Credit: Courtesy of Gerald Shatten, Oregon Health Science University, USA.

The problem with transgenic mouse models of human disease is just that — they are mouse models. And although they can be very useful, the many differences between us and mice have proved problematic for the fine-tuning of some therapies. So a goal of the past few years has been to produce transgenic (non-human) primates, but a major obstacle to this has been getting conventional gene-transfer methods to work. Now, a team of Oregon scientists have overcome this problem to produce the first live-born and (so far) healthy transgenic monkey.

Chan et al. overcame the technical barrier to transgenic success in primates by adapting a vector that they had previously used to good effect in cattle. The key to this protocol is that the transgene-carrying retroviral vector is introduced into oocytes and not into embryos. Often retroviral vectors give rise to transgenic mosaics because they only integrate into dividing cells — cells in which the nuclear envelope is degraded during mitosis, so allowing the retroviral pre-integration complex access to the host cell DNA. Chan et al. therefore reasoned that, rather than targeting embryos, they would introduce their vector into metaphase II oocytes, which have no nuclear envelope. This timing should allow vector complexes to access the DNA. Additionally, as the genes are inserted before fertilization, the resulting offspring should not be mosaic.

So how successful was this strategy when transferred to rhesus monkeys? The authors injected their green fluorescent protein-encoding retroviral vector — either under the control of the cytomegalovirus early promoter or the human elongation factor-1 alpha promoter — into 224 mature rhesus oocytes. Six hours later, they fertilized them. Of these 224 fertilized oocytes, 126 developed to 4-cell stage embryos, 40 of which were selected by their morphology for transfer, in pairs, to 20 surrogate mothers. Five pregnancies resulted, three of which produced healthy males. One pregnancy miscarried fraternal twins mid-gestation, possibly because rhesus monkeys rarely sustain twin pregnancies.

Transgene integration, transcription and expression analyses revealed that the miscarried twins and one of the liveborn males were transgenic and that this male does not express the transgene. The authors named him ANDi (for inserted DNA in a reverse-transcribed direction, see picture), but they will have a four-year wait until ANDi hits puberty before they can test for germline transmission. The other two males also require further testing to see whether they are transgenic mosaics.

Although successful gene-targeting in primates has many more barriers to overcome, this method of producing transgenic monkeys could be combined with other approaches to hasten progress in this field. But the questions that remain are not all of a technical nature — the advanced cognitive awareness of our primate relatives requires careful thought as to what is and what is not ethically appropriate when it comes to genetically modifying them.