Coby M. Laarakkers

Advances in Quantitative Hepcidin Measurements by Time-of-Flight Mass Spectrometry

Assays for the detection of the iron regulatory hormone hepcidin in plasma or urine have not yet been widely available, whereas quantitative comparisons between hepcidin levels in these different matrices were thus far even impossible due to technical restrictions. To circumvent these limitations, we here describe several advances in time-of flight mass spectrometry (TOF MS), the most important of which concerned spiking of a synthetic hepcidin analogue as internal standard into serum and urine samples. This serves both as a control for experimental variation, such as recovery and matrix-dependent ionization and ion suppression, and at the same time allows value assignment to the measured hepcidin peak intensities. The assay improvements were clinically evaluated using samples from various patients groups and its relevance was further underscored by the significant correlation of serum hepcidin levels with serum iron indices in healthy individuals. Most importantly, this approach allowed kinetic studies as illustrated by the paired analyses of serum and urine samples, showing that more than 97% of the freely filtered serum hepcidin can be reabsorbed in the kidney. Thus, the here reported advances in TOF MS-based hepcidin measurements represent critical steps in the accurate quantification of hepcidin in various body fluids and pave the way for clinical studies on the kinetic behavior of hepcidin in both healthy and diseased states.

Immunochemical and Mass-Spectrometry–Based Serum Hepcidin Assays for Iron Metabolism Disorders

BACKGROUND Hepcidin is an iron-regulatory peptide hormone that consists of 3 isoforms: bioactive hepcidin-25, and inactive hepcidin-22 and hepcidin-20. Hepcidin is instrumental in the diagnosis and monitoring of iron metabolism disorders, but reliable methods for its quantification in serum are sparse, as is knowledge of their relative analytical strengths and clinical utility. METHODS We developed a competitive (c)-ELISA and an immunocapture TOF mass-spectrometry (IC-TOF-MS) assay. Exploiting these 2 methods and our previously described weak cation exchange (WCX)-TOF-MS assay, we measured serum hepcidin concentrations in 186 patients with various disorders of iron metabolism and in 23 healthy controls. RESULTS We found that (a) the relative differences in median hepcidin concentrations in various diseases to be similar, although the absolute concentrations measured with c-ELISA and WCX-TOF-MS differed; (b) hepcidin isoforms contributed to differences in hepcidin concentrations between methods, which were most prominent in patients with chronic kidney disease; and (c) hepcidin concentrations measured by both the c-ELISA and IC-TOF-MS correlated with ferritin concentrations <60 μg/L, and were suitable for distinguishing between iron deficiency anemia (IDA) and the combination of IDA and anemia of chronic disease. CONCLUSIONS c-ELISA is the method of choice for the large-scale quantification of serum hepcidin concentrations, because of its low limit of detection, low cost, and high-throughput. Because of its specificity for bioactive hepcidin-25, WCX-TOF-MS can be regarded as a valuable special-purpose assay for disorders with variable concentrations of hepcidin isoforms, such as chronic kidney disease.

(Pre)analytical imprecision, between-subject variability, and daily variations in serum and urine hepcidin: implications for clinical studies.

The utility of urine and serum hepcidin measurements in the clinic depends on their reproducibility. We sought to expand our previous work on the within-subject variability and between-subject variability of this novel iron parameter in the serum and urine of 24 healthy controls by time-of-flight mass spectrometry at four different time points during the day. A linear mixed model for repeated data was used to distinguish three components of the total variability in the measurements: within-day/within-subject variability, between-subject variability, and additional residual or (pre)analytical variability. Differences in diurnal hepcidin patterns were observed between urine and serum. Urine levels remained similar during the course of the morning and increased significantly during the afternoon, whereas serum levels increased significantly throughout both the morning and afternoon. Furthermore, in serum the (pre)analytical variability (28.6%) was smaller than the between-subject (48.1%) and within-day/within-subject variability (30.3%) compared with urine variability (97.2% vs. 67.7 and 77.3%, respectively). High serum ferritin levels were associated with higher serum hepcidin levels but not with urine levels. Transferrin saturation did not correlate with hepcidin levels. To minimize variability, we recommend (i) standardizing for sampling time and (ii) measuring serum hepcidin levels.

Serum hepcidin-25 levels in patients with chronic kidney disease are independent of glomerular filtration rate

BACKGROUND Hepcidin is a key regulator of iron homeostasis and levels are elevated in patients with chronic kidney disease (CKD). Hepcidin may explain the often observed imbalance in iron metabolism in patients with CKD. We evaluated the influence of estimated glomerular filtration rate (eGFR) on serum levels of hepcidin-25 and its isoforms in patients with renal dysfunction. METHODS Serum levels of the biologically active hepcidin-25 and its isoforms were determined in CKD and dialysis patients by a mass spectrometry-based assay. RESULTS In 83 patients with CKD not requiring dialysis, serum hepcidin-25 levels were not significantly increased (5.1 nM versus 4.2 nM, P = 0.30) and positively correlated with ferritin (r = 0.74, P < 0.01). Multiple regression analysis showed ferritin to be the only significant predictor of hepcidin-25 levels. Serum hepcidin-25 levels were not dependent on eGFR. In contrast, hepcidin-20 and total hepcidin levels showed an independent significant inverse correlation with eGFR. In 48 haemodialysis patients, median hepcidin-25 levels were significantly higher than in CKD patients (9.4 nM versus 5.1 nM, P < 0.001) and again strongly correlated with ferritin (r = 0.79, P < 0.001). CONCLUSIONS eGFR is not a major determinant of serum hepcidin-25 levels. In contrast, the hepcidin isoforms hepcidin-20 and hepcidin-22 accumulate in patients with renal impairment.

Serum hepcidin levels are innately low in HFE-related haemochromatosis but differ between C282Y-homozygotes with elevated and normal ferritin levels.

HFE C282Y‐homozygosity has been associated with low hepcidin expression, leading to increased ferritin levels. However, serum hepcidin protein levels have not been documented in humans. In the current study, we compared serum hepcidin levels of newly diagnosed HFE C282Y‐homozygotes with (N = 15) and without (N = 7) elevated serum ferritin levels to levels of 40 controls (20 heterozygotes and 20 wild types). In addition, hepcidin levels of four C282Y homozygotes were investigated during the course of all phlebotomy treatment phases. Serum hepcidin levels were lower in HFE C282Y‐homozygotes (median; 25th–75th percentile: 1·88; 0·78–2·77 nmol/l) compared to controls (2·74; 1·45–5·39). Hepcidin/ferritin ratios were also lower in homozygotes. Homozygotes with an elevated serum ferritin had a higher serum hepcidin but a lower hepcidin/ferritin ratio than those with normal ferritin (2·28; 1·62–3·23 nmol/l hepcidin vs. 0·80; 0·60–1·29 and 3·63; 2·72–7·59 pmol hepcidin/μg ferritin vs. 13·2; 5·15–14·2). Serum hepcidin decreased during the depletion phase of phlebotomy and remained low during maintenance. This study showed that serum hepcidin is innately low in HFE‐related haemochromatosis. Elevated ferritin levels were associated with increased hepcidin levels while erythropoiesis lead to lower hepcidin levels. During depletion, therefore, hepcidin levels are decreased, which may exacerbate intestinal iron absorption.

Effect of the antihepcidin Spiegelmer lexaptepid on inflammation-induced decrease in serum iron in humans

Increased hepcidin production is key to the development of anemia of inflammation. We investigated whether lexaptepid, an antihepcidin l-oligoribonucleotide, prevents the decrease in serum iron during experimental human endotoxemia. This randomized, double-blind, placebo-controlled trial was carried out in 24 healthy males. At T = 0 hours, 2 ng/kg Escherichia coli lipopolysaccharide was intravenously administered, followed by an intravenous injection of 1.2 mg/kg lexaptepid or placebo at T = 0.5 hours. The lipopolysaccharide-induced inflammatory response was similar in subjects treated with lexaptepid or placebo regarding clinical and biochemical parameters. At T = 9 hours, serum iron had increased by 15.9 ± 9.8 µmol/L from baseline in lexaptepid-treated subjects compared with a decrease of 8.3 ± 9.0 µmol/L in controls (P < .0001). This study delivers proof of concept that lexaptepid achieves clinically relevant hepcidin inhibition enabling investigations in the treatment of anemia of inflammation. This trial was registered at www.clinicaltrial.gov as #NCT01522794.

Hepcidin-25 is a marker of the response rather than resistance to exogenous erythropoietin in chronic kidney disease/chronic heart failure patients

Erythropoietin (EPO) resistance, an important cause of anaemia in patients with heart and renal failure, is associated with increased mortality. The hypothesis of the present study was that exogenous EPO decreases hepcidin levels and that the decrease in hepcidin levels upon EPO treatment is related to the bone marrow response.

Improved mass spectrometry assay for plasma hepcidin: detection and characterization of a novel hepcidin isoform

Mass spectrometry (MS)-based assays for the quantification of the iron regulatory hormone hepcidin are pivotal to discriminate between the bioactive 25-amino acid form that can effectively block the sole iron transporter ferroportin and other naturally occurring smaller isoforms without a known role in iron metabolism. Here we describe the design, validation and use of a novel stable hepcidin-25+40 isotope as internal standard for quantification. Importantly, the relative large mass shift of 40 Da makes this isotope also suitable for easy-to-use medium resolution linear time-of-flight (TOF) platforms. As expected, implementation of hepcidin-25+40 as internal standard in our weak cation exchange (WCX) TOF MS method yielded very low inter/intra run coefficients of variation. Surprisingly, however, in samples from kidney disease patients, we detected a novel peak (m/z 2673.9) with low intensity that could be identified as hepcidin-24 and had previously remained unnoticed due to peak interference with the formerly used internal standard. Using a cell-based bioassay it was shown that synthetic hepcidin-24 was, like the -22 and -20 isoforms, a significantly less potent inducer of ferroportin degradation than hepcidin-25. During prolonged storage of plasma at room temperature, we observed that a decrease in plasma hepcidin-25 was paralleled by an increase in the levels of the hepcidin-24, -22 and -20 isoforms. This provides first evidence that all determinants for the conversion of hepcidin-25 to smaller inactive isoforms are present in the circulation, which may contribute to the functional suppression of hepcidin-25, that is significantly elevated in patients with renal impairment. The present update of our hepcidin TOF MS assay together with improved insights in the source and preparation of the internal standard, and sample stability will further improve our understanding of circulating hepcidin and pave the way towards further optimization and standardization of plasma hepcidin assays.

Growth differentiation factor 15 in patients with congenital dyserythropoietic anaemia (CDA) type II

Congenital dyserythropoietic anaemias (CDAs) are heterogeneous, hereditary disorders hallmarked by ineffective erythropoiesis and tissue iron overload. Growth differentiation factor 15 (GDF15) was suggested to mediate iron overload in iron-loading anaemias, such as the thalassaemias and CDAI by suppressing hepcidin, the key regulator of iron absorption. Here, we show that serum GDF15 concentrations are elevated in subjects with CDAI and CDAII. Despite similar disease characteristics, CDAI patients present with significantly higher GDF15 concentrations compared to CDAII patients. Hepcidin concentrations are inappropriately low in CDAII patients considering the severe hepatic iron overload associated with this disorder. GDF15 significantly correlates with the degree of anaemia (Hb), the response of erythropoiesis (reticulocyte index) as well as with iron availability in the serum (transferrin saturation). The observation that GDF15 is elevated in CDAII patients is consistent with the proposal that GDF15 is among the erythroid factors down-regulating hepcidin and contributing to iron overload in conditions of dyserythropoiesis.

Hepcidin Serum Levels and Resistance to Recombinant Human Erythropoietin Therapy in Haemodialysis Patients

This title was also presented orally at Clinical Biochemistry workshop,Faro, 29 Janeiro, 2010