A downstream microdeletion associated with reduced BCL11A expression contains several putative erythroid enhancers. The region deleted in patient 1 is shown in red. Shown are DNase sequencing tracks for adult erythroblasts (derived from peripheral blood CD34+ cells), other hematopoietic cells in which BCL11A is expressed (B cells) or not expressed (T cells), and the developing fetal brain. Sequencing tracks have been emboldened for clarity. Potential erythroid regulatory elements are highlighted with vertical bars and are bound in human pro-erythroblasts by different combinations of noted erythroid transcription factors (GATA1, TAL1, KLF1, and NFE2) as evidenced by chromatin immunoprecipitation (ChIP) sequencing. These regions are also marked by H3K4me1, typically associated with active enhancer elements. Of note, regions highlighted in pink display erythroid DNase hypersensitivity that is not present in the other hematopoietic cells, suggesting that they are erythroid-specific sites. Regions in gray signify DNase-hypersensitive sites at an alternative VRK2 promoter and upstream element, which are present in other tissues. The segment in blue indicates a known erythroid enhancer in BCL11A intron 2, to which SNPs associated with HbF levels are localized.9 A second cluster of high HbF-associated SNPs (marked by an asterisk) lies downstream of BCL11A but outside the deleted region. The fetal brain exhibits widespread DNase hypersensitivity across the deleted region, which might account for the neurological phenotype of this patient.
