Scientists have resurrected an ancient gene that can jump between chromosomes in the cells of zebra fish, salmon, and humans. Their work could overcome a stumbling block for gene therapy by ensuring that a gene is actually inserted into a target cell's chromosome, where it will order the production of therapeutic proteins. Experts say the new method, reported in the current issue of Cell, could also be a useful tool for studying embryonic development.
Nobel Laureate Barbara McClintock realized in the 1940s that certain genes can "jump" from one chromosome to another. These so-called transposable elements, or transposons, squeeze themselves into host chromosomes. McClintock found them in the corn genome, for example, and they have counterparts in fruit flies. In 1994, the first vertebrate transposons were discovered in zebra fish, but they were inactive and appeared to have been silenced millions of years ago.
To resurrect a working vertebrate transposon, molecular geneticist Perry Hackett at the University of Minnesota, Twin Cities, and his colleagues Zoltan Ivics, Zsuzsanna Izsvak, and Ronald Plasterk turned to the salmon genome, where transposons had been active more recently in evolutionary time. The team trawled the salmon genome for examples of inactive transposons, then eliminated apparent mutations. Finally, they patched together a transposon based on computer alignments and their own judgment from their knowledge of similar sequences in other genes. Not only could their new construction, dubbed "Sleeping Beauty," slip into chromosomes, but a small test gene spliced into the transposon was also imported into the DNA of fish and human cells. When combined with lipids to deliver genes to cells, the transposon chalked up a 20-fold increase of efficiency at inserting genes into chromosomes.
"This work is a real surprise," says Jef Boeke, a molecular geneticist at Johns Hopkins University, who adds that it represents a "really important 'integration machine' for getting foreign genes into target cells." But several hoops remain before Sleeping Beauty is ready for gene therapy. The transposon inserts itself almost randomly into the host cell's chromosomes, and thus, it could land in the middle of a desirable gene and cause a harmful mutation. It will also have to be adapted to carry genes large enough to code for many therapeutic proteins.
In the meantime, Boeke says Sleeping Beauty could provide a new and better way to create mutants to study embryonic development, since the inactivated gene can be "tagged" using a short DNA sequence inserted by the transposon. Sleeping Beauty "could be an almost universal mutagenesis tool" for vertebrates, says Boeke. As such, the new discovery could accelerate manifold the understanding of how an embryo becomes an adult organism.