Haploinsufficiency for the erythroid transcription factor KLF1 causes hereditary persistence of fetal hemoglobin

Compound heterozygosity for KLF1 mutations associated with remarkable increase of fetal hemoglobin and red cell protoporphyrin

The persistence of high fetal hemoglobin level in adults may ameliorate the clinical phenotype of beta-thalassemia and sickle cell anemia. Several genetic variants responsible for hereditary persistence of fetal hemoglobin, linked and not linked to the beta globin gene cluster, have been identified in patients and in normal individuals. Monoallelic loss of KLF1, a gene with a key role in erythropoiesis, has been recently reported to be responsible for persistence of high levels of fetal hemoglobin. In a Sardinian family, high levels of HbF (22.1-30.9%) were present only in compound heterozygotes for the S270X nonsense and K332Q missense mutations, while the isolated S270X nonsense (haploinsufficiency) or K332Q missense mutation were associated with normal HbF levels (<1.5%). Functionally, the K332Q Klf1 mutation impairs binding to the BCl11A gene and activation of the γ- and β-globin promoters. Moreover, we report for the first time the association of KLF1 mutations with very high levels of zinc protoporphyrin.

Targeted therapeutic strategies for fetal hemoglobin induction

Increased levels of fetal hemoglobin (HbF) can ameliorate the severity of the β-hemoglobin disorders, sickle cell disease (SCD) and β-thalassemia, which are major sources of morbidity and mortality worldwide. As a result, there has been a longstanding interest in developing therapeutic approaches for inducing HbF. For more than 3 decades, the majority of HbF inducers developed were based on empiric observations and have had limited success. Recently, human genetic approaches have provided insight into previously unappreciated regulators of the fetal-to-adult hemoglobin switch and HbF silencing, revealing molecular targets to induce HbF. This article reviews these developments and discusses how molecules including BCL11A, KLF1, MYB, SOX6, miRNAs 15a and 16-1, and histone deacetylase 1 and 2 (HDAC1/2) could be important targets for HbF induction in humans. The current understanding of how these molecules function and the benefits and drawbacks of each of these potential therapeutic targets are also examined. The identification of these regulators of HbF expression is extremely promising and suggests that rationally designed approaches targeting the very mechanisms mediating this switching process could lead to better, less toxic, and more effective strategies for HbF induction.

Human globin knock-in mice complete fetal-to-adult hemoglobin switching in postnatal development

Elevated levels of fetal γ-globin can cure disorders caused by mutations in the adult β-globin gene. This clinical finding has motivated studies to improve our understanding of hemoglobin switching. Unlike humans, mice do not express a distinct fetal globin. Transgenic mice that contain the human β-globin locus complete their fetal-to-adult hemoglobin switch prior to birth, with human γ-globin predominantly restricted to primitive erythroid cells. We established humanized (100% human hemoglobin) knock-in mice that demonstrate a distinct fetal hemoglobin (HbF) stage, where γ-globin is the dominant globin chain produced during mid- to late gestation. Human γ- and β-globin gene competition is evident around the time of birth, and γ-globin chain production diminishes in postnatal life, with transient production of HbF reticulocytes. Following completion of the γ- to-β-globin switch, adult erythroid cells synthesize low levels of HbF. We conclude that the knock-in globin genes are expressed in a pattern strikingly similar to that in human development, most notably with postnatal resolution of the fetal-to-adult hemoglobin switch. Our findings are consistent with the importance of BCL11A in hemoglobin switching, since removal of intergenic binding sites for BCL11A results in human γ-globin expression in mouse definitive erythroid cells.

A dominant mutation in the gene encoding the erythroid transcription factor KLF1 causes a congenital dyserythropoietic anemia

The congenital dyserythropoietic anemias (CDAs) are inherited red blood cell disorders whose hallmarks are ineffective erythropoiesis, hemolysis, and morphological abnormalities of erythroblasts in bone marrow. We have identified a missense mutation in KLF1 of patients with a hitherto unclassified CDA. KLF1 is an erythroid transcription factor, and extensive studies in mouse models have shown that it plays a critical role in the expression of globin genes, but also in the expression of a wide spectrum of genes potentially essential for erythropoiesis. The unique features of this CDA confirm the key role of KLF1 during human erythroid differentiation. Furthermore, we show that the mutation has a dominant-negative effect on KLF1 transcriptional activity and unexpectedly abolishes the expression of the water channel AQP1 and the adhesion molecule CD44. Thus, the study of this disease-causing mutation in KLF1 provides further insights into the roles of this transcription factor during erythropoiesis in humans.

Putting a finger on the switch

The transition from fetal to adult beta-like globin expression is a key step in the maturation of the red blood cell lineage. Two new studies show that the KLF1 zinc finger protein uses direct and indirect means to regulate the final switch from fetal to adult globin expression and that monoallelic loss of KLF1 expression leads to persistence of fetal hemoglobin.