A p53 defect sensitizes various stages of B cell development to lymphomagenesis in mice carrying an IgH 3' regulatory region-driven c-myc transgene

Mouse Models of c-myc Deregulation Driven by IgH Locus Enhancers as Models of B-Cell Lymphomagenesis

Chromosomal translocations linking various oncogenes to transcriptional enhancers of the immunoglobulin heavy chain (IgH) locus are often implicated as the cause of B-cell malignancies. Two major IgH transcriptional enhancers have been reported so far. The E_μ enhancer located upstream of the C_μ gene controls early events in B-cell maturation such as VDJ recombination. The 3' regulatory region (3'RR) located downstream from the C_α gene controls late events in B-cell maturation such as IgH transcription, somatic hypermutation, and class switch recombination. Convincing demonstrations of the essential contributions of both E_μ and 3'RR in B-cell lymphomagenesis have been provided by transgenic and knock-in animal models which bring the oncogene c-myc under E_μ/3'RR transcriptional control. This short review summarizes the different mouse models so far available and their interests/limitations for progress in our understanding of human c-myc-induced B-cell lymphomagenesis.

Enhancers and MYC interplay in hematopoiesis

Transcription requires the fine interplay between enhancers and transcription factors. Enhancers are able to activate transcription of genes involved in normal cell biology, whereas aberrant enhancer activity leads to oncogenesis. MYC is a well-established proto-oncogene involved in half of human cancers amplifying the output of its targets. The crosstalk between MYC and enhancers is known for many years since the discovery of IgH enhancer juxtaposition with MYC in high-grade lymphomas. Here, we focus mainly in the enhancers surrounding MYC in the 8q24 locus. That region comprises several enhancers that associate with other transcription factors, transmembrane receptors, and fusion genes composing complex regulatory networks aberrantly expressed in almost all types of hematological malignancies. Understanding the nature of these interactions in normal blood cells and in leukemias/lymphomas will expand MYC targeting options in the armamentarium against hematological cancers.

Eμ and 3'RR transcriptional enhancers of the IgH locus cooperate to promote c-myc-induced mature B-cell lymphomas

Numerous B-cell lymphomas feature translocations linking oncogenes to different locations in the immunoglobulin heavy chain (IgH) locus. During Burkitt lymphoma (BL), IgH breakpoints for c-myc translocation stand either close to JH segments or within switch regions. Transcription, accessibility, and remodeling of the IgH locus are under the control of the 2 potent cis-acting enhancer elements: Eμ and the 3' regulatory region (3'RR). To ensure their respective contributions to oncogene deregulation in the context of the endogenous IgH locus, we studied transgenic mice harboring a knock-in of c-myc in various positions of the IgH locus (3' to JH segments, 5' to Cμ with Eμ deletion and Cα). The observed spectrum of tumors, kinetics of emergence, and transcriptome analysis provide strong evidence that both Eμ and 3'RR deregulate c-myc and cooperate together to promote B-cell lymphomagenesis. Transgenics mimicking endemic BL (with c-myc placed 3' to JH segments) exhibited the highest rate of B-cell lymphoma emergence, the highest Ki67 index of proliferation, and the highest transcriptomic similarities to human BL. The 3'RR enhancer alone deregulated c-myc and initiated the development of BL-like lymphomas, suggesting that its targeting would be of therapeutic interest to reduce c-myc oncogenicity in vivo.

The IgH 3' regulatory region and c-myc-induced B-cell lymphomagenesis

Deregulation and mutations of c-myc have been reported in multiple mature B-cell malignancies such as Burkitt lymphoma, myeloma and plasma cell lymphoma. After translocation into the immunoglobulin heavy chain (IgH) locus, c-myc is constitutively expressed under the control of active IgH cis-regulatory enhancers. Those located in the IgH 3 regulatory region (3RR) are master control elements of transcription. Over the past decade numerous convincing demonstrations of 3RRs contribution to mature c-myc-induced lymphomagenesis have been made using transgenic models with various types of IgH-c-myc translocations and transgenes. This review highlights how IgH 3RR physiological functions play a critical role in c-myc deregulation during lymphomagenesis.

Efficient role of IgH 3' regulatory region deficient B-cells in the development of oil granulomas

Functional B-cells are essential for the formation of oil granulomas. The IgH 3′ regulatory region (3′RR) activates important check-points during B-cell maturation. We investigated if 3′RR-deficient B-cells remain efficient to develop oil granulomas in response to pristine. B-cells expressing an IgH 3′RR-deficient allele were similarly recruited to wild type allele expressing B-cells in the granuloma. No differences were observed between 3′RR-deficient mice and control mice for granuloma numbers, cellular composition and ability to express mRNA transcripts for several pro- and anti-inflammatory cytokines. Altogether these results suggest a normal role for 3′RR-deficient B-cells in the development of an acute B-cell-mediated inflammatory response.

The IgH 3' regulatory region influences lymphomagenesis in Igλ-Myc mice

The IgH 3′regulatory region (3′RR), encompassing the four transcriptional enhancers hs3a-hs1,2-hs3b-hs4, has a key role on class switch recombination, somatic hypermutation, IgH transcription and B-cell fate. In plasma cells, transcribed IgH and IgL loci often colocalized in transcription factories and an IgL transcription defect might translate into lowered IgH transcription. We explored whether the 3′RR would affect lymphomagenesis in Igλ-Myc transgenic mice prone to lymphoproliferations. Breeding Igλ-Myc transgenics in a background deficient for the 3′RR influences lymphomagenesis toward less mature lymphomas (16% vs 54%, p = 0.01, Z test for two population proportions). In a 3′RR-deficient background mature tumors less often expressed the CD43 antigen (54% vs 0%, p = 0.02), a membrane glycoprotein expressed on activated mature B-cells. In contrast, in a 3′RR-deficient background tumors more often expressed the CD5 antigen (32% vs 12%, p = 0.05) that may serve to control autoimmunity and that is suspected to play a role in leukemic transformation. Lymphoma myc transcript levels, the Ki67 index of proliferation, the clonality, the usage of V(D)J segments, and their somatic hypermutation status were not affected in the 3′RR-deficient background. In conclusion, most probably through its action during the maturation process, the 3′RR can influence lymphomagenesis even when not linked with an oncogene.

The class-specific BCR tonic signal modulates lymphomagenesis in a c-myc deregulation transgenic model

Deregulation of c-myc by translocation onto immunoglobulin (Ig) loci can promote B cell malignant proliferations with phenotypes as diverse as acute lymphoid leukemia, Burkitt lymphoma, diffuse large B cell lymphoma, myeloma… The B cell receptor (BCR) normally providing tonic signals for cell survival and mitogenic responses to antigens, can also contribute to lymphomagenesis upon sustained ligand binding or activating mutations. BCR signaling varies among cell compartments and BCR classes. For unknown reasons, some malignancies associate with expression of either IgM or class-switched Ig. We explored whether an IgA BCR, with strong tonic signaling, would affect lymphomagenesis in c-myc IgH 3′RR transgenic mice prone to lymphoproliferations. Breeding c-myc transgenics in a background where IgM expression was replaced with IgA delayed lymphomagenesis. By comparison to single c-myc transgenics, lymphomas from double mutant animals were more differentiated and less aggressive, with an altered transcriptional program. Larger tumor cells more often expressed CD43 and CD138, which culminated in a plasma cell phenotype in 10% of cases. BCR class-specific signals thus appear to modulate lymphomagenesis and may partly explain the observed association of specific Ig classes with human B cell malignancies of differential phenotype, progression and prognosis.

Fluorescent biosensors for drug discovery new tools for old targets--screening for inhibitors of cyclin-dependent kinases

Cyclin-dependent kinases play central roles in regulation of cell cycle progression, transcriptional regulation and other major biological processes such as neuronal differentiation and metabolism. These kinases are hyperactivated in most human cancers and constitute attractive pharmacological targets. A large number of ATP-competitive inhibitors of CDKs have been identified from natural substances, in high throughput screening assays, or through structure-guided approaches. Alternative strategies have been explored to target essential protein/protein interfaces and screen for allosteric inhibitors that trap inactive intermediates or prevent conformational activation. However this remains a major challenge given the highly conserved structural features of these kinases, and calls for new and alternative screening technologies. Fluorescent biosensors constitute powerful tools for the detection of biomolecules in complex biological samples, and are well suited to study dynamic processes and highlight molecular alterations associated with pathological disorders. They further constitute sensitive and selective tools which can be readily implemented to high throughput and high content screens in drug discovery programmes. Our group has developed fluorescent biosensors to probe cyclin-dependent kinases and gain insight into their molecular behaviour in vitro and in living cells. These tools provide a means of monitoring subtle alterations in the abundance and activity of CDK/Cyclins and can respond to compounds that interfere with the conformational dynamics of these kinases. In this review we discuss the different strategies which have been devised to target CDK/Cyclins, and describe the implementation of our CDK/Cyclin biosensors to develop HTS/HCS assays in view of identifying new classes of inhibitors for cancer therapeutics.

Targeting the oncogene B lymphoma deregulator IgH 3' regulatory region does not impede the in vivo inflammatory response in mice

The IgH 3′ regulatory region (3′RR), encompassing the four transcriptional enhancers hs3a-hs1,2-hs3b-hs4, is a potent lymphoma oncogene deregulator but its role in B cell-mediated inflammatory responses is unknown. We investigated the 3′RR involvement in the in vivo pristane-induced inflammatory response in BALB/c mice. The lack of the 3′RR in BALB/c mice had no wide effect on the incidence, the kinetic of development and the cellular composition of peritoneal ascites. Ascite pro-inflammatory cytokines levels (IL-6, IL-21, IL-12/23, TNF-α) were unchanged while anti-inflammatory cytokines levels (IL-10, interferon-γ) were slightly increased in 3′RR-deficient BALB/c mice as compared to wt BALB/c mice. In conclusion, the 3′RR is dispensable for the efficient recruitment of immune cells and the normal development of an inflammatory response in the in vivo pristane-induced inflammatory model. The 3′RR might be considered as a potential suitable target for anti-lymphoma pharmacological therapy without potent adverse effect on normal immune and inflammatory responses.

Elucidation of the enigmatic IgD class-switch recombination via germline deletion of the IgH 3' regulatory region

Classical class-switch recombination (cCSR) substitutes the Cμ gene with Cγ, Cε, or Cα, thereby generating IgG, IgE, or IgA classes, respectively. This activation-induced deaminase (AID)-driven process is controlled by the IgH 3' regulatory region (3'RR). Regulation of rare IgD CSR events has been enigmatic. We show that μδCSR occurs in mouse mesenteric lymph node (MLN) B cells and is AID-dependent. AID attacks differ from those in cCSR because they are not accompanied by extensive somatic hypermutation (SHM) of targeted regions and because repaired junctions exhibit features of the alternative end-joining (A-EJ) pathway. In contrast to cCSR and SHM, μδCSR is 3'RR-independent, as its absence affects neither breakpoint locations in Sμ- and Sδ-like (σ(δ)) nor mutation patterns at Sμ-σ(δ) junctions. Although mutations occur in the immediate proximity of the μδ junctions, SHM is absent distal to the junctions within both Sμ and rearranged VDJ regions. In conclusion, μδCSR is active in MLNs, occurs independently of 3'RR-driven assembly, and is even dramatically increased in 3'RR-deficient mice, further showing that its regulation differs from cCSR.

Somatic inactivation of Tp53 in hematopoietic stem cells or thymocytes predisposes mice to thymic lymphomas with clonal translocations

TP53 protects cells from transformation by responding to stresses including aneuploidy and DNA double-strand breaks (DSBs). TP53 induces apoptosis of lymphocytes with persistent DSBs at antigen receptor loci and other genomic loci to prevent these lesions from generating oncogenic translocations. Despite this critical function of TP53, germline Tp53(-/-) mice succumb to immature T-cell (thymic) lymphomas that exhibit aneuploidy and lack clonal translocations. However, Tp53(-/-) mice occasionally develop B lineage lymphomas and Tp53 deletion in pro-B cells causes lymphomas with oncogenic immunoglobulin (Ig) locus translocations. In addition, human lymphoid cancers with somatic TP53 inactivation often harbor oncogenic IG or T-cell receptor (TCR) locus translocations. To determine whether somatic Tp53 inactivation unmasks translocations or alters the frequency of B lineage tumors in mice, we generated and analyzed mice with conditional Tp53 deletion initiating in hematopoietic stem cells (HSCs) or in lineage-committed thymocytes. Median tumor-free survival of each strain was similar to the lifespan of Tp53(-/-) mice. Mice with HSC deletion of Tp53 predominantly succumbed to thymic lymphomas with clonal translocations not involving Tcr loci; however, these mice occasionally developed mature B-cell lymphomas that harbored clonal Ig translocations. Deletion of Tp53 in thymocytes caused thymic lymphomas with aneuploidy and/or clonal translocations, including oncogenic Tcr locus translocations. Our data demonstrate that the developmental stage of Tp53 inactivation affects karyotypes of lymphoid malignancies in mice where somatic deletion of Tp53 initiating in thymocytes is sufficient to cause thymic lymphomas with oncogenic translocations.

Mantle cell lymphoma-like lymphomas in c-myc-3'RR/p53+/- mice and c-myc-3'RR/Cdk4R24C mice: differential oncogenic mechanisms but similar cellular origin

Mantle cell lymphoma (MCL) is a malignant lymphoproliferative B-cell disorder that does not occur spontaneously in mice but experimental mice model have been developed. Recently two different mice models prone to develop MCL-like lymphomas were generated: c-myc-3'RR/Cdk4(R24C) mice and c-myc-3'RR/p53+/- mice. Comparison of their gene expression profiles does not highlight specific differences other than those in relation with their specific mutational status (i.e., Cdk4(R24C) mutation or p53 mutations). We propose that similarly to typical human MCL and its blastoid or cyclin-D1 variants that correspond to the same genetic entity, MCL-like lymphomas of c-myc-3'RR/ p53+/- mice and c-myc-3'RR/Cdk4(R24C) mice represent a spectrum of the same entity.

A defect of the INK4-Cdk4 checkpoint and Myc collaborate in blastoid mantle cell lymphoma-like lymphoma formation in mice

Mantle cell lymphoma (MCL) is a B-cell malignancy characterized by a monoclonal proliferation of lymphocytes with the co-expression of CD5 and CD43, but not of CD23. Typical MCL is associated with overexpression of cyclin D1, and blastoid MCL variants are associated with Myc (alias c-myc) translocations. In this study, we developed a murine model of MCL-like lymphoma by crossing Cdk4(R24C) mice with Myc-3'RR transgenic mice. The Cdk4(R24C) mouse is a knockin strain that expresses a Cdk4 protein that is resistant to inhibition by p16(INK4a) as well as other INK4 family members. Ablation of INK4 control on Cdk4 does not affect lymphomagenesis, B-cell maturation, and functions in Cdk4(R24C) mice. Additionally, B cells were normal in numbers, cell cycle activity, mitogen responsiveness, and Ig synthesis in response to activation. By contrast, breeding Cdk4(R24C) mice with Myc-3'RR transgenic mice prone to develop aggressive Burkitt lymphoma-like lymphoma (CD19(+)IgM(+)IgD(+) cells) leads to the development of clonal blastoid MCL-like lymphoma (CD19(+)IgM(+)CD5(+)CD43(+)CD23(-) cells) in Myc/Cdk4(R24C) mice. Western blot analysis revealed high amounts of Cdk4/cyclin D1 complexes as the main hallmark of these lymphomas. These results indicate that although silent in nonmalignant B cells, a defect in the INK4-Cdk4 checkpoint can participate in lymphomagenesis in conjunction with additional alterations of cell cycle control, a situation that might be reminiscent of the development of human blastoid MCL.