Aminolevulinate Synthase 2 Research Articles

Erythropoiesis related biomarkers to detect autologous blood micro-transfusion in antidoping field

Indirect detection of autologous blood transfusion (ABT) abuse in Anti-Doping world is done by hematological module of athlete biological passport (ABP). Actual markers (reticulocytes (%RET) and hemoglobin (HGB)) of this module are enough sensitive to detect ABT. However, a new way uses by athlete to improve their performance is micro ABT and in this case %RET and HGB have some lack of sensitivity to detect this new way of doping. Previous studies have demonstrated that erythropoiesis-related RNAs, genes 5’-aminolevulinate synthase 2 ( ALAS2) and carbonic anhydrase 1 ( CA1) in capillary dried blood spots (DBS) could be potential biomarkers to increased sensitivity of the ABP in case of micro-ABT. In the present study, we monitored ALAS2 and CA1 expression in DBS after a donation of 450ml of blood and after a micro reinfusion (130ml) of this blood. DBSs samples collected from 48 volunteers’ males and females were analyzed. Participants were divided in two groups (micro ABT and placebo). Before the beginning of the study, randomized volunteers from transfused and non-transfused cohort were monitored 2 or 8 times weekly to get baseline points. 5 samples were collected after the blood withdrawal and 6 samples after the micro-ABT. The purpose was to evaluate the performance of RNAs biomarkers for the detection of micro-ABT in a longitudinal blinded setting. Our results illustrated a clear significant increase of ALAS2 until to 300 % after withdrawal. Regarding CA1 the increase is up to 250%. Looking at micro-ABT ALAS2 and CA1 will decrease under the baseline value. Regarding actual ABP markers, %RET slightly increase after the withdrawal up to 140%. In contrast, micro-ABT seem to have any effect with value that came back to baseline value without decreasing under baseline value. Finally, HGB slightly decrease after withdrawal follow by any increase after micro-ABT. Oppositely, regarding placebo group RNA markers and actual ABP markers stay stable all along the study. Erythropoiesis-related RNAs in capillary DBS such as ALAS2 and CA1 were sensitive biomarkers for blood withdrawal detection followed by micro-ABT. Furthermore, this investigation provide evidence that micro ABT can be detected in a longitudinal blinded setting with RNA biomarkers in DBS. World Anti Doping Agency. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

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 3weeks 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.

The effects of iron injection on blood doping biomarkers in dried blood spots.

Iron supplementation is not considered as a doping method; however, it can affect the levels of several biomarkers of the hematologic module of the athlete biological passport (ABP), such as the reticulocyte percentage (%RET) and hemoglobin (HGB) level. Thus, iron injection could be a confounding factor in antidoping analyses. Previous studies have suggested that the HGB level and the expression levels of reticulocyte-related-mRNAs, such as 5'-aminolevulinate synthase 2 (ALAS2) and carbonic anhydrase 1 (CA1), could be promising biomarkers for the ABP and detectable in dried blood spots (DBSs). Therefore, in this study, we examined the impact of iron injection on the levels of these potential biomarkers in DBSs. Reticulocyte-related-mRNAs analyses were performed by RT-qPCR. Ferritin level in DBS was measured with enzyme-linked immunosorbent assay(ELISA) method. Notably, there were no significant effects of iron supplementation on the levels of ALAS2 and CA1 mRNAs but by contrast, the %RET and immature reticulocyte fraction (IRF) measured in whole blood increased significantly following iron injection. As expected, iron supplementation increased the ferritin level significantly in both serum and DBS samples. In conclusion, these findings reinforce the specificity of reticulocyte-related mRNAs in DBSs as biomarkers of blood doping to target in antidoping analyses.

ISCA2 deficiency leads to heme synthesis defects and impaired erythroid differentiation in K562 cells by indirect ROS-mediated IRP1 activation

Iron‑sulfur (Fe-S) clusters have been shown to play important roles in various cellular physiological process. Iron‑sulfur cluster assembly 2 (ISCA2) is a vital component of the [4Fe-4S] cluster assembly machine. Several studies have shown that ISCA2 is highly expressed during erythroid differentiation. However, the role and specific regulatory mechanisms of ISCA2 in erythroid differentiation and erythroid cell growth remain unclear. RNA interference was used to deplete ISCA2 expression in human erythroid leukemia K562 cells. The proliferation, apoptosis, and erythroid differentiation ability of the cells were assessed. We show that knockdown of ISCA2 has profound effects on [4Fe-4S] cluster formation, diminishing mitochondrial respiratory chain complexes, leading to reactive oxygen species (ROS) accumulation and mitochondrial damage, inhibiting cell proliferation. Excessive ROS can inhibit the activity of cytoplasmic aconitase (ACO1) and promote ACO1, a bifunctional protein, to perform its iron-regulating protein 1(IRP1) function, thus inhibiting the expression of 5′-aminolevulinate synthase 2 (ALAS2), which is a key enzyme in heme synthesis. Deficiency of ISCA2 results in the accumulation of iron divalent. In addition, the combination of excessive ferrous iron and ROS may lead to damage of the ACO1 cluster and higher IRP1 function. In brief, ISCA2 deficiency inhibits heme synthesis and erythroid differentiation by double indirect downregulation of ALAS2 expression. We conclude that ISCA2 is essential for normal functioning of mitochondria, and is necessary for erythroid differentiation and cell proliferation.

The use of RNA-based 5'-aminolevulinate synthase 2 biomarkers in dried blood spots to detect recombinant human erythropoietin microdoses.

The hematological module of the Athlete Biological Passport (ABP) is used for indirect detection of blood manipulations; however, the use of this method to detect doping, such as with microdoses of recombinant human erythropoietin (rhEPO), is problematic. For this reason, the sensitivity of ABP must be enhanced by implementing novel biomarkers. Here, we show that 5'‐aminolevulinate synthase 2 (ALAS2) mRNAs are useful transcriptomic biomarkers to improve the indirect detection of rhEPO microdosing. Moreover, the sensitivity was sufficient to distinguish rhEPO administration from exposure to hypoxic conditions. Levels of mRNAs encoding carbonate anhydrase 1 (CA1) and solute carrier family 4 member 1 (SLC4A1) RNA, as well as the linear (L) and linear + circular (LC) forms of ALAS2 mRNA, were monitored for 16 days after rhEPO microdosing and during exposure to hypoxic conditions. ALAS2 mRNAs increased by 300% compared with the baseline values after rhEPO microdosing. Moreover, ALAS2 mRNAs were not significantly increased under hypoxic conditions. By contrast, CA1 mRNA was increased after both rhEPO microdosing and hypoxia, whereas SLC4A1 mRNA did not significantly increase under either condition. Furthermore, the analyses described here were performed using dried blood spots (DBSs), which provide advantages in terms of the sample collection, transport, and storage logistics. This study demonstrates that ALAS2 mRNA levels are sensitive and specific transcriptomic biomarkers for the detection of rhEPO microdosing using the hematological module of the ABP, and this method is compatible with the use of DBSs for anti‐doping analyses.

The Use of RNA‐based 5’‐Aminolevulinate Synthase 2 Biomarkers in Dried Blood Spots to detect rhEPO Micro Doses and Hypoxic Stimulation

The hematological module of the Athlete Biological Passport (ABP) is used for indirect detection of blood manipulations; however, the use of this method to detect doping e.g. with micro doses of recombinant human erythropoietin (rhEPO) is problematic. For this reason, sensitivity of ABP have to be enhance with the implementation of new biomarkers. Here we demonstrated that RNA-based 5’-Aminolevulinate Synthase 2 (ALAS2) could be a useful biomarker to improve the indirect detection of rhEPO micro dosing. Moreover, satisfying results were obtain also for specificity in order to distinguish rhEPO administrations from a sojourn in hypoxic conditions. Samples from two different clinical studies were used to monitor levels of different RNA markers, linear-circular (LC) and linear (L), of ALAS2 RNA, Carbonate anhydrase 1 (CA1) and Solute Carrier family 4 member 1 (SLC4A1) after rhEPO microdoses administration and under hypoxic condition. Analysis of RNA based biomarkerswere performed using dried blood spots (DBSs), these ones provide many advantages in terms of the samples collection, transport and storage. In order to monitored transcirptomics biomarkers variations, manual extraction of RNA from DBSs were performed follow by a RT-qPCR. Transcriptomics biomarkerswere monitored along 16 days where five micro doses were administrated, ALAS2 markers illustrated an increase up to 300% in comparison with the baseline value and these variations were statistically different from evolution in control group. Additionally, also CA1 variated in a significant way due to micro doses administrations. Moreover, ALAS2 markers variations under hypoxic conditions are not significant when compared with control group, which is the opposite of CA1 or reticulocytes percentage, where significant differences were obtained. Finally, SLC4A1 has no significant variation in both conditions. This study demonstrated that transcriptomic biomarkers, in particular ALAS2 mRNAs, could be used to improve the sensitivity and specificity of the hematological module of the ABP, and are compatible with the use of DBSs for anti-doping analyses. The expression level of SLC4A1 was not increased by micro doses of rhEPO, indicating that it is not a beneficial biomarker to detect micro dosing. CA1 is an interesting candidate, although, unlike ALAS2, its expression level was increased also significantly following exposure to hypoxia.

Low-Density Lipoproteins, High-Density Lipoproteins (HDL), and HDL-Associated Proteins Differentially Modulate Chronic Myelogenous Leukemia Cell Viability.

Cellular lipid metabolism, lipoprotein interactions, and liver X receptor (LXR) activation have been implicated in the pathophysiology and treatment of cancer, although findings vary across cancer models and by lipoprotein profiles. In this study, we investigated the effects of human-derived low-density lipoproteins (LDL), high-density lipoproteins (HDL), and HDL-associated proteins apolipoprotein A1 (apoA1) and serum amyloid A (SAA) on markers of viability, cholesterol flux, and differentiation in K562 cells-a bone marrow-derived, stem-like erythroleukemia cell model of chronic myelogenous leukemia (CML). We further evaluated whether lipoprotein-mediated effects were altered by concomitant LXR activation. We observed that LDL promoted higher K562 cell viability in a dose- and time-dependent manner and increased cellular cholesterol concentrations, while LXR activation by the agonist TO901317 ablated these effects. LXR activation in the presence of HDL, apoA1 and SAA-rich HDL suppressed K562 cell viability, while robustly inducing mRNA expression of ATP-binding cassette transporter A1 (ABCA1). HDL and its associated proteins additionally suppressed mRNA expression of anti-apoptotic B-cell lymphoma-extra large (BCL-xL), and the erythroid lineage marker 5'-aminolevulinate synthase 2 (ALAS2), while SAA-rich HDL induced mRNA expression of the megakaryocytic lineage marker integrin subunit alpha 2b (ITGA2B). Together, these findings suggest that lipoproteins and LXR may impact the viability and characteristics of CML cells.

Mitochondria Biogenesis Modulates Iron-Sulfur Cluster Synthesis to Increase Cellular Iron Uptake.

Iron-sulfur (Fe-S) clusters are required for mitochondrial function. Fe-S cluster synthesis occurs in the mitochondria and iron uptake is required for mitochondrial biogenesis. However, Fe-S clusters inhibit the expression of the iron importer transferrin receptor 1 (TfR1), whereas lack of the Fe-S cluster stimulates TfR1 expression. Yet, it is unclear whether Fe-S cluster synthesis increases with mitochondria biogenesis and, in turn, whether this negatively modulates TfR1 expression. We manipulated peroxisome proliferator-activated receptor-gamma coactivator-1α expression to control mitochondrial biogenesis in a variety of cell types, including erythroid cells. We demonstrated that Fe-S cluster synthesis increases with mitochondria biogenesis but does not interfere with increasing TfR1 expression. In fact, TfR1 expression is stimulated through alternative means to meet iron requirement for mitochondria biogenesis. Furthermore, under enhanced mitochondria biogenesis, increased Fe-S cluster synthesis inhibits the function of iron-regulating protein (IRP)1 and hence stimulates the expression of 5'-aminolevulinate synthase 2 (ALAS2), a target of IRP1 and rate-limiting enzyme in erythroid heme biogenesis. Increased ALAS2 expression leads to enhanced heme production, hemoglobinization, and erythropoiesis. Therefore, our study also provides a mechanism to link mitochondrial biogenesis with erythropoiesis and has a potential therapeutic value in the treatment of blood disorders.

Identification of differential gene expression related to epirubicin-induced cardiomyopathy in breast cancer patients.

Epirubicin is a potent chemotherapeutic agent for the treatment of breast cancer. However, it may lead to cardiotoxicity and cardiomyopathy, and no reliable biomarker was available for the early prediction of epirubicin-induced cardiomyopathy. Global gene expression changes of peripheral blood cells were studied using high-throughput RNA sequencing in three pair-matched breast cancer patients (patients who developed symptomatic cardiomyopathy paired with patients who did not) before and after the full session of epirubicin-based chemotherapy. Functional analysis was conducted using gene ontology and pathway enrichment analysis. We identified 13 significantly differentially expressed genes between patients who developed symptomatic epirubicin-induced cardiomyopathy and their paired control who did not. Among them, the upregulated Bcl-associated X protein was related to "apoptosis," while the downregulated 5'-aminolevulinate synthase 2 (ALAS2) was related to both "glycine, serine, and threonine metabolism" and "porphyrin and chlorophyll metabolism" in pathway enrichment analysis. ALAS2 and the metabolic pathways which were involved may play an important role in the development of epirubicin-induced cardiomyopathy. ALAS2 may be useful as an early biomarker for epirubicin-induced cardiotoxicity detection.

Detection of Stimulated Erythropoiesis by the RNA-Based 5'-Aminolevulinate Synthase 2 Biomarker in Dried Blood Spot Samples

Despite implementation of the Athlete Biological Passport 10 years ago, blood doping remains difficult to detect. Thus, there is a need for new biomarkers to increase the sensitivity of the adaptive model. Transcriptomic biomarkers originating from immature reticulocytes may be reliable indicators of blood manipulations. Furthermore, the use of dried blood spots (DBSs) for antidoping purposes constitutes a complementary approach to venous blood collection. Here, we developed a method of quantifying the RNA-based 5'-aminolevulinate synthase 2 (ALAS2) biomarker in DBS. The technical, interindividual, and intraindividual variabilities of the method, and the effects of storage conditions on the production levels of ALAS2 RNA were assessed. The method was used to monitor erythropoiesis stimulated endogenously (blood withdrawal) or exogenously (injection of recombinant human erythropoietin). When measured over a 7-week period, the intra- and interindividual variabilities of ALAS2 expression in DBS were 12.5%-42.4% and 49%, respectively. Following withdrawal of 1 unit of blood, the ALAS2 RNA in DBS increased significantly for up to 15 days. Variations in the expression level of this biomarker in DBS samples were more marked than those of the conventional hematological parameters, reticulocyte percentage and immature reticulocyte fraction. After exogenous stimulation of erythropoiesis via recombinant human erythropoietin injection, ALAS2 expression in DBS increased by a mean 8-fold. Monitoring of transcriptomic biomarkers in DBS could complement the measurement of hematological parameters in the Athlete Biological Passport and aid the detection of blood manipulations.

Relevance of the iron-responsive element (IRE) pseudotriloop structure for IRP1/2 binding and validation of IRE-like structures using the yeast three-hybrid system

Iron-responsive elements (IREs) are ~35-nucleotide (nt) stem-loop RNA structures located in 5′ or 3′ untranslated regions (UTRs) of mRNAs that mediate post-transcriptional regulation by their association with IRE-binding proteins (IRPs). IREs are characterized by their apical 6-nt loop motif 5’-CAGWGH-3′ (W = A or U and H = A, C or U), the so-called pseudotriloop, of which the loop nts C1 and G5 are paired, and the none-paired C between the two stem regions. In this study, the yeast three-hybrid (Y3H) system was used to investigate the relevance of the pseudotriloop structure of ferritin light chain (FTL) for the IRE-IRP interaction and the binding affinities between variant IRE(-like) structures and the two IRP isoforms, IRP1 and 2. Destabilization of the pseudotriloop structure by a G5-to-A mutation reduced binding of IRP1 and 2, while restoring the pseudotriloop conformation by the compensatory C1-to-U mutation, restored binding to both IRPs. In particular, IRP1 showed even stronger binding to the C1U-G5A mutant than to the wildtype FTL IRE. On the other hand, deletion of the bulged-out U6 of the pseudotriloop did not significantly affect its binding to either IRP1 or 2, but substitution with C particularly enhanced the binding to IRP1. In comparison to FTL IRE, IRE-like structures of 5′-aminolevulinate synthase 2 (ALAS2) and SLC40A1 (also known as ferroportin-1) showed similar or, in the case of endothelial PAS domain protein 1 (EPAS1) IRE, slightly weaker binding affinity to IRPs. SLC11A2 (a.k.a. divalent metal transporter-1) IRE exhibited relatively weak binding to IRP1 and medium binding to IRP2. Notably, the IRE-like structure of α-synuclein showed no detectable binding to either IRP under the conditions used in this Y3H assay. Our results indicate that Y3H can be used to characterize binding between IRPs and various IRE-like structures in vivo.

EPO-R+ myelodysplastic cells with ring sideroblasts produce high erythroferrone levels to reduce hepcidin expression in hepatic cells

Recently, a new erythroid regulator, erythroferrone (ERFE), which downregulates hepatic hepcidin production, has been identified. However, the relationship between ERFE and abnormal iron metabolism in MDS is unclear. In this study, we examined the level of ERFE mRNA during ex vivo erythroid differentiation using cord blood CD34+ cells and we further analyzed whether ERFE could be produced by MDS cells using a public database (GSE58831). ERFE mRNA was increased during normal erythroid differentiation. An analysis of GSE58831 indicated that ERFE expression in bone marrow (BM) MDS cells was higher than that in healthy volunteer (HV)–derived BM cells. ERFE expression significantly and positively correlated with the expression of erythropoietin (EPO) receptors (EPO-R), ALAS2 (5’-Aminolevulinate Synthase 2), STEAP3 (STEAP family member 3) and the presence of ring sideroblasts or the SF3B1 mutation. These results suggest that EPO-R+ MDS cells with ring sideroblasts or an SF3B1 mutation produce high levels of ERFE that may be associated with a reduction in hepcidin.

Identification of Cerebral Metal Ion Imbalance in the Brain of Aging Octodon degus.

The accumulation of redox-active transition metals in the brain and metal dyshomeostasis are thought to be associated with the etiology and pathogenesis of several neurodegenerative diseases, and Alzheimer’s disease (AD) in particular. As well, distinct biometal imaging and role of metal uptake transporters are central to understanding AD pathogenesis and aging but remain elusive, due inappropriate detection methods. We therefore hypothesized that Octodon degus develop neuropathological abnormalities in the distribution of redox active biometals, and this effect may be due to alterations in the expression of lysosomal protein, major Fe/Cu transporters, and selected Zn transporters (ZnTs and ZIPs). Herein, we report the distribution profile of biometals in the aged brain of the endemic Chilean rodent O. degus—a natural model to investigate the role of metals on the onset and progression of AD. Using laser ablation inductively coupled plasma mass spectrometry, our quantitative images of biometals (Fe, Ca, Zn, Cu, and Al) appear significantly elevated in the aged O. degus and show an age-dependent rise. The metals Fe, Ca, Zn, and Cu were specifically enriched in the cortex and hippocampus, which are the regions where amyloid plaques, tau phosphorylation and glial alterations are most commonly reported, whilst Al was enriched in the hippocampus alone. Using whole brain extracts, age-related deregulation of metal trafficking pathways was also observed in O. degus. More specifically, we observed impaired lysosomal function, demonstrated by increased cathepsin D protein expression. An age-related reduction in the expression of subunit B2 of V-ATPase, and significant increases in amyloid beta peptide 42 (Aβ42), and the metal transporter ATP13a2 were also observed. Although the protein expression levels of the zinc transporters, ZnT (1,3,4,6, and 7), and ZIP7,8 and ZIP14 increased in the brain of aged O. degus, ZnT10, decreased. Although no significant age-related change was observed for the major iron/copper regulator IRP2, we did find a significant increase in the expression of DMT1, a major transporter of divalent metal species, 5′-aminolevulinate synthase 2 (ALAS2), and the proto-oncogene, FOS. Collectively, our data indicate that transition metals may be enriched with age in the brains of O. degus, and metal dyshomeostasis in specific brain regions is age-related.

Blood Transcriptomic Meta-analysis Identifies Dysregulation of Hemoglobin and Iron Metabolism in Parkinson' Disease.

Disrupted iron metabolism has been implicated in the pathogenesis of Parkinson’s disease (PD), a progressive neurodegenerative disorder that severely affects movement and coordination, yet the molecular mechanisms underlying this association remain unknown. To this end, we performed a transcriptomic meta-analysis of four blood microarrays in PD. We observed a significant downregulation of genes related to hemoglobin including, hemoglobin delta (HBD), alpha hemoglobin stabilizing protein (ASHP), genes implicated in iron metabolism including, solute carrier family 11 member 2 (SLC11A2), ferrochelatase (FECH), and erythrocyte-specific genes including erythrocyte membrane protein (EPB42), and 5′-aminolevulinate synthase 2 (ALAS2). Pathway and network analysis identified enrichment in processes related to mitochondrial membrane, oxygen transport, oxygen and heme binding, hemoglobin complex, erythrocyte development, tetrapyrrole metabolism and the spliceosome. Collectively, we identified a subnetwork of genes in blood that may provide a molecular explanation for the disrupted hemoglobin and iron metabolism in the pathogenesis of PD.

MiR-218 Inhibits Erythroid Differentiation and Alters Iron Metabolism by Targeting ALAS2 in K562 Cells.

microRNAs (miRNAs) are involved in a variety of biological processes. The regulatory function and potential role of miRNAs targeting the mRNA of the 5′-aminolevulinate synthase 2 (ALAS2) in erythropoiesis were investigated in order to identify miRNAs which play a role in erythroid iron metabolism and differentiation. Firstly, the role of ALAS2 in erythroid differentiation and iron metabolism in human erythroid leukemia cells (K562) was confirmed by ALAS2 knockdown. Through a series of screening strategies and experimental validations, it was identified that hsa-miR-218 (miR-218) targets and represses the expression of ALAS2 by binding to the 3′-untranslated region (UTR). Overexpression of miR-218 repressed erythroid differentiation and altered iron metabolism in K562 cells similar to that seen in the ALAS2 knockdown in K562 cells. In addition to iron metabolism and erythroid differentiation, miR-218 was found to be responsible for a reduction in K562 cell growth. Taken together, our results show that miR-218 inhibits erythroid differentiation and alters iron metabolism by targeting ALAS2 in K562 cells.

Aminolevulinate synthase 2 mediates erythrocyte differentiation by regulating larval globin expression during Xenopus primary hematopoiesis

Hemoglobin synthesis by erythrocytes continues throughout a vertebrate’s lifetime. The mechanism of mammalian heme synthesis has been studied for many years; aminolevulinate synthase 2 (ALAS2), a heme synthetase, is associated with X-linked dominant protoporphyria in humans. Amphibian and mammalian blood cells differ, but little is known about amphibian embryonic hemoglobin synthesis. We investigated the function of the Xenopus alas2 gene (Xalas2) in primitive amphibian erythrocytes and found that it is first expressed in primitive erythroid cells before hemoglobin alpha 3 subunit (hba3) during primary hematopoiesis and in the posterior ventral blood islands at the tailbud stage. Xalas2 is not expressed during secondary hematopoiesis in the dorsal lateral plate. Hemoglobin was barely detectable by o-dianisidine staining and hba3 transcript levels decreased in Xalas2-knockdown embryos. These results suggest that Xalas2 might be able to synthesize hemoglobin during hematopoiesis and mediate erythrocyte differentiation by regulating hba3 expression in Xenopus laevis.

Three kinships with ALAS2 P520L (c. 1559 C → T) mutation, two in association with severe iron overload, and one with sideroblastic anemia and severe iron overload

Mutations in aminolevulinate synthase 2 ( ALAS2) are usually associated with sideroblastic anemia and iron overload. The objective of this study was to determine if “mild” mutations in ALAS2 might increase the severity of primary iron overload. Direct sequencing of the ALAS2 gene was performed on 24 subjects with primary hemochromatosis and one subject with sideroblastic anemia with severe iron overload. We identified a novel mutation P520L (c. 1559 C → T) in ALAS2 in three subjects. Two had severe iron overload and no anemia: one also had HFE C282Y homozygosity, and the other was wildtype for HFE and other iron-related genes. The third subject had sideroblastic anemia with iron overload, and was hemizygous for both P520L and R560H (c. 1679 G → A) mutations in ALAS2. The P520L mutation was found at a frequency of 0.0013 (741 alleles) in white control subjects, but was not found in 158 alleles from black control subjects. The proline in this position is highly conserved across species from humans to zebrafish. However, genotype/phenotype studies of the families demonstrate that the P520L mutation alone has no iron-associated phenotype, but it may act as a modifier of iron overload in the presence of mutations in HFE or other uncharacterized hemochromatosis genes. Thus, ALAS2 mutations might contribute to more severe iron loading in persons with primary hemochromatosis.

Three Kinships with ALAS2 P520L Mutations, Two in Association with Severe Iron Overload, and One with Sideroblastic Anemia and Severe Iron Overload.

Aminolevulinate synthase 2 (ALAS2) is an erythroid-expressed gene located on chromosome Xp11.21. Mutations in ALAS2 have been shown to be associated with sideroblastic anemia. We have identified a novel mutation in ALAS2, P520L, in three kinships. The P520L mutation was not found in 316 white male control subjects. The proline in this position is highly conserved across species from humans to zebrafish.In the C kinship, the P520L mutation was first identified in a white man who presented with severe iron overload at an early age and was a HFE C282Y homozygote. Genetic analyses of members of the C kinship suggest that the presence of P520L alone in hemizygous males, or simple heterozygosity in females, is not associated with anemia or iron overload. Females heterozygous for both HFE C282Y and ALAS2 P520L also had normal ferritin levels. Only subjects homozygous for HFE C282Y and hemizygous or heterozygous for ALAS2 P520L had severe iron overload. Sequencing studies revealed that the propositus did not have missense mutations in HAMP, HJV, FPN1, ABC7, IL6, or RAG1.In the H kinship, the propositus was a white woman with severe iron overload who was heterozygous for ALAS2 P520L; she had a wildtype HFE genotype. The pedigree of the H kinship identified 5 additional females who were heterozygous for the P520L ALAS2 mutation. Only the propositus had iron overload and none of the subjects with P520L had sideroblastic anemia. The propositus did not have missense mutations in FPN1, HAMP, HJV, TFR2, B2M, IRP2, ABC7, or SFT. We speculate that the propositus in the H kinship has a mutation in a currently unknown gene that contributes to her severe iron overload. In the S kinship, the P520L mutation was identified in a white man with sideroblastic anemia and severe iron overload. The patient had a second mutation in ALAS2, R560H, a mutation previously described in two brothers with sideroblastic anemia of variable penetrance (Blood 100:4236–4238, 2002). It is not possible to determine the effect of the P520L mutation on the penetrance of the R560H-associated sideroblastic anemia and iron overload.We conclude that the ALAS2 P520L occurs at a very low allele frequency in the white population. The present observations also suggest that there is no distinctive phenotype associated with the P520L mutation alone, but that P520L may act as a modifier of iron overload in the presence of HFE homozygosity, other missense mutations of ALAS2, or mutations of uncharacterized iron regulatory genes.

The Zebrafish Hypochromic Mutant [iItalic]Shiraz[/iItalic] Encodes a Novel Mitochondrial Glutaredoxin That Establishes a Link between Heme and Fe/S Production.

Iron is required in the mitochondria both to produce heme, which is used for hemoglobin synthesis, and to make iron-sulfur (Fe/S) clusters, which confer electron transfer or catalytic functions to proteins. Cellular iron utilization and Fe/S cluster production are thought to occur independently, yet the processes are coordinated through currently uncharacterized pathways. The shiraz (sir) zebrafish mutant manifests a hypochromic, microcytic anemia. Positional cloning of sir discovered a deletion at the locus that included the zebrafish orthologue to glutaredoxin 5 (grx5), a gene required in yeast for Fe/S cluster assembly. We found that grx5 is highly expressed in the developing blood and fetal liver of both zebrafish and mouse embryos. Antisense-mediated morpholino knockdown of grx5 prevented hemoglobin production, and overexpression of zebrafish, yeast, mouse, or human grx5 RNA in sir embryos completely rescued hemoglobin production, indicating that grx5 is the gene responsible for the sir phenotype. Expression of zebrafish grx5 was found to rescue Fe/S protein production in the yeast Δgrx5 strain, demonstrating that the role of grx5 in Fe/S cluster assembly is conserved among eukaryotes. The surprising finding that mutating a gene necessary for Fe/S cluster assembly caused a lack of hemoglobin synthesis suggested that we had discovered a connection between these pathways. In vertebrates, iron regulatory protein 1 (IRP1) acts as a sensor of intracellular iron levels and controls cellular iron homeostasis via posttranscriptional regulation of iron uptake, storage, and utilization genes. For instance, IRP1 binds to the 5′ iron response element (IRE) in the aminolevulinate synthase 2 (ALAS2) mRNA, blocking translation when cellular iron is low. However, when cellular iron is replete, IRP1 binds a Fe/S cluster and its RNA-binding activity is abolished. We hypothesized that the loss of Fe/S cluster assembly in sir would activate IRP1 and block ALAS2 synthesis, resulting in hypochromia. In support of this model, overexpression of ALAS2 RNA without the 5′ IRE rescued sir hypochromia, while overexpression of ALAS2 including the IRE did not facilitate rescue. Furthermore, antisense morpholino knockdowns of IRP1 caused rescue of hemoglobin synthesis in sir embryos. The combination of these data indicate that hemoglobin production in the differentiating red cell is monitored through Fe-S cluster assembly as a mechanism to gauge iron levels and accordingly direct heme synthesis. This finding illustrates a crucial role for the mitochondrial Fe/S cluster assembly machinery during hemoglobin production, and has broad implications for the role of such genes in human hypochromic anemias.