Activation of an enhancerless gene by chromosomal integration

Neuron specificity of the neurofilament light promoter in transgenic mice requires the presence of DNA unwinding elements

Three reporter genes, the chloramphenicol acetyltransferase (CAT), the lacZ, and the intronless NF-L DNA, were used to test the activity of the proximal promoter region (-292 bp) of the human neurofilament light (hNF-L) gene in transgenic mice. Surprisingly, the hNF-L/CAT construct was highly sensitive to position effect, and its expression was found at low levels in several tissues of adult transgenic mice (Beaudet, L., Charron, G., Houle, D., Tretjakoff, I. Peterson, A., and Julien, J.-P. (1992) Gene (Amst.) 116, 205-214). In contrast, the hNF-L/lacZ or the hNF-L/intronless constructs were expressed exclusively in the nervous system during embryonic development and in adult animals. The DNA sequences analysis of the different reporter genes revealed the presence of matrix attachment regions (MARs) within the 3'-untranslated regions of all three transgenes. DNA unwinding elements were found within the MARs of lacZ and hNF-L gene constructs but not in the CAT gene construct. When this element was removed from the lacZ construct, expression of the hNF-L/lacZ transgene became susceptible to position effect and was no longer tissue-specific. These results indicate that DNA unwinding elements are essential for position effect independence conferred by MARs to the hNF-L basal promoter.

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.

Entrapment Vectors: A New Tool for Mammalian Genetics

The use of mouse embryonic stem (ES) cells has facilitated the creation of mouse strains carrying desired genetic alterations. Recently, "entrapment" vectors have been developed to identify and mutate genes active during embryogenesis. These vectors in combination with ES cells may provide an efficient screening strategy to find genes that control mouse development.

A novel octamer binding transcription factor is differentially expressed in mouse embryonic cells

We have identified a novel octamer binding factor (Oct-3) in P19 embryonal carcinoma cells. Oct-3, which recognizes the typical octamer motif (ATTTGCAT) as well as the AT-rich sequence TTAAAATTCA, is present in P19 stem cells but disappears when the cells are induced to differentiate by retinoic acid (RA). Cloned cDNA corresponding to Oct-3 encodes a protein of 377 amino acids. Oct-3 has a conserved POU domain, but the remaining part is distinct from other POU domain-containing proteins such as Oct-1 and Oct-2. mRNA of 1.5 kb coding for Oct-3 is abundant in P19 stem cells but is dramatically repressed during RA-induced differentiation. Repression of the 1.5 kb mRNA is rapid and specific to RA. In mouse, oct-3 mRNA is undetectable in all the adult organs examined. The N-terminal proline-rich region of Oct-3, when fused to the DNA binding domain of c-Jun, functions as a transcriptional activating domain. We suggest that Oct-3 is a novel octamer binding transcription factor that is developmentally regulated during mouse embryogenesis.

Identification of cellular promoters by using a retrovirus promoter trap

A retrovirus vector has been developed that selects for instances in which the provirus integrates in close proximity to cellular promoters. Coding sequences for a selectable marker (histidinol dehydrogenase) were inserted into the 3' long terminal repeat (LTR) of an enhancerless Molony murine leukemia virus. Although 1.8 kilobase pairs in size, the elongated LTR did not appear to interfere with virus replication or integration. Thus, when the virus was passaged, the elongated LTRs efficiently duplicated, placing histidinol dehydrogenase-coding sequences in the 5' LTR just 30 nucleotides from the flanking cellular DNA. Selection for histidinol expression generated cell clones in which histidinol gene sequences in the 5' LTR were invariably expressed on transcripts initiating in nearby cellular sequences. The efficiency of transducing histidinol resistance was 2,500-fold lower than the efficiency of transducing neomycin resistance when the neomycin phosphotransferase gene was located within the retrovirus and expressed from an independent promoter. By tagging transcriptionally active sites, the vector provides a means to identify and isolate promoters active in different cell types. Furthermore, the virus may be useful as an insertional mutagen, since selection for cell populations containing proviruses only in expressed sites is expected to reduce the number of intergrants needed to screen for loss of gene function.

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.

Transgenes as probes for active chromosomal domains in mouse development

Embryonic development entails a well defined temporal and spatial programme of gene expression, which may be influenced by active chromosomal domains. These chromosomal domains can be detected using transgenes which integrate randomly throughout the genome, as their expression can be affected by chromosomal position. Position effects are probably exerted most strongly on transgenes that do not contain strong promoters, enhancers or other modulating sequences. Here we have systematically explored position effects using a transgene with the weak herpes-simplex-virus thymidine-kinase promoter, linked to the readily visualized lacZ indicator gene (HSV-TK-lacZ). Each transgenic fetus with detectable expression displayed a unique lacZ staining pattern. Thus expression of this construct is apparently dictated entirely by its chromosomal position, without any construct specificity. Furthermore the transgene is faithfully transmitted to subsequent generations, allowing for systematic mapping of changes in expression during development and in adult life. These results demonstrate that transgenes can indeed be powerful tools to probe the genome for active chromosomal regions, with the potential for identifying endogenous genes involved in organogenesis and pattern formation.

Ataxia telangiectasia resists gene cloning: an account of parameters determining gene transfer into human recipient cells

A subclone of an SV40-transformed fibroblast cell line from a patient with Ataxia telangiectasia (AT) with a relatively high rate of DNA uptake was isolated. However, more than 65000 independent genomic transfectants (using wild-type human DNA) did not contain the functional AT gene. This number represents the statistical distribution of an amount of DNA equivalent to more than three times the haploid human genome. The transfectants were screened by an X ray selection protocol that could rescue a single wild-type cell out of a population of 10(6) AT cells. This suggests a reversion frequency for AT of below 10(-8). The DNA uptake into human cells is compared with that into NIH3T3 cells and future possibilities for the isolation of human repair genes are discussed.