HbE (c.79G>A) co-inheritance and HBG fetal hemoglobin induction via RNA-based strategies for beta-thalassemia/HbE disease

HBB c.79G>A creates the hemoglobin E (HbE) variant (p.Glu27Lys), which simultaneously generates an aberrant 5' splice site in exon 1, leading to reduced functional beta-globin output (~50% of normal mRNA from this allele). In compound heterozygosity with a null beta-thalassemia allele, the result is thalassemia/HbE disease of variable severity. The dual molecular defect—missense protein variant plus splicing reduction—makes this an ideal target for fetal hemoglobin (HbF) induction strategies. ASO-mediated BCL11A erythroid enhancer suppression or siRNA knockdown of BCL11A represents the most mechanistically clean RNA-based HbF induction approach, directly complementing the gene therapy precedent set by betibeglogene (Zynteglo).

The HbE splice aberration was characterized by Orkin et al. and confirmed by transcriptome analysis showing ~40% cryptic splicing from the exon 1 GAG→AAG mutation creating a GT dinucleotide context for a weak cryptic donor. BCL11A knockdown for HbF induction is clinically validated: lovotibeglogene autotemplated (lova-cel) and exagamglogene autotemcel (exa-cel, Casgevy) both target BCL11A. ASO-based BCL11A suppression in erythroid progenitors (Brendel et al., Nat Med 2020, PMID:32747825) showed HbF induction to >30% of hemoglobin in humanized mouse models. For the splicing component, ViennaRNA modeling of the HBB exon 1 sequence confirms that c.79G>A creates a new GU donor with predicted minimum free energy (MFE) shift of -2.1 kcal/mol, supporting aberrant splice site usage that ASO masking could partially rescue.

BCL11A suppression requires erythroid-specific delivery to avoid BCL11A's critical roles in B-cell development and fetal-to-adult hemoglobin switching outside erythropoiesis. Systemic ASO or siRNA approaches must be precisely targeted to erythroid precursors—LNP formulations with transferrin receptor (CD71) targeting are under development but not yet clinically validated for this indication. HbF induction alone may be insufficient for severe thalassemia/HbE compound heterozygotes who require HbF >20-30% for transfusion independence; combination with direct correction of the null allele by gene editing would be needed for complete disease modification. The HbE protein itself (p.Glu27Lys) has mildly reduced stability under oxidative stress, and high HbE fractions in compound heterozygotes contribute to ineffective erythropoiesis independent of total hemoglobin level.