RARalpha-PLZF overcomes PLZF-mediated repression of CRABPI, contributing to retinoid resistance in t(11;17) acute promyelocytic leukemia

Retinoic acid signaling in development and differentiation commitment and its regulatory topology

Retinoic acid (RA), the derivative of vitamin A/retinol, is a signaling molecule with important implications in health and disease. It is a well-known developmental morphogen that functions mainly through the transcriptional activity of nuclear RA receptors (RARs) and, uncommonly, through other nuclear receptors, including peroxisome proliferator-activated receptors. Intracellular RA is under spatiotemporally fine-tuned regulation by synthesis and degradation processes catalyzed by retinaldehyde dehydrogenases and P450 family enzymes, respectively. In addition to dictating the transcription architecture, RA also impinges on cell functioning through non-genomic mechanisms independent of RAR transcriptional activity. Although RA-based differentiation therapy has achieved impressive success in the treatment of hematologic malignancies, RA also has pro-tumor activity. Here, we highlight the relevance of RA signaling in cell-fate determination, neurogenesis, visual function, inflammatory responses and gametogenesis commitment. Genetic and post-translational modifications of RAR are also discussed. A better understanding of RA signaling will foster the development of precision medicine to improve the defects caused by deregulated RA signaling.

Function of PML-RARA in Acute Promyelocytic Leukemia

The transformation of acute promyelocytic leukemia (APL) from the most fatal to the most curable subtype of acute myeloid leukemia (AML), with long-term survival exceeding 90%, has represented one of the most exciting successes in hematology and in oncology. APL is a paradigm for oncoprotein-targeted cure.APL is caused by a 15/17 chromosomal translocation which generates the PML-RARA fusion protein and can be cured by the chemotherapy-free approach based on the combination of two therapies targeting PML-RARA: retinoic acid (RA) and arsenic. PML-RARA is the key driver of APL and acts by deregulating transcriptional control, particularly RAR targets involved in self-renewal or myeloid differentiation, also disrupting PML nuclear bodies. PML-RARA mainly acts as a modulator of the expression of specific target genes: genes whose regulatory elements recruit PML-RARA are not uniformly repressed but also may be upregulated or remain unchanged. RA and arsenic trioxide directly target PML-RARA-mediated transcriptional deregulation and protein stability, removing the differentiation block at promyelocytic stage and inducing clinical remission of APL patients.

An Acute Promyelocytic Leukemia Resistant to All-Trans Retinoic Acid: A Case Report of the ZBTB16::RARa Variant and Review of the Literature

Introduction

Acute promyelocytic leukemia (APL) is characterized by the PML::RARa gene fusion and treatment consists of all-trans retinoic acid (ATRA). Rarely, genetic APL variants have been described which are insensitive to ATRA treatment and are therefore associated with a worse prognosis. Rapid identification of the APL variant is essential to start the correct treatment.

Case Presentation

Here, we present a case of a 66-year-old male patient with weight loss and arthralgia. Laboratory results showed an anemia and mild leukocytosis with predominantly monocytes. Bone marrow investigation unexpectedly revealed a t(11;17)(q23;q21). This raised suspicion of an ATRA-resistant APL. By demonstrating the ZBTB16::RARa gene fusion, the diagnosis was confirmed.

Conclusion

This case study emphasizes the importance of integrated diagnostics and provides guidance to recognize the ZBTB16::RARa APL, which is the most prevalent ATRA-resistant APL. Furthermore, an overview of other genetic APL variants is presented and how to treat these uncommon diseases in clinical practice.

The MYCN inhibitor BGA002 restores the retinoic acid response leading to differentiation or apoptosis by the mTOR block in MYCN-amplified neuroblastoma

BACKGROUND

Neuroblastoma is a deadly childhood cancer, and MYCN-amplified neuroblastoma (MNA-NB) patients have the worst prognoses and are therapy-resistant. While retinoic acid (RA) is beneficial for some neuroblastoma patients, the cause of RA resistance is unknown. Thus, there remains a need for new therapies to treat neuroblastoma. Here we explored the possibility of combining a MYCN-specific antigene oligonucleotide BGA002 and RA as therapeutic approach to restore sensitivity to RA in NB.

METHODS

By molecular and cellular biology techniques, we assessed the combined effect of the two compounds in NB cell lines and in a xenograft mouse model MNA-NB.

RESULTS

We found that MYCN-specific inhibition by BGA002 in combination with RA (BGA002-RA) act synergistically and overcame resistance in NB cell lines. BGA002-RA also reactivated neuron differentiation (or led to apoptosis) and inhibited invasiveness capacity in MNA-NB. Moreover, we found that neuroblastoma had the highest level of mRNA expression of mTOR pathway genes, and that BGA002 led to mTOR pathway inhibition followed by autophagy reactivation in MNA-NB cells, which was strengthened by BGA002-RA. BGA002-RA in vivo treatment also eliminated tumor vascularization in a MNA-NB mouse model and significantly increased survival.

CONCLUSION

Taken together, MYCN modulation mediates the therapeutic efficacy of RA and the development of RA resistance in MNA-NB. Furthermore, by targeting MYCN, a cancer-specific mTOR pathway inhibition occurs only in MNA-NB, thus avoiding the side effects of targeting mTOR in normal cells. These findings warrant clinical testing of BGA002-RA as a strategy for overcoming RA resistance in MNA-NB.

CRABP1 in Non-Canonical Activities of Retinoic Acid in Health and Diseases

In this review, we discuss the emerging role of Cellular Retinoic Acid Binding Protein 1 (CRABP1) as a mediator of non-canonical activities of retinoic acid (RA) and relevance to human diseases. We first discuss the role of CRABP1 in regulating MAPK activities and its implication in stem cell proliferation, cancers, adipocyte health, and neuro-immune regulation. We then discuss an additional role of CRABP1 in regulating CaMKII activities, and its implication in heart and motor neuron diseases. Through molecular and genetic studies of Crabp1 knockout (CKO) mouse and culture models, it is established that CRABP1 forms complexes with specific signaling molecules to function as RA-regulated signalsomes in a cell context-dependent manner. Gene expression data and CRABP1 gene single nucleotide polymorphisms (SNPs) of human cancer, neurodegeneration, and immune disease patients implicate the potential association of abnormality in CRABP1 with human diseases. Finally, therapeutic strategies for managing certain human diseases by targeting CRABP1 are discussed.

PLZF Acetylation Levels Regulate NKT Cell Differentiation

The transcription factor promyelocytic leukemia zinc finger (PLZF) is encoded by the BTB domain-containing 16 (Zbtb16) gene. Its repressor function regulates specific transcriptional programs. During the development of invariant NKT cells, PLZF is expressed and directs their effector program, but the detailed mechanisms underlying PLZF regulation of multistage NKT cell developmental program are not well understood. This study investigated the role of acetylation-induced PLZF activation on NKT cell development by analyzing mice expressing a mutant form of PLZF mimicking constitutive acetylation (PLZF^ON) mice. NKT populations in PLZF^ON mice were reduced in proportion and numbers of cells, and the cells present were blocked at the transition from developmental stage 1 to stage 2. NKT cell subset differentiation was also altered, with T-bet⁺ NKT1 and RORγt⁺ NKT17 subsets dramatically reduced and the emergence of a T-bet^-RORγt^- NKT cell subset with features of cells in early developmental stages rather than mature NKT2 cells. Preliminary analysis of DNA methylation patterns suggested that activated PLZF acts on the DNA methylation signature to regulate NKT cells' entry into the early stages of development while repressing maturation. In wild-type NKT cells, deacetylation of PLZF is possible, allowing subsequent NKT cell differentiation. Interestingly, development of other innate lymphoid and myeloid cells that are dependent on PLZF for their generation is not altered in PLZF^ON mice, highlighting lineage-specific regulation. Overall, we propose that specific epigenetic control of PLZF through acetylation levels is required to regulate normal NKT cell differentiation.

Current views on the genetic landscape and management of variant acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is characterized by the accumulation of promyelocytes in bone marrow. More than 95% of patients with this disease belong to typical APL, which express PML-RARA and are sensitive to differentiation induction therapy containing all-trans retinoic acid (ATRA) and arsenic trioxide (ATO), and they exhibit an excellent clinical outcome. Compared to typical APL, variant APL showed quite different aspects, and how to recognize, diagnose, and treat variant APL remained still challenged at present. Herein, we drew the genetic landscape of variant APL according to recent progresses, then discussed how they contributed to generate APL, and further shared our clinical experiences about variant APL treatment. In practice, when APL phenotype was exhibited but PML-RARA and t(15;17) were negative, variant APL needed to be considered, and fusion gene screen as well as RNA-sequencing should be displayed for making the diagnosis as soon as possible. Strikingly, we found that besides of RARA rearrangements, RARB or RARG rearrangements also generated the phenotype of APL. In addition, some MLL rearrangements, NPM1 rearrangements or others could also drove variant APL in absence of RARA/RARB/RARG rearrangements. These results indicated that one great heterogeneity existed in the genetics of variant APL. Among them, only NPM1-RARA, NUMA-RARA, FIP1L1-RARA, IRF2BP2-RARA, and TFG-RARA have been demonstrated to be sensitive to ATRA, so combined chemotherapy rather than differentiation induction therapy was the standard care for variant APL and these patients would benefit from the quick switch between them. If ATRA-sensitive RARA rearrangement was identified, ATRA could be added back for re-induction of differentiation. Through this review, we hoped to provide one integrated view on the genetic landscape of variant APL and helped to remove the barriers for managing this type of disease.

Emerging Cancer Epigenetic Mechanisms Regulated by All-Trans Retinoic Acid

All-trans retinoic acid (RA), which is the dietary bioactive derivative obtained from animal (retinol) and plant sources (beta-carotene), is a physiological lipid signal of both embryonic and postembryonic development. During pregnancy, either RA deficiency or an excessive RA intake is teratogenic. Too low or too high RA affects not only prenatal, but also postnatal, developmental processes such as myelopoiesis and mammary gland morphogenesis. In this review, we mostly focus on emerging RA-regulated epigenetic mechanisms involving RA receptor alpha (RARA) and Annexin A8 (ANXA8), which is a member of the Annexin family, as well as ANXA8 regulatory microRNAs (miRNAs). The first cancer showing ANXA8 upregulation was reported in acute promyelocytic leukemia (APL), which induces the differentiation arrest of promyelocytes due to defective RA signaling caused by RARA fusion genes as the PML-RARA gene. Over the years, ANXA8 has also been found to be upregulated in other cancers, even in the absence of RARA fusion genes. Mechanistic studies on human mammary cells and mammary glands of mice showed that ANXA8 upregulation is caused by genetic mutations affecting RARA functions. Although not all of the underlying mechanisms of ANXA8 upregulation have been elucidated, the interdependence of RA-RARA and ANXA8 seems to play a relevant role in some normal and tumorigenic settings.

PLZF-RARα, NPM1-RARα, and Other Acute Promyelocytic Leukemia Variants: The PETHEMA Registry Experience and Systematic Literature Review

It has been suggested that 1–2% of acute promyelocytic leukemia (APL) patients present variant rearrangements of retinoic acid receptor alpha (RARα) fusion gene, with the promyelocytic leukaemia zinc finger (PLZF)/RAR_α being the most frequent. Resistance to all-trans-retinoic acid (ATRA) and arsenic trioxide (ATO) has been suggested in PLZF/RAR_α and other variant APLs. Herein, we analyze the incidence, characteristics, and outcomes of variant APLs reported to the multinational PETHEMA (Programa para el Tratamiento de Hemopatias Malignas) registry, and we perform a systematic review in order to shed light on strategies to improve management of these extremely rare diseases. Of 2895 patients with genetically confirmed APL in the PETHEMA registry, 11 had variant APL (0.4%) (9 PLZF-RAR_α and 2 NPM1-RAR_α), 9 were men, with median age of 44.6 years (3 months to 76 years), median leucocytes (WBC) 16.8 × 10⁹/L, and frequent coagulopathy. Eight patients were treated with ATRA plus chemotherapy-based regimens, and 3 with chemotherapy-based. As compared to previous reports, complete remission and survival was slightly better in our cohort, with 73% complete remission (CR) and 73% survival despite a high relapse rate (43%). After analyzing our series and performing a comprehensive and critical review of the literature, strong recommendations on appropriate management of variant APL are not possible due to the low number and heterogeneity of patients reported so far.

Classic and Variants APLs, as Viewed from a Therapy Response

Most acute promyelocytic leukemia (APL) are caused by PML-RARA, a translocation-driven fusion oncoprotein discovered three decades ago. Over the years, several other types of rare X-RARA fusions have been described, while recently, oncogenic fusion proteins involving other retinoic acid receptors (RARB or RARG) have been associated to very rare cases of acute promyelocytic leukemia. PML-RARA driven pathogenesis and the molecular basis for therapy response have been the focus of many studies, which have now converged into an integrated physio-pathological model. The latter is well supported by clinical and molecular studies on patients, making APL one of the rare hematological disorder cured by targeted therapies. Here we review recent data on APL-like diseases not driven by the PML-RARA fusion and discuss these in view of current understanding of "classic" APL pathogenesis and therapy response.

Genetics and functions of the retinoic acid pathway, with special emphasis on the eye

Retinoic acid (RA) is a potent morphogen required for embryonic development. RA is formed in a multistep process from vitamin A (retinol); RA acts in a paracrine fashion to shape the developing eye and is essential for normal optic vesicle and anterior segment formation. Perturbation in RA-signaling can result in severe ocular developmental diseases—including microphthalmia, anophthalmia, and coloboma. RA-signaling is also essential for embryonic development and life, as indicated by the significant consequences of mutations in genes involved in RA-signaling. The requirement of RA-signaling for normal development is further supported by the manifestation of severe pathologies in animal models of RA deficiency—such as ventral lens rotation, failure of optic cup formation, and embryonic and postnatal lethality. In this review, we summarize RA-signaling, recent advances in our understanding of this pathway in eye development, and the requirement of RA-signaling for embryonic development (e.g., organogenesis and limb bud development) and life.

Acute Promyelocytic Leukemia: Update on the Mechanisms of Leukemogenesis, Resistance and on Innovative Treatment Strategies

This review highlights new findings that have deepened our understanding of the mechanisms of leukemogenesis, therapy and resistance in acute promyelocytic leukemia (APL). Promyelocytic leukemia-retinoic acid receptor α (PML-RARa) sets the cellular landscape of acute promyelocytic leukemia (APL) by repressing the transcription of RARa target genes and disrupting PML-NBs. The RAR receptors control the homeostasis of tissue growth, modeling and regeneration, and PML-NBs are involved in self-renewal of normal and cancer stem cells, DNA damage response, senescence and stress response. The additional somatic mutations in APL mainly involve FLT3, WT1, NRAS, KRAS, ARID1B and ARID1A genes. The treatment outcomes in patients with newly diagnosed APL improved dramatically since the advent of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). ATRA activates the transcription of blocked genes and degrades PML-RARα, while ATO degrades PML-RARa by promoting apoptosis and has a pro-oxidant effect. The resistance to ATRA and ATO may derive from the mutations in the RARa ligand binding domain (LBD) and in the PML-B2 domain of PML-RARa, but such mutations cannot explain the majority of resistances experienced in the clinic, globally accounting for 5-10% of cases. Several studies are ongoing to unravel clonal evolution and resistance, suggesting the therapeutic potential of new retinoid molecules and combinatorial treatments of ATRA or ATO with different drugs acting through alternative mechanisms of action, which may lead to synergistic effects on growth control or the induction of apoptosis in APL cells.

Traffic lights for retinoids in oncology: molecular markers of retinoid resistance and sensitivity and their use in the management of cancer differentiation therapy

For decades, retinoids and their synthetic derivatives have been well established anticancer treatments due to their ability to regulate cell growth and induce cell differentiation and apoptosis. Many studies have reported the promising role of retinoids in attaining better outcomes for adult or pediatric patients suffering from several types of cancer, especially acute myeloid leukemia and neuroblastoma. However, even this promising differentiation therapy has some limitations: retinoid toxicity and intrinsic or acquired resistance have been observed in many patients. Therefore, the identification of molecular markers that predict the therapeutic response to retinoid treatment is undoubtedly important for retinoid use in clinical practice. The purpose of this review is to summarize the current knowledge on candidate markers, including both genetic alterations and protein markers, for retinoid resistance and sensitivity in human malignancies.

Molecular Malfeasance Mediating Myeloid Malignancies: The Genetics of Acute Myeloid Leukemia

A remarkable number of different, but recurrent, structural cytogenetic abnormalities have been observed in AML, and the 2016 WHO AML classification system incorporates numerous distinct entities associated with translocations or inversions, as well as others associated with single gene mutations into a category entitled "AML with recurrent genetic abnormalities." The AML classification is heavily reliant on cytogenetic and molecular information based on conventional genetic techniques (including karyotype, fluorescence in situ hybridization, reverse transcriptase polymerase chain reaction, single gene sequencing), but large-scale next generation sequencing is now identifying novel mutations. With targeted next generation sequencing panels now clinically available at many centers, detection of mutations, as well as alterations in epigenetic modifiers, is becoming part of the routine diagnostic evaluation of AML and will likely impact future classification schemes.

Eya2, a Target Activated by Plzf, Is Critical for PLZF-RARA-Induced Leukemogenesis

PLZF is a transcription factor that confers aberrant self-renewal in leukemogenesis, and the PLZF-RARA fusion gene causes acute promyelocytic leukemia (APL) through differentiation block. However, the molecular mechanisms of aberrant self-renewal underlying PLZF-mediated leukemogenesis are poorly understood. To investigate these mechanisms, comprehensive expression profiling of mouse hematopoietic stem/progenitor cells transduced with Plzf was performed, which revealed the involvement of a key transcriptional coactivator, Eya2, a target molecule shared by Plzf and PLZF-RARA, in the aberrant self-renewal. Indeed, PLZF-RARA as well as Plzf rendered those cells immortalized through upregulation of Eya2. Eya2 also led to immortalization without differentiation block, while depletion of Eya2 suppressed clonogenicity in cells immortalized by PLZF-RARA without influence on differentiation and apoptosis. Interestingly, cancer outlier profile analysis of human samples of acute myeloid leukemia (AML) in The Cancer Genome Atlas (TCGA) revealed a subtype of AML that strongly expressed EYA2 In addition, gene set enrichment analysis of human AML samples, including TCGA data, showed that this subtype of AML was more closely associated with the properties of leukemic stem cells in its gene expression signature than other AMLs. Therefore, EYA2 may be a target for molecular therapy in this subtype of AML, including PLZF-RARA APL.

Role of Vitamin A/Retinoic Acid in Regulation of Embryonic and Adult Hematopoiesis

Vitamin A is an essential micronutrient throughout life. Its physiologically active metabolite retinoic acid (RA), acting through nuclear retinoic acid receptors (RARs), is a potent regulator of patterning during embryonic development, as well as being necessary for adult tissue homeostasis. Vitamin A deficiency during pregnancy increases risk of maternal night blindness and anemia and may be a cause of congenital malformations. Childhood Vitamin A deficiency can cause xerophthalmia, lower resistance to infection and increased risk of mortality. RA signaling appears to be essential for expression of genes involved in developmental hematopoiesis, regulating the endothelial/blood cells balance in the yolk sac, promoting the hemogenic program in the aorta-gonad-mesonephros area and stimulating eryrthropoiesis in fetal liver by activating the expression of erythropoietin. In adults, RA signaling regulates differentiation of granulocytes and enhances erythropoiesis. Vitamin A may facilitate iron absorption and metabolism to prevent anemia and plays a key role in mucosal immune responses, modulating the function of regulatory T cells. Furthermore, defective RA/RARα signaling is involved in the pathogenesis of acute promyelocytic leukemia due to a failure in differentiation of promyelocytes. This review focuses on the different roles played by vitamin A/RA signaling in physiological and pathological mouse hematopoiesis duddurring both, embryonic and adult life, and the consequences of vitamin A deficiency for the blood system.

Molecular Characteristics and Clinical Significance of 12 Fusion Genes in Acute Promyelocytic Leukemia: A Systematic Review

Acute promyelocytic leukemia (APL) is characterized by the generation of the promyelocytic leukemia-retinoic acid (RA) receptor α (PML-RARα) fusion gene. PML-RARα is the central leukemia-initiating event in APL and is directly targeted by all-trans-RA (ATRA) as well as arsenic. In classic APL harboring PML-RARα transcripts, more than 90% of patients can achieve complete remission when treated with ATRA combined with arsenic trioxide chemotherapy. In the last 20 years, more than 10 variant fusion genes have been found and identified in APL patients. These variant APL cases present different clinical phenotypes and treatment outcomes. All variant APL cases show a similar breakpoint within the RARα gene, whereas its partner genes are variable. These fusion proteins have the ability to repress rather than activate retinoic targets. These chimeric proteins also possess different molecular characteristics, thereby resulting in variable sensitivities to ATRA and clinical outcomes. In this review, we comprehensively analyze various rearrangements in variant APL cases that have been reported in the literature as well as the molecular characteristics and functions of the fusion proteins derived from different RARα partner genes and their clinical implications.

The dynamic genome: transposons and environmental adaptation in the nervous system

Classically thought as genomic clutter, the functional significance of transposable elements (TEs) has only recently become a focus of attention in neuroscience. Increasingly, studies have demonstrated that the brain seems to have more retrotransposition and TE transcription relative to other somatic tissues, suggesting a unique role for TEs in the central nervous system. TE expression and transposition also appear to vary by brain region and change in response to environmental stimuli such as stress. TEs appear to serve a number of adaptive roles in the nervous system. The regulation of TE expression by steroid, epigenetic and other mechanisms in interplay with the environment represents a significant and novel avenue to understanding both normal brain function and disease.

Molecular Monitoring as a Path to Cure Acute Promyelocytic Leukemia

Acute promyelocytic leukemia (APL) is a molecularly well-defined disease, characterized by a specific chromosomal translocation; the improvement in biologic and clinical knowledge and subsequent introduction of molecularly targeted therapies have transformed the management of APL, with survival rates now exceeding 80%. Minimal residual disease (MRD) assessment in APL is the most important tool for its treatment; the prognostic role of the molecular detection of promyelocytic leukemia retinoic acid receptor α (PML-RARα) transcript after consolidation therapy in the early identification of the following hematologic relapse is now well established and guides preemptive therapy. First experiences performed with a qualitative polymerase chain reaction (PCR) approach were replaced with more accurate real-time quantitative PCR (RQ-PCR), which guarantees a numeric quantification of MRD. The identification of arsenic trioxide (ATO) as a valid therapy not only in relapsed patients but also as an alternative to standard therapy alone or in association with all-trans-retinoic acid enlarges the setting of validation of MRD evaluation in APL patients, considering a possible different clearance of PML-RARα with innovative therapy different from the standard ones. MRD monitoring demonstrated its validity also in the setting of relapsed patients with interesting results in the autologous and allogeneic stem cell transplantation setting or with the use of other biological agents. The aim of this review is to report and discuss the actual state of the art of MRD in APL.

Post transcriptional control of the epigenetic stem cell regulator PLZF by sirtuin and HDAC deacetylases

BACKGROUND

The transcriptional repressor promyelocytic leukemia zinc finger protein (PLZF) is critical for the regulation of normal stem cells maintenance by establishing specific epigenetic landscape. We have previously shown that CBP/p300 acetyltransferase induces PLZF acetylation in order to increase its deoxynucleotidic acid (DNA) binding activity and to enhance its epigenetic function (repression of PLZF target genes). However, how PLZF is inactivated is not yet understood.

RESULTS

In this study, we demonstrate that PLZF is deacetylated by both histone deacetylase 3 and the NAD+ dependent deacetylase silent mating type information regulation 2 homolog 1 (SIRT1). Unlike other PLZF-interacting deacetylases, these two proteins interact with the zinc finger domain of PLZF, where the activating CBP/p300 acetylation site was previously described, inducing deacetylation of lysines 647/650/653. Overexpression of histone deacetylase 3 (HDAC3) and SIRT1 is associated with loss of PLZF DNA binding activity and decreases PLZF transcriptional repression. As a result, the chromatin status of the promoters of PLZF target genes, involved in oncogenesis, shift from a heterochromatin to an open euchromatin environment leading to gene expression even in the presence of PLZF.

CONCLUSIONS

Consequently, SIRT1 and HDAC3 mediated-PLZF deacetylation provides for rapid control and fine-tuning of PLZF activity through post-transcriptional modification to regulate gene expression and cellular homeostasis.

Retinoic acid signaling in cancer: The parable of acute promyelocytic leukemia

Inevitably fatal some 40 years, acute promyelocytic leukemia (APL) can now be cured in more than 95% of cases. This clinical success story is tightly linked to tremendous progress in our understanding of retinoic acid (RA) signaling. The discovery of retinoic acid receptor alpha (RARA) was followed by the cloning of the chromosomal translocations driving APL, all of which involve RARA. Since then, new findings on the biology of nuclear receptors have progressively enlightened the basis for the clinical efficacy of RA in APL. Reciprocally, the disease offered a range of angles to approach the cellular and molecular mechanisms of RA action. This virtuous circle contributed to make APL one of the best-understood cancers from both clinical and biological standpoints. Yet, some important questions remain unanswered including how lessons learnt from RA-triggered APL cure can help design new therapies for other malignancies.

The acute promyelocytic leukaemia success story: curing leukaemia through targeted therapies

The recent finding that almost all patients with acute promyelocytic leukaemia (APL) may be cured using a combination of retinoic acid (RA) and arsenic trioxide (As(2)O(3)) (N Engl J Med, 369, 2013 and 111) highlights the progress made in our understanding of APL pathogenesis and therapeutic approaches over the past 25 years. The study of APL has revealed many important lessons related to transcriptional control, nuclear organization, epigenetics and the role of proteolysis in biological control. Even more important has been the clinical demonstration that molecularly targeted therapy can eradicate disease.

Activation of a promyelocytic leukemia-tumor protein 53 axis underlies acute promyelocytic leukemia cure

Acute promyelocytic leukemia (APL) is driven by the promyelocytic leukemia (PML)-retinoic acid receptor-α (PML-RARA) fusion protein, which interferes with nuclear receptor signaling and PML nuclear body (NB) assembly. APL is the only malignancy definitively cured by targeted therapies: retinoic acid (RA) and/or arsenic trioxide, which both trigger PML-RARA degradation through nonoverlapping pathways. Yet, the cellular and molecular determinants of treatment efficacy remain disputed. We demonstrate that a functional Pml-transformation-related protein 53 (Trp53) axis is required to eradicate leukemia-initiating cells in a mouse model of APL. Upon RA-induced PML-RARA degradation, normal Pml elicits NB reformation and induces a Trp53 response exhibiting features of senescence but not apoptosis, ultimately abrogating APL-initiating activity. Apart from triggering PML-RARA degradation, arsenic trioxide also targets normal PML to enhance NB reformation, which may explain its clinical potency, alone or with RA. This Pml-Trp53 checkpoint initiated by therapy-triggered NB restoration is specific for PML-RARA-driven APL, but not the RA-resistant promyelocytic leukemia zinc finger (PLZF)-RARA variant. Yet, as NB biogenesis is druggable, it could be therapeutically exploited in non-APL malignancies.

RARα-PLZF oncogene inhibits C/EBPα function in myeloid cells

In acute promyelocytic leukemia, granulocytic differentiation is arrested at the promyelocyte stage. The variant t(11;17) translocation produces two fusion proteins, promyelocytic leukemia zinc finger-retinoic acid receptor α (PLZF-RARα) and RARα-PLZF, both of which participate in leukemia development. Here we provide evidence that the activity of CCAAT/enhancer binding protein α (C/EBPα), a master regulator of granulocytic differentiation, is severely impaired in leukemic promyelocytes with the t(11;17) translocation compared with those associated with the t(15;17) translocation. We show that RARα-PLZF inhibits myeloid cell differentiation through interactions with C/EBPα tethered to DNA, using ChIP and DNA capture assays. Furthermore, RARα-PLZF recruits HDAC1 and causes histone H3 deacetylation at C/EBPα target loci, thereby decreasing the expression of C/EBPα target genes. In line with these results, HDAC inhibitors restore in part C/EBPα target gene expression. These findings provide molecular evidence for a mechanism through which RARα-PLZF acts as a modifier oncogene that subverts differentiation in the granulocytic lineage by associating with C/EBPα and inhibiting its activity.

The epigenetic regulator PLZF represses L1 retrotransposition in germ and progenitor cells

Germ cells and adult stem cells maintain tissue homeostasis through a finely tuned program of responses to both physiological and stress-related signals. PLZF (Promyelocytic Leukemia Zinc Finger protein), a member of the POK family of transcription factors, acts as an epigenetic regulator of stem cell maintenance in germ cells and haematopoietic stem cells. We identified L1 retrotransposons as the primary targets of PLZF. PLZF-mediated DNA methylation induces silencing of the full-length L1 gene and inhibits L1 retrotransposition. Furthermore, PLZF causes the formation of barrier-type boundaries by acting on inserted truncated L1 sequences in protein coding genes. Cell stress releases PLZF-mediated repression, resulting in L1 activation/retrotransposition and impaired spermatogenesis and myelopoiesis. These results reveal a novel mechanism of action by which, PLZF represses retrotransposons, safeguarding normal progenitor homeostasis.

The histone demethylase PHF8 governs retinoic acid response in acute promyelocytic leukemia

While all-trans retinoic acid (ATRA) treatment in acute promyelocytic leukemia (APL) has been the paradigm of targeted therapy for oncogenic transcription factors, the underlying mechanisms remain largely unknown, and a significant number of patients still relapse and become ATRA resistant. We identified the histone demethylase PHF8 as a coactivator that is specifically recruited by RARα fusions to activate expression of their downstream targets upon ATRA treatment. Forced expression of PHF8 resensitizes ATRA-resistant APL cells, whereas its downregulation confers resistance. ATRA sensitivity depends on the enzymatic activity and phosphorylation status of PHF8, which can be pharmacologically manipulated to resurrect ATRA sensitivity to resistant cells. These findings provide important molecular insights into ATRA response and a promising avenue for overcoming ATRA resistance.

BTB-ZF factors recruit the E3 ligase cullin 3 to regulate lymphoid effector programs

The differentiation of several T and B cell effector programs in the immune system is directed by signature transcription factors that induce rapid epigenetic remodeling. We report that PLZF, the BTB-ZF transcription factor directing the innate-like effector program of NKT thymocytes ^1,2 was prominently associated with cullin 3 (Cul3), an E3 ubiquitin ligase previously shown to use BTB domain-containing proteins as adaptors for substrate binding ^3–7. PLZF transported Cul3 to the nucleus where the two proteins were associated within a chromatin modifying complex. Furthermore, PLZF expression resulted in selective changes of ubiquitination of multiple components of this complex. Cul3 was also found associated with another BTB-ZF transcription factor, Bcl6, which directs the B cell germinal center and the T follicular helper programs. Conditional deletion in mice demonstrated an essential role of Cul3 for the development of PLZF- and Bcl6-dependent lineages. We conclude that distinct lineage-specific BTB-ZF transcription factors recruit Cul3 to alter the ubiquitination pattern of their associated chromatin modifying complex. We propose that this novel function is essential to direct the differentiation of several T and B lymphocyte effector programs, and may also be involved in the oncogenic role of PLZF and Bcl6 in leukemias and lymphomas ^8,9.

Expression of the promyelocytic leukemia zinc-finger in T-lymphoblastic lymphoma and leukemia has strong implications for their cellular origin and greater association with initial bone marrow involvement

The promyelocytic leukemia zinc-finger (PLZF) is essential for the development of innate T cells (as represented by natural killer T cells) for acquisition of their unique innate immune properties. We evaluated the PLZF protein expression in a variety of immature and mature lymphoid malignancies. PLZF was preferentially expressed in T-lymphoblastic lymphoma/acute lymphoblastic leukemia (T-LBL/ALL) in 50% of the 54 cases. Among 51 cases of peripheral T-cell lymphoma not otherwise specified, only one (2%) expressed PLZF. One mycosis fungoides case expressed PLZF in lymph node involved by tumor. Otherwise, PLZF was not detected in any other type of lymphoma. In T-LBL/ALL, PLZF expression was more common in CD4/CD8 double-negative (67%) or CD8 single-positive subtypes (73%) than in CD4/CD8 double-positive (13%) and CD4 single-positive subtypes (0%) (P=0.001). Importantly, PLZF and CD1a expression were mutually exclusive in T-LBL/ALL (P=0.001). This was also the case for T-cell receptor βF1 expression (P=0.000). Most (96%) of the PLZF-positive T-LBL/ALL cases showed initial bone marrow involvement compared with 39% of PLZF-negative cases (P=0.000). Based on these findings, we suggest that T-LBL/ALLs that express PLZF arise from early immature double-negative thymocytes when the T-cell receptor β chain has not yet expressed or innate T-cell precursors, and strongly imply bone marrow involvement.

The importance of molecular cytogenetic analysis prior to using cell lines in research: The case of the KG-1a leukemia cell line

KG-1 and its less differentiated subline KG-1a are leukemia cell lines used in research in a number of laboratories. The karyotypes of the two lines were initially identical. In the following years, further analysis revealed that the cell lines had acquired additional karyotypical abnormalities and differed in the presence of certain typical chromosomal rearrangements. To obtain cytogenetic authentication prior to the use of the two cell lines, we analyzed their karyotype by combining DAPI- and CMA-chromosome bandings and a fluorescence in situ hybridization (FISH)-based approach by using BAC clones useful for the identification of chromosome regions of interest. Sequences of the MYC, PLZF, RARA and BCR genes, that are known to play a critical role in leukemogenesis, and certain BAC clones mapped to five known common fragile sites (CFS) were used for the FISH analysis. A telomeric probe (TTAGGG)n and a set of BAC clones were used to characterize the marker chromosome der(1) that was observed in the cell line KG-1a. The existence of notable differences between the karyotype of the KG-1a cell line previously described, and that described in this study, demonstrate that the use of established cancer cell lines should be preceded by cytogenetic and/or molecular characterization.

Flow cytometric immunobead assay for fast and easy detection of PML-RARA fusion proteins for the diagnosis of acute promyelocytic leukemia

The PML–RARA fusion protein is found in approximately 97% of patients with acute promyelocytic leukemia (APL). APL can be associated with life-threatening bleeding complications when undiagnosed and not treated expeditiously. The PML–RARA fusion protein arrests maturation of myeloid cells at the promyelocytic stage, leading to the accumulation of neoplastic promyelocytes. Complete remission can be obtained by treatment with all-trans-retinoic acid (ATRA) in combination with chemotherapy. Diagnosis of APL is based on the detection of t(15;17) by karyotyping, fluorescence in situ hybridization or PCR. These techniques are laborious and demand specialized laboratories. We developed a fast (performed within 4–5 h) and sensitive (detection of at least 10% malignant cells in normal background) flow cytometric immunobead assay for the detection of PML–RARA fusion proteins in cell lysates using a bead-bound anti-RARA capture antibody and a phycoerythrin-conjugated anti-PML detection antibody. Testing of 163 newly diagnosed patients (including 46 APL cases) with the PML–RARA immunobead assay showed full concordance with the PML–RARA PCR results. As the applied antibodies recognize outer domains of the fusion protein, the assay appeared to work independently of the PML gene break point region. Importantly, the assay can be used in parallel with routine immunophenotyping for fast and easy diagnosis of APL.

Diagnostic and prognostic value of cytogenetics in acute myeloid leukemia

The last 4 decades have seen major advances in understanding the genetic basis of acute myeloid leukemia (AML), and substantial improvements in survival of children and young adults with the disease. A key step forward was the discovery that AML cells harbor recurring cytogenetic abnormalities. The identification of the genes involved in chromosomal rearrangements has provided insights into the regulation of normal hematopoiesis and how disruption of key transcription factors and epigenetic modulators promote leukemic transformation. Cytogenetics has been widely adopted to provide the framework for development of risk-stratified treatment approaches to patient management.

CRABP-II- and FABP5-independent all-trans retinoic acid resistance in COLO 16 human cutaneous squamous cancer cells

The effect of all-trans retinoic acid (ATRA) on cutaneous squamous cell carcinomas (c-SCC) has been poorly described. Because the imbalance of CRABP-II-mediated anticancer signalling and FABP5-mediated growth-promoting signalling was supposed to be related with ATRA sensitivities of cancer cells, COLO16 human c-SCC cell line was selected to check underlying mechanism leading to ATRA resistance by multiple experimental approaches. The results revealed that COLO 16 cells were resistant to 15 μm ATRA treatment. FABP5 as well as the elements related with CRABP-II signalling (CYP26A1, CYP26B1, CRABP-I, RARα/β/γ and RXRα/β/γ) and with FABP5 signalling (PPARβ/δ) were expressed, but CRABP-II was undetectable in COLO 16 cells. 5-Aza treatment enhanced CRABP-II expression but further bisulfite sequencing PCR-DNA sequencing revealed no methylation in CRABP-II promoter region. Transfection of CRABP-II-expressing plasmids or FABP5 siRNA or both successfully manipulated the level(s) of target gene expression but failed to overcome ATRA resistance in the transfectants. In conclusion, CRABP-II and FABP5 expression were imbalanced in ATRA-resistant COLO 16 cells. 5-Aza-enhanced CRABP-II expression and unmethylation in CRABP-II promoter region suggest the methylation of certain CRABP-II regulatory gene(s) in COLO 16 cells. As neither restoration of CRABP-II expression nor the increased CRABP-II versus FABP5 ratio can overcome ATRA resistance of COLO 16 cells, additional ATRA-resistant mechanism(s) may present in human c-SCCs and COLO 16 cells would be of value in addressing this issue.

Development of real-time quantitative polymerase chain reaction assays to track treatment response in retinoid resistant acute promyelocytic leukemia

Molecular detection of minimal residual disease (MRD) has become established to assess remission status and guide therapy in patients with ProMyelocytic Leukemia-RARA+ acute promyelocytic leukemia (APL). However, there are few data on tracking disease response in patients with rarer retinoid resistant subtypes of APL, characterized by PLZF-RARA and STAT5b-RARA. Despite their rarity (<1% of APL) we identified 6 cases (PLZF-RARA, n = 5; STAT5b-RARA, n = 1), established the respective breakpoint junction regions and designed reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) assays to detect leukemic transcripts. The relative level of fusion gene expression in diagnostic samples was comparable to that observed in t(15;17) - associated APL, affording assay sensitivities of ∼1 in 10(4)-10(5). Serial samples were available from two PLZF-RARA APL patients. One showed persistent polymerase chain reaction positivity, predicting subsequent relapse, and remains in CR2, ∼11 years post-autograft. The other, achieved molecular remission (CRm) with combination chemotherapy, remaining in CR1 at 6 years. The STAT5b-RARA patient failed to achieve CRm following frontline combination chemotherapy and ultimately proceeded to allogeneic transplant on the basis of a steadily rising fusion transcript level. These data highlight the potential of RT-qPCR detection of MRD to facilitate development of more individualized approaches to the management of rarer molecularly defined subsets of acute leukemia.

Characterisation of genome-wide PLZF/RARA target genes

The PLZF/RARA fusion protein generated by the t(11;17)(q23;q21) translocation in acute promyelocytic leukaemia (APL) is believed to act as an oncogenic transcriptional regulator recruiting epigenetic factors to genes important for its transforming potential. However, molecular mechanisms associated with PLZF/RARA-dependent leukaemogenesis still remain unclear.

We searched for specific PLZF/RARA target genes by ChIP-on-chip in the haematopoietic cell line U937 conditionally expressing PLZF/RARA. By comparing bound regions found in U937 cells expressing endogenous PLZF with PLZF/RARA-induced U937 cells, we isolated specific PLZF/RARA target gene promoters. We next analysed gene expression profiles of our identified target genes in PLZF/RARA APL patients and analysed DNA sequences and epigenetic modification at PLZF/RARA binding sites. We identify 413 specific PLZF/RARA target genes including a number encoding transcription factors involved in the regulation of haematopoiesis. Among these genes, 22 were significantly down regulated in primary PLZF/RARA APL cells. In addition, repressed PLZF/RARA target genes were associated with increased levels of H3K27me3 and decreased levels of H3K9K14ac. Finally, sequence analysis of PLZF/RARA bound sequences reveals the presence of both consensus and degenerated RAREs as well as enrichment for tissue-specific transcription factor motifs, highlighting the complexity of targeting fusion protein to chromatin. Our study suggests that PLZF/RARA directly targets genes important for haematopoietic development and supports the notion that PLZF/RARA acts mainly as an epigenetic regulator of its direct target genes.

Expression and Function of Kruppel Like-Factors (KLF) in Carcinogenesis

Krüppel-like factor (KLF) family members share a three C2H2 zinc finger DNA binding domain, and are involved in cell proliferation and differentiation control in normal as in pathological situations. Studies over the past several years support a significant role for this family of transcription factors in carcinogenesis. KLFs can both activate and repress genes that participate in cell-cycle regulation. Among them, many up-regulated genes are inhibitors of proliferation, whereas genes that promote cell proliferation are repressed. However, several studies do present KLFs as positive regulator of cell proliferation. KLFs can be deregulated in multiple cancers either by loss of heterozygosity (LOH), somatic mutation or transcriptional silencing by promoter hypermethylation. Accordingly, KLF expression was shown to mediate growth inhibition when ectopically expressed in multiple cancer-derived cell lines through the inhibition of a number of key oncogenic signaling pathways, and to revert the tumorogenic phenotype in vivo. Taken together, these observations suggest that KLFs act as tumor suppressor. However, in some occasion, KLFs could act as tumor promoters, depending on “cellular context”. Thus, this review will discuss the roles and the functions of KLF family members in carcinogenesis, with a special focus on cancers from epithelial origin.

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.

Understanding PLZF: two transcriptional targets, REDD1 and smooth muscle α-actin, define new questions in growth control, senescence, self-renewal and tumor suppression

PLZF can function as a transcriptional activator or as a transcriptional repressor. Recent studies have identified two direct transcriptional targets of PLZF, REDD1 and smooth muscle α-actin. REDD1 is activated by PLZF. It mediates the PLZF-dependent downregulation of TORC1 and is responsible for the maintenance of pluripotency in cultures of spermatogonial progenitor cells. This activity may extend to other stem-like cell types. The effect of REDD1 on TORC1 also raises the possibility that REDD1 controls cell growth, tumorigenicity and senescence. The regulatory loop extending from PLZF via REDD1 to TORC1 identifies REDD1 as a critical determinant of TOR activity. The transcription of smooth muscle α-actin is repressed by PLZF. In fibroblasts, this downregulation is accompanied by a change of cell shape and a dramatic reorganization of the cytoskeleton. It is also correlated with the acquisition of cellular resistance to oncogenic transformation. The resistance is selective, it works against some oncoproteins but not against others. The molecular mechanisms underlying the changes in the cytoskeleton and in the susceptibility to oncogenic transformation are unknown. However these changes are dependent on the activity of RAS and thus probably involve the RAC/RHO family of proteins.

BCOR as a novel fusion partner of retinoic acid receptor alpha in a t(X;17)(p11;q12) variant of acute promyelocytic leukemia

The majority of acute promyelocytic leukemia (APL) cases are characterized by the presence of a promyelocytic leukemia–retinoic acid receptor alpha(RARA) fusion gene. In a small subset, RARA is fused to a different partner, usually involved in regulating cell growth and differentiation. Here, we identified a novel RARA fusion transcript, BCOR-RARA, in a t(X;17)(p11;q12) variant of APL with unique morphologic features, including rectangular and round cytoplasmic inclusion bodies. Although the patient was clinically responsive to all-trans retinoic acid, several relapses occurred with standard chemotherapy and all-trans retinoic acid. BCOR is a transcriptional corepressor through the proto-oncoprotein, BCL6, recruiting histone deacetylases and polycomb repressive complex 1 components. BCOR-RARA was found to possess common features with other RARA fusion proteins. These included: (1) the same break point in RARA cDNA; (2) self-association; (3) retinoid X receptor alpha is necessary for BCOR-RARA to associate with the RARA responsive element; (4) action in a dominant-negative manner on RARA transcriptional activation; and (5) aberrant subcellular relocalization. It should be noted that there was no intact BCOR found in the 45,-Y,t(X;17)(p11;q12) APL cells because they featured only a rearranged X chromosome. These results highlight essential features of pathogenesis in APL in more detail. BCOR appears to be involved not only in human congenital diseases, but also in a human cancer.

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.

Atypical mRNA fusions in PML-RARA positive, RARA-PML negative acute promyelocytic leukemia

Reciprocal RARA-PML transcripts are not detected in approximately 25% of patients with PML-RARA positive acute promyelocytic leukemia (APL), but the reasons for this are poorly understood. We studied 21 PML-RARA positive/RARA-PML negative cases by bubble PCR and multiplex long template PCR to identify the genomic breakpoints. Additional RT-PCR analysis was performed based on the DNA findings. Three cases were found to have complex rearrangements involving a third locus: the first had a PML-CDC6-RARA forward DNA fusion and expressed a chimeric PML-CDC6-RARA mRNA in addition to a PML-RARA. The other two had HERC1-PML and NT_009714.17-PML genomic fusion sequences at their respective reciprocal breakpoints. Six patients were falsely classified as RARA-PML negative due to deletions on chromosome 15 and/or 17, or alternative splicing leading to atypical RARA-PML fusion transcripts, which were not identified by conventional RT-PCR assays. This study demonstrates that the frequency of RARA-PML expression has been underestimated and highlights remarkable complexity at chromosomal breakpoint regions in APL even in cases with an apparently simple balanced t(15;17)(q24;q12).

PLZF/ZBTB16, a glucocorticoid response gene in acute lymphoblastic leukemia, interferes with glucocorticoid-induced apoptosis

Glucocorticoids (GCs) cause cell cycle arrest and apoptosis in lymphoid cells which is exploited to treat lymphoid malignancies. The mechanisms of these anti-leukemic GC effects are, however, poorly understood. We previously defined a list of GC-regulated genes by expression profiling in children with acute lymphoblastic leukemia (ALL) during systemic GC monotherapy and in experimental systems of GC-induced apoptosis. PLZF/ZBTB16, a transcriptional repressor, was one of the most promising candidates derived from this screen. To investigate its role in the anti-leukemic GC effects, we performed overexpression and knock-down experiments in CCRF-CEM childhood ALL cells. Transgenic PLZF/ZBTB16 alone had no detectable effect on cell proliferation or survival, but reduced sensitivity to GC-induced apoptosis but not apoptosis induced by antibodies against Fas/CD95 or 3 different chemotherapeutics. Knock-down of ZBTB16 entailed a small, but significant, increase in cell death induction by GC. Affymetrix Exon array-based whole genome expression profiling revealed that PLZF/ZBTB16 induction did not significantly alter the expression profile, however, it interfered with the regulation of numerous GC response genes, including BCL2L11/Bim, which has previously been shown to be responsible for cell death induction in CCRF-CEM cells. Thus, the protective effect of PLZF/ZBTB16 can be attributed to interference with transcriptional regulation by GC.

Pharmacogenetic model of retinoic acid-induced dyslipidemia and insulin resistance

Aims: Therapeutic administration of retinoids is often accompanied with undesirable side effects, including an increase in lipid levels in up to 45% of treated patients. We tested the hypothesis of whether spontaneously hypertensive rat (SHR) and congenic SHR.PD-(D8Rat42-D8Arb23)/Cub (SHR-Lx) strains, differing only in a 14-gene region of chromosome 8 and previously shown to display differential sensitivity to the teratogenic effects of retinoic acid, could serve as a pharmacogenetic model set of the metabolic side effects of retinoid therapy. Materials & methods: Male, 15-week old rats (n = 12/strain) of SHR and SHR-Lx strains were fed a high-sucrose diet for 2 weeks and subsequently treated either with all-trans retinoic acid (15 mg/kg) or only with a vehicle for 16 days (n = 6/strain/treatment), while still on the high-sucrose diet. We assessed the morphometric and metabolic profiles of all groups, including glucose tolerance tests, levels of insulin, adiponectin, free fatty acids, concentrations of triglycerides and cholesterol in 20 lipoprotein fractions under conditions of both high-sucrose diet and high-sucrose diet plus all-trans retinoic acid administration. Results & conclusion: SHR-Lx displayed substantially greater sensitivity to a number of all-trans retinoic acid-induced metabolic dysregulations compared with SHR, resulting in impairment of glucose tolerance, increased visceral adiposity, and substantially greater increase of circulating triglyceride concentrations, accompanied by a shift towards their less favorable distribution into the lipoprotein fractions. These observations closely mimic the common side effects of retinoid therapy in humans, rendering SHR-Lx an experimental pharmacogenetic model of atRA-induced dyslipidemia.

Comprehensive genomic screens identify a role for PLZF-RARalpha as a positive regulator of cell proliferation via direct regulation of c-MYC

The t(11;17)(q23;q21) translocation is associated with a retinoic acid (RA)-insensitive form of acute promyelocytic leukemia (APL), involving the production of reciprocal fusion proteins, promyelocytic leukemia zinc finger-retinoic acid receptor alpha (PLZF-RARalpha) and RARalpha-PLZF. Using a combination of chromatin immunoprecipitation promotor arrays (ChIP-chip) and gene expression profiling, we identify novel, direct target genes of PLZF-RARalpha that tend to be repressed in APL compared with other myeloid leukemias, supporting the role of PLZF-RARalpha as an aberrant repressor in APL. In primary murine hematopoietic progenitors, PLZF-RARalpha promotes cell growth, and represses Dusp6 and Cdkn2d, while inducing c-Myc expression, consistent with its role in leukemogenesis. PLZF-RARalpha binds to a region of the c-MYC promoter overlapping a functional PLZF site and antagonizes PLZF-mediated repression, suggesting that PLZF-RARalpha may act as a dominant-negative version of PLZF by affecting the regulation of shared targets. RA induced the differentiation of PLZF-RARalpha-transformed murine hematopoietic cells and reduced the frequency of clonogenic progenitors, concomitant with c-Myc down-regulation. Surviving RA-treated cells retained the ability to be replated and this was associated with sustained c-Myc expression and repression of Dusp6, suggesting a role for these genes in maintaining a self-renewal pathway triggered by PLZF-RARalpha.