Chronic kidney disease (CKD) is associated with nearly universal disturbances in mineral bone disease (MBD),1Bacchetta J. Harambat J. Cochat P. Salusky I.B. Wesseling-Perry K. The consequences of chronic kidney disease on bone metabolism and growth in children.,2Defective skeletal mineralization in pediatric CKD. manifesting as bone pain,3Groothoff J.W. Offringa M. Van Eck-Smit B.L. et al.Severe bone disease and low bone mineral density after juvenile renal failure. deformities,3Groothoff J.W. Offringa M. Van Eck-Smit B.L. et al.Severe bone disease and low bone mineral density after juvenile renal failure. growth retardation in children,4Haffner D. Schaefer F. Nissel R. et al.Effect of growth hormone treatment on the adult height of children with chronic renal failure. German Study Group for Growth Hormone Treatment in Chronic Renal Failure. fractures,3Groothoff J.W. Offringa M. Van Eck-Smit B.L. et al.Severe bone disease and low bone mineral density after juvenile renal failure.,5Denburg M.R. Kumar J. Jemielita T. et al.Fracture burden and risk factors in childhood CKD: results from the CKiD Cohort Study. and vascular calcification.6Shroff R. Long D.A. Shanahan C. Mechanistic insights into vascular calcification in CKD. Although assessment of bone health is integral to the care of patients with CKD, it remains a major challenge for physicians.7Bakkaloglu S.A. Bacchetta J. Lalayiannis A.D. et al.Bone evaluation in paediatric chronic kidney disease: clinical practice points from the European Society for Paediatric Nephrology CKD-MBD and Dialysis working groups and CKD-MBD working group of the ERA-EDTA. Histomorphometric analysis of bone biopsy specimens is the gold standard for diagnosis of renal bone disease8National Kidney Foundation
K/DOQI clinical practice guidelines for bone metabolism and disease in chronic kidney disease. but is invasive and not suited for screening and longitudinal monitoring. On the other hand, radiologic bone assessment, such as conventional X-ray imaging or dual-energy X-ray absorptiometry, although noninvasive and widely available, has limited sensitivity and specificity in predicting changes in bone turnover and mineralization.9Lalayiannis A.D. Crabtree N.J. Fewtrell M. et al.Assessing bone mineralisation in children with chronic kidney disease: what clinical and research tools are available?.,10

Lalayiannis AD, Crabtree NJ, Ferro CJ, et al. Routine serum biomarkers, but not dual-energy X-ray absorptiometry, correlate with cortical bone mineral density in children and young adults with chronic kidney disease. Nephrol Dial Transplant. Published online October 23, 2020. https://doi.org/10.1093/ndt/gfaa199

Circulating biomarkers, reflecting global skeletal activity, are recognized as important tools11Biochemical markers of bone turnover: why theory, research, and clinical practice are still in conflict.,12Using biochemical markers of bone turnover in clinical practice. to detect changes in bone homeostasis, enabling the assessment of both bone formation and resorption.13Fischer D.C. Mischek A. Wolf S. et al.Paediatric reference values for the C-terminal fragment of fibroblast-growth factor-23, sclerostin, bone-specific alkaline phosphatase and isoform 5b of tartrate-resistant acid phosphatase.,14Doyon A. Fischer D.C. Bayazit A.K. et al.Markers of bone metabolism are affected by renal function and growth hormone therapy in children with chronic kidney disease. Classical formation markers include bone alkaline phosphatase (BAP), a protein found on the surface of osteoblasts reflecting their activity. Bone resorption markers include tartrate-resistant acid phosphatase 5b (TRAP5b), an enzyme produced by osteoclasts reflecting their number,15Skeletal remodeling in health and disease., 16Minireview: the OPG/RANKL/RANK system., 17Transcriptional networks controlling skeletal development., 18Kubota T. Michigami T. Ozono K. Wnt signaling in bone metabolism. and C-terminal telopeptide of type I collagen. Also studied was sclerostin, a canonical Wnt signaling pathway inhibitor.19Brandenburg V.M. D'Haese P. Deck A. et al.From skeletal to cardiovascular disease in 12 steps-the evolution of sclerostin as a major player in CKD-MBD. More recently, the effects of chronic inflammation, which is common in patients with advanced CKD, has come into focus. Several conditions characterized by chronic inflammation are associated with excessive bone resorption,20David V. Martin A. Isakova T. et al.In inflammation and functional iron deficiency regulate fibroblast growth factor 23 production.,21Tsukasaki M. Takayanagi H. Osteoimmunology: evolving concepts in bone-immune interactions in health and disease. but the effects, if any, of different dialysis modalities on the inflammatory phenotype and CKD-related bone disease have not been studied.The HDF, Hearts and Height (3H) study, a multicenter, longitudinal study in children receiving hemodiafiltration (HDF) compared with conventional hemodialysis (HD),22Shroff R. Bayazit A. Stefanidis C.J. et al.Effect of haemodiafiltration vs conventional haemodialysis on growth and cardiovascular outcomes in children–the HDF, Heart and Height (3H) study. showed that subclinical cardiovascular disease is prevalent in children on dialysis, with attenuated progression of vascular changes in children receiving HDF compared with conventional HD.23Shroff R. Smith C. Ranchin B. et al.Effects of hemodiafiltration versus conventional hemodialysis in children with ESKD: the HDF, Heart and Height Study. In a cross-over trial when children on conventional HD were switched to HDF, and all other dialysis-related parameters were kept constant, a significant improvement in inflammation, antioxidant capacity, and endothelial risk profile was seen even within a short time (3 months),24Ağbaş A. Canpolat N. Çalışkan S. et al.Hemodiafiltration is associated with reduced inflammation, oxidative stress and improved endothelial risk profile compared to high-flux hemodialysis in children. but the effect of these biochemical changes on CKD-related bone disease was not explored. We present a post hoc analysis of the 3H data to determine the changes in bone biomarkers in children on dialysis and the effect of different dialysis modalities on the evolution of MBD over a 1-year follow-up.DiscussionDialysis is known to cause a proinflammatory milieu and is associated with profound dysregulation of mineral bone metabolism, but the impact of different dialysis modalities on CKD-MBD in adults and children remains largely undescribed. This prospective observational study, a post hoc analysis of bone-related outcomes in children on maintenance dialysis, has shown that children on HDF have an attenuated inflammatory profile, increased bone formation, stable sclerostin concentrations, and lower FGF23/klotho ratios compared with those on conventional HD. The ∼25% reduction in FGF23 and a lower FGF23/klotho ratio in patients on HDF may explain the lower left ventricular mass in the 3H cohort23Shroff R. Smith C. Ranchin B. et al.Effects of hemodiafiltration versus conventional hemodialysis in children with ESKD: the HDF, Heart and Height Study. and reduced cardiovascular mortality in adult randomized studies comparing HDF with conventional HD.30Maduell F. Moreso F. Pons M. et al.High-efficiency postdilution online hemodiafiltration reduces all-cause mortality in hemodialysis patients. HDF may achieve these effects through an improved clearance of the large middle-sized molecules, including proinflammatory cytokines and other “uremic toxins,” as well as reduced production of these molecules in the more biocompatible milieu in HDF. There was no difference in residual renal function in HD and HDF cohorts, and this did not influence markers of mineral metabolism.While bone turnover is best described on bone histology from tetracycline-labeled bone biopsy specimens, circulating biomarkers are useful surrogate measures: serum BAP levels are reflective of bone formation and TRAP5b of bone resorption.7Bakkaloglu S.A. Bacchetta J. Lalayiannis A.D. et al.Bone evaluation in paediatric chronic kidney disease: clinical practice points from the European Society for Paediatric Nephrology CKD-MBD and Dialysis working groups and CKD-MBD working group of the ERA-EDTA. Unlike other bone biomarkers, BAP and TRAP5b are both unaffected by CKD stage.31The use of bone turnover markers in chronic kidney disease-mineral and bone disorders. A large prospective observational study measuring a range of biomarkers in children with CKD stages 3 to 5 showed a high bone turnover, with a clear increase in the BAP z-score and a small but significant increase of TRAP5b z-scoress.14Doyon A. Fischer D.C. Bayazit A.K. et al.Markers of bone metabolism are affected by renal function and growth hormone therapy in children with chronic kidney disease. We showed that the BAP-to-TRAP5b activity remained almost constant in children on HD, whereas children on HDF had an increase in the BAP/TRAP ratio that was comparable to that seen in healthy children.13Fischer D.C. Mischek A. Wolf S. et al.Paediatric reference values for the C-terminal fragment of fibroblast-growth factor-23, sclerostin, bone-specific alkaline phosphatase and isoform 5b of tartrate-resistant acid phosphatase. Although we did not see an association with growth hormone treatment, a previous study showed that BAP was higher and TRAP5b was lower in children receiving recombinant growth hormone than in untreated controls,14Doyon A. Fischer D.C. Bayazit A.K. et al.Markers of bone metabolism are affected by renal function and growth hormone therapy in children with chronic kidney disease. further strengthening the case for an increased BAP/TRAP5b ratio reflecting osteoanabolism in the HDF cohort.We showed that the BAP/TRAP5b ratio inversely correlated with the inflammatory markers on univariable analysis. However, on multivariable analysis, adjustment for TNF-α attenuated but did not completely remove the difference between HD and HDF groups, suggesting that a reduction in the inflammatory milieu may contribute to but is not the sole cause of the observed improvement in the BAP/TRAP5b ratio in the HDF cohort. A similar inverse association of BAP and TRAP5b with CRP has been shown in children with CKD 3 to 5, implying that the effect of inflammation on reduced bone turnover is independent of the underlying renal disease or the effect of dialysis per se.14Doyon A. Fischer D.C. Bayazit A.K. et al.Markers of bone metabolism are affected by renal function and growth hormone therapy in children with chronic kidney disease.A recent cross-sectional study in 63 children with CKD stages 2 to 5 showed that TNF-α correlated with biomarkers of CKD-MBD and showed an inverse association with the height z-score, suggesting that inflammation may contribute to growth impairment in pediatric CKD.32Meza K. Biswas S. Zhu Y.S. et al.Tumor necrosis factor-alpha is associated with mineral bone disorder and growth impairment in children with chronic kidney disease. This study, however, showed an unexpected positive correlation between TNF-α and both bone formation markers alkaline phosphatase and BAP, making the inhibitory effect of inflammatory cytokines on bone turnover difficult to reconcile.32Meza K. Biswas S. Zhu Y.S. et al.Tumor necrosis factor-alpha is associated with mineral bone disorder and growth impairment in children with chronic kidney disease.Chronic inflammation is a multisystem disorder, and there are several potential mechanisms by which inflammatory cytokines can aggravate bone disease in CKD. TNF-α inhibits the expression of RUNX2, a major transcription factor that blocks osteoblast differentiation33Ding J. Ghali O. Lencel P. et al.TNF-alpha and IL-1beta inhibit RUNX2 and collagen expression but increase alkaline phosphatase activity and mineralization in human mesenchymal stem cells. and promotes osteoclastogenesis.34Lam J. Takeshita S. Barker J.E. Kanagawa O. Ross F.P. Teitelbaum S.L. TNF-alpha induces osteoclastogenesis by direct stimulation of macrophages exposed to permissive levels of RANK ligand. Receptor activator of nuclear factor κB ligand (RANKL; also known as TNF ligand superfamily, member 11) is an essential cytokine in osteoclastogenesis but may play a paradoxical role in bone homeostasis by also increasing the expression of bone morphogenic protein 2 and stimulating the Wnt pathway.35Osta B. Benedetti G. Miossec P. Classical and paradoxical effects of TNF-alpha on bone homeostasis. Binding of RANKL to the RANK receptor leads to activation of TNF receptor-associated factors and upregulation of osteoclast target genes.36Krum S.A. Chang J. Miranda-Carboni G. Wang C.Y. Novel functions for NFkappaB: inhibition of bone formation.TNF-α inhibitors are used in chronic inflammatory disorders of childhood, such as inflammatory bowel disease and juvenile idiopathic arthritis, wherein children treated with TNF-α inhibitors showed improved trabecular bone mineral density and cortical structure,37Griffin L.M. Thayu M. Baldassano R.N. et al.Improvements in bone density and structure during anti-TNF-alpha therapy in pediatric Crohn’s disease. reduced bone loss,38Simonini G. Giani T. Stagi S. de Martino M. Falcini F. Bone status over 1 yr of etanercept treatment in juvenile idiopathic arthritis. and greater statural growth.39Tynjala P. Lahdenne P. Vahasalo P. Kautiainen H. Honkanen V. Impact of anti-TNF treatment on growth in severe juvenile idiopathic arthritis.,40Malik S. Ahmed S.F. Wilson M.L. et al.The effects of anti-TNF-alpha treatment with adalimumab on growth in children with Crohn’s disease (CD). Our data suggest that high serum phosphate levels negatively influence bone turnover. This effect may be mediated via the parathyroid hormone effect on RANKL-directed osteoclastogenesis or even via stimulating inflammation. Dietary phosphate loading increased serum TNF-α in uremic rats.41Yamada S. Tokumoto M. Tatsumoto N. et al.Phosphate overload directly induces systemic inflammation and malnutrition as well as vascular calcification in uremia. After parathyroidectomy, bone expression of TNF-α significantly decreases in patients on HD.42Santos F.R. Moyses R.M. Montenegro F.L. Jorgetti V. Noronha I.L. IL-1beta, TNF-alpha, TGF-beta, and bFGF expression in bone biopsies before and after parathyroidectomy.Sclerostin, a potent inhibitor of the Wnt/β-catenin pathway,43Drueke T.B. Lafage-Proust M.H. Sclerostin: just one more player in renal bone disease?. showed an early reduction in plasma concentrations with convective clearance.24Ağbaş A. Canpolat N. Çalışkan S. et al.Hemodiafiltration is associated with reduced inflammation, oxidative stress and improved endothelial risk profile compared to high-flux hemodialysis in children. Children on HDF in this study had sclerostin levels comparable to those with moderate to severe CKD in the series published by Doyon et al.,14Doyon A. Fischer D.C. Bayazit A.K. et al.Markers of bone metabolism are affected by renal function and growth hormone therapy in children with chronic kidney disease. although levels in both groups were lower than in age-matched healthy children.44Kirmani S. Amin S. McCready L.K. et al.Sclerostin levels during growth in children. Serum sclerostin levels in healthy children were inversely associated with cortical volumetric bone mineral density and cortical thickness,44Kirmani S. Amin S. McCready L.K. et al.Sclerostin levels during growth in children. and lower sclerostin levels were predictive of high bone turnover even in adults with CKD.45Cejka D. Herberth J. Branscum A.J. et al.Sclerostin and Dickkopf-1 in renal osteodystrophy.FGF23, a phosphaturic hormone produced by osteocytes, is a 32-kDa protein that is known to be cleared by HDF. Comparable to previous studies, we showed ∼25% lower FGF23 levels in HDF children,29Pérouse de Montclos T. Ranchin B. Leclerc A.L. et al.Online hemodiafiltration in children and hypoparathyroidism: a single-centre series of cases. Article in French. whereas levels increased by >100% in HD children. As seen with the dialytic clearance of other large middle-sized molecules compounds such as FGF23, IL-6, and TNF-α, although an early reduction in plasma concentrations is seen with convective clearance,24Ağbaş A. Canpolat N. Çalışkan S. et al.Hemodiafiltration is associated with reduced inflammation, oxidative stress and improved endothelial risk profile compared to high-flux hemodialysis in children. this is not sufficient to further reduce plasma concentrations in the long-term, perhaps because production of these compounds overwhelms the clearance capacity of thrice-weekly dialysis.28Carlson N. Mortensen O.H. Axelsen M. Pedersen R.S. Heaf J.G. Clearance of sclerostin, osteocalcin, fibroblast growth factor 23, and osteoprotegerin by dialysis. Although protective in early CKD, FGF23 is known to have several off-target effects, independent of changes in mineral metabolism, on cardiac myocytes,46Faul C. Amaral A.P. Oskouei B. et al.FGF23 induces left ventricular hypertrophy. with an increased prevalence of left ventricular hypertrophy47Gutiérrez O.M. Januzzi J.L. Isakova T. et al.Fibroblast growth factor 23 and left ventricular hypertrophy in chronic kidney disease. and premature death in adults with all stages of CKD,48Gutiérrez O.M. Mannstadt M. Isakova T. et al.Fibroblast growth factor 23 and mortality among patients undergoing hemodialysis. stressing the importance of reducing FGF23 levels by HDF. Importantly, these changes in FGF23 levels were independent of serum phosphate, which did not change in the HD or HDF cohorts. Nevertheless, serum phosphate levels are a poor proxy of phosphate exposure, and other groups have shown that phosphate control is better in patients on HDF compared with HD.49Lornoy W. De Meester J. Becaus I. et al.Impact of convective flow on phosphorus removal in maintenance hemodialysis patients. Similarly, FGF23 levels are significantly lower in patients on short daily HD compared with those on conventional thrice-weekly HD, suggesting that FGF23 levels may be a more sensitive biomarker of cumulative phosphate burden.50Zaritsky J. Rastogi A. Fischmann G. et al.Short daily hemodialysis is associated with lower plasma FGF23 levels when compared with conventional hemodialysis.FGF23 expression is markedly increased in acute inflammation, but osteocytes are able to maintain normal serum levels of biologically active FGF23, despite the rise in FGF23 transcription, by commensurately increasing FGF23 cleavage.20David V. Martin A. Isakova T. et al.In inflammation and functional iron deficiency regulate fibroblast growth factor 23 production. In contrast, chronic inflammation also increases biologically active intact FGF23 levels, perhaps because sustained periods of FGF23 overproduction overwhelm the capacity of the FGF23 cleavage apparatus in osteocytes. Measurement of FGF23 fragments may have revealed a stronger association between FGF23 concentrations and inflammation. Cytokine profiling in a cohort of adults with predialysis CKD has shown that several cytokines may play a role in increasing the production of FGF23.51Egli-Spichtig D. Imenez Silva P.H. Glaudemans B. et al.Tumor necrosis factor stimulates fibroblast growth factor 23 levels in chronic kidney disease and non-renal inflammation.,52Wallquist C. Mansouri L. Norrbäck M. et al.Associations of fibroblast growth factor 23 with markers of inflammation and leukocyte transmigration in chronic kidney disease. Furthermore, inflammation may also indirectly stimulate the production of FGF23 through the activation of hepcidin and induction of hypoferremia.20David V. Martin A. Isakova T. et al.In inflammation and functional iron deficiency regulate fibroblast growth factor 23 production. Thus, in patients on dialysis, FGF23 can aggravate the bone disease of CKD and lose any protective role it may have in countering hyperphosphatemia in the early stages of CKD.