Deficiency of the adaptor SLP-65 in pre-B-cell acute lymphoblastic leukaemia

ARID5B regulates fatty acid metabolism and proliferation at the Pre-B cell stage during B cell development

During B cell development in bone marrow, large precursor B cells (large Pre-B cells) proliferate rapidly, exit the cell cycle, and differentiate into non-proliferative (quiescent) small Pre-B cells. Dysregulation of this process may result in the failure to produce functional B cells and pose a risk of leukemic transformation. Here, we report that AT rich interacting domain 5B (ARID5B), a B cell acute lymphoblastic leukemia (B-ALL) risk gene, regulates B cell development at the Pre-B stage. In both mice and humans, we observed a significant upregulation of ARID5B expression that initiates at the Pre-B stage and is maintained throughout later stages of B cell development. In mice, deletion of Arid5b in vivo and ex vivo exhibited a significant reduction in the proportion of immature B cells but an increase in large and small Pre-B cells. Arid5b inhibition ex vivo also led to an increase in proliferation of both Pre-B cell populations. Metabolic studies in mouse and human bone marrow revealed that fatty acid uptake peaked in proliferative B cells then decreased during non-proliferative stages. We showed that Arid5b ablation enhanced fatty acid uptake and oxidation in Pre-B cells. Furthermore, decreased ARID5B expression was observed in tumor cells from B-ALL patients when compared to B cells from non-leukemic individuals. In B-ALL patients, ARID5B expression below the median was associated with decreased survival particularly in subtypes originating from Pre-B cells. Collectively, our data indicated that Arid5b regulates fatty acid metabolism and proliferation of Pre-B cells in mice, and reduced expression of ARID5B in humans is a risk factor for B cell leukemia.

Puppet masters of B-cell progenitor acute lymphoblastic leukemia: The preB cell receptor and the interleukin 7 receptor α

B-cell progenitor acute lymphoblastic leukemia (BCP-ALL) is enriched for a preB cell phenotype, hinting at a specific vulnerability of this cell stage. Two signaling pathways via the preB cell receptor (preBCR) and the interleukin 7 receptor α (IL-7Rα) chain govern the balance between differentiation and proliferation at this stage and both receptor pathways are routinely altered in human BCP-ALL. Here, we review the immunobiology of both the preBCR as well as the IL-7Rα and analyze the human BCP-ALL spectrum in the light of these signaling complexes. Finally, we present a terminology for preBCR signaling modules that distinguishes a pro-proliferative "phase-I" module from a pro-differentiative "phase-II" module. This terminology might serve as a framework to better address shared oncogenic mechanics of preB cell stage BCP-ALL.

CD137L and CD4 T cells limit BCL6-expressing pre-germinal center B cell expansion and BCL6-driven B cell malignancy

Aberrant expression of the proto-oncogene BCL6 is a driver of tumorigenesis in diffuse large B cell lymphoma (DLBCL). Mice overexpressing BCL6 from the B cell-specific immunoglobulin heavy chain μ intron promoter (Iμ-Bcl6^Tg/+ ) develop B cell lymphomas with features typical of human DLBCL. While the development of B cell lymphoma in these mice is tightly controlled by T cells, the mechanisms of this immune surveillance are poorly understood. Here we show that CD4 T cells contribute to the control of lymphoproliferative disease in lymphoma-prone Iμ-Bcl6^Tg/+ mice. We reveal that this CD4 T cell immuno-surveillance requires signaling by the co-stimulatory molecule CD137 ligand (CD137L; also known as 4-1BBL), which may promote the transition of pre-malignant B cells with an activated phenotype into the germinal center stage via reverse signaling, preventing their hazardous accumulation. Thus, CD137L-mediated CD4 T cell immuno-surveillance adds another layer of protection against B cell malignancy to that provided by CD8 T cell cytotoxicity.

C/EBPβ induces B-cell acute lymphoblastic leukemia and cooperates with BLNK mutations

BLNK (BASH/SLP‐65) encodes an adaptor protein that plays an important role in B‐cell receptor (BCR) signaling. Loss‐of‐function mutations in this gene are observed in human pre‐B acute lymphoblastic leukemia (ALL), and a subset of Blnk knock‐out (KO) mice develop pre‐B‐ALL. To understand the molecular mechanism of the Blnk mutation‐associated pre‐B‐ALL development, retroviral tagging was applied to KO mice using the Moloney murine leukemia virus (MoMLV). The Blnk mutation that significantly accelerated the onset of MoMLV‐induced leukemia and increased the incidence of pre‐B‐ALL Cebpb was identified as a frequent site of retroviral integration, suggesting that its upregulation cooperates with Blnk mutations. Transgenic expression of the liver‐enriched activator protein (LAP) isoform of Cebpb reduced the number of mature B‐lymphocytes in the bone marrow and inhibited differentiation at the pre‐BI stage. Furthermore, LAP expression significantly accelerated leukemogenesis in Blnk KO mice and alone acted as a B‐cell oncogene. Furthermore, an inverse relationship between BLNK and C/EBPβ expression was also noted in human pre‐B‐ALL cases, and the high level of CEBPB expression was associated with short survival periods in patients with BLNK‐downregulated pre‐B‐ALL. These results indicate the association between the C/EBPβ transcriptional network and BCR signaling in pre‐B‐ALL development and leukemogenesis. This study gives insight into ALL progression and suggests that the BCR/C/EBPβ pathway can be a therapeutic target.

Developmental partitioning of SYK and ZAP70 prevents autoimmunity and cancer

Even though SYK and ZAP70 kinases share high sequence homology and serve analogous functions, their expression in B and T cells is strictly segregated throughout evolution. Here, we identified aberrant ZAP70 expression as a common feature in a broad range of B cell malignancies. We validated SYK as the kinase that sets the thresholds for negative selection of autoreactive and premalignant clones. When aberrantly expressed in B cells, ZAP70 competes with SYK at the BCR signalosome and redirects SYK from negative selection to tonic PI3K signaling, thereby promoting B cell survival. In genetic mouse models for B-ALL and B-CLL, conditional expression of Zap70 accelerated disease onset, while genetic deletion impaired malignant transformation. Inducible activation of Zap70 during B cell development compromised negative selection of autoreactive B cells, resulting in pervasive autoantibody production. Strict segregation of the two kinases is critical for normal B cell selection and represents a central safeguard against the development of autoimmune disease and B cell malignancies.

Prevention of Necrotizing Enterocolitis through Milk Polar Lipids Reducing Intestinal Epithelial Apoptosis

Neonatal necrotizing enterocolitis (NEC) is a common and devastating disease. The objective of this research was to investigate the protective mechanisms of milk polar lipids (MPLs) on the attenuation of lipopolysaccharides (LPS)-induced intestinal inflammation and apoptosis. MPLs were extracted from buttermilk and analyzed using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). A neonatal NEC rat model was used to investigate the effects of MPLs on NEC and its underlying mechanisms. Hematoxylin-eosin (H&E) staining and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling (TUNEL) assay were used to observe intestinal morphological changes and intestinal epithelial cell apoptosis, which showed that MPLs could reduce NEC symptoms and intestinal apoptosis. The expressions of IL-6, IL-8, and TNF-α in the MPL group was significantly downregulated (P < 0.05), and the expression levels of IL-10 were significantly upregulated (P < 0.05). At the same time, MPLs also significantly reduced (P < 0.05) activation of the LPS-induced TLR4/NF-κB signaling pathway. Furthermore, MPLs inhibit apoptosis by reducing the expressions of Bax, caspase-9, and caspase-3 and by increasing the expression of Bcl-2. In conclusion, MPLs could reduce NEC symptoms in mice by inhibiting cell inflammation and protecting against intestinal apoptosis.

Tubulosine selectively inhibits JAK3 signalling by binding to the ATP-binding site of the kinase of JAK3

Gain- or loss-of-function mutations in Janus kinase 3 (JAK3) contribute to the pathogenesis of various haematopoietic malignancies and immune disorders, suggesting that aberrant JAK3 signalling is an attractive therapeutic target to treat these disorders. In this study, we performed structure-based computational database screening using the 3D structure of the JAK3 kinase domain and the National Cancer Institute diversity set and identified tubulosine as a novel JAK3 inhibitor. Tubulosine directly blocked the catalytic activity of JAK3 by selective interacting with the JAK3 kinase domain. Consistently, tubulosine potently inhibited persistently activated and interleukin-2-dependent JAK3, and JAK3-mediated downstream targets. Importantly, it did not affect the activity of other JAK family members, particularly prolactin-induced JAK2/signal transducer and activator of transcription 5 and interferon alpha-induced JAK1-TYK2/STAT1. Tubulosine specifically decreased survival and proliferation of cancer cells, in which persistently active JAK3 is expressed, by inducing apoptotic and necrotic/autophagic cell death without affecting other oncogenic signalling. Collectively, tubulosine is a potential small-molecule compound that selectively inhibits JAK3 activity, suggesting that it may serve as a promising therapeutic candidate for treating disorders caused by aberrant activation of JAK3 signalling.

It Takes Three Receptors to Raise a B Cell

As the unique source of diverse immunoglobulin repertoires, B lymphocytes are an indispensable part of humoral immunity. B cell progenitors progress through sequential and mutually exclusive states of proliferation and recombination, coordinated by cytokines and chemokines. Mutations affecting the crucial pre-B cell checkpoint result in immunodeficiency, autoimmunity, and leukemia. This checkpoint was previously modeled by the signaling of two opposing receptors, IL-7R and the pre-BCR. We provide an update to this model in which three receptors, IL-7R, pre-BCR, and CXCR4, work in concert to coordinate both the proper positioning of B cell progenitors in the bone marrow (BM) microenvironment and their progression through the pre-B checkpoint. Furthermore, signaling initiated by all three receptors directly instructs cell fate and developmental progression.

Prognostic value of B-cell linker protein in colorectal cancer

This study aimed to investigate the clinicopathological and prognostic impact of B-cell linker (BLNK) protein expression in colorectal cancer (CRC) as its function in CRC remains unexplored. We performed immunohistochemical staining for BLNK using tissue microarrays of 418 consecutive CRC samples; of these 10 were excluded due to inappropriate staining. The expression intensity and staining level was scored as 0-3 and 0-4, respectively, based on the percentage of positive cells. The immunoreactivity score (IRS) was calculated by multiplying these two scores. BLNK expression was observed in 222 patients (54.4 %). Lymph node metastasis (p = 0.031), right colon cancer (p = 0.026), mucinous adenocarcinoma (p < 0.001), and perineural invasion (p = 0.049) were more frequently observed in the IRS 4-12 group than in the IRS 0-3 group. At the same cutoff point, the 5-year recurrence-free survival rate of the patients with stage III was significantly lower than that observed in IRS 4-12 group (74.8 % ± 4.2 % vs. 54.2 % ± 8.5 %, p = 0.003). Multivariate analysis revealed IRS 4-12 to be an independent risk factor for recurrence (Hazard ratio 2.346, 95 % confidence interval 1.348-4.085, p = 0.003). In conclusion, overexpression of BLNK protein is an independent risk factor for CRC recurrence.

Long Noncoding RNA MIR17HG Promotes Colorectal Cancer Progression via miR-17-5p

Immune dysregulation plays a vital role in colorectal cancer initiation and progression. Long noncoding RNAs (lncRNA) exhibit multiple functions including regulation of gene expression. Here, we identified an immune-related lncRNA, MIR17HG, whose expression was gradually upregulated in adjacent, adenoma, and colorectal cancer tissue. MIR17HG promoted tumorigenesis and metastasis in colorectal cancer cells both in vitro and in vivo. Mechanistically, MIR17HG increased the expression of NF-κB/RELA by competitively sponging the microRNA miR-375. In addition, RELA transcriptionally activated MIR17HG in a positive feedback loop by directly binding to its promoter region. Moreover, miR-17-5p, one of the transcribed miRNAs from MIR17HG, reduced the expression of the tumor suppressor B-cell linker (BLNK), resulting in increased migration and invasion of colorectal cancer cells. MIR17HG also upregulated PD-L1, indicating its potential role in immunotherapy. Overall, these findings demonstrate that MIR17HG plays an oncogenic role in colorectal cancer and may serve as a promising therapeutic target. SIGNIFICANCE: These findings provide mechanistic insight into the role of the lncRNA MIR17HG and its miRNA members in regulating colorectal cancer carcinogenesis and progression.

Dynamic clonal progression in xenografts of acute lymphoblastic leukemia with intrachromosomal amplification of chromosome 21

Intrachromosomal amplification of chromosome 21 is a heterogeneous chromosomal rearrangement occurring in 2% of cases of childhood precursor B-cell acute lymphoblastic leukemia. These abnormalities are too complex to engineer faithfully in animal models and are unrepresented in leukemia cell lines. As a resource for future functional and preclinical studies, we have created xenografts from the leukemic blasts of patients with intrachromosomal amplification of chromosome 21 and characterized them by in-vivo and ex-vivo luminescent imaging, flow immunophenotyping, and histological and ultrastructural analyses of bone marrow and the central nervous system. Investigation of up to three generations of xenografts revealed phenotypic evolution, branching genomic architecture and, compared with other B-cell acute lymphoblastic leukemia genetic subtypes, greater clonal diversity of leukemia-initiating cells. In support of intrachromosomal amplification of chromosome 21 as a primary genetic abnormality, it was always retained through generations of xenografts, although we also observed the first example of structural evolution of this rearrangement. Clonal segregation in xenografts revealed convergent evolution of different secondary genomic abnormalities implicating several known tumor suppressor genes and a region, containing the B-cell adaptor, PIK3AP1, and nuclear receptor co-repressor, LCOR, in the progression of B-cell acute lymphoblastic leukemia. Tracking of mutations in patients and derived xenografts provided evidence for co-operation between abnormalities activating the RAS pathway in B-cell acute lymphoblastic leukemia and for their aggressive clonal expansion in the xeno-environment. Bi-allelic loss of the CDKN2A/B locus was recurrently maintained or emergent in xenografts and also strongly selected as RNA sequencing demonstrated a complete absence of reads for genes associated with the deletions.

Role of Bruton's tyrosine kinase in B cells and malignancies

Bruton’s tyrosine kinase (BTK) is a non-receptor kinase that plays a crucial role in oncogenic signaling that is critical for proliferation and survival of leukemic cells in many B cell malignancies. BTK was initially shown to be defective in the primary immunodeficiency X-linked agammaglobulinemia (XLA) and is essential both for B cell development and function of mature B cells. Shortly after its discovery, BTK was placed in the signal transduction pathway downstream of the B cell antigen receptor (BCR). More recently, small-molecule inhibitors of this kinase have shown excellent anti-tumor activity, first in animal models and subsequently in clinical studies. In particular, the orally administered irreversible BTK inhibitor ibrutinib is associated with high response rates in patients with relapsed/refractory chronic lymphocytic leukemia (CLL) and mantle-cell lymphoma (MCL), including patients with high-risk genetic lesions. Because ibrutinib is generally well tolerated and shows durable single-agent efficacy, it was rapidly approved for first-line treatment of patients with CLL in 2016. To date, evidence is accumulating for efficacy of ibrutinib in various other B cell malignancies. BTK inhibition has molecular effects beyond its classic role in BCR signaling. These involve B cell-intrinsic signaling pathways central to cellular survival, proliferation or retention in supportive lymphoid niches. Moreover, BTK functions in several myeloid cell populations representing important components of the tumor microenvironment. As a result, there is currently a considerable interest in BTK inhibition as an anti-cancer therapy, not only in B cell malignancies but also in solid tumors. Efficacy of BTK inhibition as a single agent therapy is strong, but resistance may develop, fueling the development of combination therapies that improve clinical responses. In this review, we discuss the role of BTK in B cell differentiation and B cell malignancies and highlight the importance of BTK inhibition in cancer therapy.

Autoimmunity checkpoints as therapeutic targets in B cell malignancies

Targeted therapy of cancer typically focuses on inhibitors (for example, tyrosine kinase inhibitors) that suppress oncogenic signalling below a minimum threshold required for survival and proliferation of cancer cells. B cell acute lymphoblastic leukaemia and B cell lymphomas originate from various stages of development of B cells, which, unlike other cell types, are under intense selective pressure. The vast majority of newly generated B cells are autoreactive and die by negative selection at autoimmunity checkpoints (AICs). Owing to ubiquitous encounters with self-antigen, autoreactive B cells are eliminated by the overwhelming signalling strength of their autoreactive B cell receptor (BCR). A series of recent findings suggests that, despite malignant transformation, AICs are fully functional in B cell malignancies. This Opinion article proposes targeted engagement of AICs as a previously unrecognized therapeutic opportunity to overcome drug resistance in B cell malignancies.

Copy number profiling of adult relapsed B-cell precursor acute lymphoblastic leukemia reveals potential leukemia progression mechanisms

The outcome of relapsed adult acute lymphoblastic leukemia (ALL) remains dismal despite new therapeutic approaches. Previous studies analyzing relapse samples have shown a high degree of heterogeneity regarding gene alterations without an evident relapse signature. Bone marrow or peripheral blood samples from 31 adult B-cell precursor ALL patients at first relapse, and 21 paired diagnostic samples were analyzed by multiplex ligation probe-dependent amplification (MLPA). Nineteen paired diagnostic and relapse samples of these 21 patients were also analyzed by SNP arrays. A trend to acquire homozygous CDKN2A/B deletions and a significant increase in the number of copy number alterations (CNA) was observed from diagnosis to first relapse. Evolution from an ancestral clone was the main pattern of clonal evolution. Relapse samples were extremely heterogeneous regarding CNA frequencies. However, CDKN2A/B, PAX5, ETV6, ATM, IKZF1, VPREB1, and TP53 deletions and duplications of 1q, 8q, 17q, 21, X/Y PAR1, and Xp were frequently detected at relapse. Duplications of genes involved in cell proliferation, drug resistance and stem cell homeostasis regulation, as well as deletions of KDM6A and STAG2 genes emerged as specific alterations at relapse. Genomics of relapsed adult B-cell precursor ALL is highly heterogeneous, although some recurrent lesions involved in essential pathways deregulation were frequently observed. Selective and simultaneous targeting of these deregulated pathways may improve the results of current salvage therapies.

Gene Expression Profiling of Acute Lymphoblastic Leukemia in Children with Very Early Relapse

BACKGROUND AND AIMS

Acute lymphoblastic leukemia (ALL) is the most common childhood cancer worldwide. Mexican patients have high mortality rates, low frequency of good prognosis biomarkers (i.e., ETV6-RUNX1) and a high proportion is classified at the time of diagnosis with a high risk to relapse according to clinical features. In addition, very early relapses are more frequently observed than in other populations. The aim of the study was to identify new potential biomarkers associated with very early relapse in Mexican ALL children through transcriptome analysis.

METHODS

Microarray gene expression profiling on bone marrow samples of 54 pediatric ALL patients, collected at time of diagnosis and/or at relapse, was performed. Eleven patients presented relapse within the first 18 months after diagnosis. Affymetrix Human Transcriptome Array 2.0 (HTA 2.0) was used to perform gene expression analysis. Annotation and functional enrichment analyses were carried out using Gene Ontology, KEGG pathway analysis and Ingenuity Pathway Analysis tools.

RESULTS

BLVRB, ZCCHC7, PAX5, EBF1, TMOD1 and BLNK were differentially expressed (fold-change >2.0 and p value <0.01) between relapsed and non-relapsed patients. Functional analysis of abnormally expressed genes revealed their important role in cellular processes related to the development of hematological diseases, cancer, cell death and survival and in cell-to-cell signaling interaction.

CONCLUSIONS

Our data support previous findings showing the relevance of PAX5, EBF1 and ZCCHC7 as potential biomarkers to identify a subgroup of ALL children in high risk to relapse.

Molecular role of the PAX5-ETV6 oncoprotein in promoting B-cell acute lymphoblastic leukemia

PAX5 is a tumor suppressor in B-ALL, while the role of PAX5 fusion proteins in B-ALL development is largely unknown. Here, we studied the function of PAX5-ETV6 and PAX5-FOXP1 in mice expressing these proteins from the Pax5 locus. Both proteins arrested B-lymphopoiesis at the pro-B to pre-B-cell transition and, contrary to their proposed dominant-negative role, did not interfere with the expression of most regulated Pax5 target genes. Pax5-Etv6, but not Pax5-Foxp1, cooperated with loss of the Cdkna2a/b tumor suppressors in promoting B-ALL development. Regulated Pax5-Etv6 target genes identified in these B-ALLs encode proteins implicated in pre-B-cell receptor (BCR) signaling and migration/adhesion, which could contribute to the proliferation, survival, and tissue infiltration of leukemic B cells. Together with similar observations made in human PAX5-ETV6+ B-ALLs, these data identified PAX5-ETV6 as a potent oncoprotein that drives B-cell leukemia development.

Pre-BCR signaling in precursor B-cell acute lymphoblastic leukemia regulates PI3K/AKT, FOXO1 and MYC, and can be targeted by SYK inhibition

Precursor-B-cell receptor (pre-BCR) signaling and spleen tyrosine kinase (SYK) recently were introduced as therapeutic targets for patients with B-cell acute lymphoblastic leukemia (B-ALL), but the importance of this pathway in B-ALL subsets and mechanism of downstream signaling have not fully been elucidated. Here, we provide new detailed insight into the mechanism of pre-BCR signaling in B-ALL. We compared the effects of pharmacological and genetic disruption of pre-BCR signaling in vitro and in mouse models for B-ALL, demonstrating exquisite dependency of pre-BCR(+) B-ALL, but not other B-ALL subsets, on this signaling pathway. We demonstrate that SYK, PI3K/AKT, FOXO1 and MYC are important downstream mediators of pre-BCR signaling in B-ALL. Furthermore, we define a characteristic immune phenotype and gene expression signature of pre-BCR(+) ALL to distinguish them from other B-ALL subsets. These data provide comprehensive new insight into pre-BCR signaling in B-ALL and corroborate pre-BCR signaling and SYK as promising new therapeutic targets in pre-BCR(+) B-ALL.

PU.1 cooperates with IRF4 and IRF8 to suppress pre-B-cell leukemia

The Ets family transcription factor PU.1 and the interferon regulatory factor (IRF)4 and IRF8 regulate gene expression by binding to composite DNA sequences known as Ets/interferon consensus elements. Although all three factors are expressed from the onset of B-cell development, single deficiency of these factors in B-cell progenitors only mildly impacts on bone marrow B lymphopoiesis. Here we tested whether PU.1 cooperates with IRF factors in regulating early B-cell development. Lack of PU.1 and IRF4 resulted in a partial block in development the pre-B-cell stage. The combined deletion of PU.1 and IRF8 reduced recirculating B-cell numbers. Strikingly, all PU.1/IRF4 and ~50% of PU.1/IRF8 double deficient mice developed pre-B-cell acute lymphoblastic leukemia (B-ALL) associated with reduced expression of the established B-lineage tumor suppressor genes, Ikaros and Spi-B. These genes are directly regulated by PU.1/IRF4/IRF8, and restoration of Ikaros or Spi-B expression inhibited leukemic cell growth. In summary, we demonstrate that PU.1, IRF4 and IRF8 cooperate to regulate early B-cell development and to prevent pre-B-ALL formation.

The pre-B-cell receptor checkpoint in acute lymphoblastic leukaemia

The B-cell receptor (BCR) and its immature form, the precursor-BCR (pre-BCR), have a central role in the control of B-cell development, which is dependent on a sequence of cell-fate decisions at specific antigen-independent checkpoints. Pre-BCR expression provides the first checkpoint, which controls differentiation of pre-B to immature B-cells in normal haemopoiesis. Pre-BCR signalling regulates and co-ordinates diverse processes within the pre-B cell, including clonal selection, proliferation and subsequent maturation. In B-cell precursor acute lymphoblastic leukaemia (BCP-ALL), B-cell development is arrested at this checkpoint. Moreover, malignant blasts avoid clonal extinction by hijacking pre-BCR signalling in favour of the development of BCP-ALL. Here, we discuss three mechanisms that occur in different subtypes of BCP-ALL: (i) blocking pre-BCR expression; (ii) activating pre-BCR-mediated pro-survival and pro-proliferative signalling, while inhibiting cell cycle arrest and maturation; and (iii) bypassing the pre-BCR checkpoint and activating pro-survival signalling through pre-BCR independent alternative mechanisms. A complete understanding of the BCP-ALL-specific signalling networks will highlight their application in BCP-ALL therapy.

Rationale for targeting the pre-B-cell receptor signaling pathway in acute lymphoblastic leukemia

Inhibitors of B-cell receptor (BCR) and pre-BCR signaling were successfully introduced into patient care for various subtypes of mature B-cell lymphoma (e.g., ibrutinib, idelalisib). Acute lymphoblastic leukemia (ALL) typically originates from pre-B cells that critically depend on survival signals emanating from a functional pre-BCR. However, whether patients with ALL benefit from treatment with (pre-) BCR inhibitors has not been explored. Recent data suggest that the pre-BCR functions as tumor suppressor in the majority of cases of human ALL. However, a distinct subset of human ALL is selectively sensitive to pre-BCR antagonists.

Checkpoints that control B cell development

B cells differentiate from pluripotent hematopoietic stem cells (pHSCs) in a series of distinct stages. During early embryonic development, pHSCs migrate into the fetal liver, where they develop and mature to B cells in a transient wave, which preferentially populates epithelia and lung as well as gut-associated lymphoid tissues. This is followed by continuous B cell development throughout life in the bone marrow to immature B cells that migrate to secondary lymphoid tissues, where they mature. At early stages of development, before B cell maturation, the gene loci encoding the heavy and light chains of immunoglobulin that determine the B cell receptor composition undergo stepwise rearrangements of variable region-encoding gene segments. Throughout life, these gene rearrangements continuously generate B cell repertoires capable of recognizing a plethora of self-antigens and non-self-antigens. The microenvironment in which these B cell repertoires develop provide signaling molecules that play critical roles in promoting gene rearrangements, proliferation, survival, or apoptosis, and that help to distinguish self-reactive from non-self-reactive B cells at four distinct checkpoints. This refinement of the B cell repertoire directly contributes to immunity, and defects in the process contribute to autoimmune disease.

Enteroviral Infection in a Patient with BLNK Adaptor Protein Deficiency

B-cell linker (BLNK) protein is a non-redundant adaptor molecule in the signaling pathway activated by (pre) B-cell antigen receptor signals. We present two siblings with a homozygous deleterious frameshift mutation in BLNK, resulting in a block of B cell development in the bone marrow at the preB1 to preB2 stage, absence of circulating B cells and agammaglobulinemia. This is the first description of an enteroviral infection associated arthritis and dermatitis in a patient with BLNK deficiency.

Therapeutic potential of spleen tyrosine kinase inhibition for treating high-risk precursor B cell acute lymphoblastic leukemia

Intensified and central nervous system (CNS)-directed chemotherapy has improved outcomes for pediatric B cell acute lymphoblastic leukemia (B-ALL) but confers treatment-related morbidities. Moreover, many patients suffer relapses, underscoring the need to develop new molecular targeted B-ALL therapies. Using a mouse model, we show that leukemic B cells require pre-B cell receptor (pre-BCR)-independent spleen tyrosine kinase (SYK) signaling in vivo for survival and proliferation. In diagnostic samples from human pediatric and adult B-ALL patients, SYK and downstream targets were phosphorylated regardless of pre-BCR expression or genetic subtype. Two small-molecule SYK inhibitors, fostamatinib and BAY61-3606, attenuated the growth of 69 B-ALL samples in vitro, including high-risk (HR) subtypes. Orally administered fostamatinib reduced heavy disease burden after xenotransplantation of HR B-ALL samples into immunodeficient mice and decreased leukemia dissemination into spleen, liver, kidneys, and the CNS of recipient mice. Thus, SYK activation sustains the growth of multiple HR B-ALL subtypes, suggesting that SYK inhibitors may improve outcomes for HR and relapsed B-ALL.

PU.1 opposes IL-7-dependent proliferation of developing B cells with involvement of the direct target gene bruton tyrosine kinase

Deletion of genes encoding the E26 transformation-specific transcription factors PU.1 and Spi-B in B cells (CD19-CreΔPB mice) leads to impaired B cell development, followed by B cell acute lymphoblastic leukemia at 100% incidence and with a median survival of 21 wk. However, little is known about the target genes that explain leukemogenesis in these mice. In this study we found that immature B cells were altered in frequency in the bone marrow of preleukemic CD19-CreΔPB mice. Enriched pro-B cells from CD19-CreΔPB mice induced disease upon transplantation, suggesting that these were leukemia-initiating cells. Bone marrow cells from preleukemic CD19-CreΔPB mice had increased responsiveness to IL-7 and could proliferate indefinitely in response to this cytokine. Bruton tyrosine kinase (BTK), a negative regulator of IL-7 signaling, was reduced in preleukemic and leukemic CD19-CreΔPB cells compared with controls. Induction of PU.1 expression in cultured CD19-CreΔPB pro-B cell lines induced Btk expression, followed by reduced STAT5 phosphorylation and early apoptosis. PU.1 and Spi-B regulated Btk directly as shown by chromatin immunoprecipitation analysis. Ectopic expression of BTK was sufficient to induce apoptosis in cultured pro-B cells. In summary, these results suggest that PU.1 and Spi-B activate Btk to oppose IL-7 responsiveness in developing B cells.

Targeting Bruton's tyrosine kinase in B cell malignancies

Bruton's tyrosine kinase (BTK) is a key component of B cell receptor (BCR) signalling and functions as an important regulator of cell proliferation and cell survival in various B cell malignancies. Small-molecule inhibitors of BTK have shown antitumour activity in animal models and, recently, in clinical studies. High response rates were reported in patients with chronic lymphocytic leukaemia and mantle cell lymphoma. Remarkably, BTK inhibitors have molecular effects that cannot be explained by the classic role of BTK in BCR signalling. In this Review, we highlight the importance of BTK in various signalling pathways in the context of its therapeutic inhibition.

Transcriptional control of pre-B cell development and leukemia prevention

The differentiation of early B cell progenitors is controlled by multiple transcriptional regulators and growth-factor receptors. The triad of DNA-binding proteins, E2A, EBF1, and PAX5 is critical for both the early specification and commitment of B cell progenitors, while a larger number of secondary determinants, such as members of the Ikaros, ETS, Runx, and IRF families have more direct roles in promoting stage-specific pre-B gene-expression program. Importantly, it is now apparent that mutations in many of these transcription factors are associated with the progression to acute lymphoblastic leukemia. In this review, we focus on recent studies that have shed light on the transcriptional hierarchy that controls efficient B cell commitment and differentiation as well as focus on the oncogenic consequences of the loss of many of the same factors.

Novel mechanism of tumor suppression by polarity gene discs large 1 (DLG1) revealed in a murine model of pediatric B-ALL

Drosophila melanogaster discs large (dlg) is an essential tumor suppressor gene (TSG) controlling epithelial cell growth and polarity of the fly imaginal discs in pupal development. A mammalian ortholog, Dlg1, is involved in embryonic urogenital morphogenesis, postsynaptic densities in neurons, and immune synapses in lymphocytes. However, a potential role for Dlg1 as a mammalian TSG is unknown. Here, we present evidence that loss of Dlg1 confers strong predisposition to the development of malignancies in a murine model of pediatric B-cell acute lymphoblastic leukemia (B-ALL). Using mice with conditionally deleted Dlg1 alleles, we identify a novel "pre-leukemic" stage of developmentally arrested early B-lineage cells marked by preeminent c-Myc expression. Mechanistically, we show that in B-lineage progenitors Dlg1 interacts with and stabilizes the PTEN protein, regulating its half-life and steady-state abundance. The loss of Dlg1 does not affect the level of PTEN mRNAs but results in a dramatic decrease in PTEN protein, leading to excessive phosphoinositide 3-kinase signaling and proliferation. Our data suggest a novel model of tumor suppression by a PDZ domain-containing polarity gene in hematopoietic cancers.

B-cell biology and development

B cells develop from hematopoietic precursor cells in an ordered maturation and selection process. Extensive studies with many different mouse mutants provided fundamental insights into this process. However, the characterization of genetic defects causing primary immunodeficiencies was essential in understanding human B-cell biology. Defects in pre-B-cell receptor components or in downstream signaling proteins, such as Bruton tyrosine kinase and B-cell linker protein, arrest development at the pre-B-cell stage. Defects in survival-regulating proteins, such as B-cell activator of the TNF-α family receptor (BAFF-R) or caspase recruitment domain-containing protein 11 (CARD11), interrupt maturation and prevent differentiation of transitional B cells into marginal zone and follicular B cells. Mature B-cell subsets, immune responses, and memory B-cell and plasma cell development are disturbed by mutations affecting Toll-like receptor signaling, B-cell antigen receptor coreceptors (eg, CD19), or enzymes responsible for immunoglobulin class-switch recombination. Transgenic mouse models helped to identify key regulatory mechanisms, such as receptor editing and clonal anergy, preventing the activation of B cells expressing antibodies recognizing autoantigens. Nevertheless, the combination of susceptible genetic backgrounds with the rescue of self-reactive B cells by T cells allows the generation of autoreactive clones found in patients with many autoimmune diseases and even in those with primary immunodeficiencies. The rapid progress of functional genomic research is expected to foster the development of new tools that specifically target dysfunctional B lymphocytes to treat autoimmunity, B-cell malignancies, and immunodeficiency.

Cutting edge: cell-autonomous control of IL-7 response revealed in a novel stage of precursor B cells

During early stages of B-lineage differentiation in bone marrow, signals emanating from IL-7R and pre-BCR are thought to synergistically induce proliferative expansion of progenitor cells. Paradoxically, loss of pre-BCR-signaling components is associated with leukemia in both mice and humans. Exactly how progenitor B cells perform the task of balancing proliferative burst dependent on IL-7 with the termination of IL-7 signals and the initiation of L chain gene rearrangement remains to be elucidated. In this article, we provide genetic and functional evidence that the cessation of the IL-7 response of pre-B cells is controlled via a cell-autonomous mechanism that operates at a discrete developmental transition inside Fraction C' (large pre-BII) marked by transient expression of c-Myc. Our data indicate that pre-BCR cooperates with IL-7R in expanding the pre-B cell pool, but it is also critical to control the differentiation program shutting off the c-Myc gene in large pre-B cells.

A novel mechanism for the autonomous termination of pre-B cell receptor expression via induction of lysosome-associated protein transmembrane 5

The expression of the pre-B cell receptor (BCR) is confined to the early stage of B cell development, and its dysregulation is associated with anomalies of B-lineage cells, including leukemogenesis. Previous studies suggested that the pre-BCR signal might trigger the autonomous termination of pre-BCR expression even before the silencing of pre-BCR gene expression to prevent sustained pre-BCR expression. However, the underlying mechanism remains ill defined. Here we demonstrate that the pre-BCR signal induces the expression of lysosome-associated protein transmembrane 5 (LAPTM5), which leads to the prompt downmodulation of the pre-BCR. While LAPTM5 induction had no significant impact on the internalization of cell surface pre-BCR, it elicited the translocation of a large pool of intracellular pre-BCR from the endoplasmic reticulum to the lysosomal compartment concomitantly with a drastic reduction of the level of intracellular pre-BCR proteins. This reduction was inhibited by lysosomal inhibitors, indicating the lysosomal degradation of the pre-BCR. Notably, the LAPTM5 deficiency in pre-B cells led to the augmented expression level of surface pre-BCR. Collectively, the pre-BCR induces the prompt downmodulation of its own expression through the induction of LAPTM5, which promotes the lysosomal transport and degradation of the intracellular pre-BCR pool and, hence, limits the supply of pre-BCR to the cell surface.

Advances in the molecular pathobiology of B-lymphoblastic leukemia

B-lymphoblastic leukemia/lymphoma, also known as B-acute lymphoblastic leukemia, is derived from B-cell progenitors. B-acute lymphoblastic leukemia occurs predominantly in children, but can occur at any age. Risk-adapted intensive chemotherapy is effective in treating most children with B-acute lymphoblastic leukemia, but this approach is less successful in adults. Recent developments in genome-wide genetic analysis in B-acute lymphoblastic leukemia have provided insights into disease pathogenesis and prognosis. B-acute lymphoblastic leukemia cases usually carry a primary genetic event, often a chromosome translocation, and a constellation of secondary genetic alterations that are acquired and selected dynamically in a nonlinear fashion. These genetic changes commonly affect cellular mechanisms that control B-cell differentiation and proliferation. The cooperative interaction between inactivation of hematopoietic transcription factors involved in differentiation (class II mutation) and activating mutations involved in cell proliferation (class I mutation) is reminiscent of the pathogenic model of acute myeloid leukemia. The resulting improved molecular understanding of B-acute lymphoblastic leukemia is helping to refine disease risk stratification and discover new therapeutic approaches for patients with refractory disease. In this review, we first summarize the clinicopathologic and immunophenotypic features of B-acute lymphoblastic leukemia and introduce current understanding of B-cell development and B-acute lymphoblastic leukemia leukemogenesis. We then focus on recent advances in genetic analysis and gene expression profiling of B-acute lymphoblastic leukemia and discuss the implications of these findings for disease evolution, risk prediction, and possible novel therapeutic approaches.

Philadelphia chromosome-positive leukemia stem cells in acute lymphoblastic leukemia and tyrosine kinase inhibitor therapy

Leukemia stem cells (LSCs), which constitute a minority of the tumor bulk, are functionally defined on the basis of their ability to transfer leukemia into an immunodeficient recipient animal. The presence of LSCs has been demonstrated in acute lymphoblastic leukemia (ALL), of which ALL with Philadelphia chromosome-positive (Ph(+)). The use of imatinib, a tyrosine kinase inhibitor (TKI), as part of front-line treatment and in combination with cytotoxic agents, has greatly improved the proportions of complete response and molecular remission and the overall outcome in adults with newly diagnosed Ph(+) ALL. New challenges have emerged with respect to induction of resistance to imatinib via Abelson tyrosine kinase mutations. An important recent addition to the arsenal against Ph(+) leukemias in general was the development of novel TKIs, such as nilotinib and dasatinib. However, in vitro experiments have suggested that TKIs have an antiproliferative but not an antiapoptotic or cytotoxic effect on the most primitive ALL stem cells. None of the TKIs in clinical use target the LSC. Second generation TKI dasatinib has been shown to have a more profound effect on the stem cell compartment but the drug was still unable to kill the most primitive LSCs. Allogeneic stem cell transplantation (SCT) remains the only curative treatment available for these patients. Several mechanisms were proposed to explain the resistance of LSCs to TKIs in addition to mutations. Hence, TKIs may be used as a bridge to SCT rather than monotherapy or combination with standard chemotherapy. Better understanding the biology of Ph(+) ALL will open new avenues for effective management. In this review, we highlight recent findings relating to the question of LSCs in Ph(+) ALL.

The role of STAT5 in lymphocyte development and transformation

STAT5 plays a crucial role in B and T lymphocyte development. However, whether STAT5 primarily plays a role as a permissive factor, involved in lymphocyte survival, or an instructive factor, involved in lymphocyte differentiation, has been unclear. In addition, while STAT5 has been suggested to act as a transcriptional repressor, the mechanism by which it represses transcription was undefined. Recent reports have begun to shed new light on these roles for STAT5 in lymphocyte development, transcriptional repression, and leukemic transformation.

Pre-B cell receptor-mediated activation of BCL6 induces pre-B cell quiescence through transcriptional repression of MYC

Initial cell surface expression of the pre-B cell receptor induces proliferation. After 2 to 5 divisions, however, large pre-BII (Fraction C') cells exit cell cycle to become resting, small pre-BII cells (Fraction D). The mechanism by which pre-BII cells exit cell cycle, however, is currently unclear. The checkpoint at the Fraction C'-D transition is critical for immunoglobulin light chain gene recombination and to prevent malignant transformation into acute lymphoblastic leukemia. Here we demonstrate that inducible activation of pre-B cell receptor signaling induces cell-cycle exit through up-regulation of the transcriptional repressor BCL6. Inducible activation of BCL6 downstream of the pre-B cell receptor results in transcriptional repression of MYC and CCND2. Hence, pre-B cell receptor-mediated activation of BCL6 limits pre-B cell proliferation and induces cellular quiescence at the small pre-BII (Fraction D) stage.

Deletion of genes encoding PU.1 and Spi-B in B cells impairs differentiation and induces pre-B cell acute lymphoblastic leukemia

The E26 transformation-specific (Ets) transcription factor PU.1 is required to generate lymphoid progenitor cells from hematopoietic stem cells, but it is not required to generate B cells from committed B-cell lineage progenitors. We hypothesized that PU.1 function in B-cell differentiation is complemented by the related Ets transcription factor Spi-B. To test this hypothesis, mice were generated lacking both PU.1 and Spi-B in the B-cell lineage. Unlike mice lacking PU.1 or Spi-B, mice deficient in both PU.1 and Spi-B in the B-cell lineage had reduced frequencies of B cells as well as impaired B-cell differentiation. Strikingly, all PU.1 and Spi-B-deficient mice developed pre-B cell acute lymphoblastic leukemia before 30 weeks of age. Pre-B cells accumulated in the thymus resulting in massive thymic enlargement and dyspnea. These findings demonstrate that PU.1 and Spi-B are essential transcriptional regulators of B-cell differentiation as well as novel tumor suppressors in the B-cell lineage.

Estrogen and insulin-like growth factor-I (IGF-I) independently down-regulate critical repressors of breast cancer growth

Although estrogen receptor alpha (ERα) and insulin-like growth factor (IGF) signaling are important for normal mammary development and breast cancer, cross-talk between these pathways, particularly at the level of transcription, remains poorly understood. We performed microarray analysis on MCF-7 breast cancer cells treated with estradiol (E2) or IGF-I for 3 or 24 h. IGF-I regulated mRNA of five to tenfold more genes than E2, and many genes were co-regulated by both ligands. Importantly, expression of these co-regulated genes correlated with poor prognosis of human breast cancer. Closer examination revealed enrichment of repressed transcripts. Interestingly, a number of potential tumor suppressors, for example, B-cell linker (BLNK), were down-regulated by IGF-I and E2. Analysis of three down-regulated genes showed that E2-mediated repression occurred independently of IGF-IR, and IGF-I-mediated repression occurred independently of ERα. However, repression by IGF-I or E2 required common kinases, such as PI3K and MEK, suggesting downstream convergence of the two pathways. In conclusion, E2 and IGF-I co-regulate a set of genes that affect breast cancer outcome. There is enrichment of repressed transcripts, and, for some genes, the down-regulation is independent at the receptor level. This may be important clinically, as tumors with active ERα and IGF-IR signaling may require co-targeting of both pathways.

Role of PI3K in the generation and survival of B cells

Engagement of the B-cell antigen receptor (BCR) or its precursor, the pre-BCR, induces a cascade of biochemical reactions that regulate the differentiation, selection, survival, and activation of B cells. This cascade is initiated by receptor-associated tyrosine kinases that activate multiple downstream signaling pathways. Since it is required for metabolism, cell growth, development, and survival, the activation of phosphoinositide 3-kinase (PI3K)-dependent pathways represents a crucial event of BCR/pre-BCR signaling. The phosphorylated substrates of the PI3K promote specific recruitment of selected signaling proteins to the plasma membrane, where important signaling complexes are formed to mediate the above-mentioned biological processes. Here, we review the principles of PI3K signaling and highlight the role of an important PI3K-driven module in VDJ recombination of immunoglobulin (Ig) genes during early B-cell development as compared with class switch recombination of Ig genes in mature B cells after activation by specific antigens. Furthermore, we discuss the role of PI3K in the survival of mature B cells, which is strictly dependent on BCR expression and basal BCR signaling.

Pre-B-cell leukemias in Btk/Slp65-deficient mice arise independently of ongoing V(D)J recombination activity

The adapter protein Slp65 and Bruton's tyrosine kinase (Btk) are key components of the precursor-B (pre-B) cell receptor (pre-BCR) signaling pathway. Slp65-deficient mice spontaneously develop pre-B-cell leukemia, expressing high levels of the pre-BCR on their cell surface. As leukemic Slp65-deficient pre-B cells express the recombination activating genes (Rag)1 and Rag2, and manifest ongoing immunoglobulin (Ig) light-chain rearrangement, it has been hypothesized that deregulated recombinase activity contributes to malignant transformation. In this report, we investigated whether Rag-induced DNA damage is involved in oncogenic transformation of Slp65-deficient B cells. We employed Btk/Slp65 double-deficient mice carrying an autoreactive 3-83μδ BCR transgene. When developing B cells in their bone marrow express this BCR, the V(D)J recombination machinery will be activated, allowing for secondary Ig light-chain gene rearrangements to occur. This phenomenon, called receptor editing, will rescue autoreactive B cells from apoptosis. We observed that 3-83μδ transgenic Btk/Slp65 double-deficient mice developed B-cell leukemias expressing both the 3-83μδ BCR and the pre-BCR components λ5/VpreB. Importantly, such leukemias were found at similar frequencies in mice concomitantly deficient for Rag1 or the non-homologous end-joining factor DNA-PKcs. We therefore conclude that malignant transformation of Btk/Slp65 double-deficient pre-B cells is independent of deregulated V(D)J recombination activity.

Activation-induced cytidine deaminase accelerates clonal evolution in BCR-ABL1-driven B-cell lineage acute lymphoblastic leukemia

Activation-induced cytidine deaminase (AID) is required for somatic hypermutation and immunoglobulin (Ig) class switch recombination in germinal center (GC) B cells. Occasionally, AID can target non-Ig genes and thereby promote GC B-cell lymphomagenesis. We recently showed that the oncogenic BCR-ABL1 kinase induces aberrant expression of AID in pre-B acute lymphoblastic leukemia (ALL) and lymphoid chronic myelogenous leukemia blast crisis. To elucidate the biological significance of aberrant AID expression, we studied loss of AID function in a murine model of BCR-ABL1 ALL. Mice transplanted with BCR-ABL1-transduced AID(-/-) bone marrow had prolonged survival compared with mice transplanted with leukemia cells generated from AID(+/+) bone marrow. Consistent with a causative role of AID in genetic instability, AID(-/-) leukemia had a lower frequency of amplifications and deletions and a lower frequency of mutations in non-Ig genes, including Pax5 and Rhoh compared with AID(+/+) leukemias. AID(-/-) and AID(+/+) ALL cells showed a markedly distinct gene expression pattern, and AID(-/-) ALL cells failed to downregulate a number of tumor-suppressor genes including Rhoh, Cdkn1a (p21), and Blnk (SLP65). We conclude that AID accelerates clonal evolution in BCR-ABL1 ALL by enhancing genetic instability and aberrant somatic hypermutation, and by negative regulation of tumor-suppressor genes.

Autoinhibition and adapter function of Syk

Development, survival, and activation of B lymphocytes are controlled by signals emanating from the B-cell antigen receptor (BCR). The BCR has an autonomous signaling function also known as tonic signaling that allows for long-term survival of B cells in the immune system. Upon binding of antigen to the BCR, the tonic signal is amplified and diversified, leading to alteration in gene expression and B-cell activation. The spleen tyrosine kinase (Syk) intimately cooperates with the signaling subunits of the BCR and plays a central role in the amplification and diversification of BCR signals. In this review, we discuss the molecular mechanisms by which Syk activity is inhibited and activated at the BCR. Importantly, Syk acts not only as a kinase that phosphorylates downstream substrates but also as an adapter that can bind to a diverse set of signaling proteins. Depending on its interactions and localization, Syk can signal opposing cell fate decisions such as proliferation or differentiation of B cells.

The SYK tyrosine kinase: a crucial player in diverse biological functions

Spleen tyrosine kinase (SYK) is known to have a crucial role in adaptive immune receptor signalling. However, recent reports indicate that SYK also mediates other, unexpectedly diverse biological functions, including cellular adhesion, innate immune recognition, osteoclast maturation, platelet activation and vascular development. SYK is activated by C-type lectins and integrins, and activates new targets, including the CARD9-BCL-10-MALT1 pathway and the NLRP3 inflammasome. Studies using Drosophila melanogaster suggest that there is an evolutionarily ancient origin of SYK-mediated signalling. Moreover, SYK has a crucial role in autoimmune diseases and haematological malignancies. This Review summarizes our current understanding of the diverse functions of SYK and how this is being translated for therapeutic purposes.

Cloning and functional characterization of chicken stem cell antigen 2

Stem cell antigen 2 (SCA2) is a Ly6 family member whose function is largely unknown. To characterize biological properties and tissue distribution of chicken SCA2, SCA2 was expressed in E. coli, purified, and a polyclonal antibody developed. Utilizing the polyclonal antibody, SCA2 is a 13 kDa cell surface protein anchored by a glycosyl-phosphatidylinositol (GPI) moiety. SCA2 is expressed in connective tissues of thymus and bursa based on immunohistochemistry, immunoprecipitation, and western blots. In bursal follicles, SCA2 is specifically expressed on the cortical-medullary epithelial cells (CMEC) surrounded by MHC class II presenting cells. Expression profiles of bursal cells induced by contact with SCA2-expressing cells shows down-regulation of numerous genes including CD79B, B cell linker (BLNK), spleen tyrosine kinase (SYK), and gamma 2-phospholipase C (PLCG2) that are involved in the B cell receptor (BCR) and immune response signaling pathways. These results suggest chicken SCA2 plays a role in regulating B lymphocytes.

NSC114792, a novel small molecule identified through structure-based computational database screening, selectively inhibits JAK3

Background

Human or animals lacking either JAK3 or the common gamma chain (γc) expression display severe combined immunodeficiency disease, indicating the crucial role of JAK3 in T-cell development and the homeostasis of the immune system. JAK3 has also been suggested to contribute to the pathogenesis of tumorigenesis. Recent studies identified activating JAK3 mutations in patients with various hematopoietic malignancies, including acute megakaryoblastic leukemia. Importantly, functional analyses of some of those JAK3 mutations have been shown to cause lethal hematopoietic malignancies in animal models. These observations make JAK3 an ideal therapeutic target for the treatment of various human diseases. To identify novel small molecule inhibitors of JAK3, we performed structure-based virtual screen using the 3D structure of JAK3 kinase domain and the NCI diversity set of compounds.

Results

We identified NSC114792 as a lead compound. This compound directly blocked the catalytic activity of JAK3 but not that of other JAK family members in vitro. In addition, treatment of 32D/IL-2Rβ cells with the compound led to a block in IL-2-dependent activation of JAK3/STAT5 but not IL-3-dependent activation of JAK2/STAT5. Consistent with the specificity of NSC114792 for JAK3, it selectively inhibited persistently-activated JAK3, but failed to affect the activity of other JAK family members and other oncogenic kinases in various cancer cell lines. Finally, we showed that NSC114792 decreases cell viability by inducing apoptosis through down-regulating anti-apoptotic gene expression only in cancer cells harboring persistently-active JAK3.

Conclusions

NSC114792 is a lead compound that selectively inhibits JAK3 activity. Therefore, our study suggests that this small molecule inhibitor of JAK3 can be used as a starting point to develop a new class of drugs targeting JAK3 activity, and may have therapeutic potential in various diseases that are caused by aberrant JAK3 activity.