Expression of the human beta-globin gene after retroviral transfer into murine erythroleukemia cells and human BFU-E cells.

Replication-defective amphotropic retrovirus vectors containing either the human beta-globin gene with introns or an intronless beta-globin minigene were constructed and used to study beta-globin expression following gene transfer into hematopoietic cells. The beta-globin genes were marked by introducing a 6-base-pair insertion into the region corresponding to the 5' untranslated region of the beta-globin mRNA to allow detection of RNA encoded by the new gene in human cells expressing normal human beta-globin RNA. Introduction of a virus containing the beta-globin gene with introns into murine erythroleukemia cells resulted in inducible expression of human beta-globin RNA and protein, while the viruses containing the minigene were inactive. The introduced human beta-globin gene was 6 to 110% as active as the endogenous mouse beta maj-globin genes in six randomly chosen cell clones. Introduction of the viruses into human BFU-E cells, followed by analysis of marked and unmarked globin RNAs in differentiated erythroid colonies, revealed that the introduced beta-globin gene was about 5% as active as the endogenous genes in these normal human erythroid cells and that again the minigene was inactive. These data are discussed in terms of the potential treatment of genetic disease by gene therapy.     

A majority of mice show long-term expression of a human beta-globin gene after retrovirus transfer into hematopoietic stem cells.

Murine bone marrow was infected with a high-titer retrovirus vector containing the human beta-globin and neomycin phosphotransferase genes. Anemic W/Wv mice were transplanted with infected marrow which in some cases had been exposed to the selective agent G418. Human beta-globin expression was monitored in transplanted animals by using a monoclonal antibody specific for human beta-globin polypeptide, and hematopoietic reconstitution was monitored by using donor and recipient mice which differed in hemoglobin type. In some experiments all transplanted mice expressed the human beta-globin polypeptide for over 4 months, and up to 50% of peripheral erythrocytes contained detectable levels of polypeptide. DNA analysis of transplanted animals revealed that virtually every myeloid cell contained a provirus. Integration site analysis and reconstitution of secondary marrow recipients suggested that every mouse was reconstituted with at least one infected stem cell which had extensive repopulation capability. The ability to consistently transfer an active beta-globin gene into mouse hematopoietic cells improves the feasibility of using these techniques for somatic cell gene therapy in humans.     

Analysis of adeno-associated virus-mediated ex vivo transferred human β-globin gene in bone marrow engrafted mice

Adeno-associated virus (AAV)-2 was developed as a useful vector for human gene therapy. In this report, we analyzed the integration and expression of AAV-mediated ex vivo transferred human beta-globin gene in bone marrow (BM) reconstituted mice. Recombinant AAV (rAAV) containing human beta-globin gene was packaged by infecting individual G418-resistant BHK-21 cell clones integrated with the plasmid AV53HS432Deltabeta2.0Neo with recombinant herpes simplex virus, which can express rep and cap genes of wild-type AAV. The titer of rAAV was determined using slot blot hybridization with a result of 10(13) virus particles/ml (genome copy number). Low-density mononuclear cells were isolated from fetal livers of embryos from pregnant C57BL/6 mice at 14-16 days of gestation and were infected with rAAV. The transduced hematopoietic cells were then reinfused into lethally irradiated C57BL/6 recipient mice via tail vein injection. To analyze the provirus in short-term and long-term BM reconstituted mice, PCR/Southern blot and RT-PCR were performed to identify the integrity of the provirus and to detect the expression of human beta-globin gene, respectively. Genomic DNA was extracted from spleen nodules of BM reconstituted mice 12 days after transplantation. Human beta-globin gene was detected in 1 out of 6 nodules using PCR combined with Southern blot. Human beta-globin gene was also detected in the BM and thymus of mouse Y6161, in the thymus and spleen of mouse Y6162 and in the BM of mice Y6211 and Y6212. RT-PCR revealed low levels of expression of human beta-globin gene in the BM of mouse Y6211. Our results suggested that the efficiency of AAV-mediated human beta-globin gene integration into hematopoietic stem/progenitor cells was very low. It is necessary to perform further research on AAV biology before applying gene therapy that requires integration of a foreign gene into host chromosomes.     

Epstein-Barr virus shuttle vector for stable episomal replication of cDNA expression libraries in human cells.

Efficient transfection and expression of cDNA libraries in human cells has been achieved with an Epstein-Barr virus-based subcloning vector (EBO-pcD). The plasmid vector contains a resistance marker for hygromycin B to permit selection for transformed cells. The Epstein-Barr virus origin for plasmid replication (oriP) and the Epstein-Barr virus nuclear antigen gene have also been incorporated into the vector to ensure that the plasmids are maintained stably and extrachromosomally. Human lymphoblastoid cells can be stably transformed at high efficiency (10 to 15%) by such plasmids, thereby permitting the ready isolation of 10(6) to 10(7) independent transformants. Consequently, entire high-complexity EBO-pcD expression libraries can be introduced into these cells. Furthermore, since EBO-pcD plasmids are maintained as episomes at two to eight copies per cell, intact cDNA clones can be readily isolated from transformants and recovered by propagation in Escherichia coli. By using such vectors, human cells have been stably transformed with EBO-pcD-hprt to express hypoxanthine-guanine phosphoribosyltransferase and with EBO-pcD-Leu-2 to express the human T-cell surface marker Leu-2 (CD8). Reconstruction experiments with mixtures of EBO-pcD plasmids demonstrated that one clone of EBO-pcD-hprt per 10(6) total clones or one clone of EBO-pcD-Leu-2 per 2 x 10(4) total clones can be recovered intact from the transformed cells. The ability to directly select for expression of very rare EBO-pcD clones and to then recover these episomes should make it possible to clone certain genes where hybridization and immunological screening methods are not applicable but where a phenotype can be scored or selected in human cell lines.     

Analysis of the differentiation-promoting potential of inducible c-fos genes introduced into embryonal carcinoma cells.

To investigate the differentiation-promoting potential of c-fos in embryonal carcinoma cells (EC cells) we have designed various human metallothionein promoter-mouse-c-fos gene constructs containing also the selectable SV40 promoter-driven neo gene. Upon transfection into F9 EC cells and selection for neo resistance, the following results were obtained. (i) With each of the constructs, colonies of morphologically altered and differentiated (i.e., TROMA-1 and TROMA-3 expressing) cells were identified. (ii) Expression of c-fos was required to affect the differentiation state of F9 cells to a significant extent, but a low level was sufficient; no enhancement of differentiation was noticeable even after 100-fold induction of c-fos expression by cadmium. (iii) F9 cell clones were isolated which, in spite of very high levels of exogenous c-fos expression, had stem cell morphology. These cells, however, continuously generated morphologically altered and differentiated cells upon subculturing. (iv) In other EC cell lines, which resemble stem cells more closely than the 'partially differentiated' F9 cells, c-fos expression showed either a less pronounced (P19 cells) or no differentiation-promoting effect at all (PC13 cells). Our results suggest that the c-fos gene product acts in concert with other, probably 'spontaneously' occurring events to promote differentiation of certain EC cell lines.     

LCR/MEL: a versatile system for high-level expression of heterologous proteins in erythroid cells.

We have used the human globin locus control region (LCR) to assemble an expression system capable of high-level, integration position-independent expression of heterologous genes and cDNAs in murine erythroleukaemia (MEL) cells. The cDNAs are inserted between the human beta-globin promoter and the second intron of the human beta-globin gene, and this expression cassette is then placed downstream of the LCR and transfected into MEL cells. The cDNAs are expressed at levels similar to those of the murine beta-globin in the induced MEL cells. Heterologous genomic sequences can also be expressed at similar levels when linked to to the LCR and beta-globin promoter. In addition we demonstrate that, after induction of differentiation, MEL cells are capable of secreting heterologous proteins over a prolonged time period, making this system suitable for use in continuous production systems such as hollow fibre bioreactors. The utility of the LCR/MEL cell system is demonstrated by the expression of growth hormone at high levels (greater than 100 mg/l) 7 days after induction. Since the expression levels seen do not depend upon gene amplification and are independent of the integration position of the expression cassette, it is possible to obtain clones with stable high-level expression within 3-4 weeks after transfection.     

Differentiation-dependent expression of the chicken delta-crystallin gene introduced into mouse teratocarcinoma stem cells.

PCC3 mouse teratocarcinoma (TCC) stem cells were cotransfected with either the plasmid p delta C-1A or p delta C-1B carrying the chicken delta-crystallin gene, and with the plasmid pSV2gpt containing the selectable bacterial xanthine-guanine phosphoribosyltransferase (XGPRT) gene, using the calcium phosphate technique. Nine transformed PCC3 stem cell lines, each of which was clonally derived from respective colonies surviving after the selection process, were isolated. Southern blot analysis revealed that all of them stably maintained delta-crystallin sequences associated with high mol. wt. cellular DNA after propagation in non-selective medium in vitro, and after the production of solid tumors in the syngenic host mice. Six cell lines contain the intact delta-crystallin gene sequence and eight contain the gpt sequence. The number of delta-crystallin DNA copies was highly variable among transformed lines, 1-500 delta-crystallin genes per diploid mouse genome. No expression of the exogenous genes was detected in the transformed cells as long as they were in the undifferentiated state. However, the synthesis of delta-crystallin in certain types of cells was detected immunohistologically in three lines after the differentiation. The positive cell types were unique to each line, skeletal muscle in Y delta-9, certain columnar epithelia in Y delta-2, and unidentified spindle-shaped cells in Y delta-3. Authentic delta-crystallin polypeptides with a mol. wt. of 48 000 are synthesized upon differentiation of line Y delta-3 in solid tumors in syngenic mice.     

Stable transformation of mouse teratocarcinoma stem cells with the dominant selective marker Eco.gpt and retention of their developmental potentialities

Transformation of PCC4 mouse teratocarcinoma stem cells was obtained using a dominant selective marker, the enzyme xanthine-guanine phosphoribosyltransferase (XGPRT), coded by the bacterial Eco.gpt gene placed under the control of the early SV40 genes in the vector pSV2gpt. An average of 20 colonies of transformed cells was obtained, using the calcium phosphate technique, 10 microg DNA vector, no carrier DNA and 1 x 10(6) recipient cells. Five independent Eco.gpt-transformed PCC4 cell lines were propagated in selective medium and assayed for XGPRT activity. All of them had the ability to convert [14C]xanthine to xanthine monophosphate. pSV2gpt sequences were present and associated with high mol. wt. cellular DNA. pSV2gpt sequences and XGPRT activity were both conserved in the three clones that were propagated in non-selective medium for 30 generations. The transformed PCC4 cells retained their ability to produce, in host mice, teratocarcinoma tumors composed of embryonal carcinoma and various differentiated tissues. Thus, pSV2gpt can be used as a dominant marker to select teratocarcinoma stem cells co-transformed with genes that are not selectable by themselves.     

Mutagenesis of retroviral vectors transducing human beta-globin gene and beta-globin locus control region derivatives results in stable transmission of an active transcriptional structure.

Retrovirus-mediated gene transfer of the human beta-globin gene into hematopoietic stem cells is an attractive approach to the therapy of human beta-globin gene disorders. However, expression of the transduced beta-globin gene linked to its proximal cis-acting sequences (-0.8 to +0.3 kb from the cap site) is considerably below the level required for a significant therapeutic effect. The discovery of the beta-locus control region (beta-LCR), organized in four major DNase I hypersensitive sites far upstream of the human beta-like globin gene cluster, provided a potential means to achieve a high level of expression of a linked human beta-globin gene, but initial attempts to incorporate beta-LCR derivatives in retroviral vectors resulted in the production of low-titer viruses with multiple rearrangements of the transmitted proviral structures. We now describe how extensive mutagenesis of the transduced beta-globin gene, eliminating a 372 bp intronic segment and multiple reverse polyadenylation and splicing signals, increases viral titer significantly and restores stability of proviral transmission upon infection of cell lines and bone marrow-repopulating cells. These optimized vectors have enabled us to analyze the expression properties of various retrovirally transduced beta-LCR derivatives in dimethylsulfoxide-induced murine erythroleukemia cells and to achieve ratios of human beta-globin/murine beta maj-globin mRNA, on a per gene basis, as high as 80%.     

Collagen synthesis in mouse embryonal carcinoma cells: effect of retinoic acid

In five lines of mouse embryonal carcinoma cells, PCC3/A1, PCC4, PCC4/Aza-R1, and F9, collagen synthesis was examined by immunofluorescence reaction using specific antibodies directed against collagen. All the embryonal carcinoma cell lines showed type IV collagen, and PCC7-S/Aza-R1 revealed the additional presence of type III collagen. When the F9 and PCC3/A1 EC cells were treated with retinoic acid and dibutyryl-cAMP, they differentiated into morphologically different cellular types. These cellular types showed new types of collagen. Thus, in treated F9 cells, type I, type III, and type V collagen were detected and in treated PCC3/A1 cells, type III and type V collagen were detected. In two established cellular strains, PYS-2 corresponding to parietal endoderm and 3TDM-1 corresponding to trophoblastoma, collagen was identified by immunological reaction and electrophoretic mobility. The trophoblastoma cell line was characterized by the production of type I, type III, and type IV collagen, whereas endodermal PYS-2 revealed type IV collagen.