Read The Epigenetics Revolution Online
Authors: Nessa Carey
Tags: #Science/Life Sciences/Genetics and Genomics
The Epigenetics Revolution (4 page)
Did John Gurdon know what this process was when he generated his new baby toads? No. Does that make his achievement any less magnificent? Not at all. Darwin knew nothing about genes when he developed the theory of evolution through natural selection. Mendel knew nothing about DNA when, in an Austrian monastery garden, he developed his idea of inherited factors that are transmitted ‘true’ from generation to generation of peas. It doesn’t matter. They saw what nobody else had seen and suddenly we all had a new way of viewing the world.
The epigenetic landscape
Oddly enough, there was a conceptual framework that was in existence when John Gurdon performed his work. Go to any conference with the word ‘epigenetics’ in the title and at some point one of the speakers will refer to something called ‘Waddington’s epigenetic landscape’. They will show the grainy image seen in
Figure 1.1.
Figure 1.1.
Conrad Waddington was a hugely influential British polymath. He was born in 1903 in India but was sent back to England to go to school. He studied at Cambridge University but spent most of his career at the University of Edinburgh. His academic interests ranged from developmental biology to the visual arts to philosophy, and the cross-fertilisation between these areas is evident in the new ways of thinking that he pioneered.
Figure 1.1
The image created by Conrad Waddington to represent the epigenetic landscape. The position of the ball represents different cell fates.
The image created by Conrad Waddington to represent the epigenetic landscape. The position of the ball represents different cell fates.
Waddington presented his metaphorical epigenetic landscape in 1957 to exemplify concepts of developmental biology
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. The landscape merits quite a bit of discussion. As you can see, there is a ball at the top of a hill. As the ball rolls down the hill, it can roll into one of several troughs towards the bottom of the hill. Visually this immediately suggests various things to us, because we have all at some point in our childhood rolled balls down hills, or stairs, or something.
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. The landscape merits quite a bit of discussion. As you can see, there is a ball at the top of a hill. As the ball rolls down the hill, it can roll into one of several troughs towards the bottom of the hill. Visually this immediately suggests various things to us, because we have all at some point in our childhood rolled balls down hills, or stairs, or something.
What do we immediately understand when we see the image of Waddington’s landscape? We know that once a ball has reached the bottom it is likely to stay there unless we do something to it. We know that to get the ball back up to the top will be harder than rolling it down the hill in the first place. We also know that to roll the ball out of one trough and into another will be hard. It might even be easier to roll it part or all of the way back up and then direct it into a new trough, than to try and roll it directly from one trough to another. This is especially true if the two troughs we’re interested in are separated by more than one hillock.
This image is incredibly powerful in helping to visualise what might be happening during cellular development. The ball at the top of the hill is the zygote, the single cell that results from the fusion of one egg and one sperm. As the various cells of the body begin to differentiate (become more specialised), each cell is like a ball that has rolled further down the hill and headed into one of the troughs. Once it has gone as far as it can go, it’s going to stay there. Unless something extraordinarily dramatic happens, that cell is never going to turn into another cell type (jump across to another trough). Nor is it going to move back up to the top of the hill and then roll down again to give rise to all sorts of different cell types.
Like the time traveller’s levers, Waddington’s landscape at first just seems like another description. But it’s more than that, it’s a model that helps us to develop ways of thinking. Just like so many of the scientists in this chapter, Waddington didn’t know the details of the mechanisms but that didn’t really matter. He gave us a way of thinking about a problem that was useful.
John Gurdon’s experiments had shown that sometimes, if he pushed hard enough, he could move a cell from the very bottom of a trough at the bottom of the hill, right the way back up to the top. From there it can roll down and become any other cell type once more. And every toad that John Gurdon and his team created taught us two other important things. The first is that cloning – the recreation of an animal from the cells of an adult – is possible, because that’s what he had achieved. The second thing it taught us is that cloning is really difficult, because he had to perform hundreds of SCNTs for every toad that he managed to generate.
That’s why there was such a furore in 1996 when Keith Campbell and Ian Wilmut at the Roslin Institute created the first mammalian clone, Dolly the sheep
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. Like John Gurdon, they used SCNT. In the case of Dolly, the scientists transferred the nucleus from a cell in the mammary gland of an adult ewe into an unfertilised sheep egg from which they had removed the original nucleus. Then they transplanted this into the uterus of a recipient ewe. Pioneers of cloning were nothing if not obsessively persistent. Campbell and Wilmut performed nearly 300 nuclear transfers before they obtained that one iconic animal, which now revolves in a glass case in the Royal Scottish Museum in Edinburgh. Even today, when all sorts of animals have been cloned, from racehorses to prize cattle and even pet dogs and cats, the process is incredibly inefficient. Two questions have remained remarkably pertinent since Dolly tottered on her soon to be prematurely arthritic legs into the pages of history. The first is why is cloning animals so inefficient? The second is why are the animals so often less healthy than ‘natural’ offspring? The answer in both cases is epigenetics, and the molecular explanations will become apparent as we move through our exploration of the field. But before we do, we’re going to take our cue from H. G. Wells’s time traveller and fast-forward over thirty years from John Gurdon in Cambridge to a laboratory in Japan, where an equally obsessive scientist has found a completely new way of cloning animals from adult cells.
6
. Like John Gurdon, they used SCNT. In the case of Dolly, the scientists transferred the nucleus from a cell in the mammary gland of an adult ewe into an unfertilised sheep egg from which they had removed the original nucleus. Then they transplanted this into the uterus of a recipient ewe. Pioneers of cloning were nothing if not obsessively persistent. Campbell and Wilmut performed nearly 300 nuclear transfers before they obtained that one iconic animal, which now revolves in a glass case in the Royal Scottish Museum in Edinburgh. Even today, when all sorts of animals have been cloned, from racehorses to prize cattle and even pet dogs and cats, the process is incredibly inefficient. Two questions have remained remarkably pertinent since Dolly tottered on her soon to be prematurely arthritic legs into the pages of history. The first is why is cloning animals so inefficient? The second is why are the animals so often less healthy than ‘natural’ offspring? The answer in both cases is epigenetics, and the molecular explanations will become apparent as we move through our exploration of the field. But before we do, we’re going to take our cue from H. G. Wells’s time traveller and fast-forward over thirty years from John Gurdon in Cambridge to a laboratory in Japan, where an equally obsessive scientist has found a completely new way of cloning animals from adult cells.
Any intelligent fool can make things bigger and more complex … It takes a touch of genius and a lot of courage to move in the opposite direction.
Albert Einstein
Let’s move on about 40 years from John Gurdon’s work, and a decade on from Dolly. There is so much coverage in the press about cloned mammals that we might think this procedure has become routine and easy. The reality is that it is still highly time-consuming and laborious to create clones by nuclear transfer, and consequently it’s generally a very costly process. Much of the problem lies in the fact that the process relies on manually transferring somatic nuclei into eggs. Unlike the amphibians that John Gurdon worked on, there’s the additional problem that mammals don’t produce very many eggs at once. Mammalian eggs also have to be extracted carefully from the body, they aren’t just ejected into a tank like toad eggs. Mammalian eggs have to be cultured incredibly delicately to keep them healthy and alive. Researchers need to remove the nucleus manually from an egg, inject in a nucleus from an adult cell (without damaging anything), then keep culturing the cells really, really carefully until they can be implanted into the uterus of another female. This is incredibly intensive and painstaking work and we can only do it one cell at a time.
For many years, scientists had a dream of how they would carry out cloning in an ideal world. They would take really accessible cells from the adult mammal they wanted to clone. A small sample of cells scraped from the skin would be a pleasantly easy option. Then they would treat these cells in the laboratory, adding specific genes, or proteins, or chemicals. This treatment would change the way the nuclei of these cells behaved. Instead of acting like the nucleus of a skin cell, they would act the same way as nuclei from newly fertilised eggs. The treatment would therefore have the same ultimate effect as transferring the nuclei from adult cells into fertilised eggs, from which their own nuclei had been removed. The beauty of such a hypothetical scheme is that we’d have bypassed most of the really difficult and time-consuming steps that require such a high level of technical skill in manipulating tiny cells. This would make it an easily accessible technique and one that could be carried out on lots of cells simultaneously, rather than just one nuclear transfer at a time.
Okay, we’d still have to find a way of putting them into a surrogate mother, but we only have to go down the surrogate mother route if we want to generate a complete individual. Sometimes this is exactly what we want – to re-create a prize bull or prize stallion, for example, but this is not what most sane people want to do with humans. Indeed cloning humans (reproductive cloning) is banned in pretty much every country which has the scientists and the infrastructure to undertake such a task. But actually for most purposes we don’t need to go as far as this stage for cloning to be useful for humans. What we need are cells that have the potential to turn into lots of other cell types. These are the cells that are known as stem cells, and they are metaphorically near the top of Waddington’s epigenetic landscape. The reason we need such cells lies in the nature of the diseases that are major problems in the developed world.
In the rich parts of our planet the diseases that kill most of us are chronic. They take a long time to develop and often they take a long time to kill us when they do. Take heart disease, for example – if someone survives the initial heart attack they don’t necessarily ever go back to having a totally healthy heart again. During the attack some of the heart muscle cells (cardiomyocytes) may become starved of oxygen and die. We might imagine this would be no problem, as surely the heart can create replacement cells? After all, if we donate blood, our bone marrow can make more red blood cells. Similarly, we have to do an awful lot of damage to the liver before it stops being able to regenerate and repair itself. But the heart is different. Cardiomyocytes are referred to as ‘terminally differentiated’ – they have gone right to the bottom of Waddington’s hill and are stuck in a particular trough. Unlike bone marrow or liver, the heart doesn’t have an accessible reservoir of less specialised cells (cardiac stem cells) that could turn into new cardiomyocytes. So, the long-term problem that follows a heart attack is that our bodies can’t make new cardiac muscle cells. The body does the only thing it can and replaces the dead cardiomyocytes with connective tissue, and the heart never beats in quite the same way it did before.
Similar things happen in so many diseases – the insulin-secreting cells that are lost when teenagers develop type 1 diabetes, the brain cells that are lost in Alzheimer’s disease, the cartilage producing cells that disappear during osteoarthritis – the list goes on and on. It would be great if we could replace these with new cells, identical to our own. This way we wouldn’t have to deal with all the rejection issues that make organ transplants such a challenge, or with the lack of availability of donors. Using stem cells in this way is referred to as therapeutic cloning; creating cells identical to a specific individual in order to treat a disease.
For over 40 years we’ve known that in theory this could be possible. John Gurdon’s work and all that followed after him showed that adult cells contain the blueprints for all the cells of the body if we can only find the correct way of accessing them. John Gurdon had taken nuclei from adult toads, put them into toad eggs and been able to push those nuclei all the way back up Waddington’s landscape and create new animals. The adult nuclei had been – and this word is critical – reprogrammed. Ian Wilmut and Keith Campbell had done pretty much the same thing with sheep. The important common feature to recognise here is that in each case the reprogramming only worked when the adult nucleus was placed inside an unfertilised egg. It was the egg that was really important. We can’t clone an animal by taking an adult nucleus and putting it into some other cell type.
Why not?
We need a little cell biology here. The nucleus contains the vast majority of the DNA/genes that encode us – our blueprint. There’s a miniscule fraction of DNA that isn’t in the nucleus, it’s in tiny structures called mitochondria, but we don’t need to worry about that here. When we’re first taught about cells in school it’s almost as if the nucleus is all powerful and the rest of the cell – the cytoplasm – is a bag of liquid that doesn’t really do much. Nothing could be further from the truth, and this is especially the case for the egg, because the toads and Dolly have taught us that the cytoplasm of the egg is absolutely key. Something, or some things, in that egg cytoplasm actively reprogrammed the adult nucleus that the experimenters injected into it. These unknown factors moved a nucleus from the bottom of one of Waddington’s troughs right back to the top of the landscape.
Nobody really understood how the cytoplasm of eggs could convert adult nuclei into ones like zygotes. There was pretty much an assumption that whatever it was must be incredibly complicated and difficult to unravel. Often in science really big questions have smaller, more manageable questions inside them. So a number of labs tackled a conceptually simpler, but technically still hugely challenging issue.
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