Use of the hereditary persistence of fetal hemoglobin 2 enhancer to increase the expression of oncoretrovirus vectors for human gamma-globin

The development of oncoretrovirus vectors for human γ-globin has been hampered by problems of low expression and gene silencing. In order to address these problems, we investigated an enhancer element identified from individuals with deletional hereditary persistence of fetal hemoglobin 2 (HPFH2), a genetic condition characterized by elevated levels of γ-globin in adults. Plasmid transfection studies in erythroid MEL (murine erythroleukemia) cells demonstrated the HPFH2 element could function synergistically with the β-globin locus control region to enhance the expression of an A γ-globin gene with a truncated −382 bp promoter. A series of oncoretrovirus vectors were subsequently generated that contain an expression cassette for A γ-globin linked to various combinations of the HPFH2 enhancer, the α-globin HS40 enhancer, and several versions of the promoter from A γ-globin or β-globin. Expression analysis in transduced MEL cell clones revealed very high levels of promoter-autonomous silencing that was at least partially abrogated by the HPFH2 enhancer. The vector containing a combination of a −201 bp A γ-globin gene promoter with the Greek HPFH −117 point mutation and both the HPFH2 and HS40 enhancers exhibited no signs of vector silencing and was expressed at 248±99% per copy of mouse α-globin (62% of total α-globin). This represents a significant improvement over previously reported oncoretrovirus vectors for A γ-globin, and demonstrates the capacity of the HPFH2 enhancer to abrogate sequence-autonomous silencing of the A γ-globin promoter in the context of a gene transfer vector.