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Creating a knockout human

Wednesday, 27 February, 2008

I said in my post about a possible genetic manipulation for viral immunity that a knockout human would “take too long and probably be unethical”. Well, I may have to eat my words (at least, the part about it taking too long). You see, I think there would be a way to make a knockout human in a shorter time than making a knockout mouse (of course, creating a knockout mouse with this method would be shorter still). I’ll explain how a knockout mouse is usually created, and then I’ll tell you the shortcut to be taken for long-lived creatures like humans.

Firstly, stem cells are isolated from a mouse embryo. These cells are genetically manipulated to insert a non-functional version of the gene of interest, hence “knocking out” the gene by overwriting it with a broken copy (a functional gene from another animal could be added to create a ‘knock in” mouse). The inserted gene is usually accompanied by a marker gene – a gene that can be detected (such as immunity to a toxic agent, which can be detected by growing the stem cells in a mixture containing that agent). Then, stem cells in which the gene has been successfully inserted are inserted into another (or the same) blastocyst, meaning that the mouse growing from that embryo will be a chimera – containing some cells that have been genetically modified and some that haven’t. If some of the germ cells (eggs or sperm) happen to have arisen from cells that have been genetically engineered, then those mice can be bred with another “knockout” mouse (or inbred) to create a mouse where both chromosomes contain genetically engineered genes.

This takes so long because to get your hands on a knockout mouse embryo requires you to wait for some mice to get old enough to breed. For mice, this is only about 5-8 weeks, but in humans it would be at minimum about 12 years and safely (and legally) at around 16/18/21 years. And not only is this a problem in terms of time, but forcing people to copulate, and specifying their partner, is a big ethical problem (even if we accept the ethical acceptability of genetic modification of humans). But this can be overcome, although it may bring up other ethical problems of its own.

The shortcut stems from the fact that we don’t actually need a man and a woman to create a child. Females would actually have all their primary oocytes (immature eggs) around 4-6 months before birth, but they mature and are released only after puberty. Likewise male embryos contain spermatogonia (although unlike in the female, they will be produced throughout life), which develop into spermatozoa after puberty. Turning primary oocytes into ova (mature eggs) is fairly simple to do in vitro, involving just a set of chemical signals (and has been done before to result in pregnancy). Turning spermatogonia into spermatozoa to achieve conception is harder, but it can be done in vivo (by putting the embryonic spermatogonia into a donor testicle, even one of a different species). So, one could possibly take a shortcut by performing some advanced IVF on germ cells harvested from the initial chimeric human embryos.

In solving this practical problem, numerous ethical issues could be raised. Firstly, one could object to creating a child whose genetic parents would have been embryos or foetuses at the time of conception (and need not even be implanted). Surely it is unethical to force two people to mate, but provided one accepts the discarding of embryos from IVF (and indeed normal reproduction), and also accepts abortion five months before birth, then why should this be an issue? If we can deny that the embryo or foetus is a person, or otherwise deny it a right to life, then we can surely deny the embryo or foetus the right to choose a sexual partner. One could also object to the xenotransplantation for maturation of human sperm (i.e. having human sperm grow in the testes of another animal), but this is likely a technical hurdle to be overcome, and so soon human sperm could be grown in a dish like we do for human eggs.

Perhaps an easier method, that gets around the above ethical hurdles, would be to transfer the nucleus of one of the genetically modified stem cells into an ovum and stimulate it to mature into a knockout human. This could negate the need to create the chimeric generation altogether. But, this is called cloning, and is frowned upon in most Western nations.

Of course, we are likely to come up with much better methods of genetic interventions in mice, and soon I expect the knockout procedure will be replaced with something more efficient. The easiest, I think, would be finding a way to dedifferentiate (turn back) the genetically modified stem cells from pluripotency to totipotency, allowing them to develop into an embryo without forming the chimera. In fact, I would not be at all surprised if such a procedure had already been done recently (but won’t that upset the pro-life crowd – “every embryonic-stem cell is sacred”?).

So, bottom line is that I was wrong. But in my defence, the press release did report that one of the researchers said something even more incorrect. So you can all look at his comment to distract yourself from the fact that I said something wrong:

Dr. Sonenberg explained that the process of knocking out genes is not possible in humans, but the researchers are optimistic new pharmaceutical therapies will evolve from their research. [emphasis mine to increase the power of the distraction]

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3 comments

  1. So what do you think a child with XSCID is?


  2. A child. One with mutations in the gene for interleukin 2 receptor gamma, leading to a severely compromised immune system. I don’t see what that has to do with creating a knockout human though.



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