Persistent expression of the ␥-globin genes in adults with deletion types of hereditary persistence of fetal hemoglobin (HPFH) is thought to be mediated by enhancer-like effects of DNA sequences at the 3 breakpoints of the deletions. A transgenic mouse model of deletion-type HPFH was generated by using a DNA fragment containing both human ␥-globin genes and HPFH-2 breakpoint DNA sequences linked to the core sequences of the locus control region (
Previous analysis of the hemoglobin phenotype of the K562 human erythroleukemia cell line showed regulated expression of the epsilon-, zeta-, gamma-, alpha-, and delta-globin genes. Expression of the beta- globin genes has not been previously detected in this cell line. In this report, we describe the isolation of a variant of the K562 cell line that actively expresses beta-globin messenger RNA (mRNA) and polypeptide and shows greatly reduced expression of the delta-globin genes. This phenotype developed spontaneously in culture while two other K562 isolates grown under the same culture conditions have not undergone the same delta- to beta-globin switch. Analysis of this unique K562 variant shows that a construct containing a beta-globin promoter is quite active upon transient transfection into these cells. This finding suggests that the activation of the endogenous beta-globin genes results from changes in the trans-acting environment of these cells. The regulation of the beta-globin genes in this variant is characterized by a paradoxical decrease in the level of beta-globin mRNA after exposure to hemin. Other globin genes of this variant are appropriately regulated and show increased expression after hemin induction. Further study of this variant may shed light on mechanisms of gene regulation that are involved in hemoglobin switching.
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