Hemoglobin synthesis in somatic cell hybrids: independent segregation of the human alpha- and beta-globin genes

Genomic evidence for independent origins of beta-like globin genes in monotremes and therian mammals

Phylogenetic reconstructions of the beta-globin gene family in vertebrates have revealed that developmentally regulated systems of hemoglobin synthesis have been reinvented multiple times in independent lineages. For example, the functional differentiation of embryonic and adult beta-like globin genes occurred independently in birds and mammals. In both taxa, the embryonic beta-globin gene is exclusively expressed in primitive erythroid cells derived from the yolk sac. However, the "epsilon-globin" gene in birds is not orthologous to the epsilon-globin gene in mammals, because they are independently derived from lineage-specific duplications of a proto beta-globin gene. Here, we report evidence that the early and late expressed beta-like globin genes in monotremes and therian mammals (marsupials and placental mammals) are the products of independent duplications of a proto beta-globin gene in each of these two lineages. Results of our analysis of genomic sequence data from a large number of vertebrate taxa, including sequence from the recently completed platypus genome, reveal that the epsilon- and beta-globin genes of therian mammals arose via duplication of a proto beta-globin gene after the therian/monotreme split. Our analysis of genomic sequence from the platypus also revealed the presence of a duplicate pair of beta-like globin genes that originated via duplication of a proto beta-globin gene in the monotreme lineage. This discovery provides evidence that, in different lineages of mammals, descendent copies of the same proto beta-globin gene may have been independently neofunctionalized to perform physiological tasks associated with oxygen uptake and storage during embryonic development.

Efficiency and limitations of the hn-cDNA library approach for the isolation of human transcribed genes from hybrid cells

The use of splice donor site consensus sequences as primers in cDNA synthesis (to make a cDNA library from heterogeneous RNA or unprocessed transcript--an hn-cDNA library) and the screening of such an hn-cDNA library with human repeat DNA probe in order to isolate human genes from somatic cell hybrids have been demonstrated. Here, we optimize and evaluate the efficiency and limitations of the approach. Computer analysis of genomic sequences of 22 randomly selected human genes indicated that hexamers CTTACC, CTCACC, and CCTACC were most efficient at beginning first-strand cDNA synthesis at donor splice sites of hnRNA and suggested that the procedure is efficient for priming cDNA synthesis of at least one exon from most every gene. Primer extension experiments established conditions in which the primers would initiate synthesis of cDNA starting from a perfectly matched position on the RNA template at more than 60-fold higher yield than any other product. By isolation of a clone containing exon III of the human DNA repair gene ERCC1, we indicate that the approach is capable of cloning exons from weakly expressed genes. Sequencing of clones revealed a structure of hn-cDNA clones consistent with the expectations of the cloning strategy and indicated the potential of the clones in detecting polymorphisms. Finally, we demonstrate that the expression of these hn-cDNA sequences in cells can be detected efficiently at the hnRNA level by reverse transcriptase-polymerase chain reaction (RT/PCR).

The problem of hemoglobin switching in premature infants and IUGR infants

Understanding the hemoglobin switching regulation mechanism in extremely premature infants is particularly important for understanding the extrauterine adaptation of the infants. In this study, we sought to identify factors influencing oxygen affinity in fetal blood and neonatal blood and obtained these results. (1) Aging is required for hemoglobin switching. (2) Switching in premature infants (27 to 32 weeks) is delayed at least 3 weeks in comparison with full-term infants. (3) A delay in the switching of fetal hemoglobin to adult hemoglobin was confirmed in IUGR (intrauterine growth retardation) infants. However, there is a compensatory increase in 2, 3-DPG for adaptation after birth. (4) A delay in 2, 3-DPG increase was observed in RDS (respiratory distress syndrome) infants, and oxygen affinity remained high.

Adult and fetal human globin genes are expressed following chromosomal transfer into MEL cells

Somatic cell hybridization of mouse erythroleukemia (MEL) cells and HEL cells, a human erythroleukemia line that produces fetal (gamma) but fails to express adult (beta) globin, was used to test whether the expression of the two human globin genes is regulated cis or trans. An experimental approach using anti-human globin monoclonal antibodies for detection, efficient cloning, and monitoring of hybrids of interest was employed. Further characterization of hybrids used isoelectric focusing for detection of human globins and S1 nuclease mapping. In contrast to the parental HEL line, all chromosome 11-retaining HEL-MEL hybrids expressed human beta-globin, suggesting that the HEL beta-globin genes (i) are transcriptionally competent, (ii) become activated in response to a positive trans-acting element within the MEL environment, and (iii) fail to express into the HEL environment because of either the absence of a positive trans-acting element or the presence of a trans-acting inhibitor of beta-globin gene expression. In addition to beta-globin, the primary HEL-MEL hybrids co-expressed gamma-globin; however, gamma-globin expression segregated by subcloning so that secondary and tertiary clones either expressed only beta-globin or co-expressed gamma- and beta-globin. The results of subcloning can be explained by assuming that gamma-globin gene expression is controlled by a HEL cell-derived transacting element encoded by a gene not syntenic to chromosome 11 or by postulating that the HEL gamma-globin genes become randomly modified during the continuous proliferation of hybrids.

The genetic relationship between the tomato mutants, flacca and lateral suppressor, with reference to abscisic acid accumulation

Two tomato mutants, Lycopersicon esculentum flacca and lateral suppressor, are assigned to map position 59 of chromosome 7. The tight linkage between these two gene loci was detected as a result of attempts to establish whether they would exhibit phenotypic interaction. The possibility that both mutants result in abnormalities of abscisic acid (ABA) accumulation is considered. ABA analysis supports the suggestion that plants homozygous for flacca have a substantially lower concentration but indicates that lateral suppressor homozygotes do not differ from normal in ABA content. An attempt is made to reconcile the results with those of Tucker (1976, New. Phytol. 77, 561-568) by suggesting that lateral suppressor plants may accumulate high levels of an ABA metabolite which is indistinguishable from ABA using the Commelina epidermal strip bioassay.

Co-expression of mouse and rat haemoglobins in interspecific hybrids of mouse and at erythroleukemia cells

A modified technique for cell fusion with lysolecithin-lipid emulsions was used to generate hybrid erythroleukemia cell lines from Friend leukemia mouse cells (FLC) and chemically transformed rat erythroleukemia cells. Chromosome analysis of the hybrid cells showed the presence of both parental genomes even after long culture periods. The hybrids were still able to undergo erythroid differentiation after dimethylsulphoxide (DMSO) stimulation. Analysis of the globin chains from the DMSO-stimulated cells showed that both the rat and the mouse erythroid phenotypes were expressed. This demonstrates the compatibility of the regulatory genetic elements for the control of erythroid differentiation in cell hybrids of erythroleukemic populations from different species.

X-linked control of globin mRNA and hemoglobin production in erythroleukemia-lymphoma cell hybrids

In somatic cell hybrids formed by the fusion of mouse erythroleukemic cells with cultured mouse lymphoma cells, retention of the X chromosome donated by the lymphoma parent is correlated with inhibition of hemoglobin accumulation in response to dimethyl sulfoxide. The inhibition of hemoglobin production was due to an inhibition of globin mRNA accumulation. Heme can partially overcome the effects of the lymphoma X chromosome and induce globin mRNA and hemoglobin accumulation in the dimethylsulfoxide-treated hybrid cells. The data suggests that the X chromosome contributed by the lymphoma cells inhibits hemoglobin production by inhibiting both inducible globin mRNA accumulation as well as inducible heme biosynthesis, most likely at a step after the formation of delta-aminolevulinic acid. The properties of erythroleukemia x lymphoma cell hybrids are compared with those of a series of erythroleukemia x bone marrow cell hybrids. The data indicate the possibility of multiply loci on the X chromosome capable of regulating the expression of erythroid characteristics.

Necessity for two human chromosomes for human chorionic gonadotropin production in human-mouse hybrids

Through a series of human-mouse hybrids we have identified that two human chromosomes, 10 and 18, must be present for production of the pregnancy protein hormone human chorionic gonadotropin (hCG). Human choriocarcinoma cells producing hCG were hybridized to mouse cells. From 49 independent clones three hybrid clones continued to produce whole hCG. Chromosomal analysis was done on the 3 producer clones and 5 nonproducer clones. The additional 41 nonproducer clones were genetically characterized by isozymes. Only when chromosomes 10 and 18 were present in a clone would the whole hCG molecule be produced. Clones with only 10 or only 18 did not produce hormone. Nine subclones of a producer clone confirmed this observation. Three subclones retaining both 10 and 18 continued to produce hCG. This study demonstrated the need to use cellular chromosome data and population enzyme data to identify two chromosomes necessary for hCG production in heterogeneous human-mouse hybrids.

Thalassemia and pregnancy: results of an antenatal screening program

A thalassemia screening program was implemented at our institution using the finding of a mean corpuscular volume less than 80 fl as the index of abnormality. Further evaluation using hemoglobin (Hb) electrophoresis and serum iron studies was carried out according to the scheme detailed below. A diagnosis of thalassemia was made in 33 women (42 pregnancies). Eight patients had alpha-thalassemia trait, 23 beta-thalassemia trait, and two Hb H disease. Thalassemia trait did not have any adverse effect on pregnancy outcome. In two couples the fetuses were at risk for homozygous disease and in one couple the fetus was at risk for sickle cell beta-thalassemia. The screening program described is an effective and inexpensive means of detecting thalassemia in an antenatal population and is applicable to most every clinic or office setting.

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.

The linkage relationships of the haemoglobin beta, delta and alpha loci with 34 genetic marker systems

An analysis of the linkage relationships of the Hbalpha and Hbbeta loci with 34 genetic marker systems is presented. No evidence of linkage of either haemoglobin locus with any of the marker loci was found. The Hbalpha locus may be excluded from approximately 7% and the Hbbeta locus from approximately one-third of the autosomal genome.

Stability of the individual globin genes during erythroid differentiation

The genes of sheep betaA, betaC, and gamma globin were all present in DNA from erythroid cells which synthesized only betaC globin. Similarly, selective excision of non-expressed genes was shown not to occur during human erythroid differentiation. In contrast, evolutionary deletion of the betaC gene accounts for the inability of many sheep to make this globin.

Study of markers of human erythroid differentiation in hybrid cells

Somatic cell hybrids exhibiting co-expression of the globin genes of two species were generated by fusion of mouse erythroleukemia cells with Chinese hamster or human marrow erythroid cells. In contrast, extinction of the mouse globin genes occurred in hybrids formed between the erythroleukemia cells and human fibroblasts. Direct detection of the human globin genes in human X mouse fibroblast hybrids was achieved by annealing of DNA from these cells to human globin complementary DNA. This method was developed to permit the chromosomal assignment of the human globin genes.

Extinction of globin gene expression in human fibroblast x mouse erythroleukemia cell hybrids

We have chosen human fibroblast x mouse erythroleukemia hybrid cells as a model system to examine regulation of unique genes. The globin genes were studied as a marker of erythroid differentiation. Three separate hybrid cell lines were incubated in 2% dimethylsulfoxide, an agent which induces erythroid differentiation of the parental erythroleukemia cells. Neither human nor mouse globin mRNA sequences could be detected by a sensitive molecular hybridization assay which utilized globin complementary D N A. However, td n a from one of the cell lines was shown to contain both the mouse and humand globin genes. Thus, loss of the genes by chromosomal segregation did not account for their failure to be expressed. Cocultivation of the mouse erythroleukemia cells with excess human fibroblasts did not prevent erythroid differentiation of the erythroleukemia cells in the presence of dimethylsulfoxide. Similarly globin gene expression was preserved in tetraploid cells generated by fusion of two erythroleukemia lines. Thus, extinction of globin geneated by fusion of two erythroleukemia lines. Thus, extinction of blobin gene expression in the human fibroblast x erythroleukemia hybrids occurred at the level of mRNA production and appeared to be due to the presence of the fibroblast genome within the hybrial cell.

Absence of messenger RNA and gene DNA for beta-globin chains in hereditary persistence of fetal hemoglobin

The relative amounts of alpha-amd beta-globin mRNA and globin gene DNA were measured in reticulocyte RNA and lymphocyte DNA of an individual with homozygous hereditary persistence of fetal hemoglobin whose red blood cells contain 100% fetal hemoglobin (hb F: alpha2gamma2.) Molecular hybridization assays used as probes full-length DNA copies of human alpha- and beta-globin messenger RNA. The results of these hybridization assays demonstrated the expected amounts of alpha-globin mRNA and gene DNA, but absence of beta-globin mRNA and absence of beta-globin gene DNA. In the individual studied, hereditary persistence of fetal hemoglobin is associated with total deletion of the beta-globin structural gene.