Hereditary Erythrocytosis: Results from Europe's Largest Idiopathic Erythrocytosis Database
Authors: Maaziz Nada , Jaillard Mathilde , Garrec Céline , Airaud Fabrice , Aral Bernard , Bezieau Stephane , Gardie Betty , Girodon Francois 
Published: December 2024

Introduction
Increased hemoglobin (Hb) and hematocrit values are frequent reasons for out patient clinic in hematology. In the majority of cases, a simple work-up leads to the diagnosis of either Polycythaemia Vera or acquired polycythaemia secondary to chronic hypoxia, EPO secreting tumour, etc. Hereditary erythrocytosis (HE) is a group of rare diseases, most often linked to mutations in genes involved in erythropoiesis. We report here on the main causes of HE, based on Europe's largest database of idiopathic erythrocytoses.
Materials and methods
Between 2015 and 2024, after written informed consent, over 900 French patients with idiopathic erythrocytosis benefited from NGS using a panel of erythrocytosis-specific genes including EPAS1 (HIF2A), EGLN1 (PHD2), VHL, EPOR, JAK2, SH2B3(LNK), BPGM, PIEZO1, HBB/HBA. This genomic exploration followed a complete diagnostic workup that included screening for the JAK2V617F and exon 12 mutations, serum EPO, blood electrolytes, iron and hemolysis status, venous (to assess P50) and arterial blood gases, measurement of red cell mass using isotope or CO-rebreathing methods, abdominal ultrasound, functional respiratory tests, search for sleep apnea syndrome, bone marrow biopsy and progenitors growth cultures if necessary.
Results
Of the 909 patients studied, with a median age of 50 years old [1-90], 86% were men. The distribution was respectively 7%, 24%, 38% and 31% for the age groups 0-19 years, 20-39 years, 40-59 years, &gt;60 years. A pathogen or likely pathogen mutation was noted in 72 (7.9%) patients (78% male, 22% female), with the following age distribution: 11 (17%), 23 (10%), 23 (6.6%), 15 (5.3%) in 0-19, 20-39, 40-59 and &gt;60 years respectively. The gene distribution was as follows: EPAS1 n=20, EGLN1 n=13, SH2B3 n= 10, HBB/HBA n=8, JAK2 n= 7, EPOR n=5, PIEZO1 n=5, VHL n=3, BPGM n=1. The discovery of a mutation in patients led to a second screening for erythrocytosis in 30 relatives. Few thrombotic complications were noted in HE patients. Surprisingly 3 patients with an EPAS1 mutation had a very low variant allelic frequency (from 1.5 to 4%) due to mosaicism, including one case associated with paragangliomas. PIEZO1 mutations were observed in patients with mild erythrocytosis, associated with splenomegaly, biological hemolysis, iron overload, moderately lowered venous P50, and a typically shifted ektacytometry curve.
Discussion
We report here the sequencing results of Europe's largest database of erythrocytoses, for which a genetic cause was identified in less than 10% of patients, and reveal HE in 30 relatives, enabling diagnosis for these individuals by targeted sequencing and saving all additional diagnostic tests. While the presence of high affinity Hb is the leading cause of HE, its low proportion (11% in our cohort) can be explained by the fact that this type of anomaly was normally detected prior to NGS by performing venous blood gases to assess P50: if this was &lt; 23 mmHg, targeted Sanger sequencing of the HBB and HBA genes was then performed without recourse to NGS. The paradoxical finding of 8 patients with high affinity Hb in our series, despite the P50 filter, can be explained either by arterial rather than venous blood gases, or by the presence of air bubbles in the syringe, which falsified the result. Similarly, the surprising finding of JAK2 mutations in 7 patients in our series, which had to be negative to benefit from NGS, can be explained by the presence of 3 atypical JAK2 mutations undetected by conventional methods, and a low allelic burden (&lt;15%) of the JAK2 exon 12 mutations undetected by Sanger sequencing in 4 patients. The use of NGS makes it possible to avoid these pitfalls. Mutations in genes involved in oxygen sensing pathway (EPAS1, EGLN1 and VHL) account for 50% of HE in our cohort, mainly represented by EPAS1 mutations (n=20, nearly 30% of total HE). Consistently, the probability of finding a mutation was inversely proportional to age, ranging from 17% for patients under 20, to 5% for those over 60.
Conclusion
The study of idiopathic erythrocytoses using NGS analysis enables the diagnosis of HE in around 10% of patients, with mutations mostly linked to oxygen sensing pathway. There is still considerable scope for progress in the discovery of other molecular markers for HE. The recent availability of HIF2a-inhibitors holds out the prospect of better management of patients with oxygen sensing pathway genes mutations thanks to these targeted therapies.