Human Gamma Globin Gene Regulation in Knock-In Mouse Models of Anemia.

The generation of transgenic and gene targeted mouse models of human hemoglobinopathies provides valuable opportunities to test mechanisms of human globin gene regulation and experimental therapies. Yet mice do not naturally have a fetal hemoglobin, challenging our ability to adequately model the developmental onset of disease. Transgenic model systems that contain the entire human β-globin locus present obstacles to the study of human globin gene switching, including a fetal to adult globin gene switch that occurs too early in development. The generation of genetically engineered mice with a delayed human γ to β hemoglobin switch has been a major topic of interest for our laboratory. Delayed γ globin gene expression improves the clinical progression in patients as well as animal models with hemoglobinopathies. However, molecular mechanisms involved in globin gene switching are not well understood. In this study the transcriptional and epigenetic regulation of human γ to β hemoglobin switching are analyzed in novel human knock-in (KI) mouse models that complete the switch from fetal to adult hemoglobin after birth. These KI mice were generated by replacement of the adult mouse β-globin genes by homologous recombination in embryonic stem cells with a delayed switching human γ to β globin gene construct. Quantitative real-time PCR and HPLC were used to measure mouse and human embryonic, fetal, and adult globin genes through development and show that we have given the mouse a true fetal hemoglobin. Heterozygous mice express human β-like globin genes at a high level comparable to the adult mouse β globin genes. Mutations responsible for hereditary persistence of fetal hemoglobin (HPFH) in the γ globin promoter recapitulate the human phenotype in KI mice, with over 50 fold γ globin gene upregulation in adults. These HPFH KI mice also display higher γ globin levels at birth and markedly delayed γ globin gene downregulation in the weeks following birth. These studies in KI mice demonstrate that human β-like globin genes interacting with the mouse LCR are regulated in a manner similar to what is seen in humans and may be used to study the mechanisms of globin gene switching. Greater understanding of γ-globin gene regulation will be required for achieving the therapeutic goal of reactivating silenced γ-globin genes to ameliorate severe human hemoglobinopathies.