Acute leukemia with promyelocytic features in PML/RARalpha transgenic mice

Acute promyelocytic leukemia: A model of molecular target based therapy

Leukemia, a group of hematological malignancies characterized by clonal expansion of hematopoietic cells with uncontrolled proliferation, decreased apoptosis and blocked differentiation, is one of the most notorious enemies of mankind which accounts for some 300,000 new cases and 222,000 deaths each year worldwide. Leukemia can be divided into acute or chronic, lymphoid or myeloid types, based on the disease progression and hematopoietic lineages involved 5. The responses of leukemia to therapies differ from one type or subtype to another. Hence, to improve the clinical outcome, the therapeutic strategies should be disease pathogenesis-based and individualized. The close collaboration between bench and bedside may not only shed new lights on leukemogenesis, gain insights into therapeutic mechanisms, but also provide opportunities for designing more rational therapies. The development of curative approaches for acute promyelocytic leukemia (APL) may serve as a paradigm.

How animal models of leukaemias have already benefited patients

The relative genetic simplicity of leukaemias, the development of which likely relies on a limited number of initiating events has made them ideal for disease modelling, particularly in the mouse. Animal models provide incomparable insights into the mechanisms of leukaemia development and allow exploration of the molecular pillars of disease maintenance, an aspect often biased in cell lines or ex vivo systems. Several of these models, which faithfully recapitulate the characteristics of the human disease, have been used for pre-clinical purposes and have been instrumental in predicting therapy response in patients. We plea for a wider use of genetically defined animal models in the design of clinical trials, with a particular focus on reassessment of existing cancer or non-cancer drugs, alone or in combination.

PML(NLS(-)) inhibits cell apoptosis and promotes proliferation in HL-60 cells

Promyelocytic leukemia (PML) is a cell-growth suppressor, and PML-retinoic acid receptor α (PML-RARα) is known as a fusion gene of acute promyelocytic leukemia (APL). Studies have reported that neutrophil elastase(NE) cleaved bcr-1-derived PML-RARα in early myeloid cells leading to the removal of nuclear localization signal (NLS) from PML. The resultant PML without NLS named PML(NLS(-)). PML(NLS(-)) mainly locates and functions in the cytoplasm. PML(NLS(-)) may act in different ways from PML, but its biological characteristics have not been reported. In this study, the PML (NLS(-)) was silenced with shRNA [HL-60/pPML(NLS(-))-shRNA] and over-expressed by preparation of recombinant adenovirus [HL-60/pAd-PML(NLS(-))]. The mRNA and protein expression of PML(NLS(-)) were detected by RT-PCR and Western blot respectively. Cell proliferation in vitro was assessed by MTT assay. Flow cytometry (FCM) was used to detect apoptotic cells. The transcription of BCL-2, BAX and C-MYC was detected in HL-60/pAd-PML(NLS(-)) cells. Our results showed that compared to the control group, the expression of PML(NLS(-)) was significantly reduced in the HL-60/pPML(NLS(-))-shRNA cells, and increased significantly in the HL-60/pAd-PML(NLS(-)) cells. The proliferation was significantly inhibited in the HL-60/pPML(NLS(-))-shRNA cells in a time-dependent manner, but markedly promoted in the HL-60/pAd-PML(NLS(-)) cells treated with 60 μmol/L emodin. FCM revealed the apoptosis increased in HL-60/pPML(NLS(-))-shRNA cells, and decreased in the HL-60/pAd-PML(NLS(-)) cells. The expression of BAX decreased significantly, while that of BCL-2 and C-MYC increased significantly in HL-60/ pAd-PML(NLS(-)) cells. Down-regulation of PML(NLS(-)) expression inhibits the proliferation and induces the apoptosis of HL-60 cells. On the contrary, over-expression of PML(NLS(-)) promotes the proliferation and reduce the emodin-induced apoptosis of HL-60 cells.

Synthesis of [(11)C]Am80 via Novel Pd(0)-Mediated Rapid [(11)C]Carbonylation Using Arylboronate and [(11)C]Carbon Monoxide

(11)C-labeled methylbenzoates [(11)C]4a-d were synthesized using Pd(0)-mediated rapid cross-coupling reactions employing [(11)C]carbon monoxide and arylboronic acid neopentyl glycol esters 3a-d under atmospheric pressure in methanol-dimethylformamide (MeOH-DMF), in radiochemical yields of 12 ± 5-26 ± 13% (decay-corrected based on [(11)C]O). The reaction conditions were highly favorable for the synthesis of [(11)C]Am80 ([(11)C]2) and [(11)C]methyl 4-((5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-yl)carbamoyl)benzoate ([(11)C]2-Me) using 4-(5,5-dimethyl-1,3,2-dioxaborinan-2-yl)-N-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-yl)benzamide (5), both of which produced a decay-corrected radiochemical yield (RCY) of 26 ± 13%, with >99% radiochemical purity and an average specific radioactivity of 44 GBq/μmol. The yields of [(11)C]4a, [(11)C]2-Me, and [(11)C]2 were improved by the use of a 2-fold excess of the solvents and reagents under the same conditions to give respective yields of 66 ± 8, 65 ± 7, and 48 ± 2%.

The APL paradigm and the "co-clinical trial" project

Tremendous advances in technologies have allowed the attainment of powerful insights into the molecular and genetic determinants that drive human cancers. However, this acquired knowledge has been translated into effective therapeutics very slowly, in part due to difficulty in predicting which drug or drug combination is likely to be effective in the complex mutational background of human cancers. To address this difficulty we have proposed and initiated the "co-clinical trial" project, in which we exploit mouse models that faithfully replicate the variety of mutational events observed in human cancers, to conduct preclinical trials that parallel ongoing human phase I/II clinical trials. Here, we focus on concepts relevant to the application of this novel paradigm and the essential components required for its implementation to ultimately achieve the rational and rapid development of new therapeutic treatments.

Expression and function of PML-RARA in the hematopoietic progenitor cells of Ctsg-PML-RARA mice

Because PML-RARA-induced acute promyelocytic leukemia (APL) is a morphologically differentiated leukemia, many groups have speculated about whether its leukemic cell of origin is a committed myeloid precursor (e.g. a promyelocyte) versus an hematopoietic stem/progenitor cell (HSPC). We originally targeted PML-RARA expression with CTSG regulatory elements, based on the early observation that this gene was maximally expressed in cells with promyelocyte morphology. Here, we show that both Ctsg, and PML-RARA targeted to the Ctsg locus (in Ctsg-PML-RARA mice), are expressed in the purified KLS cells of these mice (KLS = Kit⁺Lin⁻Sca⁺, which are highly enriched for HSPCs), and this expression results in biological effects in multi-lineage competitive repopulation assays. Further, we demonstrate the transcriptional consequences of PML-RARA expression in Ctsg-PML-RARA mice in early myeloid development in other myeloid progenitor compartments [common myeloid progenitors (CMPs) and granulocyte/monocyte progenitors (GMPs)], which have a distinct gene expression signature compared to wild-type (WT) mice. Although PML-RARA is indeed expressed at high levels in the promyelocytes of Ctsg-PML-RARA mice and alters the transcriptional signature of these cells, it does not induce their self-renewal. In sum, these results demonstrate that in the Ctsg-PML-RARA mouse model of APL, PML-RARA is expressed in and affects the function of multipotent progenitor cells. Finally, since PML/Pml is normally expressed in the HSPCs of both humans and mice, and since some human APL samples contain TCR rearrangements and express T lineage genes, we suggest that the very early hematopoietic expression of PML-RARA in this mouse model may closely mimic the physiologic expression pattern of PML-RARA in human APL patients.

Methionine-induced hyperhomocysteinemia reverts fibrinolytic pathway activation in a murine model of acute promyelocytic leukemia

Increased fibrinolysis is an important component of acute promyelocytic leukemia (APL) bleeding diathesis. APL blasts overexpress annexin II (ANXII), a receptor for tissue plasminogen activator (tPA), and plasminogen, thereby increasing plasmin generation. Previous studies suggested that ANXII plays a pivotal role in APL coagulopathy. ANXII binding to tPA can be inhibited by homocysteine and hyperhomocysteinemia can be induced by L-methionine supplementation. In the present study, we used an APL mouse model to study ANXII function and the effects of hyperhomocysteinemia in vivo. Leukemic cells expressed higher ANXII and tPA plasma levels (11.95 ng/mL in leukemic vs 10.74 ng/mL in wild-type; P = .004). In leukemic mice, administration of L-methionine significantly increased homocysteine levels (49.0 μmol/mL and < 6.0 μmol/mL in the treated and nontreated groups, respectively) and reduced tPA levels to baseline concentrations. The latter were also decreased after infusion of the LCKLSL peptide, a competitor for the ANXII tPA–binding site (11.07 ng/mL; P = .001). We also expressed and purified the p36 component of ANXII in Pichia methanolica. The infusion of p36 in wild-type mice increased tPA and thrombin-antithrombin levels, and the latter was reversed by L-methionine administration. The results of the present study demonstrate the relevance of ANXII in vivo and suggest that methionine-induced hyperhomocysteinemia may reverse hyperfibrinolysis in APL.

Sox4 cooperates with CREB in myeloid transformation

The cAMP response element-binding protein (CREB) is a nuclear transcription factor that is critical for normal and neoplastic hematopoiesis. Previous studies have demonstrated that CREB is a proto-oncogene whose overexpression promotes cellular proliferation in hematopoietic cells. Transgenic mice that overexpress CREB in myeloid cells develop a myeloproliferative disease with splenomegaly and aberrant myelopoiesis. However, CREB overexpressing mice do not spontaneously develop acute myeloid leukemia. In this study, we used retroviral insertional mutagenesis to identify genes that accelerate leukemia in CREB transgenic mice. Our mutagenesis screen identified several integration sites, including oncogenes Gfi1, Myb, and Ras. The Sox4 transcription factor was identified by our screen as a gene that cooperates with CREB in myeloid leukemogenesis. We show that the transduction of CREB transgenic mouse bone marrow cells with a Sox4 retrovirus increases survival and self-renewal of cells in vitro. Furthermore, leukemic blasts from the majority of acute myeloid leukemia patients have higher CREB, phosphorylated CREB, and Sox 4 protein expression. Sox4 transduction of mouse bone marrow cells results in increased expression of CREB target genes. We also demonstrate that CREB is a direct target of Sox4 by chromatin immunoprecipitation assays. These results indicate that Sox4 and CREB cooperate and contribute to increased proliferation of hematopoietic progenitor cells.

How to manage acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a unique subtype of acute myeloid leukemia (AML). The prognosis of APL is changing, from the worst among AML as it used to be, to currently the best. The application of all-trans-retinoic acid (ATRA) to the induction therapy of APL decreases the mortality of newly diagnosed patients, thereby significantly improving the response rate. Therefore, ATRA combined with anthracycline-based chemotherapy has been widely accepted and used as a classic treatment. It has been demonstrated that high doses of cytarabine have a good effect on the prevention of relapse for high-risk patients. However, as the indications of arsenic trioxide (ATO) for APL are being extended from the original relapse treatment to the first-line treatment of de novo APL, we find that the regimen of ATRA, combined with ATO, seems to be a new treatment option because of their targeting mechanisms, milder toxicities and improvements of long-term outcomes; this combination may become a potentially curable treatment modality for APL. We discuss the therapeutic strategies for APL, particularly the novel approaches to newly diagnosed patients and the handling of side effects of treatment and relapse treatment, so as to ensure each newly diagnosed patient of APL the most timely and best treatment.

Altered nuclear cofactor switching in retinoic-resistant variants of the PML-RARα oncoprotein of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) results from a reciprocal translocation that fuses the gene for the PML tumor suppressor to that encoding the retinoic acid receptor alpha (RARα). The resulting PML-RARα oncogene product interferes with multiple regulatory pathways associated with myeloid differentiation, including normal PML and RARα functions. The standard treatment for APL includes anthracycline-based chemotherapeutic agents plus the RARα agonist all-trans retinoic acid (ATRA). Relapse, which is often accompanied by ATRA resistance, occurs in an appreciable frequency of treated patients. One potential mechanism suggested by model experiments featuring the selection of ATRA-resistant APL cell lines involves ATRA-resistant versions of the PML-RARα oncogene, where the relevant mutations localize to the RARα ligand-binding domain (LBD). Such mutations may act by compromising agonist binding, but other mechanisms are possible. Here, we studied the molecular consequence of ATRA resistance by use of circular dichroism, protease resistance, and fluorescence anisotropy assays employing peptides derived from the NCOR nuclear corepressor and the ACTR nuclear coactivator. The consequences of the mutations on global structure and cofactor interaction functions were assessed quantitatively, providing insights into the basis of agonist resistance. Attenuated cofactor switching and increased protease resistance represent features of the LBDs of ATRA-resistant PML-RARα, and these properties may be recapitulated in the full-length oncoproteins.

Halofuginone has anti-proliferative effects in acute promyelocytic leukemia by modulating the transforming growth factor beta signaling pathway

Promyelocytic leukemia-retinoic acid receptor alpha (PML-RARα) expression in acute promyelocytic leukemia (APL) impairs transforming growth factor beta (TGFβ) signaling, leading to cell growth advantage. Halofuginone (HF), a low-molecular-weight alkaloid that modulates TGFβ signaling, was used to treat APL cell lines and non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mice subjected to transplantation with leukemic cells from human chorionic gonadotrophin-PML-RARα transgenic mice (TG). Cell cycle analysis using incorporated bromodeoxyuridine and 7-amino-actinomycin D showed that, in NB4 and NB4-R2 APL cell lines, HF inhibited cellular proliferation (P<0.001) and induced apoptosis (P = 0.002) after a 24-hour incubation. Addition of TGFβ revealed that NB4 cells were resistant to its growth-suppressive effects and that HF induced these effects in the presence or absence of the cytokine. Cell growth inhibition was associated with up-regulation of TGFβ target genes involved in cell cycle regulation (TGFB, TGFBRI, SMAD3, p15, and p21) and down-regulation of MYC. Additionally, TGFβ protein levels were decreased in leukemic TG animals and HF in vivo could restore TGFβ values to normal. To test the in vivo anti-leukemic activity of HF, we transplanted NOD/SCID mice with TG leukemic cells and treated them with HF for 21 days. HF induced partial hematological remission in the peripheral blood, bone marrow, and spleen. Together, these results suggest that HF has anti-proliferative and anti-leukemic effects by reversing the TGFβ blockade in APL. Since loss of the TGFβ response in leukemic cells may be an important second oncogenic hit, modulation of TGFβ signaling may be of therapeutic interest.

(+)α-Tocopheryl succinate inhibits the mitochondrial respiratory chain complex I and is as effective as arsenic trioxide or ATRA against acute promyelocytic leukemia in vivo

The vitamin E derivative (+)α-tocopheryl succinate (α-TOS) exerts pro-apoptotic effects in a wide range of tumors and is well tolerated by normal tissues. Previous studies point to a mitochondrial involvement in the action mechanism; however, the early steps have not been fully elucidated. In a model of acute promyelocytic leukemia (APL) derived from hCG-PML-RARα transgenic mice, we demonstrated that α-TOS is as effective as arsenic trioxide or all-trans retinoic acid, the current gold standards of therapy. We also demonstrated that α-TOS induces an early dissipation of the mitochondrial membrane potential in APL cells and studies with isolated mitochondria revealed that this action may result from the inhibition of mitochondrial respiratory chain complex I. Moreover, α-TOS promoted accumulation of reactive oxygen species hours before mitochondrial cytochrome c release and caspases activation. Therefore, an in vivo antileukemic action and a novel mitochondrial target were revealed for α-TOS, as well as mitochondrial respiratory complex I was highlighted as potential target for anticancer therapy.

Functional and structural insights into ASB2alpha, a novel regulator of integrin-dependent adhesion of hematopoietic cells

By providing contacts between hematopoietic cells and the bone marrow microenvironment, integrins are implicated in cell adhesion and thereby in control of cell fate of normal and leukemia cells. The ASB2 gene, initially identified as a retinoic acid responsive gene and a target of the promyelocytic leukemia retinoic acid receptor α oncoprotein in acute promyelocytic leukemia cells, encodes two isoforms, a hematopoietic-type (ASB2α) and a muscle-type (ASB2β) that are involved in hematopoietic and myogenic differentiation, respectively. ASB2α is the specificity subunit of an E3 ubiquitin ligase complex that targets filamins to proteasomal degradation. To examine the relationship of the ASB2α structure to E3 ubiquitin ligase function, functional assays and molecular modeling were performed. We show that ASB2α, through filamin A degradation, enhances adhesion of hematopoietic cells to fibronectin, the main ligand of β1 integrins. Furthermore, we demonstrate that a short N-terminal region specific to ASB2α, together with ankyrin repeats 1 to 10, is necessary for association of ASB2α with filamin A. Importantly, the ASB2α N-terminal region comprises a 9-residue segment with predicted structural homology to the filamin-binding motifs of migfilin and β integrins. Together, these data provide new insights into the molecular mechanisms of ASB2α binding to filamin.

PML-RARA can increase hematopoietic self-renewal without causing a myeloproliferative disease in mice

Acute promyelocytic leukemia (APL) is characterized by the t(15;17) translocation that generates the fusion protein promyelocytic leukemia-retinoic acid receptor α (PML-RARA) in nearly all cases. Multiple prior mouse models of APL constitutively express PML-RARA from a variety of non-Pml loci. Typically, all animals develop a myeloproliferative disease, followed by leukemia in a subset of animals after a long latent period. In contrast, human APL is not associated with an antecedent stage of myeloproliferation. To address this discrepancy, we have generated a system whereby PML-RARA expression is somatically acquired from the mouse Pml locus in the context of Pml haploinsufficiency. We found that physiologic PML-RARA expression was sufficient to direct a hematopoietic progenitor self-renewal program in vitro and in vivo. However, this expansion was not associated with evidence of myeloproliferation, more accurately reflecting the clinical presentation of human APL. Thus, at physiologic doses, PML-RARA primarily acts to increase hematopoietic progenitor self-renewal, expanding a population of cells that are susceptible to acquiring secondary mutations that cause progression to leukemia. This mouse model provides a platform for more accurately dissecting the early events in APL pathogenesis.

Revisiting the differentiation paradigm in acute promyelocytic leukemia

As the result of intense clinical and basic research, acute promyelocytic leukemia (APL) has progressively evolved from a deadly to a curable disease. Historically, efforts aimed at understanding the molecular bases for therapy response have repeatedly illuminated APL pathogenesis. The classic model attributes this therapeutic success to the transcriptional reactivation elicited by retinoic acid and the resulting overcoming of the differentiation block characteristic of APL blasts. However, in clinical practice, retinoic acid by itself only rarely yields prolonged remissions, even though it induces massive differentiation. In contrast, as a single agent, arsenic trioxide neither directly activates transcription nor triggers terminal differentiation ex vivo, but cures many patients. Here we review the evidence from recent ex vivo and in vivo studies that allow a reassessment of the role of differentiation in APL cure. We discuss alternative models in which PML-RARA degradation and the subsequent loss of APL cell self-renewal play central roles. Rather than therapy aimed at inducing differentiation, targeting cancer cell self-renewal may represent a more effective goal, achievable by a broader range of therapeutic agents.

The nuclear bodies inside out: PML conquers the cytoplasm

The promyelocytic leukemia (PML) protein is the core component of nuclear substructures that host more than 70 proteins, termed nuclear domains 10 or PML-nuclear bodies. PML was first identified as the gene participating in the translocation responsible for the pathogenesis of acute promyelocytic leukemia (APL). The notion that PML is a tumor suppressor gene was soon extrapolated from leukemia to solid tumors. The last decade has radically changed the view of how this tumor suppressor is regulated, how it can be therapeutically targeted, and how it functions. Notably, one of the most recent and striking features uncovered is how PML regulates cellular homeostasis outside its original niche in the nucleus. These new findings open an exciting new area of research in extra-nuclear PML functions.

Effector caspases and leukemia

Caspases, a family of aspartate-specific cysteine proteases, play a major role in apoptosis and a variety of physiological and pathological processes. Fourteen mammalian caspases have been identified and can be divided into two groups: inflammatory caspases and apoptotic caspases. Based on the structure and function, the apoptotic caspases are further grouped into initiator/apical caspases (caspase-2, -8, -9, and -10) and effector/executioner caspases (caspase-3, -6, and -7). In this paper, we discuss what we have learned about the role of individual effector caspase in mediating both apoptotic and nonapoptotic events, with special emphasis on leukemia-specific oncoproteins in relation to effector caspases.

Treatment with 5-azacytidine accelerates acute promyelocytic leukemia leukemogenesis in a transgenic mouse model

A key oncogenic force in acute promyelocytic leukemia (APL) is the ability of the promyelocytic leukemia-retinoic acid receptor α (PML-RARA) oncoprotein to recruit transcriptional repressors and DNA methyltransferases at retinoic acid-responsive elements. Pharmacological doses of retinoic acid relieve transcriptional repression inducing terminal differentiation/apoptosis of the leukemic blasts. APL blasts often harbor additional recurrent chromosomal abnormalities, and significantly, APL prevalence is increased in Latino populations. These observations suggest that multiple genetic and environmental/dietary factors are likely implicated in APL. We tested whether dietary or targeted chemopreventive strategies relieving PML-RARA transcriptional repression would be effective in a transgenic mouse model. Surprisingly, we found that 1) treatment with a demethylating agent, 5-azacytidine, results in a striking acceleration of APL; 2) a high fat, low folate/choline-containing diet resulted in a substantial but nonsignificant APL acceleration; and 3) all-trans retinoic acid (ATRA) is ineffective in preventing leukemia and results in ATRA-resistant APL. Our findings have important clinical implications because ATRA is a drug of choice for APL treatment and indicate that global demethylation, whether through dietary manipulations or through the use of a pharmacologic agent such as 5-azacytidine, may have unintended and detrimental consequences in chemopreventive regimens.

Correlation among nuclear localization of NuMA-RARα, deregulation of gene expression and leukemic phenotype of hCG-NuMA-RARα transgenic mice

Acute promyelocytic leukemia (APL) is a model system of aberrant transcription in cancer. We sought to elucidate the mechanism of action of the variant fusion NuMA-RARα in APL, using the hCG-NuMA-RARα transgenic model. We report that subcellular localization of NuMA-RARα in transgenic mice is dependent upon its protein expression and transgene dosage. Subcellular localization of the fusion is inversely correlated with extent of gene deregulation at the mRNA level for Cebpα, Cebpɛ and Pu.1. Finally, we report that phenotype onset is correlated with NuMA-RARα copy number; mice with higher copy number developing disease later than those with lower copy number.

Rara haploinsufficiency modestly influences the phenotype of acute promyelocytic leukemia in mice

RARA (retinoic acid receptor alpha) haploinsufficiency is an invariable consequence of t(15;17)(q22;q21) translocations in acute promyelocytic leukemia (APL). Retinoids and RARA activity have been implicated in hematopoietic self-renewal and neutrophil maturation. We and others therefore predicted that RARA haploinsufficiency would contribute to APL pathogenesis. To test this hypothesis, we crossed Rara(+/-) mice with mice expressing PML (promyelocytic leukemia)-RARA from the cathepsin G locus (mCG-PR). We found that Rara haploinsufficiency cooperated with PML-RARA, but only modestly influenced the preleukemic and leukemic phenotype. Bone marrow from mCG-PR(+/-) × Rara(+/-) mice had decreased numbers of mature myeloid cells, increased ex vivo myeloid cell proliferation, and increased competitive advantage after transplantation. Rara haploinsufficiency did not alter mCG-PR-dependent leukemic latency or penetrance, but did influence the distribution of leukemic cells; leukemia in mCG-PR(+/-) × Rara(+/-) mice presented more commonly with low to normal white blood cell counts and with myeloid infiltration of lymph nodes. APL cells from these mice were responsive to all-trans retinoic acid and had virtually no differences in expression profiling compared with tumors arising in mCG-PR(+/-) × Rara(+/+) mice. These data show that Rara haploinsufficiency (like Pml haploinsufficiency and RARA-PML) can cooperate with PML-RARA to influence the pathogenesis of APL in mice, but that PML-RARA is the t(15;17) disease-initiating mutation.

Interplay of protein misfolding pathway and unfolded-protein response in acute promyelocytic leukemia

Protein misfolding has traditionally been linked to the pathogenesis of various neurodegenerative diseases. However, emerging evidence from various laboratories, including ours, suggests that protein misfolding may also play a fundamental role in some malignancies, particularly those caused by fusion oncoprotein generated from chromosomal translocation. Promyelocytic leukemia (PML) fused to the retinoic acid receptor (RAR) is a fusion oncoprotein linked to the transformation of acute promyelocytic leukemia (APL), and is not only a misfolded protein itself, but also promotes misfolding of nuclear receptor corepressor (N-CoR) protein, a corepressor essential for the growth-suppressive function of several tumor-suppressor proteins. PML-RAR promotes misfolding of N-CoR by inducing aberrant post-translational modification, which destabilizes its core and promotes instability. Misfolded N-CoR, thus, contributes to differentiation arrest and survival of APL cells through loss-of-function and aberrant gain-of-function properties. Therapeutic restoration of N-CoR conformation and function with conformation-modifying agents not only releases this differentiation arrest but also sensitizes APL cells to programmed cell death. These findings illustrate the potential of the misfolded N-CoR protein as a conformation-based drugable molecular target for APL, and highlights the promise of various conformation-modifying agents as novel therapeutics for APL. Protein conformational rearrangement, resulting from an inherited or acquired genetic alteration, could be a common pathological phenomenon contributing to transformation in different types of leukemias and solid tumors and, therefore, could serve as a common ground for designing a unifying diagnostic as well as therapeutic approach for a widely diverse disease such as cancer. To that end, APL could serve as a model for the development of a novel conformation-based therapeutic approach for other malignant diseases.

A protease-resistant PML-RAR{alpha} has increased leukemogenic potential in a murine model of acute promyelocytic leukemia

Previous studies in our laboratory demonstrated that the azurophil granule protease neutrophil elastase (NE) cleaves promyelocytic leukemia-retinoic acid receptor (PML-RAR)α (PR), the fusion protein that initiates acute promyelocytic leukemia (APL). Further, NE deficiency reduces the penetrance of APL in a murine model of this disease. We therefore predicted that NE-mediated PR cleavage might be important for its ability to initiate APL. To test this hypothesis, we generated a mouse expressing NE-resistant PR. These mice developed APL indistinguishable from wild-type PR, but with significantly reduced latency (median leukemia-free survival of 274 days vs 473 days for wild-type PR, P < .001). Resistance to proteolysis may increase the abundance of full-length PR protein in early myeloid cells, and our previous data suggested that noncleaved PR may be less toxic to early myeloid cells. Together, these effects appear to increase the leukemogenicity of NE-resistant PR, contrary to our previous prediction. We conclude that NE deficiency may reduce APL penetrance via indirect mechanisms that are still NE dependent.

Acute promyelocytic leukaemia: novel insights into the mechanisms of cure

The fusion oncogene, promyelocytic leukaemia (PML)-retinoic acid receptor-α (RARA), initiates acute promyelocytic leukaemia (APL) through both a block to differentiation and increased self-renewal of leukaemic progenitor cells. The current standard of care is retinoic acid (RA) and chemotherapy, but arsenic trioxide also cures many patients with APL, and an RA plus arsenic trioxide combination cures most patients. This Review discusses the recent evidence that reveals surprising new insights into how RA and arsenic trioxide cure this leukaemia, by targeting PML-RARα for degradation. Drug-triggered oncoprotein degradation may be a strategy that is applicable to many cancers.

Pharicin B stabilizes retinoic acid receptor-α and presents synergistic differentiation induction with ATRA in myeloid leukemic cells

All-trans retinoic acid (ATRA), a natural ligand for the retinoic acid receptors (RARs), induces clinical remission in most acute promyelocytic leukemia (APL) patients through the induction of differentiation and/or eradication of leukemia-initiating cells. Here, we identify a novel natural ent-kaurene diterpenoid derived from Isodon pharicus leaves, called pharicin B, that can rapidly stabilize RAR-α protein in various acute myeloid leukemic (AML) cell lines and primary leukemic cells from AML patients, even in the presence of ATRA, which is known to induce the loss of RAR-α protein. Pharicin B also enhances ATRA-dependent the transcriptional activity of RAR-α protein in the promyelocytic leukemia-RARα-positive APL cell line NB4 cells. We also showed that pharicin B presents a synergistic or additive differentiation-enhancing effect when used in combination with ATRA in several AML cell lines and, especially, some primary leukemic cells from APL patients. In addition, pharicin B can overcome retinoid resistance in 2 of 3 NB4-derived ATRA-resistant subclones. These findings provide a good example for chemical biology-based investigations of pathophysiological and therapeutic significances of RAR-α and PML-RAR-α proteins. The effectiveness of the ATRA/pharicin B combination warrants further investigation on their use as a therapeutic strategy for AML patients.

Cryptic insertion of PML-RARA into the 3p25 locus in an acute promyelocytic leukemia with t(3;17)(p25;q21)

We studied a case of a 72-year-old man with acute promyelocytic leukemia and a t(3;17)(p25;q21). Fluorescence in situ hybridization failed to show rearrangement of the PML (promyelocytic leukemia protein) locus but did demonstrate relocalization of the retinoic acid receptor alpha (RARA) to chromosome 3. We performed a modified panhandle polymerase chain reaction analysis to investigate the unknown 5' partner. Our analysis indicates that the fusion partner is PML. This karyotype therefore results in a cryptic PML-RARA fusion inserted into the 3p25 locus. Our case highlights the need for molecular analysis of seemingly novel karyotypic abnormalities.

Pim2 cooperates with PML-RARalpha to induce acute myeloid leukemia in a bone marrow transplantation model

Although the potential role of Pim2 as a cooperative oncogene has been well described in lymphoma, its role in leukemia has remained largely unexplored. Here we show that high expression of Pim2 is observed in patients with acute promyelocytic leukemia (APL). To further characterize the cooperative role of Pim2 with promyelocytic leukemia/retinoic acid receptor alpha (PML/RARalpha), we used a well-established PML-RARalpha (PRalpha) mouse model. Pim2 coexpression in PRalpha-positive hematopoietic progenitor cells (HPCs) induces leukemia in recipient mice after a short latency. Pim2-PRalpha cells were able to repopulate mice in serial transplantations and to induce disease in all recipients. Neither Pim2 nor PRalpha alone was sufficient to induce leukemia upon transplantation in this model. The disease induced by Pim2 overexpression in PRalpha cells contained a slightly higher fraction of immature myeloid cells, compared with the previously described APL disease induced by PRalpha. However, it also clearly resembled an APL-like phenotype and showed signs of differentiation upon all-trans retinoic acid (ATRA) treatment in vitro. These results support the hypothesis that Pim2, which is also a known target of Flt3-ITD (another gene that cooperates with PML-RARalpha), cooperates with PRalpha to induce APL-like disease.

Identification of a myeloid committed progenitor as the cancer-initiating cell in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is characterized by a block in differentiation and accumulation of promyelocytes in the bone marrow and blood. The majority of APL patients harbor the t(15:17) translocation leading to expression of the fusion protein promyelocytic-retinoic acid receptor alpha. Treatment with retinoic acid leads to degradation of promyelocytic-retinoic acid receptor alpha protein and disappearance of leukemic cells; however, 30% of APL patients relapse after treatment. One potential mechanism for relapse is the persistence of cancer "stem" cells in hematopoietic organs after treatment. Using a novel sorting strategy we developed to isolate murine myeloid cells at distinct stages of differentiation, we identified a population of committed myeloid cells (CD34(+), c-kit(+), FcgammaRIII/II(+), Gr1(int)) that accumulates in the spleen and bone marrow in a murine model of APL. We observed that these cells are capable of efficiently generating leukemia in recipient mice, demonstrating that this population represents the APL cancer-initiating cell. These cells down-regulate the transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha) possibly through a methylation-dependent mechanism, indicating that C/EBPalpha deregulation contributes to transformation of APL cancer-initiating cells. Our findings provide further understanding of the biology of APL by demonstrating that a committed transformed progenitor can initiate and propagate the disease.

Hematopoiesis and Retinoids: Development and Disease

Retinoids function as activating ligands for a class of nuclear receptors that control gene expression programs for a wide range of tissues and organs during embryogenesis and throughout life. Over the years, three sets of observations have spurred interest in the function of retinoids with respect to development and disease of hematopoietic cells. Since the 1920s, epidemiological studies indicated altered hematopoiesis in vitamin A-deficient (VAD) human populations. More recently, the ability of retinoids to affect various aspects of hematopoietic development has been demonstrated in vitro. Finally, it was discovered that the gene encoding a retinoid receptor is a key target for chromosomal translocations that cause acute promyelocytic leukemia (APL). More recent investigations using targeted gene disruptions, VAD animal models, and mouse models of leukemia have continued to shed light on the function of the retinoid pathway in blood cells. It is now clear that retinoids are required for normal hematopoiesis during both yolk sac and fetal liver stages of hematopoiesis, while the pathway has at least modulatory functions for bone marrow derived progenitors. Studies of normal development and APL have provided complementary insight into the molecular control of blood cell differentiation. Here we review the evidence for retinoid requirements in hematopoiesis and also summarize current ideas regarding how this pathway is subverted in leukemia.

Mutant FLT3 signaling contributes to a block in myeloid differentiation

FLT3 is a member of the class III receptor tyrosine kinase family and is primarily expressed on hematopoietic stem/progenitor cells. Somatic mutations of FLT3 involving internal tandem duplication (ITD) of the juxtamembrane domain or point mutations in the activation loop have been identified in approximately 17 - 34% and 7 - 9% of acute myeloid leukemia (AML) patients, respectively. The ITD mutations appear to activate the tyrosine kinase domain through receptor dimerization in a FLT3 ligand-independent manner. Constitutively activated FLT3 provides cells with proliferative and anti-apoptotic advantages and portends an especially poor prognosis for patients with this mutation. FLT3/ITD mutations also contribute to a block of myeloid differentiation. FLT3 tyrosine kinase inhibitors suppress the growth and induce apoptosis and differentiation of leukemia cells expressing FLT3/ITD mutants. Therefore, FLT3 is a therapeutic target and inhibition of FLT3 tyrosine kinase activity may provide a new approach in the treatment of leukemia carrying these mutations.

Acute promyelocytic leukemia: A novelPML/RARαfusion that generates a frameshift in the RARα transcript and ATRA resistance

Acute promyelocytic leukemia (APL) is characterized by increased promyelocytes in the marrow that harbor a t(15;17) and promyelocyte leukemia (PML)/RARalpha fusion gene. The oncogenic gene product is believed to act through disruption of the transcription-modulating function of RARalpha. Differentiation of promyelocytes and remission is achieved with all transretinoic acid (ATRA) therapy usually in combination with chemotherapy. This report describes a patient with the t(15;17) who did not respond typically to ATRA and IDAC induction chemotherapy, although achieved and remains in complete remission five years following induction and one consolidation with high dose cytarabine (HIDAC). RT-PCR and sequencing revealed a novel fusion of RARalpha exon 3 to PML exon 5 that creates a frameshift and premature stop codon in the RARalpha portion of the transcript. Since none of the RARalpha functional domains are maintained, this case highlights the possibility that PML/RARalpha may directly affect promyelocyte differentiation through disruption of PML function.

High throughput digital quantification of mRNA abundance in primary human acute myeloid leukemia samples

Acute promyelocytic leukemia (APL) is characterized by the t(15;17) chromosomal translocation, which results in fusion of the retinoic acid receptor alpha (RARA) gene to another gene, most commonly promyelocytic leukemia (PML). The resulting fusion protein, PML-RARA, initiates APL, which is a subtype (M3) of acute myeloid leukemia (AML). In this report, we identify a gene expression signature that is specific to M3 samples; it was not found in other AML subtypes and did not simply represent the normal gene expression pattern of primary promyelocytes. To validate this signature for a large number of genes, we tested a recently developed high throughput digital technology (NanoString nCounter). Nearly all of the genes tested demonstrated highly significant concordance with our microarray data (P < 0.05). The validated gene signature reliably identified M3 samples in 2 other AML datasets, and the validated genes were substantially enriched in our mouse model of APL, but not in a cell line that inducibly expressed PML-RARA. These results demonstrate that nCounter is a highly reproducible, customizable system for mRNA quantification using limited amounts of clinical material, which provides a valuable tool for biomarker measurement in low-abundance patient samples.

A bcr3/short form PML-RARalpha transcript in an acute promyelocytic leukemia resulted from a derivative chromosome 17 due to submicroscopic insertion of the PML gene into the RARalpha locus

Acute promyelocytic leukemia (APL) is a subtype of acute myeloid leukemia. Submicroscopic insertion of RARalpha into PML, resulting in PML-RARalpha from derivative chromosome 15, has been rarely reported. Herein, we describe a functional PML-RARalpha transcript from the long arm of derivative chromosome 17 in a patient with microgranular APL. The conventional karyotype showed normal chromosomes 15 and 17. It is interesting that interphase and metaphase fluorescence in situ hybridizations demonstrated a fusion signal on the long arm of one chromosome 17 homolog, with both PML and RARalpha still present on chromosomes 15 and 17, respectively, although the signal on one chromosome 15 was weaker, indicating partial loss of the PML gene. Reverse transcriptase-polymerase chain reaction revealed a transcript corresponding to a break cluster region 3 (bcr3) short form PML-RARalpha. To the best of our knowledge, this is the first report of an APL with a bcr3/short form PML-RARalpha transcript generated from derivative chromosome 17 due to submicroscopic insertion of the PML gene into the RARalpha locus.

Id1 immortalizes hematopoietic progenitors in vitro and promotes a myeloproliferative disease in vivo

Id1 is frequently overexpressed in many cancer cells, but the functional significance of these findings is not known. To determine if Id1 could contribute to the development of hematopoietic malignancy, we reconstituted mice with hematopoietic cells overexpressing Id1. We showed for the first time that deregulated expression of Id1 leads to a myeloproliferative disease in mice, and immortalizes myeloid progenitors in vitro. In human cells, we demonstrate that Id genes are expressed in human acute myelogenous leukemia cells, and that knock down of Id1 expression inhibits leukemic cell line growth, suggesting that Id1 is required for leukemic cell proliferation. These findings established a causal relationship between Id1 overexpression and hematologic malignancy. Thus, deregulated expression of Id1 may contribute to the initiation of myeloid malignancy, and Id1 may represent a potential therapeutic target for early stage intervention in the treatment of hematopoietic malignancy.

New insights into the role of PML in tumour suppression

The PML gene is involved in the t(15;17) translocation of acute promyelocytic leukaemia (APL), which generates the oncogenic fusion protein PML (promyelocytic leukaemia protein)-retinoic acid receptor alpha. The PML protein localises to a subnuclear structure called the PML nuclear domain (PML-ND), of which PML is the essential structural component. In APL, PML-NDs are disrupted, thus implicating these structures in the pathogenesis of this leukaemia. Unexpectedly, recent studies indicate that PML and the PML-ND play a tumour suppressive role in several different types of human neoplasms in addition to APL. Because of PML's extreme versatility and involvement in multiple cellular pathways, understanding the mechanisms underlying its function, and therefore role in tumour suppression, has been a challenging task. In this review, we attempt to critically appraise the more recent advances in this field and propose new avenues of investigation.

Epigenetic plasticity of chromatin in embryonic and hematopoietic stem/progenitor cells: therapeutic potential of cell reprogramming

During embryonic development and adult life, the plasticity and reversibility of modifications that affect the chromatin structure is important in the expression of genes involved in cell fate decisions and the maintenance of cell-differentiated state. Epigenetic changes in DNA and chromatin, which must occur to allow the accessibility of transcriptional factors at specific DNA-binding sites, are regarded as emerging major players for embryonic and hematopoietic stem cell (HSC) development and lineage differentiation. Epigenetic deregulation of gene expression, whether it be in conjunction with chromosomal alterations and gene mutations or not, is a newly recognized mechanism that leads to several diseases, including leukemia. The reversibility of epigenetic modifications makes DNA and chromatin changes attractive targets for therapeutic intervention. Here we review some of the epigenetic mechanisms that regulate gene expression in pluripotent embryonic and multipotent HSCs but may be deregulated in leukemia, and the clinical approaches designed to target the chromatin structure in leukemic cells.

Review: genetic models of acute myeloid leukaemia

The use of genetically engineered mice (GEM) have been critical in understanding disease states such as cancer, and none more so than acute myelogenous leukaemia (AML), a disease characterized by over 100 distinct chromosomal translocations. A substantial proportion of cases exhibiting recurrent reciprocal translocations at diagnosis, such as t(8;21) or t(15;17) have been exhaustively studied and are currently employed in clinical diagnosis. However, a definitive conclusion regarding the leukaemogenic potential of defined transgenes for this disease remains elusive. While it is increasingly apparent that a number of cooperating mutations are necessary to develop a leukaemic phenotype, the number of models reflecting these synergisms remains few. Furthermore, little emphasis has been paid to the effect of chromosomal translocations other than recurrent genetic abnormalities, with no models reflecting the multiple abnormalities observed in high-risk cases of AML accounting for 8-10% of adult AML. Here we review the differing technologies employed in generation of GEM of AML. We discuss the relevance of GEM AML from embryonic stem cell-mediated (for example retinoic acid receptor-alpha fusions and AML1/ETO) models; through to the valuable retroviral-mediated gene transfer models. The latter have been used to great effect in defining the transforming properties of chromosomal translocation products such as MLL (found in 5-6% of all AML cases) and NUP98 (denoting poor prognosis in therapy-related disease) and particularly when co-transduced with bad prognostic factors such as Flt3 mutations. Finally, we comment on the emergence of newer transduction technologies, which can regulate the level of expression to defined cell lineages in both primary murine and human xenografts, and discuss how combining multiple genetic modalities, more relevant models of this complex disease are being generated.

Xenograft models for the preclinical evaluation of new therapies in acute leukemia

Major advances in understanding the pathophysiology of acute leukemia have resulted in a dramatic increase in the availability of novel compounds for clinical trials. However, since the number of new drugs far exceeds the number of clinical trials that can be conducted because of the availability of eligible patients, there is an urgent need to utilize reliable preclinical models for the prioritization of the most promising potential therapies for those clinical trials. The most widely used preclinical models for the acute leukemias are human tumor xenografts established in immune-deficient mice, and genetically engineered mouse strains. This review summarizes the recent developments and considerations in the use of xenograft models of acute lymphoblastic leukemia, acute myeloid leukemia, and acute promyelocytic leukemia for the preclinical testing of new therapies.

Treatment of acute promyelocytic leukaemia with all-trans retinoic acid and arsenic trioxide: a paradigm of synergistic molecular targeting therapy

To turn a disease from highly fatal to highly curable is extremely difficult, especially when the disease is a type of cancer. However, we can gain some insight into how this can be done by looking back over the 50-year history of taming acute promyelocytic leukaemia (APL). APL is the M3 type of acute myeloid leukaemia characterized by an accumulation of abnormal promyelocytes in bone marrow, a severe bleeding tendency and the presence of the chromosomal translocation t(15;17) or variants. APL was considered the most fatal type of acute leukaemia five decades ago and the treatment of APL was a nightmare for physicians. Great efforts have been made by scientists worldwide to conquer this disease. The first use of chemotherapy (CT) was unsuccessful due to lack of supportive care and cytotoxic-agent-related exacerbated coagulopathy. The first breakthrough came from the use of anthracyclines which improved the complete remission (CR) rate, though the 5-year overall survival could only be attained in a small proportion of patients. A rational and intriguing hypothesis, to induce differentiation of APL cells rather than killing them, was raised in the 1970s. Laudably, the use of all-trans retinoic acid (ATRA) in treating APL resulted in terminal differentiation of APL cells and a 90-95% CR rate of patients, turning differentiation therapy in cancer treatment from hypothesis to practice. The combination of ATRA with CT further improved the 5-year overall survival. When arsenic trioxide (ATO) was used to treat relapsed APL not only the patients but also the ancient drug were revived. ATO exerts dose-dependent dual effects on APL cells: at low concentration, ATO induces partial differentiation, while at relatively high concentration, it triggers apoptosis. Of note, both ATRA and ATO trigger catabolism of the PML-RARalpha fusion protein which is the key player in APL leukaemogenesis generated from t(15;17), targeting the RARalpha (retinoic acid receptor alpha) or promyelocytic leukaemia (PML) moieties, respectively. Hence, in treating APL both ATRA and ATO represent paradigms for molecularly targeted therapy. At molecular level, ATRA and ATO synergistically modulate multiple downstream pathways/cascades. Strikingly, a clearance of PML-RARalpha transcript in an earlier and more thorough manner, and a higher quality remission and survival in newly diagnosed APL are achieved when ATRA is combined with ATO, as compared to either monotherapy, making APL a curable disease. Thus, the story of APL can serve as a model for the development of curative approaches for disease; it suggests that molecularly synergistic targeted therapies are powerful tools in cancer, and dissection of disease pathogenesis or anatomy of the cancer genome is critical in developing molecular target-based therapies.

The theory of APL revisited

Acute promyelocytic leukemia (APL) is associated with reciprocal and balanced chromosomal translocations always involving the retinoic acid receptor alpha (RARa) gene on chromosome 17 and variable partner genes (X genes) on distinct chromosomes. RARalpha fuses to the PML gene in the majority of APL cases, and in a few cases to the PLZF, NPM, NuMA and STAT5b genes. As a consequence, X-RARalpha and RARalpha-X fusion genes are generated encoding aberrant chimeric proteins that exert critical oncogenic functions. Here we will integrate some of the most recent findings in APL research in a unified model and discuss some of the outstanding questions that remain to be addressed.

Mouse models of acute promyelocytic leukemia

Mouse models of acute promyelocytic leukemia have been generated through transgenic, knock-in, retroviral, and xenograft strategies. These models have been used to elucidate mechanisms underlying leukemogenesis. Among the areas investigated are the role of reciprocal fusions; effects of target cells, expression levels, and mouse strains; cooperating events; and restrictive and permissive factors. These models have also been used to gain insight into the effects of the immune system on leukemic cells and the mechanism of response to retinoic acid. Furthermore, preclinical studies utilizing these mice have advanced therapy for myeloid leukemia.

PML4 induces differentiation by Myc destabilization

Opposing functions like oncogene and tumor suppressions have been established for c-Myc and promyelocytic leukemia (PML) protein, respectively. Myc is known to inhibit differentiation of hematopoietic precursor cells, and here we report that PML promotes cell differentiation. We further demonstrate that PML and Myc form a complex in vivo. The interaction of the two proteins leads to the destabilization of Myc in a manner dependent on the really interesting new gene (RING) domain of PML. Although several PML isoforms are able to interact with Myc, the ability to destabilize Myc is specific for PML4. Importantly, the PML-induced destabilization resulted in a reduction of promoter-bound Myc on Myc-repressed genes. Thereby, PML induced the re-activation of Myc-repressed target genes including the tumor suppressive genes of the cell cycle inhibitors cdkn1a/p21 and cdkn2b/p15. Together, these results establish PML-mediated destabilization of Myc and the derepression of cell cycle inhibitor genes as an important regulatory mechanism that allows cell differentiation and prevents aberrant proliferation driven by uncontrolled Myc activity.

Cleavage of misfolded nuclear receptor corepressor confers resistance to unfolded protein response-induced apoptosis

We have recently reported that accumulation of misfolded nuclear hormone receptor corepressor (N-CoR) as insoluble protein aggregates in acute promyelocytic leukemia (APL) cells induces endoplasmic reticulum (ER) stress and activates unfolded protein response (UPR). Although accumulation of misfolded proteins is known to trigger UPR-induced cytotoxic cell death in several neurodegenerative disorders, APL cells are notably resistant to UPR-induced apoptosis. The molecular basis for the paradoxical response of APL cells to UPR is not known. Here, we report that a glycoprotease, selectively expressed in APL cells, regulates the response of APL cells to UPR-induced apoptosis through processing of misfolded N-CoR protein. Results show that misfolded N-CoR is cleaved selectively in APL cells, and cellular extracts of APL cells and human primary APL cells contain activity that cleaves N-CoR protein. Purification and spectrometric analysis of N-CoR cleaving activity from an APL cell line reveals that it is a glycoprotein endopeptidase known as OSGEP. Furthermore, the cleavage of N-CoR in APL cells could be blocked by the broad-spectrum protease inhibitor AEBSF and by RNA interference-mediated down-regulation of OSGEP expression. AEBSF selectively inhibits growth and promotes apoptosis of APL cells possibly through a mechanism involving AEBSF-induced accumulation of insoluble N-CoR protein and by triggering ER stress. Taken together, these findings suggest that selective induction of protease activity in APL cells may represent a novel cytoprotective component of UPR, which could be exploited by tumor cells to survive the toxic insult of misfolded protein(s).

Effect of stem cell turnover rates on protection against cancer and aging

Tissue stem cells are responsible for replenishing and maintaining a population of cells which make up a functioning organ. They divide by asymmetric cell division where one daughter remains a stem cell while the other daughter becomes a transit cell, which divides a defined number of times and differentiates. A fully differentiated cell has a finite life-span. A tissue can be maintained by various strategies. Stem cells can divide often and differentiated cells die often (fast turnover). Alternatively, stem cells can divide infrequently, and the differentiated cells are long lived (slow turnover). Genetic alterations and mutations can interfere with tissue homoeostasis. Mutations can induce senescence and apoptosis, and this can result in a reduction of the number of functioning tissue cells which could correlate with tissue aging. Alternatively, mutations can result in the carcinogenic transformation of cells and the formation of a tumour. Using mathematical models, I find that the cellular turnover rate affects the ability of genetic alterations to induce aging and the development of cancer. If mutations occur as a result of errors during cell division, the model suggests that a low cellular turnover rate protects both against aging and the development of cancer. On the other hand, if mutations occur independent from cell division (e.g. if DNA is hit by damaging agents), I find that a high cellular turnover rate protects against aging, while it promotes the development of cancer. Implications for optimal tissue design are discussed.

Mouse Modeling in Oncologic Preclinical and Translational Research

Through scientific and technological advancements, our ability to manipulate the mouse genome has allowed us to evaluate the effect of specific genetic alterations on in vivo tumorigenesis. This has allowed and will allow us to define molecular pathways describing the processes of tumor initiation, invasion, and progression to metastatic disease. Additionally, these models may serve as an excellent platform for the identification of novel molecular targets for therapy as well as to evaluate the efficacy of targeted therapies. Ultimately this will translate from preclinical mouse model trials to the development of clinical trials and protocols for cancer patients. Here we review the usefulness of mouse modeling in oncologic translational research.

Identification of a molecular signature for leukemic promyelocytes and their normal counterparts: focus on DNA repair genes

Acute promyelocytic leukemia (APL) is a clonal expansion of hematopoietic precursors blocked at the promyelocytic stage. Gene expression profiles of APL cells obtained from 16 patients were compared to eight samples of CD34+-derived normal promyelocytes. Malignant promyelocytes showed widespread changes in transcription in comparison to their normal counterpart and 1020 differentially expressed genes were identified. Discriminating genes include transcriptional regulators (FOS, JUN and HOX genes) and genes involved in cell cycle and DNA repair. The strong upregulation in APL of some transcripts (FLT3, CD33, CD44 and HGF) was also confirmed at protein level. Interestingly, a trend toward a transcriptional repression of genes involved in different DNA repair pathways was found in APL and confirmed by real-time polymerase chain reactor (PCR) in a new set of nine APLs. Our results suggest that both inefficient base excision repair and recombinational repair might play a role in APLs development. To investigate the expression pathways underlying the development of APL occurring as a second malignancy (sAPL), we included in our study eight cases of sAPL. Although both secondary and de novo APL were characterized by a strong homogeneity in expression profiling, we identified a small set of differentially expressed genes that discriminate sAPL from de novo cases.

The NPM-RAR fusion protein associated with the t(5;17) variant of APL does not interact with PML

The PML protein localizes to regions of the nucleus known as nuclear bodies or PODs. However, in t(15;17) Acute Promyelocytic Leukemia (APL) blasts, PML is found in a micro-punctate pattern. In order to test the hypothesis that delocalization of PML from PODs is necessary for APL, we investigated the interaction of the t(5;17) APL fusion protein NPM-RAR with PML. NPM-RAR localizes diffusely throughout the nucleoplasm. NPM-RAR does not alter the localization of PML in transfected HeLa cells, and does not associate with PML in vitro. These studies suggest that NPM-RAR does not interact with PML.