Disrupting the adult-globin promoter alleviates promoter competition and reactivates foetal-globin gene expression

The benign condition Hereditary Persistence of Foetal Haemoglobin (HPFH) is known to ameliorate symptoms when co-inherited with b-haemoglobinopathies, such as sickle cell disease and b-thalassaemia. The condition is sometimes associated with point mutations in the foetal 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 b-globin locus that all reside downstream of the foetal 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 foetal globin genes, including the deletion of silencer elements, of genes encoding non-coding RNA, and bringing downstream enhancer elements into proximity with the foetal globin gene promoters. Here we systematically analyse the deletions associated with both HPFH and a related condition known as db-thalassaemia and propose a unifying mechanism. In all cases where foetal globin is up-regulated, the proximal adult b-globin (HBB) promoter is deleted. We use CRISPR gene editing to delete or disrupt elements within the promoter and find that virtually all mutations that reduce HBB promoter activity, result in elevated foetal globin expression. These results fit with previous models where the foetal and adult globin genes compete for the distal Locus Control Region and suggest that targeting the HBB promoter might be explored to elevate foetal globin and reduce sickle globin expression as a treatment for b-haemoglobinopathies.