XB-ART-43515PLoS Genet July 1, 2011; 7 (7): e1002114.
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Loss of the BMP antagonist, SMOC-1, causes Ophthalmo-acromelic (Waardenburg Anophthalmia) syndrome in humans and mice.
Ophthalmo-acromelic syndrome (OAS), also known as Waardenburg Anophthalmia syndrome, is defined by the combination of eye malformations, most commonly bilateral anophthalmia, with post-axial oligosyndactyly. Homozygosity mapping and subsequent targeted mutation analysis of a locus on 14q24.2 identified homozygous mutations in SMOC1 (SPARC-related modular calcium binding 1) in eight unrelated families. Four of these mutations are nonsense, two frame-shift, and two missense. The missense mutations are both in the second Thyroglobulin Type-1 (Tg1) domain of the protein. The orthologous gene in the mouse, Smoc1, shows site- and stage-specific expression during eye, limb, craniofacial, and somite development. We also report a targeted pre-conditional gene-trap mutation of Smoc1 (Smoc1(tm1a)) that reduces mRNA to ∼10% of wild-type levels. This gene-trap results in highly penetrant hindlimb post-axial oligosyndactyly in homozygous mutant animals (Smoc1(tm1a/tm1a)). Eye malformations, most commonly coloboma, and cleft palate occur in a significant proportion of Smoc1(tm1a/tm1a) embryos and pups. Thus partial loss of Smoc-1 results in a convincing phenocopy of the human disease. SMOC-1 is one of the two mammalian paralogs of Drosophila Pentagone, an inhibitor of decapentaplegic. The orthologous gene in Xenopus laevis, Smoc-1, also functions as a Bone Morphogenic Protein (BMP) antagonist in early embryogenesis. Loss of BMP antagonism during mammalian development provides a plausible explanation for both the limb and eye phenotype in humans and mice.
PubMed ID: 21750680
PMC ID: PMC3131273
Article link: PLoS Genet
Species referenced: Xenopus laevis
Genes referenced: gal.2 npat rpe smoc1
Article Images: [+] show captions
|Figure 1. Mapping ophthalmo-acromelic syndrome.Clinical photographs (a,b) and radiographs (c) of patient R14C12 showing bilateral anophthalmia, in association with bilateral postaxial oligodactyly and cutaneous syndactyly of 2nd & 3rd toes. (d) Multipoint linkage analysis using 10K SNPchip data from families 1–3 showing a significant LOD score of Z = 5.3 at 14q22.3–24.2, a region also identified by autozygosity mapping (see Table 1). (e) Microsatellite marker analysis for affected individuals in Families 1–3 and Family 9 showing region of homozygosity, but no common haplotype. (f) The microsatellite data refined the OAS candidate interval to Chr14∶69.89–71.26 Mb which is shown diagrammatically with the 22 annotated genes that were sequenced in this study (UCSC assembly GRCh37).|
|Figure 2. Mutation analysis.(a) Family pedigrees and associated SMOC1 mutations identified. The pedigree for Family 1 is representative and shows segregation of a homozygous SMOC1 mutation (c.911delG; p.Asp305MetfsX59) in affected individuals with both parents (and all unaffected sibs) being heterozygous carriers. (b) Schematic of the SMOC1 gene (top) and predicted protein (below), illustrating the exon positions for all eight mutations identified in the OAS families. Coding exons are coloured black and numbered, UTRs are brown, protein domains are labeled with amino acid residue numbers. Red arrowheads indicate the position of the mutations in the peptide. Red asterisks highlight the missense changes, which are located in the second thyroglobulin domain thought to be involved in the control of proteolytic degradation (n.t.- Sample not tested).|
|Figure 3. A targeted Smoc1 mutation caused an ophthalmo-acromelic-like phenotype in mice.(a) OPT representation of wild type (WT) Smoc1 expression at embryonic day (E) 9.5 (green represents Smoc1 expression); Smoc1 is expressed in the pharyngeal (branchial) arches (BA), the rostral neural tube (NT), in the anlage of the forelimbs (FL), the fronto-nasal region (FN), and in the somites (S). (b) At E10.5, expression is maintained in the branchial arches, somites and in the frontal nasal processes, as well as extending caudally in the neural tube. (c) In E10.5 Smoc1tm1a/tm1a embryos, β-galactosidase activity was observed in tissues consistent with the OPT analysis of WT Smoc1 expression: in the dorsal hindlimbs; in the medial regions of dorsal and ventral forelimbs, the branchial arches, in the frontonasal processes, and in the somites. In addition, strong signal was observed in the eye region (scale bar = 500 µm). (d) X-gal stained sagittal sections of a representative E10.5 Smoc1tm1a/tm1a embryo in the developing eye showing that expression was restricted to ventral regions of the presumptive optic stalk (POS). (e) Examination of optic nerve morphology identified an extension of the RPE into the dorsal optic nerve in mutant animals compared to control. (f) Photographs of Smoc1tm1a/tm1a eye showing an optic fissure closure defect (arrowhead) consistent with coloboma (scale bars = 100 µm). (g) Expression in the 1st branchial arch mesenchyme was distributed in proximal regions and absent from distal areas, with positive signal also seen in the epithelial cells at the hinge region between maxilliary (MX) and mandibular (MD) components (arrowheads) (scale bar = 100 µm). (h,i) Pictomicrographs of sections through E14.5 heads showing a failure in palatal shelf (PS) fusion in the developing palate in the Smoc1tm1a/tm1a embryo (i) compared to the fully fused WT littermate (h). (j,k) Surface rendered visualization of OPT reconstructions of hindlimbs at E14.5. (j) WT embryo with normal arrangement of 5 digits in the hindlimb whereas the Smoc1tm1a/tm1a littermate (k) had hindlimb oligodactyly affecting the axial digits, with only 4 digits present. (l) Skeletal preparation of P0 Smoc1tm1a/tm1a hindlimb with osseous fusion of the phalanges of digits 3–4 (red arrow).|
References [+] :
Abouzeid, Mutations in the SPARC-related modular calcium-binding protein 1 gene, SMOC1, cause waardenburg anophthalmia syndrome. 2011, Pubmed