H. sapiens B-cell lymphoma/leukemia 11 Transcription Factor A

Reina He '26 and Rosa Kwon '26

Contents:

I. Introduction

In humans, erythropoiesis, the process of red blood cell production, is marked by an important developmental switch from γ- to β-globin expression during the transition from fetal (HbF) to adult hemoglobin (HbA). This transition is regulated by BCL11A, otherwise known as B-cell lymphoma/leukemia 11A for the gene’s role as a common retroviral insertion site in murine leukemia and how it was first identified in human B-cell non-Hodgkin lymphomas. BCL11A is a transcriptional repressor that silences γ-globin both directly through DNA binding and indirectly through protein interactions (Figure 1).


Figure 1. Known expressions and functions of BCL11A and its involvement in cellular processes.

As a result, BCL11A plays a role in the hematopoietic system due to its involvement in cell proliferation, differentiation, and apoptosis. In addition, BCL11A participates in the regulation of axon branching, development of skin, dorsal spinal neurons, and neural stem cells, and protein modification. Due to its engagement in an array of cellular processes, the varied expression and alteration of BCL11A can affect a wide range of diseases and phenotypes, as displayed above (Yin et al., 2019).

II. General Structure

The BCL11A protein is a regulatory C2H2 type zinc finger (ZF) protein that can bind DNA. The protein comprises up to thirty amino acids in the consensus sequence, which forms respectively, in the C- and N-terminal portion (Mackeh et al., 2018). These secondary structures fold into a stable assembly through hydrophobic interactions and the enclosure of a zinc ion by These loops are formed by two cysteine residues separated by four to five amino acids followed by two histidine residues. The His residues are typically located downstream in the amino acid sequence, folding back to form stabilized structures through coordination with zinc ions and the Cys residues.

In BCL11A, Zn1002 (zinc ion) forms direct metal coordination with His792, situated at the end of the Lys46 residue that forms an α-helix, establishes On the other side, Cys775, in addition to coordinating with the zinc ion, forms a Lastly, His788 not only coordinates with the zinc ion but also engages in interactions with Tyr777. Together, these interactions allow the zinc ion to be surrounded by multiple residues, forming a highly stable structure essential for the protein's function.

III. DNA Binding

As a γ-globin regulator, BCL11A binds directly to the TGACCA motif at -115 bp of the γ-globin promoter through its C-terminal zinc finger domains, of which there are three: in a sequence-specific manner ZnF4 and ZnF5 bind to DNA major grooves, while ZnF6 associates with the DNA minor groove.

ZnF4 forms with the DNA. Asn 753 forms hydrogen bond with A119; Asn756 forms hydrogen bond with A118; While ZnF4 has no sequence specific binding, it still provides a strong driving force for DNA binding.

As the most important part of DNA binding, ZnF5 forms with four sequence nucleotide-specific bindings : Gln781 and Ser783 bind to cytosine at position -117; Lys784 and Arg787 bind to guanine at position -115 and -114, respectively. Tyr777 and Thr791 also bind to DNA backbone to increase affinity for binding.

Even though ZnF6 does not directly engage in DNA bindings, it is important for enhancing the affinity of general hydrogen bonds networks. To be more specific, the hydrogen bonds between Asn753, Asn756, and His760 were enhanced by ZnF6.

IV. Clinical Significance

Hemoglobin (Hb) disorders are a category of diseases caused by abnormal globin expression. The two major types of Hb disorders are β-thalassemia and sickle cell disease. These disorders result from a deficiency or absence of HbA, which is normally composed of β-globin. Research has shown that the shortage of HbA can be compensated by HbF, which is composed of γ-globin and is typically expressed only during the fetal stage. BCL11A, a transcription factor, represses γ-globin expression in adults. Therefore, reducing BCL11A activity leads to increased γ-globin expression and the restoration of HbF, alleviating the severity of Hb disorders.

Studies examining the structure of BCL11A revealed that it undergoes structural changes influenced by the entropic contributions of its zinc finger domains ZnF4, ZnF5, and ZnF6. They addressed the function of ZnF6, which was thought to be not important previously. Future clinical applications include exploring strategies to recognize BCL11A at different stages and potentially prevent its binding to the γ-globin promoter. Another promising clinical approach for treating hemoglobin disorders involves using CRISPR-Cas9 to target BCL11A, a treatment that was successful with the Casgevy gene therapy. Removing or inactivating BCL11A has also shown to alleviate the symptoms of Hb disorders, and other techniques are currently being researched and developed.


V. References

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