A new tool for genome research, developed in the Yale laboratory of Tian Xu, Ph.D. ’90, professor and vice chair of genetics, professor of molecular oncology and development and a Howard Hughes Medical Institute investigator, promises to greatly accelerate the work of assigning purpose to thousands of unexplored human genes.
The tool is a jumping gene, a small piece of DNA called a transposon that moves around the genome, usually settling in other genes and allowing scientists to suppress the activity of existing genes or insert new ones.
Transposons are active in many plant and insect genomes and have helped to make the fruit fly Drosophila the darling of geneticists, as these mobile DNA fragments were used to decipher the role of nearly every gene in that model organism. But for decades scientists could not find an equivalent transposon for mammals.
As reported in the August 12 issue of the journal Cell, Xu and his colleagues manipulated a transposon called piggyBac, found in the cabbage looper moth, so that it can be easily cut and pasted into the genomes of higher organisms, including mice and humans. “With this transposon, we now have the ability to systematically inactivate each and every gene in a model organism like the mouse,” Xu said.
In mouse studies, scientists have traditionally used chemicals to modify genes, but this approach is painstakingly slow, and it can be difficult to locate the genes that have been mutated. ThepiggyBac transposon, when injected into fertilized mouse eggs along with an enzyme called transposase, is remarkably efficient at inserting itself into important coding regions of the genome, and as its name implies, it carries genetic tags that allow researchers to locate mutations quickly.
Moreover, piggyBac has the added feature of total reversibility, which should allow scientists to verify that particular mutations have particular effects. In the presence of transposase, piggyBaceasily hops into genes, and it remains in place in any offspring in subsequent generations that do not inherit the enzyme. But when these mice are mated with others who carry the transposase gene, piggyBac hops back out of genes without leaving a trace.
These traits make piggyBac a “dream tool” for geneticists, Xu said. “This new technology will completely change the game of using mutagenesis to understand the function of mouse genes and, by extension, their human counterparts.” PiggyBac could also be a promising new vehicle for human gene therapy, according to Xu, who said that, in addition to carrying tags, piggyBac can be engineered to carry new genes into the genome.
To demonstrate the potential of this genetic piggybacking, Xu and his colleagues used piggyBac to insert a gene for a protein that glows red under ultraviolet light into a mouse. However, many more experiments will be required to determine whether the transposon, or some variation of it, could reliably and safely transfer therapeutic genes to humans.
Xu’s immediate goal is to use piggyBac to inactivate every gene in the mouse, one by one, a project that would be unthinkable with traditional mutagenesis methods. “For the past two decades, it has routinely taken about a year to mutate one gene in a mouse, and altogether about 3,000 genes have been knocked out in mice, out of a total of about 25,000 that are in the genome,” Xu explained. With the help of piggyBac, he said, “in three months, with two students, we have done 75 genes.”