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.

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.

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).

Rapid and reproducible characterization of sickling during automated deoxygenation in sickle cell disease patients.

In sickle cell disease (SCD), sickle hemoglobin (HbS) polymerizes upon deoxygenation, resulting in sickling of red blood cells (RBCs). These sickled RBCs have strongly reduced deformability, leading to vaso-occlusive crises and chronic hemolytic anemia. To date, there are no reliable laboratory parameters or assays capable of predicting disease severity or monitoring treatment effects. We here report on the oxygenscan, a newly developed method to measure RBC deformability (expressed as Elongation Index - EI) as a function of pO2 . Upon a standardized, 22 minute, automated cycle of deoxygenation (pO2 median 16 mmHg ± 0.17) and reoxygenation, a number of clinically relevant parameters are produced in a highly reproducible manner (coefficients of variation <5%). In particular, physiological modulators of oxygen affinity, such as, pH and 2,3-diphosphoglycerate showed a significant correlation (respectively R = -0.993 and R = 0.980) with Point of Sickling (PoS5% ), which is defined as the pO2 where a 5% decrease in EI is observed during deoxygenation. Furthermore, in vitro treatment with antisickling agents, including GBT440, which alter the oxygen affinity of hemoglobin, caused a reproducible left-shift of the PoS, indicating improved deformability at lower oxygen tensions. When RBCs from 21 SCD patients were analyzed, we observed a significantly higher PoS in untreated homozygous SCD patients compared to treated patients and other genotypes. We conclude that the oxygenscan is a state-of-the-art technique that allows for rapid analysis of sickling behavior in SCD patients. The method is promising for personalized treatment, development of new treatment strategies and could have potential in prediction of complications.

Ribosomal protein mutations induce autophagy through S6 kinase inhibition of the insulin pathway.

Mutations affecting the ribosome lead to several diseases known as ribosomopathies, with phenotypes that include growth defects, cytopenia, and bone marrow failure. Diamond-Blackfan anemia (DBA), for example, is a pure red cell aplasia linked to the mutation of ribosomal protein (RP) genes. Here we show the knock-down of the DBA-linked RPS19 gene induces the cellular self-digestion process of autophagy, a pathway critical for proper hematopoiesis. We also observe an increase of autophagy in cells derived from DBA patients, in CD34+ erythrocyte progenitor cells with RPS19 knock down, in the red blood cells of zebrafish embryos with RP-deficiency, and in cells from patients with Shwachman-Diamond syndrome (SDS). The loss of RPs in all these models results in a marked increase in S6 kinase phosphorylation that we find is triggered by an increase in reactive oxygen species (ROS). We show that this increase in S6 kinase phosphorylation inhibits the insulin pathway and AKT phosphorylation activity through a mechanism reminiscent of insulin resistance. While stimulating RP-deficient cells with insulin reduces autophagy, antioxidant treatment reduces S6 kinase phosphorylation, autophagy, and stabilization of the p53 tumor suppressor. Our data suggest that RP loss promotes the aberrant activation of both S6 kinase and p53 by increasing intracellular ROS levels. The deregulation of these signaling pathways is likely playing a major role in the pathophysiology of ribosomopathies.

Novel Homozygous Mutation of the Internal Translation Initiation Start Site of VHL is Exclusively Associated with Erythrocytosis: Indications for Distinct Functional Roles of von Hippel-Lindau Tumor Suppressor Isoforms.

Congenital secondary erythrocytosis is a rare disorder characterized by increased red blood cell production. An important cause involves defects in the oxygen sensing pathway, in particular the PHD2-VHL-HIF axis. Mutations in VHL are also associated with the von Hippel-Lindau tumor predisposition syndrome. The differences in phenotypic expression of VHL mutations are poorly understood. We report on three patients with erythrocytosis, from two unrelated families. All patients show exceptionally high erythropoietin (EPO) levels, and are homozygous for a novel missense mutation in VHL: c.162G>C p.(Met54Ile). The c.162G>C mutation is the most upstream homozygous VHL mutation described so far in patients with erythrocytosis. It abolishes the internal translational start codon, which directs expression of VHLp19, resulting in the production of only VHLp30. The exceptionally high EPO levels and the absence of VHL-associated tumors in the patients suggest that VHLp19 has a role for regulating EPO levels that VHLp30 does not have, whereas VHLp30 is really the tumor suppressor isoform.

Partial pyruvate kinase deficiency aggravates the phenotypic expression of band 3 deficiency in a family with hereditary spherocytosis.

In a family with mild dominant spherocytosis, affected members showed partial band 3 deficiency. The index patient showed more severe clinical symptoms than his relatives, and his red blood cells displayed concomitant low pyruvate kinase activity. We investigated the contribution of partial PK deficiency to the phenotypic expression of mutant band 3 in this family. Pyruvate kinase deficiency and band 3 deficiency were characterized by DNA analysis. Results of red cell osmotic fragility testing, the results of cell deformability obtained by the Automated Rheoscope and Cell Analyzer and the results obtained by Osmotic Gradient Ektacytometry, which is a combination of these tests, were related to the red cell ATP content. Spherocytosis in this family was due to a novel heterozygous mutation in SLC4A1, the gene for band 3. Reduced PK activity of the index patient was attributed to a novel mutation in PKLR inherited from his mother, who was without clinical symptoms. Partial PK deficiency was associated with decreased red cell ATP content and markedly increased osmotic fragility. This suggests an aggravating effect of low ATP levels on the phenotypic expression of band 3 deficiency.

Oxygen gradient ektacytometry-derived biomarkers are associated with acute complications in sickle cell disease.

ABSTRACT: We investigated the potential of the point of sickling (PoS; the pO2 tension at which red cells start to sickle), determined by oxygen gradient ektacytometry to serve as a biomarker associated with the incidence of acute sickle cell disease-related complications in 177 children and 50 adults. In the pediatric cohort, for every 10 mmHg increase in PoS reflecting a greater likelihood of sickling, the likelihood of an individual experiencing >1 type of acute complication increased; the adjusted odds ratio (aOR) was 1.65. For every 0.1 increase in minimum elongation index (EImin; reflecting improved red blood cell deformability at hypoxia), the aOR was 0.50. In the adult cohort, for every 10 mmHg increase in PoS, we found an aOR of 3.00, although this was not significant after correcting for multiple testing. There was a trend for an association between higher PoS and greater likelihood of vaso-occlusive episodes (VOEs; children aOR, 1.35; adults aOR, 2.22). In children, only EImin was associated with VOEs (aOR, 0.68). When data of both cohorts were pooled, significant associations with PoS and/or EImin were found for all acute complications, independently and when >1 type of acute complication was assessed. These findings indicate that oxygen gradient ektacytometry generates novel biomarkers and provides a rationale for further development of these biomarkers in the assessment of clinical severity, evaluation of novel therapies, and as surrogate clinical trial end points. These biomarkers may be useful in assessing efficacy of novel therapies like pyruvate kinase activators, voxelotor, and L-glutamine.

A novel composition of endogenous metabolic modulators improves red blood cell properties in sickle cell disease.

The most common forms of sickle cell disease (SCD) are sickle cell anemia (SCA; HbSS) and HbSC disease. In both, especially the more dense, dehydrated and adherent red blood cells (RBCs) with reduced deformability are prone to hemolysis and sickling, and thereby vaso-occlusion. Based on plasma amino acid profiling in SCD, a composition of 10 amino acids and derivatives (RCitNacQCarLKHVS; Axcella Therapeutics, USA), referred to as endogenous metabolic modulators (EMMs), was designed to target RBC metabolism. The effects of ex vivo treatment with the EMM composition on different RBC properties were studied in SCD (n = 9 SCA, n = 5 HbSC disease). Dose-dependent improvements were observed in RBC hydration assessed by hemocytometry (MCV, MCHC, dense RBCs) and osmotic gradient ektacytometry (Ohyper). Median (interquartile range [IQR]) increase in Ohyper compared to vehicle was 4.9% (4.0%-5.5%), 7.5% (6.9%-9.4%), and 12.8% (11.5%-14.0%) with increasing 20×, 40×, and 80X concentrations, respectively (all p < 0.0001). RBC deformability (EImax using oxygen gradient ektacytometry) increased by 8.1% (2.2%-12.1%; p = 0.0012), 9.6% (2.9%-15.1%; p = 0.0013), and 13.3% (5.7%-25.5%; p = 0.0007), respectively. Besides, RBC adhesion to subendothelial laminin decreased by 43% (6%-68%; p = 0.4324), 58% (48%-72%; p = 0.0185), and 71% (49%-82%; p = 0.0016), respectively. Together, these results provide a rationale for further studies with the EMM composition targeting multiple RBC properties in SCD.

Metabolic blood profile and response to treatment with the pyruvate kinase activator mitapivat in patients with sickle cell disease.

Mitapivat is an investigational, oral, small-molecule allosteric activator of pyruvate kinase (PK). PK is a regulatory glycolytic enzyme that is key in providing the red blood cell (RBC) with sufficient amounts of adenosine triphosphate (ATP). In sickle cell disease (SCD), decreased 2,3-DPG levels increase the oxygen affinity of hemoglobin, thereby preventing deoxygenation and polymerization of sickle hemoglobin. The PK activator mitapivat has been shown to decrease levels of 2,3-DPG and increase levels of ATP in RBCs in patients with SCD. In this phase 2, investigator-initiated, open-label study (https://www.clinicaltrialsregister.eu/ NL8517; EudraCT 2019-003438-18), untargeted metabolomics was used to explore the overall metabolic effects of 8-week treatment with mitapivat in the dose-finding period. In total, 1773 unique metabolites were identified in dried blood spots of whole blood from ten patients with SCD and 42 healthy controls (HCs). The metabolic phenotype of patients with SCD revealed alterations in 139/1773 (7.8%) metabolites at baseline when compared to HCs (false discovery rate-adjusted p < 0.05), including increases of (derivatives of) polyamines, purines, and acyl carnitines. Eight-week treatment with mitapivat in nine patients with SCD altered 85/1773 (4.8%) of the total metabolites and 18/139 (12.9%) of the previously identified altered metabolites in SCD (unadjusted p < 0.05). Effects were observed on a broad spectrum of metabolites and were not limited to glycolytic intermediates. Our results show the relevance of metabolic profiling in SCD, not only to unravel potential pathophysiological pathways and biomarkers in multisystem diseases but also to determine the effect of treatment.

In-vitro and in-silico evidence for oxidative stress as drivers for RDW.

Red blood cell distribution width (RDW) is a biomarker associated with a variety of clinical outcomes. While anemia and subclinical inflammation have been posed as underlying pathophysiology, it is unclear what mechanisms underlie these assocations. Hence, we aimed to unravel the mechanisms in silico using a large clinical dataset and validate our findings in vitro. We retrieved complete blood counts (CBC) from 1,403,663 measurements from the Utrecht Patient Oriented Database, to model RDW using gradient boosting regression. We performed (sex-stratified) analyses in patients with anemia, patients younger/older than 50 and validation across platforms and care settings. We then validated our hypothesis regarding oxidative stress using an in vitro approach. Only percentage microcytic (pMIC) and macrocytic (pMAC) erythrocytes and mean corpuscular volume were most important in modelling RDW (RMSE = 0.40, R2 = 0.96). Subgroup analyses and validation confirmed our findings. In vitro induction of oxidative stress underscored our results, namely increased RDW and decreased erythrocyte volume, yet no vesiculation was observed. We found that erythrocyte size, especially pMIC, is most informative in predicting RDW, but no role for anemia or inflammation. Oxidative stress affecting the size of the erythrocytes may play a role in the association between RDW and clinical outcomes.

One-year safety and efficacy of mitapivat in sickle cell disease: follow-up results of a phase 2, open-label study.

Targeting the primary pathogenic event of sickle cell disease (SCD), the polymerization of sickle hemoglobin (HbS), may prevent downstream clinical events. Mitapivat, an oral pyruvate kinase (PK) activator, has therapeutic potential by increasing adenosine triphosphate (ATP) and decreasing 2,3-diphosphoglycerate (2,3-DPG), a glycolytic red blood cell (RBC) intermediate. In the previously reported 8-week dose-finding period of this phase 2, investigator-initiated, open-label study, mitapivat was well tolerated and showed efficacy in SCD. Here, the 1-year fixed-dose extension period is reported in which 9 of 10 included patients (90%) aged ≥16 years with SCD (HbSS, HbS/ß0, or HbS/ß+) continued with mitapivat. Mostly mild treatment-emergent adverse events (AEs) (most commonly, transaminase increase and headache) were still reported. Apart from the reported nontreatment-related serious AE (SAE) of a urinary tract infection in the dose-finding period, 1 nontreatment-related SAE occurred in the fixed-dose extension period in a patient who died of massive pulmonary embolism due to COVID-19. Importantly, sustained improvement in Hb level (mean increase, 1.1 ± 0.7 g/dL; P = .0014) was seen, which was accompanied by decreases in markers of hemolysis. In addition, the annualized rate of vaso-occlusive events reduced significantly from a historic baseline of 1.33 ± 1.32 to 0.64 ± 0.87 (P = .0489) when combining the dose-finding period and fixed-dose extension period. Cellularly, the ATP:2,3-DPG ratio and Hb-oxygen affinity significantly increased and RBC sickling (point of sickling) nonsignificantly reduced. Overall, this study demonstrated 1-year safety and efficacy of treatment with mitapivat in SCD, supporting further evaluation in ongoing phase 2/3 study (RISE UP, NCT05031780). This trial was registered at https://www.clinicaltrialsregister.eu/ as NL8517 and EudraCT 2019-003438-18.

Methodological aspects of oxygen gradient ektacytometry in sickle cell disease: Effects of sample storage on outcome parameters in distinct patient subgroups.

Sickle cell disease (SCD) is a genetic disorder characterized by the production of an abnormal hemoglobin (Hb), which, under deoxygenation, may polymerize and cause a mechanical distortion of red blood cell (RBC) into a crescent-like shape. Recently a method, using ektacytometry principle, has been developed to assess RBC deformability as a function of oxygen tension (pO2) and is called oxygen gradient ektacytometry (oxygenscan). However, standardization of this test is needed to properly assess the tendency of sickling of RBCs under deoxygenation and to allow comparisons between different laboratories. The study compared the oxygenscan responses during blood storage between distinct populations of SCD patients. Blood from 40 non-transfused homozygous SCD patients (HbSS), 16 chronically transfused HbSS patients, and 14 individuals with compound heterozygous hemoglobin SC disease (HbSC) at steady-state was collected in EDTA tubes. Measurements were performed within 4 hours after collection and after 24 hours of storage at 4°C. We showed that storage affected the minimum RBC deformability reached during deoxygenation (EImin) in both non-transfused HbSS and HbSC patients and the maximum RBC deformability (EImax) measured before deoxygenation (i.e., in normoxia) in the three groups. In contrast, the tendency of RBCs to sickle under deoxygenation (i.e., the point of sickling; PoS) remained rather stable between the two time of measurements. Collectively, since the time between blood sampling and analysis affects some key oxygen gradient ektacytometry-derived parameters we recommend that each laboratory performs oxygenscan measurements at a standardized time point.

Effects of Genotypes and Treatment on Oxygenscan Parameters in Sickle Cell Disease.

(1) Background: The aim of the present study was to compare oxygen gradient ektacytometry parameters between sickle cell patients of different genotypes (SS, SC, and S/ß+) or under different treatments (hydroxyurea or chronic red blood cell exchange). (2) Methods: Oxygen gradient ektacytometry was performed in 167 adults and children at steady state. In addition, five SS patients had oxygenscan measurements at steady state and during an acute complication requiring hospitalization. (3) Results: Red blood cell (RBC) deformability upon deoxygenation (EImin) and in normoxia (EImax) was increased, and the susceptibility of RBC to sickle upon deoxygenation was decreased in SC patients when compared to untreated SS patients older than 5 years old. SS patients under chronic red blood cell exchange had higher EImin and EImax and lower susceptibility of RBC to sickle upon deoxygenation compared to untreated SS patients, SS patients younger than 5 years old, and hydroxyurea-treated SS and SC patients. The susceptibility of RBC to sickle upon deoxygenation was increased in the five SS patients during acute complication compared to steady state, although the difference between steady state and acute complication was variable from one patient to another. (4) Conclusions: The present study demonstrates that oxygen gradient ektacytometry parameters are affected by sickle cell disease (SCD) genotype and treatment.

Fifteen novel mutations in PKLR associated with pyruvate kinase (PK) deficiency: structural implications of amino acid substitutions in PK.

Pyruvate kinase (PK) deficiency is a rare disease but an important cause of hereditary nonspherocytic hemolytic anemia. The disease is caused by mutations in the PKLR gene and shows a marked variability in clinical expression. We report on the molecular characterization of 38 PK-deficient patients from 35 unrelated families. Twenty-nine different PKLR mutations were detected, of which 15 are reported here for the first time. Two novel deletions are reported: c.142_159del18 is the largest in-frame deletion described thus far and predicts the loss of six consecutive amino acids (p.Thr48_Thr53del) in the N-terminal domain of red blood cell PK. The other deletion removes nearly 1.5 kb of genomic DNA sequence (c.1618+37_2064del1477) and is one of a few large deletional mutants in PKLR. In addition, 13 novel point mutations were identified: one nonsense mutant, p.Arg488X, and 12 missense mutations, predicting the substitution of a single amino acid: p.Arg40Trp, p.Leu73Pro, p.Ile90Asn, p.Gly111Arg, p.Ala154Thr, p.Arg163Leu, p.Gly165Val, p.Leu272Val, p.Ile310Asn, p.Val320Leu, p.Gly358Glu, and p.Leu374Pro. We used the three-dimensional (3D) structure of recombinant human tetrameric PK to evaluate the protein structural context of the affected residues. In addition, in selected patients red blood cell PK antigen levels were measured by enzyme-linked immunosorbent assay (ELISA). Collectively, the results provided us with a rationale for the observed enzyme deficiency and contribute to both a better understanding of the genotype-to-phenotype correlation in PK deficiency as well as the enzyme's structure and function.

Characterization of Sickling During Controlled Automated Deoxygenation with Oxygen Gradient Ektacytometry.

In sickle cell disease (SCD), a single point mutation in the gene coding for beta-globin causes the production of abnormal hemoglobin S (HbS). When deoxygenated, HbS can polymerize, forming rigid rods of hemoglobin, resulting in the sickling of red blood cells (RBCs). These sickled RBCs have significantly reduced deformability, causing vaso-occlusion, which leads to numerous SCD-related clinical complications, including pain, stroke, and organ damage. RBC deformability is also reduced by RBC dehydration, resulting in dense red blood cells that are more likely to sickle. To date, there is not a single widely available, rapid, and reproducible laboratory assay capable of predicting the disease severity or directly monitoring the treatment effects for novel, non-fetal hemoglobin inducing therapies. In this study, we describe a protocol to measure RBC deformability as a function of pO2 that allows for the quantitation of sickling behavior in SCD patients. Oxygen gradient ektacytometry measures RBC deformability, expressed as the elongation index (EI), as a function of pO2. RBCs are exposed to a fixed shear stress of 30 Pa during one round of deoxygenation and reoxygenation. Six readout parameters are produced. Of these, the point of sickling (PoS), defined as the pO2 at which maximum EI (EImax) shows a 5% decrease, and minimum EI during deoxygenation (EImin) are the most informative, reflecting an individual patient's pO2 at which sickling starts and the minimal deformability of a patient's red blood cells, respectively. PoS is associated with an individual patient's hemoglobin affinity for oxygen, whereas EImin shows a strong correlation with fetal hemoglobin levels. We conclude that oxygen gradient ektacytometry is a promising technique to monitor the treatment of patients with SCD, as a biomarker for anti-sickling agents in clinical and preclinical trials, and an important tool to study sickling behavior of RBCs from individuals with SCD and sickle cell traits.