A beta-globin gene, inactive in the K562 leukemic cell, functions normally in a heterologous expression system

The K562 human leukemia cell is an erythroid-like cell that may serve as a model for the study of globin gene expression in transcriptionally active human erythroid cells. K562 cells express all globin genes with the exception of that for beta-globin; failure to produce beta-globin could result from an acquired mutation in each of the beta-globin genes or from an alteration in the regulatory factor environment of the beta-globin gene. To uncover a possible acquired mutation, restriction endonuclease analysis of genomic K562 DNA and expression studies of a cloned K562 beta-globin gene were carried out. Restriction endonuclease analysis revealed no structural alteration of the K562 beta-globin genes. Analysis of the polymorphic Ava II site in intervening sequence 2 of the beta-globin gene showed that K562 cells contain two different beta-globin alleles, both of which are inactive. A K562 beta-globin gene was cloned, ligated into the expression vector pLTN3B, and introduced into COS cells. Transcripts were analyzed by RNA blot, dot blot, S1 nuclease mapping, and primer extension assay. The cloned K562 beta-globin gene was transcribed in COS cells as efficiently as a normal beta-globin gene introduced into COS cells; the mRNA was 10 S and polyadenylylated; the 5' and 3' termini and the processing of transcripts were identical to that of mRNA transcribed from a normal gene. Based on these data we suggest that the absence of beta-globin gene expression results not from an alteration in the beta-globin gene, but from a quantitative or qualitative alteration in a trans-acting factor important in beta-globin gene expression.