Selective synergy of immunoglobulin enhancer elements in B-cell development: a characteristic of kappa light chain enhancers, but not heavy chain enhancers

3D enhancer-promoter interactions and multi-connected hubs: Organizational principles and functional roles

The spatiotemporal control of gene expression is dependent on the activity of cis-acting regulatory sequences, called enhancers, which regulate target genes over variable genomic distances and, often, by skipping intermediate promoters, suggesting mechanisms that control enhancer-promoter communication. Recent genomics and imaging technologies have revealed highly complex enhancer-promoter interaction networks, whereas advanced functional studies have started interrogating the forces behind the physical and functional communication among multiple enhancers and promoters. In this review, we first summarize our current understanding of the factors involved in enhancer-promoter communication, with a particular focus on recent papers that have revealed new layers of complexities to old questions. In the second part of the review, we focus on a subset of highly connected enhancer-promoter "hubs" and discuss their potential functions in signal integration and gene regulation, as well as the putative factors that might determine their dynamics and assembly.

Building integrative functional maps of gene regulation

Every cell in the human body inherits a copy of the same genetic information. The three billion base pairs of DNA in the human genome, and the roughly 50 000 coding and non-coding genes they contain, must thus encode all the complexity of human development and cell and tissue type diversity. Differences in gene regulation, or the modulation of gene expression, enable individual cells to interpret the genome differently to carry out their specific functions. Here we discuss recent and ongoing efforts to build gene regulatory maps, which aim to characterize the regulatory roles of all sequences in a genome. Many researchers and consortia have identified such regulatory elements using functional assays and evolutionary analyses; we discuss the results, strengths and shortcomings of their approaches. We also discuss new techniques the field can leverage and emerging challenges it will face while striving to build gene regulatory maps of ever-increasing resolution and comprehensiveness.

Enhancing B-Cell Malignancies-On Repurposing Enhancer Activity towards Cancer

B-cell lymphomas and leukemias derive from B cells at various stages of maturation and are the 6th most common cancer-related cause of death. While the role of several oncogenes and tumor suppressors in the pathogenesis of B-cell neoplasms was established, recent research indicated the involvement of non-coding, regulatory sequences. Enhancers are DNA elements controlling gene expression in a cell type- and developmental stage-specific manner. They ensure proper differentiation and maturation of B cells, resulting in production of high affinity antibodies. However, the activity of enhancers can be redirected, setting B cells on the path towards cancer. In this review we discuss different mechanisms through which enhancers are exploited in malignant B cells, from the well-studied translocations juxtaposing oncogenes to immunoglobulin loci, through enhancer dysregulation by sequence variants and mutations, to enhancer hijacking by viruses. We also highlight the potential of therapeutic targeting of enhancers as a direction for future investigation.

Evidence for additive and synergistic action of mammalian enhancers during cell fate determination

Enhancer activity drives cell differentiation and cell fate determination, but it remains unclear how enhancers cooperate during these processes. Here we investigate enhancer cooperation during transdifferentiation of human leukemia B-cells to macrophages. Putative enhancers are established by binding of the pioneer factor C/EBPα followed by chromatin opening and enhancer RNA (eRNA) synthesis from H3K4-monomethylated regions. Using eRNA synthesis as a proxy for enhancer activity, we find that most putative enhancers cooperate in an additive way to regulate transcription of assigned target genes. However, transcription from 136 target genes depends exponentially on the summed activity of its putative paired enhancers, indicating that these enhancers cooperate synergistically. The target genes are cell type-specific, suggesting that enhancer synergy can contribute to cell fate determination. Enhancer synergy appears to depend on cell type-specific transcription factors, and such interacting enhancers are not predicted from occupancy or accessibility data that are used to detect superenhancers.

Regulatory sharing between estrogen receptor α bound enhancers

The human genome encodes an order of magnitude more gene expression enhancers than promoters, suggesting that most genes are regulated by the combined action of multiple enhancers. We have previously shown that neighboring estrogen-responsive enhancers exhibit complex synergistic contributions to the production of an estrogenic transcriptional response. Here we sought to determine the molecular underpinnings of this enhancer cooperativity. We generated genetic deletions of four estrogen receptor α (ER) bound enhancers that regulate two genes and found that enhancers containing full estrogen response element (ERE) motifs control ER binding at neighboring sites, while enhancers with pre-existing histone acetylation/accessibility confer a permissible chromatin environment to the neighboring enhancers. Genome engineering revealed that two enhancers with half EREs could not compensate for the lack of a full ERE site within the cluster. In contrast, two enhancers with full EREs produced a transcriptional response greater than the wild-type locus. By swapping genomic sequences, we found that the genomic location of a full ERE strongly influences enhancer activity. Our results lead to a model in which a full ERE is required for ER recruitment, but the presence of a pre-existing permissible chromatin environment can also be needed for estrogen-driven gene regulation to occur.

TATA box and Sp1 sites mediate the activation of c-myc promoter P1 by immunoglobulin kappa enhancers

In Burkitt's lymphoma (BL) cells the proto-oncogene c-myc is transcriptionally activated by chromosomal translocation to the immunoglobulin (Ig) gene loci. This activation is characterized by preferential transcription from the c-myc promoter P1 and accomplished by juxtaposed Ig enhancer elements. To identify promoter elements required for enhancer-activated P1 transcription, we studied the activation of c-myc reporter gene constructs by the Ig kappa intron and 3' enhancers. Deletion analysis defined the core promoter with a TATA box and two adjacent GC/GT boxes upstream sufficient for basal and enhancer-activated transcription. Gel retardation assays revealed Sp1's binding affinity to the GC/GT box proximal to the TATA box to be higher than to the distal one. This difference correlated well with the resulting levels of transcription mediated by Sp1 in contransfection experiments in BL and Sp1-deficient SL2 cells. Sp3 also bound to the core promoter in vitro, but failed to transactivate in vivo. Mutation of the distal Sp1 site moderately affected basal transcription concomitant with a modest decrease in enhancer stimulation. Mutation of the proximal Sp1 site almost entirely abolished basal as well as enhanced transcription. A considerable level of basal transcription was maintained upon mutation of the TATA box, whereas enhancer-activated transcription largely was abolished. Stable transfection of the BL cell line Raji with constructs containing core promoter mutations confirmed that the proximal Sp1 site and the TATA box are essential for the activation of promoter P1 by the Ig kappa enhancers.

Multiplex Enhancer Interference Reveals Collaborative Control of Gene Regulation by Estrogen Receptor α-Bound Enhancers

Multiple regulatory regions have the potential to regulate a single gene, yet how these elements combine to affect gene expression remains unclear. To uncover the combinatorial relationships between enhancers, we developed Enhancer-interference (Enhancer-i), a CRISPR interference-based approach that uses 2 different repressive domains, KRAB and SID, to prevent enhancer activation simultaneously at multiple regulatory regions. We applied Enhancer-i to promoter-distal estrogen receptor α binding sites (ERBS), which cluster around estradiol-responsive genes and therefore may collaborate to regulate gene expression. Targeting individual sites revealed predominant ERBS that are completely required for the transcriptional response, indicating a lack of redundancy. Simultaneous interference of different ERBS combinations identified supportive ERBS that contribute only when predominant sites are active. Using mathematical modeling, we find strong evidence for collaboration between predominant and supportive ERBS. Overall, our findings expose a complex functional hierarchy of enhancers, where multiple loci bound by the same transcription factor combine to fine-tune the expression of target genes.

Targeted overexpression of an activated N-ras gene results in B-cell and plasma cell lymphoproliferation and cooperates with c-myc to induce fatal B-cell neoplasia

Multiple myeloma is an incurable malignant expansion of plasma cells in the bone marrow. Although there is no pathognomonic genetic lesion among multiple myeloma patients, activation of the ras gene has been identified as a common mutation. We have previously described the use of the 3' κ immunoglobulin light chain enhancer (3'KE) to target transgenic expression in murine B and plasma cells, resulting in bcl-X(L) and c-myc-driven murine models of multiple myeloma. In this report, we characterize the role of activated mutant N-ras in B and plasma cells in transgenic mice. We constructed transgenic mice that use 3'KE to direct expression of a mutant activated N-ras. We also crossed the N-ras mice with mice bearing a c-myc transgene to study the cooperative effects of the transgenic constructs. Mice were sacrificed when moribund or at specific time intervals and characterized by serology, light microscopy, and flow cytometry. The transgenic N-ras animals develop B- and plasma cell lymphoproliferation, and aged mice develop immunoglobulinemia, renal hyaline tubular casts, and microscopic foci of abnormal plasma cells in extramedullary sites, including the liver and kidney. Bitransgenic 3'KE/N-Ras V12 × Eμ-c-Myc mice develop fatal B-cell neoplasia, with a median survival of 10 weeks. These data indicate that activated N-ras can play a role in B- and plasma cell homeostasis and that activated N-Ras and c-Myc can cooperate to induce B-cell neoplasia.

Systematic targeted integration to study Albumin gene control elements

To study transcriptional regulation by distant enhancers, we devised a system of easily modified reporter plasmids for integration into single-copy targeting cassettes in clones of HuH7, a human hepatocellular carcinoma. The plasmid constructs tested transcriptional function of a 35-kb region that contained the rat albumin gene and its upstream flanking region. Expression of integrants was analyzed in two orientations, and compared to transient expression of non-integrated plasmids. Enhancers were studied in their natural positions relative to the promoter and localized by deletion. All constructs were also analyzed by transient transfection assays. In addition to the known albumin gene enhancer (E1 at -10 kb), we demonstrated two new enhancers, E2 at -13, and E4 at +1.2 kb. All three enhancers functioned in both transient assays and integrated constructs. However, chromosomal integration demonstrated several differences from transient expression. For example, analysis of E2 showed that enhancer function within the chromosome required a larger gene region than in transient assays. Another conserved region, E3 at -0.7 kb, functioned as an enhancer in transient assays but inhibited the function of E1 and E2 when chromosomally integrated. The enhancers did not show additive or synergistic behavior,an effect consistent with competition for the promoter or inhibitory interactions among enhancers. Growth arrest by serum starvation strongly stimulated the function of some integrated enhancers, consistent with the expected disruption of enhancer-promoter looping during the cell cycle.

The IgH locus 3' regulatory region: pulling the strings from behind

Antigen receptor gene loci are among the most complex in mammals. The IgH locus, encoding the immunoglobulin heavy chain (IgH) in B-lineage cells, undergoes major transcription-dependent DNA remodeling events, namely V(D)J recombination, Ig class-switch recombination (CSR), and somatic hypermutation (SHM). Various cis-regulatory elements (encompassing promoters, enhancers, and chromatin insulators) recruit multiple nuclear factors in order to ensure IgH locus regulation by tightly orchestrated physical and/or functional interactions. Among major IgH cis-acting regions, the large 3' regulatory region (3'RR) located at the 3' boundary of the locus includes several enhancers and harbors an intriguing quasi-palindromic structure. In this review, we report progress insights made over the past decade in order to describe in more details the structure and functions of IgH 3'RRs in mouse and human. Generation of multiple cellular, transgenic and knock-out models helped out to decipher the function of the IgH 3' regulatory elements in the context of normal and pathologic B cells. Beside its interest in physiology, the challenge of elucidating the locus-wide cross talk between distant cis-regulatory elements might provide useful insights into the mechanisms that mediate oncogene deregulation after chromosomal translocations onto the IgH locus.

A Transgenic Mouse Model of Plasma Cell Malignancy Shows Phenotypic, Cytogenetic, and Gene Expression Heterogeneity Similar to Human Multiple Myeloma

Multiple myeloma is an incurable plasma cell malignancy for which existing animal models are limited. We have previously shown that the targeted expression of the transgenes c-Myc and Bcl-X(L) in murine plasma cells produces malignancy that displays features of human myeloma, such as localization of tumor cells to the bone marrow and lytic bone lesions. We have isolated and characterized in vitro cultures and adoptive transfers of tumors from Bcl-xl/Myc transgenic mice. Tumors have a plasmablastic morphology and variable expression of CD138, CD45, CD38, and CD19. Spectral karyotyping analysis of metaphase chromosomes from primary tumor cell cultures shows that the Bcl-xl/Myc tumors contain a variety of chromosomal abnormalities, including trisomies, translocations, and deletions. The most frequently aberrant chromosomes are 12 and 16. Three sites for recurring translocations were also identified on chromosomes 4D, 12F, and 16C. Gene expression profiling was used to identify differences in gene expression between tumor cells and normal plasma cells (NPC) and to cluster the tumors into two groups (tumor groups C and D), with distinct gene expression profiles. Four hundred and ninety-five genes were significantly different between both tumor groups and NPCs, whereas 124 genes were uniquely different from NPCs in tumor group C and 204 genes were uniquely different from NPCs in tumor group D. Similar to human myeloma, the cyclin D genes are differentially dysregulated in the mouse tumor groups. These data suggest the Bcl-xl/Myc tumors are similar to a subset of plasmablastic human myelomas and provide insight into the specific genes and pathways underlying the human disease.

Immunoglobulin kappa enhancers are differentially regulated at the level of chromatin structure

The kappa intronic and the kappa 3' enhancers synergize to regulate recombination and transcription of the Ig kappa locus. Although these enhancers have overlapping functions, the kappa i enhancer appears to predominate during receptor editing, while the kappa 3' enhancer may be more important for initiating Ig kappa germline transcription to target locus recombination and, later in development, somatic hypermutation. Changes in chromatin structure appear to regulate both enhancers, and previous reports suggest that both enhancers are packaged into an accessible chromatin structure only in B lineage cells. Why these enhancers cannot activate the demethylated, accessible, protein-associated Ig kappa allele in pro-B cells is not known. Furthermore, how the enhancers function to reactivate the locus for receptor editing or to quantitatively promote hypermutation in B cells is vague. Quantitative analysis of Ig enhancer chromatin structure in murine pro-, pre-and splenic B cells demonstrated that the kappa i enhancer maintains a highly accessible chromatin structure under a variety of conditions. This stable chromatin structure mirrored the highly accessible structure characterizing the Ig mu intronic enhancer, despite the fact that Ig mu is activated prior to Ig kappa during B cell development. Surprisingly, parallel analysis of the kappa 3' enhancer demonstrated its accessible chromatin structure is markedly unstable, as characterized by sensitivity to changes in environmental conditions. These data unexpectedly suggest that kappa locus regulation is compartmentalized along the gene in B lineage cells. Furthermore, these findings raise the possibility that environmentally dependent regulation of kappa 3' enhancer structure underlies changes in kappa activation during B cell development.

Targeted overexpression of Bcl-XL in B-lymphoid cells results in lymphoproliferative disease and plasma cell malignancies

Multiple myeloma is an incurable malignancy, and there is currently no mouse model that fully recapitulates the development and progression of the disease. We now describe a transgenic mouse that expresses a Bcl-XL transgene under the control of the 3'kappa immunoglobulin light chain enhancer, which is most active in murine B cells in late developmental stages. These mice developed nonmalignant plasma cell foci in the bone marrow and soft tissues and hyaline tubular casts in the kidneys. Median survival of the 3'KE/Bcl-XL mice was similar to littermate controls. When the 3'KE/Bcl-XL mouse was crossed to an Emu/c-Myc transgenic mouse, median survival of double transgenic progeny was 5.5 weeks. Peripheral blood and soft tissues were infiltrated with immature/mature B cells, and plasma cell lesions were identified in the bone marrow of all mice coexpressing Bcl-XL and c-Myc. These B- and plasma cell lesions demonstrated features consistent with malignancy. These results indicate that the 3'kappa immunoglobulin light chain enhancer can effectively target expression of Bcl-XL to B cells in late developmental stages, and they provide direct evidence that Bcl-XL can contribute to plasmacytomagenesis. Furthermore, this murine model serves as an important step in developing a novel genetically induced mouse model of plasma cell malignancies exhibiting bone marrow involvement.

Chromatin structural analyses of the mouse Igkappa gene locus reveal new hypersensitive sites specifying a transcriptional silencer and enhancer

To identify new regulatory elements within the mouse Igkappa locus, we have mapped DNase I hypersensitive sites (HSs) in the chromatin of B cell lines arrested at different stages of differentiation. We have focused on two regions encompassing 50 kilobases suspected to contain new regulatory elements based on our previous high level expression results with yeast artificial chromosome-based mouse Igkappa transgenes. This approach has revealed a cluster of HSs within the 18-kilobase intervening sequence, which we cloned and sequenced in its entirety, between the Vkappa gene closest to the Jkappa region. These HSs exhibit pro/pre-B cell-specific transcriptional silencing of a Vkappa gene promoter in transient transfection assays. We also identified a plasmacytoma cell-specific HS in the far downstream region of the locus, which in analogous transient transfection assays proved to be a powerful transcriptional enhancer. Deletional analyses reveal that for each element multiple DNA segments cooperate to achieve either silencing or enhancement. The enhancer sequence is conserved in the human Igkappa gene locus, including NF-kappaB and E-box sites that are important for the activity. In summary, our results pinpoint the locations of presumptive regulatory elements for future knockout studies to define their functional roles in the native locus.

Activation of c-myc promoter P1 by immunoglobulin kappa gene enhancers in Burkitt lymphoma: functional characterization of the intron enhancer motifs kappaB, E box 1 and E box 2, and of the 3' enhancer motif PU

Deregulated expression of the proto-oncogene c- myc in Burkitt lymphoma (BL) cells carrying a t(2;8) translocation is mediated by a synergistic interaction of the translocated immunoglobulin (Ig) kappa gene intron (kappaEi) and 3' (kappaE3') enhancers and characterized by a strong activation of the promoter P1. We have investigated the functional role of distinct kappa enhancer sequence motifs in P1 activation on both mini-chromosomes and reporter gene constructs. Stable and transient transfections of BL cells revealed critical roles of the kappaEi and kappaE3' elements kappaB and PU, respectively. Joint mutation of kappaB and PU completely abolished P1 activity, implying that an interaction of kappaB- and PU-binding factors is essential for the enhancer synergism. Mutation of the E box 1 and E box 2 motifs markedly decreased P1 activity in transient but not in stable transfection experiments. Co-expression of the NF-kappaB subunit p65(RelA) and Sp1, an essential factor for P1 transcription, in Drosophila melanogaster SL2 cells synergistically enhanced promoter activity. Our results support a model which proposes cross-talk between promoter and enhancer binding factors as the basic mechanism for kappa enhancer-mediated c- myc activation in BL cells.

Developmental regulation of the kappa locus involves both positive and negative sequence elements in the 3' enhancer that affect synergy with the intron enhancer

Expression of the mouse immunoglobulin kappa locus is regulated by the intron and 3' enhancers. Previously, we have reported that these enhancers can synergize at mature B cell stages. Here we present our recent studies on the identification and characterization of the 3' enhancer sequences that play important roles in this synergy. By performing mutational analyses with novel reporter constructs, we find that the 5' region of the cAMP response element (CRE), the PU. 1/PIP, and the E2A motifs of the 3' enhancer are critical for the synergy. These motifs are known to contribute to the enhancer activity. However, we also show that mutating other functionally important sequences has no significant effect on the synergy. Those sequences include the 3' region of the CRE motif, the BSAP motif, and the region 3' of the E2A motif. We have further demonstrated that either the 5'-CRE, the PU.1/PIP, or the E2A motif alone is sufficient to synergize with the intron enhancer. Moreover, the PU.1 motif appears to act as a negative element at pre-B cell stages but as a positive element at mature B cell stages. We have also identified a novel negative regulatory sequence within the 3' enhancer that contributes to the regulation of synergy, as well as developmental stage and tissue specificity of expression. While the levels of many of the 3' enhancer binding factors change very little in cell lines representing different B cell stages, the intron enhancer binding factors significantly increase at more mature B cell stages.

Transcription factor Pip can enhance DNA binding by E47, leading to transcriptional synergy involving multiple protein domains

The transcription factors E2A (E12/E47) and Pip are both required for normal B-cell development. Each protein binds to regulatory sequences within various immunoglobulin enhancer elements. Activity of E2A proteins can be regulated by interactions with other proteins which influence their DNA binding or activation potential. Similarly, Pip function can be influenced by interaction with the protein PU.1, which can recruit Pip to bind to DNA. We show here that a previously unidentified Pip binding site resides adjacent to the E2A binding site within the immunoglobulin kappa 3' enhancer. Both of these binding sites are crucial for high-level enhancer activity. We found that E47 and Pip can functionally interact to generate a very potent 100-fold transcriptional synergy. Through a series of mutagenesis experiments, we identified the Pip sequences necessary for transcriptional activation and for synergy with E47. Two synergy domains (residues 140 to 207 and 300 to 420) in addition to the Pip DNA binding domain (residues 1 to 134) are required for maximal synergy with E47. We also identified a Pip domain (residues 207 to 300) that appears to mask Pip transactivation potential. Part of the synergy mechanism between E47 and Pip appears to involve the ability of Pip to increase DNA binding by E47, perhaps by inducing a conformational change in the E47 protein. E47 may also induce a conformational change in Pip which unmasks sequences important for transcriptional activity. Based upon our results, we propose a model for E47-Pip transcriptional synergy.

Regulation of promoter and intron enhancer activity in immunoglobulin heavy-chain genes during B-cell differentiation

Chloramphenicol acetyltransferase (CAT) transgenic mice, in which the transgene is regulated by the VH promoter and heavy-chain intron enhancer (E mu), were examined to investigate the variation of activity of these cis-acting elements during the late stage of B-cell development. CAT enzyme activity decreased when resting B cells were stimulated through B-cell receptors (BCR) with goat anti-mouse IgM antibodies in vitro. On the other hand, when these B cells were stimulated by lipopolysaccharide (LPS) in vitro, they showed enhanced CAT activity, accompanied by an increase in the number of CD43+ B220+ cells (pro-plasma and plasma cells). In addition, the CAT activities in CD43+ B220+ and PNAhi B220+ cells from immunized mice were higher than those in CD43- B220+ and PNAlo B220+ cells, respectively. These results suggest that the activity of E mu in the context of VH promoter was transiently down-regulated by stimulation through the BCR but enhanced at the pro-plasma and plasma stages.

3′ IgH Enhancer Elements Shift Synergistic Interactions During B Cell Development

IgH gene expression is tightly controlled over the course of B cell development, B cell activation, and the subsequent differentiation of these cells into Ig-secreting plasmacytes. There are several transcriptional enhancers that map within and downstream of the IgH locus, and some of these have been clearly implicated in the developmental regulation of IgH gene assembly and expression. While some of the individual enhancers from this locus have been studied extensively, the functional interactions possible among this group of enhancers have been largely unexplored. In the present study, we have measured the transcriptional activities of combinations of enhancers introduced into B-lineage cell lines at several different developmental stages. We detected a developmental progression in which the 3′ enhancers are initially inactive, then become strongly active through synergistic interactions, and finally achieve a strong level of activity with little interdependency. The relative contributions of Eμ (the intron enhancer) and of the 3′ enhancers also change as a function of developmental stage. We discuss these results in light of parallel studies of developmental changes in transcription factor requirements.

Octamer independent activation of transcription from the kappa immunoglobulin germline promoter

Previous analyses of immunoglobulin V region promoters has led to the discovery of a common octamer motif which is functionally important in the tissue-specific and developmentally regulated transcriptional activation of immunoglobulin genes. The germline promoters (Ko) located upstream of the J region gene segments of the kappa locus also contain an octamer motif (containing a single base pair mutation and referred to as the variant octamer) which has been shown previously to bind Oct-1 and Oct-2 transcription factors in vitro. To further elucidate the role of this variant octamer motif in the regulation of germline transcription from the unrearranged kappa locus, we have quantitated the relative binding affinity of Oct-1 and Oct-2 for the variant octamer motif and determined the functional role of this octamer motif in transcriptional activation. We find that, although the variant octamer motif binds Oct-1 and Oct-2 in vitro with 5-fold lower affinity than the consensus octamer motif, mutation of the variant octamer motif to either a consensus octamer or non-octamer motif has no effect on transcriptional activation from the germline promoter. We also find significant differences in activation of germline and V region promoters by kappa enhancers. Our results suggest that the germline promoters and V region promoters differ in their dependence on octamer for activation and respond differently to enhancer activation. These findings have important implications in regulation of germline transcription as well as concomitant activation of the V-J recombination of the kappa light chain locus.

The Ig(kappa) enhancer influences the ratio of Ig(kappa) versus Ig(lambda) B lymphocytes

We generated mice harboring germline mutations in which the enhancer element located 9 kb 3' of the immunoglobulin kappa light chain gene (3'E kappa) was replaced either by a single loxP site (3'E kappa delta) or by a neomycin resistance gene (3'E kappa N). Mice homozygous for the 3'E(kappa delta) mutation had substantially reduced numbers of kappa-expressing B cells and increased numbers of lambda-expressing B cells accompanied by decreased kappa versus lambda gene rearrangement. In these mutant mice, kappa expression was reduced in resting B cells, but was normal in activated B cells. The homozygous 3'E(kappa)N mutation resulted in a similar but more pronounced phenotype. Both mutations acted in cis. These studies show that the 3'E(kappa) is critical for establishing the normal kappa/lambda ratio, but is not absolutely essential for kappa gene rearrangement or, surprisingly, for normal kappa expression in activated B cells. These studies also imply the existence of additional regulatory elements that have overlapping function with the 3'E(kappa) element.

Accessibility control of antigen-receptor variable-region gene assembly: role of cis-acting elements

Antigen receptor variable region genes are assembled from germline variable (V), diversity (D), and joining (J) gene segments. This process requires expression of V(D)J recombinase activity, and "accessibility" of variable gene segments to this recombinase. The exact mechanism by which variable gene segments become accessible during development is not known. However, several studies have shown that cis-acting elements that regulate transcription may also function to regulate accessibility. Here we review the evidence that transcriptional promoters, enhancers, and silencers are involved in regulation of accessibility. The manner in which these elements may combine to regulate accessibility is addressed. In addition, current and potential strategies for identifying and analyzing cis-acting elements that mediate locus accessibility are discussed.

Differential interactions between the immunoglobulin heavy chain mu intron and 3' enhancer

The immunoglobulin heavy chain 3' enhancer may be a novel type of a transcriptional regulation element in as much as its function is position dependent. We show that there are interactions between the mu intron and 3' enhancer which are differentially regulated depending on the distance between the two elements. Thus, a transcriptional repression is exerted by the 3' enhancer when juxtaposed to the intron enhancer. Whereas no or only modest synergism between the immunoglobulin mu intron and 3' enhancer has been reported to date, we show here that the stimulatory effect is substantially increased by extending the distance between the two enhancers. In our stable expression system, the mu intron enhancer insulated the test gene from neighboring chromatin.