Adjacent DNA elements dominantly restrict the ubiquitous activity of a novel chromatin-opening region to specific tissues

The 3'-Jα Region of the TCRα Locus Bears Gene Regulatory Activity in Thymic and Peripheral T Cells

Much progress has been made in understanding the important cis-mediated controls on mouse TCRα gene function, including identification of the Eα enhancer and TCRα locus control region (LCR). Nevertheless, previous data have suggested that other cis-regulatory elements may reside in the locus outside of the Eα/LCR. Based on prior findings, we hypothesized the existence of gene regulatory elements in a 3.9-kb region 5’ of the Cα exons. Using DNase hypersensitivity assays and TCRα BAC reporter transgenes in mice, we detected gene regulatory activity within this 3.9-kb region. This region is active in both thymic and peripheral T cells, and selectively affects upstream, but not downstream, gene expression. Together, these data indicate the existence of a novel cis-acting regulatory complex that contributes to TCRα transgene expression in vivo. The active chromatin sites we discovered within this region would remain in the locus after TCRα gene rearrangement, and thus may contribute to endogenous TCRα gene activity, particularly in peripheral T cells, where the Eα element has been found to be inactive.

Adapting in vitro embryonic stem cell differentiation to the study of locus control regions

Numerous locus control region (LCR) activities have been discovered in gene loci important to immune cell development and function. LCRs are a distinct class of cis-acting gene regulatory elements that appear to contain all the DNA sequence information required to establish an independently and predictably regulated gene expression program at any genomic site in native chromatin of a whole animal. As such, LCR-regulated transgenic reporter systems provide invaluable opportunities to investigate the mechanisms of gene regulatory DNA action during development. Furthermore the qualities of LCR-driven gene expression, including spatiotemporal specificity and "integration site-independence" would be highly desirable to incorporate into vectors used in therapeutic genetic engineering. Thus, advancement in the methods used to investigate LCRs is of considerable basic and translational significance. We study the LCR present in the mouse T cell receptor (TCR)-α gene locus. Until recently, transgenic mice provided the only experimental model capable of supporting the entire spectrum of LCR activities. We have recently reported complete manifestation of TCRα LCR function in T cells derived in vitro from mouse embryonic stem cells (ESC), thus validating a complete cell culture model for the full range of LCR activities seen in transgenic mice. Here we discuss the critical parameters involved in studying LCR-regulated gene expression during in vitro hematopoietic differentiation from ESCs. This advance provides an approach to speed progress in the LCR field, and facilitate the clinical application of its findings, particularly to the genetic engineering of T cells.

Complete TCR-α gene locus control region activity in T cells derived in vitro from embryonic stem cells

Locus control regions (LCRs) are cis-acting gene regulatory elements with the unique, integration site-independent ability to transfer the characteristics of their locus-of-origin's gene expression pattern to a linked transgene in mice. LCR activities have been discovered in numerous T cell lineage-expressed gene loci. These elements can be adapted to the design of stem cell gene therapy vectors that direct robust therapeutic gene expression to the T cell progeny of engineered stem cells. Currently, transgenic mice provide the only experimental approach that wholly supports all the critical aspects of LCR activity. In this study, we report the manifestation of all key features of mouse TCR-α gene LCR function in T cells derived in vitro from mouse embryonic stem cells. High-level, copy number-related TCR-α LCR-linked reporter gene expression levels are cell type restricted in this system, and upregulated during the expected stage transition of T cell development. We also report that de novo introduction of TCR-α LCR-linked transgenes into existing T cell lines yields incomplete LCR activity. These data indicate that establishing full TCR-α LCR activity requires critical molecular events occurring prior to final T lineage determination. This study also validates a novel, tractable, and more rapid approach for the study of LCR activity in T cells, and its translation to therapeutic genetic engineering.

A matter of packaging: influence of nucleosome positioning on heterologous gene expression

The organization of DNA into the various levels of chromatin compaction is the main obstacle that restricts the access of transcriptional machinery to genes. Genome-wide chromatin analyses have shown that there are common chromatin organization patterns for most genes but have also revealed important differences in nucleosome positioning throughout the genome. Such chromatin heterogeneity is one of the reasons why recombinant gene expression is highly dependent on integration sites. Different solutions have been tested for this problem, including artificial targeting of chromatin-modifying factors or the addition of DNA elements, which efficiently counteract the influence of the chromatin environment.An influence of the chromatin configuration of the recombinant gene itself on its transcriptional behavior has also been established. This view is especially important for heterologous genes since the general parameters of chromatin organization change from one species to another. The chromatin organization of bacterial DNA proves particularly dramatic when introduced into eukaryotes. The nucleosome positioning of recombinant genes is the result of the interaction between the machinery of the hosting cell and the sequences of both the recombinant genes and the promoter regions. We discuss the key aspects of this phenomenon from the heterologous gene expression perspective.

Ectopic T cell receptor-α locus control region activity in B cells is suppressed by direct linkage to two flanking genes at once

The molecular mechanisms regulating the activity of the TCRα gene are required for the production of the circulating T cell repertoire. Elements of the mouse TCRα locus control region (LCR) play a role in these processes. We previously reported that TCRα LCR DNA supports a gene expression pattern that mimics proper thymus-stage, TCRα gene-like developmental regulation. It also produces transcription of linked reporter genes in peripheral T cells. However, TCRα LCR-driven transgenes display ectopic transcription in B cells in multiple reporter gene systems. The reasons for this important deviation from the normal TCRα gene regulation pattern are unclear. In its natural locus, two genes flank the TCRα LCR, TCRα (upstream) and Dad1 (downstream). We investigated the significance of this gene arrangement to TCRα LCR activity by examining transgenic mice bearing a construct where the LCR was flanked by two separate reporter genes. Surprisingly, the presence of a second, distinct, reporter gene downstream of the LCR virtually eliminated the ectopic B cell expression of the upstream reporter observed in earlier studies. Downstream reporter gene activity was unaffected by the presence of a second gene upstream of the LCR. Our findings indicate that a gene arrangement in which the TCRα LCR is flanked by two distinct transcription units helps to restrict its activity, selectively, on its 5′-flanking gene, the natural TCRα gene position with respect to the LCR. Consistent with these findings, a TCRα/Dad1 locus bacterial artificial chromosome dual-reporter construct did not display the ectopic upstream (TCRα) reporter expression in B cells previously reported for single TCRα transgenes.

CTCF-Independent, but Not CTCF-Dependent, Elements Significantly Contribute to TCR-α Locus Control Region Activity

The mouse TCRalpha/TCRdelta/Dad1 gene locus bears a locus control region (LCR) that drives high-level, position-independent, thymic transgene expression in chromatin. It achieves this through DNA sequences that enhance transcription and protect transgene expression from integration site-dependent position effects. The former activity maps to a classical enhancer region (Ealpha). In contrast, the elements supporting the latter capacity that suppresses position effects are incompletely understood. Such elements likely play important roles in their native locus and may resemble insulator/boundary sequences. Insulators support enhancer blocking and/or chromatin barrier activity. Most vertebrate enhancer-blocking insulators are dependent on the CTCF transcription factor and its cognate DNA binding site. However, studies have also revealed CTCF-independent enhancer blocking and barrier insulator activity in the vertebrate genome. The TCRalpha LCR contains a CTCF-dependent and multiple CTCF-independent enhancer-blocking regions whose roles in LCR activity are unknown. Using randomly integrated reporter transgenes in mice, we find that the CTCF region plays a very minor role in LCR function. In contrast, we report the in vivo function of two additional downstream elements located in the region of the LCR that supports CTCF-independent enhancer-blocking activity in cell culture. Internal deletion of either of these elements significantly impairs LCR activity. These results reveal that the position-effect suppression region of the TCRalpha LCR harbors an array of CTCF-independent, positive-acting gene regulatory elements, some of which share characteristics with barrier-type insulators. These elements may help manage the separate regulatory programs of the TCRalpha and Dad1 genes.

The human desmin locus: gene organization and LCR-mediated transcriptional control

Locus control regions (LCRs) are defined by their ability to confer reproducible physiological levels of transgene expression in mice and therefore thought to possess the ability to generate dominantly a transcriptionally active chromatin structure. We report the first characterization of a muscle-cell-specific LCR, which is linked to the human desmin gene (DES). The DES LCR consists of five regions of muscle-specific DNase I hypersensitivity (HS) localized between -9 and -18 kb 5' of DES and reproducibly drives full physiological levels of expression in all muscle cell types. The DES LCR DNase I HS regions are highly conserved between humans and other mammals and can potentially bind a broad range of muscle-specific and ubiquitous transcription factors. Bioinformatics and direct molecular analysis show that the DES locus consists of three muscle-specific (DES) or muscle preferentially expressed genes (APEG1 and SPEG, the human orthologue of murine striated-muscle-specific serine/threonine protein kinase, Speg). The DES LCR may therefore regulate expression of SPEG and APEG1 as well as DES.

The TCRalpha locus control region specifies thymic, but not peripheral, patterns of TCRalpha gene expression

The molecular mechanisms ensuring the ordered expression of TCR genes are critical for proper T cell development. The mouse TCR alpha-chain gene locus contains a cis-acting locus control region (LCR) that has been shown to direct integration site-independent, lymphoid organ-specific expression of transgenes in vivo. However, the fine cell type specificity and developmental timing of TCRalpha LCR activity are both still unknown. To address these questions, we established a transgenic reporter model of TCRalpha LCR function that allows for analysis of LCR activity in individual cells by the use of flow cytometry. In this study we report the activation of TCRalpha LCR activity at the CD4-CD8-CD25-CD44- stage of thymocyte development that coincides with the onset of endogenous TCRalpha gene rearrangement and expression. Surprisingly, TCRalpha LCR activity appears to decrease in peripheral T cells where TCRalpha mRNA is normally up-regulated. Furthermore, LCR-linked transgene activity is evident in gammadelta T cells and B cells. These data show that the LCR has all the elements required to reliably reproduce a developmentally correct TCRalpha-like expression pattern during thymic development and unexpectedly indicate that separate gene regulatory mechanisms are acting on the TCRalpha gene in peripheral T cells to ensure its high level and fine cell type-specific expression.

Effect of deletion of the DNase I hypersensitive sites on the transcription of chicken Ig-beta gene and on the maintenance of active chromatin state in the Ig-beta locus

The role of DNase I hypersensitive sites (DHSs) in transcription of the B cell-specific Ig-beta gene and in maintenance of active chromatin state in the Ig-beta locus were examined. A total of 10 DHSs were divided into four regions, and each region was deleted separately in chicken B lymphocyte-derived DT40 cells. Deletion of three DHSs located between the Ig-beta promoter and its upstream Na channelgene, resulted in the absence of Ig-beta mRNA. Three regions except the region in the Na channel gene were involved in the transcription of Ig-beta gene. The enhancing activity of DHSs as determined by transient transfection assays did not always correlate with the effect of DHS deletion on the expression level of Ig-beta mRNA. In each deletion, cells contained the same DHSs as observed in the predeletion cells, indicating that deleted DHSs did not participate in the maintenance of DT40-specific DHSs. Enhanced acetylation of H3 and H4 histones at the Ig-beta promoter and at DT40-specific DHSs was observed in cells in which DHSs between the Na channel gene and Ig-beta promoter were deleted; therefore, these DHSs are prerequisite for transcription of the Ig-beta gene but not required for the maintenance of active chromatin state in the Ig-beta locus. Thus, epigenetic factors required for the maintenance of the active chromatin state are suggested to reside in other regions than those deleted in this study.

Enforcing order within a complex locus: current perspectives on the control of V(D)J recombination at the murine T-cell receptor alpha/delta locus

V(D)J recombination proceeds according to defined developmental programs at T-cell receptor (TCR) and immunoglobulin loci as a function of cell lineage and stage of differentiation. Although the molecular details are still lacking, such regulation is thought to occur at the level of accessibility of chromosomal recombination signal sequences to the recombinase. The unique and complex organization of the TCRalpha/delta locus poses intriguing regulatory challenges in this regard: embedded TCRalpha and TCRdelta gene segments rearrange at distinct stages of thymocyte development, there is a highly regulated progression of primary followed by secondary rearrangements involving Jalpha segments, and there are important developmental constraints on V gene segment usage. The locus therefore provides a fascinating laboratory in which to explore the basic mechanisms underlying developmental control. We provide here a current view of cis-acting mechanisms that enforce the TCRalpha/delta locus developmental program, and we emphasize the unresolved issues that command the attention of our and other laboratories.

Chromatin folding and gene expression: new tools to reveal the spatial organization of genes

An important aim in biology is to understand how gene expression is regulated in the context of chromatin. Much progress has been made towards cracking the 'histone code', which describes the composition and organization of chromatin at high resolution. At the lower resolution provided by microscopy, nuclear compartmentalization has been linked to the control of gene expression and silencing. I will review two new techniques able to reveal the three-dimensional organization of individual loci, providing a view of the folding of the chromatin fibre at an intermediate level of resolution. Carter and colleagues and Tolhuis and colleagues have used the new techniques to demonstrate direct physical contact between the locus control region (LCR) and expressed genes in the active murine beta-globin locus. The techniques will allow us to assess the role of locus organization when transcription is directed by distant regulatory elements. The new techniques (and their foreseeable descendants) will permit investigation of many genomic activities involving physical contact between separate regions of any genome. As such, they provide us with a new level of resolution at which to investigate the functional significance of chromatin organization as patterns of gene expression are initiated and modulated during development.

Factors Binding a Non-classical Cis-element Prevent Heterochromatin Effects on Locus Control Region Activity

A locus control region (LCR) is a cis-acting gene-regulatory element capable of transferring the expression characteristics of its gene locus of origin to a linked transgene. Furthermore, it can do this independently of the site of integration in the genome of transgenic mice. Although most LCRs contain subelements with classical transcriptional enhancer function, key aspects of LCR activity are supported by cis-acting sequences devoid of the ability to act as direct transcriptional enhancers. Very few of these "non-enhancer" LCR components have been characterized. Consequently, the sequence requirements and molecular bases for their functions, as well as their roles in LCR activity, are poorly understood. We have investigated these questions using the LCR from the mouse T cell receptor (TCR) alpha/Dad1 gene locus. Here we focus on DNase hypersensitive site (HS) 6 of the TCRalpha LCR. HS6 does not support classical enhancer activity, yet has gene regulatory activity in an in vivo chromatin context. We have identified three in vivo occupied factor-binding sites within HS6, two of which interact with Runx1 and Elf-1 factors. Deletion of these sites from the LCR impairs its activity in vivo. This mutation renders the transgene locus abnormally inaccessible in chromatin, preventing the normal function of other LCR subelements and reducing transgene mRNA levels. These data show these factor-binding sites are required for preventing heterochromatin formation and indicate that they function to maintain an active TCRalpha LCR assembly in vivo.

Ectopic T cell receptor expression causes B cell immunodeficiency in transgenic mice

Mice expressing transgenic T cell receptors (TCR) are used to explore important questions in immunity. However, transgene expression may have unexpected effects. We previously reported a B cell immunodeficiency, comprising decreased B cell numbers and diminished antibody responses, in mice that express a transgenic TCR specific for nicotinic acetylcholine receptor; the mice were generated using cassette vectors designed specifically for transgenic TCR expression [see Kouskoff et al. J. Immunol. Methods 1995. 180: 273-280]. We now show data suggesting that this defect is due to the expression and accumulation of TCR alpha and beta chains inside B cells and induction of an endoplasmic reticulum stress response, causing apoptosis at the pre B-I and later B cell stage. Thus, inappropriate transgene expression can profoundly affect B cells, leading to a previously undescribed mechanism of immunodeficiency.

Functional dissection of the mouse tyrosinase locus control region identifies a new putative boundary activity

Locus control regions (LCRs) are complex high-order chromatin structures harbouring several regulatory elements, including enhancers and boundaries. We have analysed the mouse tyrosinase LCR functions, in vitro, in cell lines and, in vivo, in transgenic mice and flies. The LCR-core (2.1 kb), located at -15 kb and carrying a previously described tissue-specific DNase I hypersensitive site, operates as a transcriptional enhancer that efficiently transactivates heterologous promoters in a cell-specific orientation-independent manner. Furthermore, we have investigated the boundary activity of these sequences in transgenic animals and cells. In mice, the LCR fragment (3.7 kb) rescued a weakly expressed reference construct that displays position effects. In Drosophila, the LCR fragment and its core insulated the expression of a white minigene reporter construct from chromosomal position effects. In cells, sequences located 5' from the LCR-core displayed putative boundary activities. We have obtained genomic sequences surrounding the LCR fragment and found a LINE1 repeated element at 5'. In B16 melanoma and L929 fibroblast mouse cells, this element was found heavily methylated, supporting the existence of putative boundary elements that could prevent the spreading of condensed chromatin from the LINE1 sequences into the LCR fragment, experimentally shown to be in an open chromatin structure.

Transgenes encompassing dual-promoter CpG islands from the human TBP and HNRPA2B1 loci are resistant to heterochromatin-mediated silencing

The genetic elements that are responsible for establishing a transcriptionally competent, open chromatin structure at a region of the genome that consists only of ubiquitously expressed, housekeeping genes are currently unknown. We demonstrate for the first time through functional analysis in stably transfected tissue culture cells that transgenes containing methylation-free CpG islands spanning the dual divergently transcribed promoters from the human TATA binding protein (TBP)-proteasome component-B1 (PSMB1) and heterogeneous nuclear ribonucleoprotein A2/B1 (HNRPA2B1)-heterochromatin protein 1Hs-gamma (chromobox homolog 3, CBX3) gene loci are sufficient to prevent transcriptional silencing and a variegated expression pattern when integrated within centromeric heterochromatin. In addition, only transgene constructs extending over both the HNRPA2B1 and the CBX3 promoters, and not the HNRPA2B1 promoter alone, were able to confer high and stable long-term EGFP reporter gene expression. These observations suggest that methylation-free CpG islands associated with dual, divergently transcribed promoters possess an independent dominant chromatin opening function and may therefore be major determinants in establishing and maintaining a region of open chromatin at housekeeping gene loci.

Sodium Butyrate Inhibits Cell Growth and Stimulates p21WAF1/CIP1 Protein in Human Colonic Adenocarcinoma Cells Independently of p53 Status

Butyric acid, one of the short-chain fatty acids produced by microbial fermentation in the colon, exhibits antiproliferative activities in various cancer cell lines. The initial objective of the study was to assess whether the effect of sodium butyrate (NaB) on cell growth differed by p53 status of the cells. Four human colorectal adenocarcinoma cell lines were used: HT29 (p53 point mutation), Caco2 (p53 truncation), LS513 (p53 wild type), and Lovo (p53 wild type). NaB significantly inhibited cell growth in all four cell lines. NaB arrested HT29 and LS513 cells in G0/G1 and Caco2 and Lovo in G2-phase. A second objective was to determine whether NaB similarly affected the cyclin-dependent kinase inhibitor, p21WAF1/CIP1. In all cell lines, p21 mRNA levels were immediately elevated after NaB exposure, and p21 protein levels were increased within 6 h. NaB increased p21 promoter activity in both Caco2 and Lovo, suggesting p53 independence. NaB did not influence p21 mRNA stability. Although three DNase I hypersensitivity sites were identified in the region of the p21 gene, induction of p21 mRNA by NaB was not accompanied by relaxation of the chromatin in the region of the p21 gene.

Locus control regions

Locus control regions (LCRs) are operationally defined by their ability to enhance the expression of linked genes to physiological levels in a tissue-specific and copy number-dependent manner at ectopic chromatin sites. Although their composition and locations relative to their cognate genes are different, LCRs have been described in a broad spectrum of mammalian gene systems, suggesting that they play an important role in the control of eukaryotic gene expression. The discovery of the LCR in the beta-globin locus and the characterization of LCRs in other loci reinforces the concept that developmental and cell lineage-specific regulation of gene expression relies not on gene-proximal elements such as promoters, enhancers, and silencers exclusively, but also on long-range interactions of various cis regulatory elements and dynamic chromatin alterations.

Distal and proximal cis-linked sequences are needed for the total expression phenotype of the mouse alcohol dehydrogenase 1 (Adh1) gene

Mouse alcohol dehydrogenase 1 (Adh1) gene expression occurs at high levels in liver and adrenal, moderate levels in kidney and intestine, low levels in a number of other tissues, and is undetectable in thymus, spleen and brain by Northern analysis. In transgenic mice, a minigene construct containing 10 kb of upstream and 1.5 kb of downstream flanking sequence directs expression in kidney, adrenal, lung, epididymis, ovary and skin but promotes ectopic expression in thymus and spleen while failing to control expression in liver, eye, intestine and seminal vesicle. Cosmids containing either 7 kb of upstream and 21 kb of downstream or 12 kb of upstream and 23 kb of downstream sequence flanking genetically marked Adh1 additionally promotes seminal vesicle expression suggesting downstream or intragenic sequence controls expression in this tissue. However, expression in liver, adrenal, or intestine is not promoted. The Adh1(a) allele on the cosmid expresses an enzyme electrophoretically distinct from that of the endogenous Adh1(b) allele, and presence of the heterodimeric enzyme in expressing tissues confirms that transgene activity occurs in the same cell-type as the endogenous gene. Transgene expression levels promoted by cosmids were at physiologically relevant amounts and exhibited greater copy-number dependence than observed with minigenes. Transgene mRNA expression correlated with expression measured at the enzyme level. A bacterial artificial chromosome containing 110 kb of 5'- and 104 kb of 3'-flanking sequence surrounding the Adh1 gene promoted expression in tissues at levels comparable to the endogenous gene most importantly including liver, adrenal and intestinal tissue where high level Adh1 expression occurs. Transgene expression in liver was in the same cell types as promoted by the endogenous gene. Although proximal elements extending 12 kb upstream and 23 kb downstream of the Adh1 gene promote expression at physiologically relevant levels in most tissues, more distal elements are additionally required to promote normal expression levels in liver, adrenal and intestinal tissue where Adh1 is most highly expressed.

Function and factor interactions of a locus control region element in the mouse T cell receptor-alpha/Dad1 gene locus

Locus control regions (LCRs) refer to cis-acting elements composed of several DNase I hypersensitive sites, which synergize to protect transgenes from integration-site dependent effects in a tissue-specific manner. LCRs have been identified in many immunologically important gene loci, including one between the TCRdelta/TCRalpha gene segments and the ubiquitously expressed Dad1 gene. Expression of a transgene under the control of all the LCR elements is T cell specific. However, a subfragment of this LCR is functional in a wide variety of tissues. How a ubiquitously active element can participate in tissue-restricted LCR activity is not clear. In this study, we localize the ubiquitously active sequences of the TCR-alpha LCR to an 800-bp region containing a prominent DNase hypersensitive site. In isolation, the activity in this region suppresses position effect transgene silencing in many tissues. A combination of in vivo footprint examination of this element in widely active transgene and EMSAs revealed tissue-unrestricted factor occupancy patterns and binding of several ubiquitously expressed transcription factors. In contrast, tissue-specific, differential protein occupancies at this element were observed in the endogenous locus or full-length LCR transgene. We identified tissue-restricted AML-1 and Elf-1 as proteins that potentially act via this element. These data demonstrate that a widely active LCR module can synergize with other LCR components to produce tissue-specific LCR activity through differential protein occupancy and function and provide evidence to support a role for this LCR module in the regulation of both TCR and Dad1 genes.

delta-Opioid receptor gene: effect of Sp1 factor on transcriptional regulation in vivo

delta-Opioid receptor (DOR) promoter exhibited a cell-type-specific expression pattern. Protein-DNA interactions in this promoter were identified by dimethyl sulfate in vivo footprinting analysis of NG108-15 cells, expressing endogenous DOR. Complete protection of the putative Sp1 cis-element and partial protection of the sequence defined as X-NotI in the basal promoter were observed only in the G0/G1 phase of the cell cycle. No protection was detected in Neuro2A cells that do not express DOR. In vivo formaldehyde cross-linking confirmed Sp1 factor binding to its cis-acting element during the G0/G1 phase. The functional significance of these Sp1 and X-NotI sites was evaluated by transient transfection analysis. Northern blot analysis and nuclear run-off assays revealed maximum DOR mRNA level and transcription rate, respectively, during the G0/G1 phase of NG108-15 cells. In summary, the protein-DNA interactions at the Sp1 and X-NotI sites are necessary for cell cycle-dependent and cell-type-specific up-regulated DOR gene expression.

Development of stable cell lines for production or regulated expression using matrix attachment regions

One of the major hurdles of isolating stable, inducible or constitutive high-level producer cell lines is the time-consuming selection procedure. Given the variation in the expression levels of the same construct in individual clones, hundreds of clones must be isolated and tested to identify one or more with the desired characteristics. Various boundary elements (BEs), matrix attachment regions, and locus control regions (LCRs) were screened for their ability to augment the expression of heterologous genes in Chinese hamster ovary (CHO) cells. Of the chromatin elements assayed, the chicken lysozyme matrix-attachment region (MAR) was the only element to significantly increase stable reporter expression. We found that the use of the MAR increases the proportion of high-producing clones, thus reducing the number of clones that need to be screened. These benefits are observed both for constructs with MARs flanking the transgene expression cassette, as well as when constructs are co-transfected with the MAR on a separate plasmid. Moreover, the MAR was co-transfected with a multicomponent regulatable beta-galactosidase expression system in C2C12 cells and several clones exhibiting regulated expression were identified. Hence, MARs are useful in the development of stable cell lines for production or regulated expression.

A new regulatory region of the IL-2 locus that confers position-independent transgene expression

Although the promoter/enhancer of the IL-2 gene mediates inducible reporter gene expression in vitro, it cannot drive consistent expression in transgenic mice. The location and existence of any regulatory elements that could open the IL-2 locus in vivo have remained unknown, preventing analysis of IL-2 regulation in developmental contexts. In this study, we report the identification of such a regulatory region, marked by novel DNase-hypersensitive sites upstream of the murine IL-2 promoter in unstimulated and stimulated T cells. Inclusion of most of these sites in an 8.4-kb IL-2 promoter green fluorescent protein transgene gives locus control region-like activity. Expression is efficient, tissue specific, and position independent. This transgene is expressed not only in peripheral T cells, but also in immature thymocytes and thymocytes undergoing positive selection, in agreement with endogenous IL-2 expression. In contrast, a 2-kb promoter green fluorescent protein transgene, lacking the new hypersensitive sites, is expressed in only a few founder lines, and expression is dysregulated in CD8(+) cells. Thus, the 6.4 kb of additional upstream IL-2 sequence contains regulatory elements that provide integration site independence and differential regulation of transgene expression in CD8 vs CD4 cells.

Characteristics of the adeno-associated virus preintegration site in human chromosome 19: open chromatin conformation and transcription-competent environment

Adeno-associated virus (AAV) establishes latency in infected cells by integrating into the cellular genome, with a high preference for a unique region, called AAVS1, of the human chromosome 19. The AAV proteins Rep78 and -68 are postulated to initiate the site-specific integration process by binding to a Rep binding site (RBS) in AAVS1. We provide further evidence to corroborate this model by demonstrating that the AAVS1 RBS in human cell lines is located near a DNase I hypersensitive "open" chromatin region and therefore is potentially easily accessible to Rep proteins. This open conformation is maintained in transgenic rats which carry an AAVS1 3. 5-kb DNA fragment and are proficient for Rep-mediated site-specific integration. Interestingly, the core of the DNAse I hypersensitive site in AAVS1 corresponds to a sequence displaying transcriptional enhancer-like properties, suggesting that AAVS1 constitutes a transcription-competent environment. The implications of our findings for AAV physiology and gene therapy are discussed.

Methylation of the cyclin A1 promoter correlates with gene silencing in somatic cell lines, while tissue-specific expression of cyclin A1 is methylation independent

Gene expression in mammalian organisms is regulated at multiple levels, including DNA accessibility for transcription factors and chromatin structure. Methylation of CpG dinucleotides is thought to be involved in imprinting and in the pathogenesis of cancer. However, the relevance of methylation for directing tissue-specific gene expression is highly controversial. The cyclin A1 gene is expressed in very few tissues, with high levels restricted to spermatogenesis and leukemic blasts. Here, we show that methylation of the CpG island of the human cyclin A1 promoter was correlated with nonexpression in cell lines, and the methyl-CpG binding protein MeCP2 suppressed transcription from the methylated cyclin A1 promoter. Repression could be relieved by trichostatin A. Silencing of a cyclin A1 promoter-enhanced green fluorescent protein (EGFP) transgene in stable transfected MG63 osteosarcoma cells was also closely associated with de novo promoter methylation. Cyclin A1 could be strongly induced in nonexpressing cell lines by trichostatin A but not by 5-aza-cytidine. The cyclin A1 promoter-EGFP construct directed tissue-specific expression in male germ cells of transgenic mice. Expression in the testes of these mice was independent of promoter methylation, and even strong promoter methylation did not suppress promoter activity. MeCP2 expression was notably absent in EGFP-expressing cells. Transcription from the transgenic cyclin A1 promoter was repressed in most organs outside the testis, even when the promoter was not methylated. These data show the association of methylation with silencing of the cyclin A1 gene in cancer cell lines. However, appropriate tissue-specific repression of the cyclin A1 promoter occurs independently of CpG methylation.

An upstream Oct-1- and Oct-2-binding silencer governs B29 (Ig beta) gene expression

The B cell-specific B29 (Igbeta) gene is activated in the earliest B cell precursors and is expressed throughout B cell development. Tissue-specific expression of the murine B29 gene is controlled by a B cell-specific promoter whose activity is governed by a cassette of upstream transcriptional silencers. This study describes a potent new silencer that is located 5' of the previously identified B29 silencer elements, FROG and TOAD. Like these known elements, the new B29 silencer is not restricted to the B29 promoter. Nuclear proteins from all cell lines tested interacted with this A+T-rich sequence, which closely resembled a noncanonical octamer binding motif and also conformed to the consensus sequence for nuclear matrix attachment regions. Interaction of Oct-1 and Oct-2 with the B29 A+T-rich sequence was confirmed using octamer-specific Abs. Oct-1/Oct-2 binding was required for the inhibitory activity of this sequence because mutations that blocked Oct-1/Oct-2 binding also eliminated inhibition of the B29 promoter. This B29 A+T-rich sequence specifically interacted with isolated nuclear matrix proteins in vitro, suggesting that it may also function as a matrix attachment region element. Maintenance of the level of B29 gene expression through the interaction of the minimal promoter and the upstream silencer elements FROG, TOAD, and the A+T-rich Oct-1/Oct-2 binding motif may be essential for normal B cell development and/or function.

Control of organ-specific demethylation by an element of the T-cell receptor-alpha locus control region

DNA methylation is important for mammalian development and the control of gene expression. Recent data suggest that DNA methylation causes chromatin closure and gene silencing. During development, tissue specifically expressed gene loci become selectively demethylated in the appropriate cell types by poorly understood processes. Locus control regions (LCRs), which are cis-acting elements providing stable, tissue-specific expression to linked transgenes in chromatin, may play a role in tissue-specific DNA demethylation. We studied the methylation status of the LCR for the mouse T-cell receptor alpha/delta locus using a novel assay for scanning large distances of DNA for methylation sites. Tissue-specific functions of this LCR depend largely on two DNase I-hypersensitive site clusters (HS), HS1 (T-cell receptor alpha enhancer) and HS1'. We report that these HS induce lymphoid organ-specific DNA demethylation in a region located 3.8 kilobases away with little effect on intervening, methylated DNA. This demethylation is impaired in mice with a germline deletion of the HS1/HS1' clusters. Using 5'-deletion mutants of a transgenic LCR reporter gene construct, we show that HS1' can act in the absence of HS1 to direct this tissue-specific DNA demethylation event. Thus, elements of an LCR can control tissue-specific DNA methylation patterns both in transgenes and inside its native locus.

Locus control regions: coming of age at a decade plus

The beta-globin locus control region (LCR) is the founding member of a novel class of cis-acting regulatory elements that confer high level, tissue-specific, site-of-integration-independent, copy number-dependent expression on linked transgenes located in ectopic chromatin sites. Knowledge from beta-globin and other LCR studies has shed light on our understanding of the long-range interaction between enhancers and promoters, the relationship between chromatin conformation and transcriptional regulation, and the developmental regulation of multiple gene loci. After over a decade of investigation and discovery, we take a retrospective look at the beta-globin LCR and other LCRs, summarize their properties and review models of LCR function.

Long-distance chromatin mechanisms controlling tissue-specific gene locus activation

Several different types of regulatory mechanisms contribute to the tissue- and development-specific regulation of a gene. It is now well established that, in addition to promoters, upstream cis-regulatory elements, which bind a variety of trans-acting factors, are essential for correct gene activation. In the last few years, however, it has become evident that the chromatin structure of eukaryotic genes is an important additional regulatory layer that is essential for correct gene expression during development. Chromatin is essentially a repressive environment for transcription factors; hence, much effort in recent years has been devoted to the elucidation of how these repressive forces are overcome during the process of gene locus activation. A particular interesting question in this context is: what are the molecular mechanisms by which extensive regions of chromatin, in many cases far outside the coding region, are reorganized during development? In this review, I summarize data from recent investigations that have uncovered a surprising variety of factors involved in this process.

A novel element upstream of the Vgamma2 gene in the murine T cell receptor gamma locus cooperates with the 3' enhancer to act as a locus control region

Transgenic expression constructs were employed to identify a cis-acting transcription element in the T cell receptor (TCR)-gamma locus, called HsA, between the Vgamma5 and Vgamma2 genes. In constructs lacking the previously defined enhancer (3'E(Cgamma1)), HsA supports transcription in mature but not immature T cells in a largely position-independent fashion. 3'E(Cgamma1), without HsA, supports transcription in immature and mature T cells but is subject to severe position effects. Together, the two elements support expression in immature and mature T cells in a copy number-dependent, position-independent fashion. Furthermore, HsA was necessary for consistent rearrangement of transgenic recombination substrates. These data suggest that HsA provides chromatin-opening activity and, together with 3'E(Cgamma1), constitutes a T cell-specific locus control region for the TCR-gamma locus.

In Vivo Overexpression of Dad1, the Defender Against Apoptotic Death-1, Enhances T Cell Proliferation But Does Not Protect Against Apoptosis

The Dad1 protein has been shown to play a role in prevention of apoptosis in certain cell types. Dad1 is also a subunit of the oligosaccharyltransferase enzyme complex that initiates N-linked glycosylation. It is encoded by a gene located adjacent to the TCR α and δ genes on mouse chromosome 14. We have investigated the role of Dad1 during T cell development and activation. We observe that endogenous Dad1 levels are modulated during T cell development to reach maximal expression in mature thymocytes. Transgenic mice that overexpress Dad1 in both the thymus and peripheral immune system have been generated. Apoptosis of thymocytes from such mice is largely unaffected, but peripheral T cells display hyperproliferation in response to stimuli. Therefore, the linkage between the TCR and Dad1 genes may have important consequences for T cell function.

Enhancer-Blocking Activity Within the DNase I Hypersensitive Site 2 to 6 Region Between the TCR α and Dad1 Genes

Although tightly linked, the TCR α and δ genes are expressed specifically in T lymphocytes, whereas the Dad1 gene is ubiquitously expressed. Between TCR α and Dad1 are eight DNase I hypersensitive sites (HS). HS1 colocalizes with the TCR α enhancer (Eα) and is T cell-specific; HS2, -3, -4, -5, and -6 map downstream of HS1 and are tissue-nonspecific. The region spanning HS2–6 was reported to display chromatin-opening activity and to confer copy number-dependent and integration site-independent transgene expression in transgenic mice. Here, we demonstrate that HS2–6 also displays enhancer-blocking activity, as it can block an enhancer from activating a promoter when located between the two in a chromatin-integrated context, and can do so without repressing either the enhancer or the promoter. Multiple enhancer-blocking elements are arrayed across HS2–6. We show that HS2–6 by itself does not activate transcription in chromatin context, but can synergize with an enhancer when located upstream of an enhancer and promoter. We propose that HS2–6 primarily functions as an insulator or boundary element that may be critical for the autonomous regulation of the TCR α and Dad1 genes.

Association of the 5'HS4 sequence of the chicken beta-globin locus control region with human EF1 alpha gene promoter induces ubiquitous and high expression of human CD55 and CD59 cDNAs in transgenic rabbits

Whatever its field of application, animal transgenesis aims at a high level of reproducible and stable transgene expression. In the case of xenotransplantation, prevention of hyperacute rejection of grafts of animal origin requires the use of organs expressing human inhibitors of complement activation such as CD55 (DAF) and CD59. Pigs transgenic for these molecules have been produced, but with low and variable levels of expression. In order to improve cDNA expression, a vector containing the 5'HS4 region from the LCR of the chicken beta-globin locus and the promoter and the first intron from the human EF1 alpha gene, was used to co-express human CD55 and CD59 cDNAs in transgenic rabbits. The transgenic lines with the 5'HS4 region displayed dramatically enhanced CD55 and CD59 mRNA concentrations in brain, heart, kidney, liver, lung, muscle, spleen and aortic endothelial cells in comparison with the transgenic lines without the 5'HS4 region. In the absence of the 5'HS4 region, only some of the transgenic lines displayed specific mRNAs and at low levels. Human CD55 and CD59 proteins were detectable in mononuclear cells from transgenic rabbits although at a lower level than in human mononuclear cells. On the other hand, primary aortic endothelial cells from a bi-transgenic line were very efficiently protected in vitro against human complement-dependent lysis. Transgenic rabbits harbouring the two human inhibitors of complement activation, CD55 and CD59, can therefore be used as new models in xenotransplantation. Moreover, the vector containing the 5'HS4 region from the LCR of the chicken beta-globin locus seems appropriate not only for xenotransplantation but also for any other studies involving transgenic animals in which cDNAs have to be expressed at a high level in all cell types.

Conservation of sequence and structure flanking the mouse and human beta-globin loci: the beta-globin genes are embedded within an array of odorant receptor genes

In mouse and human, the beta-globin genes reside in a linear array that is associated with a positive regulatory element located 5' to the genes known as the locus control region (LCR). The sequences of the mouse and human beta-globin LCRs are homologous, indicating conservation of an essential function in beta-globin gene regulation. We have sequenced regions flanking the beta-globin locus in both mouse and human and found that homology associated with the LCR is more extensive than previously known, making up a conserved block of approximately 40 kb. In addition, we have identified DNaseI-hypersensitive sites within the newly sequenced regions in both mouse and human, and these structural features also are conserved. Finally, we have found that both mouse and human beta-globin loci are embedded within an array of odorant receptor genes that are expressed in olfactory epithelium, and we also identify an olfactory receptor gene located 3' of the beta-globin locus in chicken. The data demonstrate an evolutionarily conserved genomic organization for the beta-globin locus and suggest a possible role for the beta-globin LCR in control of expression of these odorant receptor genes and/or the presence of mechanisms to separate regulatory signals in different tissues.

Hypersensitive site 2 specifies a unique function within the human beta-globin locus control region to stimulate globin gene transcription

The human beta-globin locus control region (LCR) harbors both strong chromatin opening and enhancer activity when assayed in transgenic mice. To understand the contribution of individual DNase I hypersensitive sites (HS) to the function of the human beta-globin LCR, we have mutated the core elements within the context of a yeast artificial chromosome (YAC) carrying the entire locus and then analyzed the effect of these mutations on the formation of LCR HS elements and expression of the genes in transgenic mice. In the present study, we examined the consequences of two different HS2 mutations. We first generated seven YAC transgenic lines bearing a deletion of the 375-bp core enhancer of HS2. Single-copy HS2 deletion mutants exhibited severely depressed HS site formation and expression of all of the human beta-globin genes at every developmental stage, confirming that HS2 is a vital, integral component of the LCR. We also analyzed four transgenic lines in which the core element of HS2 was replaced by that of HS3 and found that while HS3 is able to restore the chromatin-opening activity of the LCR, it is not able to functionally replace HS2 in mediating high-level globin gene transcription. These results continue to support the hypothesis that HS2, HS3, and HS4 act as a single, integral unit to regulate human globin gene transcription as a holocomplex, but they can also be interpreted to say that formation of a DNase I hypersensitive holocomplex alone is not sufficient for mediating high-level globin gene transcription. We therefore propose that the core elements must productively interact with one another to generate a unique subdomain within the nucleoprotein holocomplex that interacts in a stage-specific manner with individual globin gene promoters.

A new element within the T-cell receptor alpha locus required for tissue-specific locus control region activity

Locus control regions (LCRs) are cis-acting regulatory elements thought to provide a tissue-specific open chromatin domain for genes to which they are linked. The gene for T-cell receptor alpha chain (TCRalpha) is exclusively expressed in T cells, and the chromatin at its locus displays differentially open configurations in expressing and nonexpressing tissues. Mouse TCRalpha exists in a complex locus containing three differentially regulated genes. We previously described an LCR in this locus that confers T-lineage-specific expression upon linked transgenes. The 3' portion of this LCR contains an unrestricted chromatin opening activity while the 5' portion contains elements restricting this activity to T cells. This tissue-specificity region contains four known DNase I hypersensitive sites, two located near transcriptional silencers, one at the TCRalpha enhancer, and another located 3' of the enhancer in a 1-kb region of unknown function. Analysis of this region using transgenic mice reveals that the silencer regions contribute negligibly to LCR activity. While the enhancer is required for complete LCR function, its removal has surprisingly little effect on chromatin structure or expression outside the thymus. Rather, the region 3' of the enhancer appears responsible for the tissue-differential chromatin configurations observed at the TCRalpha locus. This region, herein termed the "HS1' element," also increases lymphoid transgene expression while suppressing ectopic transgene activity. Thus, this previously undescribed element is an integral part of the TCRalphaLCR, which influences tissue-specific chromatin structure and gene expression.

A Tailless Fas-FADD Death-Effector Domain Chimera Is Sufficient to Execute Fas Function in T Cells But Not B Cells of MRL-lpr/lpr Mice

The Fas receptor delivers signals crucial for lymphocyte apoptosis through its cytoplasmic death domain. Several Fas cytoplasmic-associated proteins have been reported and studied in cell lines. So far, only Fas-associated death domain protein (FADD), another death domain-containing molecule has been shown to be essential for Fas signals in vivo. FADD is thought to function by recruiting caspase-8 through its death-effector domain. To test whether FADD is sufficient to deliver Fas signals, we generated transgenic mice expressing a chimera comprised of the Fas extracellular domain and FADD death-effector domain. Expression of this protein in lymphocytes of Fas-deficient MRL-lpr/lpr mice completely diminishes their T cell but not their B cell abnormalities. These results suggest that FADD alone is sufficient for initiation of Fas signaling in primary T cells, but other pathways may operate in B cells.

Correct Function of the Locus Control Region May Require Passage Through a Nonerythroid Cellular Environment

The function of the β-globin locus control region (LCR) has been studied both in cell lines and in transgenic mice. We have previously shown that when a 248-kb β-locus YAC was first microinjected into L-cells and then transferred into MEL cells by fusion, the YAC loci of the LxMEL hybrids displayed normal expression and developmental regulation.To test whether direct transfer of a β-globin locus (β-YAC) into MEL cells could be used for studies of the function of the LCR, a 155-kb β-YAC that encompasses the entire β-globin locus was used. This YAC was retrofitted with a PGK-neo selectable marker and with two I-PpoI sites at the vector arm-cloned insert junctions, allowing detection of the intact globin loci on a single I-PpoI fragment by pulsed field gel electrophoresis (PFGE). ThePpo-155 β-YAC was used to directly lipofect MEL 585 cells. In 7 β-YAC MEL clones with at least one intact copy of the YAC, the levels of total human globin mRNA (ie, ɛ + γ + β) per copy of integrated β-YAC varied more than 97-fold between clones. These results indicated that globin gene expression was strongly influenced by the position of integration of the β-YAC into the MEL cell genome and suggested that the LCR cannot function properly when the locus is directly transferred into an erythroid cell environment as naked β-YAC DNA. To test whether passage of the β-YAC through L-cells before transfer into MEL cells was the reason for the previously observed correct developmental regulation of human globin genes in the LxMEL hybrid cells, we transfected the YAC into L-cells by lipofection. Three clones carried the intact 144-kb I-PpoI fragment and transcribed the human globin genes with a fetal-like pattern. Subsequent transfer of the YAC of these L(β-YAC) clones into MEL cells by fusion resulted in LxMEL hybrids that synthesized human globin mRNA. The variation in human β-globin mRNA (ie, ɛ + γ + β) levels between hybrids was 2.5-fold, indicating that globin gene expression was independent of position of integration of the transgene, as expected for normal LCR function. The correct function of the LCR when the YAC is first transferred into the L-cell environment raises the possibility that normal activation of the LCR requires interaction with the transcriptional environment of an uncommitted, nonerythroid cell. We propose that the activation of the LCR may represent a multistep process initiated by the binding of ubiquitous transcription factors early during the differentiation of hematopoietic stem cells and completed with the binding of erythroid type of factors in the committed erythroid progenitors.

Correct Function of the Locus Control Region May Require Passage Through a Nonerythroid Cellular Environment

The function of the β-globin locus control region (LCR) has been studied both in cell lines and in transgenic mice. We have previously shown that when a 248-kb β-locus YAC was first microinjected into L-cells and then transferred into MEL cells by fusion, the YAC loci of the LxMEL hybrids displayed normal expression and developmental regulation.To test whether direct transfer of a β-globin locus (β-YAC) into MEL cells could be used for studies of the function of the LCR, a 155-kb β-YAC that encompasses the entire β-globin locus was used. This YAC was retrofitted with a PGK-neo selectable marker and with two I-PpoI sites at the vector arm-cloned insert junctions, allowing detection of the intact globin loci on a single I-PpoI fragment by pulsed field gel electrophoresis (PFGE). ThePpo-155 β-YAC was used to directly lipofect MEL 585 cells. In 7 β-YAC MEL clones with at least one intact copy of the YAC, the levels of total human globin mRNA (ie, ɛ + γ + β) per copy of integrated β-YAC varied more than 97-fold between clones. These results indicated that globin gene expression was strongly influenced by the position of integration of the β-YAC into the MEL cell genome and suggested that the LCR cannot function properly when the locus is directly transferred into an erythroid cell environment as naked β-YAC DNA. To test whether passage of the β-YAC through L-cells before transfer into MEL cells was the reason for the previously observed correct developmental regulation of human globin genes in the LxMEL hybrid cells, we transfected the YAC into L-cells by lipofection. Three clones carried the intact 144-kb I-PpoI fragment and transcribed the human globin genes with a fetal-like pattern. Subsequent transfer of the YAC of these L(β-YAC) clones into MEL cells by fusion resulted in LxMEL hybrids that synthesized human globin mRNA. The variation in human β-globin mRNA (ie, ɛ + γ + β) levels between hybrids was 2.5-fold, indicating that globin gene expression was independent of position of integration of the transgene, as expected for normal LCR function. The correct function of the LCR when the YAC is first transferred into the L-cell environment raises the possibility that normal activation of the LCR requires interaction with the transcriptional environment of an uncommitted, nonerythroid cell. We propose that the activation of the LCR may represent a multistep process initiated by the binding of ubiquitous transcription factors early during the differentiation of hematopoietic stem cells and completed with the binding of erythroid type of factors in the committed erythroid progenitors.

Developmental regulation of V(D)J recombination at the TCR alpha/delta locus

The T-cell receptor (TCR) alpha/delta locus includes a large number of V, D, J and C gene segments that are used to produce functional TCR delta and TCR alpha chains expressed by distinct subsets of T lymphocytes. V(D)J recombination events within the locus are regulated as a function of developmental stage and cell lineage during T-lymphocyte differentiation in the thymus. The process of V(D)J recombination is regulated by cis-acting elements that modulate the accessibility of chromosomal substrates to the recombinase. Here we evaluate how the assembly of transcription factor complexes onto enhancers, promoters and other regulatory elements within the TCR alpha/delta locus imparts developmental control to VDJ delta and VJ alpha rearrangement events. Furthermore, we develop the notion that within a complex locus such as the TCR alpha/delta locus, highly localized and region-specific control is likely to require an interplay between positive regulatory elements and blocking or boundary elements that restrict the influence of the positive elements to defined regions of the locus.

The locus control region is necessary for gene expression in the human beta-globin locus but not the maintenance of an open chromatin structure in erythroid cells

Studies in many systems have led to the model that the human beta-globin locus control region (LCR) regulates the transcription, chromatin structure, and replication properties of the beta-globin locus. However the precise mechanisms of this regulation are unknown. We have developed strategies to use homologous recombination in a tissue culture system to examine how the LCR regulates the locus in its natural chromosomal environment. Our results show that when the functional components of the LCR, as defined by transfection and transgenic studies, are deleted from the endogenous beta-globin locus in an erythroid background, transcription of all beta-globin genes is abolished in every cell. However, formation of the remaining hypersensitive site(s) of the LCR and the presence of a DNase I-sensitive structure of the beta-globin locus are not affected by the deletion. In contrast, deletion of 5'HS5 of the LCR, which has been suggested to serve as an insulator, has only a minor effect on beta-globin transcription and does not influence the chromatin structure of the locus. These results show that the LCR as currently defined is not necessary to keep the locus in an "open" conformation in erythroid cells and that even in an erythroid environment an open locus is not sufficient to permit transcription of the beta-like globin genes.

Sequence comparisons and functional studies of the proximal promoter of the carbonic anhydrase 3 (CA3) gene

Carbonic anhydrase 3 (CA3) is a member of a gene family encoding proteins which catalyse the hydration of CO2 to generate protons and bicarbonate ions for cellular ion transport and pH homeostasis. In mouse embryos CA3 is expressed at high levels in notochord and skeletal muscle and here we demonstrate that this pattern of expression is the same in the developing human embryo. To investigate mechanisms controlling CA3 transcription, we have isolated and compared 2.8kb of sequence flanking exon 1 from the mouse and human genes. Several segments of high sequence identity >80% have been identified, the longest segments of which represent a proximal promoter region and a putative enhancer element. We have shown previously that in cultured cells the human 2.8kb promoter region imposes high level myogenic specific transcription of a reporter gene. However, we now show that while this promoter region directed muscle-specific expression in transgenic mouse embryos this was subject to position effects.

Locus control regions, chromatin activation and transcription

The past year has seen interesting advances in our understanding of the action of locus control regions. For the first time, the chromosomal distance was described in detail as a parameter in positive/negative regulation of transcription via gene competition. A number of publications have also described negative regulatory elements which restrict the action of locus control regions and other regulatory regions to specific genes and/or specific tissues. The emerging picture indicates that several very different types of negative regulation ensure that transcriptional activation occurs only in the appropriate cells.

Rescue of an MMTV transgene by co-integration reveals novel locus control properties of the ovine beta-lactoglobulin gene that confer locus commitment to heterogeneous tissues

In an attempt to enhance the frequency and level of expression of a poor-performing MMTV-driven transgene, we co-integrated this construct with the ovine beta-lactoglobulin (BLG) gene in transgenic mice. Seven lines of transgenic mice possessing co-integrated BLG and MMTV-RZ5 transgenes were compared with 12 lines of mice that possessed only the MMTV-RZ5 construct. Co-integration enhanced the frequency of expression in the mammary gland from two out of 12 lines for the MMTV-RZ5 transgene alone, to five out of seven when co-integrated with BLG. Surprisingly, co-integration also resulted in co-expression of the two transgenes in the salivary gland, lung and spleen in addition to the mammary gland. Furthermore, both transgenes were expressed in virgin animals, and throughout pregnancy and lactation, suggesting that the developmental regulation of the locus follows that of the MMTV-promoter. These findings represent a novel locus control property of the ovine BLG gene that confers commitment of the locus to the mammary gland, but also to a range of heterogeneous tissues possibly defined by the second promoter at the locus.