We're reminded almost daily that we are more like our winged, fruit-craving little friends than appearances might first suggest. The latest knock to our dignity comes from Raymond Robledo and colleagues, who have found that a family of genes homologous to those that control the outgrowth of antennae and legs in Drosophila also patterns limbs in mammals. The Distal-less ( Dll )/Dlx family of homeobox transcription factors are known for being involved in limb formation in many insects and vertebrates; the importance of this study has been to extend this function to mammals as well, including humans.
In Drosophila, there is only one Dll gene; flies without it lack the distal portion of their appendages. Mammals, by contrast, have six Dlx genes (Dlx1–Dlx6), which cluster in pairs on the genome — Robledo et al. concentrate here on the Dlx5/6 cluster. This is because, although all vertebrate Dlx genes are expressed in the developing limbs, the brain and the craniofacial primordia, the Dlx5/6 cluster also maps to a region on human chromosome 7 that is associated with a severe human limb defect called split-hand/foot malformation type 1 (SHFM1). In this dominantly inherited disorder, the central limb digits are missing, giving the extremities a claw-like appearance. The phenotype of Dlx5/6 double-knockout mice, which these authors generated, confirm their expectation that Dlx5/6 might regulate limb development. Not only do these animals have bone, inner ear and severe craniofacial defects — as could be predicted from the expression patterns of Dlx5/6 and as has been reported for other Dlx single- and double-knockout mice — but they also phenocopy the limb defects seen in human SHFM1.
However, Dlx genes can only be said to be functional homologues of Dll if the limb abnormalities of Dlx5/6−/− mice arise from defects in proximal–distal (P/D) patterning. Indeed, by embryonic day 11.5 — when most of the mutant defects become apparent — all the molecular markers for the distal medial limb are missing. The authors propose that, by reducing cell proliferation, the absence of Dlx5/6 causes loss of cells in the apical ectodermal ridge (AER), which controls P/D patterning. Furthermore, the ectopic expression of Dlx5 (alone) in the AER fully rescued the SHFM1 defect in Dlx5/6−/− mice.
Putting all this information together, Robledo and colleagues argue that all species with appendages owe these structures to the evolutionary ancient Dll/Dlx gene family. By showing that Dlx5/6 mutant mice can phenocopy SHFM1, they have extended the function of Dlx genes to humans, as well as creating a model for investigating the physiological basis and progression of this human limb abnormality.
ORIGINAL RESEARCH PAPER
Robledo, R. F. et al. The Dlx5 and Dlx6 homeobox genes are essential for craniofacial, axial, and appendicular skeletal development. Genes Dev. 16, 1089–1101 (2002)
Related links
Rights and permissions
About this article
Cite this article
Casci, T. Distal-less makes the leap. Nat Rev Genet 3, 413 (2002). https://doi.org/10.1038/nrg822
Issue Date:
DOI: https://doi.org/10.1038/nrg822