An autosomal dominant high bone mass phenotype in association with craniosynostosis in an extended family is caused by an LRP5 missense mutation

Genetic variants in the LRP5 gene associated with gain and loss of bone mineral density

The low-density lipoprotein receptor-related protein 5 (LRP5) plays a pivotal role in bone formation, influencing the proliferation and differentiation of osteoblasts and thereby impacting overall bone mass. Genetic variations stemming from non-synonymous single nucleotide polymorphisms (nsSNPs) within the LRP5 gene can lead to either enhanced or diminished function of the resultant protein, culminating in distinct phenotypic expressions such as osteoporosis-pseudoglioma syndrome (OPPG) and high bone mass (HBM). Through in silico analysis of 17 identified nsSNPs, it was observed that 14 of these variants induced damage at highly conserved sites, resulting in the destabilization of both protein function and structure. Notably, the functional alteration, be it a gain or loss, is primarily dictated by the interaction between the molecule and LRP5, rather than the specific amino acid substitution. This research offers an identification of detrimental nsSNPs within the LRP5 protein and serves as a foundation for population-based investigations into the phenotypic repercussions on a broader scale.

Enhanced insights into the genetic architecture of 3D cranial vault shape using pleiotropy-informed GWAS

Large-scale GWAS studies have uncovered hundreds of genomic loci linked to facial and brain shape variation, but only tens associated with cranial vault shape, a largely overlooked aspect of the craniofacial complex. Surrounding the neocortex, the cranial vault plays a central role during craniofacial development and understanding its genetics are pivotal for understanding craniofacial conditions. Experimental biology and prior genetic studies have generated a wealth of knowledge that presents opportunities to aid further genetic discovery efforts. Here, we use the conditional FDR method to leverage GWAS data of facial shape, brain shape, and bone mineral density to enhance SNP discovery for cranial vault shape. This approach identified 120 independent genomic loci at 1% FDR, nearly tripling the number discovered through unconditioned analysis and implicating crucial craniofacial transcription factors and signaling pathways. These results significantly advance our genetic understanding of cranial vault shape and craniofacial development more broadly.

Therapeutic targeting of Wnt antagonists by small molecules for treatment of osteoporosis

Wnt signaling is one of the key regulators of bone development and homeostasis. Wnt signaling regulates key biological events, including stem cell fate and osteoblast and osteoclast activity, leading to the maintenance of bone mass and strength. Wnt ligands are secreted glycoproteins that bind to Frizzled (FZD) receptors and their coreceptors, lipoprotein receptor-related proteins-5/6 (LRP5/6). Binding of Wnts to FZD triggers canonical (β-catenin-dependent) and noncanonical (β-catenin-independent) pathways. In canonical Wnt signaling, stabilized β-catenin translocates to the nucleus, where it promotes osteoblast differentiation by activating target genes, including Runx2 and Osterix. The negative regulators of Wnt or so-called Wnt antagonists, including CXXC5, sFRP, sclerostin, DKK1, and Notum, compete for Fzd binding, attenuating Wnt signaling. The critical roles of Wnt signaling in bone homeostasis have been established by various bone diseases caused by mutations in Wnt signaling pathways. Loss-of-function mutations in the LRP5 gene cause osteoporosis-pseudoglioma syndrome, whereas gain-of-function mutations are linked to osteopetrosis characterized by high bone density. Sclerosteosis and Van Buchem disease are caused by mutations affecting the SOST gene, which encodes sclerostin, a natural inhibitor of Wnt signalling. Loss-of-function mutations in SOST result in excessive bone growth, markedly increased bone density, and other skeletal abnormalities due to uncontrolled Wnt activity. Considering the clinical relevance of Wnt signaling, targeting Wnt inhibitors is being intensely pursued using small molecules that act by inhibiting endogenous Wnt agonists. We used a computational biology approach to review current data on pharmacophores of Wnt antagonists, assessing their potential as therapeutic candidates for postmenopausal osteoporosis.

The high-bone-mass phenotype of novel transgenic mice with LRP5 A241T mutation

Gain-of-function mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) can cause high-bone-mass (HBM) phenotype, with 19 identified mutations so far. The A242T mutation in LRP5 has been found in 9 families, making it one of the most prevalent mutations. However, the correlation between the A242T mutation and HBM phenotype remains unverified in animal models. This study aimed to investigate the bone properties in a new transgenic mouse model carrying the LRP5 A241T missense mutation, equivalent to A242T in humans. Heterozygous Lrp5A241T mice were generated using CRISPR/Cas9 genome editing. Body weight increased with age from 4 to 16 weeks, higher in males than females, with no difference between Lrp5A241T mice and wild-type control. Micro-CT showed slightly longer femur and notably elevated trabecular bone mass of the femur and fifth lumbar spine with higher bone mineral density, bone volume fraction, and trabecular thickness in Lrp5A241T mice compared to wild-type mice. Additionally, increased cortical bone thickness and volume of the femur shaft and skull were observed in Lrp5A241T mice. Three-point bending tests of the tibia demonstrated enhanced bone strength properties in Lrp5A241T mice. Histomorphometry confirmed that the A241T mutation increased bone formation without affecting osteoblast number and reduced resorption activities in vivo. In vitro experiments indicated that the LRP5 A241T mutation enhanced osteogenic capacity of osteoblasts with upregulation of the Wnt signaling pathway, with no significant impact on the resorptive activity of osteoclasts. In summary, mice carrying the LRP5 A241T mutation displayed high bone mass and quality due to enhanced bone formation and reduced bone resorption in vivo, potentially mediated by the augmented osteogenic potential of osteoblasts. Continued investigation into the regulatory mechanisms of its bone metabolism and homeostasis may contribute to the advancement of novel therapeutic strategies for bone disorders.

LRP5, Bone Mass Polymorphisms and Skeletal Disorders

The formation and maintenance of the gross structure and microarchitecture of the human skeleton require the concerted functioning of a plethora of morphogenic signaling processes. Through recent discoveries in the field of genetics, numerous genotypic variants have been implicated in pathologic skeletal phenotypes and disorders arising from the disturbance of one or more of these processes. For example, total loss-of-function variants of LRP5 were found to be the cause of osteoporosis-pseudoglioma syndrome (OPPG). LRP5 encodes for the low-density lipoprotein receptor-related protein 5, a co-receptor in the canonical WNT-β-catenin signaling pathway and a crucial protein involved in the formation and maintenance of homeostasis of the human skeleton. Beyond OPPG, other partial loss-of-function variants of LRP5 have been found to be associated with other low bone mass phenotypes and disorders, while LRP5 gain-of-function variants have been implicated in high bone mass phenotypes. This review introduces the roles that LRP5 plays in skeletal morphogenesis and discusses some of the structural consequences that result from abnormalities in LRP5. A greater understanding of how the LRP5 receptor functions in bone and other body tissues could provide insights into a variety of pathologies and their potential treatments, from osteoporosis and a variety of skeletal abnormalities to congenital disorders that can lead to lifelong disabilities.

LRP5 high bone mass (Worth-type autosomal dominant endosteal hyperostosis): case report and historical review of the literature

PURPOSE

LRP5 high bone mass (HBM) is an autosomal dominant endosteal hyperostosis caused by mutations of the low-density lipoprotein receptor-related protein 5 (LRP5) gene. Alternative names included "autosomal dominant osteosclerosis" and "Worth disease." The aim of the paper is to provide an historical overview of a disorder whose literature is complicated and confusing due to the past use of several denominations and lack of reviews.

METHODS

We collected case reports of HBM with evidence of autosomal dominant transmission preceding the identification of the LRP5 mutations in 2002 (Worth-type endosteal hyperostosis) and cases of LRP5 HBM confirmed by genetic analysis since 2002. The prevalence of relevant clinical and laboratory findings was estimated. We described an affected woman with neurological manifestations.

RESULTS

A 44-year-old Caucasian woman with torus palatinus complained of headache, hypo-/anosmia, and complete mixed deafness. Dual-energy X-ray absorptiometry (DEXA) scan revealed elevated bone mass. The A242T mutation of the LRP5 gene was detected. Including the present case, 155 patients have been reported to date. Neurological involvement and increased serum alkaline phosphatase (ALP) were present in 19.4% and 3.7% of cases, respectively. Facial changes and torus palatinus were observed in 61% and 41% of cases, respectively.

CONCLUSIONS

We present the only historical review on Worth-type endosteal hyperostosis, now known as LRP5 HBM. Neurological manifestations, previously considered absent in the disease, affect 19.4% of the patients. Genetic analysis and appropriate denomination of LRP5 HBM are fundamental for diagnosis and to mitigate the confusion that has long characterized this disease.

Novel mutation in LRP5 gene cause rare osteosclerosis: cases studies and literature review

To study the effects of low-density lipoprotein receptor-related protein 5 (LRP5) gene mutations on bone, and to open up our view of LRP5 and Wnt pathways on bone mass regulation. Three patients with increased bone mineral density or thickened bone cortex were included, who were 30-year-old, 22-year-old and 50-year-old men, respectively. The latter two patients were son and father of a same family. The characteristics of bone X-rays were evaluated in detail. Bone turnover markers were detected, such as procollagen type 1 amino-terminal peptide (P1NP), alkaline phosphatase (ALP), and type 1 collagen carboxyl terminal peptide (β-CTX). Dual energy X-ray absorptiometry (DXA) was used to measure the bone mineral density (BMD) at lumbar spine and proximal femur of the patients. The targeted next-generation sequencing (NGS) technology was used to detect pathogenic gene mutations, which were further verified by Sanger sequencing. Moreover, the gene mutation spectrum and phenotypic characteristics of reported patients with LRP5 gain-of-function mutations were summarized by reviewing the literature. The main characteristics of the first patient were headache, facial paralysis, high BMD (lumbar vertebrae 1–4: 1.877 g/cm2, Z-score: 5.8; total hip: 1.705 g/cm2, Z-score: 5.7), slightly increased P1NP (87.0 ng/mL) and β-CTX (0.761 ng/mL) level, and with thickened bone cortex, especially the cranial vault. The latter two patients showed enlargement of the mandible and enlarged osseous prominence of the tours palatinus. X-rays showed that the bone cortex of skull and long bones were thickened. The bone turnover markers and BMD were normal. All three cases carried novel missense mutations in LRP5 gene, which were mutation in exon 3 (c.586 T > G, p.Trp196Gly) of the first patient, and mutation in exon 20 (c.4240C > A, p.Arg1414Ser) of the latter two patients. Combined with the reported literature, a total of 19 gain-of-function mutations in LRP5 were detected in 113 patients from 33 families. Hotspot mutations included c.724G > A, c.512G > T and c.758C > T. Furthermore, mutations in the exon 3 of LRP5 may cause severe phenotypes. LRP5 gain-of-function mutations can lead to rare autosomal dominant osteosclerosis type Ι (ADO Ι), which was characterized by increased bone mass and thickened bone cortex. In-depth research on the Wnt pathway will be benefit for discovering important mechanisms of bone mass regulation.

De novo mutations in the BMP signaling pathway in lambdoid craniosynostosis

Lambdoid craniosynostosis (CS) is a congenital anomaly resulting from premature fusion of the cranial suture between the parietal and occipital bones. Predominantly sporadic, it is the rarest form of CS and its genetic etiology is largely unexplored. Exome sequencing of 25 kindreds, including 18 parent-offspring trios with sporadic lambdoid CS, revealed a marked excess of damaging (predominantly missense) de novo mutations that account for ~ 40% of sporadic cases. These mutations clustered in the BMP signaling cascade (P = 1.6 × 10^-7), including mutations in genes encoding BMP receptors (ACVRL1 and ACVR2A), transcription factors (SOX11, FOXO1) and a transcriptional co-repressor (IFRD1), none of which have been implicated in other forms of CS. These missense mutations are at residues critical for substrate or target sequence recognition and many are inferred to cause genetic gain-of-function. Additionally, mutations in transcription factor NFIX were implicated in syndromic craniosynostosis affecting diverse sutures. Single cell RNA sequencing analysis of the mouse lambdoid suture identified enrichment of mutations in osteoblast precursors (P = 1.6 × 10^-6), implicating perturbations in the balance between proliferation and differentiation of osteoprogenitor cells in lambdoid CS. The results contribute to the growing knowledge of the genetics of CS, have implications for genetic counseling, and further elucidate the molecular etiology of premature suture fusion.

Hereditary Metabolic Bone Diseases: A Review of Pathogenesis, Diagnosis and Management

Hereditary metabolic bone diseases are characterized by genetic abnormalities in skeletal homeostasis and encompass one of the most diverse groups among rare diseases. In this review, we examine 25 selected hereditary metabolic bone diseases and recognized genetic variations of 78 genes that represent each of the three groups, including sclerosing bone disorders, disorders of defective bone mineralization and disorder of bone matrix and cartilage formation. We also review pathophysiology, manifestation and treatment for each disease. Advances in molecular genetics and basic sciences has led to accurate genetic diagnosis and novel effective therapeutic strategies for some diseases. For other diseases, the genetic basis and pathophysiology remain unclear. Further researches are therefore crucial to innovate ways to overcome diagnostic challenges and develop effective treatment options for these orphan diseases.

Craniofacial sutures: Signaling centres integrating mechanosensation, cell signaling, and cell differentiation

Cranial sutures are dynamic structures in which stem cell biology, bone formation, and mechanical forces interface, influencing the shape of the skull throughout development and beyond. Over the past decade, there has been significant progress in understanding mesenchymal stromal cell (MSC) differentiation in the context of suture development and genetic control of suture pathologies, such as craniosynostosis. More recently, the mechanosensory function of sutures and the influence of mechanical signals on craniofacial development have come to the forefront. There is currently a gap in understanding of how mechanical signals integrate with MSC differentiation and ossification to ensure appropriate bone development and mediate postnatal growth surrounding sutures. In this review, we discuss the role of mechanosensation in the context of cranial sutures, and how mechanical stimuli are converted to biochemical signals influencing bone growth, suture patency, and fusion through mediation of cell differentiation. We integrate key knowledge from other paradigms where mechanosensation forms a critical component, such as bone remodeling and orthodontic tooth movement. The current state of the field regarding genetic, cellular, and physiological mechanisms of mechanotransduction will be contextualized within suture biology.

Notch-Wnt signal crosstalk regulates proliferation and differentiation of osteoprogenitor cells during intramembranous bone healing

Adult bone regeneration is orchestrated by the precise actions of osteoprogenitor cells (OPCs). However, the mechanisms by which OPC proliferation and differentiation are linked and thereby regulated are yet to be defined. Here, we present evidence that during intramembranous bone formation OPC proliferation is controlled by Notch signaling, while differentiation is initiated by activation of canonical Wnt signaling. The temporospatial separation of Notch and Wnt signal activation during the early stages of bone regeneration suggests crosstalk between the two pathways. In vitro and in vivo manipulation of the two essential pathways demonstrate that Wnt activation leads to initiation of osteogenic differentiation and at the same time inhibits Notch signaling, which results in termination of the proliferative phase. Here, we establish canonical Wnt signaling as a key regulator that facilitates the crosstalk between OPC proliferation and differentiation during intramembranous, primary bone healing.

Signaling Pathways in Bone Development and Their Related Skeletal Dysplasia

Bone development is a tightly regulated process. Several integrated signaling pathways including HH, PTHrP, WNT, NOTCH, TGF-β, BMP, FGF and the transcription factors SOX9, RUNX2 and OSX are essential for proper skeletal development. Misregulation of these signaling pathways can cause a large spectrum of congenital conditions categorized as skeletal dysplasia. Since the signaling pathways involved in skeletal dysplasia interact at multiple levels and have a different role depending on the time of action (early or late in chondrogenesis and osteoblastogenesis), it is still difficult to precisely explain the physiopathological mechanisms of skeletal disorders. However, in recent years, significant progress has been made in elucidating the mechanisms of these signaling pathways and genotype-phenotype correlations have helped to elucidate their role in skeletogenesis. Here, we review the principal signaling pathways involved in bone development and their associated skeletal dysplasia.

High bone mass from mutation of low-density lipoprotein receptor-related protein 6 (LRP6)

Wnt/β-catenin signaling is important for skeletal development and health. Eleven heterozygous gain-of-function missense mutations within the first β-propeller of low-density lipoprotein receptor-related protein 5 (LRP5) are known to cause the autosomal dominant disorder called high bone mass (HBM). In 2019, different heterozygous LRP6 missense mutations were identified in two American families with the HBM phenotype but including absent lateral maxillary and mandibular incisors. We report a 19-year-old Argentinian man referred for "osteopetrosis" and nine years of generalized, medium-intensity bone pain and arthralgias of both knees. His jaw and nasal bridge were broad and several teeth were missing. Routine biochemical testing, including of mineral homeostasis, was normal. Urinary deoxypyridinoline and serum CTX were slightly increased. Radiographic skeletal survey showed diffusely increased radiodensity. DXA revealed substantially elevated BMD Z-scores. Digital orthopantomography confirmed agenesis of his maxillary and mandibular lateral incisors and his second left superior premolar. Cranial magnetic resonance imaging showed diffuse thickening of the calvarium and skull base, dilation of the sheath of the optic nerves containing increased fluid and associated with subtle stenosis of the optic canal, and narrow internal auditory canals. Mutation analyses identified a heterozygous indel mutation in exon 4 of LRP6 involving a single nucleotide change and 6-nucleotide deletion (c.678T>Adel679-684, p.His226Gln-del227-228ProPhe) leading to a missense change and 2-amino acid deletion that would compromise the first β-propeller of LRP6. Experience to date indicates LRP6 HBM is indistinguishable from LRP5 HBM without mutation analysis, although in LRP6 HBM absence of adult lateral incisors may prove to be a unique feature.

The Genetic Architecture of High Bone Mass

The phenotypic trait of high bone mass (HBM) is an excellent example of the nexus between common and rare disease genetics. HBM may arise from carriage of many 'high bone mineral density [BMD]'-associated alleles, and certainly the genetic architecture of individuals with HBM is enriched with high BMD variants identified through genome-wide association studies of BMD. HBM may also arise as a monogenic skeletal disorder, due to abnormalities in bone formation, bone resorption, and/or bone turnover. Individuals with monogenic disorders of HBM usually, though not invariably, have other skeletal abnormalities (such as mandible enlargement) and thus are best regarded as having a skeletal dysplasia rather than just isolated high BMD. A binary etiological division of HBM into polygenic vs. monogenic, however, would be excessively simplistic: the phenotype of individuals carrying rare variants of large effect can still be modified by their common variant polygenic background, and by the environment. HBM disorders-whether predominantly polygenic or monogenic in origin-are not only interesting clinically and genetically: they provide insights into bone processes that can be exploited therapeutically, with benefits both for individuals with these rare bone disorders and importantly for the many people affected by the commonest bone disease worldwide-i.e., osteoporosis. In this review we detail the genetic architecture of HBM; we provide a conceptual framework for considering HBM in the clinical context; and we discuss monogenic and polygenic causes of HBM with particular emphasis on anabolic causes of HBM.

Aberrantly activated Wnt/β-catenin pathway co-receptors LRP5 and LRP6 regulate osteoblast differentiation in the developing coronal sutures of an Apert syndrome (Fgfr2S252W /+ ) mouse model

BACKGROUND

Apert syndrome is an autosomal, dominant inherited disorder characterized by craniosynostosis and syndactyly caused by gain-of-function mutations in the fibroblast growth factor receptor 2 (FGFR2) gene. Wnt/β-catenin signaling plays critical roles in regulating the skeletal development. Here, we analyzed the role of this pathway in the developing coronal sutures (CS) of a murine Apert syndrome model (Fgfr2^S252W/+ ).

RESULTS

We observed aberrantly increased mRNA expression of Lrp5 and Lrp6 in CS of Fgfr2^S252W/+ mice, whereas both wild type (WT) and Fgfr2^S252W/+ mice showed similar expression of other Wnt/β-catenin-related genes, such as Wnt3, Wnt3a, Fzd4, Fzd6, Axin2, and Dkk1 as evidenced by in situ hybridization. Significantly increased Lrp5 and Lrp6 mRNA expression was observed by quantitative PCR analysis of cultured cells isolated from CS of Fgfr2^S252W/+ mice. Phospho-LRP5, phospho-LRP6, and non-phospho-β-catenin were upregulated in Fgfr2^S252W/+ CS compared with that in WT CS. Short-interfering RNA targeting Lrp5 and Lrp6 significantly reduced runt-related transcription factor 2, collagen type 1 alpha 1, and osteocalcin mRNA expression, and alkaline phosphatase activity in cultured cells.

CONCLUSIONS

The Wnt/β-catenin pathway was activated in the CS of Fgfr2^S252W/+ mice during craniofacial development, suggesting the involvement of the Wnt/β-catenin pathway in the pathogenesis of CS synostosis in Fgfr2^S252W/+ mice.

WNT Signaling and Bone: Lessons From Skeletal Dysplasias and Disorders

Skeletal dysplasias are a diverse group of heritable diseases affecting bone and cartilage growth. Throughout the years, the molecular defect underlying many of the diseases has been identified. These identifications led to novel insights in the mechanisms regulating bone and cartilage growth and homeostasis. One of the pathways that is clearly important during skeletal development and bone homeostasis is the Wingless and int-1 (WNT) signaling pathway. So far, three different WNT signaling pathways have been described, which are all activated by binding of the WNT ligands to the Frizzled (FZD) receptors. In this review, we discuss the skeletal disorders that are included in the latest nosology of skeletal disorders and that are caused by genetic defects involving the WNT signaling pathway. The number of skeletal disorders caused by defects in WNT signaling genes and the clinical phenotype associated with these disorders illustrate the importance of the WNT signaling pathway during skeletal development as well as later on in life to maintain bone mass. The knowledge gained through the identification of the genes underlying these monogenic conditions is used for the identification of novel therapeutic targets. For example, the genes underlying disorders with altered bone mass are all involved in the canonical WNT signaling pathway. Consequently, targeting this pathway is one of the major strategies to increase bone mass in patients with osteoporosis. In addition to increasing the insights in the pathways regulating skeletal development and bone homeostasis, knowledge of rare skeletal dysplasias can also be used to predict possible adverse effects of these novel drug targets. Therefore, this review gives an overview of the skeletal and extra-skeletal phenotype of the different skeletal disorders linked to the WNT signaling pathway.

New explanation for autosomal dominant high bone mass: Mutation of low-density lipoprotein receptor-related protein 6

LRP5 encodes low-density lipoprotein receptor-related protein 5 (LRP5). When LRP5 with a Frizzled receptor join on the surface of an osteoblast and bind a member of the Wnt family of ligands, canonical Wnt/β-catenin signaling occurs and increases bone formation. Eleven heterozygous gain-of-function missense mutations within LRP5 are known to prevent the LRP5 inhibitory ligands sclerostin and dickkopf1 from attaching to LRP5's first β-propeller, and thereby explain the rare autosomal dominant (AD) skeletal disorder "high bone mass" (HBM). LRP6 is a cognate co-receptor of LRP5 and similarly controls Wnt signaling in osteoblasts, yet the consequences of increased LRP6-mediated signaling remain unknown. We investigated two multi-generational American families manifesting the clinical and routine laboratory features of LRP5 HBM but without an LRP5 defect and instead carrying a heterozygous LRP6 missense mutation that would alter the first β-propeller of LRP6. In Family 1 LRP6 c.602C>T, p.A201V was homologous to LRP5 HBM mutation c.641C>T, p.A214V, and in Family 2 LRP6 c.553A>C, p.N185H was homologous to LRP5 HBM mutation c.593A>G, p.N198S but predicting a different residue at the identical amino acid position. In both families the LRP6 mutation co-segregated with striking generalized osteosclerosis and hyperostosis. Clinical features shared by the seven LRP6 HBM family members and ten LRP5 HBM patients included a broad jaw, torus palatinus, teeth encased in bone and, reportedly, resistance to fracturing and inability to float in water. For both HBM disorders, all affected individuals were taller than average for Americans (Ps < 0.005), but with similar mean height Z-scores (P = 0.7606) and indistinguishable radiographic skeletal features. Absence of adult maxillary lateral incisors was reported by some LRP6 HBM individuals. In contrast, our 16 patients with AD osteopetrosis [i.e., Albers-Schönberg disease (A-SD)] had an unremarkable mean height Z-score (P = 0.9401) lower than for either HBM group (Ps < 0.05). DXA mean BMD Z-scores in LRP6 HBM versus LRP5 HBM were somewhat higher at the lumbar spine (+7.8 vs +6.5, respectively; P = 0.0403), but no different at the total hip (+7.9 vs +7.7, respectively; P = 0.7905). Among the three diagnostic groups, only the LRP6 HBM DXA BMD values at the spine seemed to increase with subject age (R = +0.7183, P = 0.0448). Total hip BMD Z-scores were not significantly different among the three disorders (Ps > 0.05), and showed no age effect (Ps > 0.1). HR-pQCT available only for LRP6 HBM revealed indistinct corticomedullary boundaries, high distal forearm and tibial total volumetric BMD, and finite element analysis predicted marked fracture resistance. Hence, we have discovered mutations of LRP6 that cause a dento-osseous disorder indistinguishable without mutation analysis from LRP5 HBM. LRP6 HBM seems associated with generally good health, providing some reassurance for the development of anabolic treatments aimed to enhance LRP5/LRP6-mediated osteogenesis.

Genetic Causes of Craniosynostosis: An Update

In 1993, Jabs et al. were the first to describe a genetic origin of craniosynostosis. Since this discovery, the genetic causes of the most common syndromes have been described. In 2015, a total of 57 human genes were reported for which there had been evidence that mutations were causally related to craniosynostosis. Facilitated by rapid technological developments, many others have been identified since then. Reviewing the literature, we characterize the most common craniosynostosis syndromes followed by a description of the novel causes that were identified between January 2015 and December 2017.

Mutations in Known Monogenic High Bone Mass Loci Only Explain a Small Proportion of High Bone Mass Cases

High bone mass (HBM) can be an incidental clinical finding; however, monogenic HBM disorders (eg, LRP5 or SOST mutations) are rare. We aimed to determine to what extent HBM is explained by mutations in known HBM genes. A total of 258 unrelated HBM cases were identified from a review of 335,115 DXA scans from 13 UK centers. Cases were assessed clinically and underwent sequencing of known anabolic HBM loci: LRP5 (exons 2, 3, 4), LRP4 (exons 25, 26), SOST (exons 1, 2, and the van Buchem's disease [VBD] 52-kb intronic deletion 3'). Family members were assessed for HBM segregation with identified variants. Three-dimensional protein models were constructed for identified variants. Two novel missense LRP5 HBM mutations ([c.518C>T; p.Thr173Met], [c.796C>T; p.Arg266Cys]) were identified, plus three previously reported missense LRP5 mutations ([c.593A>G; p.Asn198Ser], [c.724G>A; p.Ala242Thr], [c.266A>G; p.Gln89Arg]), associated with HBM in 11 adults from seven families. Individuals with LRP5 HBM (∼prevalence 5/100,000) displayed a variable phenotype of skeletal dysplasia with increased trabecular BMD and cortical thickness on HRpQCT, and gynoid fat mass accumulation on DXA, compared with both non-LRP5 HBM and controls. One mostly asymptomatic woman carried a novel heterozygous nonsense SOST mutation (c.530C>A; p.Ser177X) predicted to prematurely truncate sclerostin. Protein modeling suggests the severity of the LRP5-HBM phenotype corresponds to the degree of protein disruption and the consequent effect on SOST-LRP5 binding. We predict p.Asn198Ser and p.Ala242Thr directly disrupt SOST binding; both correspond to severe HBM phenotypes (BMD Z-scores +3.1 to +12.2, inability to float). Less disruptive structural alterations predicted from p.Arg266Cys, p.Thr173Met, and p.Gln89Arg were associated with less severe phenotypes (Z-scores +2.4 to +6.2, ability to float). In conclusion, although mutations in known HBM loci may be asymptomatic, they only account for a very small proportion (∼3%) of HBM individuals, suggesting the great majority are explained by either unknown monogenic causes or polygenic inheritance.

The Wnt Co-Receptor Lrp5 Is Required for Cranial Neural Crest Cell Migration in Zebrafish

During vertebrate neurulation, cranial neural crest cells (CNCCs) undergo epithelial to mesenchymal transition (EMT), delaminate from the neural plate border, and migrate as separate streams into different cranial regions. There, they differentiate into distinct parts of the craniofacial skeleton. Canonical Wnt signaling has been shown to be essential for this process at different levels but the involved receptors remained unclear. Here we show that the frizzled co-receptor low-density-lipoprotein (LDL) receptor-related protein 5 (Lrp5) plays a crucial role in CNCC migration and morphogenesis of the cranial skeleton. Early during induction and migration of CNCCs, lrp5 is expressed ubiquitously but later gets restricted to CNCC derivatives in the ventral head region besides different regions in the CNS. A knock-down of lrp5 does not interfere with induction of CNCCs but leads to reduced proliferation of premigratory CNCCs. In addition, cell migration is disrupted as CNCCs are found in clusters at ectopic positions in the dorsomedial neuroepithelium after lrp5 knock-down and transient CRISPR/Cas9 gene editing. These migratory defects consequently result in malformations of the craniofacial skeleton. To date, Lrp5 has mainly been associated with bone homeostasis in mammals. Here we show that in zebrafish, lrp5 also controls cell migration during early morphogenetic processes and contributes to shaping the craniofacial skeleton.

Canonical Wnt signalling activates TAZ through PP1A during osteogenic differentiation

TAZ, a transcriptional modulator, has a key role in cell proliferation, differentiation and stem cell self-renewal. TAZ activity is regulated by several signalling pathways, including Hippo, GPCR and Wnt signalling, but the regulatory mechanisms of TAZ activation are not yet clearly understood. In this report, we show that TAZ is regulated by canonical Wnt signalling during osteogenic differentiation. Wnt3a increases TAZ expression and an inhibitor of GSK3β, a downstream effector of Wnt signalling, induces TAZ. Wnt3a facilitates the dephosphorylation of TAZ, which stabilises TAZ and prevents it from binding 14-3-3 proteins, thus inducing the nuclear localisation of TAZ. Dephosphorylation of TAZ occurs via PP1A, and depletion of PP1A blocks Wnt3a-induced TAZ stabilisation. Wnt3a-induced TAZ activates osteoblastic differentiation and siRNA-induced TAZ depletion decreases Wnt3a-induced osteoblast differentiation. Taken together, these results show that TAZ mediates Wnt3a-stimulated osteogenic differentiation through PP1A, suggesting that the Wnt signal regulates the Hippo pathway.

Friend or foe: high bone mineral density on routine bone density scanning, a review of causes and management

A finding of high BMD on routine DXA scanning is not infrequent and most commonly reflects degenerative disease. However, BMD increases may also arise secondary to a range of underlying disorders affecting the skeleton. Although low BMD increases fracture risk, the converse may not hold for high BMD, since elevated BMD may occur in conditions where fracture risk is increased, unaffected or reduced. Here we outline a classification for the causes of raised BMD, based on identification of focal or generalized BMD changes, and discuss an approach to guide appropriate investigation by clinicians after careful interpretation of DXA scan findings within the context of the clinical history. We will also review the mild skeletal dysplasia associated with the currently unexplained high bone mass phenotype and discuss recent advances in osteoporosis therapies arising from improved understanding of rare inherited high BMD disorders.

The effect of a Beare-Stevenson syndrome Fgfr2 Y394C mutation on early craniofacial bone volume and relative bone mineral density in mice

Quantifying the craniofacial skeletal phenotype during development highlights potential effects of known mutations on bone maturation and is an informative first step for the analysis of animal models. We introduce a novel technique to easily and efficiently quantify individual cranial bone volume and relative bone mineral density across the murine skull from high resolution computed tomography images. The approach can be combined with existing quantitative morphometric methods to provide details of bone growth and bone quality, which can be used to make inferences about regulatory effects local to individual bones and identify locations and developmental times for which additional analyses are warranted. Analysis of the Fgfr2(+/Y394C) mouse model of Beare-Stevenson cutis gyrata syndrome, an FGFR-related craniosynostosis syndrome, is used to demonstrate the method. Mutants and unaffected littermates display similar bone volume and relative bone density at birth, followed by significant differences at postnatal day eight. The change in rates of bone volume growth occurs similarly for all bones of the skull, regardless of origin, location or association with craniosynostosis. These results suggest an association between low bone density, low bone volume, and Fgfr craniosynostosis mutations. Our novel technique provides an initial quantitative evaluation of local shifts in bone maturation across the skull of animal models.

Wnt5a can both activate and repress Wnt/β-catenin signaling during mouse embryonic development

Embryonic development is controlled by a small set of signal transduction pathways, with vastly different phenotypic outcomes depending on the time and place of their recruitment. How the same molecular machinery can elicit such specific and distinct responses, remains one of the outstanding questions in developmental biology. Part of the answer may lie in the high inherent genetic complexity of these signaling cascades, as observed for the Wnt-pathway. The mammalian genome encodes multiple Wnt proteins and receptors, each of which show dynamic and tightly controlled expression patterns in the embryo. Yet how these components interact in the context of the whole organism remains unknown. Here we report the generation of a novel, inducible transgenic mouse model that allows spatiotemporal control over the expression of Wnt5a, a protein implicated in many developmental processes and multiple Wnt-signaling responses. We show that ectopic Wnt5a expression from E10.5 onwards results in a variety of developmental defects, including loss of hair follicles and reduced bone formation in the skull. Moreover, we find that Wnt5a can have dual signaling activities during mouse embryonic development. Specifically, Wnt5a is capable of both inducing and repressing β-catenin/TCF signaling in vivo, depending on the time and site of expression and the receptors expressed by receiving cells. These experiments show for the first time that a single mammalian Wnt protein can have multiple signaling activities in vivo, thereby furthering our understanding of how signaling specificity is achieved in a complex developmental context.

Genetic analysis and effect of triiodothyronine and prednisone trial on bone turnover in a patient with craniotubular hyperostosis

Craniotubular hyperostosis are a group of high bone mass disorders related to mutations in the LRP5 and SOST genes, although other causative genes remain to be identified. Little is known about the bone turnover and the response to T3 or glucocorticoids in these patients. We describe a patient with craniotubular hyperostosis, including mutation analyses of the LRP5, SOST, DKK1 and KRM1 genes. We also studied bone turnover and bone mineral density (BMD), before and after a trial with T3 (75 microg/d for 28 weeks) and T3 and prednisone (T3 100 microg/d for 2 weeks, followed by 10 weeks on prednisone 10 mg/d, and a final 2 weeks period off of medicactions, completing 3 cycles in 42 weeks. Mutation analysis of the complete coding region and flanking highly conserved sequences of SOST, evaluation of the presence of the 52-kb deletion associated with Van Buchem disease in Dutch patients and mutation analysis of exons 2-4 of LRP5, and the coding regions of DKK1 and KRM1 did not reveal any disease-causing mutations. A baseline 5 to 7 fold increase in osteocalcin and in deoxypiridinoline was detected. After 4 weeks on 75 microg/d of T3, osteocalcin decreased 36%, but at week 28, it returned to basal. Deoxypiridinoline did not change. After the first cycle on T3 and prednisone, osteocalcin decreased 72%, and at the end of the third cycle it remained 44% below basal value. Deoxypiridinoline was stable and high during the three cycles; no changes in BMD were observed. As we failed to identify any disease-causing mutations in our patient with craniotubular hyperostosis, we suggest that another gene must be involved in the pathogenesis of his condition. This study provides additional data about the high bone turnover described in craniotubular hyperostosis, and also suggests an abnormal response to T3 excess in this condition.

The binding between sclerostin and LRP5 is altered by DKK1 and by high-bone mass LRP5 mutations

Low-density lipoprotein receptor-related protein 5 (LRP5), a Wnt coreceptor, plays an important role in bone metabolism as loss-of-function and gain-of-function mutations in LRP5 result in the autosomal recessive osteoporosis-pseudoglioma syndrome and autosomal dominant high-bone mass (HBM) phenotypes, respectively. Prior studies suggested that the presence of HBM-associated LRP5 mutations results in decreased antagonism of LRP5-mediated Wnt signaling. In the present study, we investigated six different HBM-LRP5 mutations and confirm that neither Dickkopf1 (DKK1) nor sclerostin efficiently inhibits HBM-LRP5 signaling. In addition, when coexpressed, DKK1 and sclerostin do not inhibit HBM-LRP5 mutants better than either inhibitor by itself. Also, DKK1 and sclerostin do not simultaneously bind to wild-type LRP5, and DKK1 is able to displace sclerostin from previously formed sclerostin-LRP5 complexes. In conclusion, our results indicate that DKK1 and sclerostin are independent, and not synergistic, regulators of LRP5 signaling and that the function of each is impaired by HBM-LRP5 mutations.

Bone regeneration is regulated by wnt signaling

Tissue regeneration is increasingly viewed as reactivation of a developmental process that, when misappropriated, can lead to malignant growth. Therefore, understanding the molecular and cellular pathways that govern tissue regeneration provides a glimpse into normal development as well as insights into pathological conditions such as cancer. Herein, we studied the role of Wnt signaling in skeletal tissue regeneration.

INTRODUCTION

Some adult tissues have the ability to regenerate, and among these, bone is one of the most remarkable. Bone exhibits a persistent, lifelong capacity to reform after injury, and continual bone regeneration is a prerequisite to maintaining bone mass and density. Even slight perturbations in bone regeneration can have profound consequences, as exemplified by conditions such as osteoporosis and delayed skeletal repair. Here, our goal was to determine the role of Wnts in adult bone regeneration.

MATERIALS AND METHODS

Using TOPgal reporter mice, we found that damage to the skeleton instigated Wnt reporter activity, specifically at the site of injury. We used a skeletal injury model to show that Wnt inhibition, achieved through adenoviral expression of Dkk1 in the adult skeleton, prevented the differentiation of osteoprogenitor cells.

RESULTS

As a result, injury-induced bone regeneration was reduced by 84% compared with controls. Constitutive activation of the Wnt pathway resulting from a mutation in the Lrp5 Wnt co-receptor results in high bone mass, but our experiments showed that this same point mutation caused a delay in bone regeneration. In these transgenic mice, osteoprogenitor cells in the injury site were maintained in a proliferative state and differentiation into osteoblasts was delayed.

CONCLUSIONS

When considered together, these data provide a framework for understanding the roles of Wnt signaling in adult bone regeneration and suggest a feasible approach to treating clinical conditions where enhanced bone formation is desired.

Wnt10b increases postnatal bone formation by enhancing osteoblast differentiation

Overexpression of Wnt10b from the osteocalcin promoter in transgenic mice increases postnatal bone mass. Increases in osteoblast perimeter, mineralizing surface, and bone formation rate without detectable changes in pre-osteoblast proliferation, osteoblast apoptosis, or osteoclast number and activity suggest that, in this animal model, Wnt10b primarily increases bone mass by stimulating osteoblastogenesis.

INTRODUCTION

Wnt signaling regulates many aspects of development including postnatal accrual of bone. Potential mechanisms for how Wnt signaling increases bone mass include regulation of osteoblast and/or osteoclast number and activity. To help differentiate between these possibilities, we studied mice in which Wnt10b is expressed specifically in osteoblast lineage cells or in mice devoid of Wnt10b.

MATERIALS AND METHODS

Transgenic mice, in which mouse Wnt10b is expressed from the human osteocalcin promoter (Oc-Wnt10b), were generated in C57BL/6 mice. Transgene expression was evaluated by RNase protection assay. Quantitative assessment of bone variables was done by radiography, muCT, and static and dynamic histomorphometry. Mechanisms of bone homeostasis were evaluated with assays for BrdU, TUNEL, and TRACP5b activity, as well as serum levels of C-terminal telopeptide of type I collagen (CTX). The endogenous role of Wnt10b in bone was assessed by dynamic histomorphometry in Wnt10b(-/-) mice.

RESULTS

Oc-Wnt10b mice have increased mandibular bone and impaired eruption of incisors during postnatal development. Analyses of femoral distal metaphyses show significantly higher BMD, bone volume fraction, and trabecular number. Increased bone formation is caused by increases in number of osteoblasts per bone surface, rate of mineral apposition, and percent mineralizing surface. Although number of osteoclasts per bone surface is not altered, Oc-Wnt10b mice have increased total osteoclast activity because of higher bone mass. In Wnt10b(-/-) mice, changes in mineralizing variables and osteoblast perimeter in femoral distal metaphyses were not observed; however, bone formation rate is reduced because of decreased total bone volume and trabecular number.

CONCLUSIONS

High bone mass in Oc-Wnt10b mice is primarily caused by increased osteoblastogenesis, with a minor contribution from elevated mineralizing activity of osteoblasts.

An 11q11-q13.3 duplication, including FGF3 and FGF4 genes, in a patient with syndromic multiple craniosynostoses

Interstitial duplications of 11q are very rare and seldom reported. In this paper we describe the first case of a duplication involving bands 11q11 and 11q12. This newly described patient has multiple craniosynostoses, congenital heart defect and developmental delay, and is a carrier of a mosaic duplication: 46,XY,dup(11)(q11-->q13.3)(29)/46,XY(6). The breakpoints were further delimited by comparative genomic hybridization microarray. We also performed fluorescent in situ hybridization analysis to determine the extension of the duplication in a patient described earlier with a duplication 11q13.5-q21. An overlapping region of less than 1.2 Mb was identified and included the duplication of genes FGF3 and FGF4 in both individuals. We discuss the possible implications of dosage effects of these genes in the onset of craniosynostosis.

The genetics of low-density lipoprotein receptor-related protein 5 in bone: a story of extremes

A few years ago, human genetic studies provided compelling evidence that the low-density lipoprotein receptor-related protein 5 (LRP5) is involved in the regulation of bone homeostasis because pathogenic LRP5 mutations were found in monogenic conditions with abnormal bone density. On the one hand, the osteoporosis pseudoglioma syndrome results from loss of function of LRP5, whereas on the other hand, gain-of-function mutations in LRP5 cause conditions with an increased bone density. On the molecular level, these types of mutations result in disturbed (respectively, decreased and increased) canonical Wnt signaling, an important metabolic pathway in osteoblasts during embryonic and postnatal osteogenesis. This signaling cascade is activated by binding of Wnt ligand to the Frizzled/LRP5 receptor complex. In addition to the involvement of LRP5 in conditions with extreme bone phenotypes, the genetic profile of this gene has also been shown to contribute to the determination of bone density in the general population. Quite a number of studies already demonstrated that common polymorphic variants in LRP5 are associated with bone mineral density and consequently osteoporosis, a multifactorial trait with low bone mass and porous bone structure. These genetic studies together with results obtained from in vitro and in vivo studies emphasize the importance of LRP5 and canonical Wnt signaling in the regulation of bone homeostasis. Therefore, unraveling the exact mechanisms of this signaling cascade has become an important area in bone research. This review focuses on the genetics of LRP5 and summarizes the findings on monogenic bone conditions as well as the current knowledge of its involvement in the pathogenesis of osteoporosis.

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.

INTRODUCTION

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.

MATERIALS AND 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.

Structure-based mutation analysis shows the importance of LRP5 beta-propeller 1 in modulating Dkk1-mediated inhibition of Wnt signaling

A single point mutation (G to T) in the low-density lipoprotein receptor related protein 5 (LRP5) gene results in a glycine to valine amino acid change (G171V) and is responsible for an autosomal dominant high bone mass trait (HBM) in two independent kindreds. LRP5 acts as a co-receptor to Wnts with Frizzled family members and transduces Wnt-canonical signals which can be antagonized by LRP5 ligand, Dickkopf 1 (Dkk1). In the presence of Wnt1, LRP5 or the HBM variant (LRP5-G171V) induces beta-catenin nuclear translocation and activates T cell factor (TCF)-luciferase reporter activity. HBM variant suppresses Dkk1 function and this results in reduced inhibition of TCF activity as compared to that with LRP5. Structural analysis of LRP5 revealed that the HBM mutation lies in the 4th blade of the first beta-propeller domain. To elucidate the functional significance and consequence of the LRP5-G171V mutation in vitro, we took a structure-based approach to design 15 specific LRP5 point mutations. These included (a) substitutions at the G171 in blade 4, (b) mutations in blades 2-6 of beta-propeller 1, and (c) mutations in beta-propellers 2, 3 and 4. Here we show that substitutions of glycine at 171 to K, F, I and Q also resulted in HBM-like activity in the presence of Wnt1 and Dkk1. This indicates the importance of the G171 site rather than the effect of specific amino acid modification to LRP5 receptor function. Interestingly, G171 equivalent residue mutations in other blades of beta-propeller 1 (A65V, S127V, L200V, A214V and M282V) resulted in LRP5-G171V-like block of Dkk1 function. However G171V type mutations in other beta-propellers of LRP5 did not result in resistance to Dkk1 function. These results indicate the importance of LRP5 beta-propeller 1 for Dkk1 function and Wnt signaling. These data and additional comparative structural analysis of the LRP5 family member LDLR suggest a potential functional role of the first beta-propeller domain through intramolecular interaction with other domains of LRP5 wherein Dkk1 can bind. Such studies may also lead to a better understanding of the mechanisms underlying the reduced function of Dkk1-like inhibitory ligands of LRP5 with HBM-like mutations and its relationship to increased bone density phenotypes.

A family with osteoporosis pseudoglioma syndrome due to compound heterozygosity of two novel mutations in the LRP5 gene

Osteoporosis pseudoglioma syndrome (OPPG) is an autosomal recessive disorder due to mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene. Here, we report two novel missense mutations found in a southern Chinese family of a non-consanguineous marriage. Three out of four children had blindness, low bone mineral density (BMD) and multiple fractures in their childhood. Genotyping by DNA sequencing demonstrated 2 new mutations in exon 7 of the LRP5 gene. Tryptophans at amino acid residue positions 478 and 504 were replaced by arginine (W478R) and cysteine (W504C), respectively. While the parents that possessed either heterozygous W478R or W504C were apparently normal, all affected subjects were compound heterozygotes for the W478R and W504C mutations in the LRP5 gene. W478R is located immediately C-terminal to the third YWTD repeat of the second YWTD/EGF domain in LRP5, while W504C is located between the third and the fourth YWTD repeats of the second YWTD/EGF domain in LRP5. Using LRP5-related proteins, such as the low-density lipoprotein receptor (LDLR) and nidogen as reference models, a homology model of LRP5 suggested that the observed mutations may affect the molecular interactions of LRP5 and so lead to the observed OPPG phenotypes.

Molecular bases of the regulation of bone remodeling by the canonical Wnt signaling pathway

Osteoporosis is a common, prevalent, and debilitating condition, particularly in postmenopausal women. Genetics play a major role in determining peak bone mass and fracture risk, but few genes have been demonstrated conclusively to be involved, much less the signaling pathways with which they are affiliated. The identification of mutations in the gene Lrp5, a Wnt coreceptor, as the cause for both osteoporotic and high-bone mass disorders implicated the canonical Wnt signaling pathway in bone mass regulation. Since Lrp5, other Wnt components have been identified as being regulators of bone mass, and Wnt target genes affecting bone homeostasis have begun to be elucidated. This chapter looks at the various components of the canonical Wnt signaling pathway and the data indicating that this pathway plays a major role in the control of both bone formation and bone resorption, the two key aspects of bone remodeling.