Rare Sighting of an Unstable Hemoglobin Variant [ Β11 (A8) Val → Asp]: Hemoglobin Windsor

Abstract

Hemoglobin (Hb) is the major blood oxygen carrier where Hb subunits bind oxygen via the heme iron buried within a hydrophobic pocket that faces the outside of HbA tetramers. Iron must be in its reduced (ferrous, Fe2+) state for Hb to bind oxygen. Oxidized or metHb (ferric, Fe3+) cannot bind oxygen and is relatively unstable. Globin mutations that alter amino acids within the ligand pocket lead to changes in affinity for oxygen, and increased rates of metHb formation and heme loss. We present the case of a patient with a rare variant “Hb Windsor” which has only been described once in the literature. The patient is a 20-year-old Mexican male with a history of hepatitis B presenting with three days of fevers, chills, nausea and vomiting. The patient reported a diagnosis of hepatitis at the age of eight in Mexico and with jaundice all his life. He had no prior hospitalizations or blood transfusions. He denied any unusual food ingestions or recent foreign travel. In the emergency room, the patient was hypoxic with a SpO2 73% and tachypneic which improved with supplemental oxygen. On physical examination, the patient was jaundiced with scleral icterus and the spleen was easily palpable to the midline. CT scan confirmed splenomegaly measuring 20 cm. Labs showed a white blood count 21.6 nl, hemoglobin 4.2 g/dl, platelet count 33 nl, lactate dehydrogenase 13 unit/L and reticulocyte count 24%, total bilirubin 16 mg/dl, direct bilirubin 4 mg/dl and methemoglobin 20.9%. The patient was administered methylene blue and transfused two units of packed red blood cells with hemoglobin increasing to 7.1 g/dl. HIV, hepatitis B and C, direct antiglobulin test, and G6PD testing were negative. The peripheral smear showed numerous nucleated red cells, increased polychromasia with red cells showing numerous microspherocytes, microanthocytes, and bubbled cells with significant basophilic stippling. No fragmented cells were present. The clinical history and lab findings suggested a chronic hemolytic anemia, and the peripheral smear suggested a possible hemoglobinopathy. Hemoglobin electrophoresis by high performance liquid chromatography showed an unknown hemoglobin of 22.5%. Isoelectric focusing showed Hb A 71.9%, F, Hb A2 4.6%, Hb F 0.3%, and an abnormal band of 23% in the hemoglobin M range. Specimens were sent for DNA sequencing and a point mutation in the beta chain was found leading to a valine to aspartate substitution in the A8 position. Protein analysis showed a valine is substituted with aspartic acid in the ligand pocket which is the unstable varian Hb Windsor. This substitution in the base of the heme pocket results in instability of the hemoglobin molecule and an increase in oxygen affinity. The residues lining the heme pocket function as a hydrophobic basket from the heme iron, preventing its oxidation to the ferric state. Valine, which occupies position A8 is a hydrophobic amino acid. However, in Hb Windsor, the valine is substituted by aspartic acid, a polar molecule. The hydrophilic nature of aspartic acid, a polar molecule, may disrupt the hydrophobic nature of the heme pocket. The valine at position A8 also acts as a contact with the H helix which is lost in Hb Windsor leading to decreased Hb stability. These substitutions cause heme iron to auto-oxidize which results in methemoglobinemia. Hb M has iron in the ferric state (Fe3+) and is unable to carry oxygen which produces cyanosis. Our patient was found to be hypoxic requiring oxygen supplementation and found to have methemoglobinemia. We present a rare case of a hemoglobin variant Hb Windsor in a Mexican adult which has only been previously documented in a nine-month-old Anglo-Saxon infant. This variant seems to be a spontaneous de novo mutation as both patients had no family history of anemia. It is hypothesized that the hemolysis was precipitated by oxidative stress of an acute infection in the setting of an unstable hemoglobin. The patient was found to have stable hemoglobin of 12 g/dl and methemoglobin of 4.9% in follow up several years later.