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The first wife to be beheaded was Anne Boleyn, the mother of the future Queen Elizabeth I. After her death, the Tudor spin doctors launched quite a smear campaign and Anne Boleyn’s physical appearance was described in such a way that she sounded like the 16th-century image of a witch. She was characterised as having a projecting tooth, a large mole under her chin and six fingers on her right hand. The story of that extra finger has passed down in folklore, although there is little if any evidence that it was true.{283}
Perhaps one of the reasons that the story has been accepted is because it’s not completely ludicrous. It’s not as if the chroniclers claimed that the former queen had three legs. There are people who are born with an extra finger, although usually they have an extra finger on each hand rather than just one.
There is a protein-coding gene that is very important in the correct development of the hands and feet.[40] The protein acts as a morphogen, meaning that it governs patterns of tissue development. The effects of the protein are very dependent on its concentration, and in the developing embryo there is a gradient effect, where high levels in one region gradually fade away to lower levels in adjacent tissues.
One of the features controlled by this morphogen is the number of fingers. If the expression levels of the protein are wrong, babies are born with extra fingers. Over ten years ago researchers discovered that some cases of extra fingers were caused by a tiny genetic change. This wasn’t in the morphogen gene, but in a region of junk DNA about a million base pairs away. They identified the change in a huge Dutch family where the presence of extra fingers was clearly inherited as a genetic trait. All 96 affected individuals had a change of just one base in the junk. Instead of a C base, these patients had a G base. None of the relatives with the normal number of digits had a C in this position. Single base changes were also found in other families where some individuals had extra fingers. These changes were in the same general region of the genome as in the Dutch family but 200–300 base pairs away from that alteration.{284}
The junk region that carries these single base changes is an enhancer of the morphogen gene.[41] In order to create the correct body pattern, the spatial and temporal control of the morphogen is very tightly controlled by a whole slew of regulators. In the people with the mutation and the extra digit, the enhancer activity was slightly abnormal. The impact of the tiny change in this one regulator shows just how important and finely tuned this control is.
Here’s some help with another quiz. What’s the connection between Dutch people who have trouble buying gloves, and one of the great figures of 20th-century American literature? No? Give up? Well, in the 1930s Ernest Hemingway was given a cat by a ship’s captain. Instead of having five toes on its front paws, this cat had six. There are now about 40 descendants of this cat at Hemingway’s home, about half of whom have six toes on their front paws. It’s easy to find pictures of these cats on the internet{285} and they are simultaneously cute and a little bit scary. The extra toe looks like a thumb, rendering the cats slightly too capable-looking for comfort.
The same group that identified the change in the enhancer region in humans with extra fingers showed that the same region was altered in Hemingway’s cats. By inserting the enhancer into another animal’s genome they confirmed that the alteration changed the expression of the morphogen. The experimental animal over-expressed the morphogen and developed an extra digit on each front paw. Rather delightfully, this effect was demonstrated by inserting feline DNA into a murine embryo. A genuine catand-mouse game.{286}
Cats with extra front paw toes have also been found in other countries, including the UK. In the British cats there is also a change in the same enhancer, but it’s not exactly the same change. It is two base pairs away from the Hemingway change, in a three-base-pair motif that is very highly conserved in evolution. The enhancer region that is involved in the extra digits on the forelimbs of humans and cats is about 800 base pairs in length and most of it is highly conserved from humans all the way down to fish. This suggests that the control of limb development is a very ancient system.
The morphogen that is responsible for finger formation is also critical for other developmental processes. One of these is the process whereby the structures of the front of the brain and the face are formed. If this process goes wrong, the effect can be very mild: simply a cleft lip. But at the other extreme, where the morphogen expression is more severely disrupted, the effects can be devastating. The brain and face may be completely abnormal, with no proper formation of brain structures. In the most severe cases the babies are born with just one malformed eye in the middle of the forehead and with severely impaired brain development. The babies never survive.
This spectrum of condition is known as holoprosencephaly.{287} A number of different protein-coding genes has been shown to be mutated in different families with this condition. Many of these genes are involved in the regulation of the same morphogen that is required for correct digit formation. In some cases, the gene for the morphogen protein itself is mutated. The developing embryo only produces half of the normal amount of the morphogen, because the functional protein is only produced from one chromosome, not two. The abnormalities in the affected individuals show that it is critical that the morphogen levels hit the right thresholds at key points in development.
Not all the mutations that cause holoprosencephaly have been identified. Researchers studied the DNA from nearly 500 individuals who were affected by the condition. They found an unexpected change in a junk DNA region of one severely affected infant. This was a single base change, from C to T, in a region over 450,000 base pairs away from the morphogen gene.{288}
The C to T change occurred in a block of ten base pairs that has been conserved since our ancestors diverged from the ancestors of frogs, over 350 million years ago. We can therefore surmise that this stretch of apparent junk has been maintained throughout evolution and has a function. In the case of this specific enhancer, the C binds a transcription factor protein.[42] Transcription factors are the proteins which are unusual because they recognise specific DNA sequences, usually in promoters, and bind to them. Binding of transcription factors to a promoter is essential for switching on a gene. The key transcription factor for this enhancer can bind to the ten-base-pair motif when the DNA contains a C in the appropriate position, but not when it contains a T.
This change from a C to a T in the enhancer wasn’t present in 450 unrelated healthy control individuals. That might make it seem very likely that this change was the cause of the problems in the patient, but it’s important to remember that it was also only seen once in about the same number of patients with the condition. The baby’s mother was unaffected, and as expected she had the normal C base on both her chromosomes. But unexpectedly, the baby’s father had the same genetic sequence at the enhancer as his child. One chromosome had a C at the relevant position and the other had a T in the same place. But the father was completely unaffected by any symptoms of holoprosencephaly.
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The protein is called Sonic Hedgehog, symbol SHH. Researchers went through a phase of giving genes apparently comic names. This is now discouraged as it’s suddenly not so amusing if a genetic counsellor has to pass on a whimsical gene name to the parents of a child with a severe genetic condition.