Hepcidin as a potential biomarker for blood doping

Athlete biological passport: longitudinal biomarkers and statistics in the fight against doping

As novel substances, short time windows, and limits of detection increasingly challenge direct methods of doping detection in sports, indirect tools inevitably take a greater role in the fight against it. One such tool is the athlete biological passport (ABP) - a longitudinal profiling of the measured haematological and biochemical biomarkers, combined with calculated scores, against the background of epidemiological data crucial for doping detection. In both of its modules, haematological and steroidal, ABP parameters are analysed with the Bayesian adaptive model, which individualises reference and cut-off values to improve its sensitivity. It takes into account the confounding factors with proven and potential influence on the biomarkers, such as race and altitude exposure. The ABP has already changed the fight against doping, but its importance will further grow with the new modules (e.g., endocrinological), parameters (e.g., plasma volume-independent parameters), and complementing indirect methods (e.g., transcriptomic).

mRNA biomarkers sensitive and specific to micro-dose erythropoietin treatment at sea level and altitude

Recombinant human erythropoietin (rhEPO) is prohibited by the World Anti-Doping Agency. rhEPO abuse can be indirectly detected via the athlete biological passport (ABP). However, altitude exposure challenges interpretation of the ABP. This study investigated whether 5'-aminolevulinate synthase 2 (ALAS2) and carbonic anhydrase 1 (CA1) in capillary dried blood spots (DBSs) are sensitive and specific markers of rhEPO treatment at altitude. ALAS2 and CA1 expression was monitored in DBS collected weekly before, during, and after a 3-week period at sea level or altitude. Participants were randomly assigned to receive 20 IU kg bw-1 epoetin alpha (rhEPO) or placebo injections every second day for 3 weeks while staying at sea level (rhEPO, n = 25; placebo, n = 9) or altitude (rhEPO, n = 12; placebo, n = 27). ALAS2 and CA1 expression increased up to 300% and 200%, respectively, upon rhEPO treatment at sea-level and altitude (P-values <0.05). When a blinded investigator interpreted the results, ALAS2 and CA1 expression had a sensitivity of 92%. Altitude did not confound the interpretation. Altitude affected ALAS2 and CA1 expression less than actual ABP markers when compared between sea level and altitude results. An individual athlete passport-like approach simulation confirmed the biomarker potential of ALAS2 and CA1. ALAS2 and CA1 were sensitive and specific biomarkers of micro-dose rhEPO treatment at sea level and altitude. Altitude seemed less a confounding factor for these biomarkers, especially when they are combined. Thus, micro-dose rhEPO injections can be detected in a longitudinal blinded setting using mRNA biomarkers in DBS.

Indirect biomarkers of blood doping: A systematic review

The detection of blood doping represents a current major issue in sports and an ongoing challenge for antidoping research. Initially focusing on direct detection methods to identify a banned substance or its metabolites, the antidoping effort has been progressively complemented by indirect approaches. The longitudinal and individual monitoring of specific biomarkers aims to identify nonphysiological variations that may be related to doping practices. From this perspective, the identification of markers sensitive to erythropoiesis alteration is key in the screening of blood doping. The current Athlete Biological Passport implemented since 2009 is composed of 14 variables (including two primary markers, i.e., hemoglobin concentration and OFF score) for the hematological module to be used for indirect detection of blood doping. Nevertheless, research has continually proposed and investigated new markers sensitive to an alteration of the erythropoietic cascade and specific to blood doping. If multiple early markers have been identified (at the transcriptomic level) or developed directly in a diagnostics' kit (at a proteomic level), other target variables at the end of the erythropoietic process (linked with the red blood cell functions) may strengthen the hematological module in the future. Therefore, this review aims to provide a global systematic overview of the biomarkers considered to date in the indirect investigation of blood doping.

Future opportunities for the Athlete Biological Passport

The Athlete Biological Passport (ABP) was introduced to complement the direct anti-doping approach by indirectly outlining the possible use of prohibited substances or methods in sports. The ABP proved its effectiveness, at least through a deterrent effect, even though the matrices used for longitudinal monitoring (urine and blood) are subject to many intrinsic (e.g., genetic) and extrinsic (e.g., environmental conditions) confounding factors. In that context, new and more specific biomarkers are currently under development to enhance both the sensitivity and the specificity of the ABP. Multiple strategies are presently being explored to improve this longitudinal monitoring, with the development of the current modules, the investigation of new strategies, or the screening of new types of doping. Nevertheless, due to the variability induced by indirect biomarkers, the consideration of confounding factors should continuously support this research. Beyond tremendous advances in analytical sensitivity, machine learning-based approaches seem inevitable to facilitate an expert interpretation of numerous biological profiles and promote anti-doping efforts. This perspective article highlights the current innovations of the Athlete Biological Passport that seem the most promising. Through different research axes, this short manuscript provides an opportunity to bring together approaches that are more widely exploited (e.g., omics strategies) and others in the early stages of investigation (e.g., artificial intelligence) seeking to develop the ABP.

Altitude and Erythropoietin: Comparative Evaluation of Their Impact on Key Parameters of the Athlete Biological Passport: A Review

The hematological module of the Athlete's Biological Passport (ABP) identifies doping methods and/or substances used to increase the blood's capacity to transport or deliver oxygen to the tissues. Recombinant human erythropoietin (rhEPOs) are doping substances known to boost the production of red blood cells and might have an effect on the blood biomarkers of the ABP. However, hypoxic exposure influences these biomarkers similarly to rhEPOs. This analogous impact complicates the ABP profiles' interpretation by antidoping experts. The present study aimed to collect and identify, through a literature search, the physiological effects on ABP blood biomarkers induced by these external factors. A total of 43 studies were selected for this review. A positive correlation (R² = 0.605, r = 0.778, p < 0.001) was identified between the hypoxic dose and the increase in hemoglobin concentration (HGB) percentage. In addition, the change in the reticulocyte percentage (RET%) has been identified as one of the most sensitive parameters to rhEPO use. The mean effects of rhEPO on blood parameters were greater than those induced by hypoxic exposure (1.7 times higher for HGB and RET% and 4 times higher for hemoglobin mass). However, rhEPO micro-doses have shown effects that are hardly distinguishable from those identified after hypoxic exposure. The results of the literature search allowed to identify temporal and quantitative evolution of blood parameters in connection with different hypoxic exposure doses, as well as different rhEPOs doses. This might be considered to provide justified and well-documented interpretations of physiological changes in blood parameters of the Athlete Biological Passport.

Hepcidin as a Prospective Individualized Biomarker for Individuals at Risk of Low Energy Availability

Hepcidin, a peptide hormone with an acknowledged evolutionary function in iron homeostasis, was discovered at the turn of the 21st century. Since then, the implications of increased hepcidin activity have been investigated as a potential advocate for the increased risk of iron deficiency in various health settings. Such implications are particularly relevant in the sporting community where peaks in hepcidin postexercise (∼3-6 hr) are suggested to reduce iron absorption and recycling, and contribute to the development of exercise-induced iron deficiency in athletes. Over the last decade, hepcidin research in sport has focused on acute and chronic hepcidin activity following single and repeated training blocks. This research has led to investigations examining possible methods to attenuate postexercise hepcidin expression through dietary interventions. The majority of macronutrient dietary interventions have focused on manipulating the carbohydrate content of the diet in an attempt to determine the health of athletes adopting the low-carbohydrate or ketogenic diets, a practice that is a growing trend among endurance athletes. During the process of these macronutrient dietary intervention studies, an observable coincidence of increased cumulative hepcidin activity to low energy availability has emerged. Therefore, this review aims to summarize the existing literature on nutritional interventions on hepcidin activity, thus, highlighting the link of hepcidin to energy availability, while also making a case for the use of hepcidin as an individualized biomarker for low energy availability in males and females.

Effects of altitude and recombinant human erythropoietin on iron metabolism: a randomized controlled trial

Current markers of iron deficiency (ID), such as ferritin and hemoglobin, have shortcomings, and hepcidin and erythroferrone (ERFE) could be of clinical relevance in relation to early assessment of ID. Here, we evaluate whether exposure to altitude-induced hypoxia (2,320 m) alone, or in combination with recombinant human erythropoietin (rHuEPO) treatment, affects hepcidin and ERFE levels before alterations in routine ID biomarkers and stress erythropoiesis manifest. Two interventions were completed, each comprising a 4-wk baseline, a 4-wk intervention at either sea level or altitude, and a 4-wk follow-up. Participants (n = 39) were randomly assigned to 20 IU·kg body wt^-1 rHuEPO or placebo injections every second day for 3 wk during the two intervention periods. Venous blood was collected weekly. Altitude increased ERFE (P ≤ 0.001) with no changes in hepcidin or routine iron biomarkers, making ERFE of clinical relevance as an early marker of moderate hypoxia. rHuEPO treatment at sea level induced a similar pattern of changes in ERFE (P < 0.05) and hepcidin levels (P < 0.05), demonstrating the impact of accelerated erythropoiesis and not of other hypoxia-induced mechanisms. Compared with altitude alone, concurrent rHuEPO treatment and altitude exposure induced additive changes in hepcidin (P < 0.05) and ERFE (P ≤ 0.001) parallel with increases in hematocrit (P < 0.001), demonstrating a relevant range of both hepcidin and ERFE. A poor but significant correlation between hepcidin and ERFE was found (R² = 0.13, P < 0.001). The findings demonstrate that hepcidin and ERFE are more rapid biomarkers of changes in iron demands than routine iron markers. Finally, ERFE and hepcidin may be sensitive markers in an antidoping context.

Induction of erythroferrone in healthy humans by micro-dose recombinant erythropoietin or high-altitude exposure

The erythropoietin (Epo)-erythroferrone (ERFE)-hepcidin axis coordinates erythropoiesis and iron homeostasis. While mouse studies have established that Epo-induced ERFE production represses hepcidin synthesis by inhibiting hepatic BMP/SMAD signaling, evidence for the role of ERFE in humans is limited. To investigate the role of ERFE as a physiological erythroid regulator in humans, we conducted two studies. First, 24 males were given six injections of saline (placebo), recombinant Epo (rhEpo) at a dose of 20 IU/kg (micro-dose) or rhEpo at 50 IU/kg (low dose). Second, we quantified ERFE in 22 subjects exposed to high altitude (3800 m) for 15 h. In the first study, total hemoglobin mass (Hbmass) increased after low- but not after micro-dose injections, when compared to the mass after placebo injections. Serum ERFE levels were enhanced by rhEpo, remaining higher than after placebo for 48 h (micro-dose) or 72 h (low-dose) after injections. Conversely, hepcidin levels decreased when Epo and ERFE rose, before any changes in serum iron parameters occurred. In the second study, serum Epo and ERFE increased at high altitude. The present results demonstrate that in healthy humans ERFE responds to slightly increased Epo levels not associated with Hbmass expansion and downregulates hepcidin in an apparently iron-independent way. Notably, ERFE flags micro-dose Epo, thus holding promise as a novel biomarker of doping.

Erythroferrone structure, function, and physiology: Iron homeostasis and beyond

Erythroferrone (ERFE) is the main erythroid regulator of hepcidin, the homeostatic hormone controlling plasma iron levels and total body iron. When the release of erythropoietin from the kidney stimulates the production of new red blood cells, it also increases the synthesis of ERFE in bone marrow erythroblasts. Increased ERFE then suppresses hepcidin synthesis, thereby mobilizing cellular iron stores for use in heme and hemoglobin synthesis. Recent mechanistic studies have shown that ERFE suppresses hepcidin transcription by inhibiting bone morphogenetic protein signaling in hepatocytes. In ineffective erythropoiesis, pathological overproduction of ERFE by an expanded population of erythroblasts suppresses hepcidin and causes iron overload, even in non-transfused patients. ERFE may be a useful biomarker of ineffective erythropoiesis and an attractive target for treating its systemic effects.

Hepcidin: Homeostasis and Diseases Related to Iron Metabolism

Iron is an essential metal for cell survival that is regulated by the peptide hormone hepcidin. However, its influence on certain diseases is directly related to iron metabolism or secondary to underlying diseases. Genetic alterations influence the serum hepcidin concentration, which can lead to an iron overload in tissues, as observed in haemochromatosis, in which serum hepcidin or defective hepcidin synthesis is observed. Another genetic imbalance of iron is iron-refractory anaemia, in which serum concentrations of hepcidin are increased, precluding the flow and efflux of extra- and intracellular iron. During the pathogenesis of certain diseases, the resulting oxidative stress, as well as the increase in inflammatory cytokines, influences the transcription of the HAMP gene to generate a secondary anaemia due to the increase in the serum concentration of hepcidin. To date, there is no available drug to inhibit or enhance hepcidin transcription, mostly due to the cytotoxicity described in the in vitro models. The proposed therapeutic targets are still in the early stages of clinical trials. Some candidates are promising, such as heparin derivatives and minihepcidins. This review describes the main pathways of systemic and genetic regulation of hepcidin, as well as its influence on the disorders related to iron metabolism.

Hepcidin: a real-time biomarker of iron need

There are numerous blood-based biomarkers for assessing iron stores, but all come with certain limitations. Hepcidin is a hormone primarily produced in the liver that has been proposed as the 'master regulator' of dietary uptake and iron metabolism, and has enormous potential to provide a 'real time' indicator of body iron levels. In this Minireview, the biochemical function of hepcidin in regulating iron levels will be discussed, with a specific focus on how hepcidin can aid in the assessment of iron stores and clinical diagnosis of iron deficiency, iron deficiency anaemia and other iron-related disorders. The role hepcidin itself plays in diseases of iron metabolism will be examined, and current efforts to translate hepcidin assays into the clinic will be critically appraised. Potential limitations of hepcidin as a marker of iron need will also be addressed, as well as the development of new therapies that directly target the hormone that sits atop the hierarchy of systemic iron metabolism.