Biallelic hexokinase 1 (HK1) variants causative of non-spherocytic haemolytic anaemia: A case series with emphasis on the HK1 promoter variant and literature review.

The hexokinase (HK) enzyme plays a key role in red blood cell energy production. Hereditary non-spherocytic haemolytic anaemia (HNSHA) caused by HK deficiency is a rare disorder with only 12 different disease-associated variants identified. Here, we describe the clinical features and genotypes of four previously unreported patients with hexokinase 1 (HK1)-related HNSHA, yielding two novel truncating HK1 variants. The patients' phenotypes varied from mild chronic haemolytic anaemia to severe infantile-onset transfusion-dependent anaemia. Three of the patients had mild haemolytic disease caused by the common HK1 promoter c.-193A>G variant combined with an intragenic HK1 variant, emphasizing the importance of including this promoter variant in the haemolytic disease gene panels. HK activity was normal in a severely affected patient with a homozygous HK1 c.2599C>T, p.(His867Tyr) variant, but the affinity for ATP was reduced, hampering the HK function. In cases of HNSHA, kinetic studies should be considered in the functional studies of HK. We reviewed the literature of previously published patients to provide better insight into this rare disease and add to the understanding of genotype-phenotype correlation.

Heterozygosity for bisphosphoglycerate mutase deficiency expressing clinically as congenital erythrocytosis: A case series and literature review.

Erythrocytosis is associated with increased red blood cell mass and can be either congenital or acquired. Congenital secondary causes are rare and include germline variants increasing haemoglobin (Hb)-oxygen affinity (e.g., Hb or bisphosphoglycerate mutase (BPGM) variants) or affecting oxygen-sensing pathway proteins. Here, we describe five adults from three kindreds with erythrocytosis associated with heterozygosity for BPGM variants, including one novel. Functional analyses showed partial BPGM deficiency, reduced 2,3-bisphosphoglycerate levels and/or increased Hb-oxygen affinity. We also review currently known BPGM variants. This study contributes to raising awareness of BPGM variants, and in particular that heterozygosity for BPGM deficiency may already manifest clinically.

Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.

Gain-of-function mutations in the EPAS1/HIF2A gene have been identified in patients with hereditary erythrocytosis that can be associated with the development of paraganglioma, pheochromocytoma and somatostatinoma. In the present study, we describe a unique European collection of 41 patients and 28 relatives diagnosed with an erythrocytosis associated with a germline genetic variant in EPAS1. In addition we identified two infants with severe erythrocytosis associated with a mosaic mutation present in less than 2% of the blood, one of whom later developed a paraganglioma. The aim of this study was to determine the causal role of these genetic variants, to establish pathogenicity, and to identify potential candidates eligible for the new hypoxia-inducible factor-2 α (HIF-2α) inhibitor treatment. Pathogenicity was predicted with in silico tools and the impact of 13 HIF-2b variants has been studied by using canonical and real-time reporter luciferase assays. These functional assays consisted of a novel edited vector containing an expanded region of the erythropoietin promoter combined with distal regulatory elements which substantially enhanced the HIF-2α-dependent induction. Altogether, our studies allowed the classification of 11 mutations as pathogenic in 17 patients and 23 relatives. We described four new mutations (D525G, L526F, G527K, A530S) close to the key proline P531, which broadens the spectrum of mutations involved in erythrocytosis. Notably, we identified patients with only erythrocytosis associated with germline mutations A530S and Y532C previously identified at somatic state in tumors, thereby raising the complexity of the genotype/phenotype correlations. Altogether, this study allows accurate clinical follow-up of patients and opens the possibility of benefiting from HIF-2α inhibitor treatment, so far the only targeted treatment in hypoxia-related erythrocytosis disease.

Characterization of genetic variants in the EGLN1/PHD2 gene identified in a European collection of patients with erythrocytosis.

Hereditary erythrocytosis is a rare hematologic disorder characterized by an excess of red blood cell production. Here we describe a European collaborative study involving a collection of 2,160 patients with erythrocytosis sequenced in ten different laboratories. We focused our study on the EGLN1 gene and identified 39 germline missense variants including one gene deletion in 47 probands. EGLN1 encodes the PHD2 prolyl 4-hydroxylase, a major inhibitor of hypoxia-inducible factor. We performed a comprehensive study to evaluate the causal role of the identified PHD2 variants: (i) in silico studies of localization, conservation, and deleterious effects; (ii) analysis of hematologic parameters of carriers identified in the UK Biobank; (iii) functional studies of the protein activity and stability; and (iv) a comprehensive study of PHD2 splicing. Altogether, these studies allowed the classification of 16 pathogenic or likely pathogenic mutants in a total of 48 patients and relatives. The in silico studies extended to the variants described in the literature showed that a minority of PHD2 variants can be classified as pathogenic (36/96), without any differences from the variants of unknown significance regarding the severity of the developed disease (hematologic parameters and complications). Here, we demonstrated the great value of federating laboratories working on such rare disorders in order to implement the criteria required for genetic classification, a strategy that should be extended to all hereditary hematologic diseases.

A novel missense variant in ATP11C is associated with reduced red blood cell phosphatidylserine flippase activity and mild hereditary hemolytic anemia.

Adenosine Triphosphatase (ATPase) Phospholipid Transporting 11C gene (ATP11C) encodes the major phosphatidylserine (PS) flippase in human red blood cells (RBCs). Flippases actively transport phospholipids (e.g., PS) from the outer to the inner leaflet to establish and maintain phospholipid asymmetry of the lipid bilayer of cell membranes. This asymmetry is crucial for survival since externalized PS triggers phagocytosis by splenic macrophages. Here we report on pathophysiological consequences of decreased flippase activity, prompted by a patient with hemolytic anemia and hemizygosity for a novel c.2365C > T p.(Leu789Phe) missense variant in ATP11C. ATP11C protein expression was strongly reduced by 58% in patient-derived RBC ghosts. Furthermore, functional characterization showed only 26% PS flippase activity. These results were confirmed by recombinant mutant ATP11C protein expression in HEK293T cells, which was decreased to 27% compared to wild type, whereas PS-stimulated ATPase activity was decreased by 57%. Patient RBCs showed a mild increase in PS surface exposure when compared to control RBCs, which further increased in the most dense RBCs after RBC storage stress. The increase in PS was not due to higher global membrane content of PS or other phospholipids. In contrast, membrane lipid lateral distribution showed increased abundance of cholesterol-enriched domains in RBC low curvature areas. Finally, more dense RBCs and subtle changes in RBC morphology under flow hint toward alterations in flow behavior of ATP11C-deficient RBCs. Altogether, ATP11C deficiency is the likely cause of hemolytic anemia in our patient, thereby underlining the physiological role and relevance of this flippase in human RBCs.

Red blood cell phenotyping from 3D confocal images using artificial neural networks.

The investigation of cell shapes mostly relies on the manual classification of 2D images, causing a subjective and time consuming evaluation based on a portion of the cell surface. We present a dual-stage neural network architecture for analyzing fine shape details from confocal microscopy recordings in 3D. The system, tested on red blood cells, uses training data from both healthy donors and patients with a congenital blood disease, namely hereditary spherocytosis. Characteristic shape features are revealed from the spherical harmonics spectrum of each cell and are automatically processed to create a reproducible and unbiased shape recognition and classification. The results show the relation between the particular genetic mutation causing the disease and the shape profile. With the obtained 3D phenotypes, we suggest our method for diagnostics and theragnostics of blood diseases. Besides the application employed in this study, our algorithms can be easily adapted for the 3D shape phenotyping of other cell types and extend their use to other applications, such as industrial automated 3D quality control.

Proton pump inhibition for secondary hemochromatosis in hereditary anemia: a phase III placebo-controlled randomized cross-over clinical trial.

Iron overload is a severe general complication of hereditary anemias. Treatment with iron chelators is hampered by important side-effects, high costs, and the lack of availability in many countries with a high prevalence of hereditary anemias. In this phase III randomized placebo-controlled trial, we assigned adults with non-transfusion-dependent hereditary anemias with mild-to-moderate iron overload to receive esomeprazole (at a dose of 40 mg twice daily) or placebo for 12 months in a cross-over design. The primary end point was change of liver iron content measured by MRI. A total of 30 participants were enrolled in the trial. Treatment with esomeprazole resulted in a statistically significant reduction in liver iron content that was 0.55 mg Fe/g dw larger than after treatment with placebo (95%CI [0.05 to 1.06]; p = 0.03). Median baseline liver iron content at the start of esomeprazole was 4.99 versus 4.49 mg Fe/g dw at start of placebo. Mean delta liver iron content after esomeprazole treatment was -0.57 (SD 1.20) versus -0.11 mg Fe/g dw (SD 0.75) after placebo treatment. Esomeprazole was well tolerated, reported adverse events were mild and none of the patients withdrew from the study due to side effects. In summary, esomeprazole resulted in a significant reduction in liver iron content when compared to placebo in a heterogeneous group of patients with non-transfusion-dependent hereditary anemias. From an international perspective this result can have major implications given the fact that proton pump inhibitors may frequently be the only realistic therapy for many patients without access to or not tolerating iron chelators.

Dried blood spot metabolomics reveals a metabolic fingerprint with diagnostic potential for Diamond Blackfan Anaemia.

The diagnostic evaluation of Diamond Blackfan Anaemia (DBA), an inherited bone marrow failure syndrome characterised by erythroid hypoplasia, is challenging because of a broad phenotypic variability and the lack of functional screening tests. In this study, we explored the potential of untargeted metabolomics to diagnose DBA. In dried blood spot samples from 18 DBA patients and 40 healthy controls, a total of 1752 unique metabolite features were identified. This metabolic fingerprint was incorporated into a machine-learning algorithm, and a binary classification model was constructed using a training set. The model showed high performance characteristics (average accuracy 91·9%), and correct prediction of class was observed for all controls (n = 12) and all but one patient (n = 4/5) from the validation or 'test' set (accuracy 94%). Importantly, in patients with congenital dyserythropoietic anaemia (CDA) - an erythroid disorder with overlapping features - we observed a distinct metabolic profile, indicating the disease specificity of the DBA fingerprint and underlining its diagnostic potential. Furthermore, when exploring phenotypic heterogeneity, DBA treatment subgroups yielded discrete differences in metabolic profiles, which could hold future potential in understanding therapy responses. Our data demonstrate that untargeted metabolomics in dried blood spots is a promising new diagnostic tool for DBA.

Comparisons of oxygen gradient ektacytometry parameters between sickle cell patients with or without α-thalassaemia.

The present study tested the impact of α-thalassaemia on oxygen gradient ektacytometry in sickle cell anaemia (SCA). Three SCA groups were compared: (i) no α-thalassaemia (four α-genes, n = 62), (ii) silent α-thalassaemia (three α-genes, n = 35) and (iii) homozygous α-thalassaemia (two α-genes, n = 12). Red blood cell (RBC) deformability measured in normoxia was not different between the three groups. The lowest RBC deformability reached at low oxygen partial pressure (pO2 ) was greater and the pO2 at which RBC started to sickle was lower in the two α-genes group compared to the other groups. Our present study showed an effect of α-thalassaemia on oxygen gradient ektacytometry in SCA.

Untargeted metabolic profiling in dried blood spots identifies disease fingerprint for pyruvate kinase deficiency.

The diagnostic evaluation and clinical characterization of rare hereditary anemia (RHA) is to date still challenging. In particular, there is little knowledge on the broad metabolic impact of many of the molecular defects underlying RHA. In this study we explored the potential of untargeted metabolomics to diagnose a relatively common type of RHA: Pyruvate Kinase Deficiency (PKD). In total, 1903 unique metabolite features were identified in dried blood spot samples from 16 PKD patients and 32 healthy controls. A metabolic fingerprint was identified using a machine learning algorithm, and subsequently a binary classification model was designed. The model showed high performance characteristics (AUC 0.990, 95%CI 0.981-0.999) and an accurate class assignment was achieved for all newly added control (13) and patient samples (6), with the exception of one patient (accuracy 94%). Important metabolites in the metabolic fingerprint included glycolytic intermediates, polyamines and several acyl carnitines. In general, the application of untargeted metabolomics in dried blood spots is a novel functional tool that holds promise for diagnostic stratification and studies on disease pathophysiology in RHA.

AG-348 (Mitapivat), an allosteric activator of red blood cell pyruvate kinase, increases enzymatic activity, protein stability, and ATP levels over a broad range of PKLR genotypes.

Pyruvate kinase (PK) deficiency is a rare hereditary disorder affecting red cell (RBC) glycolysis, causing changes in metabolism including a deficiency in ATP. This affects red cell homeostasis, promoting premature removal of RBCs from the circulation. In this study we characterized and evaluated the effect of AG-348, an allosteric activator of PK that is currently in clinical trials for treatment of PK deficiency, on RBCs and erythroid precursors from PK-deficient patients. In 15 patients ex vivo treatment with AG-348 resulted in increased enzymatic activity in all patient cells after 24 hours (mean increase 1.8-fold, range 1.2-3.4). ATP levels increased (mean increase 1.5-fold, range 1.0-2.2) similar to control cells (mean increase 1.6-fold, range, 1.4-1.8). Generally, PK thermostability was strongly reduced in PK-deficient RBCs. Ex vivo treatment with AG-348 increased residual activity 1.4 to >10-fold than residual activity of vehicle-treated samples. Protein analyses suggests that a sufficient level of PK protein is required for cells to respond to AG-348 treatment ex-vivo, as treatment effects were minimal in patient cells with very low or undetectable levels of PK-R. In half of the patients, ex vivo treatment with AG-348 was associated with an increase in RBC deformability. These data support the hypothesis that drug intervention with AG-348 effectively upregulates PK enzymatic activity and increases stability in PK-deficient RBCs over a broad range of PKLR genotypes. The concomitant increase in ATP levels suggests that glycolytic pathway activity may be restored. AG-348 treatment may represent an attractive way to correct the underlying pathologies of PK deficiency. (AG-348 is currently in clinical trials for the treatment of PK deficiency. ClinicalTrials.gov: NCT02476916, NCT03853798, NCT03548220, NCT03559699).

Recommendations for diagnosis and treatment of methemoglobinemia.

Methemoglobinemia is a rare disorder associated with oxidization of divalent ferro-iron of hemoglobin (Hb) to ferri-iron of methemoglobin (MetHb). Methemoglobinemia can result from either inherited or acquired processes. Acquired forms are the most common, mainly due to the exposure to substances that cause oxidation of the Hb both directly or indirectly. Inherited forms are due either to autosomal recessive variants in the CYB5R3 gene or to autosomal dominant variants in the globin genes, collectively known as HbM disease. Our recommendations are based on a systematic literature search. A series of questions regarding the key signs and symptoms, the methods for diagnosis, the clinical management in neonatal/childhood/adulthood period, and the therapeutic approach of methemoglobinemia were formulated and the relative recommendations were produced. An agreement was obtained using a Delphi-like approach and the experts panel reached a final consensus >75% of agreement for all the questions.

Infantile Pyknocytosis in a Premature Dichorionic Diamniotic Twin.

Infantile pyknocytosis is a rare and self-limiting cause of hemolytic anemia in neonates. It can result in severe anemia and hyperbilirubinemia. The pathogenesis is unknown: a genetic origin has been discussed; however, based on the current literature it is not clear which genetic mutations should be considered. We present a case of a premature twin, in whom genetic screening was performed. Genetic mutations in 46 genes associated with hereditary hemolytic anemia and dyserythropoietic anemia were tested. No mutations were found. In infantile pyknocytosis, a genetic defect in these genes is unlikely.

Mechanical Stress Induces Ca2+-Dependent Signal Transduction in Erythroblasts and Modulates Erythropoiesis.

Bioreactors are increasingly implemented for large scale cultures of various mammalian cells, which requires optimization of culture conditions. Such upscaling is also required to produce red blood cells (RBC) for transfusion and therapy purposes. However, the physiological suitability of RBC cultures to be transferred to stirred bioreactors is not well understood. PIEZO1 is the most abundantly expressed known mechanosensor on erythroid cells. It is a cation channel that translates mechanical forces directly into a physiological response. We investigated signaling cascades downstream of PIEZO1 activated upon transitioning stationary cultures to orbital shaking associated with mechanical stress, and compared the results to direct activation of PIEZO1 by the chemical agonist Yoda1. Erythroblasts subjected to orbital shaking displayed decreased proliferation, comparable to incubation in the presence of a low dose of Yoda1. Epo (Erythropoietin)-dependent STAT5 phosphorylation, and Calcineurin-dependent NFAT dephosphorylation was enhanced. Phosphorylation of ERK was also induced by both orbital shaking and Yoda1 treatment. Activation of these pathways was inhibited by intracellular Ca2+ chelation (BAPTA-AM) in the orbital shaker. Our results suggest that PIEZO1 is functional and could be activated by the mechanical forces in a bioreactor setup, and results in the induction of Ca2+-dependent signaling cascades regulating various aspects of erythropoiesis. With this study, we showed that Yoda1 treatment and mechanical stress induced via orbital shaking results in comparable activation of some Ca2+-dependent pathways, exhibiting that there are direct physiological outcomes of mechanical stress on erythroblasts.

The Interplay between Drivers of Erythropoiesis and Iron Homeostasis in Rare Hereditary Anemias: Tipping the Balance.

Rare hereditary anemias (RHA) represent a group of disorders characterized by either impaired production of erythrocytes or decreased survival (i.e., hemolysis). In RHA, the regulation of iron metabolism and erythropoiesis is often disturbed, leading to iron overload or worsening of chronic anemia due to unavailability of iron for erythropoiesis. Whereas iron overload generally is a well-recognized complication in patients requiring regular blood transfusions, it is also a significant problem in a large proportion of patients with RHA that are not transfusion dependent. This indicates that RHA share disease-specific defects in erythroid development that are linked to intrinsic defects in iron metabolism. In this review, we discuss the key regulators involved in the interplay between iron and erythropoiesis and their importance in the spectrum of RHA.

Identification of a new VHL exon and complex splicing alterations in familial erythrocytosis or von Hippel-Lindau disease.

Chuvash polycythemia is an autosomal recessive form of erythrocytosis associated with a homozygous p.Arg200Trp mutation in the von Hippel-Lindau (VHL) gene. Since this discovery, additional VHL mutations have been identified in patients with congenital erythrocytosis, in a homozygous or compound-heterozygous state. VHL is a major tumor suppressor gene, mutations in which were first described in patients presenting with VHL disease, which is characterized by the development of highly vascularized tumors. Here, we identify a new VHL cryptic exon (termed E1') deep in intron 1 that is naturally expressed in many tissues. More importantly, we identify mutations in E1' in 7 families with erythrocytosis (1 homozygous case and 6 compound-heterozygous cases with a mutation in E1' in addition to a mutation in VHL coding sequences) and in 1 large family with typical VHL disease but without any alteration in the other VHL exons. In this study, we show that the mutations induced a dysregulation of VHL splicing with excessive retention of E1' and were associated with a downregulation of VHL protein expression. In addition, we demonstrate a pathogenic role for synonymous mutations in VHL exon 2 that altered splicing through E2-skipping in 5 families with erythrocytosis or VHL disease. In all the studied cases, the mutations differentially affected splicing, correlating with phenotype severity. This study demonstrates that cryptic exon retention and exon skipping are new VHL alterations and reveals a novel complex splicing regulation of the VHL gene. These findings open new avenues for diagnosis and research regarding the VHL-related hypoxia-signaling pathway.

The variable manifestations of disease in pyruvate kinase deficiency and their management.

Pyruvate kinase deficiency (PKD) is the most common cause of chronic hereditary non-spherocytic hemolytic anemia and results in a broad spectrum of disease. The diagnosis of PKD requires a high index of suspicion and judicious use of laboratory tests that may not always be informative, including pyruvate kinase enzyme assay and genetic analysis of the PKLR gene. A significant minority of patients with PKD have occult mutations in non-coding regions of PKLR which are missed on standard genetic tests. The biochemical consequences of PKD result in hemolytic anemia due to red cell pyruvate and ATP deficiency while simultaneously causing increased red cell 2,3-diphosphoglycerate, which facilitates oxygen unloading. This phenomenon, in addition to numerous other factors such as genetic background and differences in splenic function result in a poor correlation between symptoms and degree of anemia from patient to patient. Red cell transfusions should, therefore, be symptom-directed and not based on a hemoglobin threshold. Patients may experience specific complications, such as paravertebral extramedullary hematopoiesis and chronic debilitating icterus, which require personalized treatment. The decision to perform splenectomy or hematopoietic stem cell transplantation is nuanced and depends on disease burden and long-term outlook given that targeted therapeutics are in development. In recognition of the complicated nature of the disease and its management and the limitations of the PKD literature, an international working group of ten PKD experts convened to better define the disease burden and manifestations. This article summarizes the conclusions of this working group and is a guide for clinicians and investigators caring for patients with PKD.

Density, heterogeneity and deformability of red cells as markers of clinical severity in hereditary spherocytosis.

Hereditary spherocytosis (HS) originates from defective anchoring of the cytoskeletal network to the transmembrane protein complexes of the red blood cell (RBC). Red cells in HS are characterized by membrane instability and reduced deformability and there is marked heterogeneity in disease severity among patients. To unravel this variability in disease severity, we analyzed blood samples from 21 HS patients with defects in ankyrin, band 3, α-spectrin or ß-spectrin using red cell indices, eosin-5-maleimide binding, microscopy, the osmotic fragility test, Percoll density gradients, vesiculation and ektacytometry to assess cell membrane stability, cellular density and deformability. Reticulocyte counts, CD71 abundance, band 4.1 a:b ratio, and glycated hemoglobin were used as markers of RBC turnover. We observed that patients with moderate/severe spherocytosis have short-living erythrocytes of low density and abnormally high intercellular heterogeneity. These cells show a prominent decrease in membrane stability and deformability and, as a consequence, are quickly removed from the circulation by the spleen. In contrast, in mild spherocytosis less pronounced reduction in deformability results in prolonged RBC lifespan and, hence, cells are subject to progressive loss of membrane. RBC from patients with mild spherocytosis thus become denser before they are taken up by the spleen. Based on our findings, we conclude that RBC membrane loss, cellular heterogeneity and density are strong markers of clinical severity in spherocytosis.

Genotype-phenotype correlation and molecular heterogeneity in pyruvate kinase deficiency.

Pyruvate kinase (PK) deficiency is a rare recessive congenital hemolytic anemia caused by mutations in the PKLR gene. This study reports the molecular features of 257 patients enrolled in the PKD Natural History Study. Of the 127 different pathogenic variants detected, 84 were missense and 43 non-missense, including 20 stop-gain, 11 affecting splicing, five large deletions, four in-frame indels, and three promoter variants. Within the 177 unrelated patients, 35 were homozygous and 142 compound heterozygous (77 for two missense, 48 for one missense and one non-missense, and 17 for two non-missense variants); the two most frequent mutations were p.R510Q in 23% and p.R486W in 9% of mutated alleles. Fifty-five (21%) patients were found to have at least one previously unreported variant with 45 newly described mutations. Patients with two non-missense mutations had lower hemoglobin levels, higher numbers of lifetime transfusions, and higher rates of complications including iron overload, extramedullary hematopoiesis, and pulmonary hypertension. Rare severe complications, including lower extremity ulcerations and hepatic failure, were seen more frequently in patients with non-missense mutations or with missense mutations characterized by severe protein instability. The PKLR genotype did not correlate with the frequency of complications in utero or in the newborn period. With ICCs ranging from 0.4 to 0.61, about the same degree of clinical similarity exists within siblings as it does between siblings, in terms of hemoglobin, total bilirubin, splenectomy status, and cholecystectomy status. Pregnancy outcomes were similar across genotypes in PK deficient women. This report confirms the wide genetic heterogeneity of PK deficiency.

Congenital Hemolytic Anemia Because of Glucose Phosphate Isomerase Deficiency: Identification of 2 Novel Missense Mutations in the GPI Gene.

Glucose phosphate isomerase (GPI) deficiency is the second most common red blood cell enzymopathy involving the glycolysis pathway. It is an autosomal recessive disorder. Chronic hemolytic anemia is a common manifestation. The most severe one can present as hydrops fetalis. It can also be associated with neurologic dysfunction. We report a girl with severe hemolytic anemia at birth because of GPI deficiency. Enzyme activity assays were inconclusive because of previous blood transfusions. She was found to be compound heterozygous for 2 novel missense mutations, c.490C>A p.(Pro164Thr) and c.817C>T p.(Arg273Cys), in the GPI gene. Other than the chronic hemolytic anemia, she also has mild fine motor, gross motor delay, and developed cerebella ataxia since 5 years old.