Function of the BCL11A enhancer

The erythroid-specific enhancer for BCL11A acts as a genetic switch, turning off the production of fetal hemoglobin (HbF) so that it can be replaced by adult hemoglobin (HbA). Located within intron 2 of the BCL11A gene, this switch only affects hemoglobin production, it does not affect other functions of this gene. This explains why it can be targeted by CRISPR therapy without affecting other critical roles of the BCL11A protein (e.g. development of B-cells and dendritic cells, brain and nervous system development).

The mechanical process detailing how this enhancer controls the genetic shift works through a precise series of cellular events:

Step 1: Erythroid-Specific Activation (Turning the Switch On)

In developing adult erythroblasts, the BCL11A intron 2 enhancer shifts from tightly packed chromatin into an open, highly sensitive configuration characterized by three distinct DNase I hypersensitive sites (DHS) located +55, +58, and +62 kilobases from the gene’s transcription start site. 

  • Transcription Factor Recruitment: Erythroid transcription factors such as GATA1 bind to these open enhancer domains, helping activate enhancer transcription.
  • 3D Chromatin Architecture: Transcription of enhancer RNAs (eRNAs) from these DHSs recruits the cohesin loader NIPBL, enabling cohesin ring loading and assembly of an enhancer-dependent chromatin rosette. This rosette stabilizes loop‑based chromatin structures that maintain high BCL11A expression.

Step 2: Physical DNA Looping to the Promoter

Once anchored by GATA1, the intron 2 enhancer undergoes a dramatic architectural pivot, bending backward to interact directly with the promoter region of the BCL11A gene itself.

  • This localized looping generates an insulated transcription hub.
  • It forces the continuous recruitment of RNA Polymerase II, triggering high-volume transcription of the BCL11A gene in adult red blood cell precursors.

Step 3. Expression of the Repressor

Because the enhancer facilitates robust physical loop connection, the cell synthesizes significant quantities of the BCL11A protein, an important transcriptional repressor in the blood and nervous systems.