Varied interactions between proviruses and adjacent host chromatin

A novel, immortal, and multipotent human neural stem cell line generating functional neurons and oligodendrocytes

The discovery and study of neural stem cells have revolutionized our understanding of the neurogenetic process, and their inherent ability to adopt expansive growth behavior in vitro is of paramount importance for the development of novel therapeutics based on neural cell replacement. Recent advances in high-throughput assays for drug development and gene discovery dictate the need for rapid, reproducible, long-term expansion of human neural stem cells (hNSCs). In this view, the complement of wild-type cell lines currently available is insufficient. Here we report the establishment of a stable human neural stem cell line (immortalized human NSCs [IhNSCs]) by v-myc-mediated immortalization of previously derived wild-type hNSCs. These cells demonstrate three- to fourfold faster proliferation than wild-type cells in response to growth factors but retain rather similar properties, including multipotentiality. By molecular biology, biochemistry, immunocytochemistry, fluorescence microscopy, and electrophysiology, we show that upon growth factor removal, IhNSCs completely downregulate v-myc expression, cease proliferation, and differentiate terminally into three major neural lineages: astrocytes, oligodendrocytes, and neurons. The latter are functional, mature cells displaying clear-cut morphological and physiological features of terminally differentiated neurons, encompassing mostly the GABAergic, glutamatergic, and cholinergic phenotypes. Finally, IhNSCs produce bona fide oligodendrocytes in fractions up to 20% of total cell number. This is in contrast to the negligible propensity of hNSCs to generate oligodendroglia reported so far. Thus, we describe an immortalized hNSC line endowed with the properties of normal hNSCs and suitable for developing the novel, reliable assays and reproducible high-throughput gene and drug screening that are essential in both diagnostics and cell therapy studies.

Kinetics of fluorescence expression in nonhuman primates transplanted with GFP retrovirus-modified CD34 cells

Downregulation and loss of proviral expression have been demonstrated to occur in a variety of in vitro studies and in mouse models. Here we evaluated the kinetics of proviral expression after transplantation in a competitive repopulating model in the baboon. Transgene persistence and green fluorescent protein (GFP) expression in peripheral blood leukocytes (PBL) were analyzed in four animals by semiquantitative PCR and flow cytometry for up to 80 weeks (range 17-80). All animals were transplanted with cells transduced with EGFP or EYFP reporters driven by Moloney murine leukemia virus (MoMuLV) or a modified promoter/enhancer, (MND) respectively. Simultaneous dual-color analysis of fluorescence levels in granulocyte and lymphocyte subsets following hematopoietic reconstitution demonstrated progressive loss of fluorescence intensity occurring predominantly early after transplant in cells transduced with both retrovirus backbones and at serial time points. In addition, we carried out PCR analysis of DNA extracted from sorted EGFP(-)/EYFP(-) cells and confirmed the presence of cells genetically marked by either vector in this population, indicating the persistence of cells that have downregulated or lost retroviral gene expression. In comparison to mouse studies, however, we did not detect substantial differences between MND and MoMuLV backbones.

Single cell analysis and selection of living retrovirus vector-corrected mucopolysaccharidosis VII cells using a fluorescence-activated cell sorting-based assay for mammalian beta-glucuronidase enzymatic activity

Mutations in the acid beta-glucuronidase gene lead to systemic accumulation of undegraded glycosaminoglycans in lysosomes and ultimately to clinical manifestations of mucopolysaccharidosis VII (Sly disease). Gene transfer by retrovirus vectors into murine mucopolysaccharidosis VII hematopoietic stem cells or fibroblasts ameliorates glycosaminoglycan accumulation in some affected tissues. The efficacy of gene therapy for mucopolysaccharidosis VII depends on the levels of beta-glucuronidase secreted by gene-corrected cells; therefore, enrichment of transduced cells expressing high levels of enzyme prior to transplantation is desirable. We describe the development of a fluorescence-activated cell sorter-based assay for the quantitative analysis of beta-glucuronidase activity in viable cells. Murine mucopolysaccharidosis VII cells transduced with a beta-glucuronidase retroviral vector can be isolated by cell sorting on the basis of beta-glucuronidase activity and cultured for further use. In vitro analysis revealed that sorted cells have elevated levels of beta-glucuronidase activity and secrete higher levels of cross-correcting enzyme than the population from which they were sorted. Transduced fibroblasts stably expressing beta-glucuronidase after subcutaneous passage in the mucopolysaccharidosis VII mouse can be isolated by cell sorting and expanded ex vivo. A relatively high percentage of these cells maintain stable expression after secondary transplantation, yielding significantly higher levels of enzymatic activity than that generated in the primary transplant.

The vagaries of variegating transgenes

Expression of transgenes in mice, when examined with assays that can distinguish individual cells, is often found to be heterocellular, or variegated. Line-to-line variations in expression of a transgene may be due largely to differences in the proportion of cells in which it is expressed. Variegated silencing by centromeric heterochromatin is well described, but other factors may also affect transgene silencing in mice. Tandem arrays of transgenes themselves form heterochromatin, and some cell lineages may tend to silence transgenes because of extensive facultative heterochromatin in their nuclei. The cis-acting transcriptional control elements within a transgene inhibit silencing, and strain-specific differences in chromatin proteins may strongly influence the extent of variegation. The accessibility of multiple differentiated cell lineages in mice suggests that they may provide a tool for dissecting the role of chromatin-mediated silencing in cell differentiation and tissue-specific gene expression.

A rapid PCR-based test for the endogenous viral element ev3 of chickens

A short fragment of chicken genomic DNA encompassing the insertion site of the endogenous avian leucosis viral element ev3 was isolated using the inverse polymerase chain reaction (inverse PCR) technique. The nucleotide sequence of the unoccupied site was used to design PCR primers that can be used to unambiguously determine the genetic status of any chicken, with respect to ev3. Screening of a small number of individuals from exotic breeds of chickens suggested that the frequency of ev3 is highly variable. The ev3 integration site shows a high degree of sequence homology with the macrophage-specific tyrosine kinase gene, bmk, in mice.

Structure and expression of endogenous retroviral sequences in the permanent LMH chicken cell line

From DNA mapping data, four endogenous proviral loci have been observed in the chicken permanent cell line LMH. The locus corresponding to endogenous virus (ev) ev1 is present in duplicate whereas the locus corresponding to ev3 is present in one copy. The other loci are probably ev6 and a solitary long terminal repeat. A RNA Northern blot analysis revealed both ev3 and ev6 transcripts but no ev1 transcript was detected. Using avian leukosis virus (ALV)-based vectors, transcomplementing assays were performed. They demonstrate the correct expression and maturation of endogenous env proteins and the absence of production of functional gag and pol components, indicating that these cells are not competent for viral production.

Lack of correlation between basal expression levels and susceptibility to transcriptional shutdown among single-gene murine leukemia virus vector proviruses

Integrated retroviruses or retroviral vectors may be transcriptionally inactive although their promoter-enhancer regions are able to direct transcription in the host cell. We have used single-gene retroviral vectors with a long terminal repeat-directed neo marker gene to determine if the level of transcription relates to the susceptibility of a provirus to inactivation. We used two isogenic vectors, carrying long terminal repeats with a strong and a weak transcriptional enhancer derived from SL3-3 and Akv murine leukemia viruses, respectively. Nonselected cell clones of the murine lymphoid cell line L691 with single integrated vector proviruses exhibiting a 20-fold range of initial expression levels were studied. The basal expression level of a given cell clone with a single provirus did not show any pattern of correlation with the long-term stability of expression, as monitored for periods up to 150 days. Our results thus indicate that the inactivation mechanism operates independently of the initial transcriptional activity of the provirus.

Insertional mutations in mammals and mammalian cells

The retroposon sequences, their mechanisms of transposition and the occurrence of insertional mutation in the mammalian genome are reviewed. Insertional mutations fall into two broad categories: those due to the disruption of a gene following the physical integration of a foreign DNA sequence result in loss of gene product and would be expected to be associated with a recessive mutation. A second class of insertional mutation is well documented in which upon integration the promoter/enhancer activities inherent in the retroposon genome exert their influence on neighboring genes. This promoter/enhancer activity of integrated retroposons may have effects over relatively long distances and thus limit the possibilities of establishing an association between retroposon integration and mutation. It is emphasized that a systematic search for insertional mutations in the mammalian genome involves an extensive two-dimensional array of possible retroposon sequences and mutant alleles. Present results represent only a small portion of the total array. Future studies promise to be fruitful in efforts to isolate genes through insertional tagging, to characterize the mechanisms of retroposon transposition, as well as to study the stability of the mammalian genome.

Chromatin structure as a molecular marker of cell lineage and developmental potential in neural crest-derived chromaffin cells

Adrenal medullary chromaffin cells have the capacity to transdifferentiate into sympathetic neurons. We show here that SCG10, a neural-specific gene that is induced during this transdifferentiation, is maintained in mature chromaffin cells in a potentially active chromatin conformation marked by two DNAase I hypersensitive sites (HSS). A low level of transcription is associated with this conformation. The HSS are also present in neurons expressing high levels of SCG10, but not in nonneuronal cells. Experiments using transgenic mice suggest that these HSS can in principle form in any cell type expressing the gene, but that a cis-repression mechanism normally prevents their assembly in nonneuronal cells. We suggest that the SCG10 HSS may represent a molecular marker of the lineage and phenotypic plasticity of chromaffin cells.

Effect of DNA damage on stable transformation of mammalian cells with integrative and episomal plasmids

The efficiency of stable transformation of human cells by integrative (non-replicating) plasmids carrying a selectable gene has been shown to be markedly enhanced by the introduction into the plasmid DNA of bulky damage, such as cyclobutane pyrimidine dimers or psoralen photoadducts. Enhanced transformation (ET) occurs in all human cells tested, including DNA repair-deficient cells from the hereditary syndrome xeroderma pigmentosum, but significantly less, if at all, in rodent cells. ET has been observed with a variety of integrative plasmid constructs, suggesting the generality of the phenomenon; as expected, ET is due to an increase in the number of cells carrying integrated plasmid sequences. In contrast to integrative plasmids, stable transformation by episomal (autonomously replicating) plasmids derived from the Epstein-Barr virus is only depressed by the introduction of photoproducts; furthermore, pronounced inactivation of transformation mediated by episomal plasmids becomes apparent in xeroderma pigmentosum cells. Altogether, these results suggest that DNA damage increases the probability of stable insertion of heterologous non-replicating DNA into human chromosomes. Moreover, the differential sensitivity to DNA damage of human cell transformation mediated by integrative versus episomal plasmids suggests caution in using such assay to measure host cell reactivation capacity; processing of DNA damage in mammalian cells might differ significantly between intra- versus extra-chromosomal DNA. Since ET may be induced by damage outside the selectable gene carried on integrative plasmids, we propose a model that involves local disruption of chromatin structure by helix-distorting DNA lesions flanking actively transcribed sequences; alternatively, reorganization of such altered DNA structure might be favored by the presence of topoisomerase-like activities in the proximity of active genes. Because ET can also be induced by DNA damage to the recipient cells, it is speculated that similar mechanism(s) might be involved in the generation of other types of non-homologous DNA recombination in damaged human chromosomes, including oncogenic cell transformation mediated by integrative DNA viruses.

Expression of the Epstein-Barr virus encoded EBNA-1 gene in stably transfected human and murine cell lines

Five murine and 3 human tumor cell lines were transfected with a retroviral vector that carries the EBV encoded EBNA-1 gene. All cell lines expressed intranuclear EBNA-1 as detected by anticomplement immunofluorescence and Western blot assays. The cell lines differed in the level of EBNA-1 expression and the size of the protein. The internal major late promoter of adenovirus was efficient in directing the transcription of EBNA-1 in the human lymphoma line BJAB, the murine T-cell lymphoma Tikaut, RBL-5, EL-4 and in the mouse sarcoma line MSWBS but was less efficient in Ramos, an EBV negative Burkitt lymphoma line, the human T-cell leukemia line 1301TK and the P815-X2 mouse mastocytoma line. All transfected lines except MSWBS contained EBNA-1 in a truncated form. The truncated EBNA-1 polypeptide reacted with the conventional human antibody reagents in an EBNA specific fashion but failed to bind rabbit or human antibody directed against the glycine-alanine repeat sequence. MSWBS contained a truncated as well as a full size EBNA-1 polypeptide. It also reacted with antibody directed against the glycine-alanine repeat. This indicates that the repeat sequence is regularly affected by the truncation.

Expression and rescue of a nonselected marker from an integrated AAV vector

We used rep+ and rep- recombinant AAV-plasmid vectors containing the nonselectable marker chloramphenicol acetyltransferase (CAT) driven by the AAV p40 promoter, and having a selectable marker, neo, inserted in the plasmid genome, and driven by a herpesvirus thymidine kinase gene promoter. Each vector was transfected into human 293 cells or HeLa cells and the neo gene was used to select geneticin-resistant (genr) cells containing integrated vectors. The genr cells were then screened for expression of the unselected marker CAT. For 293 cells, most clones from the rep- vector gave high CAT expression whereas only 50% of those from the rep+ vector expressed CAT, generally at low level. For HeLa cells about 25% of the clones derived from either the rep+ or rep- vector expressed CAT, and several clones from the rep+ vector gave very high yields. We also analyzed integrated rep+ vectors by rescue after superinfection with adenovirus and by Southern blotting. The AAV-CAT genome could be rescued from 50% of HeLa cell clones but not from 293 cell clones. Lack of rescuability reflected rearrangement of the AAV genome termini or the rep gene. Western blotting showed low level constitutive expression of rep protein in one 293 cell clone and two HeLa cell clones. Thus, the AAV p40 promoter (as well as p5 and p19) can function in integrated vectors to express unselected markers which can subsequently be rescued. Expression and rescue depended upon several parameters including the cell type, the initial structure of the vector (rep+ or rep-) but not continued expression of rep, and possibly global effects of the surrounding chromatin.

Expression of a single transfected cDNA converts fibroblasts to myoblasts

5-azacytidine treatment of mouse C3H10T1/2 embryonic fibroblasts converts them to myoblasts at a frequency suggesting alteration of one or only a few closely linked regulatory loci. Assuming such loci to be differentially expressed as poly(A)+ RNA in proliferating myoblasts, we prepared proliferating myoblast-specific, subtracted cDNA probes to screen a myocyte cDNA library. Based on a number of criteria, three cDNAs were selected and characterized. We show that expression of one of these cDNAs transfected into C3H10T1/2 fibroblasts, where it is not normally expressed, is sufficient to convert them to stable myoblasts. Myogenesis also occurs, but to a lesser extent, when this cDNA is expressed in a number of other cell lines. The major open reading frame encoded by this cDNA contains a short protein segment similar to a sequence present in the myc protein family.