December 02, 2015
Everyone knows about genies. They have almost infinite power, can grant you three wishes, and are kept under control by the owner of their lamp.
And as we saw in Disney’s Aladdin, it is a good thing that the lamp is around! When the evil sorcerer Jafar was given the powers of a genie, he began to take over the world. Until, that is, Aladdin forced him back into his lamp where he could be kept under control.
In the last few years, scientists have come up with their own genies. While not as powerful as the “real” ones, these gene drives can still pack quite a punch. And maybe even grant us a few wishes.
Gene drives can force genes to spread quickly through a population whether those genes are good for a species or not. This means we might be able, for example, to force a “bad” gene to spread through the mosquitoes that transmit malaria. By causing the mosquito population to crash, our wish to save hundreds of thousands of lives each year would be granted!
But just like a genie, we need to keep gene drives under control. We do not want something that overrides natural selection to escape and wreak havoc with ecosystems.
Which is where, as usual, our friend yeast can help! In a new study out in Nature Biotechnology, DiCarlo and colleagues use yeast to test two different strategies to make gene drives safe enough to use. And, they argue, safe enough to research.
Gene drives are based on the idea of homing endonucleases. Basically, if a gene associated with a gene drive is on just one of the two chromosomes in a pair, the gene drive will copy and insert the gene into the other chromosome through a precise DNA cut.
Now both chromosomes end up with a copy of the gene. Which of course means all of the offspring will get the altered gene too. This copying will happen generation after generation until the new gene has swept through the population.
The idea for gene drives has been around since 2003 but really only became practical with the discovery of the CRISPR/Cas9 system. This genome editing tool, which is ludicrously simple to program to target most any DNA sequence, allows scientists to create most any gene drive they want.
The CRISPR/Cas9 system has two parts. One part is the guide RNA which leads the second part, the endonuclease Cas9, to the right spot in the genome to cut. What makes the system so powerful is that you just need to make a different guide RNA to target different sequences in the genome.
One easy way to help control a gene drive is to keep these two parts separate. Do not have the guide RNA and the Cas9 on the same piece of DNA. Then, if one part were to escape, it couldn’t do anything on its own.
This is of course easy to do in yeast. Just integrate one part into a chromosome and keep the second part on a plasmid.
This is just what DiCarlo and coworkers did. And they showed that this separation can be very effective.
They integrated a guide RNA into the ADE2 gene of a haploid yeast to create a gene drive designed to disrupt ADE2. As expected, this strain produced red colonies on adenine limiting media.
They next mated this strain to a wild type haploid. All of the resulting diploids were cream colored. This is what would be expected as both copies of ADE2 need to be disrupted to see red colonies in a diploid.
When these diploids were sporulated, the researchers got the expected 2:2 ratio of red to cream colored haploids. This all changed when they introduced a Cas9 containing plasmid into the experiment.
In the presence of Cas9, more than 99% of the resulting diploids were red. And when sporulated, these diploids produced all red haploid colonies.
The two parts of CRISPR/Cas9 together drove the disrupted ADE2 through the population. But importantly, just having the guide RNA integrated into ADE2 had no effect on how the two alleles were passed down. Once one part is removed, the gene drive stalls out.
And genetic background did not significantly affect how well this ADE2 gene drive worked. When they mated their haploid to six different strains of yeast they saw no loss in efficiency.
So separating the two parts of the gene drive is a pretty good failsafe. But of course nothing is perfect.
Ideally we need some way to shut the system down if all of our safety features fail. We want to be able to get rid of Jafar and the lamp entirely if possible.
DiCarlo and coworkers showed that they could create a gene drive that could overwrite and correct the ADE2 they had disrupted with the guide RNA. This new gene drive targeted a synthetic sequence in the original gene which means that it would only affect altered yeast. So even if things go awry, we may be able to erase the changes we made.
These two strategies should help keep gene drives in check both in the wild and the lab. But of course, again, it is important to keep in mind that nothing is foolproof.
At the end of Aladdin, they buried Jafar and his lamp deep in the desert to keep him from causing any more trouble. But his lamp was found and Jafar reemerged to wreak havoc in the second Aladdin movie, reminding us that we must be very careful when unleashing powerful forces.
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight