An Evolutionarily Conserved Enhancer Regulates Bmp4 Expression in Developing Incisor and Limb Bud

To elucidate the transcriptional regulation of Bmp4 expression during organogenesis, we used phylogenetic footprinting and transgenic reporter analyses to identify Bmp4 cis-regulatory modules (CRMs). These analyses identified a regulatory region located ∼46 kb upstream of the mouse Bmp4 transcription start site that had previously been shown to direct expression in lateral plate mesoderm. We refined this regulatory region to a 396-bp minimal enhancer, and show that it recapitulates features of endogenous Bmp4 expression in developing mandibular arch ectoderm and incisor epithelium during the initiation-stage of tooth development. In addition, this enhancer directs expression in the apical ectodermal ridge (AER) of the developing limb and in anterior and posterior limb mesenchyme. Transcript profiling of E11.5 mouse incisor dental lamina, together with protein binding microarray (PBM) analyses, allowed identification of a conserved DNA binding motif in the Bmp4 enhancer for Pitx homeoproteins, which are also expressed in the developing mandibular and incisor epithelium. In vitro electrophoretic mobility shift assays (EMSA) and in vivo transgenic reporter mutational analyses revealed that this site supports Pitx binding and that the site is necessary to recapitulate aspects of endogenous Bmp4 expression in developing craniofacial and limb tissues. Finally, Pitx2 chromatin immunoprecipitation (ChIP) demonstrated direct binding of Pitx2 to this Bmp4 enhancer site in a dental epithelial cell line. These results establish a direct molecular regulatory link between Pitx family members and Bmp4 gene expression in developing incisor epithelium

A Nonsense Mutation in MSX1 Causes Witkop Syndrome

Witkop syndrome, also known as tooth and nail syndrome (TNS), is a rare autosomal dominant disorder. Affected individuals have nail dysplasia and several congenitally missing teeth. To identify the gene responsible for TNS, we used candidate-gene linkage analysis in a three-generation family affected by the disorder. We found linkage between TNS and polymorphic markers surrounding the MSX1 locus. Direct sequencing and restriction-enzyme analysis revealed that a heterozygous stop mutation in the homeodomain of MSX1 cosegregated with the phenotype. In addition, histological analysis of Msx1-knockout mice, combined with a finding of Msx1 expression in mesenchyme of developing nail beds, revealed that not only was tooth development disrupted in these mice, but nail development was affected as well. Nail plates in Msx1-null mice were defective and were thinner than those of their wild-type littermates. The resemblance between the tooth and nail phenotype in the human family and that of Msx1-knockout mice strongly supports the conclusions that a nonsense mutation in MSX1 causes TNS and that Msx1 is critical for both tooth and nail development

<i>Bmp4^lacZneo</i> and <i>Tg^CONS3</i> β-galactosidase activity in the limb and orofacial region.

<p>(<b>A</b>) Lateral views of developing fore- or hindlimbs of <i>Bmp4^lacZneo</i> control mouse embryos (<i>upper row</i>). <i>Tg^CONS3</i> transgenic embryos from permanent transgenic line TL3459 (<i>lower</i> row). The CONS3 transgene expression largely recapitulates endogenous <i>Bmp4</i> expression in the AER from E9.5–13.5. (<b>B</b>) Frontal views of the developing head of <i>Bmp4^lacZneo</i> and <i>Tg^CONS3</i> transgenic embryos from permanent line TL3459. From E9.5 to E11.0, <i>Bmp4</i> is expressed in the epithelium overlying the distal first branchial arch, maxillary and mandibular processes and medial and lateral nasal processes. At these stages, the CONS3 enhancer recapitulates endogenous expression in the ectoderm overlying the distal part of the first branchial arch at E9.5 and the mandibular process at E10.5. At E12.5, endogenous <i>Bmp4</i> expression shifts to the mesenchyme, while CONS3 expression persists in the epithelium overlying the mandibular arch and premaxilla, including incisor tooth germs. At E12.5 and 13.5, endogenous <i>Bmp4</i> is concentrated in the mesenchyme of the midface including the whisker follicles. CONS3 transgene expression persists in the epithelium that overlies the mandible, pre-maxilla and nasal pits, and fails to shift to the underlying mesenchyme. Abbreviations: MxP, maxillary process; MdP, mandibular process; MNP, medial nasal process; LNP, lateral nasal process; NP, nasal pit; Mx, maxilla; Md, mandible.</p

Conserved region driven β-galactosidase activity in the orofacial region and limb.

<p>(<b>A</b>) Schematic of the transgenic reporter construct used in transient transgenic analyses. The black rectangle denotes the location of CONS region insertions upstream of the human beta globin promoter in pGLKS (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038568#s4" target="_blank">Materials and Methods</a>). (<b>B</b>) Schematic of the <i>Bmp4^lacZneo</i> allele <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038568#pone.0038568-Lawson1" target="_blank">[45]</a> (white boxes, exons; gray boxes, coding regions). (<b>C</b>) Number of transient transgenic embryos with various CONS derivative constructs that supported β-galactosidase expression in limb or orofacial tissue.</p

Pitx binds to the minimal <i>Bmp4</i> enhancer CRM <i>in vitro</i> and the Pitx binding site is necessary for enhancer activity.

<p>(<b>A</b>) Electrophoretic Mobility Shift Assay (EMSA) exhibits robust binding of Pitx1 protein to both a positive control <i>bicoid</i> DNA sequence and to the consensus Pitx1/2 binding site in with a 25 bp probe sequence in the CONS3.8 sequence. Competition with specific or non-specific unlabelled probes indicates sequence-specific binding of GST:Pitx1 fusion protein to the consensus Pitx1/2 DNA binding motif. (<b>B</b>) Number of transient transgenic embryos that supported β-galactosidase expression in forelimbs, hindlimbs, or orofacial tissue. Asterisk indicates statistically significant differences (p<0.05) compared to wild-type CONS3 by Fisher’s exact test.</p

Pitx binds the minimal <i>Bmp4</i> enhancer <i>in vivo</i>.

<p>(<b>A</b>) Schematic representation of the ChIP assay design. The 396-bp <i>Bmp4</i> CRM containing the Pitx1/2 binding site is shown along with two control regions, C1 and C2. C1 and C2 are located upstream of the <i>Bmp4</i> transcription start site and do not contain Pitx1/2 binding sites (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038568#s4" target="_blank">Materials and Methods</a>). The transcriptional start site (TSS) is marked as +1 and Pr is the <i>Bmp4</i> proximal promoter. Chromatin Immunoprecipitation (ChIP) assays were performed in LS-8 cells. Lane 1 is a no template control using the test primers, lane 2 is the Pitx2 antibody immunoprecipitated (IP-Pitx2) chromatin, lane 3 is the normal rabbit immunoglobulin G (IgG) immunoprecipitated chromatin, and lane 4 is the input chromatin amplified using the test primers. The top gel (<i>Bmp4</i> enhancer) represents the test primers encompassing Pitx1/2 motif in the 396-bp <i>Bmp4</i> CRM. Lane 2 exhibits a 338 amplicon of the expected size. The lower two gels reveal the absence of amplicon products from the two control regions C1 and C2. An amplicon from these regions are only detected in the input control groups. (<b>B</b>) Quantitative real-time PCR was performed using the same conditions for the ChIP assays as described in (A). The 4.7-fold enrichment of the region containing the 396-bp <i>Bmp4</i> CRM in the Pitx2 antibody immunoprecipitated (Pitx2-IP) chromatin is shown in the bar graph. Significance is denoted by <i>p</i> = 0.0018 for the enrichment of amplified product in Pitx2-IP versus an IgG control set at 1.0. (<b>C</b>) Sequencing analysis demonstrates the presence of a Pitx1/2 binding site in the product amplified using the <i>Bmp4</i> minimal CRM region-specific primers and Pitx2-IP chromatin as template.</p

Deletion analysis of the <i>Bmp4</i> enhancer CONS3.

<p>(<b>A</b>) Schematic of the four regions (CONS1-4; black boxes), each containing multiple human-mouse sequence conservation blocks, tested in a transient transgenic mouse reporter assay in the same reporter shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038568#pone-0038568-g002" target="_blank">Figure 2</a>. The CONS3 enhancer was systemically narrowed down to 758 bp region, CONS3.5, which contains the Fugu-Mouse upstream conservation block, and then to a minimal 396 bp minimal enhancer. (<b>B</b>) The number of E11.5 transgenic mouse embryos showing endogenous <i>Bmp4</i> orofacial and limb expression (in the AER and mesenchyme region of fore- and hindlimbs) is reported over the total number of transgenic embryos analyzed (No. Stained/No. Tg).</p