Importance of Sequencing HBA1, HBA2 and HBB Genes to Confirm the Diagnosis of High Oxygen Affinity Hemoglobin.

Muriel Giansily-Blaizot,François Girodon,Mathieu Wemeau,Betty Gardie,Mathilde Filser,Patricia Aguilar-Martinez

doi:10.3390/genes13010132

Abstract

High oxygen affinity hemoglobin (HOAH) is the main cause of constitutional erythrocytosis. Mutations in the genes coding the alpha and beta globin chains (HBA1, HBA2 and HBB) strengthen the binding of oxygen to hemoglobin (Hb), bringing about tissue hypoxia and a secondary erythrocytosis. The diagnosis of HOAH is based upon the identification of a mutation in HBA1, HBA2 or HBB in specialized laboratories. Phenotypic studies of Hb are also useful, but electrophoretic analysis can be normal in 1/3 of cases. The establishment of the dissociation curve of Hb can be used as another screening test, a shift to the left indicating an increased affinity for Hb. The direct measurement of venous P50 using a Hemox Analyzer is of great importance, but due to specific analytic conditions, it is only available in a few specialized laboratories. Alternatively, an estimated measurement of the P50 can be obtained in most of the blood gas analyzers on venous blood. The aim of our study was therefore to determine whether a normal venous P50 value could rule out HOAH. We sequenced the HBB, HBA1 and HBA2 genes of 75 patients with idiopathic erythrocytosis. Patients had previously undergone an exhaustive medical check-up after which the venous P50 value was defined as normal. Surprisingly, sequencing detected HOAH in three patients (Hb Olympia in two patients, and Hb St Nazaire in another). A careful retrospective examination of their medical files revealed that (i) one of the P50 samples was arterial; (ii) there was some air in another sample; and (iii) the P50 measurement was not actually done in one of the patients. Our study shows that in real life conditions, due to pre-analytical contingencies, a venous P50 value that is classified as being normal may not be sufficient to rule out a diagnosis of HOAH. Therefore, we recommend the systematic sequencing of the HBB, HBA1 and HBA2 genes in the exploration of idiopathic erythrocytosis.

Highlights

Erythrocytosis and polycythemia are generic terms referring to red cell diseases characterized by an increase in hematocrit (Ht) and/or hemoglobin (Hb) concentration
As the initial next generation sequencing (NGS) panel did not include HBB, HBA1 and HBA2 genes encoding the globin chains, DNA samples were tested in a second step for all patients, regardless the P50 value, using another NGS erythrocyte gene panel that included HBB, HBA1 and HBA2 genes
Most of the patients were being treated with phlebotomy for their erythrocytosis

Summary

Introduction

Erythrocytosis and polycythemia are generic terms referring to red cell diseases characterized by an increase in hematocrit (Ht) and/or hemoglobin (Hb) concentration. Among this group of diseases, polycythemia vera arises from a primary acquired clonal abnormality of the red blood cell lineage, leading to a myeloproliferative neoplasm. Congenital secondary erythrocytosis can result from mutations in the genes encoding (i) high oxygen affinity Hb, (ii) proteins involved in the oxygen-sensing pathway, or, rarely, (iii) bisphosphoglycerate mutase (BPGM), leading to bisphosphoglycerate mutase deficiency. Despite the use of generation sequencing (NGS), an etiology is only found in about 25% of idiopathic erythrocytosis cases [4,5]

Objectives

Methods

Discussion

Conclusion