Beta-haemoglobinopathies, such as sickle cell disease and beta-thalassaemia are caused by mutations in the adult beta-globin gene. They are amongst the most commonly inherited disorders, with the World Health Organisation estimating that at least 5% of adults are carriers for a haemoglobin disorder. With current therapeutic strategies carrying various limitations, reactivating the developmentally silenced foetal globin gene is an attractive approach for treating beta-haemoglobinopathies.
A rare group of individuals with a benign condition called the Hereditary Persistence of Foetal Haemoglobin (HPFH), continue to express foetal haemoglobin into adult life. Co-inheriting a HPFH mutation with a beta-haemoglobinopathy mutation leads to alleviated symptoms of the disease. HPFH is caused by single point mutations within the promoter of the foetal globin gene. The -115 site of the foetal globin promoter contains four reported HPFH mutations and a small 13 base pair deletion. These point mutations are hypothesised to disrupt the binding of a transcriptional repressor which would normally silence the foetal globin gene around the time of birth. Our aim was to identify this repressor.
We studied a range of potential transcription factors to identify a DNA-binding domain which could directly bind to the -115 site of the foetal globin promoter in vitro using electrophoretic mobility shift assays (EMSA). We compared binding with and without the HPFH mutations and identified BCL11A as a candidate repressor. Furthermore, introduction of the naturally occurring HPFH mutations into erythroid cells by CRISPR/Cas9 genome editing disrupts BCL11A binding and raise foetal globin expression. These results identify BCL11A as the repressor which binds to the -115 site of the foetal globin promoter and reveal the mechanism of how the HPFH mutations operate, providing insights into new potential therapeutic targets for treating beta-haemoglobinopathies.