Mutants (42 page)

95
Writing of the ‘calculator of fate’.
For supernumerary eyelashes or distichiasis (
126300
) see Cockayne (1933) P-330 who notes, incidentally, that hedgehogs normally have two rows of eyelashes. For the seven-hearted chicken see Taussig (1988) following a 1904 report by the pathologist Verocay who happened to be staying at the inn and who managed to secure the viscera, but not the rest of the chicken, for study. Isidore Geoffroy Saint-Hilaire (1832–37) volume 1 pp.723–9 discusses various putative cases of heart duplications in humans but can come up with only one possibly authentic example, an early-eighteenth-century case of a grossly deformed infant. See Lickert et al. (2002) for extra hearts in ?-
catenin
-conditional null mutant mice.

96
And then there is Disorganisation.
Disorganisation (
223200
). The
Disorganisation
mutation was first studied by Hummel (1958, 1959), then by Crosby et al. (1992). The possibility of a human homologue of Disorganisation was mooted by Winter and Donnai (1989) and Donnai and Winter (1989), who proposed its existence to explain children whose malformations had been previously attributed to a miscellany of other cause. Disorganisation is caused by a dominant mutation on mouse chromosome 14.

97
The power of the homeotic genes.
The evolution of snake limblessness is discussed by Cohn and Tickle (1999). This explanation for the loss of fore-limbs in snakes does not account for the loss of the hind-limbs – which is due to a failure of the limb-buds to grow. Many studies have shown comparable changes in Hox gene expression patterns, particularly in arthropods. And some very recent studies have actually demonstrated that mutations in Hox genes are directly responsible for evolutionary changes in morphology (as a change in Hox gene expression, the snake example implies a mutation in some upstream regulatory factor). Mutations in Emx2 (
600035
), a human homologue of ems, is responsible for schizencephaly (
269160
). For a discussion of ems and other conserved brain genes see Reichert and Simeone (2001). Mutations in Pax6 (607108), a human homologue of eyeless, mutations cause aniridia (106210) (Ton et al. 1991). For a review of the conservation of eyeless/Pax6 in eye evolution see Gehring and Ikeo (1999).

99
In the cyclical way of intellectual fashion.
Geoffroy’s major ideas on what we now call homology can be found in his
Philosophic anatomique. Des organes respiratoires sous de la déermination et de l’dentité de leurs pièces osseuses
(1818) and
Considerations generates. Sur la vertèbre
(1822) which have been collected by Le Guyader (1998) who also discusses the dispute with Cuvier as does Appel (1987). The revival of the dorso-ventral inversion hypothesis is due to, among others, De Robertis and Sasai (1996); for a sceptical update see Gerhart (2000).

CHAPTER IV: CLEPPIES

105
For the mark of Cain see Friedman (1981) pp.87–107. The football coach (p. 106) was Glenn Hoddle. He was sacked (
The Times
, London, 1 February 1999).

107
As recently as 1900.
The story of the Cleppies is told by the British geneticist Karl Pearson (1908) in one of the first studies of a ‘lobster-claw’ family. Most British historians, Macaulay among them, accept that the Wigtown martyrs existed, but some such as Irving (1862) have noted that there are no eyewitness accounts and doubts that the whole thing happened, the graves notwithstanding. The best account is Fraser (1877). Irving and Fraser also note that another legend has another officer, the Provost, saying to maid Wilson, ‘Hech, my hearty! tak anither drink,’ only to find himself evermore afflicted with an unappeasable thirst. The uncertain nature of the
story is made even clearer by the reference to ‘Good King Charlie’ – Charles II – who at the time of the execution, 11 May 1685, had been dead for two months. Historians generally blame his successor, the Roman Catholic James II, for unleashing the army on the Scottish Lowlands. The etymology of ‘clep’ is also confusing. W.A. Craigie, in his
Dictionary of the older Scottish tongue
(1931), gives ‘clep’ as ‘call’, but Pearson’s story suggests that ‘clepped’ also means to have a limb deformity. The clepped families themselves are described by Pearson (1908), McMullen and Pearson (1913) and Lewis and Embleton (1908). Pearson’s papers are of particular historical interest for he uses them to advance the agenda of the biometricians against the Mendelians by showing that this apparently dominant gene does not segregate in Mendelian ratios. While his campaign against Mendelianism proved futile, he was partly right about this trait: it looks as though at least one ectrodactyly allele is over-represented in male progeny, an apparent case of meiotic drive, the only one known in a human pathology (Jarvik et al. 1994).

109
The fragments of myth
. Euterpe Bazopoulou-Kyrkanidou (1997) argues persuasively that Hephaestus’ lameness was usually represented as club-feet. Aterman (1999) proposes that Hephaestus’ deformity is related to the achondroplasia of the Egyptian deity Ptah – on which more in Chapter V – and, later, to arsenic neuritis, an acquired disease associated with smiths. These points of view are not necessarily inconsistent as the iconography clearly evolved over time. For the origins of the story of the Ostrich-Footed Wadoma see Gelfland et al. (1974); Roberts (1974); articles resurrecting the myth (e.g. Barrett and McCann 1980) and genetic investigations (Farrell 1984, Viljoen and Beighton 1984). Limb defects are second only to congenital heart defects in frequency (Bamshad et al. 1999).

111
One of the strange things about limbs
. Pearson (1908) and Lewis and Embleton (1908) recount the manual dexterity of ‘lobster-claw’ families. See Hermann Unthan’s (1935) memoirs for an edifying account of armlessness. The goat is described by Slijper (1942).

113
What induces a limb-bud to grow out into space
? The original description of the apical ectodermal ridge (or AER) and its experimental removal is described by Saunders (1948). Acheiropody (
200500
) is described by Freire-Maia (1975, 1981).

115
The apical ectodermal ridge is the sculptor of the limb. ‘
Lobster-Claw syndrome’ and ‘ectrodactyly’ are both now less commonly used than ‘split-hand-split-foot-malformation’ syndrome (SHFM). The disorder occurs in 1 in 18,000 newborns; inheritance is usually dominant. There are at least three distinct SHFM loci in humans: SHFM1 at 7q21.3-q22.1 (
183600
); SHFM2 at Xq26 (
313350
), SHFM3 at 10q24 (
600095
), and we can add a fourth, ectrodactyly, ectodermal dysplasia and cleft lip syndrome (EEC) at 3q27 (
129900
) (Celli et al. 1999). There are many other related syndromes besides. Celli et al. (1999) identify the EEC gene as p63, a close relative of the tumor
suppressor gene, P53; Yang et al. (1999) and Mills et al. (1999) study its function in mice. Another ectrodactyly gene in mice, Dactylplasia, encodes an F-box/WD40 family protein thought to be involved in protein destruction, and although the human homologue of this gene maps near to SHFM3, it has not yet been shown to be causually involved (Crackower et al. 1998; Sidow et al. 1999). The same is true for two distal-less related genes, DLX5 and DLX6, thought to be responsible for SHFM1 (Merlo et al. 2002). Both P63 and Dactylplasia are involved in the maintenance of the AER; among their many other skin defects, p63-homozyous mice have no limbs at all.

116
Action at a distance in the embryo.
Developmental biologists will notice that the account given here, which focuses on the AER’s role in promoting the growth of the limb-bud, is not that given in textbooks. There is no mention of how the AER patters the proximo-distal axis of the limb-bud via the ‘Progress-Zone clock’ (Wolpert 1971). This is because a pair of recent papers (Sun et al. 2002; Dudley et al. 2002) have convincingly shown that the Progress-Zone clock model is wrong. This is fascinating, but a bit upsetting, since it seems to throw the question of proximo-distal patterning open again. Niswander et al. (1993) describe how beads soaked in FGF can replace the apical ectodermal ridge. Sun et al. (2002) also give the most recent account of what is now a plethora of engineered FGF mutations in mice which have shed light on how they work.

116
Ridge FGFs not only keep mesodermal cells proliferating.
The role of FGFs in regulating cell death is shown by Dudley et al. (2002). See Zou and Niswander (1996) for the role of cell death in eliminating inter-digital webbing in chickens but not ducks. Webbing in humans, more precisely syndactyly, is sometimes the result of an excess of FGF signalling caused by gain-of-function mutations in the FGF receptor, FGFR2, as in Apert syndrome (
101200; 176943
) (Wilkie et al. 1995).

118
This account of the making of our limbs.
The role of thalidomide in phocomelia was first reported by McBride (1961) and Lenz (1962). Phocomelia appears in Roberts’s syndrome (
268300
) and SC Phocomelia syndrome A (
269000
), which may be the same disorder and are known as ‘pseudothalidomide’ syndromes; the genetic basis of neither is known. Goya’s sketch of a phocomelic infant is in the Louvre; Vrolik (1844–49) depicts Pepin; a brief account of his life is given in Gould and Pyle (1897) p.263.

120
How does thalidomide have its devastating effects?
Stephens et al. (2000) reviews some of the voluminous literature on thalidomide. He firmly discounts recent sensationalistic claims that thalidomide-induced phocomelics (who are now in their late thirties) are giving birth to phocomelic children – which, if true, would imply the existence of some form of Lamarkian inheritance. In principle, however, thalidomide might be a general mutagen causing high frequencies of all sorts of genetic disorders in second-generation infants. Exhaustive studies have failed to show that this is so. Until recently, the best
account of the action of thalidomide on limb formation was given by Tabin (1998). His explanation, which he convincingly defended against others (Neubert et al. 1999; Tabin 1999), rested on the idea that thalidomide causes a disassociation between proliferation and proximal-distal specification of limb-buds. In other words, it was couched in terms of the ‘Progress Zone’ model of limb specification. With the demise of that model (Sun et al. 2002; Dudley et al. 2002) the specificity of thalidomide becomes a little more difficult to explain but still probably depends on the abnormal inhibition of proliferation in particular populations of bone-precursors. It is striking that FGF8-conditional limb mutants in mice have phocomelia (Lewandoski et al. 2000; Moon and Capecchi 2000).

121
Metric, with its base 10 units.
Until recently it was held that
all
modern vertebrates (living or not) have no more than five digits (Shubin et al. 1997). True, some creatures such as pandas and moles
appeared
to have six, but they could be dismissed as not being true fingers, but rather modified wrist bones (the radial sesamoid in pandas and falciform bone in moles). Polydactyly can, however, evolve in flippers such as the paddles of the icthyosaur,
Opthalmosaurus
, which appear to conceal eight digits (Hinchliffe and Johnson 1980 p.56), and those of the
vaquita
dolphin, which have six (Ortega-Ortiz and Villa-Ramirez 2000). Alberch (1986) discusses polydactylous dogs; Lloyd (1986) does so for cats; and Wright (1935) for guinea pigs. Galis (2001) reviews the question of why, despite the frequency of polydactylous mutations, so few species exist with more than five digits per limb. Polydactly in humans (603596) and many other entries). Frequencies and kinds of Polydactyly from Flatt (1994); in the Ruhe family (Glass 1947); in the Scipion family (Manoiloff 1931).

122
If the apical ectodermal ridge.
For the discovery of the zone of polarising activity see Saunders and Gasseling (1968); for its interpretation see Tickle et al. (1975). Sonic hedgehog (
600725
) was first identified as the gene encoding the morphogen by Riddle et al. (1993). Since then, some (Yang et al. 1997) have argued that it is not the morphogen since it does not form a gradient in the limb. More recent evidence suggests that it does (Zeng et al. 2001).

126
This catalogue of mutations.
Many polydactyly genes have been identified in mice and humans, and many are transcription factors. For example, mutations in GH3 (
165240
), a zinc-finger transcription factor, cause Greig’s cephalopolysyndactyly (
175700
), Pallister-Hall syndrome (
146150
) and postaxial polydactyly (
174200; 174700
). See Manouvrier-Hanu et al. (1999) for a brief review of others. On-line Mendelian Inheritance in Man (August 2002) lists ninety-seven disorders with polydactyly in the clinical synopsis. How many of these are genuinely different is an interesting question, but the suggestion is certainly that more than ten genes are involved in correctly determining Shh activity. The Shh regulatory mutation causes extra thumbs and index fingers, more broadly, preaxial polydactyly (
190605; 174500
). The
genetics are complicated. Zguricas et al. (1999) mapped the mutations, deletions and translocations to 7q36, close to the Shh gene. Clark et al. (2001) showed that these mutations deleted a portion of Lmbri (
605522
), a gene near sonic hedgehog, and inferred that Lmbri was causal. Lettice et al. (2002), whose interpretation I follow here, provide evidence that 7q36 Polydactyly mutations are due to deletions of sonic hedgehog cis-acting regulatory elements that lie within a Lmbri intron rather than Lmbri itself. Achieropody (
200500
), which also maps to 7q36, has a similarly complex history. Achieropody mutations also delete Lmbri and, again, this gene was thought to be causal (Ianakiev et al. 2001; Clark et al. 2001), but is also probably due to a Shh regulatory mutation – though the jury is still out (Lettice et al. 2002). Certainly, the similarity of acheiropody to the pawless limbs of Shh-null mice is striking (Chiang et al. 1996; Chiang et al. 2001).