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Keywords:

  • SCLEROSTIN;
  • OSTEOPOROSIS;
  • FRACTURE;
  • MEN;
  • BONE MINERAL DENSITY

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Disclosures
  8. Acknowledgements
  9. References

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.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Disclosures
  8. Acknowledgements
  9. References

Identification of men at high risk of fracture is a major challenge in the clinical management of osteoporosis in older men. Low bone mineral density (BMD) measured by dual-energy X-ray absorptiometry (DXA) identifies fewer men at high risk of fracture compared with women.1–3 History of fracture, history of fall, and severe abdominal aortic calcification (AAC) may improve fracture prediction; however, data are not fully consistent.4–6 Biochemical bone turnover markers (BTM), quantitative computed tomography, and ultrasounds do not improve fracture prediction in men compared with BMD.7–10 This situation warrants search for new parameters to improve fracture prediction in men.

Sclerostin is a glycoprotein secreted mainly by osteocytes.11 It binds to low-density lipoprotein receptor-related protein 5 and 6 on the osteoblastic cells and inhibits activity of osteoblasts. Loss-of-function mutations of the sclerostin gene are characterized by higher bone mass.11 Serum sclerostin level correlated positively with age and BMD.12–15 In older men and women and in dialysis patients, sclerostin levels correlated positively with volumetric BMD (vBMD) (mainly trabecular) as well as trabecular number and thickness.15, 16

Data on the link between serum sclerostin and fracture risk are scanty and discordant. In a nested case-cohort study (SOF), women in the highest sclerostin quartile had higher risk of hip fracture versus the lowest quartile after adjustment for BMD.17 In a cohort of postmenopasal women followed up prospectively for 5 years, higher sclerostin levels were associated with higher fracture risk after adjustment for BMD and other confounders.18 In postmenopausal women (OFELY), circulating sclerostin was associated neither with the prevalent nor the incident fracture.18 Adjustment for BMD did not influence the results.

However, the association between serum sclerostin and fracture risk in men has not been elucidated. Therefore, we studied the association of the baseline serum sclerostin levels with BMD, BTM levels, and fracture incidence in a cohort of older men followed up prospectively for 10 years. Because the association between sclerostin levels and fracture incidence could be biased by different mortality rates according to the sclerostin level, we assessed the link between baseline sclerostin levels and all-cause mortality in the cohort.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Disclosures
  8. Acknowledgements
  9. References

Cohort

MINOS is a prospective cohort study of male osteoporosis.19 Its aim was to assess predictors of bone loss and fractures in men. Participants were recruited in 1995 to 1996 from the Société de Secours Minière de Bourgogne (SSMB) rolls in Montceau les Mines. The study was accepted by the ethics committee and performed in accord with the Helsinki Declaration of 1975 as revised in 1983. Letters inviting participation were sent to a randomly selected sample of 3400 clients of SSMB aged 50 to 85 years. Among them, 841 agreed to participate and provided informed consent. Forty-three men refused bone densitometry or had radiographs of poor quality. The cohort was followed up for 7.5 years (questionnaire and DXA every 18 months, spine radiograph after 3 and 7.5 years). Then, for 2.5 years, the men were followed up to obtain information on incident nonvertebral fractures. This study was carried out in 710 men who had BMD measurement, lateral spine radiograph, blood collection at baseline (1995 to 1996) and assay of serum sclerostin. Sclerostin was not measured in 88 men because of the insufficient quantity of serum. Men were followed to the first of the following: fracture, last contact, death, or end of follow-up. For 158 men who died (of 710 who had sclerostin assay), dates of death were provided by the SSMB.20 For two men, we did not obtain data on their life status after 5 years of follow-up. In other survivors, data on mortality were collected for 10 years. Furthermore, during 7.5 years, 69 men self-reported incident major cardiovascular events (34 myocardial infarctions, 37 strokes).21

Assessment of fractures

Among the men who had sclerostin assays, 96 reported 128 prior fractures (vertebra, 67 in 60 men; clavicle, 3; humerus, 3; elbow, 3; distal radius, 16 in 14 men; rib, 13 in 11 men; pelvis, 1; distal femur, 1; leg, 7; ankle, 13; metatarse, 1). During 7.5 years, 25 vertebral fractures occurred in 24 men. A decrease of vertebral height by 3 mm or 15% was considered an incident vertebral fracture. During the 10-year follow-up, 59 low-trauma peripheral fractures occurred in 55 men (clavicle, 1; humerus, 6; distal radius, 15; ulna, 1; rib, 11; multiple ribs, 4; pelvis, 2; hip, 5; distal femur, 2; tibia, 5; calcaneum, 1; metatarse, 6). Jointly, 84 fractures occurred in 75 men.

Bone mineral density

BMD was measured at the lumbar spine and hip by DXA (Hologic QDR-1500, Hologic Inc., Bedford, MA, USA).19 For the spine, coefficient of variation (CV) was 0.33% using a Hologic phantom and 0.62% using a human lumbar spine embedded in methyl metacrylate. For the hip and its parts, CV was 0.81% to 0.94% using a Hologic hip phantom. In volunteers, CV for lumbar spine and hip was 1.6% to 2.8%. BMD of the distal forearm was measured using single-energy X-ray absorptiometry (Osteometer DTX100, Rodovre, Denmark). CV was 0.47% using a calibration standard. Three hip scans and three forearm scans with positioning errors were excluded. Average bone width in the measured region was calculated as the projected area of the bone divided by the length of the scanned area (1.5 cm for the femoral neck, 2 cm for distal radius).22

Biochemical and hormonal measurements

Fasting serum and 24-hour urine were collected at baseline (8 a.m.) and stored at –80°C. Serum sclerostin was measured by an ELISA using two antibodies against human recombinant sclerostin (TECO Sclerostin EIA Kit, TECOmedical, Sissach, Switzerland).18, 23 The intra- and interassay CV were <10%. Detection limit was 0.13 ng/mL. Bone formation was assessed by serum osteocalcin, N-terminal propeptide of type I procollagen and bone-specific alkaline phosphatase.24 Bone resorption was assessed by 24-hour urinary excretion of total and free deoxypyridinoline and by serum and urinary C-terminal telopeptide of type I collagen (CTX-I).24 Urinary BTMs are expressed per mmol creatinine (cr). Testosterone and 17β-estradiol were measured by tritiated RIA after diethyether extraction.25 Sex hormone-binding globulin was measured by immunoradiometric assay (125-I SBP Coatria, Bio-Mérieux, Marcy l'Etoile, France). Apparent free testosterone concentration (AFTC) and bioavailable 17β-estradiol (bio-17β-estradiol) were calculated as described previously.26, 27 Serum 25-hydroxycholecalciferol was measured by RIA (Incstar Corp., Stillwater, MN, USA) after acetonitril extraction.28 Serum parathyroid hormone (PTH) using immunochemoluminometric assay (Magic Lite, Ciba Corning Diagnostic, Medfield, MA, USA).28 Glomerular filtration rate (GFR) was estimated by the MDRD equation.29

Assessement of the abdominal aortic calcification (AAC) score

AAC was assessed by a semiquantitative method on lateral radiographs of lumbar spine using the semiquantitative score described by Kauppila.30, 31 AAC score was dichotomized (>6 versus 0 to 6) because this threshold was associated with lower BMD and a higher risk of fracture.31

Assessment of covariates

Men completed interviewer-administered questionnaires to assess lifestyle and health status. Physical activity was calculated using the overall amount of time spent walking, gardening, and practicing leisure sport activity including seasonal activities. Men self-reported falls that occurred during the year before the recruitment. Body weight and height were measured by standard devices. Men self-reported diseases present at baseline (diabetes, hypertension, ischemic heart disease, parkinsonism, stroke, prostate cancer, digestive diseases, pulmonary diseases) and medications.20 The assessment of diseases was based on questions such as, “Did your doctor tell you that you had…?” Whenever possible, medications were also checked.

Statistical methods

We used SAS 9.1 software (Cary, NC, USA). For bivariate comparisons, we used a t test (variables with Gaussian distribution), Mann-Whitney's test (variables with skewed distribution), and chi-square test (classes). Correlation was assessed by Pearson's simple and partial correlation coefficient. The association of sclerostin levels with BMD and BTM levels were assessed using multivariable regression adjusted for age, weight, GFR and bio-17β-estradiol (all p < 0.1). Variables with skewed distribution were log-transformed. Differences in BMD and BTM levels across the sclerostin quintiles were calculated using analysis of covariance adjusted for age, weight, GFR, and bio-17β-estradiol (p < 0.1 for all). Fracture-free survival and survival according to the sclerostin levels (continuous, classes) were analyzed by Kaplan-Meier curves and by Cox model after checking the assumption of proportional hazards. For vertebral fractures, we set the date as half of the follow-up period during which the fracture occurred, eg, for a fracture detected after 3 years, the date was set on day 525. We selected the covariates by backward selection. The variables with p < 0.15 for at least one DXA skeletal site were retained in the final model (age, weight, BMD, history of fracture, history of fall, AAC >6, ischemic heart disease, leisure physical activity). The interaction between BMD and sclerostin level was assessed for each model. Then, we calculated HRs of fracture in four groups created to represent men according to BMD and sclerostin level: 1) three lower quintiles of BMD and of sclerostin (reference); 2) three lower BMD quintiles and two upper sclerostin quintiles; 3) two upper BMD quintiles and three lower sclerostin quintiles; 4) two upper quintiles of BMD and of sclerostin. The models were adjusted as above. In the Cox model equations for the assessment of the association between sclerostin level and survival, the covariables (age, BMI, AAC score, smoking, history of fall, physical activity, ischemic heart disease, diabetes, Parkinson's disease, creatinine clearance) were retained in the most powerful form as described previously.20 In the Cox model equations for the assessment of the link between sclerostin level and the risk of major cardiovascular event, the covariables (age, BMI, AAC score, smoking, physical activity, education level, hip BMD, urinary CTX-I, history of ischemic heart disease, diabetes, and hypertension) were retained in the most powerful form as described previously.21

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Disclosures
  8. Acknowledgements
  9. References

Correlation of serum sclerostin with bone parameters

Men who had sclerostin assay did not differ from those who did not except for a higher PTH level and greater mortality rate (Table 1). Unadjusted serum sclerostin correlated positively with age, weight, and total and bio-17β-estradiol but negatively with GFR and AFTC (Table 2). After adjustment for age and weight, serum sclerostin correlated positively with total and bio-17β-estradiol and negatively with GFR. After adjustment for age, weight, GFR, and bio-17β-estradiol, all BTM correlated negatively with serum sclerostin. BTM levels decreased by 0.10 to 0.26 SD/SD increase in sclerostin. Variation in sclerostin explained 1% to 6% of their variability. BTM levels were 0.40 to 0.78 SD higher in the lowest versus the highest sclerostin quintile (p < 0.05 to p < 0.001).

Table 1. Descriptive Analyses of Men Who Did or Did Not Have Sclerostin Measurements
 Had sclerostin measurement (n = 710)Did not have sclerostin measurement (n = 88)p
  1. BMI = body mass index; AAC = abdominal aortic calcification; AFTC = apparent free testosterone concentration; Bio-17βE2 = bioavailable 17β-estradiol; SHBG = sex hormone-binding globulin; 25OHD = 25-hydroxycholecalciferol; PTH = parathyroid hormone; GRF = glomerular filtration rate; BSAP = bone-specific alkaline phosphatase; PINP = N-terminal propeptide of type I procollagen; DPD = deoxypyridinoline; CTX-I = C-terminal telopeptide of type I collagen; CV event = incident major cardiovascular event (myocardial infarction, stroke).

Age (years)65 ± 765 ± 80.14
Weight (kg)80 ± 1381 ± 120.81
Height (cm)169 ± 6169 ± 70.18
BMI (kg/m2)28.0 ± 3.728.3 ± 3.50.43
Multiple falls72 (10.0%)9 (10.3%)0.94
Prior fractures97 (13.6%)16 (18.6%)0.21
AAC score2 [0; 6]3 [0; 7]0.34
AAC score >6166 (24.0%)24 (28.2%)0.39
Bone mineral density (g/cm2)
 Lumbar spine1.032 ± 0.1841.040 ± 0.2060.72
 Femoral neck0.841 ± 0.1200.845 ± 0.1350.76
 Trochanter0.737 ± 0.1090.729 ± 0.1210.59
 Total hip0.962 ± 0.1290.957 ± 0.1430.74
 Distal forearm0.522 ± 0.0660.514 ± 0.0640.29
 Distal radius0.553 ± 0.0700.544 ± 0.0690.26
 Ultradistal radius0.428 ± 0.0660.423 ± 0.0660.48
Average bone width
 Femoral neck4.09 ± 0.324.08 ± 0.320.73
 Distal radius2.47 ± 0.212.48 ± 0.210.86
Biological measurements
 Testosterone (nmol/L)17.7 ± 7.017.5 ± 6.90.82
 AFTC (pmol/L)200.1 ± 80.0188.7 ± 69.40.16
 17β-estradiol (pmol/L)113.3 ± 28.8116.0 ± 31.90.45
 Bio-17βE2 (pmol/L)63.8 ± 18.863.8 ± 18.60.95
 SHBG (nmol/L)84.5 ± 44.186.6 ± 40.20.64
 25OHD (ng/mL)27.3 ± 11.524.9 ± 11.70.09
 PTH (pg/mL)36 [28, 47]42 [29, 57]0.01
 GFR (mL/min)99.6 ± 34.097.3 ± 34.40.34
 Sclerostin (ng/mL)0.60 [0.49, 0.75]
 Osteocalcin (ng/mL)18.4 [14.7, 22.5]18.1 [14.0, 21.0]0.08
 Bone ALP15.8 [13.0, 19.3]15.7 [13.5, 19.0]0.86
 PINP33 [27, 43]31 [24, 39]0.89
 Total DPD6.55 [5.27, 8.21]6.74 [5.48, 8.23]0.57
 Free DPD3.23 [2.69, 4.03]3.24 [2.66, 4.01]0.78
 Urinary CTX-I112 [81, 151]108 [69, 147]0.28
 Serum CTX-I2.21 [1.64, 4.10]2.12 [1.59, 2.94]0.60
Mortality rate
 No. of deaths158 (23%)26 (31%)<0.05
 No. of CV events69 (10%)10 (11%)0.74
Table 2. Simple and Partial Correlation Coefficients of Serum Sclerostin Level With Other Parameters
 Simple correlation coefficientAdjusted for age and weightMultiple adjusted
  • Multiple adjusted = adjusted for age, weight, glomerular filtration rate, and bio-17βE2; AFTC = apparent free testosterone concentration; Bio-17βE2 = bioavailable 17β-estradiol; SHBG = sex hormone-binding globulin; 25OHD = 25-hydroxycholecalciferol; GFR = glomerular filtration rate; BSAP = bone-specific alkaline phosphatase; PINP = N-terminal propeptide of type I procollagen; DPD = deoxypyridinoline; CTX-I = C-terminal telopeptide of type I collagen.

  • a

    p < 0.05.

  • b

    p < 0.01.

  • c

    p < 0.005.

  • d

    p < 0.001.

Age0.30d  
Weight0.10b
Hormones
 Total testosterone−0.070.02 
 AFTC−0.09a0.06
 17β-estradiol0.13d0.17d
 Bio-17βE20.10a0.17d
 SHBG0.05–0.01
 25OHD−0.060.05
 Parathyroid hormone0.03–0.05
 GFR−0.22d–0.18d
Biochemical bone turnover markers
 Osteocalcin−0.16d−0.19d−0.19d
 BSAP−0.06−0.12c−0.09a
 PINP−0.13d−0.17d−0.14d
 Total DPD−0.06−0.16d−0.13c
 Free DPD−0.05−0.17d−0.14d
 Urinary CTX-I−0.25d−0.28d−0.21d
 Serum CTX-I−0.22d−0.23d−0.23d
Bone mineral density
 Lumbar spine0.38d0.34d0.31d
 Femoral neck0.20d0.25d0.26d
 Trochanter0.29d0.35d0.35d
 Total hip0.25d0.34d0.35d
 Distal forearm0.16d0.28d0.25d
 Distal radius0.16d0.27d0.24d
 Ultradistal radius0.20d0.30d0.27d
Average bone width
 Femoral neck0.03−0.05−0.05
 Distal radius0.04−0.05−0.02

BMD of all the skeletal sites correlated positively with serum sclerostin, also after adjustment for the confounders. BMD increased by 0.26 to 0.53 SD/SD increase in sclerostin. Variation in serum sclerostin explained 5% to 12% of the BMD variability. BMD was 0.62 to 1.13 SD lower in the lowest sclerostin quintile compared with the highest quintile (7.5% to 16.3%, p < 0.001).

By contrast, sclerostin levels did not differ according to the presence of AAC or of the ischemic heart disease (p > 0.15 adjusted for age, weight, smoking, hypertension, diabetes, and BMD).

Association between baseline sclerostin level and fracture incidence

Men who self-reported fractures before the recruitment had similar sclerostin levels compared with those who did not (p = 0.79 after adjustment for age, weight, hip BMD, and AAC score).

Seventy-five men (10.6%) sustained fragility fracture. They were lighter, self-reported more prior fractures, and had lower BMD, lower average width of tubular bones, and lower serum sclerostin (Table 3). The fracture incidence decreased with increasing sclerostin concentration and the trend was significant after adjustment for clinical confounders (Table 4). However, it lost significance after additional adjustment for BMD.

Table 3. Bivariate Comparisons of Men According to the Incidence of Fracture
 Fracture (−) (n = 635)Fracture (+) (n = 75)p
  1. AAC = abdominal aortic calcification; AFTC = apparent free testosterone concentration; Bio-17βE2 = bioavailable 17β-estradiol; SHBG = sex hormone-binding globulin; 25OHD = 25-hydroxycholecalciferol; PTH = parathyroid hormone; GFR = glomerular filtration rate; BSAP = bone-specific alkaline phosphatase; PINP = N-terminal propeptide of type I procollagen; DPD = deoxypyridinoline; CTX-I = C-terminal telopeptide of type I collagen; CV event = incident major cardiovascular event (myocardial infarction, stroke).

Age (years)65 ± 765 ± 70.81
Weight (kg)80 ± 1377 ± 12<0.05
Height (cm)169 ± 6168 ± 50.44
BMI (kg/m2)28.1 ± 3.727.1 ± 3.6<0.05
Current smoking, n (%)64 (10%)6 (8%)0.53
Physical activity (h/wk)22 ± 1320 ± 100.25
Multiple falls60 (9.4%)12 (15.8%)0.08
Prior fractures78 (12.4%)18 (24.0%)0.005
AAC score2 [0; 6]3 [1; 8]0.18
AAC score >6140 (21.7%)24 (31.5%)0.08
Bone mineral density (g/cm2)
 Lumbar spine1.041 ± 0.1860.958 ± 0.153<0.001
 Femoral neck0.845 ± 0.1210.805 ± 0.112<0.01
 Trochanter0.742 ± 0.1090.694 ± 0.108<0.001
 Total hip0.969 ± 0.1290.909 ± 0.127<0.001
 Distal forearm0.526 ± 0.0660.491 ± 0.063<0.001
 Distal radius0.557 ± 0.0690.522 ± 0.067<0.001
 Ultradistal radius0.432 ± 0.0650.396 ± 0.064<0.001
Average bone width
 Femoral neck4.10 ± 0.324.01 ± 0.32<0.05
 Distal radius2.48 ± 0.212.40 ± 0.21<0.005
Hormones
 Testosterone (nmol/L)17.6 ± 6.918.3 ± 7.40.41
 AFTC (pmol/L)200.1 ± 79.6201.9 ± 85.80.85
 17β-estradiol (pmol/L)113.2 ± 25.2113.3 ± 25.60.97
 Bio-17βE2 (pmol/L)63.8 ± 18.861.4 ± 18.50.32
 SHBG (nmol/L)83.6 ± 42.491.9 ± 56.20.12
 25OHD (ng/mL)27.2 ± 11.527.7 ± 11.60.76
 PTH (pg/mL)36 [28, 46]37 [29, 48]0.67
 GFR (mL/min)100.2 ± 36.294.7 ± 42.30.35
 Sclerostin (ng/mL)0.61 [0.49, 0.77]0.54 [0.47, 0.64]<0.005
 Osteocalcin (ng/mL)18.8 [14.4, 23.3]18.4 [14.7, 22.5]0.53
 BSAP15.7 [13.0, 19.3]16.3 [12.9, 21.0]0.27
 PINP33 [28, 44]33 [26, 43]0.51
 Total DPD6.48 [5.24, 8.14]6.97 [5.55, 8.35]0.12
 Free DPD3.23 [2.68, 3.99]3.33 [2.73, 4.14]0.35
 Urinary CTX-I119 [85, 162]149 [110, 207]0.24
 Serum CTX-I2.23 [1.67, 3.03]2.07 [1.48, 2.95]0.44
No. of deaths140 (22%)18 (24%)0.85
No. of CV events61 (10%)8 (11%)0.76
Table 4. Bivariate and Multivariable Association of the Baseline Sclerostin Concentration and the Risk of Osteoporotic Fracture During a 10-Year Follow-up in 710 Men From the MINOS Cohort
 All fractures (n = 75)Major fragility fractures (n = 47)
Per 1 SD increaseq4-q5 vs. q1-q3Per 1 SD increaseq4-q5 vs. q1-q3
HR (95% CI)HR (95% CI)HR (95% CI)HR (95% CI)
  • a

    p < 0.05.

  • b

    p < 0.01.

  • c

    p < 0.005.

  • Major fragility fractures include vertebra, hip, femoral shaft, tibia, pelvis, proximal humerus, and multiple ribs.

Sclerostin0.74 (0.59, 0.93)b0.48 (0.28, 0.83)b0.70 (0.53, 0.93)a0.34 (0.16, 0.71)b
 + age, weight0.72 (0.57, 0.91)b0.47 (0.27, 0.82)b0.67 (0.50, 0.89)b0.31 (0.14, 0.67)c
 + age, weight, history of falls, fractures, and ischemic heart disease, leisure physical activity, AAC score >60.73 (0.58, 0.92)a0.47 (0.27, 0.82)b0.67 (0.50, 0.91)b0.32 (0.15, 0.70)c
 + all above and BMD of one of the skeletal sites itemized below
Lumbar spine0.79 (0.61, 1.02)0.54 (0.30, 0.95)a0.74 (0.54, 1.03)0.39 (0.17, 0.90)a
Total hip0.82 (0.63, 1.06)0.55 (0.31, 0.96)a0.79 (0.57, 1.11)0.43 (0.19, 0.97)a
Trochanter0.81 (0.63, 1.05)0.55 (0.31, 0.96)a0.80 (0.57, 1.12)0.44 (0.20, 0.99)a
Femoral neck + width0.75 (0.58, 0.96)a0.53 (0.30, 0.93)a0.69 (0.50, 0.95)a0.36 (0.16, 0.79)a
Distal forearm0.81 (0.63, 1.03)0.53 (0.31, 0.92)a0.74 (0.55, 0.99)a0.39 (0.18, 0.86)a
Distal radius + width0.79 (0.61, 1.02)0.55 (0.32, 0.95)a0.74 (0.55, 0.99)a0.39 (0.17, 0.86)a
Ultradistal radius0.80 (0.62, 1.03)0.54 (0.31, 0.93)a0.76 (0.55, 1.06)0.41 (0.19, 0.92)a

Crude fracture incidence was similar in the three lower sclerostin quintiles and lower in the two highest quintiles (p < 0.05) (Fig. 1). Fracture risk was lower in the two upper quintiles combined versus three lower quintiles combined (6.1% versus 13.5%, p < 0.01). This trend remained significant after adjustment for age, weight, other clinical confounders, and BMD of most of the skeletal sites (including additional adjustment for the average width of tubular bones).

thumbnail image

Figure 1. (A) Fracture incidence according to the quintiles of the serum sclerostin concentration in the MINOS cohort. (B) Fracture-free survival in men in the three lower quintiles of the sclerostin concentration (solid line) and in the two higher quintiles (dotted line).

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In the analysis limited to 47 men who sustained major fragility fractures (vertebra, hip, femoral shaft, tibia, pelvis, proximal humerus, multiple ribs), the results were similar. Fracture incidence decreased with increasing sclerostin levels, and the trend remained significant after adjustment for clinical confounders (Table 4). However, it lost significance after additional adjustment for BMD of most of the skeletal sites. The risk of major fracture was lower in the two higher quintiles of sclerostin. The trend remained significant after adjustment for age, weight, clinical confounders, and BMD of all the skeletal sites (including adjustment for the average width of tubular bones).

In all the above analyses, the interaction between sclerostin level and BMD was not significant.

Finally, we assessed respective contributions of higher BMD and higher serum sclerostin (two upper quintiles each) to the protection against the fracture (Table 5). The fracture risk was the lowest in men who were simultaneously in the two higher quintiles of sclerostin and of BMD regardless of the skeletal site. The same trend was found for the major fractures.

Table 5. Lower Risk of Fragility Fractures Associated With Higher Bone Mineral Density (Two Upper Quintiles) and Higher Serum Sclerostin Concentration (Two Upper Quintiles) in Older Men From the MINOS Cohort
Bone mineral densitySclerostinAll fractures (n = 75)Major fractures (n = 47)
IncidenceHR (95% CI)IncidenceHR (95% CI)
  • a

    p < 0.05 versus the reference group.

  • b

    p < 0.01 versus the reference group.

  • c

    p < 0.005 versus the reference group.

  • Major fragility fractures include vertebra, hip, femoral shaft, tibia, pelvis, proximal humerus, and multiple ribs.

Lumbar spine
 <1.060 g/cm2 (q1-q3)≤0.56 ng/mL (q1-q3)46/306 (15.0%)1.0032/306 (10.5%)1.00
 ≥1.060 g/cm2 (q4-q5)≤0.56 ng/mL (q1-q3)12/128 (9.4%)0.68 (0.34, 1.34)7/128 (5.5%)0.53 (0.22, 1.30)
 <1.060 g/cm2 (q1-q3)>0.56 ng/mL (q4-q5)10/122 (8.2%)0.56 (0.27, 1.18)5/122 (4.1%)0.33 (0.12, 0.94)a
 ≥1.060 g/cm2 (q4-q5)>0.56 ng/mL (q4-q5)7/154 (4.5%)0.35 (0.16, 0.80)a3/154 (1.9%)0.21 (0.06, 0.71)a
Distal forearm
 <0.539 g/cm2 (q1-q3)≤0.56 ng/mL (q1-q3)47/279 (16.8%)1.0030/279 (10.8%)1.00
 ≥0.539 g/cm2 (q4-q5)≤0.56 ng/mL (q1-q3)11/152 (7.2%)0.39 (0.19, 0.78)b9/152 (5.9%)0.59 (0.26, 1.34)
 <0.539 g/cm2 (q1-q3)>0.56 ng/mL (q4-q5)12/144 (8.3%)0.49 (0.25, 0.94)a7/144 (4.9%)0.42 (0.18, 0.98)a
 ≥0.539 g/cm2 (q4-q5)>0.56 ng/mL (q4-q5)5/135 (3.7%)0.24 (0.10, 0.62)c1/135 (0.8%)0.08 (0.01, 0.58)a

Sensitivity analysis

Trends were similar when fracture risk was studied in quartiles or tertiles of circulating sclerostin. In the multivariable models, fracture risk was lower in the highest versus the two lowest quartiles combined (5.4 versus 13.8%, hazard ratio [HR] = 0.47, 95% confidence interval [CI] 0.23 to 0.97; p < 0.05). In similar models, fracture risk was lower in the highest versus the lowest sclerostin tertile (6.0 versus 13.5%, HR = 0.51, 95% CI 0.26 to 0.98; p < 0.05).

Association between baseline sclerostin and mortality rate

After adjustment for confounders, sclerostin level was not associated with all-cause mortality (HR = 0.96 per 1 SD increase, 95% CI 0.79 to 1.16; p = 0.64). The mortality did not differ between the two highest and the three lowest sclerostin quintiles (multi-adjusted HR = 1.12, 95% CI 0.78 to 1.63; p = 0.54). The risk of cardiovascular event did not vary according to the sclerostin levels (HR = 0.73 per 1 SD increase, 95% CI 0.31 to 1.68; p = 0.45). The cardiovascular risk did not differ in the two highest versus three lowest sclerostin quintiles (HR = 0.74, 95% CI 0.43 to 2.19; p = 0.29).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Disclosures
  8. Acknowledgements
  9. References

We found that older men with higher sclerostin levels had lower fracture risk after adjustment for age, BMD, bone width, history of falls and fractures, severe AAC, ischemic heart disease, and leisure physical activity. Sclerostin levels were not associated with mortality or cardiovascular risk. We confirm that serum sclerostin is correlated positively with age and BMD but negatively with BTM levels.

The fracture risk was consistently significantly lower in men with higher sclerostin levels, even after adjustment for the confounders known to predict fracture. The difference between our results and the previous studies is intriguing. Garnero suggested that their results were negative because of the poor statistical power.18 However, because there was no trend of the fracture incidence across the sclerostin quartiles, it is not certain whether a higher number of fractures would change the result. Interestingly, women in the highest sclerostin quartile reported fewer fractures (nonsignificantly). The difference between our study and those of Arasu and colleagues and of Ardawi and colleagues17, 18 is not clear. The link between fracture risk and BTM was similar in cohort studies and case-control studies.32 Similar trends were found for hip fracture and all fractures.32 Differences between men and women in the regulation of sclerostin secretion by sex hormones are weak.33 Thus, we cannot provide convincing explanation of this discrepancy.

Men with higher sclerostin levels had lower fracture risk, whereas sclerostin levels did not differ between the men who did or did not report prior fracture. This might suggest that this relation is not genetically determined but were rather caused by factors that occurred later in life.

Men with higher sclerostin levels had lower fracture risk regardless of BMD. Thus, factors not detected by DXA may explain this relation, eg, bone microarchitecture.15, 16 Men with fracture had poor bone microarchitecture.34–36 Patients with hip fracture had fewer osteocytes, more apoptotic osteocytes, and lower sclerostin gene expression.37 However, the osteocyte number did not differ according to the hip fracture status in another study.38 Thus, further studies are required to elucidate the mechanisms underlying the relation between sclerostin and bone fragility.

The positive correlation between BMD and circulating sclerostin may reflect greater number of osteocytes.17 Serum sclerostin may also be a marker of bone strength. Mechanical loading is associated with higher strains and lower sclerostin secretion in a dose-dependent way.39, 40 Conversely, mechanical unloading is associated with higher sclerostin secretion.39, 41, 42 Thus, circulating sclerostin may reflect the adaptation of metabolic activity of osteocytes to the existing bone strains. Men with higher BMD (and better bone microarchitecture) may have greater bone strength, lower mechanical strains, and higher sclerostin levels. Conversely, men with lower BMD (and poor bone microarchitecture) may have higher mechanical strains in the remaining bone, lower sclerostin synthesis, and higher fracture risk.

Other results are also consistent with this speculation. Loss-of-function mutations of sclerostin gene are associated with high bone mass, whereas sclerostin-transgenic mice are osteopenic.11, 43 Thus, the positive relation of serum sclerostin with BMD and bone microarchitecture can be interpreted as an increased sclerostin synthesis in more solid bone submitted to low mechanical strain. Consequently, the positive association between BMD and sclerostin would be driven by lower sclerostin secretion in men with lower BMD. This speculation is consistent with the finding that serum sclerostin correlated positively with bone mass mainly in the elderly (who have lower BMD) but not in young people (who have higher BMD).15 However, this hypothesis is difficult to interpret in the context of the age-related increase in the circulating sclerostin, unless sclerostin production by individual osteocyte increases with aging.

Correlation between sclerostin and BTM levels was negative. A similar trend was described in postmenopausal women15, 18 but was weak in men and in a group composed mainly of younger people.13, 15 The negative correlation between bone formation and sclerostin may be because of inhibition of osteoblasts by sclerostin. However, sclerostin was reported to stimulate osteoclast differentiation and bone resorption, whereas antisclerostin antibodies decreased bone resorption.44, 45 The negative correlation between serum sclerostin and bone resorption cannot be interpreted as the direct effect of sclerostin on bone resorption. However, these findings are consistent with the speculation presented above—lower BTMs were associated with higher BMD and better bone microarchitecture.24, 46

This speculation is supported by other data. Sclerostin expression was greater in more mature osteocytes and quiescent osteons, which is consistent with lower bone turnover.47 Neridronate increased BMD and sclerostin levels but decreased BTM levels.48, 49 Decrease in bone turnover rate and increase in BMD may improve bone strength. However, such trend was not found in a small group of retrospectively investigated osteoporotic postmenopausal women treated with bisphosphonates.50

In addition, our data suggest that the lower fracture risk in men with higher sclerostin levels is not biased by higher mortality or higher cardiovascular risk in this group.

Our study has limitations. Our cohort may not be representative of the French population. Sclerostin was assayed in sera stored for 16 years. Sclerostin is a large glycoprotein stabilized by disulfide bonds51 and may be preserved at –80°C. However, during the long storage, partial loss of protein was unavoidable, and the sclerostin levels in our study were slightly lower than in other groups of men.13–16 To our knowledge, long-term stability of sclerostin has not been explored. Because all samples were stored together, the loss was probably similar, leading to a systematic error. Therefore, the long-term storage had probably limited impact on the investigated association. By contrast, these sclerostin levels cannot be used as the reference values. In 11% of the cohort, sclerostin was not measured because of the insufficient amount of serum. However, these men did not differ substantially from those who had assays. Incident fractures were confirmed using medical records but without formal adjudication. Because there were few incident fractures, we assessed vertebral and nonvertebral fractures jointly. For the same reason, we tried to find a cutoff of the sclerostin level discriminating the best men with higher fracture risk from those with lower risk. Therefore, we applied a cutoff different from those used traditionally in epidemiological studies. Importantly, the use of other cutoffs provided somewhat weaker but similar trends. History of fracture, history of fall, and comorbidities were self-reported without ascertainment. The information could have been influenced by the recall bias (eg, for peripheral fracture). The diagnosis of comorbidities could also be biased by the judgment of the physician.

In conclusion, men with higher serum sclerostin levels had lower fracture risk after adjustment for BMD and other confounders. Given the low number of incident fractures in our study, further studies are required to validate these findings and to define the potential utility of sclerostin to identify men at high risk of fracture. It is plausible that low sclerostin levels are associated with higher fragility, but our study does not have sufficient statistical power to detect this trend. Higher BMD and lower BTM levels and fracture risk in men with higher sclerostin levels and the experimental data suggest that inhibition of sclerostin secretion may be found in men with higher bone fragility and may trigger bone formation to counteract this deterioration of bone strength. However, this speculation requires further studies using an experimental approach.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Disclosures
  8. Acknowledgements
  9. References

Supported in part by a contract INSERM/Merck Sharp & Dohme Chibret and by Societe de Secours Miniere de Bourgogne.

Authors' roles: PS was in charge of the MINOS study during its entire duration, collected the data, and performed the analyses. CB and OB were responsible for all biochemical measurements including measurements of sclerostin. FM was local coordinator of the MINOS study in Montceau les Mines and in charge of the contacts with the authorities of SSMB. RC contributed to the design of the study and obtained the necessary funds for measurements of sclerostin. All the authors contributed to the interpretation of the data, critically revised the content of the manuscript, and approved its final version. PS takes the responsibility for the integrity of the data analysis.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Disclosures
  8. Acknowledgements
  9. References