Globin vector regulatory elements are active in early hematopoietic progenitor cells

The globin genes are archetypal tissue-specific genes that are silent in most tissues but for late-stage erythroblasts upon terminal erythroid differentiation. The transcriptional activation of the beta-globin gene is under the control of proximal and distal regulatory elements located on chromosome 11p15.4, including the beta-globin locus control region (LCR). The incorporation of selected LCR elements in lentiviral vectors encoding beta and beta-like globin genes has enabled successful genetic treatment of the beta-thalassemias and sickle cell disease. However, recent occurrences of benign clonal expansions in thalassemic patients and myelodysplastic syndrome in patients with sickle cell disease call attention to the non-erythroid functions of these powerful vectors. Here we demonstrate that lentivirally encoded LCR elements, in particular HS1 and HS2, can be activated in early hematopoietic cells including hematopoietic stem cells and myeloid progenitors. This activity is position-dependent and results in the transcriptional activation of a nearby reporter gene in these progenitor cell populations. We further show that flanking a globin vector with an insulator can effectively restrain this non-erythroid activity without impairing therapeutic globin expression. Globin lentiviral vectors harboring powerful LCR HS elements may thus expose to the risk of trans-activating cancer-related genes, which can be mitigated by a suitable insulator.

Automated summary

The globin genes are activated in the late-stage erythroblasts during erythroid differentiation and lentiviral vectors incorporating beta-globin locus control region (LCR) elements have been successfully used for genetic treatment of the beta-thalassemias and sickle cell disease. However, recent cases of benign clonal expansions and myelodysplastic syndrome raise concerns about non-erythroid functions of these vectors. This study shows that lentivirally encoded LCR elements can be activated in early hematopoietic cells, and the use of an insulator can restrain this non-erythroid activity without affecting therapeutic globin expression.