Microarray analysis of Lyn-deficient B cells reveals germinal center-associated nuclear protein and other genes associated with the lymphoid germinal center

Carcinogenesis caused by transcription-coupled DNA damage through GANP and other components of the TREX-2 complex

Perturbation of genes is important for somatic hypermutation to increase antibody affinity during B-cell immunity; however, it may also promote carcinogenesis. Previous studies have revealed that transcription is an important process that can induce DNA damage and genomic instability. Transciption-export-2 (TREX-2) complex, which regulates messenger RNA (mRNA) nuclear export, has been studied in the budding yeast Saccharomyces cerevisiae; however, recent studies have started investigating the molecular function of the mammalian TREX-2 complex. The central molecule in the TREX-2 complex, that is, germinal center-associated nuclear protein (GANP), is closely associated with antibody affinity maturation as well as cancer etiology. In this review, we focus on carcinogenesis, lymphomagenesis, and teratomagenesis caused by transcription-coupled DNA damage through GANP and other components of the TREX-2 complex. We review the basic machinery of mRNA nuclear export and transcription-coupled DNA damage. We then briefly describe the immunological relationship between GANP and the affinity maturation of antibodies. Finally, we illustrate that the aberrant expression of the components of the TREX-2 complex, especially GANP, is associated with the etiology of various solid tumors, lymphomas, and testicular teratoma. These components serve as reliable predictors of cancer prognosis and response to chemotherapy.

The critical role of germinal center-associated nuclear protein in cell biology, immunohematology, and hematolymphoid oncogenesis

Germinal center-associated nuclear protein (GANP) is a unique and multifunctional protein that plays a critical role in cell biology, neurodegenerative disorders, immunohematology, and oncogenesis. GANP is an orthologue of Saccharomyces Sac3, one of the components of the transcription export 2 (TREX-2) complex and a messenger RNA (mRNA) nuclear export factor. GANP is widely conserved in all mammals, including humans. Although GANP was originally discovered as a molecule upregulated in the germinal centers of secondary lymphoid follicles in peripheral lymphoid organs, it is expressed ubiquitously in many tissues. It serves numerous functions, including making up part of the mammalian TREX-2 complex; mRNA nuclear export via nuclear pores; prevention of R-loop formation, genomic instability, and hyper-recombination; and B-cell affinity maturation. In this review, we first overview the extensive analyses that have revealed the basic functions of GANP and its ancestor molecule Sac3, including mRNA nuclear export and regulation of R-loop formation. We then describe how aberrant expression of GANP is significantly associated with cancer development. Moreover, we discuss a crucial role for GANP in B-cell development, especially affinity maturation in germinal centers. Finally, we illustrate that overexpression of GANP in B cells leads to lymphomagenesis resembling Hodgkin lymphoma derived from germinal center B cells, and that GANP may be involved in transdifferentiation of B cells to macrophages, which strongly affects Hodgkin lymphomagenesis.

A Novel Cytological Model of B-Cell/Macrophage Biphenotypic Cell Hodgkin Lymphoma in Ganp-Transgenic Mice

Hodgkin lymphoma (HL) is one of the most difficult neoplasms in terms of cytopathological research owing to the lack of established cytological murine models. Although HL is believed to be of lymphoid germinal center B-cell origin, HL cells exhibit unique biphenotypic characteristics of B cells and macrophages. B-cell/macrophage biphenotypic cells have also been identified in the spleen of Lyn-deficient mice. Moreover, Lyn-targeting germinal center-associated nuclear protein (GANP)-transgenic mice (Ig-ganp^Tg mice) spontaneously develop a lymphoid tumor. We aimed to investigate whether the lymphoid tumor developed in Ig-ganp^Tg mice exhibit biphenotypic characteristics of B cells/macrophages that correspond to human HL. Here, we demonstrated GANP overexpression in human HL cells and found that it may regulate transdifferentiation between B cells and macrophages. We also demonstrated that tumors were comparable with B-cell/macrophage biphenotypic Hodgkinoid lymphomas. The tumor cells expressed macrophage-related F4/80, CD68, and CD204 as well as cytoplasmic B220 and µ-/κ-chains; in addition, these cells exhibited phagocytic activity. These cells also expressed transcripts of CD30; c-fms; and the cytokines monocyte chemoattractant protein (MCP)-1, MCP-5, RANTES, tumor necrosis factor-α and thrombopoietin associated with macrophages as well as granulocyte/macrophage colony-stimulating factor, interleukin (IL)-4, IL-10, IL-12, and IL-13. Ig-ganp^Tg mice represent a novel cytological model for the study of cytopathological etiology and oncogenesis of HL.

Germinal Center B-Cell-Associated Nuclear Protein (GANP) Involved in RNA Metabolism for B Cell Maturation

Germinal center B-cell-associated nuclear protein (GANP) is upregulated in germinal center B cells against T-cell-dependent antigens in mice and humans. In mice, GANP depletion in B cells impairs antibody affinity maturation. Conversely, its transgenic overexpression augments the generation of high-affinity antigen-specific B cells. GANP associates with AID in the cytoplasm, shepherds AID into the nucleus, and augments its access to the rearranged immunoglobulin (Ig) variable (V) region of the genome in B cells, thereby precipitating the somatic hypermutation of V region genes. GANP is also upregulated in human CD4(+) T cells and is associated with APOBEC3G (A3G). GANP interacts with A3G and escorts it to the virion cores to potentiate its antiretroviral activity by inactivating HIV-1 genomic cDNA. Thus, GANP is characterized as a cofactor associated with AID/APOBEC cytidine deaminase family molecules in generating diversity of the IgV region of the genome and genetic alterations of exogenously introduced viral targets. GANP, encoded by human chromosome 21, as well as its mouse equivalent on chromosome 10, contains a region homologous to Saccharomyces Sac3 that was characterized as a component of the transcription/export 2 (TREX-2) complex and was predicted to be involved in RNA export and metabolism in mammalian cells. The metabolism of RNA during its maturation, from the transcription site at the chromosome within the nucleus to the cytoplasmic translation apparatus, needs to be elaborated with regard to acquired and innate immunity. In this review, we summarize the current knowledge on GANP as a component of TREX-2 in mammalian cells.

Regulation of monocyte differentiation by specific signaling modules and associated transcription factor networks

Monocyte/macrophages are important players in orchestrating the immune response as well as connecting innate and adaptive immunity. Myelopoiesis and monopoiesis are characterized by the interplay between expansion of stem/progenitor cells and progression towards further developed (myelo)monocytic phenotypes. In response to a variety of differentiation-inducing stimuli, various prominent signaling pathways are activated. Subsequently, specific transcription factors are induced, regulating cell proliferation and maturation. This review article focuses on the integration of signaling modules and transcriptional networks involved in the determination of monocytic differentiation.

B cell-specific loss of Lyn kinase leads to autoimmunity

The Lyn tyrosine kinase regulates inhibitory signaling in B and myeloid cells: loss of Lyn results in a lupus-like autoimmune disease with hyperactive B cells and myeloproliferation. We have characterized the relative contribution of Lyn-regulated signaling pathways in B cells specifically to the development of autoimmunity by crossing the novel lyn(flox/flox) animals with mice carrying the Cre recombinase under the control of the Cd79a promoter, resulting in deletion of Lyn in B cells. The specific deletion of Lyn in B cells is sufficient for the development of immune complex-mediated glomerulonephritis. The B cell-specific Lyn-deficient mice have no defects in early bone marrow B cell development but have reduced numbers of mature B cells with poor germinal centers, as well as increased numbers of plasma and B1a cells, similar to the lyn(-/-) animals. Within 8 mo of life, B cell-specific Lyn mutant mice develop high titers of IgG anti-Smith Ag ribonucleoprotein and anti-dsDNA autoantibodies, which deposit in their kidneys, resulting in glomerulonephritis. B cell-specific Lyn mutant mice also develop myeloproliferation, similar to the lyn(-/-) animals. The additional deletion of MyD88 in B cells, achieved by crossing lyn(flox/flox)Cd79a-cre mice with myd88(flox/flox) animals, reversed the autoimmune phenotype observed in B cell-specific Lyn-deficient mice by blocking production of class-switched pathogenic IgG autoantibodies. Our results demonstrate that B cell-intrinsic Lyn-dependent signaling pathways regulate B cell homeostasis and activation, which in concert with B cell-specific MyD88 signaling pathways can drive the development of autoimmune disease.

IL-21 promotes the production of anti-DNA IgG but is dispensable for kidney damage in lyn-/- mice

The autoimmune disease systemic lupus erythematosus is characterized by loss of tolerance to nuclear Ags and a heightened inflammatory environment, which together result in end organ damage. Lyn-deficient mice, a model of systemic lupus erythematosus, lack an inhibitor of B-cell and myeloid cell activation. This results in B-cell hyper-responsiveness, plasma cell accumulation, autoantibodies, and glomerulonephritis (GN). IL-21 is associated with autoimmunity in mice and humans and promotes B-cell differentiation and class switching. Here, we explore the role of IL-21 in the autoimmune phenotypes of lyn(-/-) mice. We find that IL-21 mRNA is reduced in the spleens of lyn(-/-) IL-6(-/-) and lyn(-/-) Btk(lo) mice, neither of which produce pathogenic autoantibodies or develop significant GN. While IL-21 is dispensable for plasma cell accumulation and IgM autoantibodies in lyn(-/-) mice, it is required for anti-DNA IgG antibodies and some aspects of T-cell activation. Surprisingly, GN still develops in lyn(-/-) IL-21(-/-) mice. This likely results from the presence of IgG autoantibodies against a limited set of non-DNA Ags. These studies identify a specific role for IL-21 in the class switching of anti-DNA B cells and demonstrate that neither IL-21 nor anti-DNA IgG is required for kidney damage in lyn(-/-) mice.

2D DIGE analysis of the bursa of Fabricius reveals characteristic proteome profiles for different stages of chicken B-cell development

Antibody producing B-cells are an essential component of the immune system. In contrast to human and mice where B-cells develop in the bone marrow, chicken B-cells develop in defined stages in the bursa of Fabricius, a gut associated lymphoid tissue. In order to gain a better understanding of critical biological processes like immigration of B-cell precursors into the bursa anlage, their differentiation and final emigration from the bursa we analyzed the proteome dynamics of this organ during embryonic and posthatch development. Samples were taken from four representative developmental stages (embryonic day (ED) 10, ED18, day 2, and day 28) and compared in an extensive 2D DIGE approach comprising six biological replicates per time point. Cluster analysis and PCA demonstrated high reliability and reproducibility of the obtained data set and revealed distinctive proteome profiles for the selected time points, which precisely reflect the differentiation processes. One hundred fifty three protein spots with significantly different intensities were identified by MS. We detected alterations in the abundance of several proteins assigned to retinoic acid metabolism (e.g. retinal-binding protein 5) and the actin-cytoskeleton (e.g. vinculin and gelsolin). By immunohistochemistry, desmin was identified as stromal cell protein associated with the maturation of B-cell follicles. Strongest protein expression difference (10.8-fold) was observed for chloride intracellular channel 2. This protein was thus far not associated with B-cell biology but our data suggest an important function in bursa B-cell development.

Dasatinib therapy results in decreased B cell proliferation, splenomegaly, and tumor growth in a murine model of lymphoma expressing Myc and Epstein-Barr virus LMP2A

Epstein-Barr virus (EBV) infection and latency has been associated with malignant diseases including nasopharyngeal carcinoma, Hodgkin lymphoma, Burkitt lymphoma, and immune deficiency associated lymphoproliferative diseases. EBV-encoded latent membrane protein 2A (LMP2A) recruits Lyn and Syk kinases via its SH2-domain binding motifs, and modifies their signaling pathways. LMP2A transgenic mice develop hyperproliferative bone marrow B cells and immature peripheral B cells through modulation of Lyn kinase signaling. LMP2A/λ-MYC double transgenic mice develop splenomegaly and cervical lymphomas starting at 8 weeks of age. We reasoned that targeting Lyn in LMP2A-expressing B cells with dasatinib would provide a therapeutic option for EBV-associated malignancies. Here, we show that dasatinib inhibits B cell colony formation by LMP2A transgenic bone marrow cells, and reverses splenomegaly and tumor growth in both a pre-tumor and a syngeneic tumor transfer model of EBV-associated Burkitt lymphoma. Our data support the idea that dasatinib may prove to be an effective therapeutic molecule for the treatment of EBV-associated malignancies.

Epstein-Barr virus latent membrane protein 2A preferentially signals through the Src family kinase Lyn

Latent membrane protein 2A (LMP2A) is a viral protein expressed during Epstein-Barr virus (EBV) latency in EBV-infected B cells both in cell culture and in vivo. LMP2A has important roles in modulating B-cell receptor signal transduction and provides survival and developmental signals to B cells in vivo. Although Lyn has been shown to be important in mediating LMP2A signaling, it is still unclear if Lyn is used preferentially or if LMP2A associates promiscuously with other Src family kinase (SFK) members. To investigate the role of various SFKs in LMP2A signaling, we crossed LMP2A transgenic mice (TgE) with Lyn(-/-), Fyn(-/-), or Blk(-/-) mice. TgE Lyn(-/-) mice had a larger immunoglobulin M (IgM)-positive B-cell population than TgE mice, suggesting that the absence of Lyn prevents LMP2A from delivering survival and developmental signals to the B cells. Both TgE Fyn(-/-) and TgE Blk(-/-) mice have an IgM-negative population of splenic B cells, similar to the TgE mice. LMP2A was also transiently transfected into the human EBV-negative B-cell line BJAB to determine which SFK members associate with LMP2A. Lyn was detected in LMP2A immunoprecipitates, whereas Fyn was not. Both Lyn and Fyn were able to bind to an LMP2A mutant which contained a sequence shown previously to bind tightly to the SH2 domain of multiple SFK members. From these results, we conclude that LMP2A preferentially associates with and signals through Lyn compared to its association with other SFKs. This preferential association is due in part to the SH2 domain of Lyn associating with LMP2A.

Engagement of the B-cell antigen receptor activates STAT through Lyn in a Jak-independent pathway

Engagement of the B-cell antigen receptor (BCR) initiated by the Src kinase Lyn triggers rapid signaling cascades, leading to proliferation, differentiation or growth arrest of B cells. The Janus kinase (JAK)-STAT (signal transducer and activator of transcription) pathway, activated through cytokine receptors, mediates similar responses. Hypothesizing that Src and JAK pathways engage in crosstalk in B-cell signaling, we studied wild-type and Lyn-null B-cell lines, which express BCR. We found that activated BCR results in tyrosine phosphorylation of JAK-STAT, which required Lyn. To confirm that STAT activation is not due to JAK, we cloned the chicken homologs of JAK1 and JAK2 and made their antisense constructs. In cells expressing antisense JAK1 and JAK2, tyrosine phosphorylation of STAT was not inhibited following BCR stimulation. Using activation loop-specific phosphotyrosine antibodies, we did not detect phospho-JAK1 and phospho-JAK2 after BCR stimulation. The JAK inhibitor AG490 did not inhibit the tyrosine phosphorylation of Lyn or STAT after BCR simulation. An in vitro phosphorylation assay showed that Lyn directly phosphorylates STAT3. In an electrophoretic mobility shift assay, BCR stimulation led to enhanced DNA binding of the STAT3 in DT40, but not in the Lyn-null cells. We conclude that BCR engagement activates the STAT pathway via Lyn, independent of JAK.

Chicken genomics resource: sequencing and annotation of 35,407 ESTs from single and multiple tissue cDNA libraries and CAP3 assembly of a chicken gene index

Its accessibility, unique evolutionary position, and recently assembled genome sequence have advanced the chicken to the forefront of comparative genomics and developmental biology research as a model organism. Several chicken expressed sequence tag (EST) projects have placed the chicken in 10th place for accrued ESTs among all organisms in GenBank. We have completed the single-pass 5′-end sequencing of 37,557 chicken cDNA clones from several single and multiple tissue cDNA libraries and have entered 35,407 EST sequences into GenBank. Our chicken EST sequences and those found in public databases (on July 1, 2004) provided a total of 517,727 public chicken ESTs and mRNAs. These sequences were used in the CAP3 assembly of a chicken gene index composed of 40,850 contigs and 79,192 unassembled singlets. The CAP3 contigs show a 96.7% match to the chicken genome sequence. The University of Delaware (UD) EST collection (43,928 clones) was assembled into 19,237 nonredundant sequences (13,495 contigs and 5,742 unassembled singlets). The UD collection contains 6,223 unique sequences that are not found in other public EST collections but show a 76% match to the chicken genome sequence. Our chicken contig and singlet sequences were annotated according to the highest BlastX and/or BlastN hits. The UD CAP3 contig assemblies and singlets are searchable by nucleotide sequence or key word ( http://cogburn.dbi.udel.edu ), and the cDNA clones are readily available for distribution from the chick EST website and clone repository ( http://www.chickest.udel.edu ). The present paper describes the construction and normalization of single and multiple tissue chicken cDNA libraries, high-throughput EST sequencing from these libraries, the CAP3 assembly of a chicken gene index from all public ESTs, and the identification of several nonredundant chicken gene sets for production of custom DNA microarrays.

Increased expression of genes linked to FcepsilonRI Signaling and to cytokine and chemokine production in Lyn-deficient mast cells

Cross-linking the high-affinity IgE receptor, FcepsilonRI, on mast cells activates signaling pathways leading to the release of preformed inflammatory mediators and the production of cytokines and chemokines associated with allergic disorders. Bone marrow-derived mast cells (BMMCs) from Lyn-deficient (Lyn-/-) mice are hyperresponsive to FcepsilonRI cross-linking with multivalent Ag. Previous studies linked the hyperresponsive phenotype in part to increased Fyn kinase activity and reduced SHIP phosphatase activity in the Lyn-/- BMMCs in comparison with wild-type (WT) cells. In this study, we compared gene expression profiles between resting and Ag-activated WT and Lyn-/- BMMCs to identify other factors that may contribute to the hyperresponsiveness of the Lyn-/- cells. Among genes implicated in the positive regulation of FcepsilonRI signaling, mRNA for the tyrosine kinase, Fyn, and for several proteins contributing to calcium regulation are more up-regulated following Ag stimulation in Lyn-/- BMMCs than in WT BMMCs. Conversely, mRNA for the low-affinity IgG receptor (FcgammaRIIB), implicated in negative regulation of FcepsilonRI-mediated signaling, is more down-regulated in Ag-stimulated Lyn-/- BMMCs than in WT BMMCs. Genes coding for proinflammatory cytokines and chemokines (IL-4, IL-6, IL-13, CSF, CCL1, CCL3, CCL5, CCL7, CCL9, and MIP1beta) are all more highly expressed in Ag-stimulated Lyn-/- mast cells than in WT cells. These microarray data identify Lyn as a negative regulator in Ag-stimulated BMMCs of the expression of genes linked to FcepsilonRI signaling and also to the response pathways that lead to allergy and asthma.

Generation of high-affinity antibody against T cell-dependent antigen in the Ganp gene-transgenic mouse

Generation of high-affinity Ab is impaired in mice lacking germinal center-associated DNA primase (GANP) in B cells. In this study, we examined the effect of its overexpression in ganp transgenic C57BL/6 mice (Ganp(Tg)). Ganp(Tg) displayed normal phenotype in B cell development, serum Ig levels, and responses against T cell-independent Ag; however, it generated the Ab with much higher affinity against nitrophenyl-chicken gammaglobulin in comparison with C57BL/6. To further examine the affinity increase, we established hybridomas producing high-affinity mAbs and compared their affinities using BIAcore. C57BL/6 generated high-affinity anti-nitrophenyl mAbs (K(D) approximately 2.50 x 10(-7) M) of IgG1/lambda1 and contained the V(H)186.2 region with W33L mutation. Ganp(Tg) generated much higher affinity (K(D) > 1.57 x 10(-9) M) by usage of V(H)186.2 as well as noncanonical V(H)7183 regions. Ganp(Tg) also generated exceptionally high-affinity anti-HIV-1 (V3 peptide) mAbs (K(D) > 9.90 x 10(-11) M) with neutralizing activity. These results demonstrated that GANP is involved in V region alteration generating high-affinity Ab.