Hemoglobin E Syndromes

Abstract

Abstract Hemoglobin (Hb) E is one of the world’s most common and important mutations. It results in a heterogeneous group of disorders whose phenotype range from asymptomatic to severe. Hb E trait and Hb EE are mild disorders. The combination of Hb E and Hb S (Hb SE) results in a sickle cell disease syndrome similar to sickle β+ thalassemia. It is important to distinguish Hb E disorders diagnostically because of this marked difference in clinical course among different genotypes. Screening tests, including hemoglobin electrophoresis and high-pressure liquid chromatography (HPLC), may suggest other mutations, unless one is familiar with the findings. E β-thalassemia, the most serious form of E syndromes, affects a million people worldwide and is increasing in North America. Its phenotype ranges from mild anemia to severe transfusion-dependent thalassemia major. Several genetic modifiers affect the phenotype, including the type of β-thalassemia mutation, Hb F levels, and co-inheritance of α-thalassemia. However, the cause of the phenotypic variability is largely unknown. A prospective natural history study of E β-thalassemia in Sri Lanka suggests that environmental modifiers are prognostically important. The clinical course of E β-thalassemia is punctuated by acute and chronic complications that may cause serious morbidity and mortality. Recent studies indicate these patients are at high risk for thromboembolism secondary to a hypercoagulable state increased by splenectomy. Morbidity from iron overload in nontransfused patients secondary to increased gastrointestinal iron absorption is common. Cardiopulmonary disease, including pulmonary hypertension, requires ongoing monitoring and is secondary to iron overload, thromboembolism, and hemolysis-induced nitric oxide deficiency. These patients are excellent candidates for Hb F–modulating agents because moderate changes in hemoglobin may result in marked improvement in phenotype. Recent studies with hydroxyurea indicate 40% of patients will clinically improve with hydroxyurea.