approach, the sequence specificity of the Cas9 nuclease is not an intrinsic property of the nuclease itself but is provided by a short and easily 'designable' guide-RNA. Thus, any desired gene can be targeted with relative ease. In their recent paper, Esvitt and colleagues discuss pros and cons in depth, and they don't overlook the bioethical implications of any possible application of gene drives based on the CRISPR/Cas9 technique outside the lab (Figure 3). And finally, they propose safeguards that can, in principle, reverse gene drives already spread in nature ‒ but this would need another blog post to explain in detail...
Christoph Weigel is lecturer at the Life Science Engineering faculty of HTW, Berlin’s University for Applied Sciences and an Associate Blogger for STC