With its transparent, externally-fertilised eggs, frequent reproduction cycle and fast growth, the experimentally-friendly zebrafish has become a major player in the study of vertebrate biology and human disease. These helpful characteristics make it a useful genetic model with which to study gene behaviour and function.

Now, researchers, led by Stephen Ekker at the Mayo Clinic in Minnesota, have for the first time been able to make custom changes to parts of the zebrafish genome, using artificial enzymes to cut out portions of DNA at targeted positions within a gene sequence, and replace it with synthetic DNA[1]. Targeted genome editing has been shown to work in cell cultures[2] and in other species[3], but such manipulation has always used enzymes known as ZFNs – zinc finger nucleases – or morpholinos, molecules that can regulate how a gene is expressed. Ekker and his colleagues used different enzymes known as transcription activator-like effector nucleases, or TALENs. In vivo modifications have never before been achieved using this method.

Zebrafish DNA has been adapted in various ways, including deliberate mutation at targeted locations using ZFNs, and manipulation of gene behaviour using morpholinos. TALENs have also been used before, though in a similar way to ZFNs – to introduce mutations, not to rewrite genetic material.

TALENs have several advantages over the other techniques; they are cheaper, more efficient, and have the potential to work on any DNA sequence. And while the effects of morpholinos are temporary, TALENs cause permanent modifications. They also allow faster analysis of induced mutations – in some cases, it is possible to examine injected animals immediately. The researchers used a more developed and active form of TALENs, further increasing their efficiency.

One of the sequences that Ekker and colleagues inserted into the zebrafish DNA was even controllable, meaning it can be switched on and off. This controllability could enable researchers to examine the effect of a gene both during early development and later in life. Traditional gene alterations do not allow this, as a fish with a mutated version of the gene could not survive until adulthood.

This successful use of TALENs in fish research "opens up the possibility to do many great experiments using zebrafish," says Jason Rihel from the zebrafish research group at University College London, who was not involved with the study. “The ability to directly rewrite native genetic sequences would give us the precise control of genes needed to refine zebrafish models of complex diseases.” Such models could be used to probe “the behavioural functions of specific brain neurons, or to tease apart the network of signals that orchestrate vertebrate development,” he adds.

Ekker says the work will find uses in other model organisms beyond zebrafish. “Although we focused on zebrafish, there are notable implications for using this same approach in other model systems, including rats, mice, flies, and worms,” he says. Research using species such as mice relies on in vitro modification of stem cells, which are then cultivated into a full mutant adult. This technique is species-limited and could not enable genetic modification of a human with a disease, for example.

But Rihel sees potential in the TALEN technique for humans as well. “To pull just one pie-in-the-sky idea off the top of my head—we could potentially use targeted DNA editing in the retina to repair a human blindness gene, for example.”