Pathogenic BCL11A variants provide insights into the mechanisms of human fetal hemoglobin silencing.

Santhosh Girirajan,Vijay G. Sankaran,Yong Shen,Rick Li,Kara E. Montbleau,Richard A. Voit,Kristian Teichert,Jeffrey M. Verboon

doi:10.1371/journal.pgen.1009835

Abstract

Increased production of fetal hemoglobin (HbF) can ameliorate the severity of sickle cell disease and β-thalassemia. BCL11A has been identified as a key regulator of HbF silencing, although its precise mechanisms of action remain incompletely understood. Recent studies have identified pathogenic mutations that cause heterozygous loss-of-function of BCL11A and result in a distinct neurodevelopmental disorder that is characterized by persistent HbF expression. While the majority of cases have deletions or null mutations causing haploinsufficiency of BCL11A, several missense variants have also been identified. Here, we perform functional studies on these variants to uncover specific liabilities for BCL11A’s function in HbF silencing. We find several mutations in an N-terminal C2HC zinc finger that increase proteasomal degradation of BCL11A. We also identify a distinct C-terminal missense variant in the fifth zinc finger domain that we demonstrate causes loss-of-function through disruption of DNA binding. Our analysis of missense variants causing loss-of-function in vivo illuminates mechanisms by which BCL11A silences HbF and also suggests potential therapeutic avenues for HbF induction to treat sickle cell disease and β-thalassemia.

Highlights

Functional follow up on the results of genome-wide association studies examining human variation in fetal hemoglobin (HbF) expression has led to the identification of BCL11A as a key HbF silencing factor and regulator of the developmental fetal-to-adult hemoglobin switch [1,2,3,4,5,6]
Clinical trials are currently seeking to target BCL11A for induction of fetal hemoglobin to treat individuals impacted by sickle cell disease and β-thalassemia
We have examined the functional consequences of pathogenic missense mutations in BCL11A and gained insight into how these mutations can alter the function of this key factor

Summary

Introduction

Functional follow up on the results of genome-wide association studies examining human variation in fetal hemoglobin (HbF) expression has led to the identification of BCL11A as a key HbF silencing factor and regulator of the developmental fetal-to-adult hemoglobin switch [1,2,3,4,5,6]. Recent studies have provided additional details on the mechanisms by which BCL11A acts to silence HbF These studies have identified co-factors that cooperate with BCL11A [4,7], DNA binding sites for BCL11A within the HbF-encoding HBG1/2 (γ-globin) promoters [8,9], and potential long-range interactions mediated by BCL11A [10,11]. Recent functional analyses through combinatorial genome editing in single erythroid progenitor cells have revealed how BCL11A acts at both the HBG1/2 promoter elements and through distal interactions to effectively silence HbF [12]. The precise mechanisms by which BCL11A silences HBG1/2 expression remain to be fully defined

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Conclusion