Position effects lead to variable globin vector expression in vivo.
(A) Percent F cells versus average PBL vector copy number in animals that received transplants of β-thalassemic BM cells transduced with γ-globin lentiviral vectors. Although γ-globin expression is vector copy–related, linear regression analysis showed a correlation coefficient (r2) of 0.47, indicating some variability in correlation. Some animals with similar vector copy numbers show large differences in the proportion of expressing cells. (B) FACS analysis of γ-globin expression in PB red cells and in TER119-positive, clonal erythroid cells derived from secondary CFU-Ss of 2 animals that received transplants of globin vector transduced β-thalassemic BM cells. Top panels show γ-globin expression (solid lines) in the red cells of animals no. 21 and no. 24, with histograms of γ-globin expression (solid lines) in unique, clonal erythroid cells of each animal shown below. The number of vector copies present in each clone, as determined by Southern blot analysis, is indicated. Dotted lines in the histograms represent staining of a negative control sample.
