Achondroplasia, the most common form of dwarfism in man, is a dominant genetic disorder caused by a point mutation (G380R) in the transmembrane region of fibroblast growth factor receptor 3 (FGFR3). We used gene targeting to introduce the human achondroplasia mutation into the murine FGFR3 gene. Heterozygotes for this point mutation that carried the neo cassette were normal whereas neo ؉ homozygotes had a phenotype similar to FGFR3-deficient mice, exhibiting bone overgrowth. This was because of interference with mRNA processing in the presence of the neo cassette. Removal of the neo selection marker by Cre͞loxP recombination yielded a dominant dwarf phenotype. These mice are distinguished by their small size, shortened craniofacial area, hypoplasia of the midface with protruding incisors, distorted brain case with anteriorly shifted foramen magnum, kyphosis, and narrowed and distorted growth plates in the long bones, vertebrae, and ribs. These experiments demonstrate that achondroplasia results from a gain-of-FGFR3-function leading to inhibition of chondrocyte proliferation. These achondroplastic dwarf mice represent a reliable and useful model for developing drugs for potential treatment of the human disease.Four fibroblast growth factor receptors (FGFRs) are known (1), and Ͼ50 mutations in three of them (FGFR1, 2, and 3) recently have been implicated in congenital skeletal and cranial disorders (reviewed in refs. 2-4). Achondroplasia, the most common form of dwarfism, was shown to be linked to a single point mutation, G380R, in the transmembrane region of FGFR3 (5, 6). FGFR3 is expressed mainly by developing bones, brain, lung, and spinal cord (7,8), and FGFR3-deficient mice show enhanced endochondral bone growth, expansion of their growth plate, and increased chondrocyte proliferation (9, 10). Thus, FGFR3 is a negative regulator of bone growth. Several experiments at the cellular level indicated that the Ach mutation (G380R) results in a constitutive activation of the receptor in a ligand-independent manner (11-13). It was suggested that this is because of stabilization of receptor dimers, a prerequisite for signal transduction in these receptors (14). This is also consistent with the constitutive activation by dimer formation described for an erbB2 (neu) receptor mutant, which carries a Val-to-Glu mutation in an analogous position to that of the FGFR3 variant in its transmembrane region (15). It is likely that, in many of the other mutations in FGFR1-3, the underlying mechanism of receptor activation is also through stabilization of receptor dimers because many of these mutations result in unpaired cysteines that may enhance inter-receptor disulfide bonds (2-4).To generate an animal model for this type of mutation and to study the role of the mutated FGFR3 in vivo, we used gene targeting to introduce the achondroplasia mutation (G380R) into murine FGFR3. This resulted in a dominant dwarf phenotype that exhibits many of the features of human achondroplasia. This is an indication from in vivo data for ...

The aim of this study was to clarify the role of Fgfr2 during later stages of embryonic development. Of two previously reported gene-targeting experiments, the more extensive Fgfr2 deletion was lethal shortly after implantation, because of trophoblast defects, whereas the less extensive one survived until midgestation with placental insufficiency and defective limb outgrowth [Xu, X., Weinstein, M., Li, C., Naski, M., Cohen, R. I., Ornitz, D. M., Leder, P. & Deng, C. (1998) Development (Cambridge, U.K.) 125, 753-765]. Fgfr2 in the early embryo is expressed in the trophectoderm, and this extra-embryonic localization persists into mid-and late gestation, when Fgfr2 also is expressed in multiple developing organs. To gain insight into the later functions of Fgfr2, fusion chimeras were constructed from homozygous mutant embryonic stem cells and wild-type tetraploid embryos. This allowed survival until term and revealed that Fgfr2 is required for both limb outgrowth and branching lung morphogenesis. The use of fusion chimeras demonstrated that early lethality was indeed because of trophectoderm defects and indicated that in the embryonic cell lineages Fgfr2 activity manifests in limb and lung development. Highly similar lung and limb phenotypes were detected recently in the loss of function mutation of Fgf10, a ligand of Fgfr2. It is likely, therefore, that whereas during early development Fgfr2 interacts with Fgf4, in limb and lung development interactions between Fgf10 and Fgfr2 may be required. Possible epithelial-mesenchymal interactions between the splicing alternatives of Fgfr2 and their specific ligands will be discussed.

The Dkk family of secreted cysteine-rich proteins regulates Wnt/β-catenin signaling by interacting with the Wnt co-receptor Lrp5/6. Here, we show that Dkk2-mediated repression of the Wnt/β-catenin pathway is essential to promote differentiation of the corneal epithelial progenitor cells into a non-keratinizing stratified epithelium. Complete transformation of the corneal epithelium into a stratified epithelium that expresses epidermal-specific differentiation markers and develops appendages such as hair follicles is achieved in the absence of the Dkk2 gene function. We show that Dkk2 is a key regulator of the corneal versus epidermal fate of the ocular surface epithelium.

Vertebrate limb development is controlled by three signaling centers that regulate limb patterning and growth along the proximodistal (PD),anteroposterior (AP) and dorsoventral (DV) limb axes. Coordination of limb development along these three axes is achieved by interactions and feedback loops involving the secreted signaling molecules that mediate the activities of these signaling centers. However, it is unknown how these signaling interactions are processed in the responding cells. We have found that distinct LIM homeodomain transcription factors, encoded by the LIM homeobox(LIM-HD) genes Lhx2, Lhx9 and Lmx1b integrate the signaling events that link limb patterning and outgrowth along all three axes. Simultaneous loss of Lhx2 and Lhx9 function resulted in patterning and growth defects along the AP and the PD limb axes. Similar, but more severe, phenotypes were observed when the activities of all three factors, Lmx1b, Lhx2 and Lhx9, were significantly reduced by removing their obligatory co-factor Ldb1. This reveals that the dorsal limb-specific factor Lmx1b can partially compensate for the function of Lhx2 and Lhx9 in regulating AP and PD limb patterning and outgrowth. We further showed that Lhx2and Lhx9 can fully substitute for each other, and that Lmx1bis partially redundant, in controlling the production of output signals in mesenchymal cells in response to Fgf8 and Shh signaling. Our results indicate that several distinct LIM-HD transcription factors in conjunction with their Ldb1 co-factor serve as common central integrators of distinct signaling interactions and feedback loops to coordinate limb patterning and outgrowth along the PD, AP and DV axes after limb bud formation.