UMMS scientists develop multicolored labeling system to track genomic locations in live cells
“Most people are using CRISPR for editing genomes. We are using it to label DNA and track the movement of DNA in live cells,” said research specialist Hanhui Ma, PhD, who coauthored the study with Thoru Pederson, PhD, the Vitold Arnett professor of cell biology and professor of biochemistry and molecular pharmacology.
Knowing the precise location of genomic elements in live cells is critical to understanding chromosome dynamics because the genes that control our biology and health do so according to their location in 3-dimensional space, said Drs. Pederson and Ma. For a gene to be transcribed and expressed, it must be accessible on the chromosome. Where DNA is positioned in the crowded nucleus plays an important role in everything from embryonic development to cancer.
Current technologies, however, are only capable of following, at most, three genomic locations at a time in live cells. Labeling more sites requires that cells be fixed by bathing them in formaldehyde, thus killing them and making it impossible to observe how the chromosome’s structure changes over time or in response to stimuli.
To overcome this technological hurdle, Pederson and Ma turned to CRISPR/Cas9. To tag specific locations along the genome using the CRISPR/Cas9 complex, they created a Cas9 mutation that makes the nuclease inactive so it only binds to the DNA and doesn’t cut the genome. Once deactivated, the CRISPR/Cas9 element is ferried to a specific location on the genome by a guide RNA that can be programmed by the researchers.
Here’s a question that occurs only to madmen and geneticists: How do you get a gene that kills an organism to spread through a whole population of that organism?
You can either make your gene deadly, and thus impossible to pass on, or not, and thus useless as a vector of attack. The solution has long been to try “silent” genes that can spread with no negative effects, either introducing a deadly weakness to a man-made chemical we withhold for a while, or by waiting for deadly activation by such a chemical. But recently, with the advent of advanced new in vivo gene editing technology, it’s become possible to make genes that seem to defy evolution — and that means we could soon start releasing animals carrying doomsday genes that spread with astonishing speed, quickly killing entire populations.
Such an animal is currently sitting in a laboratory at Imperial College London, an apocalypse mosquito carrying a gene that could one day end its entire species. It represents a controversial proposal to end the scourge of malaria, which kills hundreds of thousands of people each and every year, by wiping out the mosquitoes that spread the disease. It also represents a fundamentally new ability for humanity: the power to easily and selectively snuff out a subcategory of life on Earth. The name for this power is called gene drive.
Gene drive is simply the use of some strategy to artificially increase a gene’s inheritance rate. Such strategies are found all over nature, but despite decades of theorizing, nobody had a really viable way for mankind to harness this functionality through biotechnology. That’s changed thanks to the incredible advances in direct gene editing we’ve seen over the past half-decade, in particular the CRISPR/Cas9 gene editing suite.
These “molecular scissors” are actually borrowed from viruses, allowing scientists to swap out a gene in a living organism for one of their choice, edit it right into the genome so it will be passed on as the cells reproduce. If you can get your gene spliced into the “germ cells,” the pre-sperm or -egg cells of these organisms, then you can even introduce a chance that it will be passed on to the next generation — classically, without gene drive, you can introduce a 50% chance.
The chance is 50% because germ cells, like virtually all other cell types in humans and mosquitoes, have two copies of our genome. When we splice in our attack gene, it will end up sitting across from a second, totally normal copy of the gene it just replaced. This means that when the two copies get pulled apart to form the half-genomes of two new, separate sperm cells, only one of those new sperm cells will have our spliced-in sequence. The other will carry the same gene it would have, regardless.
So, if our spliced-in gene lowers evolutionary fitness, then all that will happen is the other half of the offspring will thrive, and the infected individuals will be quickly bred out of the population. And even if it’s a seemingly harmless silent gene that does nothing at first, it will still spread too slowly to change the overall population much at all.
Our mosquito doomsday device gets around these problems by applying two innovations.