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

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.

JAK2 unmutated erythrocytosis: 2023 Update on diagnosis and management

“…53 A left shift of the oxygen dissociation curve, that is venous P50 <24 mmHg is a good screening test for HOAV, although a small proportion of cases could be missed if arterial instead of venous blood gas is analyzed and if analytical conditions are compromised with the presence of air bubbles in the syringe. 54 Therefore, it is best to confirm that P50 was obtained on venous blood through venous oxygen saturation levels between 30% and 55%. At the Mayo Clinic, P50 testing is no longer available due to assay issues and detection of HOAV is accomplished through cation exchange high-performance liquid chromatography (HPLC), capillary electrophoresis, and mass spectrometry.…”

“…Historically, venous P50 measurement (oxygen tension at which Hgb is 50% saturated) was recommended which can also be calculated from venous blood gas using the following mathematical formula https://www.medsci.org/v04/p0232/ijmsv04p0232s1.xls 53 . A left shift of the oxygen dissociation curve, that is venous P50 <24 mmHg is a good screening test for HOAV, although a small proportion of cases could be missed if arterial instead of venous blood gas is analyzed and if analytical conditions are compromised with the presence of air bubbles in the syringe 54 . Therefore, it is best to confirm that P50 was obtained on venous blood through venous oxygen saturation levels between 30% and 55%.…”

Idiopathic erythrocytosis: A diagnostic and management challenge with emerging areas for exploration

“…Upon further examination, these patients had a family history of erythrocytosis as well as low p50 measurements on retesting. 30 A recent study identified PIEZO1 mutations in 4% of IE patients. 31 This gene is known to be affected in hereditary xerocytosis but had not previously been evaluated in the context of erythrocytosis.…”

“…More comprehensive genetic studies conducted in IE patients have identified potential germline mutations in 6%-23% of participants with low suspicion of an inherited cause and as many as 67% of patients with a high suspicion. [23][24][25][26][27][28][29][30][31][32] In a study using a 13-gene panel for inherited erythrocytosis, 18% (6/34) of IE patients were found to have variants of undetermined significance. No patients had a family history of erythrocytosis, but most were under the age of 35.…”

“…Access to comprehensive genetic testing may be limited to well‐characterized mutations such as those involving the erythropoietin receptor ( EPOR ) or von Hippel Lindau ( VHL ) protein. More comprehensive genetic studies conducted in IE patients have identified potential germline mutations in 6%–23% of participants with low suspicion of an inherited cause and as many as 67% of patients with a high suspicion 23–32 . In a study using a 13‐gene panel for inherited erythrocytosis, 18% (6/34) of IE patients were found to have variants of undetermined significance.…”