title: Defective somite patterning in mouse embryos with reduced levels of Tbx6.
abstract: During vertebrate embryogenesis, paraxial mesoderm gives rise to somites, which subsequently develop into the dermis, skeletal muscle, ribs and vertebrae of the adult. Mutations that disrupt the patterning of individual somites have dramatic effects on these tissues, including fusions of the ribs and vertebrae. The T-box transcription factor, Tbx6, is expressed in the paraxial mesoderm but is downregulated as somites develop. It is essential for the formation of posterior somites, which are replaced with ectopic neural tubes in Tbx6-null mutant embryos. We show that partial restoration of Tbx6 expression in null mutants rescues somite development, but that rostrocaudal patterning within them is defective, ultimately resulting in rib and vertebral fusions, demonstrating that Tbx6 activity in the paraxial mesoderm is required not simply for somite specification but also for their normal patterning. Somite patterning is dependent upon Notch signaling and we show that Tbx6 genetically interacts with the Notch ligand, delta-like 1 (Dll1). Dll1 expression, which is absent in the Tbx6-null mutant, is restored at reduced levels in the partially rescued mutants, suggesting that Dll1 is a target of Tbx6. We also identify the spontaneous mutation rib-vertebrae as a hypomorphic mutation in Tbx6. The similarity in the phenotypes we describe here and that of some human birth defects, such as spondylocostal dysostosis, raises the possibility that mutations in Tbx6 or components of this pathway may be responsible for these defects.
authors.affiliation: Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260, USA.
authors.forename: Phillip H
authors.initials: PH
authors.lastname: White
authors.forename: Deborah R
authors.initials: DR
authors.lastname: Farkas
authors.forename: Erin E
authors.initials: EE
authors.lastname: McFadden
authors.forename: Deborah L
authors.initials: DL
authors.lastname: Chapman
mesh_terms: D000818:Animals; D019521:Body Patterning; D016207:Cytokines; D004622:Embryo, Mammalian / Q000502:physiology*; D006801:Humans; D017403:In Situ Hybridization; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009024:Morphogenesis / Q000502:physiology; D009154:Mutation; D018008:Myogenin / Q000235:genetics / Q000378:metabolism; D010641:Phenotype; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D012272:Ribs / Q000196:embryology / Q000473:pathology; D019170:Somites / Q000502:physiology*; D013131:Spine / Q000196:embryology / Q000473:pathology; D020825:T-Box Domain Proteins; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D019076:Transgenes
chemical_list: C110586:Cer1 protein, mouse; D016207:Cytokines; D047908:Intracellular Signaling Peptides and Proteins; C503799:MYOG protein, human; D008565:Membrane Proteins; C503801:Myog protein, mouse; D018008:Myogenin; D011506:Proteins; D020825:T-Box Domain Proteins; C491182:TBX6 protein, human; C102932:Tbx6 protein, mouse; D014157:Transcription Factors; C064045:delta protein