Bone Turnover Markers in Children: From Laboratory Challenges to Clinical Interpretation

Bone turnover markers (BTMs) have been developed many years ago to study, in combination with imaging techniques, bone remodeling in adults. In children and adolescents, bone metabolism differs from adults since it implies both growth and bone remodeling, suggesting an age- and gender-dependent BTM concentration. Therefore, specific studies have evaluated BTMs in not only physiological but also pathological conditions. However, in pediatrics, the use of BTMs in clinical practice is still limited due to these many children-related specificities. This review will discuss about physiological levels of BTMs as well as their modifications under pathological conditions in children and adolescents. A focus is also given on analytical and clinical challenges that restrain BTM usefulness in pediatrics.

Advances in bone turnover markers

Bone fragility fractures remain an important worldwide health and economic problem due to increased morbidity and mortality. The current methods for predicting fractures are largely based on the measurement of bone mineral density and the utilization of mathematical risk calculators based on clinical risk factors for bone fragility. Despite these approaches, many bone fractures remain undiagnosed. Therefore, current research is focused on the identification of new factors such as bone turnover markers (BTM) for risk calculation. BTM are a group of proteins and peptides released during bone remodeling that can be found in serum or urine. They derive from bone resorptive and formative processes mediated by osteoclasts and osteoblasts, respectively. Potential use of BTM in monitoring these phenomenon and therefore bone fracture risk is limited by physiologic and pathophysiologic factors that influence BTM. These limitations in predicting fractures explain why their inclusion in clinical guidelines remains limited despite the large number of studies examining BTM.

Monthly intravenous alendronate treatment can maintain bone strength in osteogenesis imperfecta patients following cyclical pamidronate treatment

Objectives Osteogenesis imperfecta (OI) is a skeletal dysplasia characterized by recurrent fractures due to congenital bone fragility. The only bisphosphonate approved for OI in Japan is pamidronate (PAM). To investigate whether monthly intravenous alendronate (ALN) infusions can maintain bone strength in OI children following cyclical PAM treatment. Methods A prospective and non-inferiority study was conducted. Eight school-age OI patients aged 8.5±2.0 years who were treated with cyclical PAM for 6.0±2.3 years were enrolled and switched to monthly intravenous ALN (0.030 mg/kg/month). Changes in L1-4 bone mineral density (BMD) Z-scores, fracture rates, and bone turnover markers for 12 months were analyzed. Results Average BMD Z-scores were -3.0±1.9, -2.9±2.0, and -2.2±2.0 in 12 months before enrollment, at enrollment, and after 12 months of ALN treatment, respectively. BMD Z-scores increased significantly during treatment with both PAM and ALN (p=0.012), and the effect of ALN was not inferior to that of PAM (p=0.67). There was no change in fracture rates (p=0.86) and bone turnover markers during the 12 months before and after enrollment. Additionally, ALN showed no remarkable side effects. Conclusions Our results suggest that monthly intravenous ALN can maintain bone strength after primary usage of cyclical PAM. We concluded that monthly intravenous ALN as a maintenance treatment following cyclical PAM administration can be an option for OI children.

Skeletal deterioration in COL2A1-related spondyloepiphyseal dysplasia occurs prior to osteoarthritis

OBJECTIVE

Spondyloepiphyseal dysplasia, a combination of progressive arthropathy with variable signs of skeletal dysplasia, can be a result of mutations in the collagen, type II, alpha 1 (COL2A1) gene. However, the bone involvement (e.g., density, microstructure) in this disorder has hitherto not been studied.

DESIGN

A 50-year-old female patient and her 8-year-old son with flattening of vertebral bodies and early-onset osteoarthritis were genetically tested using a custom designed gene bone panel including 386 genes. Bone microstructure and turnover were assessed using high-resolution peripheral quantitative computed tomography (HR-pQCT) and serum bone turnover markers, respectively. Furthermore, the bone and cartilage phenotype of male mice heterozygous for the loss-of-function mutation of Col2a1 (Col2a1^+/d) was analyzed compared to wildtype littermates using μ-CT and histomorphometry.

RESULTS

We identified a dominant COL2A1 mutation (c.620G > A p.(Gly207Glu)) indicating spondyloepiphyseal dysplasia in the female patient and her son, both being severely affected by skeletal deterioration. Although there was no osteoarthritis detectable at first visit, the son was affected by trabecular osteopenia, which progressed over time. In an iliac crest biopsy obtained from the mother, osteoclast indices were remarkably increased. Col2a1^+/d mice developed a moderate skeletal phenotype expressed by reduced cortical and trabecular parameters at 4 weeks. Importantly, no articular defects could be observed in the knee joints at 4 weeks, while osteoarthritis was only detectable in 12-week-old mice.

CONCLUSIONS

Our results indicate that collagen type II deficiency in spondyloepiphyseal dysplasia leads to skeletal deterioration with early-onset in humans and mice that occurs prior to the development of osteoarthritis.

Bone Turnover Markers in the Diagnosis and Monitoring of Metabolic Bone Disease

BACKGROUND

Disorders of bone metabolism, most notably osteoporosis, are highly prevalent and predispose to fractures, causing high patient morbidity and mortality. Diagnosis and monitoring of bone metabolic defects can present a major challenge as these disorders are largely asymptomatic and radiographic measures of bone mass respond slowly to changes in bone physiology.

CONTENT

Bone turnover markers (BTMs) are a series of protein or protein derivative biomarkers released during bone remodeling by osteoblasts or osteoclasts. BTMs can offer prognostic information on fracture risk that supplements radiographic measures of bone mass, but testing using BTMs has to take into account the large number of preanalytic factors and comorbid clinical conditions influencing BTM levels. BTMs respond rapidly to changes in bone physiology, therefore, they have utility in determining patient response to and compliance with therapies for osteoporosis.

SUMMARY

BTMs are a useful adjunct for the diagnosis and therapeutic monitoring of bone metabolic disorders, but their use has to be tempered by the known limitations in their clinical utility and preanalytic variables complicating interpretation.

Two years' experience with denosumab for children with osteogenesis imperfecta type VI

Background

Osteogenesis imperfecta (OI) is a hereditary disease causing reduced bone mass, increased fracture rate, long bone deformities and vertebral compressions. Additional non skeletal findings are caused by impaired collagen function and include hyperlaxity of joints and blue sclera. Most OI cases are caused by dominant mutations in COL1A1/2 affecting bone formation. During the last years, recessive forms of OI have been identified, mostly affecting posttranslational modification of collagen. In 2011, mutations in SERPINF1 were identified as the molecular cause of OI type VI, and thereby a novel pathophysiology of the disease was elucidated. The subgroup of patients with OI type VI are affected by an increased bone resorption, leading to the same symptoms as observed in patients with an impaired bone formation. Severely affected children are currently treated with intravenous bisphosphonates regardless of the underlying mutation and pathophysiology. Patients with OI type VI are known to have a poor response to such a bisphosphonate treatment.

Method

Deciphering the genetic cause of OI type VI in our 4 patients (three children and one adolescent) led to an immediate translational approach in the form of a treatment with the monoclonal RANKL antibody Denosumab (1 mg/kg body weight every 12 weeks).

Results

Short-term biochemical response to this treatment was reported previously. We now present the results after 2 years of treatment and demonstrate a long term benefit as well as an increase of bone mineral density, a normalization of vertebral shape, an increase of mobility, and a reduced fracture rate.

Conclusion

This report presents the first two-year data of denosumab treatment in patients with Osteogenesis imperfecta type VI and in Osteogenesis imperfecta in general as an effective and apparently safe treatment option.

A nonclassical IFITM5 mutation located in the coding region causes severe osteogenesis imperfecta with prenatal onset

Osteogenesis imperfecta (OI) is a hereditary connective tissue disorder characterized by a wide range of skeletal symptoms. Most patients have dominantly inherited or de novo mutations in COL1A1 or COL1A2. Up to 5% of patients have OI type V, characterized by hyperplastic callus formation after fractures, calcification of the interosseous membrane of the forearm, and a mesh-like lamellation pattern observed in bone histology. Recently, a heterozygous mutation in the 5'-untranslated region (UTR) of IFITM5 (c.-14C > T) was identified as the underlying cause of OI type V, and only this specific mutation was subsequently identified in all patient cohorts with this OI subtype. We now present a case of a heterozygous mutation within the coding region of IFITM5 (c.119C > T; p.S40L). The mutation occurred de novo in the patient and resulted in severe OI with prenatal onset and extreme short stature. At the age of 19 months, the typical clinical hallmarks of OI type V were not present. Our finding has important consequences for the genetic "work-up" of patients suspected to have OI, both in prenatal and in postnatal settings: The entire gene-not only the 5'-UTR harboring the "classical" OI type V mutation-has to be analyzed to exclude a causal role of IFITM5. We propose that this should be part of the initial diagnostic steps for genetic laboratories performing SANGER sequencing in OI patients.

Hyperosteoidosis and hypermineralization in the same bone: bone tissue analyses in a boy with a homozygous BMP1 mutation

Recently, homozygous mutations in BMP1 were identified as a cause of bone fragility in children with high areal bone mineral density. We examined iliac bone tissue from a 12-year-old boy with a homozygous mutation that leads to a p.Gly12Arg change in the signal peptide of BMP1, an enzyme that cleaves C-propeptide off the procollagen type I molecule. Histomorphometric analyses revealed marked hyperosteoidosis, with osteoid volume per bone volume at approximately 11 SD above the mean value for controls. At the same time, quantitative backscattered electron imaging showed drastic hypermineralization of mineralized bone matrix. CaPeak, representing the most frequently observed calcium content of mineralized matrix in trabecular bone, was 9 SD above the mean for the control population, corresponding to a 21 % higher calcium content in the patient specimen than in the average control sample. These results are similar to those that were previously reported in an individual who had a mutation in the C-propeptide cleavage site of procollagen type I. It thus appears that disturbed C-propeptide cleavage impairs mineralization in two ways: first, the onset of mineralization is delayed, leading to an increased amount of unmineralized osteoid, and second, once mineralization starts, too much mineral is incorporated into the bone matrix, resulting in hypermineralization.

Amino-terminal propeptide of C-type natriuretic peptide (NTproCNP) predicts height velocity in healthy children

OBJECTIVE

C-type natriuretic peptide (CNP) is a paracrine regulatory factor of the growth plate and plays a key role in endochondral growth. Its amino-terminal propeptide (NTproCNP) is an equimolar product of CNP biosynthesis and is easily measured in plasma. Preliminary studies suggest that NTproCNP levels correlate with height velocity in sheep and children. The objectives of the study were to correlate NTproCNP levels with height velocity and to define the reference range for plasma CNP and NTproCNP across childhood.

DESIGN

This was a prospective, cross-sectional, observational study of healthy children.

PATIENTS

Participants were 258 healthy children between 2 months and 20 years of age.

MEASUREMENTS

Anthropometrics were obtained and CNP and NTproCNP levels were determined by radioimmunoassay.

RESULTS

For both sexes, CNP and NTproCNP levels were high in infancy, lower in early childhood, rising during puberty, then falling to low adult levels. Levels of NTproCNP peaked at 14·1 years in boys and 11·9 years in girls, coincident with the age of peak height velocity. Levels of NTproCNP varied with pubertal status, peaking at genital Tanner stage IV in boys and III in girls. There was a highly significant correlation between NTproCNP and height velocity.

CONCLUSIONS

C-type natriuretic peptide plays an integral role in endochondral growth. We show here that CNP synthesis (as measured by NTproCNP levels in plasma) is closely related to linear growth in healthy children at all ages. We propose NTproCNP as a biomarker of linear growth.

Simultaneous free and glycosylated pyridinium crosslink determination in urine: validation of an HPLC-fluorescence method using a deoxypyridinoline homologue as internal standard

Pyridinoline (Pyr), deoxypyridinoline (D-Pyr), galactosyl-pyridinoline (Gal-Pyr) and glucosyl-galactosyl pyridinoline (GluGal-Pyr) are enzymatic mature pyridinium crosslinks. Generally, only total Pyr and D-Pyr urinary amounts (free+bound forms) are evaluated by HPLC as indices of bone resorption. This report describes the validation of an HPLC-fluorescence method for the simultaneous evaluation of free Pyr and D-Pyr, together with GluGal-Pyr and Gal-Pyr, in urine of healthy women (n=20, aged 27-41) and girls (n=20, aged 5-10). The use of an unnatural D-Pyr homologue, here proposed for the first time as internal standard, and of pure Pyr, D-Pyr, GluGal-Pyr and Gal-Pyr synthesized to be used as primary calibrators, guarantees method specificity and correct crosslink quantification. Urine, spiked with IS, was solid-phase extracted prior to HPLC analysis. Total Pyr and D-Pyr amounts were also evaluated after urine hydrolysis. The HPLC method was validated for selectivity, sensitivity, linearity, precision, accuracy, recovery and stability for all measured crosslinks. Both free and total Pyr and D-Pyr as well as GluGal-Pyr and Gal-Pyr amounts were significantly higher in girls than in women (p<0.0001), indicating an increased collagen turnover rather than only bone turnover. Gal-Pyr, for the first time evaluated in girls, was under its lower quantification limit (<LLOQ, <21.20 pmol/mL) in women. The measurement of free and glycosylated pyridinium crosslinks might provide more information on the degradation of various types of collagen than only that of total Pyr and D-Pyr. Moreover, this validated method could be a useful non-invasive technique for studying pathological conditions characterized by modified glycosylation enzyme activity and for more clinical investigation on bone fragility.

Predicting growth in response to growth hormone treatment

The use of growth hormone (GH) to treat children who have disturbances of growth is complicated by variability both within and across diagnostic groups, and at the start of and throughout treatment. Growth prediction models are important tools in the effort to account for these sources of variability and tailor GH treatment to each patient's needs. This review considers the methodological approach taken to the development of models from data in large databases such as the Pfizer International Growth Database (KIGS); it also assesses the limitations of these models and their data sources, and the potential for improvements. While all aspects of model development bear continued scrutiny and improvement, the incorporation of more predictors is key if treatment outcomes are to be optimized in terms of efficacy, safety and cost.

The relationship between body composition and the urinary excretion of deoxypyridinoline and galactosyl-hydroxylysine in children and adolescents

The study intends to investigate the relationship of body composition (%fat, percent body fat; FM, fat mass; FFM, fat free mass; FA and MA cross-sectional fat and muscle area) to the urinary excretion of deoxypyridinoline (DPD) and galactosyl-hydroxylysine (Gal-Hyl). 231 healthy children and adolescents (age 5-19 years; 112 males) of the DONALD study were analyzed for FM and FFM by measuring 4 skinfold thicknesses, for DPD and Gal-Hyl in urine samples and for bone parameters, FA and MA at the forearm by peripheral quantitative computed tomography. In contrast to adrenarchal females, adrenarchal males with low %fat had low levels of DPD and Gal-Hyl. %fat was correlated with DPD in pre-adrenarchal males (r = 0.290) and females (r = 0.298). Cortical bone mineral density (BMDcort) was correlated with DPD (r = -0.351) in adrenarchal males. Controlled for BMDcort, FM was correlated with DPD in pre-adrenarchal males (r = 0.348), and FA was correlated with DPD in pre-adrenarchal females (r = 0.294). FFM was negatively correlated with Gal-Hyl in adrenarchal males (r = -0.436) and females (r = -0.338). Less than 40% of variance of excreted DPD and Gal-Hyl was explained by regression models based on parameters of body composition. The effect of body composition explains the minor part of variance of the urinary excretion of DPD and Gal-Hyl. The association of body composition to excreted DPD and Gal-Hyl was not explained by the effect of adipose tissue on bone formation and bone resorption.

Automated HPLC assay for urinary collagen cross-links: effect of age, menopause, and metabolic bone diseases

BACKGROUND

The pyridinium cross-links pyridinoline (PYD) and deoxypyridinoline (DPD) are established markers of bone resorption. We evaluated the analytical and clinical performance of a commercially available PYD HPLC assay and established reference intervals in children and adults.

METHODS

We used a commercially available reagent set (Chromsystems Instruments & Chemicals) to measure PYD and DPD in 319 healthy controls (156 premenopausal women, 80 healthy men, and 83 healthy children age 1 month to 14 years) and 397 patients with metabolic bone diseases (postmenopausal osteoporosis, n = 175; male osteoporosis, n = 176; hyperparathyroidism, n = 17; hyperthyroidism, n = 19; Paget disease, n = 10).

RESULTS

The mean intraassay and interassay CVs were <6% and <8% for both PYD and DPD, respectively. The reference interval was constant for premenopausal women in the age group 20-49 years. In men, cross-link values peaked at 20-29 years and decreased thereafter. Women with postmenopausal osteoporosis had significantly higher PYD (51%) and DPD (58%) values compared to premenopausal women. Similar results were found in osteoporotic men. In children the highest values were found in the first weeks and months after birth, followed by a decrease of 50%-60% at age 11-14 years. In metabolic bone diseases cross-link concentrations were significantly increased. The DPD:PYD ratio (mean value approximately 0.2) was remarkably constant in all populations evaluated.

CONCLUSIONS

The automated HPLC assay is a precise and convenient method for PYD and DPD measurement. We established reference intervals for adult women and men and for children up to 14 years old. The cross-link concentrations we determined by use of this HPLC method confirm its clinical value in enabling identification of increased bone resorption in patients with metabolic bone diseases.

Markers of bone turnover are associated with growth and development in young subjects with sickle cell anemia

Children with sickle cell anemia (SCA) have low bone mass though bone turnover has not been well described. In this study, growth and pubertal development were assessed twice, 1 year apart, in 80 young subjects with type-SS SCA, while whole body bone mineral content (BMC) and density where measured in a subset (n = 46). Markers of bone turnover were measured at the second visit. Bone formation (alkaline phosphatase) was elevated, whereas bone resorption (deoxypyridinoline) was decreased compared to published data in healthy children. Markers of bone turnover correlated with growth velocity and pubertal development but not with changes in bone mass.

Sex- and age-specific reference curves for serum markers of bone turnover in healthy children from 2 months to 18 years

INTRODUCTION

This study aimed to establish sex- and age-specific reference curves enabling the calculation of z-scores and to examine correlations between bone markers and anthropometric data.

METHODS

Morning blood samples were obtained from 572 healthy children and adolescents (300 boys) aged 2 months to 18 yr. Height, weight, and pubertal stage were recorded. Serum osteocalcin (OC), bone-specific alkaline phosphatase (BALP), type-1 collagen degradation markers [carboxyterminal telopeptide region of type I collagen (ICTP), carboxyterminal telopeptide alpha1 chain of type I collagen (CTX)], and tartrate-resistant acid phosphatase (TRAP5b) were measured. Cross-sectional centile charts were created for the 3rd, 50th, and 97th centiles.

RESULTS

Apart from TRAP5b, all bone markers were nonnormally distributed, requiring logarithmic (BALP, OC, ICTP) or square root (CTX) transformation. Back-transformed centile curves for age and sex are presented for practical use. All bone markers varied with age and pubertal stage (P < 0.001). Significant correlations were found between sd score (SDS) for bone formation markers BALP and OC (r = 0.13; P = 0.004), SDS for collagen degradation markers ICTP and CTX (r = 0.14; P = 0.002), and SDS for the phosphatases (r = 0.34, P < 0.001). Height and weight SDS correlated weakly with some bone marker SDS, particularly with lnBALP SDS (r = 0.20 and 0.24, respectively; both P < 0.001).

CONCLUSION

This study provides reference curves for OC, BALP, CTX, ICTP, and TRAP5b in healthy children. Taller and heavier individuals for age had greater bone marker concentrations, likely reflecting greater growth velocity. SDS for markers of bone formation, collagen degradation, and phosphatases were each independently correlated, suggesting they derive from the same biological processes. The possibility of calculating SDS will facilitate monitoring of antiresorptive therapy or disease progression in children with metabolic bone disease.

Urinary excretion of deoxypyridinoline in Perthes' disease: a prospective, controlled comparative study in 83 children

Our aim was to investigate the relationship between urinary excretion of deoxypyridinoline (DPD) as a marker of bone resorption, and Perthes' disease. There were 39 children with Perthes' disease in the florid stage who collected first-morning urine samples at regular intervals of at least three months. The level of urinary DPD was analysed by chemiluminescence immunoassay and was correlated with the radiological stage of the disease as classified by Waldenström, and the severity of epiphyseal involvement according to the classification systems of Catterall and Herring. The urinary DPD levels of a group of 44 healthy children were used as a control. The median urinary DPD/creatinine (CREA) ratio was significantly reduced (p < 0.0001) in the condensation stage and increased to slightly elevated values at the final stage (p = 0.05) when compared with that of the control group. Herring-C patients showed significantly lower median DPD/CREA ratios than Herring-B patients (p = 0.03). The significantly decreased median DPD/CREA ratio in early Perthes' disease indicated a reduced bone turnover and supports the theory of a systemic aetiology. Urinary levels of DPD may therefore be used to monitor the course of Perthes' disease.

Pediatric reference intervals for bone markers

Bone markers are specific bone-derived molecules that reflect bone remodeling activity and can be classified into two categories: bone formation and bone resorption markers. Children have significantly elevated bone marker levels due to high skeletal growth velocity and rapid bone turnover during childhood growth. Many physiological and pathological processes may influence bone metabolism and bone marker concentrations during childhood growth. Measurements of bone markers may be useful in investigating skeletal diseases in children and monitoring the response to treatment. This review documents recent advances in analytical methods, preanalytical considerations related to each marker and particularly highlights the most valuable bone formation markers, bone alkaline phosphatase and osteocalcin, and bone resorption markers, pyridinium cross-links and cross-linked telopeptides. Age- and sex-specific pediatric reference intervals and their limitations in clinical application are also discussed.

Seasonality of growth response to GH therapy in prepubertal children with idiopathic growth hormone deficiency

OBJECTIVE

Longitudinal growth of children exhibits seasonal variation. In both healthy children and in children with growth hormone (GH) deficiency (GHD) receiving GH therapy, growth rate is maximal during spring and summer. In the present study, we analyzed the growth response to GH therapy in children with GHD as a function of the season when therapy was started.

SUBJECTS AND METHODS

Anthropometric measurements and biochemical analyses of GH secretion status and bone formation were longitudinally assessed in a cohort of 52 prepubertal children with GHD (14 girls, mean age 7.6 years) who were treated with a fixed dose of GH (0.025 mg/kg/day).

RESULTS

Auxological assessments over the 2-year observation period revealed a significantly better growth response to GH therapy in children who started therapy between the spring and summer (group 1) compared with children who started in the autumn or winter (group 2). The difference was largest in the initial 3-month treatment period (35%; P<0.01). The initial better gain in height of group 1 was sustained during the study period. Baseline peak GH levels during stimulation tests and insuin-like growth factor-I levels did not differ between the two groups. However, group 1 had significantly higher bone resorption and formation markers, either at the start or shortly after initiation of GH treatment. This suggests that children with GHD have higher bone turnover during spring and early summer, irrespective of GH therapy.

CONCLUSIONS

In summary, this study suggests that the season of GH initiation is a determinant of the initial growth response to GH replacement in prepubertal children with GHD.

Collagen turnover in bone diseases

PURPOSE OF REVIEW

The primary aim of this review is to highlight recent advances in techniques available for assessing collagen turnover, particularly in relation to the diagnosis and management of bone disorders. As collagen is the major protein constituent of bone, its metabolites form the basis of most of the biochemical markers, but their efficacy needs to be viewed in the context of other non-collagenous markers, for which methodology is also advancing rapidly.

RECENT FINDINGS

New markers of bone metabolism have been developed utilising the age-dependent isomerisation and racemisation of aspartyl residues at the C-terminal end of collagen. These methods allow measurement of the ratio between newly synthesised and mature collagen: this ratio appears to provide a novel indicator of the fracture risk for osteoporosis. Other studies have led to an improved understanding of biological variability, particularly in relation to the effects of feeding. Bone resorption assays have been applied to a wide range of diseases and have been especially useful in monitoring the efficacy of novel therapies.

SUMMARY

New assays have been developed to facilitate better monitoring of collagen metabolism in bone diseases. A more complete understanding of biological variability, particularly the effects of feeding, have led to improved clinical applicability of these assays in detecting disease and monitoring therapy. Part of the future challenge, however, is to ensure that commercial assay developments keep pace with clinical expectations.