Extraembryonic expression of the human MHC class I gene HLA-G in transgenic mice. Evidence for a positive regulatory region located 1 kilobase 5' to the start site of transcription

Trophoblast, the only fetal tissue in direct contact with maternal cells, fails to express the polymorphic HLA class I molecules HLA-A and -B, but does express the nonpolymorphic class I molecule HLA-G. It is thought that HLA-G may provide some of the functions of a class I molecule without stimulating maternal immune rejection of the fetal semiallograft. As a first step in identifying the cis-acting DNA regulatory elements involved in the control of class I expression by extraembryonic tissue, several types of transgenic mice were produced. Two HLA-G genomic fragments were used, 5.7 and 6.0 kb in length. These included the entire HLA-G coding region, 1 kb of 3' flanking sequence, and 1.2 or 1.4 kb of 5' flanking sequence, respectively. A hybrid transgene, HLA-A2/G, was produced by replacing the 5' flanking sequence, first exon, and early first intron of HLA-G with the corresponding elements of HLA-A. Comparison of transgene mRNA expression patterns seen in HLA-A2/G and HLA-G transgenic mice suggests that 5' flanking sequences are largely responsible for the differing patterns of expression typical of the classical class I and HLA-G genes. Studies comparing the extraembryonic HLA-G expression levels of founder embryos transgenic for either the 5.7- or 6.0-kb HLA-G transgene showed that the 6.0-kb transgene directed HLA-G expression far more efficiently than did the 5.7-kb HLA-G transgene, producing extraembryonic HLA-G mRNA levels similar to those seen in human extraembryonic tissues. The results of these studies suggest that the 250-bp fragment present at the extreme 5' end of the 6.0-kb HLA-G transgene and absent from the 5.7-kb HLA-G transgene contains an important positive regulatory element. This 250-bp fragment lies further upstream than any of the previously documented class I regulatory regions and may function as a locus control region.     

Distinct regulatory elements control muscle-specific, fiber-type-selective, and axially graded expression of a myosin light-chain gene in transgenic mice

The fast alkali myosin light chain 1f/3f (MLC1f/3f) gene is developmentally regulated, muscle specific, and preferentially expressed in fast-twitch fibers. A transgene containing an MLC1f promoter plus a downstream enhancer replicates this pattern of expression in transgenic mice. Unexpectedly, this transgene is also expressed in a striking (approximately 100-fold) rostrocaudal gradient in axial muscles (reviewed by J. R. Sanes, M. J. Donoghue, M. C. Wallace, and J. P. Merlie, Cold Spring Harbor Symp. Quant. Biol. 57:451-460, 1992). Here, we analyzed the expression of mutated transgenes to map sites necessary for muscle-specific, fiber-type-selective, and axially graded expression. We show that two E boxes (myogenic factor binding sites), a homeodomain (hox) protein binding site, and an MEF2 site, which are clustered in an approximately 170-bp core enhancer, are all necessary for maximal transgene activity in muscle but not for fiber-type- or position-dependent expression. A distinct region within the core enhancer promotes selective expression of the transgene in fast-twitch muscles. Sequences that flank the core enhancer are also necessary for high-level activity in transgenic mice but have little influence on activity in transfected cells, suggesting the presence of regions resembling matrix attachment sites. Truncations of the MLC1f promoter affected position-dependent expression of the transgene, revealing distinct regions that repress transgene activity in neck muscles and promote differential expression among intercostal muscles. Thus, the whole-body gradient of expression displayed by the complete transgene may reflect the integrated activities of discrete elements that regulate expression in subsets of muscles. Finally, we show that transgene activity is not significantly affected by deletion or overexpression of the myoD gene, suggesting that intermuscular differences in myogenic factor levels do not affect patterns of transgene expression. Together, our results provide evidence for at least nine distinct sites that exert major effects on the levels and patterns of MLC1f expression in adult muscles.     

Purification and nucleic-acid-binding properties of a Saccharomyces cerevisiae protein involved in the control of ploidy

The mouse H19 gene is expressed exclusively from the maternal allele. The imprinted expression of the endogenous gene can be recapitulated in mice by using a 14-kb transgene encompassing 4 kb of 5'-flanking sequence, 8 kb of 3'-flanking sequence, which includes the two endoderm-specific enhancers, and an internally deleted structural gene. We have generated multiple transgenic lines with this 14-kb transgene and found that high-copy-number transgenes most closely follow the imprinted expression of the endogenous gene. To determine which sequences are important for imprinted expression, deletions were introduced into the transgene. Deletion of the 5' region, where a differentially methylated sequence proposed to be important in determining parental-specific expression is located, resulted in transgenes that were expressed and hypomethylated, regardless of parental origin. A 6-kb transgene, which contains most of the differentially methylated sequence but lacks the 8-kb 3' region, was not expressed and also not methylated. These results indicate that expression of either the H19 transgene or a 3' DNA sequence is key to establishing the differential methylation pattern observed at the endogenous locus. Finally, methylation analysis of transgenic sperm DNA from the lines that are not imprinted reveals that the transgenes are not capable of establishing and maintaining the paternal methylation pattern observed for imprinted transgenes and the endogenous paternal allele. Thus, the imprinting of the H19 gene requires a complex set of elements including the region of differential methylation and the 3'-flanking sequence.     

Early myeloid cell-specific expression of the human cathepsin G gene in transgenic mice

The human cathepsin G (CG) gene is expressed only in promyelocytes and encodes a neutral serine protease that is packaged in the azurophil (primary) granules of myeloid cells. To define the cis-acting DNA elements that are responsible for promyelocyte-specific "targeting," we injected a 6-kb transgene containing the entire human CG gene, including coding sequences contained in a 2.7-kb region, approximately 2.5 kb of 5' flanking sequence, and approximately 0.8 kb of 3' flanking sequence. Seven of seven "transient transgenic" murine embryos revealed human CG expression in the fetal livers at embryonic day 15. Stable transgenic founder lines were created with the same 6-kb fragment; four of five founder lines expressed human CG in the bone marrow. The level of human CG expression was relatively low per gene copy when compared with the endogenous murine CG gene, and expression was integration-site dependent; however, the level of gene expression correlated roughly with gene copy number. The human CG transgene and the endogenous murine CG gene were coordinately expressed in the bone marrow and the spleen. Immunohistochemical analysis of transgenic bone marrow revealed that the human CG protein was expressed exclusively in myeloid cells. Expression of human CG protein was highest in myeloid precursors and declined in mature myeloid cells. These data suggest that the human CG gene was appropriately targeted and developmentally regulated, demonstrating that the cis-acting DNA sequences required for the early myeloid cell-specific expression of human CG are present in this small genomic fragment.