Table 1 Major sources of variability in biochemical markers of bone turnover
From: The clinical utility of bone marker measurements in osteoporosis
Bone marker (Abbreviation) | Source | Action | Advantages | Disadvantages | Analysis and sample type |
|---|---|---|---|---|---|
Formation markers | |||||
Bone Alkaline Phosphatase (BAP) | Enzyme present in osteoblast plasma membranes | Enzymatic degradation of the mineralisation inhibitor pyrophosphate at alkaline pH | Low intra-individual variability <10% [15] Not affected by renal function [15] Food has little effect [16] Long circulatory half-life 1–2 days [17] Sample stability [18] Cheap | Up to 20% cross reactivity with liver isoforms [14] Changes with therapy minimal i.e. less than LSC of 25% [15] 2 peaks at 14:00 and 23:30 hrs Nadir 30% ↓at 06:30 [19] Multiple methodologies, can measure mass or activity [20] | Automated and manual immunoassays Serum, EDTA plasma |
Osteocalcin (OC) | Major non-collagen bone Gla protein. Produced by osteoblasts during bone formation and bound to hydroxyapatite | Influences osteoid mineralisation Provides negative feedback during remodelling process | EDTA sample more stable [21] Late marker of osteoblast activity [15] | Intact molecule unstable [15] Large inter-lab variation [20] Released during formation and resorption [17] Short half-life of a few minutes [22, 23] Influenced by Vit K status, renal function and circadian variability [15, 17] OC gene regulated at transcriptional level by 1,25-OH2 Vit D Vit K essential co-factor for γ-carboxylation of OC resulting in ↑ affinity for Ca and hydroxyapatite [14] | Automated and manual immunoassays Multiplex microarray Serum, EDTA plasma |
Procollagen type 1 Carboxy-terminal Propeptide (P1CP) | Specific product of proliferating osteoblasts and fibroblasts. | Cleaved from type 1 pro-collagen by proteases during type 1 collagen formation | Quantitative measure of newly formed type 1 collagen Thermostability [14] | Short half-life 6-8mins [14] Cleared in liver by mannose receptor so sensitive to thyroid hormones and IGF-1 [20] Highest levels 01:30 – 04:30, up to 20% higher than nadir 11:00 – 15:00 [19] Lacks sensitivity to changes during menopause [14] | Automated and manual immunoassays Serum, EDTA plasma |
*Procollagen type 1 amiNo-terminal Propeptide (P1NP) | Specific product of proliferating osteoblasts and fibroblasts. | Cleaved from type 1 pro-collagen by proteases during type 1 collagen formation | Low intra-individual variability[20] Small circadian rhythm[15] Stable at room temp[21] Good assay precision[20] Superior for PMO monitoring - change from baseline ↑up to 80% with anti-resorptive and ↓up to 200% with PTH medication within 3months[15] | Total assay affected by delayed clearance of monomeric fraction e.g. in renal failure or metastatic bone disease[24] Expensive | Automated and manual immunoassays Multiplex microarray Total or Intact fractions Serum, EDTA plasma |
Resorption markers Collagen derived | |||||
*Carboxy-Terminal cross-linked telopeptides of type 1 collagen (CTX) | Type 1 collagen mainly bone Isomerisation to β aspartyl occurs in mature collagen | Cleaved from type 1 collagen by cathepsin-K during bone resorption | Variability↓ fasting[25] Sample stability, especially EDTA[18][21] Substantial ↓ post anti-resorptive treatment[26] Blood sample now preferential | Large circadian variation – highest 01:30 – 04:30 approx 2x nadir 11:00–15:00[27] | Automated and manual immunoassays Multiplex microarray Urine, serum, EDTA plasma |
Carboxy-Terminal cross-linked telopeptides of type 1 collagen (ICTP or CTX-MMP) | Newly synthesised type 1 collagen predominantly bone | Cleaved from type 1 collagen by MMP during bone resorption | Â | Large circadian variation [20] Influenced by renal and liver function [20] Not responsive to usual osteoporotic treatments [20] | Manual immunoassay |
amiNo-Terminal cross-linked telopeptides of type 1 collagen (NTX) | Type 1 collagen mainly bone | Cleaved from type 1 collagen by cathepsin-K during bone resorption | Urine sample stable [15] uNTX sig predictor of fracture risk in postmenopausal women [28] Small dietary influence, although fasting blood sample preferred [15] | Large circadian variation Influenced by renal and liver function [20] Units based on manufacturer’s calibrator i.e. bone collagen equivalents [15] | Automated and manual immunoassays Urine, serum, EDTA plasma |
Type 1 collagen alpha 1 helicoidal peptide (HELP) | Type 1 collagen Amino acid 620–633 sequence of the α chain | Cleaved from helical region of type 1 collagen by cathepsin-K during bone resorption | High correlation to other markers of collagen degradation [14] | 24 hr collection – hard to collect 2nd morning void with creatinine correction – additional analytical variability Clinical validity needs further investigation | Manual immunoassay Urinary marker |
Deoxypyridinoline (DPD) | Mature type 1 collagen | Cross link released when mature type 1 collagen breaks down Mechanically stabilise the molecule | Reflect degradation of mature collagen only Specific to bone [14] Independent of dietary sources [20] Less invasive than blood | 24 hr collection – hard to collect 2nd morning void with creatinine correction – additional analytical variability Circadian variation [20] | Automated and manual immunoassays Urinary marker |
Pyridinoline (PYD) | Mature type 1 and 11 collagen | Cross link released when mature collagen type 1 and 11 breaks down Mechanically stabilise the molecule | Reflect degradation of mature collagen only Independent of dietary sources [20] | Non-specific 24 hr collection – hard to collect 2nd morning void with creatinine correction – additional analytical variability Circadian variation [20] Influenced by liver function [20] | Automated and manual immunoassays Urinary marker |
Resorption markers Osteoclastic Enzymes | |||||
Tartrate Resistant Acid Phosphatase –isoform 5b (TRAP5b) | Isoform of acid phosphatase, resistant to tartrate, cleaved by proteases into 5b, present in ruffled border of osteoclasts | Cleaves type 1 collagen into fragments | Characteristic of osteoclastic activity [14] | Unstable at room temperature [22, 23] Circadian variability ↑ immediately after exercise [29] | Automated and manual immunoassays Serum |
Cathepsin K | Cysteine protease present in ruffled border of actively resorbing osteoclasts | Cleaves telopeptide and helical regions of type 1 collagen | Specific biomarker of osteoclastic activity [14] | Unstable at room temp Clinical validity needs further investigation | Manual immunoassay Serum, EDTA plasma |
Osteocyte activity markers | |||||
Receptor Activator of Nuclear factor Kappa B Ligand (RANKL) | Produced by osteoblasts, activated by B and T cells | Binds to RANK, which is expressed on osteoclasts and their precursors, stimulating their differentiation and activity | Novel biomarker Provide safety, efficacy and pharmacokinetics data to confirm drug mechanisms and mode of action for future use | Analytical problems Can measure free or OPG-bound [30] Circulating levels may not reflect bone microenvironment [31] Affected by thyroid function [32] Research method only Clinical and analytical validity needs further investigation | Manual research –grade immunoassay Total or soluble forms in serum |
Osteoprotegerin (OPG) | Secreted by osteoblasts | Decoy receptor to RANKL reduces bone resorption by binding to RANK and preventing osteoclastogenesis | Novel biomarker Provide safety, efficacy and pharmacokinetics data to confirm drug mechanisms and mode of action for future use | Affected by thyroid function [32] Research method only Clinical and analytical validity needs further investigation | Manual research-grade immunoassay Serum |
Dickkopf-related protein 1 (DKK1) | Produced by osteocytes | Inhibition of Wnt signalling pathway through binding to LRP5/6, blocking the Wnt effects on osteoblasts and decreasing bone formation | Key role in regulation of bone turnover | Research method only Clinical and analytical validity needs further investigation | Manual research –grade immunoassay Serum |
Sclerostin (SCL) | Secreted by osteocytes | Inhibition of Wnt signalling pathway through binding to LRP5/6, blocking the Wnt effects on osteoblasts and decreasing bone formation | Significant ↓ with PTH therapy [33] | Research method only Affected by immobility [34] ↑ in type 1 and 2 diabetes [35, 36] Clinical and analytical validity needs further investigation | Manual research-grade immunoassaySerum |