TBLR1 fuses to retinoid acid receptor α in a variant t(3;17)(q26;q21) translocation of acute promyelocytic leukemia

Two decades of a protooncogene TBL1XR1: from a transcription modulator to cancer therapeutic target

Transducin beta-like 1X-related protein 1 (TBL1XR1) was discovered two decades ago and was implicated as part of the nuclear transcription corepressor complex. Over the past 20 years, the emerging oncogenic function of TBL1XR1 in cancer development has been discovered. Recent studies have highlighted that the genetic aberrations of TBL1XR1 in cancers, especially in hematologic tumors, are closely associated with tumorigenesis. In solid tumors, TBL1XR1 is proposed to be a promising prognostic biomarker due to the correlation between abnormal expression and clinicopathological parameters. Post-transcriptional and post-translational modification are responsible for the expression and function of TBL1XR1 in cancer. TBL1XR1 exerts its functional role in various processes that involves cell cycle and apoptosis, cell proliferation, resistance to chemotherapy and radiotherapy, cell migration and invasion, stemness and angiogenesis. Multitude of cancer-related signaling cascades like Wnt-β-catenin, PI3K/AKT, ERK, VEGF, NF-κB, STAT3 and gonadal hormone signaling pathways are tightly modulated by TBL1XR1. This review provided a comprehensive overview of TBL1XR1 in tumorigenesis, shedding new light on TBL1XR1 as a promising diagnostic biomarker and druggable target in cancer.

Hematopoietic Stem Cell Transplantation in Acute Promyelocytic Leukemia in the Era of All-Trans Retinoic Acid (ATRA) and Arsenic Trioxide (ATO)

Acute promyelocytic leukemia (APL) currently represents one of the malignant hemopathies with the best therapeutic responses, following the introduction of all-trans retinoic acid (ATRA) and subsequently of arsenic trioxide (ATO) treatment. As a result, a large proportion of patients with APL achieve long-term responses after first-line therapy, so performing a hematopoietic stem cell transplant as consolidation of first complete remission (CR) is no longer necessary. Even in the case of relapses, most patients obtain a new remission as a result of therapy with ATO and ATRA, but an effective consolidation treatment is necessary to maintain it. The experience accumulated from studies published in the last two decades shows the effectiveness of hematopoietic stem cell transplantation (HSCT) in improving the outcome of patients who achieve a new CR. Thus, the expert groups recommend transplantation as consolidation therapy in patients with a second CR, with the indication for autologous HSCT in cases with molecular CR and for allogeneic HSCT in patients with the persistence of minimal residual disease (MRD) or with early relapse. However, there is a variety of controversial aspects related to the role of HSCT in APL, ranging from the fact that outcome data are obtained almost exclusively from retrospective studies and historical analyses to questions related to the type of transplantation, the impact of minimal residual disease, conditioning regimens, or the role of other therapeutic options. All these questions justify the need for controlled prospective studies in the following years.

Hyperthermia promotes degradation of the acute promyelocytic leukemia driver oncoprotein ZBTB16/RARα

The acute promyelocytic leukemia (APL) driver ZBTB16/RARα is generated by the t(11;17) (q23;q21) chromosomal translocation, which is resistant to combined treatment of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) or conventional chemotherapy, resulting in extremely low survival rates. In the current study, we investigated the effects of hyperthermia on the oncogenic fusion ZBTB16/RARα protein to explore a potential therapeutic approach for this variant APL. We showed that Z/R fusion protein expressed in HeLa cells was resistant to ATO, ATRA, and conventional chemotherapeutic agents. However, mild hyperthermia (42 °C) rapidly destabilized the ZBTB16/RARα fusion protein expressed in HeLa, 293T, and OCI-AML3 cells, followed by robust ubiquitination and proteasomal degradation. In contrast, hyperthermia did not affect the normal (i.e., unfused) ZBTB16 and RARα proteins, suggesting a specific thermal sensitivity of the ZBTB16/RARα fusion protein. Importantly, we found that the destabilization of ZBTB16/RARα was the initial step for oncogenic fusion protein degradation by hyperthermia, which could be blocked by deletion of nuclear receptor corepressor (NCoR) binding sites or knockdown of NCoRs. Furthermore, SIAH2 was identified as the E3 ligase participating in hyperthermia-induced ubiquitination of ZBTB16/RARα. In short, these results demonstrate that hyperthermia could effectively destabilize and subsequently degrade the ZBTB16/RARα fusion protein in an NCoR-dependent manner, suggesting a thermal-based therapeutic strategy that may improve the outcome in refractory ZBTB16/RARα-driven APL patients in the clinic.

Molecular Heterogeneity of Pediatric AML with Atypical Promyelocytes Accumulation in Children—A Single Center Experience

Acute promyelocytic leukemia (APL) pathogenesis is based on RARA gene translocations, which are of high importance in the diagnosis of and proper therapy selection for APL. However, in some cases acute myeloid leukemia (AML) demonstrates APL-like morphological features such as atypical promyelocytes accumulation. This type of AML is characterized by the involvement of other RAR family members or completely different genes. In the present study, we used conventional karyotyping, FISH and high-throughput sequencing in a group of 271 de novo AML with atypical promyelocytes accumulation. Of those, 255 cases were shown to carry a typical chromosomal translocation t(15;17)(q24;q21) with PML::RARA chimeric gene formation (94.1%). Other RARA-positive cases exhibited cryptic PML::RARA fusion without t(15;17)(q24;q21) (1.8%, n = 5) and variant t(5;17)(q35;q21) translocation with NPM1::RARA chimeric gene formation (1.5%, n = 4). However, 7 RARA-negative AMLs with atypical promyelocytes accumulation were also discovered. These cases exhibited TBL1XR1::RARB and KMT2A::SEPT6 fusions as well as mutations, e.g., NPM1 insertion and non-recurrent chromosomal aberrations. Our findings demonstrate the genetic diversity of AML with APL-like morphological features, which is of high importance for successful therapy implementation.

Case report: A rare case of TBL1XR1-RARB positive acute promyelocytic leukemia in child and review of the literature

Some forms of acute myelogenous leukemia (AML) share typical morphological and immunophenotypic features of acute promyelocytic leukemia (APL) but are negative for promyelocytic leukemia-retinoic acid receptor alpha (PML-RARA) fusion. These forms of AML are known as variant APL. Some variants of APL present with retinoic acid receptor beta (RARB) fused or rearranged with partner genes. RARB-positive APL is very rare, resistant to all-trans retinoic acid (ATRA), and associated with poor prognosis. Here, we reported one case with TBL1XR1-RARB positive APL, featured by early onset and no apparent bleeding tendency or coagulation dysfunction. This patient was resistant to ATRA and arsenic trioxide (ATO), but was good responsive to conventional chemotherapy for AML. The case report was followed by a literature review.

Arsenic trioxide induces proteasome dependent TBLR1-RARα degradation to improve leukemia eradication through cell differentiation enhancement

Background: Acute promyelocytic leukemia (APL) mainly harbors PML-RARα fusion gene, which is sensitive to all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) treatment. However, APL harboring other RARα fusion genes exhibit different drug sensitivity. Here, we investigated the role and mechanism of TBLR1-RARα, a rare RARα fusion gene, on ATO treatment in leukemia cells.

Methods: By constructing two cell models of leukemia cell line HL-60 and U937 with overexpressed TBLR1-RARα, we detected the cell differentiation in the two cell models after ATO treatment by flow cytometry and Wright staining. Meanwhile, cell viability, colony formation and apoptosis were also determined after ATO treatment.

Results: We found that TBLR1-RARα enhanced ATO-induced apoptosis and cell proliferation inhibition. Besides, TBLR1-RARα also promoted ATO-induced cell differentiation. Furthermore, we found that the mitochondrial caspase pathway was involved in the apoptosis induced by ATO treatment in TBLR1-RARα positive leukemia cells. Moreover, ATO mediated TBLR1-RARα protein degradation via proteasome pathway, which accounts for the transcriptional activation of RARα target gene and is further involved in cell differentiation of TBLR1-RARα positive leukemia cells.

Conclusions: Our study provides evidence that TBLR1-RARα positive APL patients may benefit from ATO treatment, thereby improving the appropriate management in TBLR1-RARα positive APL.

Atypical Rearrangements in APL-Like Acute Myeloid Leukemias: Molecular Characterization and Prognosis

Acute promyelocytic leukemia (APL) accounts for 10–15% of newly diagnosed acute myeloid leukemias (AML) and is typically caused by the fusion of promyelocytic leukemia with retinoic acid receptor α (RARA) gene. The prognosis is excellent, thanks to the all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) combination therapy. A small percentage of APLs (around 2%) is caused by atypical transcripts, most of which involve RARA or other members of retinoic acid receptors (RARB or RARG). The diagnosis of these forms is difficult, and clinical management is still a challenge for the physician due to variable response rates to ATRA and ATO. Herein we review variant APL cases reported in literature, including genetic landscape, incidence of coagulopathy and differentiation syndrome, frequent causes of morbidity and mortality in these patients, sensitivity to ATRA, ATO, and chemotherapy, and outcome. We also focus on non-RAR rearrangements, complex rearrangements (involving more than two chromosomes), and NPM1-mutated AML, an entity that can, in some cases, morphologically mimic APL.

Identification of the global miR-130a targetome reveals a role for TBL1XR1 in hematopoietic stem cell self-renewal and t(8;21) AML

Gene expression profiling and proteome analysis of normal and malignant hematopoietic stem cells (HSCs) point to shared core stemness properties. However, discordance between mRNA and protein signatures highlights an important role for post-transcriptional regulation by microRNAs (miRNAs) in governing this critical nexus. Here, we identify miR-130a as a regulator of HSC self-renewal and differentiation. Enforced expression of miR-130a impairs B lymphoid differentiation and expands long-term HSCs. Integration of protein mass spectrometry and chimeric AGO2 crosslinking and immunoprecipitation (CLIP) identifies TBL1XR1 as a primary miR-130a target, whose loss of function phenocopies miR-130a overexpression. Moreover, we report that miR-130a is highly expressed in t(8;21) acute myeloid leukemia (AML), where it is critical for maintaining the oncogenic molecular program mediated by the AML1-ETO complex. Our study establishes that identification of the comprehensive miRNA targetome within primary cells enables discovery of genes and molecular networks underpinning stemness properties of normal and leukemic cells.

A novel fusion protein TBLR1-RARα acts as an oncogene to induce murine promyelocytic leukemia: identification and treatment strategies

Acute promyelocytic leukemia (APL) is characterized by a specific chromosome translocation involving RARα and its fusion partners. For decades, the advent of all-trans retinoic acid (ATRA) synergized with arsenic trioxide (As₂O₃) has turned most APL from highly fatal to highly curable. TBLR1-RARα (TR) is the tenth fusion gene of APL identified in our previous study, with its oncogenic role in the pathogenesis of APL not wholly unraveled. In this study, we found the expression of TR in mouse hematopoietic progenitors induces blockade of differentiation with enhanced proliferative capacity in vitro. A novel murine transplantable leukemia model was then established by expressing TR fusion gene in lineage-negative bone marrow mononuclear cells. Characteristics of primary TR mice revealed a rapid onset of aggressive leukemia with bleeding diathesis, which recapitulates human APL more accurately than other models. Despite the in vitro sensitivity to ATRA-induced cell differentiation, neither ATRA monotherapy nor combination with As₂O₃ confers survival benefit to TR mice, consistent with poor clinical outcome of APL patients with TR fusion gene. Based on histone deacetylation phenotypes implied by bioinformatic analysis, HDAC inhibitors demonstrated significant survival superiority in the survival of TR mice, yielding insights into clinical efficacy against rare types of APL.

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.

Acute promyelocytic leukemia drug - arsenic trioxide in the presence of eugenol shows differential action on leukemia cells (HL-60) and cardiomyocytes (H9c2) - inference from NMR study

The increased concern of cardiovascular dysfunction by cancer therapeutics has led to more effective treatment strategies. Arsenic trioxide (As₂O₃) is a potential chemotherapeutic agent for acute promyelocytic leukemia (APL), but the effectiveness is affected by potential cardiotoxicity. Researchers have been trying to find out novel modalities to manage the adverse effects of As₂O₃. In our study, the antioxidant molecule eugenol showed protective action against the destructive impact of As₂O₃ on cardiomyocytes (H9c2) without compromising the anti-cancer property As₂O₃ on leukemia cells (HL-60). We have studied the interaction between arsenic and eugenol in physiological and acidic pH to understand the molecular mechanism of differential action of As₂O₃ in the presence of eugenol using NMR spectroscopy. The study observed that at physiological pH, arsenic and eugenol interact to form an inactive product, positively affecting H9c2 cardiomyocytes. Still, there is no such interaction in acidic pH evidenced by the useful anti-cancer property of As₂O₃. The result concludes that the antioxidant molecule eugenol is an efficient protective agent against the adverse effect of As₂O₃ on cardiomyocytes.

Identification of a novel TNRC18-RARA fusion in acute promyelocytic leukemia lacking t(15;17)(q24;q12)/PML-RARA

Acute promyelocytic leukemia (APL) is a unique disease entity in acute myeloid leukemia, characterized by PML-RARA fusion gene, which is generated by chromosomal translocation t(15;17)(q24;q21). We identified TNRC18-RARA as novel RARA fusion in resembling APL. Our study highlights the importance of combining multiple molecular techniques to characterize and optimally manage APL lacking classic t(15;17)(q24;q12)/PML-RARA fusion.

Genotypic and Phenotypic Characteristics of Acute Promyelocytic Leukemia Translocation Variants

Acute promyelocytic leukemia (APL) is a special disease entity of acute myeloid leukemia (AML). The clinical use of all-trans retinoic acid (ATRA) has transformed APL into the most curable form of AML. The majority of APL cases are characterized by the fusion gene PML-RARA. Although the PML-RARA fusion gene can be detected in almost all APL cases, translocation variants of APL have been reported. To date, this is the most comprehensive review of these translocations, discussing 15 different variants. Reviewed genes involved in APL variants include: ZBTB16, NPM, NuMA, STAT5b, PRKAR1A, FIP1L1, BCOR, NABP1, TBLR1, GTF2I, IRF2BP2, FNDC3B, ADAMDTS17, STAT3, and TFG. The genotypic and phenotypic features of APL translocations are summarized. All reported studies were either case reports or case series indicating the rarity of these entities and limiting the ability to drive conclusions regarding their characteristics. However, reported variants have shown variable clinical and morphological features, with diverse responsiveness to ATRA.

NLS-RARα blocks cell differentiation by inhibiting the retinoic acid signalling pathway

A majority of acute promyelocytic leukaemia (APL) cases are characterized by the PML-RARα fusion gene. Previous studies have shown that neutrophil elastase (NE) can cleave PML-RARα and is important for the development of APL. Here, we demonstrate that one of the cleavage products of PML-RARα, NLS-RARα, can block cell differentiation by repressing the expression of the target genes within the retinoic acid signalling pathway. The results of reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot analysis showed that NLS-RARα depressed the expression of the cell differentiation marker protein, CD11b and CEBPβ, as well as the retinoic acid signalling pathway target genes, RARβ and CEBPε. Studies have shown that NLS-RARα forms heterodimers with retinoid X receptor α(RXRα) and interacts with SMRT. When treated with all-trans retinoic acid (ATRA), NLS-RARα exhibits diminished transcriptional activity compared to RARα. Moreover, in the presence of high doses of ATRA, NLS-RARα could be degraded along with the consequent transactivation of retinoic acid signalling pathway target genes and cell differentiation induction in a dose- and time-dependent manner. Together, these results indicate that NLS-RARα blocks cell differentiation by inhibiting the retinoic acid signalling pathway.

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.

Acute Promyelocytic Leukemia: A Constellation of Molecular Events around a Single PML-RARA Fusion Gene

Although acute promyelocytic leukemia (APL) is one of the most characterized forms of acute myeloid leukemia (AML), the molecular mechanisms involved in the development and progression of this disease are still a matter of study. APL is defined by the PML-RARA rearrangement as a consequence of the translocation t(15;17)(q24;q21). However, this abnormality alone is not able to trigger the whole leukemic phenotype and secondary cooperating events might contribute to APL pathogenesis. Additional somatic mutations are known to occur recurrently in several genes, such as FLT3, WT1, NRAS and KRAS, whereas mutations in other common AML genes are rarely detected, resulting in a different molecular profile compared to other AML subtypes. How this mutational spectrum, including point mutations in the PML-RARA fusion gene, could contribute to the 10%–15% of relapsed or resistant APL patients is still unknown. Moreover, due to the uncertain impact of additional mutations on prognosis, the identification of the APL-specific genetic lesion is still the only method recommended in the routine evaluation/screening at diagnosis and for minimal residual disease (MRD) assessment. However, the gene expression profile of genes, such as ID1, BAALC, ERG, and KMT2E, once combined with the molecular events, might improve future prognostic models, allowing us to predict clinical outcomes and to categorize APL patients in different risk subsets, as recently reported. In this review, we will focus on the molecular characterization of APL patients at diagnosis, relapse and resistance, in both children and adults. We will also describe different standardized molecular approaches to study MRD, including those recently developed. Finally, we will discuss how novel molecular findings can improve the management of this disease.

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.

Acute promyelocytic leukemia with a cryptic insertion of RARA into TBL1XR1

Acute promyelocytic leukemia (APL) is cytogenetically characterized by the t(15;17) (q24;q21), although cases without this translocation exist. These cases are referred to as "cryptic" or "masked" translocations. Additionally, fewer than 5% of APL cases have another partner gene fused to the RARA gene. The TBL1XR1-RARA fusion gene has recently been reported as a novel RARA-associated fusion gene. We report a case with TBL1XR1-RARA and a masked translocation that was not detected by conventional tests for RARA-associated translocations. Three-year-old girl was diagnosed with APL based morphological findings, although conventional tests for RARA-associated chimeric genes were negative. She received all-trans retinoic acid treatment, but that was not effective. She achieved a complete remission (CR) by conventional multidrug chemotherapy, but had extramedullary relapse 2 years after onset. She underwent cord blood transplantation (CBT) in her second CR and is currently alive. To investigate the underlying pathogenesis of this unique case, we performed whole-genome sequencing and found a cryptic insertion of RARA gene into the TBL1XR1 gene. The transcript of the chimeric gene, TBL1XR1-RARA, was confirmed as an in-frame fusion by RT-PCR. In conclusion, we found using next-generation sequencing (NGS) a TBL1XR1-RARA fusion in a child with variant APL without the classic karyotype. Cryptic insertion could also occur in cases other than APL with PML-RARA. Variant APL has many variants and NGS analysis should therefore be considered for APL variant cases, even for those without RARA translocation detected by conventional analysis.

Management of acute promyelocytic leukemia: updated recommendations from an expert panel of the European LeukemiaNet

Since the comprehensive recommendations for the management of acute promyelocytic leukemia (APL) reported in 2009, several studies have provided important insights, particularly regarding the role of arsenic trioxide (ATO) in frontline therapy. Ten years later, a European LeukemiaNet expert panel has reviewed the recent advances in the management of APL in both frontline and relapse settings in order to develop updated evidence- and expert opinion-based recommendations on the management of this disease. Together with providing current indications on genetic diagnosis, modern risk-adapted frontline therapy, and salvage treatment, the review contains specific recommendations for the identification and management of the most important complications such as the bleeding disorder APL differentiation syndrome, QT prolongation, and other all-trans retinoic acid- and ATO-related toxicities, as well as recommendations for molecular assessment of the response to treatment. Finally, the approach to special situations is also discussed, including management of APL in children, elderly patients, and pregnant women. The most important challenges remaining in APL include early death, which still occurs before and during induction therapy, and optimizing treatment in patients with high-risk disease.

RNF8 is responsible for ATRA resistance in variant acute promyelocytic leukemia with GTF2I/RARA fusion, and inhibition of the ubiquitin-proteasome pathway contributes to the reversion of ATRA resistance

Background

GTF2I-RARA is a newly identified RARA fusion gene in variant acute promyelocytic leukemia (APL) patients with t(7;17)(q11;q21). Clinical manifestation in the patient showed that it is a sort of ATRA-insensitive oncogene and is different from the classic PML-RARA in terms of therapeutic reaction.

Methods

To reveal the functional characteristics and regulating mechanism of the GTF2I-RARA fusion gene, we established a GTF2I-RARA-transfected HL60 cell model and examined its sensitivity to ATRA by western blot, MTT assay, flow cytometry, and Wright-Giemsa staining. Coimmunoprecipitation and confocal microscopy were used to examine the binding of GTF2I-RARA and transcriptional corepressors. We also performed ChIP-seq to search for potential target genes. Immunoprecipitation, ubiquitination assay, western blot, luciferase assay, and real-time PCR were used to analyze the effects of RNF8 on RARA. Flow cytometry and Wright-Giemsa staining were used to study the effect of MG132 and ATRA on the GTF2I-RARA-transfected HL60 cell model.

Result

We confirmed resistance of GTF2I-RARA to ATRA. Compared with PML-RARA, GTF2I-RARA has a higher affinity to HDAC3 under ATRA treatment. Using the ChIP-sequencing approach, we identified 221 GTF2I-RARA binding sites in model cells and found that the RING finger protein 8 (RNF8) is a target gene of GTF2I-RARA. RNF8 participates in disease progression and therapy resistance in APL with the GTF2I-RARA transcript. Elevated RNF8 expression promotes the interaction between RARA and RNF8 and induces RARA Lys-48 linkage ubiquitylation and degradation, resulting in attenuated transcriptional activation of RARA.

Conclusion

Our results suggest that RNF8 is a key GTF2I-RARA downstream event. Using the combination of MG132 and ATRA to treat GTF2I-RARA-HL60 cells, a synergistic effect leading to GTF2I-RARA-HL60 cell differentiation was confirmed. Taken together, the targeting of RNF8 may be an alternative choice for treatment in variant APL with GTF2I-RARA fusion.

Electronic supplementary material

The online version of this article (10.1186/s12935-019-0803-4) contains supplementary material, which is available to authorized users.

The genetics and clinical characteristics of children morphologically diagnosed as acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is characterized by t(15;17)(q22;q21), resulting in a PML-RARA fusion that is the master driver of APL. A few cases that cannot be identified with PML-RARA by using conventional methods (karyotype analysis, FISH, and RT-PCR) involve abnormal promyelocytes that are fully in accordance with APL in morphology, cytochemistry, and immunophenotype. To explore the mechanisms involved in pathogenesis and recurrence of morphologically diagnosed APL, we performed comprehensive variant analysis by next-generation sequencing in 111 pediatric patients morphologically diagnosed as APL. Structural variant (SV) analysis in 120 DNA samples from both diagnosis and relapse stage identified 95 samples with RARA rearrangement (including 94 with PML-RARA and one with NPM-RARA) and two samples with KMT2A rearrangement. In the eligible 13 RNA samples without any RARA rearrangement at diagnosis, one case each with CPSF6-RARG, NPM1-CCDC28A, and TBC1D15-RAB21 and two cases with a TBL1XR1-RARB fusion were discovered. These uncovered fusion genes strongly suggested their contributions to leukemogenesis as driver alternations and APL phenotype may arise by abnormalities of other members of the nuclear receptor superfamily involved in retinoid signaling (RARB or RARG) or even by mechanisms distinct from the formation of aberrant retinoid receptors. Single-nucleotide variant (SNV) analysis in 77 children (80 samples) with RARA rearrangement showed recurrent alternations of primary APL in FLT3, WT1, USP9X, NRAS, and ARID1A, with a strong potential for involvement in pathogenesis, and WT1 as the only recurrently mutated gene in relapsed APL. WT1, NPM1, NRAS, FLT3, and NSD1 were identified as recurrently mutated in 17 primary samples without RARA rearrangement and WT1, NPM1, TP53, and RARA as recurrently mutated in 9 relapsed samples. The survival of APL with RARA rearrangement is much better than without RARA rearrangement. Thus, patients morphologically diagnosed as APL that cannot be identified as having a RARA rearrangement are more reasonably classified as a subclass of AML other than APL, and individualized treatment should be considered according to the genetic abnormalities.

Immune cells in the tumour: new routes of retinoids for chemoprevention and chemotherapeutics

Retinoids, vitamin A and its natural and synthetic analogues have various functions, including being involved in cell proliferation and differentiation and participating in the formation of vertebrate morphology. In addition, they may activate certain tumour suppressor genes that then act as tumour inhibitors. In the past decades, retinoids have been regarded as promising chemotherapeutic and chemopreventive agents; however, their mechanisms are still not fully understood. Immune cells that participate in or are associated with the immune response play vital roles in the initiation and development of many cancers. Interestingly, recent studies have demonstrated that retinoids can also exert various effects on immune cells including macrophages, T cells and dendritic cells in tumour tissues to execute anti-tumour actions, providing new insights into chemoprevention and chemotherapeutics. In this review, we focus on the effects of retinoids on immune cells in the tumour, which may provide new approaches for antineoplastic strategies.

Recurrent RARB Translocations in Acute Promyelocytic Leukemia Lacking RARA Translocation

Translocations of retinoic acid receptor-α (RARA), typically PML-RARA, are a genetic hallmark of acute promyelocytic leukemia (APL). However, because a small fraction of APL lack translocations of RARA, we focused here on APL cases without RARA translocation to elucidate the molecular etiology of RARA-negative APL. We performed whole-genome sequencing, PCR, and FISH for five APL cases without RARA translocations. Four of five RARA-negative APL cases had translocations involving retinoic acid receptor-β (RARB) translocations, and TBL1XR1-RARB was identified as an in-frame fusion in three cases; one case had an RARB rearrangement detected by FISH, although the partner gene could not be identified. When transduced in cell lines, TBL1XR1-RARB homodimerized and diminished transcriptional activity for the retinoic acid receptor pathway in a dominant-negative manner. TBL1XR1-RARB enhanced the replating capacity of mouse bone marrow cells and inhibited myeloid maturation of human cord blood cells as PML-RARA did. However, the response of APL with RARB translocation to retinoids was attenuated compared with that of PML-RARA, an observation in line with the clinical resistance of RARB-positive APL to ATRA. Our results demonstrate that the majority of RARA-negative APL have RARB translocations, thereby forming a novel, distinct subgroup of APL. TBL1XR1-RARB as an oncogenic protein exerts effects similar to those of PML-RARA, underpinning the importance of retinoic acid pathway alterations in the pathogenesis of APL.Significance: These findings report a novel and distinct genetic subtype of acute promyelocytic leukemia (APL) by illustrating that the majority of APL without RARA translocations harbor RARB translocations. Cancer Res; 78(16); 4452-8. ©2018 AACR.

The discussion of t(1;17)(p11;q21) translocation in acute promyelocytic leukemia patient on molecular remission

Some chromosomal aberrations emerging in the course of treatment are probably not related to disease progression, but attribute to the germline alteration. Therefore, the dynamic genetic tests should be performed during the whole treatment process, which is significantly essential for efficacy evaluation and treatment decision‐ making.

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.

Pathways of retinoid synthesis in mouse macrophages and bone marrow cells

In vivo pathways of natural retinoid metabolism and elimination have not been well characterized in primary myeloid cells, even though retinoids and retinoid receptors have been strongly implicated in regulating myeloid maturation. With the use of a upstream activation sequence-GFP reporter transgene and retrovirally expressed Gal4-retinoic acid receptor α in primary mouse bone marrow cells, we identified 2 distinct enzymatic pathways used by mouse myeloid cells ex vivo to synthesize retinoic acid receptor α ligands from free vitamin A metabolites (retinyl acetate, retinol, and retinal). Bulk Kit(+) bone marrow progenitor cells use diethylaminobenzaldehyde-sensitive enzymes, whereas bone marrow-derived macrophages use diethylaminobenzaldehyde-insensitive enzymes to synthesize natural retinoic acid receptor α-activating retinoids (all-trans retinoic acid). Bone marrow-derived macrophages do not express the diethylaminobenzaldehyde-sensitive enzymes Aldh1a1, Aldh1a2, or Aldh1a3 but instead, express Aldh3b1, which we found is capable of diethylaminobenzaldehyde-insensitive synthesis of all trans-retinoic acid. However, under steady-state and stimulated conditions in vivo, diverse bone marrow cells and peritoneal macrophages showed no evidence of intracellular retinoic acid receptor α-activating retinoids, despite expression of these enzymes and a vitamin A-sufficient diet, suggesting that the enzymatic conversion of retinal is not the rate-limiting step in the synthesis of intracellular retinoic acid receptor α-activating retinoids in myeloid bone marrow cells and that retinoic acid receptor α remains in an unliganded configuration during adult hematopoiesis.

The leukemic oncoprotein NPM1-RARA inhibits TP53 activity

The variant acute promyelocytic leukemia (APL) translocation t(5;17)(q35;q21) fuses the N-terminus of nucleophosmin (NPM1) to the retinoic acid receptor alpha (RARA). We found that ectopic NPM1-RARA expression decreased TP53 protein levels in target cells. NPM1-RARA impaired TP53-dependent transcription. Cells expressing NPM1-RARA were more resistant to apoptotic stimuli. This work identifies the TP53 tumor suppressor as a novel target through which NPM1-RARA impacts leukemogenesis, and confirms the importance of impairment of TP53 in establishment of the APL phenotype.

Fusion of the TBL1XR1 and HMGA1 genes in splenic hemangioma with t(3;6)(q26;p21)

RNA-sequencing of a splenic hemangioma with the karyotype 45~47,XX,t(3;6)(q26;p21) showed that this translocation generated a chimeric TBL1XR1-HMGA1 gene. This is the first time that this tumor has been subjected to genetic analysis, but the finding of an acquired clonal chromosome abnormality in cells cultured from the lesion and the presence of the TBL1XR1-HMGA1 fusion in them strongly favor the conclusion that splenic hemangiomas are of a neoplastic nature. Genomic PCR confirmed the presence of the TBL1XR1-HMGA1 fusion gene, and RT-PCR together with Sanger sequencing verified the presence of the fusion transcripts. The molecular consequences of the t(3;6) would be substantial. The cells carrying the translocation would retain only one functional copy of the wild-type TBL1XR1 gene while the other, rearranged allele could produce a putative truncated form of TBL1XR1 protein containing the LiSH and F-box-like domains. In the TBL1XR1-HMGA1 fusion transcript, furthermore, untranslated exons of HMGA1 are replaced by the first 5 exons of the TBL1XR1 gene. The result is that the entire coding region of HMGA1 comes under the control of the TBL1XR1 promoter, bringing about dysregulation of HMGA1. This is reminiscent of similar pathogenetic mechanisms involving high mobility genes in benign connective tissue tumors such as lipomas and leiomyomas.

Synergistic targeted therapy for acute promyelocytic leukaemia: a model of translational research in human cancer

Acute promyelocytic leukaemia (APL), the M3 subtype of acute myeloid leukaemia, was once a lethal disease, yet nowadays the majority of patients with APL can be successfully cured by molecularly targeted therapy. This dramatic improvement in the survival rate is an example of the advantage of modern medicine. APL is characterized by a balanced reciprocal chromosomal translocation fusing the promyelocytic leukaemia (PML) gene on chromosome 15 with the retinoic acid receptor α (RARα) gene on chromosome 17. It has been found that all-trans-retinoic acid (ATRA) or arsenic trioxide (ATO) alone exerts therapeutic effect on APL patients with the PML-RARα fusion gene, and the combination of both drugs can act synergistically to further enhance the cure rate of the patients. Here, we provide an insight into the pathogenesis of APL and the mechanisms underlying the respective roles of ATRA and ATO. In addition, treatments that lead to more effective differentiation and apoptosis of APL cells, including leukaemia-initiating cells, and more thorough eradication of the disease will be discussed. Moreover, as a model of translational research, the development of a cure for APL has followed a bidirectional approach of 'bench to bedside' and 'bedside to bench', which can serve as a valuable example for the diagnosis and treatment of other malignancies.

NPM-RAR binding to TRADD selectively inhibits caspase activation, while allowing activation of NFκB and JNK

The t(5;17) variant of acute promeylocytic leukemia (APL) expresses a fusion of nucleophosmin (NPM) with the retinoic acid receptor alpha (RARA). We have previously shown that NPM-RAR is a binding partner of the tumor necrosis factor (TNF) receptor type-I-associated DEATH domain protein, TRADD. Binding of TNF to its receptor, TNF-R, induces recruitment of TRADD, and subsequent recruitment of a cascade of proteins that ultimate activate caspase 3, nuclear factor κB (NFκB) and c-Jun N-terminal kinase (JNK). We have previously shown that NPM-RAR interaction with TRADD blocks TNF activation of caspase 3, caspase 8, poly(ADP-ribose) polymerase (PARP) cleavage and, ultimately, apoptosis. We now report that NPM-RAR expression is permissive for TNF activation of NFκB and JNK. We propose that inhibition of TNF activation of apoptosis, while preserving TNF activation of NFκB and JNK pathways that stimulate cell growth and survival, represents a novel mechanism through which NPM-RAR contributes to development of the leukemic phenotype.

MLL-ELL fusion gene in an acute myelomonocytic leukemia patient transformed from acute promyelocytic leukemia

We report an extremely rare case of acute myelomonocytic leukemia (M4) with an MLL-ELL fusion gene lacking the PML-RARα rearrangement that transformed from hypergranular acute promyelocytic leukemia (APL) without showing any karyotypic evolution. The treatment was effective with chemotherapy for M4 and idarubicin plus a cytarabine-based chemotherapy protocol without ATRA.