Reprogramming erythroleuekmia cells to terminal differentiation and terminal cell division

In vitro differentiation of murine erythroleuekemia cells recapitulates many aspects of the erythroid terminal differentiation program, including hemoglobin synthesis and proliferation arrest. It also provides an opportunity to study the changes occurring during reprogramming of tumor cells into their normal differentiation program. This review is focused on the recent progress made in understanding the key events occurring during the reprogramming of erythroleukemia cells. We discuss the contributions of PU.1 to the block to terminal differentiation exhibited by the erythroleukemia cells as well as the role of GATA-1 in restoring normal differentiation. We also discuss the role of certain cell cycle regulators in the decision to resume normal differentiation and in the resulting terminal cell divisions and arrest.

Spi-1/PU.1 is a positive regulator of the Fli-1 gene involved in inhibition of erythroid differentiation in friend erythroleukemic cell lines

Spi-1/PU.1 and Fli-1 are two members of the ETS family of transcription factors whose expression is deregulated by proviral insertion in most erythroleukemic cell lines induced by the spleen focus-forming virus (SFFV) and Friend murine leukemia virus (F-MuLV) components of the Friend viral complex, respectively. In this study, we present evidence that transcription of the Fli-1 gene is positively regulated by Spi-1/PU.1 in SFFV-transformed cell lines: (i) all SFFV-transformed cell lines expressing Spi-1/PU.1 are characterized by a specific pattern of Fli-1 gene transcripts initiated in the -200 region instead of position -400 as reported for F-MuLV-transformed cell lines; (ii) these Fli-1 transcripts initiated in the -200 region are downregulated in parallel with that of Spi-1/PU.1 during hexamethylenebisacetamide (HMBA) induced differentiation; and (iii) Fli-1 transcription is upregulated in SFFV cells lines following stable transfection of a Spi-1/PU.1 expression vector. Furthermore, we found by transient transfection assays that the -270/-41 region of the Fli-1 gene displays promoter activity which is transactivated by Spi-1/PU.1. This promoter is strictly dependent on the integrity of two highly conserved ETS DNA binding sites that bind the Spi-1/PU.1 protein in vitro. Finally, we show that transfection of constitutive or inducible Fli-1 expression vectors in SFFV-transformed cells inhibits their erythroid differentiation induced by HMBA. Overall, these data indicate that Fli-1 is a target gene of the Spi-1/PU.1 transcription factor in SFFV-transformed cell lines. We further suggest that deregulated synthesis of Fli-1 may trigger a common mechanism contributing to erythroleukemia induced by either SFFV or F-MuLV.

Isolation and characterization of a human gene containing a nuclear localization signal from the critical region for velo-cardio-facial syndrome on 22q11

Velo-cardio-facial syndrome (VCFS) and DiGeorge syndrome are congenital disorders characterized by craniofacial anomalies, conotruncal heart defects, immune deficiencies, and learning disabilities. Both diseases are associated with similar hemizygous 22q11 deletions, indicating that haploinsufficiency of a gene(s) in 22q11 is responsible for their etiology. We describe here a new gene called NLVCF, which maps to the critical region for VCFS on 22q11 between the genes HIRA and UFD1L. NLVCF encodes a putative protein of 206 amino acids. The coding region encompasses four exons that span a genomic interval of 3.4 kb. Coding sequence analysis revealed that NLVCF is a novel gene that contains two consensus sequences for nuclear localization signals. The Nlvcf mouse homolog is 75% identical in amino acid sequence and maps to the orthologous region on mouse chromosome 16. The human NLVCF transcript is 1.3 kb in size and is expressed at varying levels in many fetal and adult tissues. Whole-mount in situ hybridization showed that Nlvcf is expressed in most structures of 9.5-dpc mouse embryos, with especially high expression in the head as well as in the first and second pharyngeal arches. NLVCF and HIRA are divergently transcribed, and their start codons lie approximately 1 kb apart in both humans and mice. Interestingly, the two genes exhibit a similar expression pattern in mouse embryos, suggesting that they may share common regulatory elements. The pattern of expression of NLVCF and its localization in the critical region suggest that NLVCF may contribute to the etiology of VCFS.

Characterization and mutation analysis of goosecoid-like (GSCL), a homeodomain-containing gene that maps to the critical region for VCFS/DGS on 22q11

Velocardiofacial syndrome (VCFS) is a developmental disorder characterized by conotruncal heart defects, craniofacial anomalies, and learning disabilities. VCFS is phenotypically related to DiGeorge syndrome (DGS) and both syndromes are associated with hemizygous 22q11 deletions. Because many of the tissues and structures affected in VCFS/DGS derive from the pharyngeal arches of the developing embryo, it is believed that haploinsufficiency of a gene(s) involved in embryonic development may be responsible for its etiology. A homeodomain-containing gene, Goosecoidlike (GSCL), has been recently described, and it resides in the critical region for VCFS/DGS on 22q11. GSCL is related to the Goosecoid gene (GSC) in both sequence of the homeodomain and genomic organization. Gsc in the mouse is expressed during early and midembryogenesis and is required for craniofacial rib, and limb development. The chick homolog of GSCL, termed GSX, is expressed during early chick embryogenesis. We detected GSCL expression in human embryos and biphasic expression in mouse embryos. It is possible that the vertebrate GSCL gene is also required for embryonic development. Due to its location in the critical region on 22q11, GSCL is an excellent candidate gene for VCFS/DGS. The vertebrate GSC protein has the same DNA binding specificity as the Drosophila morphogen, bicoid. Upon examination of the putative GSCL promoter, we found three sequence elements with an exact match to the reverse complement of the bicoid DNA recognition motif, suggesting that GSC, or possibly GSCL itself, regulates the transcription of GSCL. Sequence analysis of the putative promoter and the coding region of GSCL was performed on the DNA template from 17 VCFS patients who did not have a detectable 22q11 deletion to identify mutations. We did not detect a mutation in this set of VCFS patients. A polymorphism was detected in codon 47 of exon 1.

Identification, characterization, and precise mapping of a human gene encoding a novel membrane-spanning protein from the 22q11 region deleted in velo-cardio-facial syndrome

Velo-cardio-facial syndrome (VCFS) and DiGeorge syndrome (DGS) are characterized by a wide spectrum of phenotypes including cleft palate, conotruncal heart defects, and facial dysmorphology. Hemizygosity for a portion of chromosome 22q11 has been detected in 80-85% of VCFS/DGS patients. Using a cDNA selection protocol, we have identified a new gene, TMVCF (transmembrane protein deleted in VCFS), which maps to the deleted interval. The genomic locus is positioned between polymorphic markers D22S944 and D22S941. TMVCF encodes a small protein of 219 amino acids that is predicted to contain two membrane-spanning domains. TMVCF is expressed abundantly in human adult lung, heart, and skeletal muscle, and transcripts can be detected at least as early as Day 9 of mouse development.

Identification of a new human catenin gene family member (ARVCF) from the region deleted in velo-cardio-facial syndrome

Velo-cardio-facial syndrome (VCFS) and DiGeorge syndrome (DGS) are characterized by a wide spectrum of phenotypes, including conotruncal heart defects, cleft palate, and facial dysmorphology. Hemizygosity for a portion of chromosome 22q11 has been detected in 80-85% of VCFS/DGS patients. Both syndromes are thought to be the result of a developmental field defect. Using two independent gene-isolation procedures, we isolated a new catenin family member termed ARVCF (armadillo repeat gene deleted in VCFS) from the interval deleted in VCFS. ARVCF encodes a protein of 962 amino acids that contains a coiled coil domain and 10 tandem armadillo repeats. The primary structure of the protein is most closely related to the murine catenin p120CAS, which suggests a role for ARVCF in protein-protein interactions at adherens junctions. ARVCF is expressed ubiquitously in all fetal and adult tissues examined. This gene is hemizygous in all VCFS patients with interstitial deletions. Based on the physical location and potential functions of ARVCF, we suggest that hemizygosity at this locus may play a role in the etiology of some of the phenotypes associated with VCFS.

An integrated approach for identifying and mapping human genes

We have developed a method for generating expressed-sequence maps of human chromosomes. The method involves several steps that begin with libraries of highly representative short cDNAs prepared by using random oligomers as primers. The cDNA inserts are amplified by PCR with flanking vector primers. Chromosomal region-specific cDNA packets are prepared by hybridization of the cDNA inserts to DNA derived from yeast artificial chromosomes (YACs) assigned to defined regions of human chromosomes. The cDNA packets are cloned into yeast chromosome fragmentation vectors and used for transformation of yeast bearing the YAC used for affinity purification. Sequences in the cDNAs undergo homologous recombination with the corresponding exons in the genomic DNA yielding a set of truncated YACs. Each unique truncation specifies the location of an exon in the YAC. Since all of the truncation events end with the same vector sequence, it is possible to rescue and sequence these ends to generate expressed sequence tags. The method couples rapid purification of region-specific cDNAs with precise mapping of their genes on YACs. Appropriately truncated YACs also provide easy access to gene regulatory sequences. We describe the feasibility of individual steps of the method using the factor IX (F9) gene as a model system and we present the mapping of several expressed sequences corresponding to a 330-kb YAC containing DNA from human chromosome 6p21. In addition, we obtained the sequence, including an intron-exon junction, flanking a particular truncation event.

A dominant positive and negative selectable gene for use in mammalian cells

We have constructed three different fusion genes containing the herpes simplex virus thymidine kinase (HSV tk) and the bacterial neomycin phosphotransferase (neo) genes. All three fusion genes utilize the HSV tk promoter but differ at the junction of their components. We have determined if the fusion genes are bifunctional by introducing them into mammalian cells and testing for function of the individual components. One of the fusion genes, TNFUS 69, produced a bicistronic message and a fusion protein that has TK and NEO protein functions. This and other fusion genes of a similar nature could serve as dominant positive and negative selectable markers in mammalian cells.

Regulation of a human cardiac actin gene introduced into rat L6 myoblasts suggests a defect in their myogenic program

The rat myogenic cell line L6E9 induces skeletal but not cardiac alpha-actin mRNA upon fusion to form myotubes. However, when a human cardiac alpha-actin gene was introduced into L6E9 myoblasts, differentiation of the cells led to the accumulation of human gene transcripts in parallel with those derived from the endogenous skeletal alpha-actin gene. This result demonstrates that factors which direct rat myogenesis can regulate a muscle gene from another species and that the L6E9 cells may have a defect in their ability to activate endogenous cardiac actin gene expression.

Cotransfer of circular and linear prokaryotic and eukaryotic DNA sequences into mouse cells

We have attempted to introduce some eukaryotic and prokaryotic DNA sequences into mouse fibroblasts. Purified herpes thymidine kinase gene (tk) was introduced into mouse cells. The presence of the herpes tk gene was established by gel electrophoresis, sensitivity to the purine analog acyloguanosine, and Southern blot hybridization. We utilized two different methods to introduce nonselectable markers into mouse cells. Bacterial plasmid pBR322 was ligated to herpes tk and used for transfection. All cells that were TK+ also contained the plasmid sequences. In the second method, pBR322 DNA was mixed with herpes tk DNA and presented to mouse cells. TK+ cells were tested for pBR322 sequences by blot hydridization. The frequency of unlinked cotransfer was greater than 40%. When the circular plasmid containing pBR322 and tk was used for transfection, each of the resulting transfectants acquired several copies of the plasmid. Most of the copies were associated with high molecular weight DNA in the cell. In addition, we found that some of the plasmid molecules may exist as free circular molecules. Using the nonligated cotransfer method, we introduced purified human beta-globin sequences into the recipient cells. We were unable to detect any transcripts of the human beta-globin gene at a level greater than or equal to 10 molecules per cell.

Effect of interferon on dimethyl sulfoxide-stimulated Friend erythroleukemic cells: ultrastructural and biochemical study

Treatment of dimethyl sulfoxide-stimulated Friend erythroleukemic cells (clone 745) with mouse interferon (50 U/ml) led to the following changes: (i) a net decrease (40 to 60%) in both the total number of apparently newly synthesized virion particles per cell section and in the average number of cell sections containing one or more virion particles; (ii) a large decrease (80 to 90%) in the number of particles released into the supernatant fluid, as assayed by reverse transcriptase activity; (iii) an initial increase in the number of "immature" or "enveloped A-type" virions followed by an increase in the accumulation of empty, core shell-like particles; and (iv) a decrease in the number of cytoplasmic vacuolar structures, which have been implicated as a major site of virus production and which we show here by serial sectioning to be, in several instances, invaginations of the plasma membrane. The effects on virus production were noticeable 2 h after interferon addition and reached their full extent by 13 h. We conclude from these observations that interferon acts upon the late stage(s) of virion maturation, leading both to a decrease in virion production as well as to the formation of defective particles. In contrast, a small but significant increase in the rate at which globin mRNA and hemoglobin accumulate is observed after interferon treatment.

Somatic cell hybrids between Friend erythroleukemia cells and mouse hepatoma cells

Somatic cell hybrids between hepatoma and Friend erythroleukemia parental cells were studied for the expression of liver-specific and erythroid properties. Several independent clones were isolated using HAT selection and were shown to be true hybrids by isozyme and chromosome analysis. All displayed a complete extinction of hemoglobin and globin mRNA production, but a retention of albumin and transferrin secretion. The data suggest that erythroid differntiation is being actively inhibited by the hepatoma genome. Possible mechanisms that might explain these results are discussed in the light of current hypotheses regarding the mechanism of cell differentiation.

Maternal histone messenger RNA: detection by molecular hybridization

A competition-hybridization assay has been developed for the majority of sequences present in embryonically synthesized histone messenger RNA. The assay permits the first direct demonstration of specific "maternal" messengers in unfertilized sea-urchin eggs. The molecular size of histone messenger RNA stored in the egg appears to be the same as that of histone messenger RNA synthesized by the embryo. Maternal histone messenger RNA is found in the soluble phase of egg homogenates, in the form of ribonucleoprotein particles, unassociated with ribosomes.

Quantitative deficiency of chain-specific globin messenger ribonucleic acids in the thalassemia syndromes

A hybridization assay procedure was devised that makes possible quantitation of the ratio of mRNA of alpha to mRNA of beta globin chains in an RNA sample. The assay uses the radioactive synthetic DNA copies obtained by incubation of RNA-dependent DNA polymerase of avian myeloblastosis virus with rabbit globin mRNA that is 80-90% enriched in mRNA specific for synthesis of alpha or beta globin chains. The rabbit alpha-chain mRNA is obtained from the postribosomal supernatant of rabbit reticulocyte lysates; the rabbit beta-chain mRNA is obtained from the largest polysomes of rabbit reticulocytes treated with L-O-methylthreonine. Sufficient homology exists between rabbit and human globin chains and globin mRNAs that the synthetic DNA copies of chain-specific rabbit globin mRNA hybridize with human globin mRNA. Applied to the study of globin mRNA isolated from reticulocytes of humans with alpha and beta thalassemia, the technique revealed marked quantitative deficiency of alpha-chain mRNA relative to beta-chain mRNA in alpha thalassemia and similar deficiency of beta-chain mRNA relative to alpha-chain mRNA in beta thalassemia. The thalassemia syndromes are therefore characterized by true quantitative deficiency of the mRNA specific for the affected globin chain.