Selective restriction endonuclease cleavage of human globin genes

Double-stranded human globin DNA synthesized in vitro from sickle cell mRNA has been used as a substrate for a series of restriction endonucleases. The double-stranded DNA contained full length transcripts of the alpha- and beta- globin genes. Of the 10 enzymes tested, only 3 (Hpa I, Sal I, and Kpn I) failed to cleave either alpha- or beta-DNA; 2 (Eco RI and Bam HI) cleaved only beta-DNA; 3 (HindIII, Hpa II, and Hha I) cleaved only alpha-DNA; and 2 (Hae III and Alu I) cleaved both alpha- and beta-DNAs. The selective cleavage of human globin genes by restriction endonucleases should provide a strategy for the identification and purification of DNA fragments of genomic DNA containing globin genes plus their flanking sequences, simplify the preparation of pure, chain-specific globin probes, and permit the isolation of DNA probes for specific regions of the globin genes.

Differentiation of murine erythroleukemic (Friend) cells: an in vitro model of erythropoiesis

Normal erythropoiesis involves differentiation of uncommitted stem cells through committed erythroid precursors into cells specialized for hemoglobin synthesis. Several aspects of this developmental sequence may be studied in murine erythroleukemic cells infected with Friend virus complex. These cells are arrested at the proerythroblast stage, yet capable of continuous growth in vitro. Maturation along an erythroid pathway is induced after treatment with a variety of agents (e.g. dimethylsulfoxide, butyric acid, hemin, ouabain). Following induction, the cells morphologically resemble normoblasts, accumulate globin mRNAs and strain-specific globins, increase heme synthesis and acquire erythrocyte membrane antigens. Cloned populations of erythroleukemic cells mature in a nonhomogeneous fashion upon induction, indicative of a stochastic response in the inductive process. This "probability of differentiation" phenotype is formally analogous to stem cell development in which hematopoietic precursor cells form a constant, dividing population from which cells are continuously maturing. Although the sequence of events involved in triggering differentiation is uncertain, cloning and cell hybridization experiments demonstrate that this phenotype is under rather stable genetic (or epigenetic) control. Recent molecular analysis shows that induced differentiation is accompanied by transcriptional activation of the globin genes rather than posttranscriptional stabilization of the globin RNAs. Further application of cellular, molecular and genetic approaches in this system may help to define specific control mechanisms in erythroid development.

In vitro synthesis of a DNA probe for antisense globin sequences

A DNA probe specific for sequences complementary to globin RNA has been prepared in vitro from globin complementary DNA (cDNA). Globin cDNA was used as the template for the synthesis of a complementary strand (ccDNA) by avian myeloblastosis virus DNA polymerase. After treatment with single strand-specific S1 nuclease, double-stranded globin DNA was denatured in the presence of a vast excess of globin RNA. After hybridization of the cDNA with globin RNA, the ccDNA was isolated by hydroxyapatite chromatography. The ccDNA probe hybridized efficiently to globin cDNA, but not at all to globin RNA. This probe should prove useful in assessing the asymmetry of gene expression in cell-free transcriptional systems.

Globin RNA synthesis in vitro by isolated erythroleukemic cell nuclei: direct evidence for increased transcription during erythroid differentiation

Murine erythroleukemic cells accumulate cytoplasmic globin mRNA during differentiation induced in tissue culture by dimethyl sulfoxide. Cellular accumulation of globin RNA may reflect transcriptional activation of the globin genes and/or posttranscriptional stabilization of globin RNA during differentiation. To evaluate possible transcriptional controls directly; globin RNA synthesis by isolated erythroleukemic cell nuclei was studied. Conditions were established for optimal nuclear RNA synthesis in vitro in the presence of a mercurinucleotide (Hg-CTP). Mercurated RNA synthesized in vitro was purified free of endogenous RNA by affinity chromatography on sulfhydryl-Sepharose, and analyzed for the presence of newly synthesized globin RNA sequences by molecular hybridization to globin complementary [32P]DNA. The results demonstrate markedly increased synthesis of globin RNA by nuclei isolated from dimethyl sulfoxide-treated cells, even within 5 min of nuclear transcription in vitro. These findings are most consistent with transcriptional activation of the globin genes upon induction of differentiation.

Differentiation of erythroleukemic cells in the presence of inhibitors of DNA synthesis

Erythroid differentiation can be readily induced by butyric acid in cultured erythroleukemic cells in the presence of inhibitors of DNA synthesis and in the absence of cell division. This result appears to rule out more complex models for globin gene expression which require gene replication or cell division (or both).

Differential expression of alpha- and beta-globin genes during differentiation of cultured erythroleukemic cells

Murine erythroleukemic cells induced to differentiate in vitro with dimethylsulfoxide provide a model for events involved in the regulated expression of the globin genes. Here we examine alpha- and beta-globin gene expression in such cells which contain no detectable globin RNA prior to induction. To quantitate alpha- and beta-globin RNAs in cellular RNA samples by molecular hybridization techniques, highly radioactive complementary DNAs were synthesized using mouse alpha- and beta-globin RNAs purified by formamide gel electrophoresis. Maximally induced erythroleukemic cells and mouse reticulocytes contain nearly equal relative amounts of alpha- and beta-globin RNA. During the period in which globin RNA accumulates in differentiating erythroleukemic cells, however, alpha- and beta-globin RNAs are not present in equivalent amounts. alphaRNA is present in substantial excess (alpha/beta ratio 3.7) early in induction, and the alpha/beta RNA ratio progressively approaches 1 as differentiation proceeds further. These observations directly suggest that the alpha- and beta-globin genes are differentially expressed during cellular differentiation and raise questions as to how relative expression of globin genes is controlled during normal development.

Differentiation in erythroleukemic cells and their somatic hybrids

Clones of erythroleukemic cells differ in the extent to which they (1) undergo differentiation spontaneously and (2) can be induced to differentiate in the presence of dimethylsulfoxide. Here we demonstrate that relative differences in globin gene expression within and between clones largely reflect differences in the proportion of cells participating in differentiation rather than uniform differences in the extent to which all cells in these clones undergo differentiation. We call this phenotype of a clone its characteristic probability of differentiation, a property that reflects the likelihood that a cell of this clone will undergo erythrodifferentiation under given conditions. We have examined somatic hybrid cells formed between similar erythroleukemic lines, between phenotypically different erythroleukemic lines, between phenotypically different erythroleukemic lines, and between erythroleukemic cells and mouse fibroblasts. Results of these experiments demonstrate that the spontaneous and induced probabilities of differentiation may be altered in an uncoupled fashion, suggesting that each is determined at different steps leading to a common pathway of globin gene expression.

Lambda-chain production in human lymphoblast-mouse fibroblast hybrids

Mutant human lymphoblast cells deficient in hypoxanthine phosphoribosyltransferase (EC 2.4.2.8) activity were hybridized with thymidine kinase (EC 2.7.1.21)-deficient mouse fibroblasts. Hybrid cells were readily selected, as both parental lines were nonreverting and eliminated by hypoxanthine-amethopterinthymidine medium. Human lambda (lambda) chain was the only immunoglobulin chain produced by the lymphoblast parent, as determined by immunofluorescent techniques. Two independent hybrid clones chosen for detailed study synthesized human lambda chain, and continued to do so after prolonged culture. As in both parental lines, no human immunoglobulin heavy chains, complements C3 or C4, or alpha(1)-antitrypsin, or mouse immunoglobulin chains or complement C5 were detectable in the hybrids. Selection against thymidine kinase-containing hybrid cells with 5-bromodeoxyuridine did not eliminate positive lambda-chain reactivity, suggesting that the kinase and lambda-chain loci are not linked. The continued production of an immunoglobulin chain by human lymphoblast-mouse fibroblast hybrids contrasts with the extinction of other differentiated functions in several hybrid systems, and indicates that gene localization and linkage analysis for human immunoglobulin chains should be feasible with this system.