Antisense intergenic transcription precedes Igh D-to-J recombination and is controlled by the intronic enhancer Emu

Chromatin architecture and the generation of antigen receptor diversity

The adaptive immune system generates a specific response to a vast spectrum of antigens. This remarkable property is achieved by lymphocytes that each express single and unique antigen receptors. During lymphocyte development, antigen receptor coding elements are assembled from widely dispersed gene segments. The assembly of antigen receptors is controlled at multiple levels, including epigenetic marking, nuclear location, and chromatin topology. Here, we review recently uncovered mechanisms that underpin long-range genomic interactions and the generation of antigen receptor diversity.

Regulatory roles of natural antisense transcripts

Mammalian genomes encode numerous natural antisense transcripts, but the function of these transcripts is not well understood. Functional validation studies indicate that antisense transcripts are not a uniform group of regulatory RNAs but instead belong to multiple categories with some common features. Recent evidence indicates that antisense transcripts are frequently functional and use diverse transcriptional and post-transcriptional gene regulatory mechanisms to carry out a wide variety of biological roles.

RAG: a recombinase diversified

During B cell and T cell development, the lymphoid-specific proteins RAG-1 and RAG-2 act together to initiate the assembly of antigen receptor genes through a series of site-specific somatic DNA rearrangements that are collectively called variable-diversity-joining (V(D)J) recombination. In the past 20 years, a great deal has been learned about the enzymatic activities of the RAG-1-RAG-2 complex. Recent studies have identified several new and exciting regulatory functions of the RAG-1-RAG-2 complex. Here we discuss some of these functions and suggest that the RAG-1-RAG-2 complex nucleates a specialized subnuclear compartment that we call the 'V(D)J recombination factory'.

Cutting edge: SWI/SNF mediates antisense Igh transcription and locus-wide accessibility in B cell precursors

The stepwise process of Ag receptor gene assembly, termed V(D)J recombination, is coordinated during lymphocyte development by sweeping changes in chromatin that permit or deny access to a single recombinase enzyme. We now show that switching/sucrose nonfermenting (SWI/SNF) chromatin remodeling complexes are recruited to the Igh locus by an enhancer-dependent process and that these complexes are essential for generating recombinase accessibility throughout the locus. Depletion of SWI/SNF in pro-B cells also inhibits antisense transcription through all clusters of Igh gene segments, a pioneering process that has been implicated in the initial opening of chromatin. We conclude that SWI/SNF complexes play multiple roles in Igh gene assembly, ranging from initial locus activation to the spreading and maintenance of chromatin accessibility over large V(H), D(H), and J(H) domains.

A mouse model for chronic lymphocytic leukemia based on expression of the SV40 large T antigen

The simian virus 40 (SV40) T antigen is a potent oncogene able to transform many cell types and has been implicated in leukemia and lymphoma. In this report, we have achieved sporadic SV40 T-antigen expression in mature B cells in mice, by insertion of a SV40 T antigen gene in opposite transcriptional orientation in the immunoglobulin (Ig) heavy (H) chain locus between the D and JH segments. SV40 T-antigen expression appeared to result from retention of the targeted germline allele and concomitant antisense transcription of SV40 large T in mature B cells, leading to chronic lymphocytic leukemia (CLL). Although B-cell development was unperturbed in young mice, aging mice showed accumulation of a monoclonal B-cell population in which the targeted IgH allele was in germline configuration and the wild-type IgH allele had a productive V(D)J recombination. These leukemic B cells were IgDlowCD5+ and manifested nonrandom usage of V, D, and J segments. VH regions were either unmutated, with preferential usage of the VH11 family, or manifested extensive somatic hypermutation. Our findings provide an animal model for B-CLL and show that pathways activated by SV40 T antigen play important roles in the pathogenesis of B-CLL.

A 220-nucleotide deletion of the intronic enhancer reveals an epigenetic hierarchy in immunoglobulin heavy chain locus activation

A tissue-specific transcriptional enhancer, Eμ, has been implicated in developmentally regulated recombination and transcription of the immunoglobulin heavy chain (IgH) gene locus. We demonstrate that deleting 220 nucleotides that constitute the core Eμ results in partially active locus, characterized by reduced histone acetylation, chromatin remodeling, transcription, and recombination, whereas other hallmarks of tissue-specific locus activation, such as loss of H3K9 dimethylation or gain of H3K4 dimethylation, are less affected. These observations define Eμ-independent and Eμ-dependent phases of locus activation that reveal an unappreciated epigenetic hierarchy in tissue-specific gene expression.

Monoallelic gene expression in mammals

Three systems of monoallelic gene expression in mammals are known, namely, X-chromosome inactivation, imprinting, and allelic exclusion. In all three systems, monoallelic expression is regulated epigenetically and is frequently directed by long non-coding RNAs (ncRNAs). This review briefs all three systems of monoallelic gene expression in mammals focusing on chromatin modifications, spatial chromosome organization in the nucleus, and the functioning of ncRNAs.

Germline transcription: a key regulator of accessibility and recombination

The developmental control of V(D)J recombination is imposed at the level of chromatin accessibility of recombination signal sequences (RSSs) to the recombinase machinery. Cis-acting transcriptional regulatory elements such as promoters and enhancers play a central role in the control of accessibility in vivo. However, the molecular mechanisms by which these elements influence accessibility are still under investigation. Although accessibility for V(D)J recombination is usually accompanied by germline transcription at antigen receptor loci, the functional significance of this transcription in directing RSS accessibility has been elusive. In this chapter, we review past studies outlining the complex relationship between V(D)J recombination and transcription as well as our current understanding on how chromatin structure is regulated during gene expression. We then summarize recent work that directly addresses the functional role of transcription in V(D)J recombination.

Molecular pathways and mechanisms regulating the recombination of immunoglobulin genes during B-lymphocyte development

The hallmark of B-cell development is the ordered recombination of immunoglobulin (Ig) genes. Recently, considerable progress has been achieved in assembling gene regulatory networks comprised of signaling components and transcription factors that regulate B-cell development. In this chapter we synthesize experimental evidence to explain how such signaling pathways and transcription factors can orchestrate the ordered recombination of immunoglobulin (Ig) genes. Recombination of antigen-receptor loci is regulated both by the developmentally controlled expression of the Rag1 and Rag2 genes and the accessibility of particular loci and their gene segments to recombination. A new framework has emerged that invokes nuclear compartmentalization, large-scale chromatin dynamics and localized changes in chromatin structure in regulating the accessibility of Ig loci at specific stages of B-cell development. We review this emergent framework and discuss new experimental approaches that will be needed to explore the underlying molecular mechanisms.

Analysis of intergenic transcription and histone modification across the human immunoglobulin heavy-chain locus

Ig class switch recombination (CSR) is initiated by activation-induced cytidine deaminase (AID) mediated deamination of the switch (S) regions; the resultant mismatch is processed to yield the DNA breaks required for recombination. Whereas many of the pathways involved in the mechanism of recombination have been identified, little is known about how CSR is regulated. AID action is known to require transcription of the Ig heavy-chain genes. However, it is not understood how AID is restricted to the Ig genes. Many aspects of gene expression are known to be regulated by modification of chromatin structure. In turn, chromatin is known to be regulated by several RNA-dependent activities. We have mapped the transcriptional and chromatin landscape of the human Ig heavy-chain locus to investigate the effect these activities have on CSR. We demonstrate that the Ig heavy-chain constant genes and 3'-regulatory regions are in an active chromatin conformation in unstimulated total human B cells: the locus undergoes both genic and intergenic transcription and possesses histone modifications associated with "active" chromatin (acetylated H3 and H4 and lysine 4 trimethylated H3). However, on cytokine stimulation, these modifications spread into the S regions, demonstrating a chromatin remodeling activity associated with switching. Surprisingly, after stimulation, the S regions also accumulate lysine 9 trimethylated H3, a modification previously associated with gene silencing. These data demonstrates that the Ig locus is maintained with a complex pattern of both positive and negative histone marks and suggest that some of these marks may have dual functions.

The biochemistry of somatic hypermutation

Affinity maturation of the humoral response is mediated by somatic hypermutation of the immunoglobulin (Ig) genes and selection of higher-affinity B cell clones. Activation-induced cytidine deaminase (AID) is the first of a complex series of proteins that introduce these point mutations into variable regions of the Ig genes. AID deaminates deoxycytidine residues in single-stranded DNA to deoxyuridines, which are then processed by DNA replication, base excision repair (BER), or mismatch repair (MMR). In germinal center B cells, MMR, BER, and other factors are diverted from their normal roles in preserving genomic integrity to increase diversity within the Ig locus. Both AID and these components of an emerging error-prone mutasome are regulated on many levels by complex mechanisms that are only beginning to be elucidated.

How chromatin remodelling allows shuffling of immunoglobulin heavy chain genes

Cellular identity is determined by the switching on and off of lineage-specific genes. This dynamic process is regulated by a highly co-ordinated series of chromatin remodelling mechanisms that control DNA accessibility to facilitate transcription, replication and recombination. The identity of an individual B-lymphocyte is defined by the expression of a unique antibody protein, composed of two identical immunoglobulin heavy and two identical light chain polypeptides, which recognize a single foreign antigen with high specificity. However, the mammalian adaptive immune system requires an enormous variety of antibody-expressing B cells to combat the millions of foreign antigens it may encounter. This diversity is generated primarily at the multigene immunoglobulin loci by V(D)J recombination, a specialised form of DNA recombination in which numerous variable (V), diversity (D) and joining (J) genes are cut and pasted together in a strict order to allow shuffling of immunoglobulin genes. The mouse immunoglobulin heavy chain (Igh) locus is the largest known multigene locus. It spans approximately 3 Mb and comprises more than 200 genes. Its size and complexity pose an enormous logistic challenge to the chromatin remodelling machinery, but recent major advances in our understanding of how the 200 genes are shuffled have begun to reveal an exquisitely co-ordinated set of chromatin remodelling mechanisms which exploit every aspect of nuclear dynamics, and provide a global view of multigene regulation. This review will explore the numerous processes implicated in opening up and positioning of the locus to enable shuffling of the Igh locus genes, including non-coding RNA transcription, histone modifications, transcription factors, nuclear relocation and locus contraction.

Choreography of Ig allelic exclusion

Allelic exclusion guarantees that each B or T cell only produces a single antigen receptor, and in this way contributes to immune diversity. This process is actually initiated in the early embryo when the immune receptor loci become asynchronously replicating in a stochastic manner with one early and one late allele in each cell. This distinct differential replication timing feature then serves an instructive mark that directs a series of allele-specific epigenetic events in the immune system, including programmed histone modification, nuclear localization and DNA demethylation that ultimately bring about preferred rearrangement on a single allele, and this decision is temporally stabilized by feedback mechanisms that inhibit recombination on the second allele. In principle, these same molecular components are also used for controlling monoallelic expression at other genomic loci, such as those carrying interleukins and olfactory receptor genes that require the choice of one gene out of a large array. Thus, allelic exclusion appears to represent a general epigenetic phenomenon that is modeled on the same basis as X chromosome inactivation.

Dicer ablation affects antibody diversity and cell survival in the B lymphocyte lineage

To explore the role of Dicer-dependent control mechanisms in B lymphocyte development, we ablated this enzyme in early B cell progenitors. This resulted in a developmental block at the pro- to pre-B cell transition. Gene-expression profiling revealed a miR-17 approximately 92 signature in the 3'UTRs of genes upregulated in Dicer-deficient pro-B cells; a top miR-17 approximately 92 target, the proapoptotic molecule Bim, was highly upregulated. Accordingly, B cell development could be partially rescued by ablation of Bim or transgenic expression of the prosurvival protein Bcl-2. This allowed us to assess the impact of Dicer deficiency on the V(D)J recombination program in developing B cells. We found intact Ig gene rearrangements in immunoglobulin heavy (IgH) and kappa chain loci, but increased sterile transcription and usage of D(H) elements of the DSP family in IgH, and increased N sequence addition in Igkappa due to deregulated transcription of the terminal deoxynucleotidyl transferase gene.

Antisense transcripts from immunoglobulin heavy-chain locus V(D)J and switch regions

Activation-induced cytosine deaminase (AID) is essential for both somatic hypermutation (SHM) and class switch recombination (CSR), two processes involved in antibody diversification. Previously, various groups showed both in vitro and in vivo that AID initiates SHM and CSR by deaminating cytosines in DNA in a transcription-dependent manner. Although in vivo both DNA strands are equally targeted by AID, many in vitro and bacterial experiments found that AID almost exclusively targets the nontemplate strand of a transcribed substrate. Here, we report the detection of antisense transcripts in assembled Ig heavy chain (IgH) variable region exons and their immediate downstream region, as well as in switch regions, sequences that, respectively, are targets for SHM and CSR in vivo. In contrast, we did not detect antisense transcripts from the Cmu constant region exons, which lie between the IgH variable region exons and downstream S regions and which are not normally an AID target. Expression of the antisense variable region/flanking region and the S-region transcripts were found in all lymphocytes that transcribe these sequences in the sense direction. Steady-state levels of antisense transcripts appeared very low, and start sites potentially appeared heterogeneous. We discuss the potential implications of antisense IgH locus transcription for AID targeting or other processes.