Zhongliang Ju

CCCTC-binding factor (CTCF) and cohesin influence the genomic architecture of the Igh locus and antisense transcription in pro-B cells

Compaction and looping of the ~2.5-Mb Igh locus during V(D)J rearrangement is essential to allow all VH genes to be brought in proximity with DH-JH segments to create a diverse antibody repertoire, but the proteins directly responsible for this are unknown. Because CCCTC-binding factor (CTCF) has been demonstrated to be involved in long-range chromosomal interactions, we hypothesized that CTCF may promote the contraction of the Igh locus. ChIP sequencing was performed on pro-B cells, revealing colocalization of CTCF and Rad21 binding at ~60 sites throughout the VH region and 2 other sites within the Igh locus. These numerous CTCF/cohesin sites potentially form the bases of the multiloop rosette structures at the Igh locus that compact during Ig heavy chain rearrangement. To test whether CTCF was involved in locus compaction, we used 3D-FISH to measure compaction in pro-B cells transduced with CTCF shRNA retroviruses. Reduction of CTCF binding resulted in a decrease in Igh locus compaction. Long-range interactions within the Igh locus were measured with the chromosomal conformation capture assay, revealing direct interactions between CTCF sites 5′ of DFL16 and the 3′ regulatory region, and also the intronic enhancer (Eμ), creating a DH-JH-Eμ-CH domain. Knockdown of CTCF also resulted in the increase of antisense transcription throughout the DH region and parts of the VH locus, suggesting a widespread regulatory role for CTCF. Together, our findings demonstrate that CTCF plays an important role in the 3D structure of the Igh locus and in the regulation of antisense germline transcription and that it contributes to the compaction of the Igh locus.

Evidence for physical Interaction between the immunoglobulin heavy chain variable region and the 3' regulatory region

B cell-specific expression of immunoglobulin heavy chain (IgH) genes utilizes two cis regulatory regions, the intronic enhancer (Eμ), located in the JH-Cμ intron, and a complex regulatory region that lies 3′ to the IgH gene cluster, 3′ RR. We hypothesized that the 3′ RR is involved in IgH gene transcription in plasma cells via physical interaction between distal 3′ RR enhancers and target VH sequences, with loop formation by intervening DNA. In support of this hypothesis we report sequence data at DNA recombination breakpoints as evidence for loop formation preceding DNA inversion in a plasma cell line. In addition, using the chromosome conformation capture technique, physical interactions between VH and 3′ RR were analyzed directly and detected in MPC11 plasma cells and variants and normal splenic B cells but not detected in splenic T cells or in non-B cells. VH-3′ RR interactions were present in the absence of Eμ, but when the hs1,2 enhancer was replaced by a NeoR gene in a variant cell line lacking Eμ, H chain expression was lost, and interactions between VH and 3′ RR and among the 3′ RR regulators themselves were severely disrupted. In addition, the chromosome conformation capture technique detected interactions between the myc promoter and 3′ RR elements in MPC11, which like other plasmacytomas contains a reciprocal translocation between the c-myc and the IgH locus. In sum, our data support a hypothesis that cis VH-3′ RR and myc-3′ RR interactions involve physical interactions between these DNA elements.

Pax5 and Linker Histone H1 Coordinate DNA Methylation and Histone Modifications in the 3′ Regulatory Region of the Immunoglobulin Heavy Chain Locus

ABSTRACT The 3′ regulatory region (3′ RR) of the murine immunoglobulin heavy chain (IgH) locus contains multiple DNase I-hypersensitive (hs) sites. Proximal sites hs3A, hs1.2, and hs3B are located in an extensive palindromic region and together with hs4 are associated with enhancers involved in the expression and class switch recombination of IgH genes. Distal hs5, -6, and -7 sites located downstream of hs4 comprise a potential insulator for the IgH locus. In pro-B cells, hs4 to -7 are associated with marks of active chromatin, while hs3A, hs1.2, and hs3B are not. Our analysis of DNA methylation-sensitive restriction sites of the 3′ RR has revealed a similar modular pattern in pro-B cells; hs4 to -7 sites are unmethylated, while the palindromic region is methylated. This modular pattern of DNA methylation and histone modifications appears to be determined by at least two factors: the B-cell-specific transcription factor Pax5 and linker histone H1. In pre-B cells, a region beginning downstream of hs4 and extending into hs5 showed evidence of allele-specific demethylation associated with the expressed heavy chain allele. Palindromic enhancers become demethylated later in B-cell differentiation, in B and plasma cells.

Germline Deletion of Igh 3′ Regulatory Region Elements hs 5, 6, 7 (hs5–7) Affects B Cell-Specific Regulation, Rearrangement, and Insulation of the Igh Locus

Regulatory elements located within an ∼28-kb region 3′ of the Igh gene cluster (3′ regulatory region) are required for class switch recombination and for high levels of IgH expression in plasma cells. We previously defined novel DNase I hypersensitive sites (hs) 5, 6, 7 immediately downstream of this region. The hs 5–7 region (hs5–7) contains a high density of binding sites for CCCTC-binding factor (CTCF), a zinc finger protein associated with mammalian insulator activity, and is an anchor for interactions with CTCF sites flanking the DH region. To test the function of hs5–7, we generated mice with an 8-kb deletion encompassing all three hs elements. B cells from hs5–7 knockout (KO) (hs5–7KO) mice showed a modest increase in expression of the nearest downstream gene. In addition, Igh alleles in hs5–7KO mice were in a less contracted configuration compared with wild-type Igh alleles and showed a 2-fold increase in the usage of proximal VH7183 gene families. Hs5–7KO mice were essentially indistinguishable from wild-type mice in B cell development, allelic regulation, class switch recombination, and chromosomal looping. We conclude that hs5–7, a high-density CTCF-binding region at the 3′ end of the Igh locus, impacts usage of VH regions as far as 500 kb away.

Dynamic Changes in Binding of Immunoglobulin Heavy Chain 3′ Regulatory Region to Protein Factors during Class Switching

The 3′ regulatory region (3′ RR) of the Igh locus works at long distances on variable region (VH) and switch region (I) region promoters to initiate germ line (non-coding) transcription (GT) and promote class switch recombination (CSR). The 3′ RR contains multiple elements, including enhancers (hs3a, hs1.2, hs3b, and hs4) and a proposed insulator region containing CTCF (CCCTC-binding factor) binding sites, i.e. hs5/6/7 and the downstream region (“38”). Notably, deletion of each individual enhancer (hs3a-hs4) has no significant phenotypic consequence, suggesting that the 3′ RR has considerable structural flexibility in its function. To better understand how the 3′ RR functions, we identified transcription factor binding sites and used chromatin immunoprecipitation (ChIP) assays to monitor their occupancy in splenic B cells that initiate GT and undergo CSR (LPS±IL4), are deficient in GT and CSR (p50−/−), or do not undergo CSR despite efficient GT (anti-IgM+IL4). Like 3′ RR enhancers, hs5–7 and the 38 region were observed to contain multiple Pax5 binding sites (in addition to multiple CTCF sites). We found that the Pax5 binding profile to the 3′ RR dynamically changed during CSR independent of the specific isotype to which switching was induced, and binding focused on hs1.2, hs4, and hs7. CTCF-associated and CTCF-independent cohesin interactions were also identified. Our observations are consistent with a scaffold model in which a platform of active protein complexes capable of facilitating GT and CSR can be formed by varying constellations of 3′ RR elements.

Homologous Elements hs3a and hs3b in the 3′ Regulatory Region of the Murine Immunoglobulin Heavy Chain (Igh) Locus Are Both Dispensable for Class-switch Recombination

Immunoglobulin heavy chain (IgH) genes are formed, tested, and modified to yield diverse, specific, and high affinity antibody responses to antigen. The processes involved must be regulated, however, to avoid unintended damage to chromosomes. The 3′ regulatory region of the Igh locus plays a major role in regulating class-switch recombination (CSR), the process by which antibody effector functions are modified during an immune response. Loss of all known enhancer-like elements in this region dramatically impairs CSR, but individual element deletions have no effect on this process. In the present study, we explored the hypothesis that an underlying functional redundancy in the homologous elements hs3a and hs3b was masking the importance of either element to CSR. Several transgenic mouse lines were generated, each carrying a bacterial artificial chromosome transgene that mimicked Igh locus structure but in which hs3a was missing and hs3b was flanked by loxP sites. Matings to Cyclization Recombination Enzyme-expressing mice established “pairs” of lines that differed only in the presence or absence of hs3b. Remarkably, CSR remained robust in the absence of both hs3a and hs3b, suggesting that the remaining two elements of the 3′ regulatory region, hs1.2 and hs4, although individually dispensable for CSR, are, together, sufficient to support CSR.