some brief notes on cloning and stem cells
Back in the December/January issue of the Oz science mag Cosmos there was a piece on cloning, full of breakthroughs and cautionary tales, all before the scandal in
The article informed me among other things of the first successfully cloned dog, Snuppy, created by the team of Hwang-woo Suk at
First, some points to myself to clarify different features of cloning via Cosmos magazine and this site, among others:
Somatic nuclear cell transfer was the procedure used to clone Snuppy, as well as Dolly, the first mammal known to be cloned, back in July 1996. Snuppy was born in April 2005. The procedure involves removing the nucleus from a body or somatic cell (ie a mature, differentiated cell), and placing it into an enucleated egg cell. The cell is then stimulated to divide and grow. The embryo can be implanted into a female for gestation, or have its development suspended while still at the embryonic stage, to be put to various uses, particularly medical.
Somatic nuclear cell transfer is the procedure central to therapeutic cloning. The egg with the somatic nuclear material transferred into it, is allowed to divide. The resulting blastocyst ‘has an outer layer of cells and an inner cluster called the inner cell mass. Cells from the inner cell mass are isolated and used to develop new embryonic stem cell (ESC) lines’.
The difference between embryonic stem cells (ESCs) and other stem cells is that they’re totipotent in the first couple of days and then pluripotent. In fact, there are three types or ‘levels’ of stem cell, totipotent, pluripotent and multipotent, moving from less specialized to more specialized. Adult stem cells are multipotent, though there’s now a question as to whether multipotent cells can be transformed into pluripotent cells. These are the sorts of issues causing so much excitement within stem cell research.
To return to cloning – the cloning of a dog (out of some 1095 clone embryos) is the most recent of many achievements. Sheep, cows, pigs, goats, cat, mice, horses, rabbits, deer and even a mule, all have been successfully cloned, though primates seem as yet beyond the horizon.
There’s a fair amount of debate, though, about just how successful these clones have been. One of the well-known problems these clones have faced is Large Offspring Syndrome (LOS), especially in the case of lambs and calves, but other abnormalities, some of them not easily identifiable at birth, have abounded. In fact, there’s a question as to whether any cloned animal could be described as normal (a term which consequently has come under scrutiny), though they’ve produced healthy enough offspring. It seems that our tampering with the natural process of reproduction is what’s causing the problems.
One of the pioneers of therapeutic cloning, Rudolf Jaenisch, who has worked more or less exclusively with mice, is one person who believes that a normal clone has never been produced. Further, he believes that the problems of cloning are in principle unsolvable. Work done by his group in 2002 seems to have borne out his claim.
I’ll try to summarise the problem, with the help of Cosmos. It seems that in the natural fertilization process, when an egg cell is penetrated by a sperm cell, the egg ‘reprograms’ the genetic material of the sperm cell (which must be quite specialized in order to locate and penetrate the egg). It’s of course this reprogramming ability that scientists have made use of to create ESCs.
Scientists have learned to assist in this rebooting process. They’ve learned that rebooting requires cell quiescence (that the cell not be dividing). It also requires time for the reboot to be completed before embryonic development. The egg must also be unfertilized.
Yet even with a successful reboot, clone embryos lack the ability to distinguish between maternal and paternal gene sets. This appears to be vital. To quote the Cosmos article:
Although sperm and eggs bear the same genetic code, they carry slightly different instructions as to how that code should be read when they combine – so-called ‘imprints’. For instance some genes, if they reside in the egg, remain silent; if they reside in the sperm, they are read avidly. About 50 genes receive this differential treatment, many of which dictate the growth of the placenta and foetus. The theory, according to scientists such as Jaenisch, is that, because clone embryos cannot differentiate between paternal and maternal gene sets, they read both, rather than ignoring one, with the result that the foetus grows too much. This explanation also gels with the fact that the offspring of clones, which are born through the old-fashioned union of sperm and eggs (and hence receive the proper imprints) are normal.
Now Jaenisch and others may be overly pessimistic about the possibility of overcoming these difficulties, but he’s surely right in pointing out that they must be overcome before we even consider human cloning, quite apart from the various other ethical issues swirling around it. The good news is that, due to work coming out of Jaenisch’s lab, the ethical concerns about the use of ESCs may one day be obsolete. A technique called altered nuclear transfer results in the creation of embryo-like entities that aren’t viable in utero but still yield healthy ESCs. Of course there will always be some objectors, if only on the generalized ground of ‘playing god’, but the field continues to surprise and excite.