Engineered U7 snRNA mediates sustained splicing correction in erythroid cells from β-thalassemia/HbE patients

Repair of a splicing defect of β-globin pre-mRNA harboring hemoglobin E (HbE) mutation was successfully accomplished in erythroid cells from patients with β-thalassemia/HbE disorder by a synthetic splice-switching oligonucleotide (SSO). However, its application is limited by short-term effectiveness and requirement of lifelong periodic administration of SSO, especially for chronic diseases like thalassemias. Here, we engineered lentiviral vectors that stably express U7 small nuclear RNA (U7 snRNA) carrying the splice-switching sequence of the SSO that restores correct splicing of β^E-globin pre-mRNA and achieves a long-term therapeutic effect. Using a two-step tiling approach, we systematically screened U7 snRNAs carrying splice-switching SSO sequences targeted to the cryptic 5' splice site created by HbE mutation. We tested this approach and identified the most responsive element for mediating splicing correction in engineered U7 snRNAs in HeLa-β^E cell model cell line. Remarkably, the U7 snRNA lentiviral vector (U7 βE4+1) targeted to this region effectively restored the correctly-spliced β^E-globin mRNA for at least 5 months. Moreover, the effects of the U7 βE4+1 snRNA lentiviral vector were also evident as upregulation of the correctly-spliced β^E-globin mRNA in erythroid progenitor cells from β-thalassemia/HbE patients treated with the vector, which led to improvements of pathologies in erythroid progenitor cells from thalassemia patients. These results suggest that the splicing correction of β^E-globin pre-mRNA by the engineered U7 snRNA lentiviral vector provides a promising, long-term treatment for β-thalassemia/HbE.