A Novel Domain-Specific Mutation in a Sclerosteosis Patient Suggests a Role of LRP4 as an Anchor for Sclerostin in Human Bone

Abstract

Mutations in the LRP4 gene, coding for a Wnt signaling coreceptor, have been found to cause several allelic conditions. Among these, two are characterized by a strong skeletal involvement, namely sclerosteosis and Cenani-Lenz syndrome. In this work, we evaluated the role of LRP4 in the pathophysiology of these diseases. First, we report a novel LRP4 mutation, leading to the substitution of arginine at position 1170 in glutamine, identified in a patient with sclerosteosis. This mutation is located in the central cavity of the third b-propeller domain, which is in line with two other sclerosteosis mutations we previously described. Reporter assays demonstrate that this mutation leads to impaired sclerostin inhibition of Wnt signaling. Moreover, we compared the effect of this novel variant to mutations causing Cenani-Lenz syndrome and show that impaired membrane trafficking of the LRP4 protein is the likely mechanism underlying Cenani-Lenz syndrome. This is in contrast to sclerosteosis mutations, previously shown to impair the binding between LRP4 and sclerostin. In addition, to better understand the biology of LRP4, we investigated the circulating sclerostin levels in the serum of a patient suffering from sclerosteosis owing to a LRP4 mutation. We demonstrate that impaired sclerostin binding to the mutated LRP4 protein leads to dramatic increase in circulating sclerostin in this patient. With this study, we provide the first evidence suggesting that LRP4 is responsible for the retention of sclerostin in the bone environment in humans. These findings raise potential concerns about the utility of determining circulating sclerostin levels as a marker for other bone-related parameters. Although more studies are needed to fully understand the mechanism whereby LRP4 facilitates sclerostin action, it is clear that this protein represents a potent target for future osteoporosis therapies and an interesting alternative for the antisclerostin treatment currently under study.

Multifarious effects of 17-β-estradiol on apolipoprotein E receptors gene expression during osteoblast differentiation <i>in vitro </i>

“…A number of reports were exploded to highlight the role of the Wnt/β-catenin pathway in the regulation of bone homeostasis (57)(58)(59). Recently, the most highlighted members of the LDLR family involved in the maintenance of bone metabolism were LRP5, LRP6, LRP4, and Apoer2 (26,60). Interestingly, results in the current study showed that all of these four members were induced during osteoblast differentiation (Figure 1).…”

“…ApoE has a strong affinity for and is the main ligand for members of the LDLR family. The LDLR family is a highly conserved receptor family with diverse functions in cellular physiology (shown in Table 1) (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32). LDLR is the prototype of the entire family, and members of this family are structurally and functionally related to it.…”

Role and mechanism of action of sclerostin in bone

“…Previous work demonstrated that Sclerostin directly interacts with several members of the BMP and Wnt pathways such as BMP4, BMP2 and Lrp 4/5/6 (4;77;95;96). More recently, using a sclerostin affinity capture technique and mass spectrometry, it was shown that alkaline phosphatase, carbonic anhydrase, gremlin-1, fetuin A, midkine, annexin A1 and A2, and collagen α1, bind to sclerostin (97).…”

How rare bone diseases have informed our knowledge of complex diseases

“…[72][73][74] LRP4 is another member of the Wnt pathway family of proteins important in bone mass regulation. It was first identified in genome-wide studies 6,9,19 and in both mice 75,76 and humans 11,12 and shown to regulate bone mass. LRP4 functions as a binding or docking protein for sclerostin.…”

“…LRP4 functions as a binding or docking protein for sclerostin. 11,12,75 Thus, while LRP4 was first identified as a candidate gene for bone mass from GWAS studies, the identification of human mutations with the rare bone disorders provided important confirmatory evidence for its role in human bone mass regulation.…”