Identifying News Genes Involved in Unexplained Erythrocytosis Throught a Clinical Database
Authors: Zanchetta-Balint Fabian , Maaziz Nada , Garcelon Nicolas , Cormier-Daire Valerie , Gardie Betty , Canaud Guillaume , Rahal Ilhem , Joseph Laure , Lefrere Francois , Lyonnet Stanislas , Trovati Maciel Thiago , Girodon Francois , Hermine Olivier 
Published: December 2024

Introduction
JAK2 unmutated erythrocytosis is a frequent clinical condition that remains unexplained in most cases. Our aim is to identify new genetic determinants of erythrocytosis by analysing a cohort of paediatric patients and young adults followed for monogenic genetic diseases at Imagine institute and Necker Hospital, patients who also have an associated erythrocytosis of unknown origin.
Materials and methods
We used, Dr Warehouse the database of the Imagine Institute which gather all biological data and reports of patients followed at Necker Hospital in Paris. Dr Warehouse work as a search engine. For this work, we restricted our search to patients followed only in clinical genetic units. Children erythrocytosis was defined as hemoglobin above the 99th percentile for age. For adolescents and adults we considered as significant hemoglobin levels above 16.5 g/dl for men and 16 g/dl for women according to current WHO recommendations (2016) for polycythemia vera diagnosis. Out of more than 800 000 patients, we identified 408 patients over 1 year of age with possible erythrocytosis. We excluded from the cohort all causes of secondary erythrocytosis (including chronic respiratory failure, complex malformative heart disease or kidney transplant), false erythrocytosis (high hemoglobin level caused by deshidration or sample error). After this step 45 patient were selected and among them 19 patients had a characterized genetic disease by Next generation Sequencing (NGS). We then classified genes mutants into different groups according to the signaling pathways to which they belong. Finally, we carried out an exhaustive review of the literature for the different genes, in order to establish a pathophysiological link between the mutations and the erythrocytosis. Based on the identified genes in this cohort we build a NGS panel and test 39 adults patients without genetic disease with unexplained erythrocytosis.
Results
We have identified 13 mutations in 19 patients with a germline or mosaic genetic disease with an associated erythrocytosis. The different groups of patients identified included mutations in genes involved in the Sonic Hedgehog (SHH) pathway, causing ciliopathies (n=3), PIK3CA pathway (somatic mosaic PIK3CA mutations as part of a Cloves syndrome, n=4), RAS/MAPK pathway (Noonan syndrome, n=2), WNT/TCF2 (TCF2 (or HNF1b) mutations, n=1), characterized by renal malformations or polycystic kidney disease), NOTCH2 activating mutation (Hadju Cheney disease, n=1), JAG1 mutation (Alagille syndrome, n=1) a gene closely linked to NOTCH2, and other mutations in EYA, BRD4, KMT2A, etc. Then, using the NGS panel designed from this genetic study, we found mutations in 22 pts (56%) in the genes described above (NOTCH2, JAG1, EYA, BRD4, CREBBP, KMT2A) out of 39 patients with unexplained erythrocytosis.
Conclusion
Using a new and innovative tool of database potential new genes have been identified that may help for the diagnosis of unexplained erythrocytosis. We have identified promising new genes and signaling pathways that may be involved in erythropoiesis. Although none of these genes had previously been described as causing erythrocytosis in human, gain of function mutations of NOTCH2 are associated enhancement of erythroid differentiation in mice. Further studies are warranted to understand the role of these variants in these new pathways. Beside diagnosis it may open new avenues for therapeutic strategies.