Growing sperm in a test tube may offer a powerful new way to genetically modify animals and potentially correct human genetic diseases before conception.
The technology offers two advantages. Firstly, it creates GM animals in one generation rather than two, unlike most conventional techniques.
Secondly, because the genes are spliced into laboratory-born sperm, it may allow scientists to do sophisticated genetic manipulations in a wide range of animals. So far these have only been possible in mice.
“The big deal here is that this opens up vast possibilities to tailor this technique for different applications,” says Shawn Burgess of the US National Institutes of Health in Bethesda, Maryland. But experts say the technology will need to be improved further before it shows real promise.
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Mosaic fish
Burgess, with colleagues in Japan, developed the technique to introduce new genes into zebrafish, a favourite animal model for geneticists. Current techniques – such as introducing DNA into eggs, sperm or embryos shortly before fertilisation or implantation – results in a high percentage of “mosaic” fish possessing a mixture of GM and normal cells.
To create animals with the new genes in every cell, researchers then need to mate the mosaic fish and screen the offspring for those whose parents possessed a GM sperm or egg.
The new technique relies on culture techniques developed by the Japanese team to grow male germ cells and turn them into sperm in the test tube. Burgess then created retroviruses to infect the sperm cells and insert a new gene into their DNA.
The sperm was then used to fertilise eggs and make new animals. Only six of 1410 eggs exposed to the altered sperm eventually developed into transgenic animals, but every cell in their body contained the new gene.
“It’s a technique that’s worth keeping an eye on,” says zebrafish geneticist Perry Hackett of the Minnesota-based biotech company Discovery Genomics.
But he points out that Burgess’s success rate was so low researchers could actually end up screening fewer animals with conventional techniques. “And for those, you don’t need special sperm cultures or viruses. So I don’t think this is ready for prime time yet.”
Gentler gene therapy
Mammalian biologists also seem unlikely to rush to the technique. For one, they have not yet developed a similar method for culturing and maturing sperm. And current transgenic technology for many mammals is already much more efficient than for zebrafish.
But Burgess says his team is already busy improving the technology. For example, they are working out ways to grow the sperm in culture longer so that sperm cells that have not incorporated DNA can be killed off – that would greatly improve the efficiency.
Researchers might then be able to disrupt or replace existing genes, something that is currently only possible in mice. Gene replacement is also a goal of some cloning research, but that technique has its own challenges to be overcome.
If the new procedure could be adapted to human sperm, Burgess says it might be used for a gentler type of gene therapy by allowing hereditary defects to be repaired before fertilisation.
The whole notion of germline gene therapy in humans remains highly controversial, but Burgess thinks this method offers some advantages. “It’s an approach where you don’t have to treat or kill embryos in order to correct a defect,” he says.
Journal reference: Proceedings of the National Academy of Sciences (vol 101, p 1263)
