Next Generation Sequencing Is a Useful Tool for the Diagnosis of Congenital/Idiopathic Erythrocytoses

Introduction: Erythrocytoses are characterized by an elevated red cell mass. The most widely studied disease is Polycythemia Vera (PV), a myeloproliferative neoplasm due to the acquired JAK2-V617F mutation. However, other types of erythrocytoses exist and are of major importance. They can be either inherited (Congenital Erythrocytosis-CE) or diagnosed in adult patients with no family history (Idiopathic Erythrocytosis-IE). CE/IE are not associated with myeloproliferation but they can be associated with severe thrombo-embolic or haemorrhagic events, pulmonary arterial hypertension and, rarely, tumours. The 8 genes identified so far as causing CE lie at the crossroads of major biological pathways (metabolism, inflammation, oncogenesis) and are implicated in multiple diseases. These genes are involved (i) in the regulation of the hypoxia pathway, PHD2 (also called EGLN1), HIF-2A (EPAS1), VHL, (ii) in proliferation and differentiation of erythroid progenitors (EPOR), or (iii) in mature cell function, haemoglobins (HBB, HBA1, HBA2) or bisphosphoglyceratemutase (BPGM). However, in 80% of cases the cause remains unknown meaning that no proper diagnosis can be made, no prognosis or advice can be provided to CE/IE patients and their families, and no curative treatment exists.

Method: We created and developed a national network in France to (i) identify, (ii) collect and (iii) analyze the genomic abnormalities in patients suspected of CE/IE. The selection of patients was performed using a clinical and biological data sheet including mandatory further tests in order to exclude patients with PV or obvious secondary erythrocytosis related to lung, cardiac or renal disorder. Next generation sequencing (NGS) has been used to analyse the presence of mutations in 17 genes (VHL, PHD1, PHD2 and PHD3, HIF1A, HIF2A, HIF3A, FH, BPGM, and 8 other candidate genes). SureDesign software (Agilent, Santa Clara, CA) was used to design the custom HaloPlex capture assay. For sequence capture, HaloPlex Target Enrichment System Kit (Agilent^R), for Illumina sequencing was used, according to the manufacturer's instructions.

Results: To date, samples from 103 patients have been recorded, among whom 46 have been tested using NGS approach. Variants in 10 (21%) patients [9 males and 1 female ; median age 50 y. (12-71)] with unknown significance have been detected, including 4 in PHD genes, 5 in HIF genes, and 1 in JAK2 gene.

In patients with variants, a familial history of erythrocytosis was noted in 2. No independent thrombotic complication was reported in the 10 patients. The proportion of variants detected (21%) was close to the classical rate of genomic abnormalities usually observed in CE/IE. In 2 patients (one with a PHD2 and one with a JAK2 variants), the erythropoietin was low, whereas for the others, the erythropoietin was normal. Of note, the median age of the patients was surprisingly high, suggesting that the diagnostic was not previously performed due to the absence of available tests. Indeed, the diagnostic approaches using NGS techniques led to a considerable time gain and facilitated the identification of certain molecular abnormalities associated with CE/IE

Conclusion: NGS is a useful tool to explore mutations in CE/IE, but identifies genetic variants in only 20% of patients with such disorder. In vitro, in cellulo and in vivo (including zebrafish models) functional studies are currently performed to validate the clinical relevance of these variants. Further exams including whole exome sequencing are planned to achieve a right diagnosis in the 80% remaining CE/IE patients without identified genomic abnormalities.