The clinical severity of hemoglobin S/Black (A γδβ)0 -thalassemia

Disrupting the adult globin promoter alleviates promoter competition and reactivates fetal globin gene expression

The benign condition hereditary persistence of fetal hemoglobin (HPFH) is known to ameliorate symptoms of co-inherited β-hemoglobinopathies, such as sickle cell disease and β-thalassemia. The condition is sometimes associated with point mutations in the fetal globin promoters that disrupt the binding of the repressors BCL11A or ZBTB7A/LRF, which have been extensively studied. HPFH is also associated with a range of deletions within the β-globin locus that all reside downstream of the fetal HBG2 gene. These deletional forms of HPFH are poorly understood and are the focus of this study. Numerous different mechanisms have been proposed to explain how downstream deletions can boost the expression of the fetal globin genes, including the deletion of silencer elements, of genes encoding noncoding RNA, and bringing downstream enhancer elements into proximity with the fetal globin gene promoters. Here we systematically analyze the deletions associated with both HPFH and a related condition known as δβ-thalassemia and propose a unifying mechanism. In all cases where fetal globin is upregulated, the proximal adult β-globin (HBB) promoter is deleted. We use clustered regularly interspaced short palindromic repeats-mediated gene editing to delete or disrupt elements within the promoter and find that virtually all mutations that reduce ΗΒΒ promoter activity result in elevated fetal globin expression. These results fit with previous models where the fetal and adult globin genes compete for the distal locus control region and suggest that targeting the ΗΒΒ promoter might be explored to elevate fetal globin and reduce sickle globin expression as a treatment of β-hemoglobinopathies.

HbS-Sicilian (δβ)0-Thalassemia: A Rare Variant of Sickle Cell

Sickle cell disease (SCD) is caused by a mutation in the sixth codon of the β-globin gene on chromosome 11, which leads to a single amino acid substitution (glutamine to valine). Sickle-(δβ)⁰-thalassemia is a rare variant of sickle cell disease (delta-beta thalassemia occurring in association with sickle hemoglobin, HbS), sparsely reported in literature, and has been associated with symptomatology necessitating careful monitoring and follow-up. We describe a patient who presented with a newborn screen reported as “FS” and a negative family history for sickle cell disease and sickle cell trait. Subsequent gene sequencing studies demonstrated the presence of Sickle-(δβ)⁰-thalassemia. Clinical course has remained relatively stable for this patient now at 18 months of age without any SCD related symptomatology or complications. As this is a rare variant of SCD with potential complications, it is important to establish diagnosis towards planning comprehensive care.