Secondary erythrocytosis often results from conditions that cause tissue hypoxia or an improper increase in erythropoietin (EPO) production. EPO, the major regulator of erythropoiesis, has a complex and tightly regulated expression during development, with a liver-to-kidney switch shortly after birth. We identified six families with erythrocytosis that was associated with circulating EPO levels within the normal range and characterized as a novel molecular and functional entity. We investigated the effect of the identified pathogenic variants using EPO promoter-driven luciferase reporter genes. Induced pluripotent stem cells (iPSCs) were generated from patient cells and differentiated into hepatocyte-like EPO-producing cells. Samples of circulating EPO from patients with hereditary erythrocytosis and from healthy newborns were analyzed by means of isoelectric focusing, and EPO activity was assessed. Three novel variants were identified in the noncoding regions of EPO. Experiments with reporter assays and iPSC-derived hepatocyte-like cells showed that the variants targeted previously uncharacterized regulatory elements of the gene, which, when the variants were present, showed high responsiveness to hypoxia. EPO samples from all the patients showed a modified isoelectric-focusing profile, identical to hepatic EPO that is expressed in premature neonates and in patients with acquired erythrocytosis associated with liver diseases. EPO that was purified from patient plasma and umbilical-cord blood samples showed enhanced EPO receptor signaling activity in vitro, which suggests a potential gain of function linked to the liver-type glycosylation of EPO. We found that secondary erythrocytosis can be related to variants in EPO that lead to the production of hepatic-like EPO with an atypical glycosylation pattern and increased activity. (Funded by Région des Pays de la Loire and others; ClinicalTrials.gov number, NCT03957863.).

Polycythemia vera (PV), a myeloproliferative neoplasm characterized by an increased red blood cell mass and increased risk of thrombosis, affects approximately 65 000 people in the US, with an annual incidence of 0.5 to 4.0 cases per 100 000 persons. Erythrocytosis (hemoglobin >16.5 mg/dL in men or >16.0 mg/dL in women) is a required diagnostic criterion, although thrombocytosis (53%) and leukocytosis (49%) are common. Patients may have pruritus (33%), erythromelalgia (5.3%), transient visual changes (14%), and splenomegaly (36%) with abdominal discomfort. More than 95% of patients have a JAK2 gene variant, which helps distinguish PV from secondary causes of erythrocytosis, such as tobacco smoking or sleep apnea. Among 7 cohorts (1545 individuals), the median survival from diagnosis was 14.1 to 27.6 years. Prior to or at the time of PV diagnosis, arterial thrombosis occurred in 16% of patients and 7% had venous thrombotic events, which could involve unusual sites, such as splanchnic veins. PV is also associated with an increased bleeding risk, especially in patients with acquired von Willebrand disease, which can occur with extreme thrombocytosis (platelet count, ≥1000 × 109/L). All patients with PV should receive therapeutic phlebotomy (goal hematocrit, <45%) and low-dose aspirin (if no contraindications). Patients who are at higher risk of thrombosis include those aged 60 years or older or with a prior thrombosis. These patients and those with persistent PV symptoms may benefit from cytoreductive therapy with hydroxyurea or interferon to lower thrombosis risk and decrease symptoms. Ruxolitinib is a Janus kinase inhibitor that can alleviate pruritus and decrease splenomegaly in patients who are intolerant of or resistant to hydroxyurea. About 12.7% of patients with PV develop myelofibrosis and 6.8% develop acute myeloid leukemia. PV is a myeloproliferative neoplasm characterized by erythrocytosis and is almost universally associated with a JAK2 gene variant. PV is associated with an increased risk of arterial and venous thrombosis, hemorrhage, myelofibrosis, and acute myeloid leukemia. To decrease the risk of thrombosis, all patients with PV should be treated with aspirin and therapeutic phlebotomy to maintain a hematocrit of less than 45%. Cytoreductive therapies, such as hydroxyurea or interferon, are recommended for patients at high risk of thrombosis.

The potential etiological factors for acquired secondary erythrocytosis (SE) include sleep apnea, smoking, and renal cysts. However, there is limited evidence to consistently support an association between these factors and SE. Additionally, identifying the genetic variants underlying SE requires specific and expensive testing methods. These diagnostic challenges mean that many cases of SE are classified as idiopathic, which complicates the development of tailored diagnostic and therapeutic strategies. This study examined 2 brothers (brother I [BI] and brother II [BII]) with idiopathic SE who were undergoing monthly phlebotomy. A diagnosis of polycythemia vera or other acquired causes, such as pulmonary disease or malignancy, was excluded. Both subjects exhibited mild to moderate sleep apnea, while their erythropoietin levels were within the normal range. To identify potential disease-causing variants shared by the brothers, gene panel exome-sequencing and further biochemical investigations were conducted. The following was identified in BI and BII: potential causative mutations in the EPAS1 gene, which were ruled out as causative factors through variant annotation and gene expression analysis; a heterozygous missense variant in the PIGV gene (p.Ala341Glu), which is known to damage proteins. The red blood cells of the brothers exhibited reduced fragility and lower hemolysis levels compared to healthy controls, with a slight increase in CD59 surface exposure. These findings suggest that red blood cells from BI and BII are more resistant to hemolysis. However, given that PIGV is involved in glycosylphosphatidylinositol biosynthesis and that CD59 exposure affects hemolysis, further investigation is required to elucidate these pathogenic mechanisms. Molecular and biochemical characterization of patients with idiopathic SE may pave the way for identifying novel mechanisms involved in the disease.

JAK2 unmutated erythrocytosis encompasses a heterogeneous spectrum of hereditary and acquired entities. The foremost step is excluding polycythemia vera (PV) with JAK2 mutation screening (exons 12-15). Apparent polycythemia such as physiological outliers or relative polycythemia secondary to volume contraction should be considered. A historical overview of hematocrit (Hct) and hemoglobin (Hgb) levels helps distinguish longstanding from acquired erythrocytosis. Serum erythropoietin (Epo) levels are variably informative. Hereditary erythrocytosis should be considered in longstanding erythrocytosis with a positive family history; causes include EPOR mutations (subnormal Epo), high oxygen affinity hemoglobin variants, PIEZO1 mutations, 2,3-bisphosphoglycerate deficiency, methemoglobinemia, and germline oxygen sensing pathway mutations (HIF2A-PHD2-VHL). Acquired erythrocytosis results from central (cardiopulmonary disease) or peripheral (renal artery stenosis) hypoxia, Epo-producing tumors (renal cell carcinoma) or drugs (testosterone, sodium glucose co-transporter-2 inhibitors (SGLT2-i), erythropoiesis stimulating agents). Idiopathic erythrocytosis is an ill-defined terminology that presumes the existence of an increased Hgb/Hct level without an identifiable etiology. Cytoreductive therapy should be avoided. Phlebotomy should be considered for symptom control. Cardiovascular risk optimization and low-dose aspirin are advised, while the role of HIF2A inhibitors remains unclear. EPO mutations which produce hyperactive, hepatic-like Epo were identified. In cases with negative workup but high clinical suspicion, an expanded next generation sequencing panel for hereditary erythrocytosis is recommended. Among drugs, SGLT2-i-associated erythrocytosis is increasingly recognized. Advances in molecular hematology are expected to improve the characterization of "idiopathic erythrocytosis". Results from prospective studies are needed to elucidate the underlying pathology and guide management.

Sex and gender have emerged as central modifiers of disease biology, phenotype, and clinical outcomes in myeloproliferative neoplasms (MPNs). This review will uniquely highlight issues affecting women with MPN and articulate their relevant determinants. A higher overall prevalence of MPN has been established in women. The incidence of essential thrombocythemia (ET) predominates, while, conversely, polycythemia vera (PV) and myelofibrosis (MF) are seen in lower frequencies as compared to men. Diagnostic criteria are dictated by sex-driven physiological variances in hemoglobin and hematocrit levels in PV, mandating separate diagnostic thresholds, respectively: > 16.0 g/dL and > 48% in women vs. > 16.5 and > 49% in men. Women with MPN harbor fewer acquired somatic mutations and a lower frequency of high-risk mutations than their male counterparts; lower JAK2V617F driver variant allele frequency and attenuated allele burden kinetics have also been reported. Women with MPN are younger at diagnosis than men and, contingent on subtype, display more indolent disease features. Importantly, validated symptom burden assessments consistently disclose higher scores in women vs. men. Women with MPN have a unique thrombotic diathesis with respect to men, more frequently involving the splanchnic venous system in those ultimately diagnosed with PV. Outcomes data depict female sex as a variable associated with more favorable clinical trajectories, including lower rates of MF/leukemic transformation and secondary cancers, as well as improved overall survival rates vis-à-vis men. Potential challenges at each significant life stage will be addressed: puberty, preconception and fertility, and perimenopause; these include issues surrounding oral contraceptives and hormone use. Prospective studies suggest overall favorable maternal and fetal outcomes with pregnancy in women with MPN. Full details on risks and reported outcomes will be discussed, as well as a risk-adapted approach to management informed by obstetric and thrombosis history. Recommendations include aspirin 81 mg daily in all patients and cytoreduction with interferon-α in those with antecedent thrombosis, as well as in low-risk cases with higher-risk features (e.g., poorly controlled hematocrit and recurrent fetal loss). Antepartum anticoagulation with low molecular weight heparin (LMWH) is recommended in cases with previous venous thromboembolism. This review highlights female sex and gender as critical drivers of MPN incidence, presentation, and natural history. It further outlines the impact and management of MPN as related to unique female reproductive phases. A sex-informed lens will be required in order to recalibrate current prognostic tools, a requisite to refining patient counselling and clinical decision-making in line with precision medicine. Moreover, while several mechanisms underpinning sex-defined discrepancies have been defined, these mandate further prospective study. Finally, sex and gender-based differences must be weighted in clinical trials with systematized procedures to correct participation imbalances in favor of sex and gender equity.