Only bioactive forms of PTH (n-oxPTH and Met18(ox)-PTH) inhibit synthesis of sclerostin - evidence from in vitro and human studies

Sclerostin (SOST) is produced by osteocytes and is known as a negative regulator of bone homeostasis. Parathyroid hormone (PTH) regulates calcium, phosphate as well as vitamin D metabolism, and is a strong inhibitor of SOST synthesis in vitro and in vivo. PTH has two methionine amino acids (positions 8 and 18) which can be oxidized. PTH oxidized at Met18 (Met18(ox)-PTH) continues to be bioactive, whereas PTH oxidized at Met8 (Met8(ox)-PTH) or PTH oxidized at Met8 and Met18 (Met8, Met18(di-ox)-PTH) has minor bioactivity. How non-oxidized PTH (n-oxPTH) and oxidized forms of PTH act on sclerostin synthesis is unknown. The effects of n-oxPTH and oxidized forms of PTH on SOST gene expression were evaluated in UMR106 osteoblast-like cells. Moreover, we analyzed the relationship of SOST with n-oxPTH and all forms of oxPTH in 516 stable kidney transplant recipients using an assay system that can distinguish in clinical samples between n-oxPTH and the sum of all oxidized PTH forms (Met8(ox)-PTH, Met18(ox)-PTH, and Met8, Met18(di-ox)-PTH). We found that both n-oxPTH and Met18(ox)-PTH at doses of 1, 3, 20, and 30 nmol/L significantly inhibit SOST gene expression in vitro, whereas Met8(ox)-PTH and Met8, Met18(di-ox)-PTH only have a weak inhibitory effect on SOST gene expression. In the clinical cohort, multivariate linear regression showed that only n-oxPTH, but not intact PTH (iPTH) nor oxPTH, is independently associated with circulating SOST after adjusting for known confounding factors. In conclusion, only bioactive PTH forms such as n-oxPTH and Met18(ox)-PTH, inhibit SOST synthesis.

Inverse correlation of intact PTH, oxidized PTH as well as non-oxidized PTH with 25-hydroxyvitamin D3 in kidney transplant recipients

Background

25-hydroxyvitamin D (25(OH)D) and potentially also 1,25-dihydroxyvitamin D (1,25(OH)2D) inhibits the synthesis of parathyroid hormone (PTH) in the chief cells of the parathyroid gland. Clinical studies showing a negative correlation between (25(OH)D and PTH are in good agreement with these findings in basic science studies. However, PTH was measured in these studies with the currently clinically used 2nd or 3rd generation intact PTH (iPTH) assay systems. iPTH assays cannot distinguish between oxidized forms of PTH and non-oxidized PTH. Oxidized forms of PTH are the by far most abundant form of PTH in the circulation of patients with impaired kidney function. Oxidation of PTH causes a loss of function of PTH. Given that the clinical studies done so far were performed with an PTH assay systems that mainly detect oxidized forms of PTH, the real relationship between bioactive non-oxidized PTH and 25(OH)D as well as 1,25(OH)2D is still unknown.

Methods

To address this topic, we compared for the first time the relationship between 25(OH)D as well as 1,25(OH)2D and iPTH, oxPTH as well as fully bioactive n-oxPTH in 531 stable kidney transplant recipients in the central clinical laboratories of the Charité. Samples were assessed either directly (iPTH) or after oxPTH (n-oxPTH) was removed using a column that used anti-human oxPTH monoclonal antibodies, a monoclonal rat/mouse parathyroid hormone antibody (MAB) was immobilized onto a column with 500 liters of plasma samples. Spearman correlation analysis and Multivariate linear regression were used to evaluate the correlations between the variables.

Results

There was an inverse correlation between 25(OH)D and all forms of PTH, including oxPTH (iPTH: r=-0.197, p<0.0001; oxPTH: r=-0.203, p<0.0001; n-oxPTH: r=-0.146, p=0.001). No significant correlation was observed between 1,25(OH)2D and all forms of PTH. Multiple linear regression analysis considering age, PTH (iPTH, oxPTH and n-oxPTH), serum calcium, serum phosphor, serum creatinine, fibroblast growth factor 23 (FGF23), osteoprotegerin (OPG), albumin, and sclerostin as confounding factors confirmed these findings. Subgroup analysis showed that our results are not affected by sex and age.

Conclusion

In our study, all forms of PTH are inversely correlated with 25-hydroxyvitamin D (25(OH)D). This finding would be in line with an inhibition of the synthesis of all forms of PTH (bioactive n-oxPTH and oxidized forms of PTH with minor or no bioactivity) in the chief cells of the parathyroid glad.

Unapparent systemic effects of regional anticoagulation with citrate in continuous renal replacement therapy: a narrative review

The use of citrate, through reversible binding of calcium, has become the preferred choice for anticoagulation in continuous renal replacement therapy in the critically ill patient. Though generally considered as very efficacious in acute kidney injury, this type of anticoagulation can cause acid-base disorders as well as citrate accumulation and overload, phenomena which have been well described. The purpose of this narrative review is to provide an overview of some other, non-anticoagulation effects of citrate chelation during its use as anticoagulant. We highlight the effects seen on the calcium balance and hormonal status, phosphate and magnesium balance, as well as oxidative stress resulting from these unapparent effects. As most of these data on these non-anticoagulation effects have been obtained in small observational studies, new and larger studies documenting both short- and long-term effects should be undertaken. Subsequent future guidelines for citrate-based continuous renal replacement therapy should take not only the metabolic but also these unapparent effects into account.

Preanalytical considerations in parathyroid hormone measurement

Automated immunoassays used to evaluate parathyroid function are vulnerable to different types of interference, which can affect clinical practices. This review provides a detailed overview of the six main types of interference known to affect the measurement of parathyroid hormone (PTH): heterophilic antibodies, biotin, PTH fragments, oxidized PTH (oxPTH), phosphorylated PTH, and some preanalytical factors. Because the prevalence of some of these conditions has been reported to approach 11.7%, and the frequency of testing for parathyroid function is important, the scale of the problem might be tremendous. Potential interference in parathyroid function testing should always be suspected whenever clinical or biochemical discrepancies arise. Their identification typically relies on additional laboratory tests, including method comparison, serial dilution, blocking reagent studies, affinity adsorption, and polyethylene glycol precipitation. Moreover, some of these issues can be mitigated with the development of mass spectrometry. This review also evaluated the clinical impact of parathyroid interference on immunoassays, including misdiagnosis, inappropriate parathyroidectomy; and delay in receiving appropriate therapy. Hence, strong communication should be maintained between the clinician and laboratory to avoid such scenarios.

High-Resolution Mass Spectrometry for the Measurement of PTH and PTH Fragments: Insights into PTH Physiology and Bioactivity

Full-length parathyroid hormone (PTH 1-84) is crucial for the regulation of calcium and phosphate homeostasis and bone remodeling. PTH 1-84 is metabolized into various PTH fragments, which are measured with varying levels of efficiency by PTH immunoassays. These PTH fragments, which increase in serum as CKD progresses, could potentially modulate the effects of PTH 1-84 and contribute to CKD-associated bone disorders. To obtain a true biologic representation of total PTH bioactivity, it is necessary to measure not only PTH 1-84 but also PTH fragments that are present in circulation. Traditional second-generation PTH immunoassays collectively measure PTH 1-84, PTH fragments, and post-translationally modified PTH 1-84, making it difficult to accurately predict the character of underlying renal osteodystrophy. This review highlights current advances in methods available for PTH measurement and the clinical relevance of PTH fragments in CKD. We emphasize the usefulness of mass spectrometry as a potential reference method for PTH measurement.

Mutation of Methionine to Asparagine but Not Leucine in Parathyroid Hormone Mimics the Loss of Biological Function upon Oxidation

Human parathyroid hormone (PTH) is an 84-amino acid peptide that contains two methionine (Met) residues located at positions 8 and 18. It has long been recognized that Met residues in PTH are subject to oxidation to become Met sulfoxide, resulting in a decreased biological function of the peptide. However, the mechanism of the lost biological function of PTH oxidation remains elusive. To characterize whether the shift from the hydrophobic nature of the native Met residue to the hydrophilic nature of Met sulfoxide plays a role in the reduction of biological activity upon PTH oxidation, we conducted in silico and in vitro site-directed mutagenesis of Met-8 and Met-18 to the hydrophilic residue asparagine (Asn) or to the hydrophobic residue leucine (Leu) and compared the behavior of these mutated peptides with that of PTH oxidized at Met-8 and/or Met-18. Our results showed that the biological activity of the Asn-8 and Asn-8/Asn-18 mutants was significantly reduced, similar to Met-8 sulfoxide and Met-8/Met-18 sulfoxide analogues, while the functions of Asn-18, Leu-8, Leu-8/Leu-18 mutants, or Met-18 sulfoxide analogues were similar to wild-type PTH. This is rationalized from molecular modeling and immunoprecipitation assay, demonstrating disruption of hydrophobic interactions between Met-8 and Met-18 of PTH and type-1 PTH receptor (PTHR1) upon mutation or oxidation. Thus, these novel findings support the notion that the loss of biological function of PTH upon oxidation of Met-8 is due, at least in part, to the conversion from a hydrophobic to a hydrophilic residue that disrupts direct hydrophobic interaction between PTH and PTHR1.

Citrate dose for continuous hemofiltration: effect on calcium and magnesium balance, parathormone and vitamin D status, a randomized controlled trial

Background

Regional citrate anticoagulation may cause a negative calcium balance, systemic hypocalcemia and parathormone (PTH) activation but randomzed studies are not available. Aim was to determine the effect of citrate dose on calcium (Ca) and magnesium (Mg) balance, PTH and Vitamin D.

Methods

Single center prospective randomized study. Patients, requiring continuous venovenous hemofiltration (CVVH) with citrate, randomized to low dose citrate (2.5 mmol/L) vs. high dose (4.5 mmol/L) for 24 h, targeting post-filter ionized calcium (pfiCa) of 0.325–0.4 mmol/L vs. 0.2–0.275 mmol/L, using the Prismaflex® algorithm with 100% postfilter calcium replacement. Extra physician-ordered Ca and Mg supplementation was performed aiming at systemic iCa > 1.0 mmol/L. Arterial blood, effluent and post-filter aliquots were taken for balance calculations (area under the curve), intact (i), oxidized (ox) and non-oxidized (nox) PTH, 25-hydroxy-Vitamin D (25D) and 1,25-dihydroxy-Vitamin D (1,25D).

Results

35 patients were analyzed, 17 to high, 18 to low citrate. Mean 24-h Ca balance was - 9.72 mmol/d (standard error 1.70) in the high vs − 1.18 mmol/d (se 1.70)) (p = 0.002) in the low citrate group and 24-h Mg-balance was − 25.99 (se 2.10) mmol/d vs. -17.63 (se 2.10) mmol/d (p = 0.008) respectively. Physician-ordered Ca supplementation, higher in the high citrate group, resulted in a positive Ca-balance in both groups. iPTH, oxPTH or noxPTH were not different between groups. Over 24 h, median PTH decreased from 222 (25th–75th percentile 140–384) to 162 (111–265) pg/ml (p = 0.002); oxPTH from 192 (124–353) to 154 pg/ml (87–231), p = 0.002. NoxPTH did not change significantly. Mean 25 D (standard deviation), decreased from 36.5 (11.8) to 33.3 (11.2) nmol/l (p = 0.003), 1,25D rose from 40.9 pg/ml (30.7) to 43.2 (30.7) pg/ml (p = 0.046), without differences between groups.

Conclusions

A higher citrate dose caused a more negative CVVH Ca balance than a lower dose, due to a higher effluent Calcium loss. Physician-ordered Ca supplementation, targeting a systemic iCa > 1.0 mmol/L, higher in the high citrate group, resulted in a positive Ca-balance in both groups. iPTH and oxPTH declined, suggesting decreased oxidative stress, while noxPTH did not change. 25D decreased while 1,25-D rose. Mg balance was negative in both groups, more so in the high citrate group.

Trial registration

ClinicalTrials.gov: NCT02194569. Registered 18 July 2014.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02598-2.

Should clinical laboratories adapt to the reality of chronic kidney disease in the determination of parathyroid hormone?

Objectives

The contribution of the clinical laboratory to diagnostics is increasingly important since a great deal of clinical decisions rely on laboratory test results.

Content

Parathyroid hormone (PTH) measurement presents a considerable analytical variability due to the heterogeneity of its circulating forms and the antigenic configuration of the different assays commercially available. Such variability may have an impact on pathological conditions associated with significant increases in circulating PTH, as it is the case of chronic kidney disease (CKD).

Summary

Despite the recent identification of new molecules involved in bone and mineral disorders associated with CKD, such as klotho or the fibroblastic factor 23 (FGF23), nephrologists still base their clinical decisions on PTH concentrations. The problem is that unawareness of these analytical considerations may cause errors in the clinical interpretation of test results.

Outlook

This systematic review addresses these issues from the clinical laboratory perspective and proposes new approaches related to PTH method selection and result expression. These new strategies will help laboratory medicine specialists and nephrologist better determine the status of CKD patients.

Parathyroid hormone oxidation in chronic kidney disease: clinical relevance?

In chronic kidney disease, parathyroid hormone (PTH), like all proteins, can undergo post-translational modifications, including oxidation. This can lead to structural and functional changes of the hormone. It has been hypothesized that currently used PTH measurement methods do not adequately reflect PTH-related bone and cardiovascular abnormalities in chronic kidney disease owing to the presence of oxidized, biologically inactive PTH in the circulation. Ursem et al. now report a strong correlation between serum non-oxidized and total PTH, and comparable associations with histomorphometric and circulating bone turnover markers, pleading against this hypothesis.

Relationship between GFR, intact PTH, oxidized PTH, non-oxidized PTH as well as FGF23 in patients with CKD

Fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH) are regulators of renal phosphate excretion and vitamin D metabolism. In chronic kidney disease (CKD), circulating FGF23 and PTH concentrations progressively increase as renal function declines. Oxidation of PTH at two methionine residues (positions 8 and 18) causes a loss of function. The impact of n-oxPTH and oxPTH on FGF23 synthesis, however, and how n-oxPTH and oxPTH concentrations are affected by CKD, is yet unknown. The effects of oxidized and non-oxidized PTH 1-34 on Fgf23 gene expression were analyzed in UMR106 osteoblast-like cells. Furthermore, we investigated the relationship between n-oxPTH and oxPTH, respectively, with FGF23 in two independent patients' cohorts (620 children with CKD and 600 kidney transplant recipients). While n-oxPTH stimulated Fgf23 mRNA synthesis in vitro, oxidation of PTH in particular at Met8 led to a markedly weaker stimulation of Fgf23. The effect was even stronger when both Met8 and Met18 were oxidized. In both clinical cohorts, n-oxPTH-but not oxPTH-was significantly associated with FGF23 concentrations, independent of known confounding factors. Moreover, with progressive deterioration of kidney function, intact PTH (iPTH) and oxPTH increased substantially, whereas n-oxPTH increased only moderately. In conclusion, n-oxPTH, but not oxPTH, stimulates Fgf23 gene expression. The increase in PTH with decreasing GFR is mainly due to an increase in oxPTH in more advanced stages of CKD.

Sclerostin is an independent risk factor for all-cause mortality in kidney transplant recipients

Background

Sclerostin is a hormone contributing to the bone-vascular wall cross talk and has been implicated in cardiovascular events and mortality in patients with chronic kidney disease (CKD). We analyzed the relationship between sclerostin and mortality in renal transplant recipients.

Methods

600 stable renal transplant recipients (367men, 233 women) were followed for all-cause mortality for 3 years. Blood and urine samples for analysis and clinical data were collected at study entry. We performed Kaplan–Meier survival analysis and Cox regression models considering confounding factors such as age, eGFR, cold ischemia time, HbA1c, phosphate, calcium, and albumin. Optimal cut-off values for the Cox regression model were calculated based on ROC analysis.

Results

Sixty-five patients died during the observation period. Nonsurvivors (n = 65; sclerostin 57.31 ± 30.28 pmol/L) had higher plasma sclerostin levels than survivors (n = 535; sclerostin 47.52 ± 24.87 pmol/L) (p = 0.0036). Kaplan–Meier curve showed that baseline plasma sclerostin concentrations were associated with all-cause mortality in stable kidney transplant recipients (p = 0.0085, log-rank test). After multiple Cox regression analysis, plasma levels of sclerostin remained an independent predictor of all-cause mortality (hazard ratio, 1.011; 95% CI 1.002–1.020; p = 0.0137).

Conclusions

Baseline plasma sclerostin is an independent risk factor for all-cause mortality in patients after kidney transplantation.

Electronic supplementary material

The online version of this article (10.1007/s10157-020-01956-y) contains supplementary material, which is available to authorized users.

Parathyroid hormone

Parathyroid hormone is an essential regulator of extracellular calcium and phosphate. PTH enhances calcium reabsorption while inhibiting phosphate reabsorption in the kidneys, increases the synthesis of 1,25-dihydroxyvitamin D, which then increases gastrointestinal absorption of calcium, and increases bone resorption to increase calcium and phosphate. Parathyroid disease can be an isolated endocrine disorder or part of a complex syndrome. Genetic mutations can account for diseases of parathyroid gland formulation, dysregulation of parathyroid hormone synthesis or secretion, and destruction of the parathyroid glands. Over the years, a number of different options are available for the treatment of different types of parathyroid disease. Therapeutic options include surgical removal of hypersecreting parathyroid tissue, administration of parathyroid hormone, vitamin D, activated vitamin D, calcium, phosphate binders, calcium-sensing receptor, and vitamin D receptor activators to name a few. The accurate assessment of parathyroid hormone also provides essential biochemical information to properly diagnose parathyroid disease. Currently available immunoassays may overestimate or underestimate bioactive parathyroid hormone because of interferences from truncated parathyroid hormone fragments, phosphorylation of parathyroid hormone, and oxidation of amino acids of parathyroid hormone.

Bone microarchitecture in patients undergoing parathyroidectomy for management of secondary hyperparathyroidism

Background

Secondary hyperparathyroidism (SHPT) in patients with chronic kidney disease (CKD) leads to complex bone disease, affecting both trabecular and cortical bone, and increased fracture risk. Optimal assessment of bone in patients with CKD is yet to be determined. High-resolution magnetic resonance imaging (MRI) can provide three-dimensional assessment of bone microarchitecture, as well as determination of mechanical strength with finite element analysis (FEA).

Methods

We conducted a single-centre, cross-sectional study to determine bone microarchitecture with MRI in CKD patients with SHPT undergoing parathyroidectomy. Within two weeks of surgery, MRI was performed at the distal tibia and biochemical markers of SHPT (parathyroid hormone [PTH] and alkaline phosphatase [ALP]) were collected. Trabecular and cortical topological parameters as well as bone mechanical competence using FEA were assessed. Correlation of MRI findings of bone was made with biochemical markers.

Results

Twenty patients with CKD (15 male, 5 female) underwent MRI at the time of parathyroidectomy (16 on dialysis, 3 with functioning kidney transplant, one pre-dialysis with CKD stage 5). Median PTH at the time of surgery was 138.5 pmol/L [39.6–186.7 pmol/L]. MRI parameters in patients were consistent with trabecular deterioration, with erosion index (EI) 1.01 ± 0.3, and trabecular bone volume (BV/TV) 10.8 ± 2.9%, as well as poor trabecular network integrity with surface-to-curve ratio (S/C) 5.4 ± 2.3. There was also evidence of reduced cortical thickness, with CTh 2.698 ± 0.630 mm, and FEA demonstrated overall poor bone mechanical strength with mean elastic modulus of 2.07 ± 0.44.

Conclusion

Patients with severe SHPT requiring parathyroidectomy have evidence of significant changes in bone microarchitecture with trabecular deterioration, low trabecular and cortical bone volume, and reduced mechanical competence of bone.

Oxidation of parathyroid hormone

Parathyroid hormone (PTH) is the key hormone regulating calcium homeostasis and, as such, is an important diagnostic and prognostic marker. Although the measurement of PTH has greatly improved over the past few decades, oxidation status thereof is unaccounted for in currently used assays. PTH can be oxidized on methionine residues located at amino acid positions 8 and 18. This is a relevant post-translational modification as, due to refolding of the molecule, it results in the decreased ability to activate the PTH1 receptor. Although this loss of activity after oxidation was observed as early as 1934, only recently a method was developed to measure and distinguish non-oxidized PTH (n-oxPTH) from oxidized PTH. This method creates exciting possibilities for studying more specifically the role of n-oxPTH in physiology and pathology. Therefore, it can now be explored what the clinical implications of measuring n-oxPTH will be. Herein, we review the available evidence of the effect of oxidation on the biological activity of PTH. We also discuss studies examining the mechanism of PTH oxidation in vivo and efforts to stabilize synthetic PTH ex vivo for therapeutic applications. Lastly, the available studies regarding the clinical significance of n-oxPTH are evaluated and future directions discussed.

Treatment of hyperphosphatemia: the dangers of high PTH levels

The control of secondary hyperparathyroidism (SHPT) in pediatric chronic kidney disease is of utmost importance. Even though parathyroid hormone (PTH) is an important biomarker of mineral and bone disorders associated to CKD (CKD-MBD), calcium, phosphate, alkaline phosphatase, and vitamin D are also crucial and should be assessed together. In pediatric dialysis, high PTH levels have been associated with impaired longitudinal growth, bone disease, cardiovascular comorbidities, left ventricular hypertrophy, anemia, and even mortality (when PTH levels were above 500 pg/mL, i.e., 8.3-fold the upper normal limit (UNL)). As such, high PTH levels are for sure deleterious, but too low PTH levels have also been shown to impair growth and to promote vascular calcifications because of the underlying adynamic bone. This manuscript is part of a pros and cons debate for keeping PTH levels within the normal range in pediatric CKD, focusing on the pros. High bone turnover lesions can occur at lower PTH levels than "current" guidelines would suggest; thus, PTH alone is not a good predictor of the underlying osteodystrophy. PTH results can vary locally depending on the assay. Existing guidelines for PTH targets are conflicting and based on a very little evidence. However, the 120-180 pg/mL (2- to 3-fold the UNL) range is common to most of the guidelines; it seems to be a reasonable target in children undergoing dialysis, even though it does not correspond to "normal" PTH levels. As always, the philosophy of PTH levels in pediatric dialysis may be balanced, i.e., "not too low, not too high, and keep phosphate under control."

Cardiovascular risk factors in children on dialysis: an update

Cardiovascular disease (CVD) is a life-limiting comorbidity in patients with chronic kidney disease (CKD). In childhood, imaging studies have demonstrated early phenotypic characteristics including increases in left ventricular mass, carotid artery intima-media thickness, and pulse wave velocity, which occur even in young children with early stages of CKD. Vascular calcifications are the signature of an advanced phenotype and are mainly found in adolescents and young adults treated with dialysis. Association studies have provided valuable information regarding the significance of a multitude of risk factors in promoting CVD in children with CKD by using intermediate endpoints of measurements of surrogate parameters of CVD. Dialysis aggravates pre-existing risk factors and accelerates the progression of CVD with additional dialysis-related risk factors. Coronary artery calcifications in children and young adults with CKD accumulate in a time-dependent manner on dialysis. Identification of risk factors has led to improved understanding of principal mechanisms of CKD-induced damage to the cardiovascular system. Treatment strategies include assessment and monitoring of individual risk factor load, optimization of treatment of modifiable risk factors, and intensified hemodialysis if early transplantation is not possible.

Clinical Guidelines and PTH Measurement: Does Assay Generation Matter?

PTH is an important regulator of calcium and phosphate homeostasis and bone remodeling. It is metabolized into PTH fragments, which are measured to a different extent by PTH assays of different generations because of differences in fragments recognized and lack of assay standardization. PTH is measured in the workup of several conditions, and clinical guidelines provide recommendations concerning these measurements. This review provides an overview of the impact of differences between PTH assays, applying distinct clinical guidelines for primary and secondary hyperparathyroidism and perioperative use of PTH measurements. Guidelines deal with PTH measurement in different ways, recommending either trend monitoring, the use of a fold increase of the upper reference limit, or an absolute PTH cutoff value. For classic primary hyperparathyroidism (PHPT), the type of PTH assay used will not affect diagnosis or management because the precise concentration of PTH is less relevant. In chronic kidney disease, the guideline recommends treating secondary hyperparathyroidism above a twofold to ninefold PTH increase, which will result in different clinical decisions depending on the assay used. For patients after bariatric surgery, guidelines state absolute cutoff values for PTH, but the impact of different generation assays is unknown because direct comparison of PTH assays has never been performed. During parathyroid surgery, PTH measurements with a third-generation assay reflect treatment success more rapidly than second-generation assays. Increased awareness among clinicians regarding the complexity of PTH measurements is warranted because it can affect clinical decisions.

PTH determination in hemodialyzed patients-A laboratory perspective

Parathyroid hormone (PTH) is a key player of bone remodelling in patients suffering from Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD). Serum PTH concentrations are thus frequently measured in CKD patients. Nevertheless, this determination is far from simple. PTH stability can be an issue and degradation of the peptide can be important if storage is not properly done. Biologically active PTH circulates together with fragments, which can be detected by some immunoassays. There is, up to now, no standardization of the assays available on the market, which can lead to some confusion when patients are followed with different methods. The upper end of the reference ranges provided by some manufacturers have not been properly established and are sometimes far too high. Finally, PTH can be oxidized in vivo and thus become inactive, while still quantified by immunoassays. In this Editorial, we will try to highlight some of these issues on PTH measurement.

Parathyroid Hormone Measurement in Chronic Kidney Disease: From Basics to Clinical Implications

Accurate measurement of parathyroid hormone (PTH) is crucial for therapeutic decision-making in patients with chronic kidney disease-mineral and bone disorder (CKD-MBD). The second-generation PTH assays, often referred to as “intact PTH” assays, are the current standard and most available assays in clinical practice. However, intact PTH assays measure both full-length biologically active PTH and heterogeneous PTH fragments in the circulation, providing the equivocal value of PTH measurement in patients with CKD-MBD. Due to the variability of PTH assays, preanalytical sample errors, and the phenomenon of end-organ PTH hyporesponsiveness, current CKD-MBD guidelines recommend a wide range for serum PTH targets (2–9 the upper normal limit of the intact PTH assay) in dialysis patients to diminish the risk of developing adynamic bone disease. Nevertheless, a sizeable proportion of CKD patients still experience renal osteodystrophy despite having serum PTH levels within the recommended range. The primary cause of this inconsistency is the analytical interference of various PTH fragments and oxidized PTH forms that considerably accumulate in CKD patients. Therefore, a new mass spectrometry-based assay, which is capable of specifically measuring the whole spectra of PTH fragments, can potentially improve diagnostic accuracy for renal osteodystrophy. However, the effects of different PTH fragments on bone metabolism, vascular calcification, and mortality in CKD patients warrant further research.

Kidney graft survival of >25 years: a single center report including associated graft biopsy results

Only few centers have reported their observations on patients with very long-term kidney graft survival of more than 25 years. Eighty-six subjects were identified in our center with graft survival of >25 years. Donor age was 31.3 ± 18.5 years. Mean duration of transplantation was 30.3 ± 3.6 years. At last follow-up, the cystatin C clearance was 47 ± 23 ml/min. Transplant biopsies for cause were performed in 30 subjects at a median of 28.4 years (19.1-40.3) after transplantation. Acute or chronic active T cell-mediated rejection was present in five cases and histological characteristics of acute or chronic active humoral rejection in eight cases. More than 80% of biopsies had inflammatory infiltrates in nonatrophic or atrophic cortical areas. The number of HLA mismatches were higher in biopsied subjects (3.0 ± 1.8 vs. 2.2 ± 1.7 without biopsy). Immunosuppressive therapy was adapted in most biopsied subjects; impaired graft function and proteinuria was unchanged at last follow-up. Sixty percent of all subjects had hyperparathyroidism (iPTH of the whole group: 132 ± 157 pg/ml), which was predominantly secondary, as judged by serum calcium and graft function. Young donor age was certainly a prerequisite of longterm graft survival. Nonetheless, inflammation or rejection in most biopsied patients suggests an important role of alloreactivity even in this late course.

The effect of vitamin D supplementation on plasma non-oxidised PTH in a randomised clinical trial

OBJECTIVE

PTH can be oxidised in vivo, rendering it biologically inactive. Non-oxidised PTH (n-oxPTH) may therefore give a better image of the hormonal status of the patient. While vitamin D supplementation decreases total PTH (tPTH) concentration, the effect on n-oxPTH concentration is unexplored. We investigated the effect of vitamin D on n-oxPTH concentration in comparison to tPTH and compared the correlations between parameters of calcium, bone and lipid metabolism with n-oxPTH and tPTH.

METHODS

N-oxPTH was measured in 108 vitamin D-insufficient (25(OH)D <75 nmol/L) hypertensive patients, treated with vitamin D (2800 IE daily) or placebo for 8 weeks in the Styrian Vitamin D Hypertension Trial (NCT02136771). We calculated the treatment effect and performed correlation analyses of n-oxPTH and tPTH with parameters of calcium, bone and lipid metabolism and oxidative stress.

RESULTS

After treatment, compared to placebo, 25(OH)D concentrations increased, tPTH decreased by 9% (P < 0.001), n-oxPTH by 7% (P = 0.025) and the ratio of n-oxPTH/tPTH increased (P = 0.027). Changes in phosphate and HDL concentration correlated with changes in n-oxPTH, but not tPTH.

CONCLUSIONS

tPTH and n-oxPTH decrease upon vitamin D supplementation. Our study suggests that vitamin D supplementation reduces the oxidation of PTH, as we observed a small but significant increase in the non-oxidised proportion of PTH upon treatment. In addition, we found that changes in phosphate and HDL concentration showed a relationship with changes in n-oxPTH, but not tPTH. This may be explained by the biological activity of n-oxPTH. Further research should be carried out to establish the clinical relevance of n-oxPTH.

Secondary Hyperparathyroidism: Pathogenesis and Latest Treatment

The classic pathogenesis of secondary hyperparathyroidism (SHPT) began with the trade-off hypothesis based on parathyroid hormone hypersecretion brought about by renal failure resulting from a physiological response to correct metabolic disorder of calcium, phosphorus, and vitamin D. In dialysis patients with failed renal function, physiological mineral balance control by parathyroid hormone through the kidney fails and hyperparathyroidism progresses. In this process, many significant genetic findings have been established. Abnormalities of Ca-sensing receptor and vitamin D receptor are associated with the pathogenesis of SHPT, and fibroblast growth factor 23 has also been shown to be involved in the pathogenesis. Vitamin D receptor activators (VDRAs) are widely used for treatment of SHPT. However, VDRAs have calcemic and phosphatemic effects that limit their use to a subset of patients, and calcimimetics have been developed as alternative drugs for SHPT. Hyperphosphatemia also affects progression of SHPT, and control of hyperphosphatemia is, therefore, thought to be fundamental for control of SHPT. Currently, a combination of a VDRA and a calcimimetic is recognized as the optimal strategy for SHPT, and for other outcomes such as reduced cardiovascular disease and improved survival. The latest findings on the pathogenesis and treatment of SHPT are summarized in this review.

PTH assays in dialysis patients: Practical considerations

Parathyroid hormone (PTH) 1-84 is the main biologically active hormone produced by the parathyroid cells. Circulating PTH molecules include the whole PTH 1-84 along with amino (N) and carboxyl (C) terminal fragments. While PTH is the best available noninvasive biomarker to assess bone turnover in dialysis patients, the biological roles of individual circulating PTH fragments are still not completely known. The understanding that there is an enormous variation in the target specificity of currently available PTH assays for different circulating forms of PTH has led to the evolution of assays from first to second then third generation. With a reduction in kidney function, there is a preferential increase in circulating C fragments and non-PTH 1-84 forms, resulting in a decrease in the ratio of PTH 1-84/non-PTH 1-84. However, there are also substantial differences in between-assay measurements, with several fold variations in results. Targets based on multiples of the upper limit of normal (ULN) should be used rather than PTH ranges using absolute iPTH values. To date, the second-generation PTH remains the most widely used assay. Current guidelines recommend following iPTH trends rather than absolute values. Herein, we highlight problems and challenges in PTH assays/measurements and their interpretations in dialysis patients.

Reference intervals for measured and calculated free 25-hydroxyvitamin D in normal pregnancy

The determination of free 25-hydroxyvitamin D (25(OH)D) as compared to the analysis of total 25-hydroxyvitamin D might reflect better the vitamin D status during pregnancy, since vitamin D-binding protein (DBP) concentrations increase throughout pregnancy and the vast majority of 25(OH)D is tightly bound to DBP thus strongly influencing total 25(OH)D. The concentration of the biologically active free 25(OH)D - on the other hand - is much less dependent on the DBP concentrations. The study was conducted in May-June 2016 in 368 Caucasian pregnant healthy women - residents of Northeastern Germany. Free 25(OH)D was either measured directly by commercial ELISA kit or assessed by calculation via total 25(OH)D, DBP, and albumin serum concentrations. Regardless of the detection method, free 25(OH)D lowers in the 3rd trimester comparing to the 1st trimester (by 12% and 21%, p < 0.05 and p < 0.001, for measured and calculated free 25(OH)D, respectively), whereas total 25(OH)D was not decreased in late pregnancy. DBP rises with gestational age. Total 25(OH)D was not correlated with serum calcium (p = 0.251), whereas free 25(OH)D was significantly (p = 0.007 for measured free 25(OH)D and p < 0.001 for calculated free 25(OH)D) positively correlated with calcium. All 25(OH)D isoforms were significantly negatively correlated with bone-specific alkaline phosphatase (BSAP), however the correlation strength was the lowest with total 25(OH)D (rho = -0.108, p = 0.038), whereas both measured and calculated free 25(OH)D revealed better associations with BSAP (rho = -0.203 and rho = -0.211 for measured and calculated free 25(OH)D, respectively, p < 0.001 for both). We established pregnancy trimester-specific reference intervals for free measured and calculated 25(OH)D and DBP. Both measured and calculated free 25(OH)D showed better correlations with parameters of the endocrine vitamin D system (calcium and BSAP). Both ways of measuring free 25(OH)D in pregnant women are suitable as novel laboratory parameter for vitamin D status monitoring during human pregnancy and might replace in the future the routine total 25(OH)D assessment.

The Relationship Between Pulse Pressure, the Estimated Glomerular Filtration Rate, and Urine Microalbumin/Creatinine Ratio in Korean Adults

BACKGROUND/AIMS

Pulse pressure (PP) is a predictor of adverse outcomes in patients on haemodialysis. Thus, the present study was conducted to assess the relationship between PP, estimated glomerular filtration rate (eGFR), and urine microalbumin/creatinine ratio (uACR) in Korean adults.

METHODS

Data of 9,409 adults (4,206 men and 5,203 women) aged ≥ 20 years from the Sixth Korean National Health and Nutrition Examination Survey (2013-2014) were analyzed.

RESULTS

A multivariate analysis revealed that systolic blood pressure (SBP) (β = -0.170, 95% confidence interval [CI], -0.216 to -0.159), diastolic blood pressure (DBP) (β = 0.088, 95% CI 0.108-0.200; p < 0.001), and PP (β = -0.134, 95% CI -0.215 to -0.157) were significant factors determining eGFR. In contrast, SBP (β = 0.152, 95% CI, 0.985-1.456; p < 0.001), DBP (β = -0.062, 95% CI -1.141 to -0.442; p < 0.001), and PP (β = 0.118, 95% CI 0.965-1.436; p < 0.001) were the significant factors determining uACR. The odds ratios (ORs) of a high PP (PP ≥ 60 mmHg) with a normal group [eGFR ≥ 60 ml/min/1.73 m2 and uACR < 30 mg/g] as a reference were significant for decreased eGFR [eGFR < 60 ml/min/1.73 m2, 1.484 (95% CI, 1.003-2.196)], elevated uACR [uACR ≥ 30 mg/g, 2.592 (95% CI, 2.085-3.223)], and decreased eGFR plus elevated uACR [eGFR < 60 ml/min/1.73 m2 and uACR ≥ 30 mg/g, 3.889 (95% CI, 2.519-6.004)].

CONCLUSION

Enhanced PP was associated with a decreased eGFR and an increase in uACR in Korean adults. In addition, the PP increased greatly when a decrease in eGFR and an increase in uACR appeared simultaneously.

Oxidation of PTH: in vivo feature or effect of preanalytical conditions?

Background:

Posttranslational oxidation of parathyroid hormone (PTH) modifies its biological activity. Measurement of non-oxidized PTH (n-oxPTH) could be an improvement in assessing PTH status, as intact PTH may rather reflect oxidative stress. However, it is debated whether oxidation of PTH occurs in vivo, or whether it is mainly an in vitro artifact. The aim of this study was to investigate the influence of different preanalytical conditions on the oxidation of PTH within a wide range of plasma PTH concentrations and oxidation propensity.

Methods:

n-oxPTH was separated from its oxidized form using an affinity column capturing the oxidized PTH. n-oxPTH was measured in eluate using commercially available PTH assays. The study included ethylenediaminetetraacetic acid plasma samples from 17 patients undergoing hemodialysis and 32 healthy subjects. We determined effects of storage temperature, time until centrifugation and freeze-thaw cycles. PTH and n-oxPTH concentrations were measured in each sample using six different immunoassays.

Results:

n-oxPTH concentrations remained unchanged up to 180 min until centrifugation, two freeze-thaw cycles or after storage at −20°C or −80°C up to 79 days. Various methods for n-oxPTH and PTH measurements yielded highly comparable results, apart from standardization differences between various PTH and n-oxPTH assays.

Conclusions:

n-oxPTH concentrations were stable under our study conditions, indicating negligible ex vivo oxidation of PTH. In addition, PTH immunoassays have a different sensitivity for n-oxPTH than for total PTH. For this reason, the n-oxPTH/total PTH ratio cannot be used in absence of a n-oxPTH standard. Clinical implications of determining n-oxPTH require additional study.

Hope for CKD-MBD Patients: New Diagnostic Approaches for Better Treatment of CKD-MBD

BACKGROUND

Chronic kidney disease-mineral and bone disorder (CKD-MBD) patients have a huge morbidity and mortality. Only relatively minor progress in therapeutic strategies has been made in the past decades. This is at least partially due to a lack of predictive diagnostic tools allowing personalized treatment of CKD-MBD patients.

SUMMARY

In this review we describe recent progress in the diagnosis of disturbances of calcium and phosphate metabolism in patients with CKD-MBD, measuring biological active nonoxidized parathyroid hormone as well as the overall likelihood of a patient to get calcified.

KEY MESSAGE

There is hope. The new tools have the potential of allowing personalized therapy for the treatment of CKD-MBD and hence improving outcome.

A case report of severe calciphylaxis - suggested approach for diagnosis and treatment

BACKGROUND

Calciphylaxis is a serious complication in patients with chronic kidney disease associated mineral and bone disorder. It can occur in conditions with low and high bone turnover. So far, there are no definite diagnostic and therapeutic guidelines which may prevent the devastating outcome in many calciphylaxis patients. We report a case which clearly illustrates that knowledge of the underlying bone disorder is essential for a directed treatment. Based on this experience we discuss a systematic diagnostic and therapeutic approach in patients with calciphylaxis.

CASE PRESENTATION

We report a patient with severe calciphylaxis. Initial evaluation showed an elevated serum parathormone concentration and a bone-specific alkaline phosphatase activity in the upper normal range; however, the bone biopsy clearly showed adynamic bone disease. Extended dialysis with low calcium dialysate concentration and citrate anticoagulation, and administration of teriparatide led to a further increase in bone-specific alkaline phosphatase activity and most importantly, resulted in an activated bone turnover as confirmed by a second bone biopsy 11 weeks later.

CONCLUSIONS

This case illustrates that laboratory tests cannot reliably differentiate between high and low bone turnover in calciphylaxis patients. More importantly, this case highlights the fact that specific therapies that alter bone metabolism cannot be applied without knowledge of the bone status. On this background, we suggest that bone biopsies should be an integral part in the diagnosis and therapeutic decision in these patients and should be evaluated in further studies.

Renal osteodystrophy in children: pathogenesis, diagnosis and treatment

PURPOSE OF REVIEW

Disturbances in calcium-phosphate homeostasis play an important role in children with chronic kidney disease, and not only cause renal osteodystrophy but also result in increased cardiovascular morbidity and mortality. This review outlines the current aspects in the pathogenesis, diagnostic approach and treatment of renal osteodystrophy.

RECENT FINDINGS

The pathogenesis of renal osteodystrophy is under strong influence of the fibroblast growth factor 23/Klotho system, which is able to enhance phosphate excretion and reduce calcitriol synthesis in the kidney. Fibroblast growth factor 23 increases tissue calcinosis and is cardiotoxic, and is independently associated with mortality. Despite improvement in diagnostic imaging (bone density measurements), determination of biomarkers, mainly parathyroid hormone, still plays a central role. New treatment options resulted in improved bone health and also a reduction in mortality was achieved in adults with calcium-free phosphate binders. Substitution of active and inactive vitamin D is important and also has a beneficial effect on proteinuria.

SUMMARY

Knowledge about the biochemical and molecular mechanisms of renal osteodystrophy is increasing dramatically and has an impact not only to bone health but also overall morbidity and mortality. This will ultimately translate into further improved diagnostic approaches and novel treatment options.