INTRODUCTION

The Genotypic Complexity of Sickle Cell Disease

The sickle hemoglobin gene (HBB glu6val) and β thalassemia alleles are often geographically coincident. Consequently, compound heterozygotes with HbS–β thalassemia are commonplace. The incidence of this genotype is dependent on the populations' ratio of carriers of an HbS gene to carriers of β thalassemia. In the United States, and in Africa where the HbS predominates, sickle cell anemia (homozygous for HBB glu6val) is the prevailing genotype of sickle cell disease. Wherever β thalassemia is more frequent than sickle cell trait (HbAS) – Greece, Italy, and other Mediterranean countries are examples – HbS–β thalassemia predominates. Some β thalassemia variants, such as δβ thalassemia and Hb Lepore and gene deletion hereditary persistence of fetal hemoglobin (HPFH), can interact with HbS producing different interesting phenotypes. α Thalassemia is present in 30% of African Americans with sickle cell anemia and might be even more common in some populations from the Middle East and Indian subcontinent. Individuals with sickle cell anemia–α thalassemia have distinctive hematological and clinical features. Compound heterozygotes of HbS with other mutant α- or β-globin genes are also present in any population with a high prevalence of HbS. Depending on the other globin gene mutation, these conditions can resemble the clinically benign HbAS or have a phenotype with hemolytic anemia and vasoocclusive complications. Rare variants, often with intriguing phenotypes, have both the sickle mutation and an additional mutation in the same β-globin gene.