Targeting Genetic Modifiers of HBG Gene Expression in Sickle Cell Disease: The miRNA Option

Abstract

Sickle cell disease (SCD) is one of the most common inherited hemoglobinopathy disorders that affects millions of people worldwide. Reactivation of HBG (HBG1, HBG2) gene expression and induction of fetal hemoglobin (HbF) is an important therapeutic strategy for ameliorating the clinical symptoms and severity of SCD. Hydroxyurea is the only US FDA-approved drug with proven efficacy to induce HbF in SCD patients, yet serious complications have been associated with its use. Over the last three decades, numerous additional pharmacological agents that reactivate HBG transcription in vitro have been investigated, but few have proceeded to FDA approval, with the exception of arginine butyrate and decitabine; however, neither drug met the requirements for routine clinical use due to difficulties with oral delivery and inability to achieve therapeutic levels. Thus, novel approaches that produce sufficient efficacy, specificity, and sustainable HbF induction with low adverse effects are desirable. More recently, microRNAs (miRNAs) have gained attention for their diagnostic and therapeutic potential to treat various diseases ranging from cancer to Alzheimer's disease via targeting oncogenes and their gene products. Thus, it is plausible that miRNAs that target HBG regulatory genes may be useful for inducing HbF as a treatment for SCD. Our laboratory and others have documented the association of miRNAs with HBG activation or suppression via silencing transcriptional repressors and activators, respectively, of HBG expression. Herein, we review progress made in understanding molecular mechanisms of miRNA-mediated HBG regulation and discuss the extent to which molecular targets of HBG might be suitable prospects for development of SCD clinical therapy. Lastly, we discuss challenges with the application of miRNA delivery in vivo and provide potential strategies for overcoming barriers in the future.

Fig. 1

Effect of miRNAs and transcription factors on fetal globin gene expression during hemoglobin switching. Shown is the HBB gene locus on chromosome 11, which consists of a locus control region with hypersensitive sites and the five developmentally regulated globin genes, including HBE (purple), fetal HBG1 and HBG2 (brown), HBD (blue), and adult HBB (orange). Silencing of HBG genes is mediated by transcriptional repressors, such as MYB, which activates KLF1, and which in turn activates the repressor BCL11A, which mediates transcriptional silencing of HBG. STAT3 inhibits HBG while KLF1 activates HBB. The HBG promoters become hypermethylated and silenced by DNA methylation. The DNMT3 proteins, DNMT3A and DNMT3B, are required for long-term methylation and silencing of the HBG gene promoters during adult erythropoiesis. MiR-29b, a DNMT3 inhibitor, also inhibits MYB expression, resulting in HBG activation. Several miRNAs have been shown to activate or inhibit HBG expression. miRNAs shown to inhibit (red line) or induce (green line) HBG expression via their target gene(s) are shown. DNMT DNA methyltransferase, BCL11A B-cell lymphoma 11A, HSs hypersensitive sites, KLF1 Krüppel-like factor 1, KLF3 Krüppel-like factor 3, LCR locus control region, miR microRNA, MYB MYB proto-oncogene, transcription factor