The Epigenetics Revolution полностью

John Gurdon used non-fertilised toad eggs in his work. Any of us who has ever kept a tank full of frogspawn and watched this jelly-like mass develop into tadpoles and finally tiny frogs, has been working, whether we thought about it in these terms or not, with fertilised eggs, i.e. ones into which sperm have entered and created a new complete nucleus. The eggs John Gurdon worked on were a little like these, but hadn’t been exposed to sperm.

There were good reasons why he chose to use toad eggs in his experiments. The eggs of amphibians are generally very big, are laid in large numbers outside the body and are see-through. All these features make amphibians a very handy experimental species in developmental biology, as the eggs are technically relatively easy to handle. Certainly a lot better than a human egg, which is hard to obtain, very fragile to handle, is not transparent and is so small that we need a microscope just to see it.

John Gurdon worked on the African clawed toad (Xenopus laevis, to give it its official title), one of those John Malkovich ugly-handsome animals, and investigated what happens to cells as they develop and differentiate and age. He wanted to see if a tissue cell from an adult toad still contained all the genetic material it had started with, or if it had lost or irreversibly inactivated some as the cell became more specialised. The way he did this was to take a nucleus from the cell of an adult toad and insert it into an unfertilised egg that had had its own nucleus removed. This technique is called somatic cell nuclear transfer (SCNT), and will come up over and over again. ‘Somatic’ comes from the Greek word for ‘body’.

After he’d performed the SCNT, John Gurdon kept the eggs in a suitable environment (much like a child with a tank of frogspawn) and waited to see if any of these cultured eggs hatched into little toad tadpoles.

The experiments were designed to test the following hypothesis: ‘As cells become more specialised (differentiated) they undergo an irreversible loss/inactivation of genetic material.’ There were two possible outcomes to these experiments:

Either

The hypothesis was correct and the ‘adult’ nucleus has lost some of the original blueprint for creating a new individual. Under these circumstances an adult nucleus will never be able to replace the nucleus in an egg and so will never generate a new healthy toad, with all its varied and differentiated tissues.

Or

The hypothesis was wrong, and new toads can be created by removing the nucleus from an egg and replacing it with one from adult tissues.

Other researchers had started to look at this before John Gurdon decided to tackle the problem – two scientists called Briggs and King using a different amphibian, the frog Rana pipiens. In 1952 they transplanted the nuclei from cells at a very early stage of development into an egg lacking its own original nucleus and they obtained viable frogs. This demonstrated that it was technically possible to transfer a nucleus from another cell into an ‘empty’ egg without killing the cell. However, Briggs and King then published a second paper using the same system but transferring a nucleus from a more developed cell type and this time they couldn’t create any frogs. The difference in the cells used for the nuclei in the two papers seems astonishingly minor – just one day older and no froglets. This supported the hypothesis that some sort of irreversible inactivation event had taken place as the cells differentiated. A lesser man than John Gurdon might have been put off by this. Instead he spent over a decade working on the problem.

The design of the experiments was critical. Imagine we have started reading detective stories by Agatha Christie. After we’ve read our first three we develop the following hypothesis: ‘The killer in an Agatha Christie novel is always the doctor.’ We read three more and the doctor is indeed the murderer in each. Have we proved our hypothesis? No. There’s always going to be the thought that maybe we should read just one more to be sure. And what if some are out of print, or unobtainable? No matter how many we read, we may never be entirely sure that we’ve read the entire collection. But that’s the joy of disproving hypotheses. All we need is one instance in which Poirot or Miss Marple reveal that the doctor was a man of perfect probity and the killer was actually the vicar, and our hypothesis is shot to pieces. And that is how the best scientific experiments are designed – to disprove, not to prove an idea.