The high oxygen affinity haemoglobin Nantes: a family case description.

Absolute erythrocytosis is characterised by the expansion of the erythrocyte compartment in peripheral blood and is classified into primary and secondary forms which can be of either congenital or acquired origin1. Primary erythrocytosis is caused by acquired or inherited mutations leading to expansion of the erythrocyte compartment in the peripheral blood independently of extrinsic factors; it includes polycythemia vera and rare familial variants. Secondary erythrocytosis is caused by a circulating factor, usually erythropoietin, which stimulates erythropoiesis. The increased production of erythropoietin is normally related to a physiological response to hypoxia but may also be caused by an abnormal autonomous erythropoietin production (e.g. erythropoietin-secreting tumour) or an alteration of the oxygen-dependent modulation of erythropoietin synthesis (e.g. increase in the oxygen affinity of haemoglobin [Hb]). Approximately 100 haemoglobin variants with high oxygen affinity have been described in the literature2,3. Haemoglobin is a tetrameric globular protein composed of four subunits, two alpha (α1, α2) and two beta (β1, β2), which form two identical halves named α1β1 and α2β2. These dimers exist in equilibrium in two conformations with different oxygen affinity: form T (tense) and form R (relaxed) with low and high oxygen affinity, respectively. The transition from the T form to the R form is characterised by a large change in interactions between the subunits α1 and β2, while the α1β1 intra-dimer interface remains almost unaffected4,5. The α1β2 contact, also named “sliding contact”, interconnects the two dimers (α1β1 and α2β2) and involves the “switch” region, so named because it experiences the largest displacements as a result of the dimer rotation between the R and T forms. Another important contact is in the “hinge” region where the R → T shift is limited to a change in orientation6. The increased oxygen affinity determines erythrocytosis in the peripheral blood and is caused by alterations in haemoglobin structure. The molecular classification of haemoglobins with increased oxygen affinity recognises three different structural modifications concerning the main contacts involved in the transition from the deoxy- to the oxy-state, the 2,3-diphosphoglycerate binding sites and the heme pocket. In the first group the T structure cannot be achieved. These variants affect the “sliding contact” and the “switch region” and lead to increased oxygen affinity with markedly reduced cooperativity, and to a reduced ability to maintain the T conformation (e.g. Hb Coimbra, Hb Hiroshima, Hb Osler, Hb Nancy, Hb Mckees, Hb Kochi, Hb Cochin-Port Royal). Differently, structural changes that occur within the “hinge region” prevent the formation of the correct bonds stabilising the T structure, but this leads equally to a severe increase in oxygen affinity (e.g. Hb Nantes and Hb Pitie-Salpetriere)7. Hb Nantes [β34(B16)Val → Leu (HbVar: A Database of Human Hemoglobin Variants and Thalassemias) HBB:c.103G>C (HGVS nomenclature)] was found as a de novo mutation in a 38-year old woman and first reported by Wajcman et al.8. Hb Nantes shows increased oxygen affinity and causes a disease with a dominant phenotype. Residue Val β34 contributes to both the α1β1 and α1β2 interfaces9; a mutation at this site leads to alteration of the α1β1 contact area and impairs the interactions stabilising the T state. The mode of transmission of the high oxygen affinity haemoglobin variants is autosomal dominant6. The amount of the abnormal haemoglobin present in the red blood cells modulates the haematological consequences of a high oxygen affinity variant. It is important to diagnose haemoglobin with increased oxygen affinity because, although usually well tolerated in young patients, it frequently leads to thrombotic complications in older patients10,11 or when it is associated with another factor that increases thrombotic risk. Patients carrying a high oxygen affinity haemoglobin variant are most frequently identified because of an unexplained erythrocytosis with haemoglobin values varying from 16 to 20 g/dL. Routine electrophoretic or chromatographic studies may reveal an abnormal haemoglobin that is further recognised as having increased oxygen affinity. Sometimes routine electrophoretic studies fail to demonstrate the presence of a haemoglobin variant and further evaluations may be necessary, including isoelectric focusing, high performance liquid chromatography (HPLC) and electrophoresis of haemoglobin and globins under various experimental conditions12. Nevertheless, it should be kept in mind that nowadays the molecular analysis of beta globin genes is much faster and often cheaper than the biochemical approach, and also gives a definitive diagnosis.