scholarly journals IL-7 receptor signaling is necessary for stage transition in adult B cell development through up-regulation of EBF

2005 ◽  
Vol 201 (8) ◽  
pp. 1197-1203 ◽  
Author(s):  
Kazu Kikuchi ◽  
Anne Y. Lai ◽  
Chia-Lin Hsu ◽  
Motonari Kondo
Keyword(s):  
Transcription Factor ◽  
B Cells ◽  
B Cell ◽  
Target Genes ◽  
Cell Development ◽  
B Cell Development ◽  
Receptor Signaling ◽  
Cell Stage ◽  
Transcription Factor Network ◽  
Stage Transition

Cytokine receptor signals have been suggested to stimulate cell differentiation during hemato/lymphopoiesis. Such action, however, has not been clearly demonstrated. Here, we show that adult B cell development in IL-7−/− and IL-7Rα2/− mice is arrested at the pre–pro-B cell stage due to insufficient expression of the B cell–specific transcription factor EBF and its target genes, which form a transcription factor network in determining B lineage specification. EBF expression is restored in IL-7−/− pre–pro-B cells upon IL-7 stimulation or in IL-7Rα−/− pre–pro-B cells by activation of STAT5, a major signaling molecule downstream of the IL-7R signaling pathway. Furthermore, enforced EBF expression partially rescues B cell development in IL-7Rα−/− mice. Thus, IL-7 receptor signaling is a participant in the formation of the transcription factor network during B lymphopoiesis by up-regulating EBF, allowing stage transition from the pre–pro-B to further maturational stages.

scholarly journals Induction of metabolic quiescence defines the transitional to follicular B cell switch

2019 ◽  
Vol 12 (604) ◽  
pp. eaaw5573 ◽  
Author(s):  
Jocelyn R. Farmer ◽  
Hugues Allard-Chamard ◽  
Na Sun ◽  
Maimuna Ahmad ◽  
Alice Bertocchi ◽  
...  
Keyword(s):  
B Cells ◽  
Cell Surface ◽  
B Cell ◽  
Cell Development ◽  
B Cell Development ◽  
Ampk Activation ◽  
Cell Stage ◽  
Extracellular Adenosine ◽  
Transitional B Cells ◽  
Follicular B Cells

Transitional B cells must actively undergo selection for self-tolerance before maturing into their resting follicular B cell successors. We found that metabolic quiescence was acquired at the follicular B cell stage in both humans and mice. In follicular B cells, the expression of genes involved in ribosome biogenesis, aerobic respiration, and mammalian target of rapamycin complex 1 (mTORC1) signaling was reduced when compared to that in transitional B cells. Functional metabolism studies, profiling of whole-cell metabolites, and analysis of cell surface proteins in human B cells suggested that this transition was also associated with increased extracellular adenosine salvage. Follicular B cells increased the abundance of the cell surface ectonucleotidase CD73, which coincided with adenosine 5′-monophosphate–activated protein kinase (AMPK) activation. Differentiation to the follicular B cell stage in vitro correlated with surface acquisition of CD73 on human transitional B cells and was augmented with the AMPK agonist, AICAR. Last, individuals with gain-of-function PIK3CD (PI3Kδ) mutations and increased pS6 activation exhibited a near absence of circulating follicular B cells. Together, our data suggest that mTORC1 attenuation may be necessary for human follicular B cell development. These data identify a distinct metabolic switch during human B cell development at the transitional to follicular stages, which is characterized by an induction of extracellular adenosine salvage, AMPK activation, and the acquisition of metabolic quiescence.

Identification of Cyclin D3 as a Direct Transcriptional Target of E2A Using Damid.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 1618-1618
Author(s):  
John K. Choi ◽  
Siyuan Song ◽  
Jonathan Cooperman ◽  
Danielle L. Letting ◽  
Gerd A. Blobel
Keyword(s):  
Transcription Factor ◽  
Dna Binding ◽  
B Cell ◽  
Target Genes ◽  
Cell Development ◽  
Binding Activity ◽  
B Cell Development ◽  
Cyclin D3 ◽  
Dna Binding Activity ◽  
E Boxes

Abstract The transcription factor E2A is required for very early B cell development. The exact mechanism by which E2A promotes B cell development is unclear and cannot be explained by the known E2A targets, components of the pre-B cell receptor and cyclin dependent kinase inhibitors, indicating additional pathways and targets remain to be identified. We had previously reported that E2A can promote precursor B cell expansion, promote G1 cell cycle progression, and induce the expressions of multiple G1 phase cyclins including cyclin D3, suggesting that E2A induction of these genes may contribute to early B cell development. To better understand the mechanism by which E2A induces these cyclins, we characterized the relationship between E2A and the cyclin D3 gene promoter. E2A transactivated a luciferase reporter plasmid containing the 1kb promoter of cyclin D3 that contains two consensus E2A binding sites (E-boxes); however, deletion of the E-boxes did not disrupt the transactivation by E2A. We hypothesized three possible mechanisms: 1) indirect activation of cyclin D3 via another transcription factor, 2) binding of E2A to cryptic non-E-boxes, or 3) recruitment of E2A to the promoter via interaction with other DNA binding factor. To test the first possibility, promoter occupancy was examined using the DamID approach. In this approach, a fusion protein consisting of E. coli DNA adenosine methyltransferase (DAM) and a transcription factor of interest is expressed at low levels, resulting in specific methylation of adenosine residues within 2–5 kb of the transcription factor target sites. A fusion construct composed of E2A and DAM (E47Dam), was subcloned in lentiviral vectors, and used to transduce precursor B cell lines. The methylated adenosine residues were detected using a sensitive ligation-mediated PCR (LM-PCR) assay that required only 1 ug of genomic DNA and can detect methylation even if only 3% of the cells express E47Dam; no methylated adenosines were detected in control cells, indicating that all methylated residues resulted from E47Dam. Specific adenosine methylation was identified at the IgH intronic enhancer, a known E2A target site, but not at the non-target sites, CD19, HPRT, and GAPDH promoters. Specific methylation was detected at the cyclin D3 promoter but not 10 kb down-stream, despite similar concentrations of E-boxes at both sites. Chromatin immunoprecipitation analysis confirmed the DamID findings and further localized the binding to within 1 kb of the two E-boxes in the cyclin D3 promoter. To distinguish between the two remaining mechanisms (cryptic non-E-boxes versus recruitment via other DNA binding factors), two point mutations were introduced into E47Dam that disrupted its DNA binding activity. The mutated E47Dam continued to methylate at the cyclin D3 promoter. We conclude that E2A can be recruited to the cyclin D3 promoter, independent of E-boxes or E2A DNA binding activity. Our findings raise the possibility that some direct E2A target genes may lack functional E-boxes. Furthermore, mutated E2A, lacking an E2A DNA binding domain, that is seen in 6% of pediatric ALLs may still activate a subset of E2A target genes. Finally, our application of lentiviral vectors and LM-PCR to the DamID approach should permit analysis of primary human precursor B cells, despite the limitations in cell number and transduction efficiency.

The Proto-Oncogene LRF Is Essential for Normal Mature B Cell Development and Germinal Center Formation.

Blood ◽  
2008 ◽  
Vol 112 (11) ◽  
pp. 1788-1788
Author(s):  
Nagisa Sakurai ◽  
Manami Maeda ◽  
Sung-UK Lee ◽  
Julie Teruya-Feldstein ◽  
Takahiro Maeda
Keyword(s):  
B Cells ◽  
B Cell ◽  
Cell Development ◽  
B Cell Development ◽  
Cell Stage ◽  
Transitional B Cells ◽  
Notch Pathways ◽  
Secondary Lymphoid Organs

Abstract LRF (Leukemia/Lymphoma Related Factor, also known as Pokemon, FBI-1, OCZF and ZBTB7a) was originally identified as an interaction partner of the oncoprotein BCL6. LRF can act as a proto-oncogene by repressing the tumor suppressor ARF and cooperates with BCL6 in MEF (mouse embryonic fibroblasts) immortalization. It is highly expressed in human Non-Hodgkin Lymphoma (NHL) cases, in the pathogenesis of which BCL6 is known to be involved (Maeda et al. Nature 2005). Inducible inactivation of the LRF gene in mouse Hematopoietic Stem Cells (HSCs) results in complete block of early B cell development at the HSC/progenitor stages and concomitant development of double positive (DP) T cells in the bone marrow (BM) (Maeda et al. Science 2007). While these findings clearly illustrate key roles of LRF in normal and malignant B cell development, it is not fully identified as to which B cell stages LRF is required during normal B cell development. To elucidate the role of LRF in B cells in vivo, we established and characterized B cell-specific LRF conditional knockout (KO) mice. We took advantage of mb-1 Cre knock-in mice, in which Cre expression is restricted to the B cells after the ProB cell stage. B cell compartments in the BM (PreProB, ProB, PreB and immatureB) are grossly normal in LRFF/ Fmb1-Cre mice. The LRF gene was efficiently eliminated in BM CD19+ B cells revealed by quantitative real-time PCR assay. Furthermore, LRF protein was not detected in purified CD19+ B cells, but seen in CD19-non-B cells, confirming the specific inactivation of the LRF gene in B cells. Thus, despite its critical role at the HSC/progenitor stages, LRF was found to be dispensable for the survival of normal BM B cells. These findings are consistent with the fact that GSI treatment (Maeda et al. Science 2007) or Notch1 loss (Lee and Maeda, unpublished) rescues the defects in early B cell development seen in LRFF/FMx1-Cre+ mice. Notch signaling is necessary for the transitional B cells to commit to the marginal zone B cells (MZB). Inactivation of the component of the Notch pathways in mice results in no MZB development. On the contrary, deletion of the MINT/SHARP gene, a suppressor of Notch signaling, leads to increase of MZB cells and concomitant reduction of follicular B (FOB) cells, indicating that Notch induces MZB cell fate at the transitional B cell stage. Given that LRF is a potent Notch suppressor at the HSC/progenitor stages, we hypothesized that LRF opposes Notch pathway in mature B cells as well. To test this hypothesis, we characterized mature B cell development in LRFF/Fmb1-Cre mice. While transitional B cells were largely unaffected in LRFF/Fmb1-Cre mice, we observed a slight but statistically significant reduction of follicular (FO) B cells (B220+CD19+AA4.1-CD1d-CD23+) and concomitant increase of MZB cells (B220+CD19+AA4.1-CD1d+CD23-) as seen in MINT/SHARP knockout mice. Thus, LRF may also oppose Notch pathways at the branching point for the FOB vs. MZB fate decision. Finally, to determine the role of LRF in Germinal Center (GC) formation in vivo, we characterized secondary lymphoid organs of LRFF/Fmb1-Cre mice after antigen stimulation. Both spleen and Peyer’s Patches were analyzed two weeks after immunization with Chicken Gamma Globulin (NP-CGG). While a GC reaction was robustly induced in control mice upon immunization, GC formation was significantly impaired in LRFF/Fmb1-Cre mice as revealed by immuno-histochemical analysis (IHC) and FACS. Only few GC cells (B220+CD19+FAS+CD38-PNA+) were observed in spleens, and the absolute numbers of GC cells were drastically reduced in LRFF/Fmb1-Cre mice. Residual LRF-deficient GC B cells were mostly negative for CXCR4, which is predominantly expressed in proliferating centroblasts within GCs, suggesting that LRF-deficient GC B cells may have defects in cellular proliferation in response to antigen stimuli. Our data indicates that LRF plays key roles in mature B cell development in the secondary lymphoid organs, but dispensable for the maintenance of early BM B cells.

scholarly journals Essential role of EBF1 in the generation and function of distinct mature B cell types

2012 ◽  
Vol 209 (4) ◽  
pp. 775-792 ◽  
Author(s):  
Bojan Vilagos ◽  
Mareike Hoffmann ◽  
Abdallah Souabni ◽  
Qiong Sun ◽  
Barbara Werner ◽  
...  
Keyword(s):  
B Cells ◽  
B Cell ◽  
Intracellular Signaling ◽  
Ectopic Expression ◽  
Cell Types ◽  
Cell Development ◽  
B Cell Development ◽  
Cell Stage ◽  
Cell Immunity ◽  
Cell Commitment

The transcription factor EBF1 is essential for lineage specification in early B cell development. In this study, we demonstrate by conditional mutagenesis that EBF1 is required for B cell commitment, pro–B cell development, and subsequent transition to the pre–B cell stage. Later in B cell development, EBF1 was essential for the generation and maintenance of several mature B cell types. Marginal zone and B-1 B cells were lost, whereas follicular (FO) and germinal center (GC) B cells were reduced in the absence of EBF1. Activation of the B cell receptor resulted in impaired intracellular signaling, proliferation and survival of EBF1-deficient FO B cells. Immune responses were severely reduced upon Ebf1 inactivation, as GCs were formed but not maintained. ChIP- and RNA-sequencing of FO B cells identified EBF1-activated genes that encode receptors, signal transducers, and transcriptional regulators implicated in B cell signaling. Notably, ectopic expression of EBF1 efficiently induced the development of B-1 cells at the expense of conventional B cells. These gain- and loss-of-function analyses uncovered novel important functions of EBF1 in controlling B cell immunity.

scholarly journals Increased Frequency of Pre–Pro B Cells in the Bone Marrow of New Zealand Black (NZB) Mice: Implications for aDevelopmental Block in B Cell Differentiation

2002 ◽  
Vol 9 (1) ◽  
pp. 35-45 ◽  
Author(s):  
Zhe-Xiong Lian ◽  
Hiroto Kita ◽  
Tomoyuki Okada ◽  
Tom Hsu ◽  
Leonard D. Shultz ◽  
...  
Keyword(s):  
New Zealand ◽  
Cell Differentiation ◽  
B Cells ◽  
B Cell ◽  
Cell Receptor ◽  
Cell Development ◽  
B Cell Development ◽  
B Cell Differentiation ◽  
Differentiation Markers ◽  
Cell Stage

Reductions in populations of both Pre-B cell (Hardy fractions D) and Pro-B cells (Hardy fractions B–C) have been described in association with murine lupus. Recent studies of B cell populations, based on evaluation of B cell differentiation markers, now allow the enumeration and enrichment of other stage specific precursor cells. In this study we report detailed analysis of the ontogeny of B cell lineage subsets in New Zealand black (NZB) and control strains of mice. Our data suggest that B cell development in NZB mice is partially arrested at the fraction A Pre–Pro B cell stage. This arrest at the Pre-Pro B cell stage is secondary to prolonged lifespan and greater resistance to spontaneous apoptosis. In addition, expression of the gene encoding the critical B cell development transcription factor BSAP is reduced in the Pre–Pro B cell stage in NZB mice. This impairment may influence subsequent B cell development to later stages, and thereby accounts for the down-regulation of the B cell receptor componentIgα(mb-1). Furthermore, levels of expression of theRug2, λ5andIgβ(B29) genes are also reduced in Pre–Pro B cells of NZB mice. The decreased frequency of precursor B cells in the Pre–Pro B cell population occurs at the most primitive stage of B cell differentiation.

scholarly journals Complete Block of Early B Cell Differentiation in Mice Lacking the Endosomal Adaptor Protein p14

Blood ◽  
2015 ◽  
Vol 126 (23) ◽  
pp. 1026-1026
Author(s):  
Marcin Lyszkiewicz ◽  
Daniel Kotlarz ◽  
Natalia Zietara ◽  
Gudrun Brandes ◽  
Jana Diestelhorst ◽  
...  
Keyword(s):  
B Cells ◽  
B Cell ◽  
Target Genes ◽  
Conflicts Of Interest ◽  
Adaptor Protein ◽  
Cell Development ◽  
B Cell Development ◽  
B Cell Differentiation ◽  
Knock Out ◽  
And Function

Abstract Human primary immunodeficiency caused by a point mutation in the 3' untranslated region of the endosomal adaptor protein p14 (also known as Lamtor2) resulted in severely impaired function of neutrophils, B cells, T cells and melanocytes. However, complexity of the phenotype and scarcity of human material preclude in-depth studies. Therefore, to gain insight into the role of p14 in B cell development and function, we generated loxP conditional knock-out mice. Using mb-1-Cre mice we demonstrated that loss of p14 at the preB1 stage lead to a complete block of B cell development, resulting in the absence of IgM-positive B cells. Further, to test the significance of p14 deficiency in peripheral organs, we took advantage of CD19-Cre mice, which have limited efficiency in deleting target genes in the bone marrow, but reach up to 95% efficiency in spleen. Thus, we could demonstrate that later in B cell development, p14 was essential for the generation and activation of mature B lymphocytes. While B1 cell development was maintained, splenic follicular B cells were massively reduced in the absence of p14. Furthermore, activation of B cell receptor (BCR) resulted in impaired intracellular signalling and proliferation of p14 deficient B cells. In particular, lack of p14 lead to delayed internalization of BCR and endosomal processing associated with impaired mobilization of Ca++ from intracellular stores as well as aberrant phosphorylation of BCR-associated kinases. In conclusion, our data revealed that p14 is a critical regulator of B cell development and function, which acts by modulating BCR signalling. Disclosures No relevant conflicts of interest to declare.

scholarly journals The mitochondrial iron transporter ABCB7 is required for B cell development, proliferation, and class switch recombination in mice

eLife ◽  
2021 ◽  
Vol 10 ◽  
Author(s):  
Michael Jonathan Lehrke ◽  
Michael Jeremy Shapiro ◽  
Matthew J Rajcula ◽  
Madeleine M Kennedy ◽  
Shaylene A McCue ◽  
...  
Keyword(s):  
Bone Marrow ◽  
B Cells ◽  
B Cell ◽  
Class Switch Recombination ◽  
Cell Development ◽  
B Cell Development ◽  
Intracellular Iron ◽  
Cell Stage ◽  
Class Switch ◽  
Mitochondrial Transporter

Iron-sulfur (Fe-S) clusters are cofactors essential for the activity of numerous enzymes including DNA polymerases, helicases, and glycosylases. They are synthesized in the mitochondria as Fe-S intermediates and are exported to the cytoplasm for maturation by the mitochondrial transporter ABCB7. Here, we demonstrate that ABCB7 is required for bone marrow B cell development, proliferation, and class switch recombination, but is dispensable for peripheral B cell homeostasis in mice. Conditional deletion of ABCB7 using Mb1-cre resulted in a severe block in bone marrow B cell development at the pro-B cell stage. The loss of ABCB7 did not alter expression of transcription factors required for B cell specification or commitment. While increased intracellular iron was observed in ABCB7-deficient pro-B cells, this did not lead to increased cellular or mitochondrial reactive oxygen species, ferroptosis, or apoptosis. Interestingly, loss of ABCB7 led to replication-induced DNA damage in pro-B cells, independent of VDJ recombination, and these cells had evidence of slowed DNA replication. Stimulated ABCB7-deficient splenic B cells from CD23-cre mice also had a striking loss of proliferation and a defect in class switching. Thus, ABCB7 is essential for early B cell development, proliferation, and class switch recombination.

B Cell Development in the Absence of PU.1.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 226-226 ◽  
Author(s):  
Min Ye ◽  
Olga Ermaermakova-Cirilli ◽  
Thomas Graf
Keyword(s):  
Bone Marrow ◽  
B Cells ◽  
B Cell ◽  
Target Genes ◽  
Fetal Liver ◽  
Cell Formation ◽  
Cell Development ◽  
B Cell Development ◽  
B Lineage

Abstract Mice deficient of the ETS-family transcription factor PU.1 lack B cells as well as macrophages. While most macrophage specific genes are known to be regulated by high levels of PU.1, the reason for the defect in B cell formation is not known. Here we analyzed a mouse strain in which a floxed version of the PU.1 gene, surrounding exon 4 and 5, which encode the DNA, binding and PEST domains (developed by C. Somoza and D. Tenen), was excised by Cre mediated recombination. As expected, this strain lacks both B cells and macrophages and die at birth. Surprisingly, however, we were able to establish lymphoid cell lines from fetal livers of these mice (day 14 to day 18), which proliferated on S17 stromal cells supplemented with IL-7 and stem cell factor. These cells expressed the B lineage cell surface markers CD19, CD43, BP-1 and CD24, but not B220. They also expressed B cell transcription factors, EBF, E47, Pax5, and their target genes, Rag1, IL7R, λ5 and v-preB, as detected by RT-PCR, exhibited DJ and VDJ immunoglobulin heavy chain rearrangements, and expressed IgM after IL-7 withdrawal. We then tested the effect of PU.1 deletion in B cells in adult animals by crossing the floxed PU.1 strain with a CD19 Cre mouse line. The spleen and peripheral blood (but not bone marrow) of these mice contained B cells that were CD19+ IgMlow, IgDhigh but B220 negative and instead expressed CD43. Thus PU.1 is not essential for immunoglobulin production and late B cell development. Although PU.1−/− fetal liver cells can give rise to cells, resembling Pre-B in vitro, the process of B cell formation was delayed by almost 12 days, compared with wt fetal liver, and the efficiency was reduced approximately 25-fold. In addition, PU.1 deficient B cells demonstrated an impaired ability to engraft into the bone marrow, when injected into irradiated SCID mice. We have found that PU.1 deficient B progenitors showed reduced or undetectable levels of the SDF1 receptor CXCR4, a receptor that has been implicated in B cell homing. Taken together, our observations suggest that PU.1 plays two different roles during B cell development: for early B cell formation and for proper migration and engraftment, which might be mediated through regulation of CXCR4 expression.

Conditional Knockout of Sox4 Markedly Affects B Lymphopoiesis in Adult Mice.

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 2524-2524
Author(s):  
Baohua Sun ◽  
Saradhi Mallampati ◽  
Yun Gong ◽  
Donghai Wang ◽  
M. James You ◽  
...  
Keyword(s):  
Stem Cells ◽  
B Cells ◽  
B Cell ◽  
Cell Development ◽  
B Cell Development ◽  
Conditional Knockout ◽  
B Lymphopoiesis ◽  
Cell Stage ◽  
Hematopoietic Stem ◽  
Adult Mice

Abstract Abstract 2524 Poster Board II-501 B cells constitute an integral part of the immune system. The development of mature B cells from hematopoietic stem cells is a complex process that is regulated in a hierarchical order by various proteins, particularly transcription factors. Sox4 is a SRY-related HMG box containing transcription factor and is known to be involved in B cell development. However, the role of Sox4 in various stages of B cell development has not been systematically investigated. In this study we used a conditional knockout mouse strain and studied the effect of Sox4 deletion in B lymphopoiesis in adult mice. We crossed the Sox4-floxed mice with different Cre mouse strains that were expected to delete the floxed Sox4 gene at different B cell developmental stages. These Cre strains included Vav-iCre (expressed in hematopoietic stem cell stage, starting from early embryos), MX1-Cre (expression in hematopoietic stem cells, induced by pIpC injection in adults), MB1-Cre (expressed in B cells, starting from early progenitor cells in embryos), and CD21-Cre (expressed in mature B cells). We demonstrated that deletion of Sox4 caused an arrest of B lymphopoiesis at the transition from pre-pro-B cell (fraction A) stage to pro-B cell stage (fraction B): fraction A cells are slightly reduced in number whereas fraction B and later stage cells are nearly absent. The pre-pro-B cells from the Sox4 knockout mice retain a population of AA4.1+ cells, which are considered to be developed into B cells. Deletion of Sox4 in early embryonic stage (Vav-iCre) or in adults (Mx1-Cre) results in a similar phenotype on B lymphopoiesis, except that peritoneal B1 cells appear to be affected with Vav-iCre, but not with Mx1-Cre. MB1-Cre gave rise to similar results as did Vav-iCre, but the arrest was not as dramatic as with Vav-iCre. CD21-Cre produced no significant difference in B cell phenotype. These data suggested that Sox4 is required for early B cell development at the transition from pre-pro-B cells to pro-B cells and is not required for mature B cells. We are currently investigating the transcription program of this transcription factor in B cell development. Disclosures: No relevant conflicts of interest to declare.

Tumor Suppressors BTG1 and BTG2 Fulfill Both Unique and Overlapping Functions During Normal B Lymphocyte Development

Blood ◽  
2012 ◽  
Vol 120 (21) ◽  
pp. 1303-1303
Author(s):  
Esther J.H. Tijchon ◽  
Liesbeth van Emst ◽  
Jørn Havinga ◽  
Jean-Pierre Rouault ◽  
Felice Tirone ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Transcription Factors ◽  
B Cells ◽  
B Cell ◽  
Tumor Suppressors ◽  
Cell Development ◽  
B Cell Development ◽  
B Cell Differentiation ◽  
B Cell Malignancies ◽  
Cell Stage

Abstract Abstract 1303 B-cell precursor acute lymphoblastic leukemia (BCP-ALL) is the most common form of cancer in children, characterized by genetic aberrations affecting master regulators of lymphoid differentiation, such as RUNX1, IKZF1, TCF3, and PAX5, as well as tumor suppressor genes that control the cell cycle, including RB1 and CDKN2A. Another gene frequently altered in BCP-ALL is BTG1, which displays highly clustered mono-allelic deletions in childhood BCP-ALL (9%) and adult ALL (6%). The frequency of BTG1 deletions is two- to three-fold higher in ETV6-RUNX1- and BCR-ABL1-positive leukemias. BTG1, and its close homologue BTG2 regulate gene expression, for instance by associating with protein arginine methyltransferase 1 (PRMT1), affecting the activity of a variety of transcription factors, including several nuclear hormone receptors and HoxB9. In addition, BTG1 and BTG2 have been implicated in regulating mRNA stability by interacting with the Ccr4-Not complex. Recent studies have also identified missense point mutations in BTG1 and BTG2 in about 20% of non-Hodgkin lymphomas, arguing that altered function of these genes contributes to B cell malignancies. To investigate a role of BTG1 and BTG2 in B cell development, we studied the phenotype of Btg1 and Btg2 single knockout (KO) and Btg1;Btg2 double KO mice. Animals deficient for either BTG1 or BTG2 displayed a mild B cell phenotype with a moderate reduction of ∼20% in the total amount of B220+ progenitor B cells in bone marrow, while splenic B cells were present at normal frequencies. More detailed analyses revealed that Btg1−/− and Btg2−/− mice both showed a partial block at the pre-pro-B cell stage (Hardy fraction A). Methylcellulose colony assays in the presence of interleukin-7 (IL-7) demonstrated 30% fewer colonies using bone marrow from Btg2−/− mice, whereas 70% fewer colonies were obtained using bone marrow derived from Btg1−/− mice. To assess whether BTG1 and BTG2 fulfill redundant functions during B cell development, we analyzed the phenotype of Btg1−/−;Btg2−/− mice. Hence we observed that the combined loss of BTG1 and BTG2 led to a much stronger block in B cell differentiation, with the majority of progenitor B cells arrested at the pre-pro-B cell stage. In the spleens of these double knockout mice we observed a roughly 50% reduction in B220+ IgM+ B cells, suggesting that these genes act to modify the activity of B lineage transcription factors rather than to fully block their activities. This is consistent with a role for these genes as modifiers of transcriptional activity. Current studies are aimed at defining the molecular targets regulated by BTG1 and BTG2 during early B cell development using RNA sequencing and protein interaction experiments. In conclusion, our data demonstrate that BTG1 and BTG2 act as important regulators of normal B cell differentiation, and that this function might be critical for their role as tumor suppressors in (early) B cell malignancies. Disclosures: No relevant conflicts of interest to declare.

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