Hb Johnstown [β109(G11)Val→Leu]: a High Oxygen Affinity Variant Associated with β⁰‐Thalassemia

Abstract

Hb Johnstown [β109(G11)Val→Leu], a high oxygen affinity hemoglobin (Hb) variant associated with β⁰‐thalassemia (thal) [IVS‐I‐1 (G→A)], was identified in an 8‐year‐old girl referred to our laboratory because of erythrocytosis and a left‐shifted oxygen dissociation curve (ODC). The phylogenetic tree showed that the mother was heterozygous for the Hb variant and the father was a β⁰‐thal carrier. This Hb variant, with normal electrophoresis, was characterized at the DNA level by β gene sequencing. The amino acid substitution potentially disrupts α1β1 contacts in the deoxyHb conformation, thus shifting the equilibrium towards the high affinity oxyHb conformation. The erythrocytosis and low values for actual P₅₀ due to Hb Johnstown were more marked due to the co‐inheritance of the β⁰‐thal.

• Erythrocytosis
• Abnormal hemoglobin (Hb)
• Oxygen affinity
• Thalassemia (thal)