Expression of Fetal Hemoglobin among Adult Human Erythroblasts Is Associated with an Altered Expression Pattern for the Transcription Factor c-Myb.

The c-Myb transcription factor plays a central role in regulating growth and differentiation during definitive erythropoiesis. In addition, c-Myb expression has been previously linked to GATA-1 expression and hereditary persistence of fetal hemoglobin. Here we examined a potential role for c-Myb in signaled expression of fetal hemoglobin (HbF) among primary adult human erythroblasts. Initial experiments were performed to determine the expression pattern of c-Myb under conditions of low fetal hemoglobin production (CD34 cells cultured over 14 days in media supplemented with erythropoietin (EPO) alone). Quantitative PCR performed on cells sampled on days 2, 5, 8, 11, and 14 demonstrated highly regulated c-Myb expression. A dramatic increase (>30 fold) in c-Myb mRNA was measured between days 5 and 8 as the committed progenitor cells differentiated to the proerythroblast stage. That increase was followed by a 3-fold reduction in c-Myb during terminal differentiation. Western blotting of nuclear extracts obtained on days 7 and 14 in EPO cultures corroborated with the quantitative PCR analyses. Electrophoretic mobility shift assays performed using day 7 and day 14 nuclear extracts showed a shifted major band with a c-Myb consensus probe. That major band was competed by unlabeled c-Myb probe on both days. Moreover, day 14 extracts repeatedly revealed two additional bands that were not competed suggesting the possibility of other complexes binding to the Myb motif or changes in c-Myb conformation during terminal differentiation. Chromatin immunoprecipitation assays also revealed preliminary evidence for Myb binding directly to beta globin locus. When compared to low HbF culture conditions (EPO; HbF <1%), cells grown in high HbF conditions (erythropoietin + stem cell factor + TGF-beta (EST); HbF>35%) demonstrated a distinct pattern of c-Myb expression. Relative to the low HbF conditions, high HbF expressing cells had reduced c-Myb levels on day 7 followed by increased levels on day 14. These results suggest that c-Myb possesses highly regulated patterns of expression and DNA binding during erythroid maturation. In addition, the association of changes in c-Myb expression patterns with HbF production suggests that this growth-related transcription factor may help regulate fetal hemoglobin synthesis in patients with beta-hemoglobinopathies.