Novel LRP5 missense mutation in a patient with a high bone mass phenotype results in decreased DKK1-mediated inhibition of Wnt signaling.
We found a novel heterozygous missense mutation (M282V) in the LRP5 gene in a patient with a high bone mass phenotype. In vitro studies suggest that a reduced antagonistic effect of DKK1 on canonical Wnt signaling contributes to the molecular effect of this mutation and its pathogenic consequence.


BACKGROUND
Gain-of-function mutations in the gene encoding LDL receptor-related protein 5 (LRP5) cause high bone mass. Recent studies revealed that a reduced inhibition of canonical Wnt signaling by Dickkopf 1 (DKK1) contributes to the pathophysiology of this disease phenotype.


METHODS
We report on a 55-yr-old female patient with a high bone mass phenotype. Sequencing of exons 2-4 of the LRP5 gene was carried out to screen for disease-associated mutations in genomic DNA of the patient. The effect of the identified mutation on LRP5 membrane trafficking was studied by immunoblotting of a truncated form of LRP5. Additionally, Wnt signal activation in the absence and presence of DKK1 was assessed using a TCF4-based reporter gene assay in Saos-2 cells.


RESULTS
Our patient presents with dense bones (Z-scores > +6), and radiographic examination showed a generalized thickening of the skeleton. BMD at the hip and lumbar spine significantly decreased through the passage to menopause, indicating no protection to bone loss. Further clinical evaluation revealed torus palatinus. Mutation analysis showed the presence of a novel heterozygous missense variant (844A-->G; M282V) in LRP5, located in the first beta-propeller domain of the extracellular portion. Although protein secretion seemed to be impaired, this mutant was able to transduce Wnt signals at levels comparable with wildtype LRP5. We additionally observed a less efficient inhibition of canonical Wnt signaling by DKK1.


CONCLUSIONS
Like all high BMD-associated gain-of-function LRP5 mutations described thus far, the M282V variant affects an amino acid located in the first beta-propeller domain, underlining the functional importance of this region in the pathophysiology of these conditions. This mutation most likely alters a region important for LRP5 modulation by DKK.
Balemans
Wendy
W
Department of Medical Genetics, University and University Hospital of Antwerp, Antwerp, Belgium. wendy.balemans@ua.ac.be
Devogelaer
Jean-Pierre
JP
Cleiren
Erna
E
Piters
Elke
E
Caussin
Emanuelle
E
Van Hul
Wim
W
D019943:Amino Acid Substitution; D015519:Bone Density / Q000235:genetics*; D001847:Bone Diseases / Q000000981:diagnostic imaging / Q000235:genetics* / Q000378:metabolism / Q000503:physiopathology; D002460:Cell Line; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism; D004252:DNA Mutational Analysis; D005260:Female; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism*; D026502:LDL-Receptor Related Proteins / Q000235:genetics* / Q000378:metabolism; D060492:Low Density Lipoprotein Receptor-Related Protein-5; D008875:Middle Aged; D020125:Mutation, Missense; D010641:Phenotype; D017434:Protein Structure, Tertiary / Q000235:genetics; D021381:Protein Transport / Q000235:genetics; D011859:Radiography; D015398:Signal Transduction / Q000235:genetics*; D051153:Wnt Proteins / Q000378:metabolism*
C501126:DKK1 protein, human; D036341:Intercellular Signaling Peptides and Proteins; D026502:LDL-Receptor Related Proteins; C555482:LRP5 protein, human; D060492:Low Density Lipoprotein Receptor-Related Protein-5; D051153:Wnt Proteins