Inhibition of LILRB2 by a Novel Blocking Antibody Designed to Reprogram Immunosuppressive Macrophages to Drive T Cell Activation in Tumors

Tumor-associated macrophages (TAMs) play an important role in maintaining the immunosuppressive state of the tumor microenvironment (TME). High levels of CD163+ TAMs specifically are associated with poor prognosis in many solid tumor types. Targeting TAMs may represent a key approach in development of the next generation of cancer immune therapeutics. Members of the leukocyte immunoglobulin-like receptor B (LILRB) family, including LILRB2 (ILT4), are known to transmit inhibitory signals in macrophages and other myeloid cells. Leveraging bulk and single cell RNAseq datasets, as well as extensive immunophenotyping of human tumors, we found that LILRB2 is highly expressed on CD163+ CD11b+ cells in the TME and that LILRB2 expression correlates with CD163 expression across many tumor types. To target LILRB2, we have developed JTX-8064, a highly potent and selective antagonistic monoclonal antibody. JTX-8064 blocks LILRB2 binding to its cognate ligands, including classical and non-classical MHC molecules. In vitro, JTX-8064 drives the polarization of human macrophages and dendritic cells toward an immunostimulatory phenotype. As a result, human macrophages treated with a LILRB2 blocker are reprogrammed to increase the activation of autologous T cells in co-culture systems. Furthermore, JTX-8064 significantly potentiates the activity of anti-PD-1 in allogeneic mixed lymphocyte reaction. In a human tumor explant culture, pharmacodynamic activity of JTX-8064 was observed in monotherapy and in combination with anti-PD-1. Collectively, our work provides strong translational and pre-clinical rationale to target LILRB2 in cancer.

Differential expression of CCR8 in tumors versus normal tissue allows specific depletion of tumor-infiltrating T regulatory cells by GS-1811, a novel Fc-optimized anti-CCR8 antibody

The presence of T regulatory (Treg) cells in the tumor microenvironment is associated with poor prognosis and resistance to therapies aimed at reactivating anti-tumor immune responses. Therefore, depletion of tumor-infiltrating Tregs is a potential approach to overcome resistance to immunotherapy. However, identifying Treg-specific targets to drive such selective depletion is challenging. CCR8 has recently emerged as one of these potential targets. Here, we describe GS-1811, a novel therapeutic monoclonal antibody that specifically binds to human CCR8 and is designed to selectively deplete tumor-infiltrating Tregs. We validate previous findings showing restricted expression of CCR8 on tumor Tregs, and precisely quantify CCR8 receptor densities on tumor and normal tissue T cell subsets, demonstrating a window for selective depletion of Tregs in the tumor. Importantly, we show that GS-1811 depleting activity is limited to cells expressing CCR8 at levels comparable to tumor-infiltrating Tregs. Targeting CCR8 in mouse tumor models results in robust anti-tumor efficacy, which is dependent on Treg depleting activity, and synergizes with PD-1 inhibition to promote anti-tumor responses in PD-1 resistant models. Our data support clinical development of GS-1811 to target CCR8 in cancer and drive tumor Treg depletion in order to promote anti-tumor immunity.

ICOS is upregulated on T cells following radiation and agonism combined with radiation results in enhanced tumor control

Multiple preclinical studies have shown improved outcomes when radiation therapy is combined with immune modulating antibodies. However, to date, many of these promising results have failed to translate to successful clinical studies. This led us to explore additional checkpoint and co-stimulatory pathways that may be regulated by radiation therapy. Here, we demonstrate that radiation increases the expression of inducible T cell co-stimulator (ICOS) on both CD4 and CD8 T cells in the blood following treatment. Moreover, when we combined a novel ICOS agonist antibody with radiation we observed durable cures across multiple tumor models and mouse strains. Depletion studies revealed that CD8 T cells were ultimately required for treatment efficacy, but CD4 T cells and NK cells also partially contributed to tumor control. Phenotypic analysis showed that the combination therapy diminished the increased infiltration of regulatory T cells into the tumor that typically occurs following radiation alone. Finally, we demonstrate in a poorly immunogenic pancreatic tumor model which is resistant to combined radiation and anti-PD1 checkpoint blockade that the addition of this novel ICOS agonist antibody to the treatment regimen results in tumor control. These findings identify ICOS as part of a T cell pathway that is modulated by radiation and targeting this pathway with a novel ICOS antibody results in durable tumor control in preclinical models.

ICOS agonism by JTX-2011 (vopratelimab) requires initial T cell priming and Fc cross-linking for optimal T cell activation and anti-tumor immunity in preclinical models

Immunotherapy checkpoint inhibitors, such as antibodies targeting PD-1 and CTLA-4, have demonstrated the potential of harnessing the immune system to treat cancer. However, despite encouraging results particularly with respect to survival, only a minority of patients benefit from these therapies. In clinical studies aimed at understanding changes in the immune system following immunotherapy treatment, ICOS (Inducible T cell CO-Stimulator) was shown to be significantly up-regulated on CD4+ T cells and this was associated with clinical activity, indicating that ICOS stimulatory activity may be beneficial in the treatment of solid tumors. In this report, we describe the generation of specific, species cross-reactive, agonist antibodies to ICOS, including the humanized clinical candidate, JTX-2011 (vopratelimab). Preclinical studies suggest that the ICOS stimulating antibodies require Fc receptor cross-linking for optimal agonistic activity. Notably, the ICOS antibodies do not exhibit superagonist properties but rather require T cell receptor (TCR)-mediated upregulation of ICOS for agonist activity. Treatment with the ICOS antibodies results in robust anti-tumor benefit and long-term protection in preclinical syngeneic mouse tumor models. Additional benefit is observed when the ICOS antibodies are administered in combination with anti-PD-1 and anti-CTLA-4 therapies. Based on the preclinical data, JTX-2011 is currently being developed in the clinical setting for the treatment of solid tumors.

Abstract 5536: ICOS hi CD4 T cells emerging on vopratelimab treatment have Th1 central memory characteristics and may contribute to durability of clinical responses

Background: Inducible T cell Co-stimulator (ICOS) is a costimulatory molecule expressed primarily on T lymphocytes that is upregulated upon cell activation. Vopratelimab (vopra) is a novel ICOS agonist antibody whose primary mechanism of action is the stimulation of primed CD4 T effector cells. Clinical responses in the Phase 1/2 ICONIC trial (NCT02904226) were associated with the emergence of ICOS hi CD4 T cells. In a separate study of PD-1/L1 monotherapy, this phenotype was not observed, demonstrating ICOS hi CD4 T cell emergence in ICONIC was due to vopra. Ex vivo stimulation of antigen-primed ICOS hi CD4 T cells by soluble vopra demonstrated induction of a polyfunctional cytokine response. Previous studies have shown that sustained ICOS upregulation was associated with clinical benefit in subjects treated with anti-CTLA-4 inhibitors, with preclinical data confirming a role for ICOS signaling in optimal anti-tumor activity. Vopra is currently being tested as a sequenced combination with ipilimumab (ipi) in the Phase 2 EMERGE trial (NCT03989362).

Methods: Assessment of phenotype and function of ICOS hi CD4 T cells was conducted using serial collections of peripheral blood mononuclear cells (PBMCs) from a subset of evaluable subjects in the ICONIC trial. A mouse syngeneic tumor model was developed to assess the kinetics of ICOS hi emergence as well as determine functionality and combination efficacy. Biological activity was assessed through ex vivo functional assays, with phenotypic assessments by flow cytometry-based profiling and genomic analysis.

Results: Phenotypic profiling of ICOS hi CD4 T cells demonstrated a T central memory phenotype with Th1 characteristics, and ICOS hi CD4 T cells increased as a % of the CD4 compartment longitudinally. Potent effector molecules, including perforin and granzyme, were significantly upregulated in isolated ICOS hi CD4 T cells relative to ICOS lo CD4 T cells from subjects. In a syngeneic tumor-bearing mouse system, ICOS hi CD4 T cells were induced in lymphoid organs following ipi treatment, with emergence detectable at day 3 and peaking at day 7 post-treatment, where up to 40% of CD4 T cells were ICOS hi. Relative to monotherapy, enhanced anti-tumor efficacy was observed in mice treated with combination ICOS agonist and ipilimumab.

Conclusion: Emergence of a peripheral ICOS hi CD4 T cell population correlates with response to vopra treatment and occurs independent of anti-PD-1 activity. Transcriptional profiling of ICOS hi CD4 T cells further defined the phenotype as Tcm cells that express cytotoxic effector molecules, which may contribute to observed clinical benefit. Treatment with ipi in combination with an ICOS agonist resulted in enhanced efficacy in mice, and follow-up studies exploring sequential dosing are ongoing.

Citation Format: Amadna Hanson, Abha Daneshwar, Heather Cohen, Yasmin Hashambhoy-Ramsay, Kristin O'Malley, Krithi Bala, Lara McGrath, Kristen Leone, Courtney Hart, Rachel McComb, Johan Baeck, Ellen Hooper, Elizabeth Trehu, Haley Laken, Monica Gostissa, Christopher Harvey. ICOS hi CD4 T cells emerging on vopratelimab treatment have Th1 central memory characteristics and may contribute to durability of clinical responses [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5536.

Abstract 5007: Preclinical evaluation of JTX-8064, an anti-LILRB2 antagonist antibody, for reprogramming tumor-associated macrophages

Introduction: Jounce has generated cell type-specific gene signatures as a means of probing The Cancer Genome Atlas and other large datasets to identify targets that may be important immune checkpoints. Using a tumor-associated macrophage (TAM) gene signature, we have found a strong correlation and coherence between TAMs and LILRB2 (leukocyte immunoglobulin like receptor B2; ILT4) across multiple tumors types. LILRB2 is a myeloid cell surface receptor containing four extracellular immunoglobulin domains, a transmembrane domain, and three cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Ligation of LILRB2 on myeloid cells, via its endogenous ligands (classical MHC I molecules [e.g. HLA-A, HLA-B] and non-classical MHC I molecules [e.g. HLA-G]), provides a negative signal that inhibits stimulation of an immune response. HLA-G is recognized as an important immunosuppressive molecule playing a role in maternal-fetal tolerance and being overexpressed in cancer - often associated with advanced disease stage and poor prognosis. As tumor-associated macrophages are known to suppress the anti-cancer immune response, these findings provide rationale for targeting LILRB2.

Methods and Results: We have generated a panel of monoclonal antibodies that bind specifically to LILRB2, but not other LILR family members, and can block binding of LILRB2 to MHC I molecules (i.e. HLA-A and HLA-G). In vitro differentiated monocyte-derived macrophages (MDMs) cultured for 24h in the presence of anti-LILRB2 antibodies and lipopolysaccharide (LPS) show polarization toward a more inflammatory phenotype - secreting higher levels of TNF-α and IL-6 with decreased amounts of IL-10 and CCL2 as compared to an isotype control antibody. NanoString mRNA analysis revealed that, in the absence of LPS or any additional stimuli, MDMs cultured with anti-LILRB2 antibodies showed gene changes consistent with inflammatory or M1-like polarization of macrophages. Anti-LILRB2 antibodies were also evaluated in human tumor histoculture and induced pharmacodynamic responses consistent with macrophage and T cell activation in a variety of tumor types. While mice do not express LILRB2 specifically, they do express a LILRB-like molecule known as Pirb. Mice that are deficient in Pirb display resistance to mouse colon 38 (MC-38) tumor growth suggesting this pathway functions as immune checkpoint in cancer.

Conclusions: Based on these preclinical data, JTX-8064, a high affinity LILRB2-specific humanized antagonist monoclonal antibody, is being developed as an immunotherapeutic to reprogram suppressive macrophages within the tumor microenvironment.

Citation Format: Heather Cohen, Yasmin Hashambhoy-Ramsay, Lauren R. Pepper, Jeffrey Y. Smith, Margaret Willer, Kevin Guay, Vikki Spaulding, Kristin O'Malley, Monica Gostissa, Abha Dhaneshwar, Edward C. Stack, Alessandro Mora, Donald R. Shaffer. Preclinical evaluation of JTX-8064, an anti-LILRB2 antagonist antibody, for reprogramming tumor-associated macrophages [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5007.

Predicting the response to CTLA-4 blockade by longitudinal noninvasive monitoring of CD8 T cells

Rashidian et al. show that ⁸⁹Zr-PEGylated single-domain antibodies that target CD8⁺ T cells can be used to monitor and evaluate the response to immunotherapy as a predictive tool.

Immunotherapy using checkpoint-blocking antibodies against targets such as CTLA-4 and PD-1 can cure melanoma and non–small cell lung cancer in a subset of patients. The presence of CD8 T cells in the tumor correlates with improved survival. We show that immuno–positron emission tomography (immuno-PET) can visualize tumors by detecting infiltrating lymphocytes and, through longitudinal observation of individual animals, distinguish responding tumors from those that do not respond to therapy. We used ⁸⁹Zr-labeled PEGylated single-domain antibody fragments (VHHs) specific for CD8 to track the presence of intratumoral CD8⁺ T cells in the immunotherapy-susceptible B16 melanoma model in response to checkpoint blockade. A ⁸⁹Zr-labeled PEGylated anti-CD8 VHH detected thymus and secondary lymphoid structures as well as intratumoral CD8 T cells. Animals that responded to CTLA-4 therapy showed a homogeneous distribution of the anti-CD8 PET signal throughout the tumor, whereas more heterogeneous infiltration of CD8 T cells correlated with faster tumor growth and worse responses. To support the validity of these observations, we used two different transplantable breast cancer models, yielding results that conformed with predictions based on the antimelanoma response. It may thus be possible to use immuno-PET and monitor antitumor immune responses as a prognostic tool to predict patient responses to checkpoint therapies.

A Rapid Embryonic Stem Cell-Based Mouse Model for B-cell Lymphomas Driven by Epstein-Barr Virus Protein LMP1

The Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) contributes to oncogenic human B-cell transformation. Mouse B cells conditionally expressing LMP1 are not predisposed to B-cell malignancies, as LMP1-expressing B cells are eliminated by T cells. However, mice with conditional B-cell LMP1 expression and genetic elimination of α/β and γ/δ T cells ("CLT" mice) die early in association with B-cell lymphoproliferation and lymphomagenesis. Generation of CLT mice involves in-breeding multiple independently segregating alleles. Thus, although introduction of additional activating or knockout mutations into the CLT model is desirable for further B-cell expansion and immunosurveillance studies, doing such experiments by germline breeding is time-consuming, expensive, and sometimes unfeasible. To generate a more tractable model, we generated clonal CLT embryonic stem (ES) cells from CLT embryos and injected them into RAG2-deficient blastocysts to generate chimeric mice, which, like germline CLT mice, harbor splenic CLT B cells and lack T cells. CLT chimeric mice generated by this RAG2-deficient blastocyst complementation ("RDBC") approach die rapidly in association with B-cell lymphoproliferation and lymphoma. Because CLT lymphomas routinely express the activation-induced cytidine deaminase (AID) antibody diversifier, we tested potential AID roles by eliminating the AID gene in CLT ES cells and testing them via RDBC. We found that CLT and AID-deficient CLT ES chimeras had indistinguishable phenotypes, showing that AID is not essential for LMP1-induced lymphomagenesis. Beyond expanding accessibility and utility of CLT mice as a cancer immunotherapy model, our studies provide a new approach for facilitating generation of genetically complex mouse cancer models.

Mechanisms that can promote peripheral B-cell lymphoma in ATM-deficient mice

The Ataxia Telangiectasia-mutated (ATM) kinase senses DNA double-strand breaks (DSB) and facilitates their repair. In humans, ATM deficiency predisposes to B- and T-cell lymphomas, but in mice it leads only to thymic lymphomas. We tested the hypothesis that increased DSB frequency at a cellular oncogene could promote B-cell lymphoma by generating ATM-deficient mice with a V(D)J recombination target (DJβ cassette) within c-myc intron 1 ("DA" mice). We also generated ATM-deficient mice carrying an Eμ-Bcl-2 transgene (AB mice) to test whether enhanced cellular survival could promote B-cell lymphomas. About 30% of DA or AB mice and nearly 100% of mice harboring the combined genotypes (DAB mice) developed mature B-cell lymphomas. In all genotypes, B-cell tumors harbored oncogenic c-myc amplification generated by breakage-fusion-bridge (BFB) from dicentric chromosomes formed through fusion of IgH V(D)J recombination-associated DSBs on chromosome 12 to sequences downstream of c-myc on chromosome 15. AB tumors demonstrate that B lineage cells harboring spontaneous DSBs leading to IgH/c-myc dicentrics are blocked from progressing to B-cell lymphomas by cellular apoptotic responses. DA and DAB tumor translocations were strictly linked to the cassette, but occurred downstream, frequently in a 6-kb region adjacent to c-myc that harbors multiple cryptic V(D)J recombination targets, suggesting that bona fide V(D)J target sequences may activate linked cryptic targets. Our findings indicate that ATM deficiency allows IgH V(D)J recombination DSBs in developing B cells to generate dicentric translocations that, via BFB cycles, lead to c-myc-activating oncogenic translocations and amplifications in mature B cells.

Developmental propagation of V(D)J recombination-associated DNA breaks and translocations in mature B cells via dicentric chromosomes

Mature IgM(+) B-cell lymphomas that arise in certain ataxia telangiectasia-mutated (ATM)-deficient compound mutant mice harbor translocations that fuse V(D)J recombination-initiated IgH double-strand breaks (DSBs) on chromosome 12 to sequences downstream of c-myc on chromosome 15, generating dicentric chromosomes and c-myc amplification via a breakage-fusion-bridge mechanism. As V(D)J recombination DSBs occur in developing progenitor B cells in the bone marrow, we sought to elucidate a mechanism by which such DSBs contribute to oncogenic translocations/amplifications in mature B cells. For this purpose, we applied high-throughput genome-wide translocation sequencing to study the fate of introduced c-myc DSBs in splenic IgM(+) B cells stimulated for activation-induced cytidine deaminase (AID)-dependent IgH class switch recombination (CSR). We found frequent translocations of c-myc DSBs to AID-initiated DSBs in IgH switch regions in wild-type and ATM-deficient B cells. However, c-myc also translocated frequently to newly generated DSBs within a 35-Mb region downstream of IgH in ATM-deficient, but not wild-type, CSR-activated B cells. Moreover, we found such DSBs and translocations in activated B cells that did not express AID or undergo CSR. Our findings indicate that ATM deficiency leads to formation of chromosome 12 dicentrics via recombination-activating gene-initiated IgH DSBs in progenitor B cells and that these dicentrics can be propagated developmentally into mature B cells where they generate new DSBs downstream of IgH via breakage-fusion-bridge cycles. We propose that dicentrics formed by joining V(D)J recombination-associated IgH DSBs to DSBs downstream of c-myc in ATM-deficient B lineage cells similarly contribute to c-myc amplification and mature B-cell lymphomas.

Genome-wide translocation sequencing reveals mechanisms of chromosome breaks and rearrangements in B cells

Whereas chromosomal translocations are common pathogenetic events in cancer, mechanisms that promote them are poorly understood. To elucidate translocation mechanisms in mammalian cells, we developed high-throughput, genome-wide translocation sequencing (HTGTS). We employed HTGTS to identify tens of thousands of independent translocation junctions involving fixed I-SceI meganuclease-generated DNA double-strand breaks (DSBs) within the c-myc oncogene or IgH locus of B lymphocytes induced for activation-induced cytidine deaminase (AID)-dependent IgH class switching. DSBs translocated widely across the genome but were preferentially targeted to transcribed chromosomal regions. Additionally, numerous AID-dependent and AID-independent hot spots were targeted, with the latter comprising mainly cryptic I-SceI targets. Comparison of translocation junctions with genome-wide nuclear run-ons revealed a marked association between transcription start sites and translocation targeting. The majority of translocation junctions were formed via end-joining with short microhomologies. Our findings have implications for diverse fields, including gene therapy and cancer genomics.

Mechanisms that promote and suppress chromosomal translocations in lymphocytes

Recurrent chromosomal translocations are characteristic features of many types of cancers, especially lymphomas and leukemias. Several basic mechanistic factors are required for the generation of most translocations. First, DNA double-strand breaks (DSBs) must be present simultaneously at the two participating loci. Second, the two broken loci must either be in proximity or be moved into proximity to be joined. Finally, cellular DNA repair pathways must be available to join the two broken loci to complete the translocation. These mechanistic factors can vary in different normal and mutant cells and, as a result, substantially influence the frequency at which particular translocations are generated in a given cell type. Ultimately, however, appearance of recurrent oncogenic translocations in tumors is, in most cases, strongly influenced by selection for the translocated oncogene during the tumorigenesis process. In this review, we discuss in depth the factors and pathways that contribute to the generation of translocations in lymphocytes and other cell types. We also discuss recent findings regarding mechanisms that underlie the appearance of recurrent translocations in tumors.

Excision of reprogramming transgenes improves the differentiation potential of iPS cells generated with a single excisable vector

The residual presence of integrated transgenes following the derivation of induced pluripotent stem (iPS) cells is highly undesirable. Here we demonstrate efficient derivation of iPS cells free of exogenous reprogramming transgenes using an excisable polycistronic lentiviral vector. A novel version of this vector containing a reporter fluorochrome allows direct visualization of vector excision in living iPS cells in real time. We find that removal of the reprogramming vector markedly improves the developmental potential of iPS cells and significantly augments their capacity to undergo directed differentiation in vitro. We further propose that methods to efficiently excise reprogramming transgenes with minimal culture passaging, such as those demonstrated here, are critical since we find that iPS cells may acquire chromosomal abnormalities, such as trisomy of chromosome 8, similar to embryonic stem cells after expansion in culture. Our findings illustrate an efficient method for the generation of transgene-free iPS cells and emphasize the potential beneficial effects that may result from elimination of integrated reprogramming factors. In addition, our results underscore the consequences of long-term culture that will need to be taken into account for the clinical application of iPS cells.

Mechanisms promoting translocations in editing and switching peripheral B cells

Variable, diversity and joining gene segment (V(D)J) recombination assembles immunoglobulin heavy or light chain (IgH or IgL) variable region exons in developing bone marrow B cells, whereas class switch recombination (CSR) exchanges IgH constant region exons in peripheral B cells. Both processes use directed DNA double-strand breaks (DSBs) repaired by non-homologous end-joining (NHEJ). Errors in either V(D)J recombination or CSR can initiate chromosomal translocations, including oncogenic IgH locus (Igh) to c-myc (also known as Myc) translocations of peripheral B cell lymphomas. Collaboration between these processes has also been proposed to initiate translocations. However, the occurrence of V(D)J recombination in peripheral B cells is controversial. Here we show that activated NHEJ-deficient splenic B cells accumulate V(D)J-recombination-associated breaks at the lambda IgL locus (Igl), as well as CSR-associated Igh breaks, often in the same cell. Moreover, Igl and Igh breaks are frequently joined to form translocations, a phenomenon associated with specific Igh-Igl co-localization. Igh and c-myc also co-localize in these cells; correspondingly, the introduction of frequent c-myc DSBs robustly promotes Igh-c-myc translocations. Our studies show peripheral B cells that attempt secondary V(D)J recombination, and determine a role for mechanistic factors in promoting recurrent translocations in tumours.

Oncogenic transformation in the absence of Xrcc4 targets peripheral B cells that have undergone editing and switching

Nonhomologous end-joining (NHEJ) repairs DNA double-strand breaks (DSBs) during V(D)J recombination in developing lymphocytes and during immunoglobulin (Ig) heavy chain (IgH) class switch recombination (CSR) in peripheral B lymphocytes. We now show that CD21-cre–mediated deletion of the Xrcc4 NHEJ gene in p53-deficient peripheral B cells leads to recurrent surface Ig-negative B lymphomas (“CXP lymphomas”). Remarkably, CXP lymphomas arise from peripheral B cells that had attempted both receptor editing (secondary V[D]J recombination of Igκ and Igλ light chain genes) and IgH CSR subsequent to Xrcc4 deletion. Correspondingly, CXP tumors frequently harbored a CSR-based reciprocal chromosomal translocation that fused IgH to c-myc, as well as large chromosomal deletions or translocations involving Igκ or Igλ, with the latter fusing Igλ to oncogenes or to IgH. Our findings reveal peripheral B cells that have undergone both editing and CSR and show them to be common progenitors of CXP tumors. Our studies also reveal developmental stage-specific mechanisms of c-myc activation via IgH locus translocations. Thus, Xrcc4/p53-deficient pro–B lymphomas routinely activate c-myc by gene amplification, whereas Xrcc4/p53-deficient peripheral B cell lymphomas routinely ectopically activate a single c-myc copy.

Conditional mouse osteosarcoma, dependent on p53 loss and potentiated by loss of Rb, mimics the human disease

Osteosarcoma is the most common primary malignant tumor of bone. Analysis of familial cancer syndromes and sporadic cases has strongly implicated both p53 and pRb in its pathogenesis; however, the relative contribution of these mutations to the initiation of osteosarcoma is unclear. We describe here the generation and characterization of a genetically engineered mouse model in which all animals develop short latency malignant osteosarcoma. The genetically engineered mouse model is based on osteoblast-restricted deletion of p53 and pRb. Osteosarcoma development is dependent on loss of p53 and potentiated by loss of pRb, revealing a dominance of p53 mutation in the development of osteosarcoma. The model reproduces many of the defining features of human osteosarcoma including cytogenetic complexity and comparable gene expression signatures, histology, and metastatic behavior. Using a novel in silico methodology termed cytogenetic region enrichment analysis, we demonstrate high conservation of gene expression changes between murine osteosarcoma and known cytogentically rearranged loci from human osteosarcoma. Due to the strong similarity between murine osteosarcoma and human osteosarcoma in this model, this should provide a valuable platform for addressing the molecular genetics of osteosarcoma and for developing novel therapeutic strategies.

H2AX prevents DNA breaks from progressing to chromosome breaks and translocations

Histone H2AX promotes DNA double-strand break (DSB) repair and immunoglobulin heavy chain (IgH) class switch recombination (CSR) in B-lymphocytes. CSR requires activation-induced cytidine deaminase (AID) and involves joining of DSB intermediates by end joining. We find that AID-dependent IgH locus chromosome breaks occur at high frequency in primary H2AX-deficient B cells activated for CSR and that a substantial proportion of these breaks participate in chromosomal translocations. Moreover, activated B cells deficient for ATM, 53BP1, or MDC1, which interact with H2AX during the DSB response, show similarly increased IgH locus breaks and translocations. Thus, our findings implicate a general role for these factors in promoting end joining and thereby preventing DSBs from progressing into chromosomal breaks and translocations. As cellular p53 status does not markedly influence the frequency of such events, our results also have implications for how p53 and the DSB response machinery cooperate to suppress generation of lymphomas with oncogenic translocations.

MDC1 maintains genomic stability by participating in the amplification of ATM-dependent DNA damage signals

MDC1 functions in checkpoint activation and DNA repair following DNA damage. To address the physiological role of MDC1, we disrupted the MDC1 gene in mice. MDC1-/- mice recapitulated many phenotypes of H2AX-/- mice, including growth retardation, male infertility, immune defects, chromosome instability, DNA repair defects, and radiation sensitivity. At the molecular level, H2AX, MDC1, and ATM form a positive feedback loop, with MDC1 directly mediating the interaction between H2AX and ATM. MDC1 binds phosphorylated H2AX through its BRCT domain and ATM through its FHA domain. Through these interactions, MDC1 accumulates activated ATM flanking the sites of DNA damage, facilitating further ATM-dependent phosphorylation of H2AX and the amplification of DNA damage signals. In the absence of MDC1, many downstream ATM signaling events are defective. These results suggest that MDC1, as a signal amplifier of the ATM pathway, is vital in controlling proper DNA damage response and maintaining genomic stability.

Direct p53 transcriptional repression: in vivo analysis of CCAAT-containing G2/M promoters

In response to DNA damage, p53 activates G(1)/S blocking and apoptotic genes through sequence-specific binding. p53 also represses genes with no target site, such as those for Cdc2 and cyclin B, key regulators of the G(2)/M transition. Like most G(2)/M promoters, they rely on multiple CCAAT boxes activated by NF-Y, whose binding to DNA is temporally regulated during the cell cycle. NF-Y associates with p53 in vitro and in vivo through the alphaC helix of NF-YC (a subunit of NF-Y) and a region close to the tetramerization domain of p53. Chromatin immunoprecipitation experiments indicated that p53 is associated with cyclin B2, CDC25C, and Cdc2 promoters in vivo before and after DNA damage, requiring DNA-bound NF-Y. Following DNA damage, p53 is rapidly acetylated at K320 and K373 to K382, histones are deacetylated, and the release of PCAF and p300 correlates with the recruitment of histone deacetylases (HDACs)-HDAC1 before HDAC4 and HDAC5-and promoter repression. HDAC recruitment requires intact NF-Y binding sites. In transfection assays, PCAF represses cyclin B2, and a nonacetylated p53 mutant shows a complete loss of repression potential, despite its abilities to bind NF-Y and to be recruited on G(2)/M promoters. These data (i) detail a strategy of direct p53 repression through associations with multiple NF-Y trimers that is independent of sequence-specific binding of p53 and that requires C-terminal acetylation, (ii) suggest that p53 is a DNA damage sentinel of the G(2)/M transition, and (iii) delineate a new role for PCAF in cell cycle control.

DNA repair, genome stability, and aging

Aging can be defined as progressive functional decline and increasing mortality over time. Here, we review evidence linking aging to nuclear DNA lesions: DNA damage accumulates with age, and DNA repair defects can cause phenotypes resembling premature aging. We discuss how cellular DNA damage responses may contribute to manifestations of aging. We review Sir2, a factor linking genomic stability, metabolism, and aging. We conclude with a general discussion of the role of mutant mice in aging research and avenues for future investigation.

The transcriptional repressor hDaxx potentiates p53-dependent apoptosis

p53 and its homologues p73 and p63 are transcription factors that play an essential role in modulating cell cycle arrest and cell death in response to several environmental stresses. The type and intensity of these responses, which can be different depending on the inducing stimulus and on the overall cellular context, are believed to rely on the activation of defined subsets of target genes. The proper activation of p53 family members requires the coordinated action of post-translational modifications and interaction with several cofactors. In this study, we demonstrate that the multifunctional protein hDaxx interacts with p53 and its homologues, both in vitro and in vivo, and modulates their transcriptional activity. Moreover, we show that hDaxx, which has been implicated in several apoptotic pathways, increases the sensitivity to DNA damage-induced cell death and that this effect requires the presence of p53. Although hDaxx represses p53-dependent transcription of the p21 gene, it does not affect the activation of proapoptotic genes, and therefore acts by influencing the balance between cell cycle arrest and proapoptotic p53 targets. Our results therefore underline the central role of hDaxx in modulating the apoptotic threshold upon several stimuli and identify it as a possible integrating factor that coordinates the response of p53 family members.

Pin1 links the activities of c-Abl and p300 in regulating p73 function

Activation of p73 upon genotoxic treatment triggers apoptosis of tumor cells lacking functional p53 and involves the activities of c-Abl and p300. Here, we demonstrate that conformational changes of p73 catalyzed by the prolyl isomerase Pin1 are crucial in this pathway. Lack of Pin1 reduces p73 stability, hampering its accumulation upon genotoxic stress. Indeed, we show that upon treatment with chemotherapeutic drugs c-Abl enhances the phosphorylation-dependent interaction between Pin1 and p73, and this in turn promotes p73 acetylation by p300. Consistently, the ability of c-Abl and p300 to increase p73 stability and transcriptional activity requires Pin1. As a consequence, Pin1 appears to be essential for activation of the apoptotic response by endogenous p73.

KeePin' the p53 family in good shape

The tumor suppressor p53 and the prolyl isomerase Pin1 are both highly connected proteins, lying at the crossroads between many signaling pathways that control cell proliferation and transformation. By catalyzing conformational changes in a large number of phosphorylated proteins, Pin1 has been implicated in the regulation of major cellular events, such as cell cycle progression, transcription, proliferation and differentiation. Recently, a role for Pin1 has emerged also in the DNA damage response, through modulation of p53 functions upon genotoxic stress. A further level of control has now been unveiled by showing that also the p53 sibling p73 requires Pin1 for its apoptotic activity.

Modification of the erythroid transcription factor GATA-1 by SUMO-1

The activity of transcription factors is tightly modulated by posttranslational modifications affecting stability, localization, and protein-protein interactions. Conjugation to SUMO is a reversible posttranslational modification that has been shown to regulate important transcription factors involved in cell proliferation, differentiation, and tumor suppression. Here, we demonstrate that the erythroid transcription factor GATA-1 is sumoylated in vitro and in vivo and map the single lysine residue involved in SUMO-1 attachment. We show that the nuclear RING finger protein PIASy promotes sumoylation of GATA-1 and dramatically represses its transcriptional activity. We present evidence that a nonsumoylatable GATA-1 mutant is indistinguishable from the WT protein in its ability to transactivate a reporter gene in mammalian cells and in its ability to trigger endogenous globin expression in Xenopus explants. These observations open interesting questions about the biological role of this posttranslational modification of GATA-1.

Transcriptional activation of the cyclin A gene by the architectural transcription factor HMGA2

The HMGA2 protein belongs to the HMGA family of architectural transcription factors, which play an important role in chromatin organization. HMGA proteins are overexpressed in several experimental and human tumors and have been implicated in the process of neoplastic transformation. Hmga2 knockout results in the pygmy phenotype in mice and in a decreased growth rate of embryonic fibroblasts, thus indicating a role for HMGA2 in cell proliferation. Here we show that HMGA2 associates with the E1A-regulated transcriptional repressor p120(E4F), interfering with p120(E4F) binding to the cyclin A promoter. Ectopic expression of HMGA2 results in the activation of the cyclin A promoter and induction of the endogenous cyclin A gene. In addition, chromatin immunoprecipitation experiments show that HMGA2 associates with the cyclin A promoter only when the gene is transcriptionally activated. These data identify the cyclin A gene as a cellular target for HMGA2 and, for the first time, suggest a mechanism for HMGA2-dependent cell cycle regulation.

Adenosine deaminase, a key enzyme in DNA precursors control, is a new p73 target

The discovery of the p73 and p63 genes, homologous to p53 tumor suppressor has uncovered a family of transcription factors and widened the scenario of cell cycle control and apoptosis. We have identified a putative p53-responsive element in the human adenosine deaminase (ADA) gene, an important enzyme involved in nucleotide metabolism, the deficit of which causes the inhibition of DNA synthesis and repair. Here, we demonstrate that the ectopic expression of p73 isoforms leads to the ADA gene upregulation, showing for the first time a correlation between p73 and ADA. We found that p73 promotes ADA gene expression following a dNTP unbalance generated by ADA enzyme deficiency and 2'deoxyadenosine accumulation. The abrogation of p73 transcriptional activity by the specific dominant-negative p73DD abolishes ADA induction. By contrast, the ADA gene does not appear to be a functional p53 target in the physiological conditions we tested. On the whole, our results contribute to the emerging picture that p73 could play a different role from p53 in normal growth and development by inducing alternative target genes, which are not shared by p53.

Regulation of p53 functions: let's meet at the nuclear bodies

The p53 tumour suppressor is crucial for the ability of the cell to either arrest cell cycle progression or activate apoptosis in response to stimuli that may impinge on genomic stability. p53 activation is controlled by mechanisms involving post-translational modifications, protein interactions and modulation of subcellular localisation. Recently, p53 was identified within nuclear bodies, particular subnuclear structures that can provide a 'platform' where interaction of p53 with specific cofactors is favoured. Modulation of recruitment/release of some of these components and modifications might be required for directing p53 toward one or another of its downstream response pathways.

The prolyl isomerase Pin1 reveals a mechanism to control p53 functions after genotoxic insults

The tumour suppressor p53 is important in the cell decision to either arrest cell cycle progression or induce apoptosis in response to a variety of stimuli. p53 post-translational modifications and association with other proteins have been implicated in the regulation of its stability and transcriptional activities. Here we report that, on DNA damage, p53 interacts with Pin1, a peptidyl-prolyl isomerase, which regulates the function of many proteins involved in cell cycle control and apoptosis. The interaction is strictly dependent on p53 phosphorylation, and requires Ser 33, Thr 81 and Ser 315. On binding, Pin1 generates conformational changes in p53, enhancing its transactivation activity. Stabilization of p53 is impaired in UV-treated Pin1(-/-) cells owing to its inability to efficiently dissociate from Mdm2. As a consequence, a reduced p53-dependent response was detected in Pin1(-/-) cells, and this correlates with a diminished transcriptional activation of some p53-regulated genes. Our results suggest that, following stress-induced phosphorylation, p53 needs to form a complex with Pin1 and to undergo a conformational change to fulfil its biological roles.

Homeodomain-interacting protein kinase-2 phosphorylates p53 at Ser 46 and mediates apoptosis

Phosphorylation of p53 at Ser 46 was shown to regulate p53 apoptotic activity. Here we demonstrate that homeodomain-interacting protein kinase-2 (HIPK2), a member of a novel family of nuclear serine/threonine kinases, binds to and activates p53 by directly phosphorylating it at Ser 46. HIPK2 localizes with p53 and PML-3 into the nuclear bodies and is activated after irradiation with ultraviolet. Antisense inhibition of HIPK2 expression reduces the ultraviolet-induced apoptosis. Furthermore, HIPK2 and p53 cooperate in the activation of p53-dependent transcription and apoptotic pathways. These data define a new functional interaction between p53 and HIPK2 that results in the targeted subcellular localization of p53 and initiation of apoptosis.

Regulation of p53 activity in nuclear bodies by a specific PML isoform

Covalent modification of the promyelocytic leukaemia protein (PML) by SUMO-1 is a prerequisite for the assembly of nuclear bodies (NBs), subnuclear structures disrupted in various human diseases and linked to transcriptional and growth control. Here we demonstrate that p53 is recruited into NBs by a specific PML isoform (PML3) or by coexpression of SUMO-1 and hUbc9. NB targeting depends on the direct association of p53, through its core domain, with a C-terminal region of PML3. The relocalization of p53 into NBs enhances p53 transactivation in a promoter-specific manner and affects cell survival. Our results indicate the existence of a cross-talk between PML- and p53-dependent growth suppression pathways, implying an important role for NBs and their resident proteins as modulators of p53 functions.

p53 is involved in the p120E4F-mediated growth arrest

Control of cell growth and division by the p53 tumor suppressor protein requires its abilities to transactivate and repress specific target genes and to associate in complex with other proteins. Here we demonstrate that p53 binds to the E1A-regulated transcription factor p120E4F, a transcriptional repressor of the adenovirus E4 promoter. The interaction involves carboxy-terminal half of p120E4F and sequences located at the end of the sequence-specific DNA-binding domain of p53. Ectopic expression of p120E4F leads to a block of cell proliferation in several human and murine cell lines and this effect requires the association with wild-type (wt) p53. Although p120E4F can also bind to mutant p53, the growth suppression induced by overexpression of the protein is severely reduced in a cell line that contains mutant p53. These data suggest that p120E4F may represent an important element within the complex network of p53 checkpoint functions.

Activation of p53 by conjugation to the ubiquitin-like protein SUMO-1

The growth-suppressive properties of p53 are controlled by posttranslational modifications and by regulation of its turnover rate. Here we show that p53 can be modified in vitro and in vivo by conjugation to the small ubiquitin-like protein SUMO-1. A lysine residue at amino acid position 386 of p53 is required for this previously undescribed modification, strongly suggesting that this lysine residue serves as the major attachment site for SUMO-1. Unlike ubiquitin, attachment of SUMO-1 does not appear to target proteins for rapid degradation but rather, has been proposed to change the ability of the modified protein to interact with other cellular proteins. Accordingly, we provide evidence that conjugation of SUMO-1 to wild-type p53 results in an increased transactivation ability of p53. We suggest that posttranslational modification of p53 by SUMO-1 conjugation provides a novel mechanism to regulate p53 activity.

The product of a gas6 splice variant allows the release of the domain responsible for Axl tyrosine kinase receptor activation

The product of gas6 (Gas6) is a growth factor with high level of similarity to protein S and was identified as the ligand for Axl family of tyrosine kinase receptors. Gas6 contains an N-terminal gamma-carboxylated domain (Gla), four epidermal growth factor like domains and a large C-terminal D region. An alternative Gas6 spliced form (Gas6SV) having an additional 43 amino acids between fourth EFG like and D domain was characterised. Here we show data indicating that Gas6SV is specifically cleaved within the inserted sequence, thereby splitting the D domain from the remaining part of the protein. The resulting two proteolytic products of 36 kDa and 50 kDa were separated and the 50 kDa fragment corresponding to region D was shown to be responsible for Axl receptor activation. Furthermore a deletion mutant of Gas6 containing only the D domain was shown to similarly activate Axl receptor phosphorylation unequivocally demonstrating that D domain can act as a signalling molecule. The possible roles of the proteolytic processing of Gas6SV in the regulation of growth factor availability are discussed.

Identification and tissue expression of a splice variant for the growth arrest-specific gene gas6

The growth arrest-specific gene gas6 encodes a secreted protein (Gas6) which is a member of the vitamin K-dependent protein family and was identified as a ligand for the Ax1 tyrosine kinase receptor family. Gas6 shares significant similarity with protein S and a similar domain organisation: an extensively gamma-carboxylated amino-terminal, four epidermal growth factor-like motifs and a large carboxy-terminal region, known as the D domain. Here we report on the isolation of a splice variant (gas6SV) characterised by an in-frame 129 bp insertion between the fourth EGF domain and the D domain. The gene gas6 was previously mapped on chromosome 13. The genomic organisation of gas6 has been investigated demonstrating the presence of alternative splicing consensus sites. Expression of gas6SV has been investigated in various human tissues and found to have a similar distribution pattern as gas6, with the exception of the spleen where gas6SV seems to be the predominant form.