The story of Sonic Hedgehog
Bob Hill: Image courtesy of Bob Hill
Click here to listen to the full podcast episode
Bob: I am Bob Hill. I'm a professor at the MRC Human Genetics Unit associated with the University of Edinburgh. And I am a geneticist and a developmental biologist.
Bob: So we got interested in extra toed cats because of mutations we had uncovered in humans. It really started from the aspect of understanding mutations that cause extra digits in humans. And so from there we knew that there were chickens with extra toes. I had learned that there were cats running around, especially in the US that had extra toes. And one of the really popular kind of group of cats that people knew a lot about were the Hemingway cats. So the Hemingway cats are a group of cats that are taken care of and live around the Hemingway home in Key West Florida.
Bob: The original six toed cat was a gift to Ernest Hemingway by an old sea captain apparently. An old sea captain friend of his gave him this cat that had extra toes - I think it was called Snow White or Snowball. And so Hemingway had kept that cat - actually, I think he got it in Cuba and when he moved to Key West he took the cat with him - and of course it bred with other cats in the area and kind of bred this defect that causes extra toes. So around Hemingway's house is a group of about 50 or 60 cats. About half of those have extra toes on their front paws, and so those are the Hemingway cats.
Bob: Across the US there are other groups of six toed cats. And one of the main ones is a Maine Coon cat, which are these huge cats, originally probably derived in Maine and therefore the name. And so they're from the Northeast side of the United States. And one of the kind of variants that had been bred into these Maine Coons is six toes. So it's a common variant that you see in Maine as well. And then there are others like the Pixie Bob that also have six toes.
Kat: Before we go any further I just want to briefly tell the story Bob told me about why these so-called Hemingway cats were so popular with seafarers. One idea was that their large paws made them exceptionally good mousers - always useful to have on board a boat. Another was that their extra toe could act a bit like a thumb, enabling them to grab onto the rigging when the seas got rough, preventing them from being swept overboard.
Bob: But probably the most likely explanation is that sailors are just really superstitious, and what’s more lucky that a cat with five toes? A cat with six toes. But what’s actually going on? Bob and his team set about finding out.
Bob: We were interested - this must be 15, 20 years ago - we were interested in very specific developmental processes. And we were interested in developmental processes that are involved in defining discrete patterns during development. And of course, digits is a really nice pattern to study because digits or fingers or toes - and in the mouse, we did all this work in the mouse, of course the toes on the paw - there is a very nice pattern of of a bone, and then there's a space and there's another bone which bones which make up the finger or the toe then a space, then another set of bones and a space. So it's a nice pattern and that is obviously genetically defined. Probably around in the 1990s, actually other groups established the genes that are involved in defining that pattern and that gene was the Sonic Hedgehog gene.
Bob: So we became very interested in Sonic Hedgehog because this fit into our interests very well. So one way of trying to look at pattern of course, is trying to look at mutant forms, and in the mouse there are probably five or six really good mutations that give you extra toes. So the question was okay, now how do you get extra toes? We know that Sonic Hedgehog defines the normal pattern. Is it involved in this abnormal pattern? And we found that, of course it is. So it's a process that causes production of Sonic Hedgehog in the wrong place in the limb during development.
Bob: But what's really interesting about this work was it wasn't a mutation in the gene. It's a mutation in a stretch of DNA that decides where the gene is going to be expressed in the limb bud. And you have a mutation in this little stretch of DNA. The protein is now made in the wrong part of the limb and the limb loses some of its information and starts making too many toes. And we can go deeper into that as well.
Bob: So essentially it was mis-production of Sonic Hedgehog that was causing the extra toes. So we found that first in mouse, and then we could show that in human. And then we got hold of some six-toed cats and show that it was the same type of mutations that was causing the defect and cats. So one series of mutations that cause it in mouse, humans, cats, we now know is also in chickens. Most recently we looked at Guinea pigs with extra toes and that they have mutations in this same stretch of DNA.
Kat: The most puzzling thing for Bob and his team to figure out when they were searching for the six-toe mutation was the fact that the change, or mutation, isn’t actually in the Sonic Hedgehog gene itself, but in something known as an enhancer - a stretch of DNA that acts as a kind of control switch, which turns the gene on at the right time and in the right place to make the right number of toes in a developing limb. And that wasn’t easy to find.
Bob: So original work had mapped where this mutation should be. And we started working on this and actually, we worked for several years before, and knowing where the mutation was, before we realised that what we were looking at was a switch for Sonic Hedgehog. And it's because the mutations in the switch are a long distance from the gene. So the gene produces the protein. This switch decides when that gene is going to be producing the protein and that switch is a million base pairs away. And that is a very long distance. And it actually, not only that, but it also sits inside another gene. So that's really very confusing.
Bob: So you have this switch sitting inside another gene, and you start working on this other gene thinking that is probably the one that's causing the extra digits. And of course it's not. So it's physically linked, but a very long distance. And so somehow it has to convey all the information that is in this switch to this gene, which is so far away, which is a million bases away. And that was astonishing at that point. That was really quite astonishing. And we're still working on that as well.
Kat: We don’t have time here to go into all the intricacies of how this long-distance control works - you’ll definitely have to read my book if you want to know more about that. But anyway, by this point you’re probably thinking that Sonic Hedgehog is some kind of ‘toe gene’, right? Wrong.
Bob: Sonic Hedgehog has a role in lung development, gut development, brain development, limb development, the development of your vertebra, left-right asymmetry of your organ systems. It just has a huge multitude of roles.
Kat: This multi-faceted role in development comes down to the kind of gene Sonic Hedgehog actually is. It’s not a gene for making toes, lungs, brains or anything specific. Instead, the gene encodes the instructions for making a kind of molecular messenger, known as a signalling molecule, which seeps out of the cells where it’s made and tells their neighbours what to do. Cells nearby get a big dose, so they do one thing, but cells further away get a smaller amount of Sonic, so they do something else.
Bob: As you might expect for such an important developmental regulator, genetic alterations that affect the Sonic Hedgehog gene can have profound consequences. A single DNA ‘letter’ change in the control switch that activates Sonic Hedgehog in the developing limb can give a cat, or a human, an extra digit. But bigger changes in this switch can lead to a complete lack of limbs altogether. So it’s not surprising that significant changes in the gene and its control switches have significant effects.
Bob: It causes a condition called holoprosencephaly, and that affects predominantly the face and the head and the brain. So most of the anterior part of the animal. And because Sonic Hedgehog is expressed along the midline of most of these organ systems, what happens is the midline is essentially missing and it brings all those structures laterally closer together.
Bob: So you can imagine if you're affecting the midline and the face, you're going to be missing all those midline structures. And what it does is bring the two eyes together until they essentially fuse so you have a single eye. And that's called cyclopia and that's the most severe form of the mutation of the loss of Sonic Hedgehog of this condition called holoprosencephaly.
Kat: It’s this cyclops condition - cyclopia - that starts another strand of the Sonic Hedgehog story, taking us back to an Idaho sheep farmer’s field in the 1950s.
Bob: There were farms along this certain hillside in Idaho, where there were picking up a lot of defects and stillbirths in sheep during the lambing season and the worst defects that they were seeing were cyclopia. So it's really severe form of holoprosencephaly. And this, of course, was before we had the molecular biology or the genetics.
Bob: And the USDA decided to try to see what, if they could establish what was happening. And it turns out it was due to the fact that there was an abundance of this plant called a corn lily and a corn lily makes tons of a molecule called cyclopamine, named, obviously because it causes cyclopia and the sheep were eating that. And it turns out, as we learned later, is that it's a block of Sonic Hedgehog function.
Kat: We’ll come back to why the discovery of chemicals that can interfere with Sonic Hedgehog are so useful a little later on. But first, we really need to address the elephant - or rather, the hedgehog - in the room. Why on earth is this gene called Sonic Hedgehog in the first place?
Bob: So Hedgehog was originally described in a very large genetic screen done in Drosophila, in the fruit fly, the common fruit fly Drosophila melanogaster, which is a very common and very popular genetic system. And it was discovered as a mutation in Drosophila in which the embryo looked like a hedgehog.
Bob: So normally a Drosophila embryo is kind of elongated and has a nice group banding pattern of bristles. If there is no Hedgehog, if there's mutations in Hedgehog, what happens is it’s shortened. It becomes almost rugby ball shaped and bristles cover the whole thing. And so it looks like a hedgehog. And so it was called Hedgehog.
Kat: Once the Hedgehog gene had been found in fruit flies, the hunt was on to see if it was in mammals too. It turns out that mice have three versions of the hedgehog gene. One became known as Desert Hedgehog, another was dubbed Indian Hedgehog, and the third…?
Bob: At about the same time, this new, at that point what was a new computer game, Sonic the Hedgehog had just come out. And so, and they say in their paper that they named it Sonic Hedgehog based on this new computer game. So it's actually named after Sonic the Hedgehog, the computer game.
Bob: But, you know, we could have ended up with Hedgehog one, two, and three, and that would have been really boring. So I would take silly over boring, I think, any day.
Kat: We can thank Bob Riddle, one of the researchers who discovered the mammalian Hedgehog genes for this one, as he apparently spotted the infamous blue character in one of his daughter’s comic books. And there is even a rather sweet, if possibly apocryphal, story about one of Riddle’s colleagues being taken aback by seeing that their new genetic discovery was so important that McDonald’s had based a Happy Meal on it…
Bob: While this moniker was amusing, it caused a lot of discussion in the genetics world about the implications of naming a gene with such profound impacts on human development after a cartoon character. So as is the case with most of our genes, the human version of Sonic Hedgehog goes by the more staid initials SHH.
Bob: The fact that we find Sonic Hedgehog, or versions of it, in everything from fruit flies to mice to sheep to humans tells us that it’s probably pretty important. And that it has probably been around for a fair old while.
Bob: The Hedgehog function has been conserved from arthropods, so we're talking about insects, so from Drosophila as I was talking about earlier, all the way up to human, all the way to mouse, human, to all the vertebrates. So it has a fundamental role in development in all these animals. And so it's been conserved for hundreds and hundreds of millions of years. If we talk about fish amphibians, reptiles, and birds and mammals, it has a central role in development of all these and a very similar role in all of these classes of vertebrates.
Kat: Where things get really interesting is when you realise that not only is Sonic Hedgehog doing the same kind of thing in all the organisms it’s found in, but that the way it’s controlled is highly conserved across species too. This goes deep.
Bob: What's interesting is how you regulate the production of Sonic Hedgehog. And that's what's interesting in evolution. And so you have this constant really, which is the gene, but how you make that gene and where you make that gene in the embryo, that's, what's important for evolution. And a really good example of that... actually there's two very good examples of this. So we were talking earlier about the enhancer that's responsible for fingers and toes. So this is the enhancer that turns on Sonic Hedgehog, that makes sure that Sonic Hedgehog is produced during limb development, that enhancer you find in fish. And of course fish don’t have fingers and toes, but if you don't have that enhancer in fish, then you don't get fin buds and they don't get fins. So the program that drives your fingers and toes is actually found in fish that don't have fingers or toes.
Bob: Secondly, another really very interesting aspect of the evolution is that when you look at snakes and this is only in the boas and the boa constrictors and the pythons. If you look at snakes, they have inactivating mutations in this enhancer. And that's one of the driving mutations as to why those snakes don't have limbs, why they are limbless, and it all falls on this one, single enhancer. So evolution is interesting. It's about how and where you produce the protein more than how you change the protein.
Kat: And that, right there, is why I love developmental genetics and evolution. The same gene, turned on in different times and places as an organism grows in the womb, or in an egg, or as a larva, is playing a fundamental role in patterning body parts, and has helped to shape species throughout evolution. And that leads to some pretty interesting experiments you can do thanks to genetic engineering, if you start switching these bits up and mixing them across species.
Bob: This is an experiment we’ve done and others have done, if you take that element out of the fish and replace the mouse element with that, you get normal fingers and toes. So it has all the information. From 400 million years ago, it has all the information it needs to give you fingers and toes, even though the fish doesn't have them. So to me, that's amazing.
Kat: And this leads us neatly on to another mix-and-match experiment that Bob and his team tried.
Bob: Now bats - bats are weird, bats are really, really neat animals and of course they have this highly specialised limb, which is a wing and quite different from a bird wing. And it really is kind of a modified forelimb. And so the bones that make up predominantly make up the wing are simply from the toes, so elongated toes and then the skin between them. What happens in bats is when you look at Sonic Hedgehog, is it that they do something a bit different with Sonic. So if you look at their limb buds, Sonic Hedgehog comes on in the same place and at the same time as you would expect to see it in mouse in the developing limb bud. Then it goes off and then it comes back on. And so it comes on in a different place in the limb bud.
Bob: So we were very interested in this. And then if you look at the enhancer, if you look at their enhancer that drives the limb expression, you find that there's some changes, there's about 14 different nucleotide changes in the bat enhancer compared to the mouse enhancer. So we decided, okay, what would happen if we actually took the bat enhancer and put it in mouse? Could we make mice with extra long digits that would fly around their cages? And so we put that into the mouse and what we did find was that we got the same pattern of expression as we got in the bat. So in the mouse now Sonic Hedgehog comes in the right place, but then it comes on again, more distally. It comes on where you'd see it in bat, but it doesn't have the same effect. And the really weird thing that we've got with the animals is that we got mice that had two elbows. So what it was affecting was the long bones, rather than the digits.
Bob: I guess the bottom line really is Sonic is very potent. It's a very potent signaling molecule. And if you express it in the wrong place in the species, it just, does some very weird things.
Kat: The potent ability of Sonic Hedgehog to make cells do things is both a blessing and a curse. In development, switched on at the right time and in the right place, it’s essential for building a baby. But if it’s activated in the wrong place, at the wrong time, then that causes trouble.
Bob: The result of that, especially in childhood is that it can cause childhood tumours. So Hedgehog and its pals, the things that operate with Hedgehog that make sure Hedgehog can function, mutations in these in children can cause a tumour called medulloblastoma, which is a brain tumour. It causes rhabdomyosarcoma which is a tumour of soft tissue and mis-production of Sonic Hedgehog can cause these tumours. And if you have drugs that can interfere with the process, then you can treat these tumours.
Kat: Remember cyclopamine, the chemical in the corn lilies in that Idaho field, which inactivated Sonic Hedgehog signalling and caused cyclops lambs? Well, tweak it a little bit and you’ve got vismodegib - a powerful drug for switching off aberrant Sonic Hedgehog in rare childhood brain tumours and much more common adult basal cell skin cancers. And that’s not the only medical application of this very special gene.
Bob: Sonic Hedgehog is obviously a very interesting gene, but you know, there are a number of genes that are expressed approximately the same time that really have very profound functions in the embryo. And so trying to understand how these genes work and how they interact and the network that these genes act in during these times in development will help us understand how our organ systems developed, what is the basis of the development of organ systems. And of course, this is very important in terms of therapeutics. If you want to start thinking about it, you know, can we make extra bits of tissue to do therapeutics.
Bob: At the moment, we can go from single stem cells and if you treat them right, and you put them in the right nutrients and stuff, and you can make them grow into eyes, you can make them grow into very small eyes, very small brains. We're trying to make them turn into limb buds, into limbs, into kidneys and livers, people are particularly interested in. So it's important to understand the whole gamut of genes that are involved in that differentiation to understand that you are actually making the type of tissues that you want to make. And if it's not, whether you can manipulate those systems so that you can guide cells into the right differentiation process.
Bob: So it's important to understand the basis of all of early embryogenesis, of early development, to understand some of these processes. If we're ever going to go into the tissue replacement field, I think we need to understand the basis of what happens to cells.
Kat: We’ve talked a lot about the Sonic Hedgehog gene, and its impact not just in medicine but in development. But as we started to wrap up our conversation I wanted to ask Bob about the impact on people with alterations in their Sonic Hedgehog gene - who are more common than you might think.
Bob: So the percentage of people that are born with limb abnormalities is about one in a couple of hundred births and probably with polydactyly it's about one in 2000. So it's a relatively frequent birth defect. A lot of people who do have the extra digits have them removed. And so surgically they can be removed and they can be surgically fixed and certainly so they're not unsightly and so it doesn't look strange.
Bob: There is a family in Brazil, there was a BBC documentary on this family in Brazil in which half the children - it’s about three generations - and about half the children have extra digits. And they see this as a good thing because they're musicians. So they're showing one of the children has six fingers, plays a guitar. And so he's learning six finger picking rather than five finger picking, you know? And so he can do more with the guitar than a normal person. One of the children plays the piano and that's quite a reach across the keyboard. One plays football, plays soccer, and he has to have special gloves made for him, he has an extra finger and he feels this is an advantage.
Bob: So there are, you know, there are some communities that see it as a good thing, and that it's good that they have extra digits. But the, the problem with some of the mutations that cause polydactyly is that it's not just a simple, you add an extra digit. There can be other bone abnormalities, depending on the mutation, other more severe bone abnormalities that can’t be fixed by the surgeon. So in that case, yes, they can be quite detrimental.
Kat: Finally, I want to tell you about what happened one day when I was giving a public talk about the Hemingway Cats and their genes, probably in a pub somewhere. A man called George Malynicz came up afterwards and told me a wonderful story about his research with feral pigs in Papua New Guinea back in the 1980s. Some of them were polydactyl, born with extra toes like the Hemingway Cats, while occasionally he would see what he described in his paper as otocephalic homozygous monsters - in other words, cyclops piglets. It sounded to me like a classic case of a Sonic Hedgehog mutation, so I put him in touch with Bob Hill to see if there were any samples available for DNA testing.
Alas, George had unfortunately kept only one sample from the pigs, which had been boiled during the preservation process, making it unlikely that DNA could be successfully extracted. And because he hadn’t been back to the island for several decades, it seemed unlikely that he’d be able to get another sample.
So - if anyone out there knows a scientist on Papua New Guinea with access to a DNA sequencer and fancies going to search for polydactyl pigs, please do drop me a line at podcast@geneticsunzipped.com and we can try to solve this genetic mystery and maybe get a paper out of it!
If you’re curious to know more about Sonic Hedgehog, and how our genes work in general then do check out my first book, Herding Hemingway’s Cats, which is available in paperback, ebook and audiobook from all good bookstores (and the evil ones too).
And you can also check out the short programme I made about this so-called Cyclops gene for my BBC Radio 4 series Ingenious - you can find the link in the page for this podcast at GeneticsUnzipped.com
