title: Signal peptide mutations in RANK prevent downstream activation of NF-κB.
abstract: Familial expansile osteolysis and related disorders are caused by heterozygous tandem duplication mutations in the signal peptide region of the gene encoding receptor activator of NF-κB (RANK), a receptor critical for osteoclast formation and function. Previous studies have shown that overexpression of these mutant proteins causes constitutive activation of NF-κB signaling in vitro, and it has been assumed that this accounts for the focal osteolytic lesions that are seen in vivo. We show here that constitutive activation of NF-κB occurred in HEK293 cells overexpressing wild-type or mutant RANK but not in stably transfected cell lines expressing low levels of each RANK gene. Importantly, only cells expressing wild-type RANK demonstrated ligand-dependent activation of NF-κB. When overexpressed, mutant RANK did not localize to the plasma membrane but localized to extensive areas of organized smooth endoplasmic reticulum, whereas, as expected, wild-type RANK was detected at the plasma membrane and in the Golgi apparatus. This intracellular accumulation of the mutant proteins is probably the result of lack of signal peptide cleavage because, using two in vitro translation systems, we demonstrate that the mutations in RANK prevent cleavage of the signal peptide. In conclusion, signal peptide mutations lead to accumulation of RANK in the endoplasmic reticulum and prevent direct activation by RANK ligand. These results strongly suggest that the increased osteoclast formation/activity caused by these mutations cannot be explained by studying the homozygous phenotype alone but requires further detailed investigation of the heterozygous expression of the mutant RANK proteins.
authors.affiliation: Musculoskeletal Research Program, University of Aberdeen Medical School, Institute of Medical Sciences, Foresterhill, Aberdeen, United Kingdom. j.c.crockett@abdn.ac.uk
authors.forename: Julie C
authors.initials: JC
authors.lastname: Crockett
authors.forename: David J
authors.initials: DJ
authors.lastname: Mellis
authors.forename: Kathleen Ij
authors.initials: KI
authors.lastname: Shennan
authors.forename: Angela
authors.initials: A
authors.lastname: Duthie
authors.forename: John
authors.initials: J
authors.lastname: Greenhorn
authors.forename: Debbie I
authors.initials: DI
authors.lastname: Wilkinson
authors.forename: Stuart H
authors.initials: SH
authors.lastname: Ralston
authors.forename: Miep H
authors.initials: MH
authors.lastname: Helfrich
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Rogers
mesh_terms: D001483:Base Sequence; D002460:Cell Line; D004254:DNA Nucleotidyltransferases / Q000378:metabolism; D005786:Gene Expression Regulation; D057809:HEK293 Cells; D006801:Humans; D008969:Molecular Sequence Data; D008970:Molecular Weight; D050505:Mutant Proteins / Q000378:metabolism / Q000648:ultrastructure; D009154:Mutation / Q000235:genetics*; D016328:NF-kappa B / Q000378:metabolism*; D010010:Osteoclasts / Q000378:metabolism / Q000648:ultrastructure; D021382:Protein Sorting Signals / Q000235:genetics*; D021381:Protein Transport; D053246:Receptor Activator of Nuclear Factor-kappa B / Q000235:genetics* / Q000378:metabolism / Q000648:ultrastructure; D015203:Reproducibility of Results; D013347:Subcellular Fractions / Q000378:metabolism; D014162:Transfection
chemical_list: D050505:Mutant Proteins; D016328:NF-kappa B; D021382:Protein Sorting Signals; D053246:Receptor Activator of Nuclear Factor-kappa B; D004254:DNA Nucleotidyltransferases; C045119:FLP recombinase