Molecular analysis of the erythroid phenotype of a patient with BCL11A haploinsufficiency

The BCL11A gene encodes a transcriptional repressor with essential functions in multiple tissues during human development. Haploinsufficiency for BCL11A causes Dias-Logan syndrome (OMIM 617101), an intellectual developmental disorder with hereditary persistence of fetal hemoglobin (HPFH). Due to the severe phenotype, disease-causing variants in BCL11A occur de novo. We describe a patient with a de novo heterozygous variant, c.1453G.T, in the BCL11A gene, resulting in truncation of the BCL11A-XL protein (p.Glu485X). The truncated protein lacks the 3 C-terminal DNA-binding zinc fingers and the nuclear localization signal, rendering it inactive. The patient displayed high fetal hemoglobin (HbF) levels (12.1-18.7% of total hemoglobin), in contrast to the parents who had HbF levels of 0.3%. We used cultures of patient-derived erythroid progenitors to determine changes in gene expression and chromatin accessibility. In addition, we investigated DNA methylation of the promoters of the g-globin genes HBG1 and HBG2. HUDEP1 and HUDEP2 cells were used as models for fetal and adult human erythropoiesis, respectively. Similar to HUDEP1 cells, the patient's cells displayed Assay for Transposase-Accessible Chromatin (ATAC) peaks at the HBG1/2 promoters and significant expression of HBG1/2 genes. In contrast, HBG1/2 promoter methylation and genome-wide gene expression profiling were consistent with normal adult erythropoiesis. We conclude that HPFH is the major erythroid phenotype of constitutive BCL11A haploinsufficiency. Given the essential functions of BCL11A in other hematopoietic lineages and the neuronal system, erythroid-specific targeting of the BCL11A gene has been proposed for reactivation of g-globin expression in b-hemoglobinopathy patients. Our data strongly support this approach.