Thrombotic risk in congenital erythrocytosis due to up-regulated hypoxia sensing is not associated with elevated hematocrit

Induction of erythropoietin by dietary medium-chain triacylglycerol in humans

Erythropoietin (EPO) is pivotal in regulating red blood cell (erythrocyte) concentrations and is primarily synthesized in the kidney. Recent research has unveiled a possible link between elevated circulating concentrations of ketone bodies (KB) and circulating EPO concentrations; however, it is not known whether nutritionally induced endogenous ketogenesis can be a stimulus to induce EPO in humans. Therefore, this study aimed to assess whether acute and chronic intake of medium-chain fatty acid-containing triacylglycerol (MCT), which rapidly enhances endogenous circulating KB, would elevate circulating EPO concentrations in humans, as indicated by prior work with exogenous KB administration. The study followed a crossover design involving 16 young men undergoing two 8-day MCT or energy-matched long-chain fatty acid-containing triacylglycerol (LCT) interventions in a randomized order. Five-hour test days were performed before and after each intervention, in which circulating KB and EPO concentrations as well as hematological parameters were assessed. Acute intake of MCT yielded a 222% sustained 5-h elevation in KB concentrations compared with LCT-with notable peak values of 0.7 ± 0.1 mmol·L-1 (312% above basal values). Remarkably, within just 8 days of daily MCT intake an impressive 38% increase in basal, fasting plasma EPO concentrations (7.19 ± 1.14 to 9.91 ± 1.25 mIU·mL-1) was demonstrated. In conclusion, this study unveils a novel physiological stimulus of circulating EPO concentrations in humans, potentially offering a new dietary approach to counter anemia in cardiovascular diseases.NEW & NOTEWORTHY This study is the first to assess the effects of nutritionally induced ketogenesis by acute and subchronic intake of medium-chain fatty acids on plasma erythropoietin concentrations. Medium-chain fatty acid intake increases postprandial ketone body concentrations and within only 8 days of daily intake substantially enhances basal plasma erythropoietin concentrations in young men. We therefore reveal a dietary stimulus of endogenous circulating erythropoietin concentrations in humans, with the potential to counter anemia in cardiovascular diseases.

Increased Transferrin Concentrations Are Not Associated With Thrombosis in People Living at High Altitude

Background

Bolivian Andean Aymara highlanders, living at 4,000 m for 14,000 years, have evolved genetic adaptations to hypoxia. These include EGLN1 encoding prolyl hydroxylase 2 (PHD2), a regulator of transferrin transcription. Transferrin level increases in hypoxia and iron deficiency. Contrasting reports indicate that elevated transferrin is associated with experimentally induced thrombosis in mice undergoing short-stay at high altitude, but with decreased thrombosis in a congenital disorder of hypoxia-sensing.

Methods

A retrospective study was conducted in people living at high altitude (3,650 - 4,150 m). We analyzed serum transferrin concentration and thrombosis history in Aymara patients with high-altitude erythrocytosis (n = 149, median age 55 years, female gender 30%, iron deficiency 23%) or high-altitude anemia (n = 137, median age 43 years, female gender 86%, iron deficiency 57%).

Results

The median transferrin concentration was 339 mg/dL in erythrocytosis patients versus 310 mg/dL in anemia patients (P = 0.037); it was 367 mg/dL in iron deficient versus 312 mg/dL in iron replete patients (P < 0.001). Thrombosis history was present in 13% of erythrocytosis and 8% of anemia patients (P = 0.25) and was present in 16% of iron deficient and 7% of iron replete patients (P = 0.017). After adjustment for erythrocytosis and iron deficiency in multivariate regression analysis, the mean (95% confidence interval) transferrin concentration was 277 (237 - 316) mg/dL in 30 patients with thrombosis history versus 324 (306 - 341) mg/dL in 256 patients without thrombosis history (P = 0.018). Similar trends occurred for the subgroups of arterial thrombosis history (P = 0.044) and venous thrombosis history (P = 0.22).

Conclusions

In individuals with extreme environmental hypoxia, we found no evidence that increased transferrin is associated with increased thrombosis history. Rather, we observed a trend to decreased thrombosis history with increased transferrin levels.

[Familial erythrocytosis type 2 due to VHL germline mutations: a case report and literature review]

To enhance the understanding of familial erythrocytosis type 2 (ECYT2) resulting from compound heterozygous mutations in the VHL gene. Methods: We conducted a retrospective analysis of the case data from a patient with ECYT2 to investigate its pathogenesis, clinical features, diagnosis and treatment options, as well as prognosis, while also reviewing the relevant literature. Results: A 31-year-old man was admitted to the hospital due to facial and hand flushing that had persisted for 29 years. Whole exome sequencing revealed compound heterozygous mutations in VHL p.P81L and p.N90T. Both of his parents were found to carry only one of these heterozygous mutations, yet they exhibited normal phenotypes. Based on the patient's hematological tests, a clear diagnosis of ECYT2 was established. Following treatment with erythrocytapheresis and daily administration of aspirin at a dosage of 100 mg, the patient experienced relief from dizziness and headaches associated with blood hyperviscosity, without any thrombotic or bleeding complications during this period. Conclusions: ECYT2 is a rare group of autosomal recessive genetic disorders. This case of ECYT2, resulting from compound heterozygous mutations in the VHL gene, represents the first report in China. Clinically, it is characterized by elevated red cell mass, normal or increased serum erythropoietin levels, and normal hemoglobin oxygen affinity levels. These factors contribute to thrombotic and bleeding complications that can lead to early mortality.

Germline genetic variants that predispose to myeloproliferative neoplasms and hereditary myeloproliferative phenotypes

Epidemiological evidence of familial predispositions to myeloid malignancies and myeloproliferative neoplasms (MPN) has long been recognised, but recent studies have added to knowledge of specific germline variants in multiple genes that contribute to the familial risk. These variants may be common risk alleles in the general population but have low penetrance and cause sporadic MPN, such as the JAK2 46/1 haplotype, the variant most strongly associated with MPN. Association studies are increasingly identifying other MPN susceptibility genes such as TERT, MECOM, and SH2B3, while some common variants in DDX41 and RUNX1 appear to lead to a spectrum of myeloid malignancies. RBBP6 and ATM variants have been identified in familial MPN clusters and very rare germline variants such as chromosome 14q duplication cause hereditary MPN with high penetrance. Rarely, there are hereditary non-malignant diseases with an MPN-like phenotype. Knowledge of those genes and germline genetic changes which lead to MPN or diseases that mimic MPN helps to improve accuracy of diagnosis, aids with counselling regarding familial risk, and may contribute to clinical decision-making. Large scale population exome and genome sequencing studies will improve our knowledge of both common and rare germline genetic contributions to MPN.

Testosterone therapy-induced erythrocytosis: can phlebotomy be justified?

Erythrocytosis, or elevated hematocrit, is a common side effect of testosterone therapy (TTh) in male hypogonadism. Testosterone stimulates erythropoiesis through an initial rise in erythropoietin (EPO), the establishment of a new EPO/hemoglobin 'set point', and a parallel decrease in the master iron regulator protein hepcidin, as well as several other potential mechanisms. Evidence shows an increased thrombotic risk associated with TTh-induced erythrocytosis. Several guidelines by endocrine organizations for the treatment of male hypogonadism recommend against starting TTh in patients presenting with elevated hematocrit at baseline or stopping TTh when its levels cannot be controlled. Besides dose adjustments, therapeutic phlebotomy or venesection is mentioned as a means of reducing hematocrit in these patients. However, evidence supporting the efficacy or safety of therapeutic phlebotomy in lowering hematocrit in TTh-induced erythrocytosis is lacking. In light of this dearth of evidence, the recommendation to lower hematocrit using therapeutic phlebotomy is notable, as phlebotomy lowers tissue oxygen partial pressure (pO2) and eventually depletes iron stores, thereby triggering various biological pathways which might increase thrombotic risk. The potential pros and cons should therefore be carefully weighed against each other, and shared decision-making is recommended for initiating therapeutic phlebotomy as a treatment in patients on TTh who present with increased hematocrit.

Endothelial PHD2 deficiency induces apoptosis resistance and inflammation via AKT activation and AIP1 loss independent of HIF2α

In hypoxic and pseudohypoxic rodent models of pulmonary hypertension (PH), hypoxia-inducible factor (HIF) inhibition attenuates disease initiation. However, HIF activation alone, due to genetic alterations or use of inhibitors of prolyl hydroxylase domain (PHD) enzymes, has not been definitively shown to cause PH in humans, indicating the involvement of other mechanisms. Given the association between endothelial cell dysfunction and PH, the effects of pseudohypoxia and its underlying pathways were investigated in primary human lung endothelial cells. PHD2 silencing or inhibition, while activating HIF2α, induced apoptosis-resistance and IFN/STAT activation in endothelial cells, independent of HIF signaling. Mechanistically, PHD2 deficiency activated AKT and ERK, inhibited JNK, and reduced AIP1 (ASK1-interacting protein 1), all independent of HIF2α. Like PHD2, AIP1 silencing affected these same kinase pathways and produced a similar dysfunctional endothelial cell phenotype, which was partially reversed by AKT inhibition. Consistent with these in vitro findings, AIP1 protein levels in lung endothelial cells were decreased in Tie2-Cre/Phd2 knockout mice compared with wild-type controls. Lung vascular endothelial cells from patients with pulmonary arterial hypertension (PAH) showed IFN/STAT activation. Lung tissue from both SU5416/hypoxia PAH rats and patients with PAH all showed AKT activation and dysregulated AIP1 expression. In conclusion, PHD2 deficiency in lung vascular endothelial cells drives an apoptosis-resistant and inflammatory phenotype, mediated by AKT activation and AIP1 loss independent of HIF signaling. Targeting these pathways, including PHD2, AKT, and AIP1, holds the potential for developing new treatments for endothelial dysfunction in PH.NEW & NOTEWORTHY HIF activation alone does not conclusively lead to human PH, suggesting that HIF-independent signaling may also contribute to hypoxia-induced PH. This study demonstrated that PHD2 silencing-induced pseudohypoxia in human lung endothelial cells suppresses apoptosis and activates STAT, effects that persist despite HIF2α inhibition or knockdown and are attributed to AKT and ERK activation, JNK inhibition, and AIP1 loss. These findings align with observations in lung endothelial cells and tissues from PAH rodent models and patients.

A patent review on hypoxia-inducible factor (HIF) modulators (2021-2023)

INTRODUCTION

Hypoxia-inducible factor (HIF) is a central regulatory factor in detecting and adapting to cellular oxygen stress. Dysregulation of HIF is associated with various human diseases. Seven HIF modulators, including six prolyl hydroxylase (PHD) inhibitors and one HIF-2α inhibitor, have already been approved for the treatment of renal anemia and cancer, respectively.

AREAS COVERED

This review summarizes HIF modulators patented in the 2021-2023 period. This review provides an overview of HIF downregulators, including HIF-1α inhibitors, HIF-2α inhibitors, and HIF-2α degraders, as well as HIF upregulators, including PHD, FIH, and VHL inhibitors, and HIF-2α and HIF-3α agonists.

EXPERT OPINION

Efforts should be made to address the adverse clinical effects associated with approved HIF-modulating drugs, including PHD inhibitors and HIF-2α inhibitors. Identification of the specific buried cavity in the HIF-2α and an opened pocket in HIF-3α offer an avenue for designing novel modulators for HIF-2α or HIF-3α. Given the similarities observed in the binding cavities of HIF-2α and HIF-3α, it should be considered whether the approved HIF-2α inhibitors also inhibit HIF-3α. A comprehensive understanding of the HIF signaling pathway biology would lead to the development of novel small-molecule HIF modulators as innovative therapeutic approaches for a wide range of human diseases.

Benefit of phlebotomy and low-dose aspirin in the prevention of vascular events in patients with EPOR primary familial polycythemia on the island of New Caledonia

Not available.

Are coagulation profiles in Andean highlanders with excessive erythrocytosis favouring hypercoagulability?

Chronic mountain sickness is a maladaptive syndrome that affects individuals living permanently at high altitude and is characterized primarily by excessive erythrocytosis (EE). Recent results concerning the impact of EE in Andean highlanders on clotting and the possible promotion of hypercoagulability, which can lead to thrombosis, were contradictory. We assessed the coagulation profiles of Andeans highlanders with and without excessive erythrocytosis (EE+ and EE-). Blood samples were collected from 30 EE+ and 15 EE- in La Rinconada (Peru, 5100-5300 m a.s.l.), with special attention given to the sampling pre-analytical variables. Rotational thromboelastometry tests were performed at both native and normalized (40%) haematocrit using autologous platelet-poor plasma. Thrombin generation, dosages of clotting factors and inhibitors were measured in plasma samples. Data were compared between groups and with measurements performed at native haematocrit in 10 lowlanders (LL) at sea level. At native haematocrit, in all rotational thromboelastometry assays, EE+ exhibited hypocoagulable profiles (prolonged clotting time and weaker clot strength) compared with EE- and LL (all P < 0.01). At normalized haematocrit, clotting times were normalized in most individuals. Conversely, maximal clot firmness was normalized only in FIBTEM and not in EXTEM/INTEM assays, suggesting abnormal platelet activity. Thrombin generation, levels of plasma clotting factors and inhibitors, and standard coagulation assays were mostly normal in all groups. No highlanders reported a history of venous thromboembolism based on the dedicated survey. Collectively, these results indicate that EE+ do not present a hypercoagulable profile potentially favouring thrombosis.

Idiopathic erythrocytosis: A diagnostic and management challenge with emerging areas for exploration

Despite published algorithms for approaching the work-up of erythrocytosis, a significant proportion of patients are left with uncertainty as to its aetiology and prognosis. The term 'idiopathic erythrocytosis' (IE) is applied when known primary and secondary aetiologies have been ruled out. However, the assignment of secondary aetiologies is not always straightforward or evidence based, which can lead to misdiagnosis and heterogeneity in cohort studies. Furthermore, new studies have identified germline or somatic mutations that may affect prognosis. Epidemiological and cohort data are inconsistent as to whether IE increases the risk for complications such as arterial and venous thromboembolism, clonal transformation or comorbid conditions. Randomized trials assessing the role of phlebotomy for long-term management of IE have not been performed, so treatment remains a vexing problem for clinicians. Standardization of terminology and testing strategies, including comprehensive genetic screening in clinical research, are key to refining our understanding of IE.

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.

Increased transferrin protects from thrombosis in Chuvash erythrocytosis

Von Hippel-Lindau protein (VHL) is essential to hypoxic regulation of cellular processes. VHL promotes proteolytic clearance of hypoxia-inducible transcription factors (HIFs) that have been modified by oxygen-dependent HIF-prolyl hydroxylases. A homozygous loss-of-function VHLR200W mutation causes Chuvash erythrocytosis, a congenital disorder caused by augmented hypoxia-sensing. Homozygous VHLR200W results in accumulation of HIFs that increase transcription of the erythropoietin gene and raise hematocrit. Phlebotomies reduce hematocrit and hyperviscosity symptoms. However, the major cause of morbidity and mortality in Chuvash erythrocytosis is thrombosis. Phlebotomies cause iron deficiency, which may further elevate HIF activity and transferrin, the HIF-regulated plasma iron transporter recently implicated in thrombogenesis. We hypothesized that transferrin is elevated in Chuvash erythrocytosis, and that iron deficiency contributes to its elevation and to thrombosis. We studied 155 patients and 154 matched controls at steady state and followed them for development of thrombosis. Baseline transferrin was elevated, and ferritin reduced in patients. VHLR200W homozygosity and lower ferritin correlated with higher erythropoietin and transferrin. During 11 years of follow-up, risk of thrombosis increased 8.9-fold in patients versus controls. Erythropoietin elevation, but not hematocrit or ferritin, correlated with thrombosis risk. Unexpectedly, transferrin elevation associated with reduced rather than increased thrombosis risk. The A allele of the promoter EPO single nucleotide polymorphisms (SNP), rs1617640, associated with elevated erythropoietin and increased thrombosis risk, whereas the A allele of the intronic TF SNP, rs3811647, associated with higher transferrin and protection from thrombosis in patients. Our findings suggest an unexpected causal relationship between increased transferrin and protection from thrombosis in Chuvash erythrocytosis.

JAK2 unmutated erythrocytosis: 2023 Update on diagnosis and management

DISEASE OVERVIEW

JAK2 unmutated or non-polycythemia vera (PV) erythrocytosis encompasses a heterogenous spectrum of hereditary and acquired entities.

DIAGNOSIS

Foremost in the evaluation of erythrocytosis is the exclusion of PV through JAK2 (inclusive of exons 12-15) mutation screening. Initial assessment should also include gathering of previous records on hematocrit (Hct) and hemoglobin (Hgb) levels, in order to streamline the diagnostic process by first distinguishing longstanding from acquired erythrocytosis; subsequent subcategorization is facilitated by serum erythropoietin (Epo) measurement, germline mutation screening, and review of historical data, including comorbid conditions and medication list. Hereditary erythrocytosis constitutes the main culprit in the context of longstanding erythrocytosis, especially when associated with a positive family history. In this regard, a subnormal serum Epo level suggests EPO receptor mutation. Otherwise, considerations include those associated with decreased (high oxygen affinity Hgb variants, 2,3-bisphosphoglycerate deficiency, PIEZO1 mutations, methemoglobinemia) or normal oxygen tension at 50% Hgb saturation (P50). The latter include germline oxygen sensing pathway (HIF2A-PHD2-VHL) and other rare mutations. Acquired erythrocytosis commonly results from central (e.g., cardiopulmonary disease, high-altitude habitat) or peripheral (e.g., renal artery stenosis) hypoxia. Other noteworthy conditions associated with acquired erythrocytosis include Epo-producing tumors (e.g., renal cell carcinoma, cerebral hemangioblastoma) and drugs (e.g., testosterone, erythropoiesis stimulating agents, sodium-glucose cotransporter-2 inhibitors). Idiopathic erythrocytosis is an ill-defined terminology that presumes the existence of an increased Hgb/Hct level without an identifiable etiology. Such classification often lacks accounting for normal outliers and is marred by truncated diagnostic evaluation.

MANAGEMENT

Current consensus treatment guidelines are not supported by hard evidence and their value is further undermined by limited phenotypic characterization and unfounded concerns for thrombosis. We are of the opinion that cytoreductive therapy and indiscriminate use of phlebotomy should be avoided in the treatment of non-clonal erythrocytosis. However, it is reasonable to consider therapeutic phlebotomy if one were to demonstrate value in symptom control, with frequency determined by symptoms rather than Hct level. In addition, cardiovascular risk optimization and low dose aspirin is often advised.

FUTURE DIRECTIONS

Advances in molecular hematology might result in better characterization of "idiopathic erythrocytosis" and expansion of the repertoire for germline mutations in hereditary erythrocytosis. Prospective controlled studies are needed to clarify potential pathology from JAK2 unmutated erythrocytosis, as well as to document the therapeutic value of phlebotomy.

The transcriptional and regulatory identity of erythropoietin producing cells

Erythropoietin (Epo) is the master regulator of erythropoiesis and oxygen homeostasis. Despite its physiological importance, the molecular and genomic contexts of the cells responsible for renal Epo production remain unclear, limiting more-effective therapies for anemia. Here, we performed single-cell RNA and transposase-accessible chromatin (ATAC) sequencing of an Epo reporter mouse to molecularly identify Epo-producing cells under hypoxic conditions. Our data indicate that a distinct population of kidney stroma, which we term Norn cells, is the major source of endocrine Epo production in mice. We use these datasets to identify the markers, signaling pathways and transcriptional circuits characteristic of Norn cells. Using single-cell RNA sequencing and RNA in situ hybridization in human kidney tissues, we further provide evidence that this cell population is conserved in humans. These preliminary findings open new avenues to functionally dissect EPO gene regulation in health and disease and may serve as groundwork to improve erythropoiesis-stimulating therapies.

Regulation of Erythropoiesis by the Hypoxia-Inducible Factor Pathway: Effects of Genetic and Pharmacological Perturbations

Red blood cells transport O₂ from the lungs to body tissues. Hypoxia stimulates kidney cells to secrete erythropoietin (EPO), which increases red cell mass. Hypoxia-inducible factors (HIFs) mediate EPO gene transcriptional activation. HIF-α subunits are subject to O₂-dependent prolyl hydroxylation and then bound by the von Hippel-Lindau protein (VHL), which triggers their ubiquitination and proteasomal degradation. Mutations in the genes encoding EPO, EPO receptor, HIF-2α, prolyl hydroxylase domain protein 2 (PHD2), or VHL cause familial erythrocytosis. In addition to O₂, α-ketoglutarate is a substrate for PHD2, and analogs of α-ketoglutarate inhibit hydroxylase activity. In phase III clinical trials evaluating the treatment of anemia in chronic kidney disease, HIF prolyl hydroxylase inhibitors were as efficacious as darbepoetin alfa in stimulating erythropoiesis. However, safety concerns have arisen that are focused on thromboembolism, which is also a phenotypic manifestation of VHL or HIF-2α mutation, suggesting that these events are on-target effects of HIF prolyl hydroxylase inhibitors.

Hypoxia-inducible factors: cancer progression and clinical translation

Hypoxia-inducible factors (HIFs) are master regulators of oxygen homeostasis that match O2 supply and demand for each of the 50 trillion cells in the adult human body. Cancer cells co-opt this homeostatic system to drive cancer progression. HIFs activate the transcription of thousands of genes that mediate angiogenesis, cancer stem cell specification, cell motility, epithelial-mesenchymal transition, extracellular matrix remodeling, glucose and lipid metabolism, immune evasion, invasion, and metastasis. In this Review, the mechanisms and consequences of HIF activation in cancer cells are presented. The current status and future prospects of small-molecule HIF inhibitors for use as cancer therapeutics are discussed.

Hematocrit, White Blood Cells, and Thrombotic Events in the Veteran Population With Polycythemia Vera

Background

Patients with polycythemia vera (PV), a chronic myeloproliferative neoplasm, have a greater morbidity and mortality risk than the general population, largely due to a high incidence of thrombotic events.

Observations

Two recently published retrospective analyses from Parasuraman and colleagues used Veterans Health Administration (VHA) data to replicate, in a real-world population, findings from the prospective, randomized Cytoreductive Therapy in Polycythemia Vera (CYTO-PV) study. In the CYTO-PV study, hematocrit (Hct) level and white blood cell (WBC) count were shown to be independently associated with thrombotic event risk in patients with PV. In the VHA analysis, patients with Hct levels < 45% were found to have a significantly lower rate of thrombotic events compared to those with levels ≥ 45% (hazard ratio [HR], 1.61; P = .04). For WBC counts and thrombosis, patients with WBC ≥ 8.5 × 109/L were found to have a higher rate of thrombotic events compared to the reference cohort of WBC < 7 × 109/L (HR, 1.47; P < .01), and the rates were higher for those with WBC ≥ 11 × 109/L (HR 1.87; P < .001).

Conclusions

The results from these analyses suggest the need for managing Hct appropriately to maintain levels < 45% and offer further support for the consideration of WBC counts in determining risk of thrombotic events. Studies are needed to clearly establish an optimal WBC count to inform updates to treatment guidelines.

Epidemiologic and Genetic Associations of Erythropoietin With Blood Pressure, Hypertension, and Coronary Artery Disease

[Figure: see text].

Thrombosis in myeloproliferative neoplasms: update in pathophysiology

PURPOSE OF REVIEW

This review summarizes high-impact research in myeloproliferative neoplasms (MPN) from the last 18 months, with a particular focus on basic science findings.

RECENT FINDINGS

A pseudo-hypoxia state with stabilization of hypoxia-inducible factor (HIFα exists that is central to cell growth, cell renewal, inflammation, and thrombotic potential in MPN hematopoietic cells.

SUMMARY

HIFα and inflammatory pathways are new therapeutic targets in MPN, with the potential to ameliorate thrombotic risk and perhaps eradicate mutant progenitor cells.

Breakthrough Science: Hypoxia-Inducible Factors, Oxygen Sensing, and Disorders of Hematopoiesis

Hypoxia-inducible factors (HIF) were discovered as activators of erythropoietin gene transcription in response to reduced O2 availability. O2-dependent hydroxylation of HIFs on proline and asparagine residues regulates protein stability and transcription activity, respectively. Mutations in genes encoding components of the oxygen sensing pathway cause familial erythrocytosis. Several small molecule inhibitors of HIF prolyl hydroxylases are currently in clinical trials as erythropoiesis stimulating agents. HIFs are overexpressed in bone marrow neoplasms, and the development of HIF inhibitors may improve outcome in these disorders.

JAK2 unmutated erythrocytosis: current diagnostic approach and therapeutic views

JAK2 unmutated or non-polycythemia vera (PV) erythrocytosis encompasses both hereditary and acquired conditions. A systematic diagnostic approach begins with documentation of historical hematocrit (Hct)/hemoglobin (Hgb) measurements and classification of the process as life-long/unknown duration or acquired. Further investigation in both categories is facilitated by determination of serum erythropoietin level (EPO). Workup for hereditary/congenital erythrocytosis requires documentation of family history and laboratory screening for high-oxygen affinity hemoglobin variants, 2, 3 biphosphoglycerate deficiency, and germline mutations that are known to alter cellular oxygen sensing (e.g., PHD2, HIF2A, VHL) or EPO signaling (e.g., EPOR mutations); the latter is uniquely associated with subnormal EPO. Acquired erythrocytosis is often elicited by central or peripheral hypoxia resulting from cardiopulmonary disease/high-altitude dwelling or renal artery stenosis, respectively; EPO in the former instance is often normal (compensated by negative feed-back). Other conditions associated with acquired erythrocytosis include EPO-producing tumors and the use of drugs that promote erythropoiesis (e.g., testosterone, erythropoiesis stimulating agents). "Idiopathic erythrocytosis" loosely refers to an otherwise not explained situation. Historically, management of non-PV erythrocytosis has been conflicted by unfounded concerns regarding thrombosis risk, stemming from limited phenotypic characterization, save for Chuvash polycythemia, well-known for its thrombotic tendency. In general, cytoreductive therapy should be avoided and phlebotomy is seldom warranted where frequency is determined by symptom control rather than Hct threshold. Although not supported by hard evidence, cardiovascular risk optimization and low-dose aspirin use are often advised. Application of modern genetic tests and development of controlled therapeutic intervention trials are needed to advance current clinical practice.

Reevaluation of excessive erythrocytosis in diagnosing chronic mountain sickness in men from the world's highest city

The diagnosis of chronic mountain sickness (CMS) is based on a score including 7 clinical features (breathlessness, sleep disturbance, cyanosis, venous dilatation, paresthesia, headache, and tinnitus) in the setting of extreme erythrocytosis. Examining individuals in La Rinconada, Peru, the highest city in the world, the authors demonstrated that CMS at extreme altitude is not linked to elevation of hemoglobin, since CMS+ and CMS− individuals had similar levels of erythrocytosis.