Van Buchem disease: Clinical,biochemical, and densitometric features of patients and disease carriers

Van Buchem disease (VBD) is a rare bone sclerosing dysplasia caused by the lack of a regulatory element of the SOST gene, which encodes for sclerostin, an osteocyte‐derived negative regulator of bone formation. We studied the demographic, clinical, biochemical, and densitometric features of 15 patients with VBD (12 adults and 3 children) and 28 related carriers of the gene mutation. The most common clinical findings in patients were facial palsy (100%) and various degrees of hearing impairment (93%); raised intracranial pressure had been documented in 20%. The clinical course of the disease appeared to stabilize in adulthood, with the majority of patients reporting no progression of symptoms or development of complications with time. Carriers of the disease had none of the clinical features or complications of the disease. Sclerostin could be detected in the serum in all but 1 VBD patients (mean 8.0 pg/mL; 95% confidence interval [CI], 4.9–11.0 pg/mL), and were lower than those of carriers (mean 28.7 pg/mL; 95% CI, 24.5–32.9 pg/mL; p < 0.001) and healthy controls (mean 40.0 pg/mL; 95% CI, 34.5–41.0 pg/mL; p < 0.). Serum procollagen type 1 amino‐terminal propeptide (P1NP) levels were also significantly higher in adult patients (mean 96.0; 95% CI, 54.6–137.4 ng/mL versus mean 47.8; 95% CI, 39.4–56.2 ng/mL, p = 0.003 in carriers and mean 37.8; 95% CI, 34.5–41.0 ng/mL, p = 0.028 in healthy controls) and declined with age. Bone mineral density (BMD) was markedly increased in all patients (mean Z‐score 8.7 ± 2.1 and 9.5 ± 1.9 at the femoral neck and spine, respectively); BMD of carriers was significantly lower than that of patients but varied widely (mean Z‐scores 0.9 ± 1.0 and 1.3 ± 1.5 at the femoral neck and spine, respectively). Serum sclerostin levels were inversely correlated with serum P1NP levels (r = –0.39, p = 0.018) and BMD values (femoral neck r = –0.69, p < 0.001; lumbar spine r = –0.78, p < 0.001). Our results show that there is a gene‐dose effect of the VBD deletion on circulating sclerostin and provide further in vivo evidence of the role of sclerostin in bone formation in humans. The small amounts of sclerostin produced by patients with VBD may explain their milder phenotype compared to that of patients with sclerosteosis, in whom serum sclerostin is undetectable. © 2013 American Society for Bone and Mineral Research.     

Methylation of Bone SOST,Its mRNA,and Serum Sclerostin Levels Correlate Strongly With Fracture Risk in Postmenopausal Women

Inhibition of sclerostin, a glycoprotein secreted by osteocytes, offers a new therapeutic paradigm for treatment of osteoporosis (OP) through its critical role as Wnt/catenin signaling regulator. This study describes the epigenetic regulation of SOST expression in bone biopsies of postmenopausal women. We correlated serum sclerostin to bone mineral density (BMD), fractures, and bone remodeling parameters, and related these findings to epigenetic and genetic disease mechanisms. Serum sclerostin and bone remodeling biomarkers were measured in two postmenopausal groups: healthy (BMD T‐score > –1) and established OP (BMD T‐score < –2.5, with at least one low‐energy fracture). Bone specimens were used to analyze SOST mRNAs, single nucleotide polymorphisms (SNPs), and DNA methylation changes. The SOST gene promoter region showed increased CpG methylation in OP patients (n = 4) compared to age and body mass index (BMI) balanced controls (n = 4) (80.5% versus 63.2%, p = 0.0001) with replication in independent cohorts (n = 27 and n = 36, respectively). Serum sclerostin and bone SOST mRNA expression correlated positively with age‐adjusted and BMI‐adjusted total hip BMD (r = 0.47 and r = 0.43, respectively; both p < 0.0005), and inversely to serum bone turnover markers. Five SNPs, one of which replicates in an independent population‐based genomewide association study (GWAS), showed association with serum sclerostin or SOST mRNA levels under an additive model (p = 0.0016 to 0.0079). Genetic and epigenetic changes in SOST influence its bone mRNA expression and serum sclerostin levels in postmenopausal women. The observations suggest that increased SOST promoter methylation seen in OP is a compensatory counteracting mechanism, which lowers serum sclerostin concentrations and reduces inhibition of Wnt signaling in an attempt to promote bone formation. © 2014 American Society for Bone and Mineral Research.     

Sclerostin antibody inhibits skeletal deterioration due to reduced mechanical loading

Sclerostin, a product of the SOST gene produced mainly by osteocytes, is a potent negative regulator of bone formation that appears to be responsive to mechanical loading, with SOST expression increasing following mechanical unloading. We tested the ability of a murine sclerostin antibody (SclAbII) to prevent bone loss in adult mice subjected to hindlimb unloading (HLU) via tail suspension for 21 days. Mice (n = 11–17/group) were assigned to control (CON, normal weight bearing) or HLU and injected with either SclAbII (subcutaneously, 25 mg/kg) or vehicle (VEH) twice weekly. SclAbII completely inhibited the bone deterioration due to disuse, and induced bone formation such that bone properties in HLU‐SclAbII were at or above values of CON‐VEH mice. For example, hindlimb bone mineral density (BMD) decreased –9.2% ± 1.0% in HLU‐VEH, whereas it increased 4.2% ± 0.7%, 13.1% ± 1.0%, and 30.6% ± 3.0% in CON‐VEH, HLU‐SclAbII, and CON‐SclAbII, respectively (p < 0.0001). Trabecular bone volume, assessed by micro–computed tomography (µCT) imaging of the distal femur, was lower in HLU‐VEH versus CON‐VEH (p < 0.05), and was 2‐ to 3‐fold higher in SclAbII groups versus VEH (p < 0.001). Midshaft femoral strength, assessed by three‐point bending, and distal femoral strength, assessed by micro–finite element analysis (µFEA), were significantly higher in SclAbII versus VEH‐groups in both loading conditions. Serum sclerostin was higher in HLU‐VEH (134 ± 5 pg/mL) compared to CON‐VEH (116 ± 6 pg/mL, p < 0.05). Serum osteocalcin was decreased by hindlimb suspension and increased by SclAbII treatment. Interestingly, the anabolic effects of sclerostin inhibition on some bone outcomes appeared to be enhanced by normal mechanical loading. Altogether, these results confirm the ability of SclAbII to abrogate disuse‐induced bone loss and demonstrate that sclerostin antibody treatment increases bone mass by increasing bone formation in both normally loaded and underloaded environments. © 2013 American Society for Bone and Mineral Research.     

Sclerostin Expression in Bile Ducts of Patients With Chronic Cholestasis May Influence the Bone Disease in Primary Biliary Cirrhosis

Sclerostin is involved in the regulation of osteoblastogenesis and little is known about its role in the development of bone disease in primary biliary cirrhosis (PBC), characterized by low bone formation. Therefore, we have assessed the circulating levels and the liver expression of sclerostin in this cholestatic disease. Serum sclerostin levels were measured in 79 women with PBC (mean age 60.6 ± 1.2 years) and in 80 control women. Lumbar and femoral bone mineral density (BMD), as well as parameters of mineral metabolism and bone remodeling, were measured. Moreover, sclerostin gene (SOST) expression in the liver was assessed by real‐time PCR in samples of liver tissue taken by biopsy in 11 PBC patients and in 5 normal liver specimens. Presence and distribution of sclerostin was evaluated in liver slices from 11 patients by immunohistochemistry. The severity of histologic lesions was assessed semiquantitatively in the same liver samples. PBC patients had higher sclerostin levels than controls (75.6 ± 3.9 versus 31.7 ± 1.6 pmol/L, p < 0.001). Serum sclerostin correlated inversely with markers of bone formation and resorption. Sclerostin mRNA in the liver was overexpressed compared with control samples (2.7‐fold versus healthy liver). Sclerostin was detected by immunohistochemistry in 7 of the 11 liver samples, mainly located in the bile ducts. Liver sclerostin was associated with the severity of cholangitis (p = 0.02) and indirectly with the degree of lobular inflammation (p = 0.03). Sclerostin mRNA expression was higher in samples that tested positive by immunohistochemistry and particularly in those with lobular granuloma (p = 0.02). The increased expression of sclerostin in the liver and the association with histologic cholangitis may explain the high serum levels of this protein in patients with PBC, thus suggesting that sclerostin may influence the decreased bone formation in this cholestatic disease. © 2016 American Society for Bone and Mineral Research.     

Sclerostin Deficiency Is Linked to Altered Bone Composition

High bone mass in animals and humans with sclerostin deficiency is associated with increased bone strength, which is not the case for all disorders with high bone mineral density, some of which are even associated with fragility fractures owing to unfavorable bone composition. In the current study we investigated whether alterations in bone composition may contribute to the bone strength characteristics associated with lack of sclerostin. We examined cortical bone of Sost‐knockout (KO) mice (n = 9, 16 weeks old) and sclerosteosis patients (young [4 to 14 years], n = 4 and adults [24 and 43 years], n = 2) by quantitative backscattered electron imaging and Raman microspectroscopy and compared it to bone from wild‐type mice and healthy subjects, respectively. In Sost‐KO mice endocortical bone exhibited altered bone composition, whereas subperiosteal bone was unchanged. When comparing endocortical bone tissue of identical tissue age as defined by sequential dual fluorochrome labeling the average bone matrix mineralization was reduced ?1.9% (p < 0.0001, younger tissue age) and ?1.5% (p < 0.05, older tissue age), and the relative proteoglycan content was significantly increased. Similarly, bone matrix mineralization density distribution was also shifted toward lower matrix mineralization in surgical samples of compact bone of sclerosteosis patients. This was associated with an increase in mineralization heterogeneity in the young population. In addition, and consistently, the relative proteoglycan content was increased. In conclusion, we observed decreased matrix mineralization and increased relative proteoglycan content in bone subcompartments of Sost‐KO mice—a finding that translated into sclerosteosis patients. We hypothesize that the altered bone composition contributes to the increased bone strength of patients with sclerostin deficiency. © 2014 American Society for Bone and Mineral Research.     

Levels of serotonin,sclerostin, bone turnover markers as well as bone density and microarchitecture in patients with high‐bone‐mass phenotype due to a mutation in Lrp5

Patients with an activation mutation of the Lrp5 gene exhibit high bone mass (HBM). Limited information is available regarding compartment‐specific changes in bone. The relationship between the phenotype and serum serotonin is not well documented. To evaluate bone, serotonin, and bone turnover markers (BTM) in Lrp5‐HBM patients, we studied 19 Lrp5‐HBM patients (T253I) and 19 age‐ and sex‐matched controls. DXA and HR‐pQCT were used to assess BMD and bone structure. Serum serotonin, sclerostin, dickkopf‐related protein 1 (DKK1), and BTM were evaluated. Z‐scores for the forearm, total hip, lumbar spine, forearm, and whole body were significantly increased (mean ± SD) between 4.94 ± 1.45 and 7.52 ± 1.99 in cases versus ?0.19 ± 1.19 to 0.58 ± 0.84 in controls. Tibial and radial cortical areas, thicknesses, and BMD were significantly higher in cases. In cases, BMD at the lumbar spine and forearm and cortical thickness were positively associated and trabecular area negatively associated with age (r = 0.49, 0.57, 0.74, and ?0.61, respectively, p < .05). Serotonin was lowest in cases (69.5 [29.9–110.4] ng/mL versus 119.4 [62.3–231.0] ng/mL, p < .001) and inversely associated with tibial cortical density (r = ?0.49, p < .05) and directly with osteocalcin (OC), bone‐specific alkaline phosphatase (B‐ALP), and procollagen type 1 amino‐terminal propeptide (PINP) (r = 0.52–0.65, p < .05) in controls only. OC and S‐CTX were lower and sclerostin higher in cases, whereas B‐ALP, PINP, tartrate‐resistant acid phosphatase (TRAP), and dickkopf‐related protein 1 (DKK1) were similar in cases and controls. In conclusion, increased bone mass in Lrp5‐HBM patients seems to be caused primarily by changes in trabecular and cortical bone mass and structure. The phenotype appeared to progress with age, but BTM did not suggest increased bone formation. © 2011 American Society for Bone and Mineral Research     

Bone Mineral Density and Parathyroid Hormone as Independent Risk Factors for Mortality in Community‐Dwelling Older Adults: A Population‐Based Prospective Cohort Study in Brazil. The São Paulo Ageing & Health (SPAH) Study

Previous studies have shown a relationship between osteoporosis and increased mortality risk. However, none of these studies performed a concomitant evaluation of the parathyroid hormone (PTH)‐calcium‐vitamin D axis and bone mass to accurately determine the contribution of each of these parameters to survival in older subjects. Thus, we sought to investigate the association between bone parameters and mortality in a longitudinal, prospective, population‐based cohort of 839 elderly subjects. Clinical data (including history of fractures and cardiovascular events) were assessed using a specific questionnaire. Laboratory exams, including serum 25OHD and PTH, were also performed. Bone mineral density (BMD) at the lumbar spine and hip were evaluated using DXA. All analyses were performed at baseline (2005 to 2007). Mortality was recorded during follow‐up. Multivariate Cox proportional regression was used to compute hazard ratios for all‐cause and cardiovascular mortality. Over a mean 4.06 ± 1.07 years, there were 132 (15.7%) deaths. These individuals were compared to 707 subjects who were alive at the end of the coverage period for mortality data collection. In a multivariate Cox proportional hazards model, age (HR 1.32; 95% CI, 1.13 to 1.55; p = 0.001, for each 5‐year increase), male gender (HR 1.90; 95% CI, 1.30 to 2.79; p = 0.001), recurrent falls (more than two in the previous year; HR 1.65; 95% CI, 1.06 to 2.56; p = 0.026), diabetes mellitus (HR 2.17; 95% CI, 1.46 to 3.21; p < 0.001), low physical activity score (HR 1.78; 95% CI, 1.14 to 2.79; p = 0.011), prior cardiovascular event (HR 1.76; 95% CI, 1.18 to 2.63; p = 0.006), total hip BMD (HR 1.41; 95% CI, 1.15 to 1.72; p = 0.001, per each 1 SD decrease), and intact PTH (iPTH) (HR 1.06; 95% CI, 1.04 to 1.08; p < 0.001, per each 10 pg/mL increase) were independently associated with all‐cause mortality. The subjects in the highest quartile of PTH (>49 pg/mL) were at a higher risk of cardiovascular death (HR 3.09; 95% CI, 1.36 to 6.99; p = 0.007) compared with the subjects in the lowest quartile (<26 pg/mL). Low BMD and higher PTH were significantly associated with mortality in community‐dwelling older adults. These findings support the notion that careful screening of these bone parameters might lead to better management of older patients and improve outcomes in this population. © 2016 American Society for Bone and Mineral Research.     

Low Plasma Level of Leucine‐Rich Repeat‐Containing 17 (LRRc17) Is an Independent and Additive Risk Factor for Osteoporotic Fractures in Postmenopausal Women

A novel role of leucine‐rich repeat‐containing 17 (LRRc17), an LRR protein secreted by osteoblasts, as a negative regulator of receptor activator of NF‐κB ligand–induced osteoclast differentiation was found. However, the clinical association between LRRc17 and osteoporotic fracture (OF) has not yet been investigated. We hypothesized that low circulating plasma level of LRRc17 might serve as an independent and additive risk factor for OF, including vertebral fractures (VF) and non‐vertebral fractures (non‐VF). In this case‐control study, 102 OF cases and 102 age‐ and body mass index–matched controls (mean age, 63.2 years) were analyzed among 532 postmenopausal women. VF (n = 49) and non‐VF (n = 60) participants were identified using lateral thoracolumbar radiographs and an interviewer‐assisted questionnaire, respectively. Median LRRc17 levels were significantly lower in participants with any OF (117.5 versus 197.3 pg/mL, p < 0.001), VF (93.2 versus 172.4 pg/mL, p = 0.002), and non‐VF (124.5 versus 206.9 pg/mL, p = 0.008) compared with the respective controls without fractures. The prevalence of OF increased from the highest LRRc17 tertile (≥228.5 pg/mL, 33.8%) to the lowest (<95.6 pg/mL, 63.2%). Each log unit decrease of LRRc17 was associated with greater risk of OF (odds ratio [OR] = 1.46; 95% confidence interval [CI] 1.10–1.96; p = 0.009) and VF (OR = 2.42; 95% CI 1.39–4.23; p = 0.002). Plasma LRRc17 significantly improved discrimination of OF, particularly VF, when added to models including clinical risk factors and bone mineral density according to the area under receiver operating characteristics curves (AUC 0.71 to 0.81, p = 0.036), category‐free net reclassification improvement (0.79; 95% CI 0.37–1.21; p < 0.001), and integrated discrimination improvement (0.13; 95% CI 0.06–0.20; p < 0.001). Low plasma LRRc17 was an independent risk factor for OF, which improved risk stratification, particularly in the spines of postmenopausal women. © 2016 American Society for Bone and Mineral Research.     

Sclerostin: Therapeutic Horizons Based Upon Its Actions

Inactivating mutations of the SOST gene cause a reduction in sclerostin levels and are associated with high bone mass. The clinical phenotypes, sclerosteosis and van Buchem’s disease, were described in 1950s. Much later, it was learned that both diseases are due to loss-of-function mutations in the SOST gene. As a regulator of an important osteoanabolic pathway, Wnt, inactivation of SOST leads to a stimulation of the pathway it regulates. The high bone mass in patients with either sclerosteosis or van Buchem’s disease is associated with unusual skeletal strength; they do not fracture. Knowledge of this molecule and its actions led rather quickly to the development of anti-sclerostin antibodies that lead to marked increases in bone mass in both animals and human subjects. Blocking sclerostin action with anti-sclerostin antibodies is a promising new therapeutic approach to osteoanabolic therapy of osteoporosis.     

Sclerostin Regulation,Microarchitecture, and Advanced Glycation End-Products in the Bone of Elderly Women With Type 2 Diabetes

Increased circulating sclerostin and accumulation of advanced glycation end-products (AGEs) are two potential mechanisms underlying low bone turnover and increased fracture risk in type 2 diabetes (T2D). Whether the expression of the sclerostin-encoding SOST gene is altered in T2D, and whether it is associated with AGEs accumulation or regulation of other bone formation-related genes is unknown. We hypothesized that AGEs accumulate and SOST gene expression is upregulated in bones from subjects with T2D, leading to downregulation of bone forming genes (RUNX2 and osteocalcin) and impaired bone microarchitecture and strength. We obtained bone tissue from femoral heads of 19 T2D postmenopausal women (mean glycated hemoglobin [HbA1c] 6.5%) and 73 age- and BMI-comparable nondiabetic women undergoing hip replacement surgery. Despite similar bone mineral density (BMD) and biomechanical properties, we found a significantly higher SOST (p = .006) and a parallel lower RUNX2 (p = .025) expression in T2D compared with non-diabetic subjects. Osteocalcin gene expression did not differ between T2D and non-diabetic subjects, as well as circulating osteocalcin and sclerostin levels. We found a 1.5-fold increase in total bone AGEs content in T2D compared with non-diabetic women (364.8 ± 78.2 versus 209.9 ± 34.4 μg quinine/g collagen, respectively; p < .001). AGEs bone content correlated with worse bone microarchitecture, including lower volumetric BMD (r = −0.633; p = .02), BV/TV (r = −0.59; p = .033) and increased trabecular separation/spacing (r = 0.624; p = .023). In conclusion, our data show that even in patients with good glycemic control, T2D affects the expression of genes controlling bone formation (SOST and RUNX2). We also found that accumulation of AGEs is associated with impaired bone microarchitecture. We provide novel insights that may help understand the mechanisms underlying bone fragility in T2D. © 2020 American Society for Bone and Mineral Research (ASBMR).     

Novel SOST gene mutation in a sclerosteosis patient and her parents

IntroductionSclerosteosis (OMIM 269500) is a rare autosomal recessive condition characterized by increased bone density associated with syndactyly. It is linked to a genetic defect in the SOST gene coding for sclerostin. So far, six different loss-of-function mutations in SOST have been reported in patients with sclerosteosis. Our objective was to sequence and identify mutation in the SOST and LRP5 genes which are known to be causal for craniotubular hyperostosis in a patient from India.Patient and methodsA 22 year old woman presented with typical features of sclerosteosis in form of progressive visual and hearing loss, syndactyly and radiographs revealing increased density of bone. Genomic sequencing of the SOST gene as well as exons 2, 3 and 4 of the LRP5 gene was performed.ResultsWe identified a novel homozygous mutation in the ^SOST gene, characterized as one nucleotide insertion resulting in a frame shift mutation and loss of functional sclerostin. Her parents were also found to have a similar but heterozygous mutation in the ^SOST gene.ConclusionA novel frame shift mutation in the ^SOST gene causing loss of functional sclerostin was identified in a patient with sclerosteosis and her parents.     

Lower fracture risk in older men with higher sclerostin concentration: A prospective analysis from the MINOS study

Sclerostin is synthesized by osteocytes and inhibits bone formation. We measured serum sclerostin levels in 710 men aged 50 years and older. Bone mineral density (BMD) was measured at the lumbar spine, hip, and distal forearm. Serum sclerostin increased with age (unadjusted r = 0.30, p < 0.001). After adjustment for age, weight, and bioavailable 17β‐estradiol, serum sclerostin correlated positively with BMD (r = 0.24 to 0.35, p < 0.001) and negatively with the levels of bone turnover markers (r = ? 0.09 to ? 0.23, p < 0.05 to 0.001). During a 10‐year follow‐up, 75 men sustained fragility fractures. Fracture risk was lower in the two upper quintiles of sclerostin combined versus three lower quintiles combined (6.1 versus 13.5%, p < 0.01). We compared fracture risk in the two highest quintiles combined versus three lower quintiles combined using the Cox model adjusted for age, weight, leisure physical activity, BMD, bone width (tubular bones), prevalent fracture, prevalent falls, ischemic heart disease, and severe abdominal aortic calcification. Men with higher sclerostin concentration had lower fracture risk (adjusted for hip BMD, hazard ratio [HR] = 0.55, 95% confidence interval [CI] 0.31 to 0.96, p < 0.05). The results were similar in 47 men with major fragility fractures (adjusted for lumbar spine BMD: HR = 0.39, 95% CI 0.17 to 0.90, p < 0.05). Men who had higher sclerostin and higher BMD (two highest quintiles) had lower risk of fracture compared with men who had lower BMD and lower sclerostin levels (three lower quintiles) (HR = 0.24, 95% CI 0.10 to 0.62, p < 0.005). Circulating sclerostin was not associated with mortality rate or the incidence of major cardiovascular events. Thus, in older men, higher serum sclerostin levels are associated with lower risk of fracture, higher BMD, and lower bone turnover rate. © 2013 American Society for Bone and Mineral Research.     

Serum sclerostin levels in Paget's disease and prostate cancer with bone metastases with a wide range of bone turnover

Evidence has been accumulating for the role of osteocytes as key players in the regulation of bone remodeling. One of the main products of these cells, sclerostin, inhibits bone formation and may also stimulate bone resorption. Circulating sclerostin has been evaluated in humans, but data are scarce in patients with different rates of bone turnover. To address this issue we evaluated serum sclerostin levels in patients with Paget's disease of bone (PD) and in patients with prostate cancer metastatic to the skeleton (PC). Sclerostin levels were measured in 88 patients with PD, 20 patients with PC and 237 healthy individuals (113 men and 124 women, aged 20 to 77 years). Bone turnover was evaluated by measuring serum levels of procollagen type 1 amino-terminal propeptide (P1NP) in all individuals studied and β-carboxy-terminal cross-linking telopeptide of type I collagen (β-CTX) only in patients. Patients were aged between 45 and 88 years and had a wide range of bone turnover: serum P1NP 9.2 to 1872 ng/ml and β-CTX 50 to 3120 pg/ml. Patients with PD and with PC had significantly higher mean serum sclerostin levels (53.1 ± 22.7 pg/ml and 56.6 ± 25.8 pg/ml, respectively) compared to healthy controls (38.1 ± 12.1 pg/ml) (p<0.001). Serum sclerostin levels were significantly correlated with P1NP in all (n=345) studied subjects (r=0.32, p<0.001). Circulating sclerostin levels are significantly increased in patients with increased bone turnover, regardless of underlying pathology. These increased levels may be due to a compensatory response to the increased number of osteoblasts at affected skeletal sites and may contribute to the increased bone resorption in patients with PC .     

Bone quality, as measured by trabecular bone score in patients with adrenal incidentalomas with and without subclinical hypercortisolism

Patients with adrenal incidentalomas (AIs) and subclinical hypercortisolism (SH) have increased risk of fracture independent of bone mineral density (BMD) and possibly due to reduced bone quality. The trabecular bone score (TBS) has been proposed as a index of bone microarchitecture. The aim of the study was to investigate TBS in AI. In 102 AI patients, SH was diagnosed in the presence of at least two of the following: (1) urinary free cortisol >70 µg/24 h (193.1 nmol/L); (2) cortisol after 1‐mg dexamethasone suppression test (1‐mg DST) >3.0 µg/dL (82.8 nmol/L); or (3) adrenocorticotropic hormone (ACTH) <10 pg/mL (<2.2 pmol/L). In patients and in 70 matched controls, BMD was measured at lumbar spine (LS) and femur (neck [FN] and total [FT]) by dual X‐ray absorptiometry and TBS was assessed in the region of LS‐BMD; BMD and TBS data were reported as Z‐scores. In patients, vertebral deformities were assessed by radiograph. Patients with SH (n = 34) had lower LS‐BMD (?0.31 ± 1.17), FT‐BMD (?0.29 ± 0.91), and TBS (?3.18 ± 1.21) than patients without SH (n = 68, 0.31 ± 1.42, p = 0.03; 0.19 ± 0.97, p = 0.01; ?1.70 ± 1.54, p < 0.0001, respectively) and controls (0.42 ± 1.52, p = 0.02; 0.14 ± 0.76, p = 0.02; ?1.19 ± 0.99, p < 0.0001, respectively). TBS was inversely correlated with 1‐mg DST (β = ?0.26, t = ?2.79, p = 0.006) regardless of age, LS‐BMD, body mass index (BMI), and gender. The presence of fracture was associated with low TBS alone (odds ratio [OR], 4.8; 95% confidence interval [CI], 1.85–12.42, p = 0.001) and with the cluster low TBS plus low LS‐BMD (OR, 4.37; 95% CI, 1.71–11.4, p = 0.002), after adjustment for age, BMI, and gender. Low TBS plus low LS‐BMD showed a good specificity (79%) for predicting fractures, whereas normal TBS (ie, > ?1.5) plus normal LS‐BMD high specificity (88.1%) for excluding fractures. Finally, TBS predicted the occurrence of a new fracture in 40 patients followed for 24 months (OR, 11.2; 95%CI, 1.71–71.41, p = 0.012) regardless of LS‐BMD, BMI, and age. In SH, bone quality, as measured by TBS, is altered. TBS is useful in detecting AI patients at risk of fractures. © 2012 American Society for Bone and Mineral Research.     

Patients with high‐bone‐mass phenotype owing to Lrp5‐T253I mutation have low plasma levels of serotonin

The Lrp5 gene is a major determinant of bone mass accrual. It has been demonstrated recently to achieve this function by hampering the synthesis of gut‐derived serotonin, which is a powerful inhibitor of bone formation. In this study we analyzed plasma serotonin levels in patients with a high‐bone‐mass (HBM) phenotype owing to gain‐of‐function mutation of Lrp5 (T253I). A total of 9 HBM patients were compared with 18 sex‐ and age‐matched controls. In HBM patients, the serotonin concentrations in platelet‐poor plasma were significantly lower than in the controls (mean ± SEM: 2.16 ± 0.28 ng/mL versus 3.51 ± 0.49 ng/mL, respectively, p < .05). Our data support the hypothesis that circulating serotonin levels mediate the increased bone mass resulting from gain‐of‐function mutations in Lrp5 in humans. © 2010 American Society for Bone and Mineral Research.     

Developments in Sclerostin Biology: Regulation of Gene Expression,Mechanisms of Action,and Physiological Functions

The SOST gene, which encodes the protein sclerostin, was identified through genetic linkage analysis of sclerosteosis and van Buchem’s disease patients. Sclerostin is a secreted glycoprotein that binds to the low-density lipoprotein receptor-related proteins 4, 5, and 6 to inhibit Wnt signaling. Since the initial discovery of sclerostin, much understanding has been gained into the role of this protein in the regulation of skeletal biology. In this article, we discuss the latest findings in the regulation of SOST expression, sclerostin mechanisms of action, and the potential utility of targeting sclerostin in conditions of low bone mass.     

Regulation of circulating sclerostin levels by sex steroids in women and in men

Sex steroids are important regulators of bone turnover, but the mechanisms of their effects on bone remain unclear. Sclerostin is an inhibitor of Wnt signaling, and circulating estrogen (E) levels are inversely associated with sclerostin levels in postmenopausal women. To directly test for sex steroid regulation of sclerostin levels, we examined effects of E treatment of postmenopausal women or selective withdrawal of E versus testosterone (T) in elderly men on circulating sclerostin levels. E treatment of postmenopausal women (n = 17) for 4 weeks led to a 27% decrease in serum sclerostin levels [versus +1% in controls (n = 18), p < .001]. Similarly, in 59 elderly men, we eliminated endogenous E and T production and studied them under conditions of physiologic T and E replacement, and then following withdrawal of T or E, we found that E, but not T, prevented increases in sclerostin levels following induction of sex steroid deficiency. In both sexes, changes in sclerostin levels correlated with changes in bone‐resorption, but not bone‐formation, markers (r = 0.62, p < .001, and r = 0.33, p = .009, for correlations with changes in serum C‐terminal telopeptide of type 1 collagen in the women and men, respectively). Our studies thus establish that in humans, circulating sclerostin levels are reduced by E but not by T. Moreover, consistent with recent data indicating important effects of Wnts on osteoclastic cells, our findings suggest that in humans, changes in sclerostin production may contribute to effects of E on bone resorption. © 2011 American Society for Bone and Mineral Research.     

Reversing LRP5‐Dependent Osteoporosis and SOST Deficiency–Induced Sclerosing Bone Disorders by Altering WNT Signaling Activity

The bone formation inhibitor sclerostin encoded by SOST binds in vitro to low‐density lipoprotein receptor‐related protein (LRP) 5/6 Wnt co‐receptors, thereby inhibiting Wnt/β‐catenin signaling, a central pathway of skeletal homeostasis. Lrp5/LRP5 deficiency results in osteoporosis‐pseudoglioma (OPPG), whereas Sost/SOST deficiency induces lifelong bone gain in mice and humans. Here, we analyzed the bone phenotype of mice lacking Sost (Sost^?/?), Lrp5 (Lrp5^?/?), or both (Sost^?/?;Lrp5^?/?) to elucidate the mechanism of action of Sost in vivo. Sost deficiency–induced bone gain was significantly blunted in Sost^?/?;Lrp5^?/? mice. Yet the Lrp5 OPPG phenotype was fully rescued in Sost^?/?;Lrp5^?/? mice and most bone parameters were elevated relative to wild‐type. To test whether the remaining bone increases in Sost^?/?;Lrp5^?/? animals depend on Lrp6, we treated wild‐type, Sost^?/?, and Sost^?/?;Lrp5^?/? mice with distinct Lrp6 function blocking antibodies. Selective blockage of Wnt1 class–mediated Lrp6 signaling reduced cancellous bone mass and density in wild‐type mice. Surprisingly, it reversed the abnormal bone gain in Sost^?/? and Sost^?/?;Lrp5^?/? mice to wild‐type levels irrespective of enhancement or blockage of Wnt3a class‐mediated Lrp6 activity. Thus, whereas Sost deficiency–induced bone anabolism partially requires Lrp5, it fully depends on Wnt1 class–induced Lrp6 activity. These findings indicate: first, that OPPG syndrome patients suffering from LRP5 loss‐of‐function should benefit from principles antagonizing SOST/sclerostin action; and second, that therapeutic WNT signaling inhibitors may stop the debilitating bone overgrowth in sclerosing disorders related to SOST deficiency, such as sclerosteosis, van Buchem disease, and autosomal dominant craniodiaphyseal dysplasia, which are rare disorders without viable treatment options. © 2014 American Society for Bone and Mineral Research.     

Mendelian Randomization Analysis Reveals a Causal Influence of Circulating Sclerostin Levels on Bone Mineral Density and Fractures

In bone, sclerostin is mainly osteocyte-derived and plays an important local role in adaptive responses to mechanical loading. Whether circulating levels of sclerostin also play a functional role is currently unclear, which we aimed to examine by two-sample Mendelian randomization (MR). A genetic instrument for circulating sclerostin, derived from a genomewide association study (GWAS) meta-analysis of serum sclerostin in 10,584 European-descent individuals, was examined in relation to femoral neck bone mineral density (BMD; n = 32,744) in GEFOS and estimated bone mineral density (eBMD) by heel ultrasound (n = 426,824) and fracture risk (n = 426,795) in UK Biobank. Our GWAS identified two novel serum sclerostin loci, B4GALNT3 (standard deviation [SD]) change in sclerostin per A allele (β = 0.20, p = 4.6 × 10⁻⁴⁹) and GALNT1 (β = 0.11 per G allele, p = 4.4 × 10⁻¹¹). B4GALNT3 is an N-acetyl-galactosaminyltransferase, adding a terminal LacdiNAc disaccharide to target glycocoproteins, found to be predominantly expressed in kidney, whereas GALNT1 is an enzyme causing mucin-type O-linked glycosylation. Using these two single-nucleotide polymorphisms (SNPs) as genetic instruments, MR revealed an inverse causal relationship between serum sclerostin and femoral neck BMD (β = –0.12, 95% confidence interval [CI] –0.20 to –0.05) and eBMD (β = –0.12, 95% CI –0.14 to –0.10), and a positive relationship with fracture risk (β = 0.11, 95% CI 0.01 to 0.21). Colocalization analysis demonstrated common genetic signals within the B4GALNT3 locus for higher sclerostin, lower eBMD, and greater B4GALNT3 expression in arterial tissue (probability >99%). Our findings suggest that higher sclerostin levels are causally related to lower BMD and greater fracture risk. Hence, strategies for reducing circulating sclerostin, for example by targeting glycosylation enzymes as suggested by our GWAS results, may prove valuable in treating osteoporosis. © 2019 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals, Inc.     

The Lrp4R1170Q Homozygous Knock‐In Mouse Recapitulates the Bone Phenotype of Sclerosteosis in Humans

Sclerosteosis is a rare autosomal recessive bone disorder marked by hyperostosis of the skull and tubular bones. Initially, we and others reported that sclerosteosis was caused by loss‐of‐function mutations in SOST, encoding sclerostin. More recently, we identified disease‐causing mutations in LRP4, a binding partner of sclerostin, in three sclerosteosis patients. Upon binding to sclerostin, LRP4 can inhibit the canonical WNT signaling that is known to be an important pathway in the regulation of bone formation. To further investigate the role of LRP4 in the bone formation process, we generated an Lrp4 mutated sclerosteosis mouse model by introducing the p.Arg1170Gln mutation in the mouse genome. Extensive analysis of the bone phenotype of the Lrp4R1170Q/R1170Q knock‐in (KI) mouse showed the presence of increased trabecular and cortical bone mass as a consequence of increased bone formation by the osteoblasts. In addition, three‐point bending analysis also showed that the increased bone mass results in increased bone strength. In contrast to the human sclerosteosis phenotype, we could not observe syndactyly in the forelimbs or hindlimbs of the Lrp4 KI animals. Finally, we could not detect any significant changes in the bone formation and resorption markers in the serum of the mutant mice. However, the serum sclerostin levels were strongly increased and the level of sclerostin in the tibia was decreased in Lrp4R1170Q/R1170Q mice, confirming the role of LRP4 as an anchor for sclerostin in bone. In conclusion, the Lrp4R1170Q/R1170Q mouse is a good model for the human sclerosteosis phenotype caused by mutations in LRP4 and can be used in the future for further investigation of the mechanism whereby LRP4 regulates bone formation. © 2017 American Society for Bone and Mineral Research.