Mutants (15 page)

When Saunders removed the apical ectodermal ridge from a young limb-bud, the result was total amputation. Yet if the bud was older and larger, then only the structures further down – wrists, digits – were lost. Why? Over the last fifty years, various answers have been given to this question. The latest, though surely not the last, turns on two quite new observations. The first of these is that the ridge FGFs only penetrate a short way, about two hundred microns (one fifth of a millimetre) into the mesoderm. In a young limb-bud, two hundred microns-worth of seceding cells cuts very deep as a proportion of total mass; in an older, larger limb-bud, much less so. This difference in proportion matters because limb-buds possess an invisible order. A limb-bud may look like an amorphous sack of cells, but even when newly formed, when it is no more than a bump on the foetal flank, its mesodermal cells have some foreknowledge of their fates. Some are already destined to become a humerus, others digits, yet others the parts
between. As the limb-bud grows, each of these populations of cells proliferates and expands in turn. When a young limb-bud is deprived of FGFs, all of these variously fated cell populations suffer; when an older limb-bud is deprived only those closest to the tip do, and with them future hands and feet, toes and fingers.

This account of the making of our limbs contains within it the roots of twentieth-century medicine’s most infamous blunder. In 1961 an Australian physician, William McBride, reported a sudden surge in the numbers of infants born with deformed limbs. Similar findings were reported a few months later by a German named Lenz. Both physicians suggested that the defects were caused by a sedative used to prevent morning sickness that has the chemical name phtalimido-glutarimide, but which swiftly became notorious by its trade-name, thalidomide. More reports rolled in from around the world. By the time it was all over, more than ten thousand infants in forty-six countries with thalidomide-induced teratologies had been found. Only the United States escaped the epidemic because a few sceptical FDA officials had delayed authorisation of a drug that was, at the time, the third best-selling in Europe.

The thalidomide infants had a very particular kind of limb deformity. Unlike acheiropods, their limbs did not suggest amputations in the womb, for most had reasonably formed hands and feet as well as shoulderblades and pelvises; they were simply missing everything else in between. Without long bones, their arms and feet connected almost directly to their torsos. Their limbs had the appearance of flippers – a condition dubbed phocomelia or ‘seal-limb’.

Phocomelic infants have always appeared sporadically. In the sketchbooks of Goya (1746–1828), that compassionate connoisseur of deformity, there is a lovely sepia-wash portrait of a young mother proudly displaying her deformed child to two inquisitive old women. And there are, scattered throughout the early teratological literature, any number of people with the disorder. In his
Tabulae
(1844–49), Willem Vrolik gave a portrait of a phocomelic, a famous eighteenth-century Parisian juggler, Marc Cazotte, also known as ‘Le Petit Pepin’. Vrolik also shows
Cazotte’s skeleton, which still hangs in the Musée Duputryen in Paris, though its legs, by sad irony, are now missing. These cases of phocomelia might have been caused by some chemical or other, but they may also have been due to mutations, several of which cause the disorder. But until the 1960s, phocomelics were rare, little more than anatomical curiosities. Thalidomide turned them into icons of medical hubris.

P
HOCOMELIA
. M
ARC
C
AZOTTE, A.K.A.
P
EPIN
(1757–1801). F
ROM
W
ILLEM
V
ROLIK
1844–49
T
ABULAE AD ILLUSTRANDAM EMBRYOGENESIN HOMINIS ET MAMMALIUM TAM NATURALEM QUAM ABNORMEM
.

How does thalidomide have its devastating effects? A comprehensive bibliography on the chemical and its consequences would run to about five thousand technical papers, but for all that, thalidomide is still poorly understood. Some things are clear. It is a teratogen and not a mutagen: the children of thalidomide victims are at no greater risk of congenital disorders than any others. Instead thalidomide inhibits cell proliferation. Taken by a pregnant woman during the time when she is most susceptible to morning sickness (thirty-nine to forty-two days after conception), it circulates throughout the bodies of mother and child and stops cells from dividing. This is when the earliest populations of cells that will form each part of the infant’s future limbs are establishing themselves. Depending on the exact duration of the exposure, the precursors of one or more bones will fail to multiply; the result is a limb with missing parts. It is even thought that thalidomide may impede, quite directly, the fibroblast growth factors that are so essential to limb-bud development, but this remains speculation. Whatever its exact
modus operandi
, thalidomide is clearly a powerful drug and so a perennially attractive one. The taboo that surrounds it is breaking down as proposals for its use against a variety of diseases
proliferate. In South America it is used to treat leprosy. Inevitably, infants with limb deformities are appearing once again as it is given to women who do not know that they have conceived.

GOING DIGITAL

Metric, with its base 10 units, exists only because the
savants
of the Académie Française who devised the system had ten fingers each on which they presumably learned to count. If pigs could do mathematics, they would probably measure their swill using a
Système International
devised from base 8, for they have only four digits per hoof. Horses have one digit per limb, camels have two, elephants have five, but guinea pigs have four on the fore-limbs and three behind. Cats and dogs have five on the forefeet and five on the hind feet, but one of those is small, and is called a ‘dew-claw’. Apart from some frogs and a kind of dolphin called a
vaquita
, most vertebrates never have more than five digits per limb.

Why this is so is deeply obscure. It is not as though extra digits are impossible to make. Mammals of all sorts sometimes show extra digits, but they are never common. St Bernards, Great Pyrenees, Newfoundlands and other large dogs are especially prone to having six digits on each foot – the duplication being an extra dew-claw. Ernest Hemingway’s cats were polydactylous, and their many-toed descendants still live in the grounds of his Key West house. Fifteen per cent of the feral cats of Boston are polydactylous (some have up to ten extra toes), but there are no feral polydactylous cats in New York. There are many
polydactylous strains of mice: one is called
Sasquatch
in homage to Big Foot, but most have more prosaic names such as
Doublefoot
or
Extra-Toes
. The American geneticist Sewall Wright once produced a baby guinea pig with forty-four fingers and toes in all, but it did not live.

And many people are born with extra digits. About i in 3000 Europeans is born with extra fingers or toes (or both), and about 1 in 300 Africans. Any digit can be duplicated, but in Africans it usually a little finger (pinkie), while in Europeans it tends to be a thumb. Polydactyly is usually genetic, frequently dominant, and can run for many generations in families. Long before Gregor Mendel ever lived, the French mathematician Pierre-Louis Moreau de Maupertuis (1698–1758) described the inheritance of Polydactyly in the ancestors and descendants of a Berlin physician called Jacob Ruhe. Ruhe’s grandmother had six fingers on each hand and six toes on each foot, as did his mother, as did he and three of his seven siblings, and two of his five children. Others have claimed even more impressive polydactylous pedigrees. In 1931 the Russian geneticist E.O. Manoiloff published an account of a polydactylous Georgian, Via?eslav Michailovi? de Camio Scipion, who, he said, was able to document his descent from a lineage of polydactylous forebears reaching back six centuries.

If the apical ectodermal ridge ensures that our limbs grow out into space, another equally unobtrusive piece of limb-bud ensures that we have the right number and kinds of fingers. It was again John Saunders, along with a collaborator, Mary
Gasseling, who discovered it. They found that if they transplanted a piece of mesoderm from the tailmost edge of one chicken limb-bud onto the headmost edge of another (so that the bud had two tailmost edges in opposite orientation to each other), the result was a chicken wing with twice the usual number of digits. Most remarkably of all, the experimental wings were like a particularly exotic variety of polydactyly in humans. They resembled people who, far from having just an extra digit or two, have hands and feet that are almost completely duplicated with up to ten digits each. The polydactylous wings had a peculiar mirror-image geometry, one shared by duplicated hands in humans. If each finger is given a code in which the thumb is 1, forefinger 2, index-finger 3, ring-finger 4, and pinkie 5, then a normal, five-fingered, hand has the formula ‘12345’, while a duplicated hand has the formula ‘5432112345’. It is that strangest of things, an anatomical palindrome.

M
IRROR-IMAGE POLYDACTYLY
. L
EFT HAND OF A WOMAN WITH EIGHT DIGITS
. F
ROM
W
LLLIAM
B
ATESON
1894
MATERIALS FOR THE STUDY OF VARIATION
.

Saunders and Gasseling called their potent piece of mesoderm the ‘zone of polarising activity’ or ‘ZPA’. It is thought to be the source of a morphogen. At its source, where it is most concentrated, this morphogen induces naive mesoderm to become
the little finger; further away, lower concentrations induce the ring, index, and forefinger in succession, and at the far opposite end of the limb, you get a thumb.

This account of how most of us come by our five fingers brings to mind the organiser. Like the organiser, the ZPA has the uncanny ability to impose order on its surroundings. And, just as the organiser morphogen was so eagerly sought for so long, so too, in recent years, has been the morphogen of the ZPA. It is almost certainly a signalling protein, likely a familiar one, a member of one of the great families of signalling proteins that also work elsewhere in the embryo. But limb-buds contain a plethora of such proteins, and it is hard to know which of them is the morphogen itself. In the past few years, several candidate molecules have been said to fit the bill. One of them is sonic hedgehog.

Sonic hedgehog appears in the limb-bud precisely where one would expect a morphogen to be: only in the mesoderm of the tailmost edge, exactly coincident with Saunders and Gasseling’s ZPA. It also does what one would expect a morphogen to do: shape limbs. Chicken wings can be sculpted into new and improbable forms – including duplicate mirror-image polydactylous ones – simply by manipulating the presence of sonic in the bud. And then there are the mutants. Mutations in at least ten genes cause Polydactyly in humans and all seem to affect, in some way or other, sonic’s role in the limb.

But, as we saw in the previous chapter, sonic hedgehog does not just determine how many fingers and toes we have. It also
divides our brains, decides how widely spaced our eyes will be, and regulates much else besides. It is an incorrigibly promiscuous molecule. Could we see the pattern of the sonic hedgehog gene’s activity over time, as in time-lapse photography, we would see it flashing on and off throughout the developing embryo and foetus, now in this incipient organ, now in that one.

The devices responsible for all this have a formidable task, and nowhere, given sonic’s power to direct the destiny of cells, do they have much room for error. These devices are transcription factors or ‘molecular switches’. Some of them keep sonic in check. Should they be disabled by mutation, sonic turns on in parts of the limb-bud that it otherwise would not – and the result is extra fingers and toes. Other mutations do not disable the transcription factors themselves, but rather delete the regulatory elements to which they bind. The result, however, is the same: a confusion of morphogen gradients and an embarrassment of digits.