title: Twist1- and Twist2-haploinsufficiency results in reduced bone formation.
abstract: BACKGROUND
  Twist1 and Twist2 are highly homologous bHLH transcription factors that exhibit extensive highly overlapping expression profiles during development. While both proteins have been shown to inhibit osteogenesis, only Twist1 haploinsufficiency is associated with the premature synostosis of cranial sutures in mice and humans. On the other hand, biallelic Twist2 deficiency causes only a focal facial dermal dysplasia syndrome or additional cachexia and perinatal lethality in certain mouse strains. It is unclear how these proteins cooperate to synergistically regulate bone formation.
  
  
  METHODS
  Twist1 floxed mice (Twist1(f/f)) were bred with Twist2-Cre knock-in mice (Twist2(Cre/+)) to generate Twist1 and Twist2 haploinsufficient mice (Twist1(f/+); Twist2(Cre/+)). X-radiography, micro-CT scans, alcian blue/alizarin red staining, trap staining, BrdU labeling, immunohistochemistry, in situ hybridizations, real-time PCR and dual luciferase assay were employed to investigate the overall skeletal defects and the bone-associated molecular and cellular changes of Twist1(f/+);Twist2(Cre/+) mice.
  
  
  RESULTS
  Twist1 and Twist2 haploinsufficient mice did not present with premature ossification and craniosynostosis; instead they displayed reduced bone formation, impaired proliferation and differentiation of osteoprogenitors. These mice exhibited decreased expressions of Fgf2 and Fgfr1-4 in bone, resulting in a down-regulation of FGF signaling. Furthermore, in vitro studies indicated that both Twist1 and Twist2 stimulated 4.9 kb Fgfr2 promoter activity in the presence of E12, a Twist binding partner.
  
  
  CONCLUSIONS
  These data demonstrated that Twist1- and Twist2-haploinsufficiency caused reduced bone formation due to compromised FGF signaling.
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America; The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) & Key Laboratory of Oral Biomedicine, Ministry of Education, School & Hospital of Stomatology, Wuhan University, Wuhan, China.
authors: Yanyu
authors: Y
authors: Huang
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America.
authors: Tian
authors: T
authors: Meng
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America.
authors: Suzhen
authors: S
authors: Wang
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America.
authors: Hua
authors: H
authors: Zhang
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America.
authors: Gabriele
authors: G
authors: Mues
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America.
authors: Chunlin
authors: C
authors: Qin
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America.
authors: Jian Q
authors: JQ
authors: Feng
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America; The University of Utah School of Dentistry, Salt Lake City, Utah, United States of America.
authors: Rena N
authors: RN
authors: D'Souza
authors: Department of Biomedical Sciences and Center for Craniofacial Research and Diagnosis, Texas A&M University Baylor College of Dentistry, Dallas, Texas, United States of America.
authors: Yongbo
authors: Y
authors: Lu
mesh_terms: D000818:Animals; D001846:Bone Development; D016222:Fibroblast Growth Factor 2 / Q000235:genetics / Q000378:metabolism; D057895:Haploinsufficiency; D051379:Mice; D008810:Mice, Inbred C57BL; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism; D017468:Receptors, Fibroblast Growth Factor / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics* / Q000378:metabolism; D051879:Twist-Related Protein 1 / Q000235:genetics* / Q000378:metabolism
chemical_list: D009687:Nuclear Proteins; D017468:Receptors, Fibroblast Growth Factor; D012097:Repressor Proteins; D051879:Twist-Related Protein 1; C096852:Twist2 protein, mouse; D016222:Fibroblast Growth Factor 2; C067250:Twist1 protein, mouse