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Serrate2 is disrupted in the mouse limb-development mutant syndactylism
Arend Sidow*, Monique S. Bulotsky*,Anne W. Kerrebrock*, Roderick T. Bronson,Mark J. Daly*, Mary P. Reeve*, Trevor L. Hawkins*, Bruce W. Birren*, Rudolf Jaenisch* & Eric S. Lander*
* Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, Massachusetts 02142, USA Department of Pathology, Tufts University Schools of Medicine and Veterinary Medicine, Boston, Massachusetts 02111, USA
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The mouse syndactylism (sm) mutation impairs some of the earliest aspects of limb development and leads to subsequent abnormalities in digit formation13.In sm homozygotes, the apical ectodermal ridge (AER) is hyperplastic by embryonic day 10.5, leading to abnormal dorsoventral thickening of the limb bud, subsequent merging of the skeletal condensations that give rise to cartilage and bone in the digits, and eventual fusion of digits. The AER hyperplasia and its effect on early digital patterning distinguish sm from many other syndactylies that result from later failure of cell death in the interdigital areas4,5. Here we use
positional cloning to show that the gene mutated in sm mice encodes the putative Notch ligand Serrate2. The results provide direct evidence that a Notch signalling pathway is involved in the earliest stages of limb-bud patterning and support the idea that an ancient genetic mechanism underlies both AER formation in vertebrates and wing-margin formation in ies6,7. In addition to cloning the sm gene, we have mapped three modiers of sm, for which we suggest possible candidate genes.
The adult phenotype of the syndactylism (sm) mutation, in its severest form, involves soft-tissue fusions along the entire length of the three middle digits, with distal skeletal elements converging into osseous fusions1 (Fig. 1a). The earliest embryonic sm phenotype is a hyperplastic and irregular AER that is buried in the mesenchyme, rather than protruding above. It can be readily detected by whole-mount in situ hybridization using Fgf8 as a marker (Fig. 1b, c) or by histological sections1 (Fig. 1d, e).
As the rst step towards positional cloning, we genetically mapped sm to distal chromosome 12 between the genetic markers D12Mit133 and D12Mit20 (Fig. 2) in an F2 intercross (sm [H11003] MOLF=Ei) with 2,883 progeny, providing 5,766 informative meioses. We then constructed a yeast articial chromosome (YAC) map of about 1.5 megabases (Mb) spanning the...