Kappa immunoglobulin promoters and enhancers display developmentally controlled interactions

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.

A new hypersensitive site, HS10, and the enhancers, E3' and Ed, differentially regulate Igκ gene expression

The mouse Igκ gene locus has three known transcriptional enhancers: an intronic enhancer (Ei), a 3' enhancer (E3'), and a further downstream enhancer (Ed). We previously discovered, using the chromosome conformation-capture technique, that Ei and E3' interact with a novel DNA sequence near the 3' end of the Igκ locus, specifically in B cells. In the present investigation, we examined the function of this far downstream element. The sequence is evolutionarily conserved and exhibits a plasmacytoma cell-specific DNase I-hypersensitive site in chromatin, henceforth termed HS10 in the locus. HS10 acts as a coactivator of E3' in transient transfection assays. Although HS10(-/-) mice exhibited normal patterns of B cell development, they were tested further along with E3'(-/-) and Ed(-/-) mice for their Igκ expression levels in plasma cells, as well as for both allelic and isotype exclusion in splenic B cells. HS10(-/-) and Ed(-/-), but not E3'(-/-), mice exhibited 2.5-fold lower levels of Igκ expression in antigenically challenged plasma cells. E3'(-/-) mice, but not HS10(-/-) mice, exhibited impaired IgL isotype and allelic exclusion in splenic B cells. We have suggestive results that Ed may also weakly participate in these processes. In addition, HS10(-/-) mice no longer exhibited regional chromosome interactions with E3', and they exhibited modestly reduced somatic hypermutation in the Jκ-Cκ intronic region in germinal center B cells from Peyer's patches. We conclude that the HS10, E3', and Ed differentially regulate Igκ gene dynamics.

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.

AID-dependent activation of a MYC transgene induces multiple myeloma in a conditional mouse model of post-germinal center malignancies

By misdirecting the activity of Activation-Induced Deaminase (AID) to a conditional MYC transgene, we have achieved sporadic, AID-dependent MYC activation in germinal center B cells of Vk*MYC mice. Whereas control C57BL/6 mice develop benign monoclonal gammopathy with age, all Vk*MYC mice progress to an indolent multiple myeloma associated with the biological and clinical features highly characteristic of the human disease. Furthermore, antigen-dependent myeloma could be induced by immunization with a T-dependent antigen. Consistent with these findings in mice, more frequent MYC rearrangements, elevated levels of MYC mRNA, and MYC target genes distinguish human patients with multiple myeloma from individuals with monoclonal gammopathy, implicating a causal role for MYC in the progression of monoclonal gammopathy to multiple myeloma.

Integration of embryonic and fetal skeletal myogenic programs at the myosin light chain 1f/3f locus

The genetic control of skeletal muscle differentiation at the onset of myogenesis in the embryo is relatively well understood compared to the formation of muscle during the fetal period giving rise to the bulk of skeletal muscle fibers at birth. The Mlc1f/3f (Myl1) locus encodes two alkali myosin light chains, Mlc1f and Mlc3f, from two promoters that are differentially regulated during development. The Mlc1f promoter is active in embryonic, fetal and adult fast skeletal muscle whereas the Mlc3f promoter is upregulated during fetal development and remains on in adult fast skeletal muscle. Two enhancer elements have been identified at the mammalian Mlc1f/3f locus, a 3' element active at all developmental stages and an intronic enhancer activated during fetal development. Here, using transgenesis, we demonstrate that these enhancers act combinatorially to confer the spatial, temporal and quantitative expression profile of the endogenous Mlc3f promoter. Using double reporter transgenes we demonstrate that each enhancer can activate both Mlc1f and Mlc3f promoters in vivo, revealing enhancer sharing rather than exclusive enhancer-promoter interactions. Finally, we demonstrate that the fetal activated enhancer contains critical E-box myogenic regulatory factor binding sites and that enhancer activation is impaired in vivo in the absence of myogenin but not in the absence of innervation. Together our observations provide insights into the regulation of fetal myogenesis and the mechanisms by which temporally distinct genetic programs are integrated at a single locus.

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.

Novel targeted deregulation of c-Myc cooperates with Bcl-X(L) to cause plasma cell neoplasms in mice

Deregulated expression of both Myc and Bcl-X(L) are consistent features of human plasma cell neoplasms (PCNs). To investigate whether targeted expression of Myc and Bcl-X(L) in mouse plasma cells might lead to an improved model of human PCN, we generated Myc transgenics by inserting a single-copy histidine-tagged mouse Myc gene, Myc(His), into the mouse Ig heavy-chain Calpha locus. We also generated Bcl-X(L) transgenic mice that contain a multicopy Flag-tagged mouse Bcl-x(Flag) transgene driven by the mouse Ig kappa light-chain 3' enhancer. Single-transgenic Bcl-X(L) mice remained tumor free by 380 days of age, whereas single-transgenic Myc mice developed B cell tumors infrequently (4 of 43, 9.3%). In contrast, double-transgenic Myc/Bcl-X(L) mice developed plasma cell tumors with short onset (135 days on average) and full penetrance (100% tumor incidence). These tumors produced monoclonal Ig, infiltrated the bone marrow, and contained elevated amounts of Myc(His) and Bcl-X(L)(Flag) proteins compared with the plasma cells that accumulated in large numbers in young tumor-free Myc/Bcl-X(L) mice. Our findings demonstrate that the enforced expression of Myc and Bcl-X(L) by Ig enhancers with peak activity in plasma cells generates a mouse model of human PCN that recapitulates some features of human multiple myeloma.

Genetically targeted radiotherapy for multiple myeloma

Multiple myeloma is a disseminated neoplasm of terminally differentiated plasma cells that is incurable with currently available therapies. Although the disease is radiosensitive, external beam radiation leads to significant toxicity due to sensitive end-organ damage. Thus, genetic approaches for therapy are required. We hypothesized that the incorporation of immunoglobulin promoter and enhancer elements in a self-inactivating (SIN) lentiviral vector should lead to specific and high-level transgene expression in myeloma cells. A SIN lentivector with enhanced green fluorescent protein (EGFP) expression under the control of a minimal immunoglobulin promoter as well as the Kappa light chain intronic and 3' enhancers transduced myeloma cell lines with high efficiency (30%-90%). EGFP was expressed at a high level in myeloma cells but silent in all nonmyeloma cell lines tested compared with the cytomegalovirus (CMV) promoter/enhancer. Transduction of myeloma cells with the targeted vector coding for the human sodiumiodide symporter (hNIS) led to hNIS expression by these cells allowing them to concentrate radioiodine up to 18-fold compared with controls. Tumor xenografts in severe combined immunodeficiency mice expressing hNIS could be imaged using iodine-123 (123I) and shown to retain iodide for up to 48 hours. These tumor xenografts were completely eradicated by a single dose of the therapeutic isotope iodine-131 (131I) without evidence of recurrence up to 5 months after therapy. We conclude that lentivectors can be transcriptionally targeted for myeloma cells and the use of hNIS as a therapeutic gene for myeloma in combination with 131I needs further exploration.

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.

Transcriptional Regulation of the Igκ Gene by Promoter-Proximal Pausing of RNA Polymerase II

Transcriptional regulation can occur at the level of initiation and RNA elongation. We report that the rearranged, nontranscribed Igκ gene in the pre-B cell line 70Z/3 harbors a paused RNA polymerase II (pol II) at a position between 45 and 89 bp downstream of the transcription initiation site. LPS, an inducer of NF-κB, activated Igκ gene transcription by increasing the processivity of pol II. TGF-β inhibited the LPS-induced transcription of the Igκ gene, but not initiation and pausing of pol II. A rearranged copy of the Igκ gene was introduced into 70Z/3 cells using an episomal vector system. The episomal Igκ was regulated by LPS and TGF-β like the endogenous gene and established a paused pol II, whereas a construct with a deletion of the intron enhancer and the C region did not establish a paused pol II. Two distinct functions can therefore be assigned to the deleted DNA elements: loading of pol II to its pause site and induction of processive transcription upon LPS stimulation. It had been proposed that somatic hypermutation of Ig genes is connected to transcription. The pause site of pol II described in this work resides upstream of the previously defined 5′ boundary of mutator activity at Igκ genes. The possible role of pausing of pol II for somatic hypermutation is discussed.

Regulation of V(D)J recombination by transcriptional promoters

Enhancer elements potentiate the rearrangement of antigen receptor loci via changes in the accessibility of gene segment clusters to V(D)J recombinase. Here, we show that enhancer activity per se is insufficient to target T-cell receptor beta miniloci for DbetaJbeta recombination. Instead, a promoter situated 5' to Dbeta1 (PDbeta) was required for efficient rearrangement of chromosomal substrates. A critical function for promoters in regulating gene segment accessibility was further supported by the ability of heterologous promoters to direct rearrangement of enhancer-containing substrates. Importantly, activation of a synthetic tetracycline-inducible promoter (Ptet) positioned upstream from the Dbeta gene segment was sufficient to target recombination of miniloci lacking a distal enhancer element. The latter result suggests that DNA loops, generated by interactions between flanking promoter and enhancer elements, are not required for efficient recognition of chromosomal gene segments by V(D)J recombinase. Unexpectedly, the Ptet substrate exhibited normal levels of rearrangement despite its retention of a hypermethylated DNA status within the DbetaJbeta cluster. Together, our findings support a model in which promoter activation, rather than intrinsic properties of enhancers, is the primary determinant for regulating recombinational accessibility within antigen receptor loci.

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.

The immunoglobulin lambda light chain enhancer consists of three modules which synergize in activation of transcription

V(D)J rearrangement, high level expression and somatic hypermutation of assembled Ig genes is tightly controlled by a number of regulatory sequence elements located in the vicinity of the J-, (D)-, and C-gene segments. During B cell maturation these elements become accessible to binding of trans-acting factors, reflecting the opening of the chromatin structure through an as yet unidentified mechanism. The mapping of regions of an altered chromatin structure (DNase I hypersensitivity) therefore is a powerful approach in identifying regulatory sequence elements. We here show that the human Ig lambda enhancer consists of three modules previously identified by us as DNase I-hypersensitive sites HSS-1, -2, and -3. The three sequence elements synergize in transcriptional activation of a reporter gene and together constitute a powerful tissue-specific enhancer which is a much stronger transcriptional activator than the kappa enhancers alone or in combination. We further show that the accessibility of the kappa and lambda enhancer elements for DNase I in the chromatin of a pre-B cell line (207) correlates with the transcriptional enhancer activities of kappa and lambda enhancer constructs. This finding is in support of an ordered model for Ig light chain activation (kappa before lambda).

Decreased Frequency of Rearrangement due to the Synergistic Effect of Nucleotide Changes in the Heptamer and Nonamer of the Recombination Signal Sequence of the Vκ Gene A2b, Which Is Associated with Increased Susceptibility of Navajos to Haemophilus influenzae Type b Disease

Navajos and genetically related populations have a 10-fold increased incidence of Haemophilus influenzae type b (Hib) disease compared with control populations. The Vκ gene A2 is used to encode the majority of anti-Hib Abs, and these are the highest affinity anti-Hib Abs. Navajos carry a different allele of the A2 gene segment (A2b) that is defective in its ability to undergo V-J recombination. The A2b allele has only three nucleotide changes from the commonly occurring A2a allele, two of which could potentially affect its ability to recombine. In this study we used two independent in vitro assays to test whether the nucleotide change found in the A2b promoter and/or in the A2b recombination signal sequence (RSS) might be responsible for the decrease in recombination frequency observed in vivo. Using a luciferase reporter gene assay, we found no significant difference between A2a and A2b promoter activities. However, the competition recombination substrate assay showed a 4.5-fold reduction in the relative frequency of recombination of the A2b RSS compared with A2a. We show that this decreased frequency is due to a synergistic effect of the unique nucleotide change present in the heptamer of the A2b RSS and the shared nucleotide change present in the nonamer of both A2b and A2a. This in vitro relative frequency of rearrangement is not significantly different from that observed in vivo; therefore, the A2b RSS is probably the factor associated with the increased susceptibility to Hib disease among individuals carrying the A2b allele.

A Developmental Stage-Specific Promoter Directs Germline Transcription of DβJβ Gene Segments in Precursor T Lymphocytes

The tissue- and stage-specific assembly of Ag receptor genes is regulated by transcriptional control elements positioned within Ig and TCR loci. To further understand the role of cis-acting elements in these regulatory mechanisms, we have characterized a transcriptional promoter that drives germline expression of TCRβ gene segments in vivo. The activity of this promoter, termed PDβ, is restricted to a highly conserved 400-bp region located directly upstream from Dβ1-coding sequences. Maximal PDβ activity requires a TATA element situated within the Dβ1 recombination signal sequences and consensus binding sites for the ubiquitous SP1 and the T cell-specific GATA-3 transcription factors. When linked to active enhancer elements, PDβ directs transcription in most cell types; however, the TCRβ enhancer (Eβ) stimulates PDβ function specifically in precursor T lymphocytes. These findings suggest that PDβ/Eβ interactions may contribute to differential regulation of regions within the TCRβ locus during thymocyte development.

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.

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.

Coordinate transcription and V(D)J recombination of the kappa immunoglobulin light-chain locus: NF-kappaB-dependent and -independent pathways of activation

To further elucidate the potential role of mitogens and cytokines in regulation of the kappa immunoglobulin light-chain locus, we have characterized the activation of transcription factor binding, kappa germ line transcription, DNase I hypersensitivity, and Vkappa-to-Jkappa recombination upon induction of model pre-B-cell lines. We find that both lipopolysaccharide (LPS) and gamma interferon (IFN-gamma) are capable of activating germ line transcription, DNase I hypersensitivity, and recombination of the kappa locus. We also find that transforming growth factor beta is capable of completely inhibiting LPS activation of transcription and recombination but has no apparent effect on activation of transcription factor binding, including activation of NF-kappaB. To address the functional role of NF-kappaB in LPS and IFN-gamma induction of these events, we blocked the nuclear translocation of NF-kappaB by overexpression of a dominant negative mutant of IkappaB-alpha (IkappaB deltaN). Overexpression of the IkappaB deltaN protein results in an inhibition of LPS but not IFN-gamma activation of germ line transcription, DNase I hypersensitivity, and Vkappa-to-Jkappa recombination. Our results demonstrate that activation of NF-kappaB is necessary but not sufficient for LPS activation of transcription and recombination at kappa. These results also suggest that NF-kappaB is not required for IFN-gamma activation of transcription or recombination. These results are important in establishing that there are multiple independent pathways of activation of both transcription and recombination.

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.

A potential role for Elf-1 in terminal transferase gene regulation

The terminal deoxynucleotidyltransferase (TdT) gene represents an attractive model for the analysis of gene regulation during an early phase of lymphocyte development. In previous studies, we identified a DNA element, termed D', which is essential for TdT promoter activity in immature lymphocytes, and two classes of D'-binding factors, Ikaros proteins and Ets proteins. Here, we report a detailed mutant analysis of the D' element which suggests that an Ets protein, rather than an Ikaros protein, activates TdT transcription. Since multiple Ets proteins are expressed in developing lymphocytes and are capable of binding to the D' element, DNA affinity chromatography was used to determine if one of the Ets proteins might bind to the D' element with a uniquely high affinity, thereby implicating that protein as a potential TdT activator. Indeed, one binding activity was greatly enriched in the high-salt eluates from a D' affinity column. Peptide microsequencing revealed that the enriched protein was Elf-1. Immunoblot analyses confirmed that in nuclear extracts, Elf-1 has a significantly higher affinity for the D' sequence than does another Ets protein, Ets-1. Transactivation and expression studies support the hypothesis that Elf-1 activates TdT transcription in immature T and B cells. Finally, a D' mutation which selectively reduces Elf-1 binding, but not the binding of other Ets proteins, was found to greatly reduce TdT promoter activity. Although Elf-1 previously had been implicated in the inducible activation of genes in mature T and B cells, our results suggest that it also plays an important role in regulating genes during an early phase of lymphocyte development.

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.

A developmentally modulated chromatin structure at the mouse immunoglobulin kappa 3' enhancer

Transcription of the mouse immunoglobulin kappa gene is controlled by two enhancers: the intronic enhancer (Ei) that occurs between the joining (J kappa) and constant (C kappa) exons and the 3' enhancer (E3') located 8.5 kb downstream of the gene. To understand the role of E3' in the activation of the mouse immunoglobulin kappa gene, we studied its chromatin structure in cultured B-cell lines arrested at various stages of differentiation. We found that 120 bp of the enhancer's transcriptional core becomes DNase I hypersensitive early in B-cell development. Genomic footprinting of pro-B and pre-B cells localized this chromatin alteration to B-cell-specific protections at the region including the direct repeat (DR) and the sequence downstream of the DR (DS), the PU.1-NFEM-5 site, and the core's E-box motif, identifying bound transcription factors prior to kappa gene rearrangement. Early footprints were, however, not detected at downstream sites proposed to play a negative role in transcription. The early chromatin structure persisted through the mature B-cell stage but underwent a dramatic shift in plasma cells, correlating with the loss of guanosine protection within the DR-DS junction and the appearance of novel footprints at a GC-rich motif upstream and the NF-E1 (YY1/delta)-binding site downstream. Gel shift analysis demonstrated that the DR-DS junction is bound by a factor with properties similar to those of BSAP (B-cell-specific activator protein). These results reveal developmental-stage-specific changes in the composition of nuclear factors bound to E3', clarify the role of factors that bind constitutively in vitro, and point to the differentiation of mature B cells to plasma cells as an important transitional point in the function of this enhancer. The observed changes in nuclear factor composition were accompanied by the rearrangement of positioned nucleosomes that flank the core region, suggesting a role for both nuclear factors and chromatin structure in modulating kappa E3' function during B-cell development. The functional implications of the observed chromatin alterations are discussed in the context of recent studies on kappa E3' and the factors that bind to it.

A defective Vkappa A2 allele in Navajos which may play a role in increased susceptibility to haemophilus influenzae type b disease

The antibody response to H. influenzae type b (Hib) is pauciclonal, and is dominated by antibodies using the VkappaA2 gene. Navajos have a 5-10-fold increased incidence of Hib disease compared with control populations. We hypothesized that a polymorphism in one of the genes in this oligoclonal response may lead to increased disease susceptibility. Since the predominant A2+ anti-Hib antibodies have high avidity for Hib and can be unmutated, the A2 Vkappa gene was analyzed. Over half of the Navajos studied, but only one control individual, had a new allele of A2, termed A2b, with three changes from the published A2 germline sequence. One of the changes was in the recombination signal sequence, suggesting that the A2b allele might not undergo V-J rearrangement very frequently. This possibility was confirmed by analyzing the relative frequency of non-productive A2 rearrangements in A2a/b heterozygous Navajos. Many fewer A2b rearrangements were observed, showing that the A2b allele is defective in its ability to undergo rearrangement. The prevalence of this allele in Navajos may play a role in their increased susceptibility to invasive Hib disease. If so, it would underscore the importance of the germline Ig repertoire for protective antibody responses to pathogenic bacteria in unimmunized children.

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.

The molecular basis of somatic hypermutation of immunoglobulin genes

Somatic hypermutation amplifies the variable region repertoire of immunoglobulin genes. Recent experimental evidence has thrown light on various molecular models of somatic hypermutation. A link between somatic hypermutation and transcription coupled DNA repair is shaping up.

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.

An AP1 binding site upstream of the kappa immunoglobulin intron enhancer binds inducible factors and contributes to expression

Expression of the kappa immunoglobulin light chain gene requires developmental- and tissue-specific regulation by trans-acting factors which interact with two distinct enhancer elements. A new protein-DNA interaction has been identified upstream of the intron enhancer, within the matrix-associated region of the J-C intron. The binding activity is greatly inducible in pre-B cells by bacterial lipopolysaccharide and interleukin-1 but specific complexes are found at all stages of B cell development tested. The footprinted binding site is homologous to the consensus AP1 motif. The protein components of this complex are specifically competed by an AP1 consensus motif and were shown by supershift to include c-Jun and c-Fos, suggesting that this binding site is an AP1 motif and that the Jun and Fos families of transcription factors play a role in the regulation of the kappa light chain gene. Mutation of the AP1 motif in the context of the intron enhancer was shown to decrease enhancer-mediated activation of the promoter in both pre-B cells induced with LPS and constitutive expression in mature B cells.

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

We have examined the interactions of the enhancers of the kappa immunoglobulin light chain gene as well as the interactions of the intron, mu, and 3' alpha enhancers of the heavy chain locus in mouse. We have observed that each of the kappa enhancers is very weak in comparison with the heavy chain intron enhancer. The mouse heavy chain 3' alpha enhancer is relatively weak as well. However, two kappa enhancers together synergistically activate transcription of a luciferase reporter gene to a level that is roughly equivalent to the heavy chain mu enhancer. Additionally, dimerization of either kappa enhancer results in synergistic increases in transcription. This property of synergism appears to be confined to the enhancers of the kappa locus, as addition of the 3' alpha E to mu E containing constructs increases transcription only modestly, and neither heavy chain enhancer synergizes when dimerized. We have gone on to characterize some of the minimal requirements for synergism between the kappa enhancers and find that the KB and E2 sites are required, but not the E3 site. The implications of these results for the coordinate regulation of the heavy and light chain transcription are discussed.

Elements regulating somatic hypermutation of an immunoglobulin kappa gene: critical role for the intron enhancer/matrix attachment region

Following encounter with antigen, the immunoglobulin genes in B lymphocytes undergo somatic hypermutation. Most nucleotide substitutions are introduced into a region flanked by the V gene promoter and intron enhancer. Experiments described here using transgenic mice revealed that the V kappa promoter does not contain specific signals since hypermutation was retained on substituting it by a beta-globin promoter. However, both the kappa intron and kappa 3' enhancer regions were found to be essential for full hypermutation. This dependence of hypermutation on both enhancers contrasts with transgene expression in hybridomas in which only the 3' enhancer (and not the intron enhancer) is necessary to achieve high mRNA levels. The results show that full hypermutation depends on multiple elements, removal of some of which may drastically impair but not totally abolish the process.