title: Mutations in LRP5 cause primary osteoporosis without features of OI by reducing Wnt signaling activity.
abstract: BACKGROUND
  Primary osteoporosis is a rare childhood-onset skeletal condition whose pathogenesis has been largely unknown. We have previously shown that primary osteoporosis can be caused by heterozygous missense mutations in the Low-density lipoprotein receptor-related protein 5 (LRP5) gene, and the role of LRP5 is further investigated here.
  
  
  METHODS
  LRP5 was analyzed in 18 otherwise healthy children and adolescents who had evidence of osteoporosis (manifested as reduced bone mineral density i.e. BMD, recurrent peripheral fractures and/or vertebral compression fractures) but who lacked the clinical features of osteogenesis imperfecta (OI) or other known syndromes linked to low BMD. Also 51 controls were analyzed. Methods used in the genetic analyses included direct sequencing and multiplex ligation-dependent probe amplification (MLPA). In vitro studies were performed using luciferase assay and quantitative real-time polymerase chain reaction (qPCR) to examine the effect of two novel and three previously identified mutations on the activity of canonical Wnt signaling and on expression of tryptophan hydroxylase 1 (Tph1) and 5-hydroxytryptamine (5-Htr1b).
  
  
  RESULTS
  Two novel LRP5 mutations (c.3446 T > A; p.L1149Q and c.3553 G > A; p.G1185R) were identified in two patients and their affected family members. In vitro analyses showed that one of these novel mutations together with two previously reported mutations (p.C913fs, p.R1036Q) significantly reduced the activity of the canonical Wnt signaling pathway. Such reductions may lead to decreased bone formation, and could explain the bone phenotype. Gut-derived Lrp5 has been shown to regulate serotonin synthesis by controlling the production of serotonin rate-limiting enzyme, Tph1. LRP5 mutations did not affect Tph1 expression, and only one mutant (p.L1149Q) reduced expression of serotonin receptor 5-Htr1b (p < 0.002).
  
  
  CONCLUSIONS
  Our results provide additional information on the role of LRP5 mutations and their effects on the development of juvenile-onset primary osteoporosis, and hence the pathogenesis of the disorder. The mutations causing primary osteoporosis reduce the signaling activity of the canonical Wnt signaling pathway and may therefore result in decreased bone formation. The specific mechanism affecting signaling activity remains to be resolved in future studies.
authors.affiliation: Oulu Center for Cell-Matrix Research, Biocenter and Department of Medical Biochemistry and Molecular Biology, University of Oulu, Oulu, Finland.
authors.forename: Johanna
authors.initials: J
authors.lastname: Korvala
authors.forename: Harald
authors.initials: H
authors.lastname: Jüppner
authors.forename: Outi
authors.initials: O
authors.lastname: Mäkitie
authors.forename: Etienne
authors.initials: E
authors.lastname: Sochett
authors.forename: Dirk
authors.initials: D
authors.lastname: Schnabel
authors.forename: Stefano
authors.initials: S
authors.lastname: Mora
authors.forename: Cynthia F
authors.initials: CF
authors.lastname: Bartels
authors.forename: Matthew L
authors.initials: ML
authors.lastname: Warman
authors.forename: Donald
authors.initials: D
authors.lastname: Deraska
authors.forename: William G
authors.initials: WG
authors.lastname: Cole
authors.forename: Heini
authors.initials: H
authors.lastname: Hartikka
authors.forename: Leena
authors.initials: L
authors.lastname: Ala-Kokko
authors.forename: Minna
authors.initials: M
authors.lastname: Männikkö
mesh_terms: D000818:Animals; D015519:Bone Density / Q000235:genetics; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D017930:Genes, Reporter; D006579:Heterozygote; D006801:Humans; D060492:Low Density Lipoprotein Receptor-Related Protein-5 / Q000235:genetics* / Q000378:metabolism; D020125:Mutation, Missense; D010013:Osteogenesis Imperfecta / Q000235:genetics; D010024:Osteoporosis / Q000235:genetics*; D010641:Phenotype; D060888:Real-Time Polymerase Chain Reaction; D011985:Receptors, Serotonin / Q000378:metabolism; D012701:Serotonin / Q000378:metabolism; D014162:Transfection; D014365:Tryptophan Hydroxylase / Q000378:metabolism; D051153:Wnt Proteins / Q000378:metabolism*; D060449:Wnt Signaling Pathway
chemical_list: C555482:LRP5 protein, human; D060492:Low Density Lipoprotein Receptor-Related Protein-5; D011985:Receptors, Serotonin; D051153:Wnt Proteins; D012701:Serotonin; C497467:TPH1 protein, human; D014365:Tryptophan Hydroxylase