The PRKAR1A gene is fused to RARA in a new variant acute promyelocytic leukemia

Utilization of RT-PCR and Optical Genome Mapping in Acute Promyelocytic Leukemia with Cryptic PML::RARA Rearrangement: A Case Discussion and Systemic Literature Review

BACKGROUND

Acute promyelocytic leukemia (APL) is characterized by abnormal promyelocytes and t(15;17)(q24;q21) PML::RARA. Rarely, patients may have cryptic or variant rearrangements. All-trans retinoic acid (ATRA)/arsenic trioxide (ATO) is largely curative provided that the diagnosis is established early.

METHODS

We present the case of a 36-year-old male who presented with features concerning for disseminated intravascular coagulation. Although the initial diagnostic work-up, including pathology and flow cytometry evaluation, suggested a diagnosis of APL, karyotype and fluorescence in situ hybridization (FISH), using the PML/RARA dual fusion and RARA breakapart probes, were negative. We performed real-time polymerase chain reaction (RT-PCR) and optical genome mapping (OGM) to further confirm the clinicopathological findings.

RESULTS

RT-PCR revealed a cryptic PML::RARA fusion transcript. OGM further confirmed the nature and orientation of a cryptic rearrangement with an insertion of RARA into PML at intron 3 (bcr3). In light of these findings, we performed a systematic literature review to understand the prevalence, diagnosis, and prognosis of APL with cryptic PML::RARA rearrangements.

CONCLUSIONS

This case, in conjunction with the results of our systematic literature review, highlights the importance of performing confirmatory testing in FISH-negative cases of suspected APL to enable prompt diagnosis and appropriate treatment.

Acute Promyelocytic Leukemia-like AML: Genetic Perspective and Clinical Implications

Acute promyelocytic leukemia (APL) is a rare type of AML, characterized by the t(15;17) translocation and accounting for 8-15% of cases. The introduction of target therapies, such as all-trans retinoic acid (ATRA) and arsenic trioxide (ATO), radically changed the management of APL, making it the most curable AML subtype. However, a small percentage (estimated to be 2%) of AML presenting with APL-like morphology and/or immunophenotype lacks t(15;17). This rare APL-like AML group, whose first case was described in the early 1990s, now includes over 40 entities. These diseases present great heterogeneity in terms of genetic lesions, clinical presentation, sensitivity to targeted agents and chemotherapy, and prognosis. Furthermore, the diagnosis is very challenging. Thus, in this paper, we aim to comprehensively review the literature reports and studies addressing APL-like entities, investigate the biological mechanisms of leukemogenesis, evaluate the clinical characteristics, and discuss future lines of research and possible clinical approaches.

Genetic drivers and cellular selection of female mosaic X chromosome loss

Mosaic loss of the X chromosome (mLOX) is the most common clonal somatic alteration in leukocytes of female individuals1,2, but little is known about its genetic determinants or phenotypic consequences. Here, to address this, we used data from 883,574 female participants across 8 biobanks; 12% of participants exhibited detectable mLOX in approximately 2% of leukocytes. Female participants with mLOX had an increased risk of myeloid and lymphoid leukaemias. Genetic analyses identified 56 common variants associated with mLOX, implicating genes with roles in chromosomal missegregation, cancer predisposition and autoimmune diseases. Exome-sequence analyses identified rare missense variants in FBXO10 that confer a twofold increased risk of mLOX. Only a small fraction of associations was shared with mosaic Y chromosome loss, suggesting that distinct biological processes drive formation and clonal expansion of sex chromosome missegregation. Allelic shift analyses identified X chromosome alleles that are preferentially retained in mLOX, demonstrating variation at many loci under cellular selection. A polygenic score including 44 allelic shift loci correctly inferred the retained X chromosomes in 80.7% of mLOX cases in the top decile. Our results support a model in which germline variants predispose female individuals to acquiring mLOX, with the allelic content of the X chromosome possibly shaping the magnitude of clonal expansion.

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.

Genomic Abnormalities as Biomarkers and Therapeutic Targets in Acute Myeloid Leukemia

Simple Summary

AML is a heterogenous malignancy with a variety of underlying genomic abnormalities. Some of the genetic aberrations in AML have led to the development of specific inhibitors which were approved by the Food and Drug Administration (FDA) and are currently used to treat eligible patients. In this review, we describe five gene mutations for which approved inhibitors have been developed, the response of AML patients to these inhibitors, and the known mechanism(s) of resistance. This review also highlights the significance of developing function-based screens for target discovery in the era of personalized medicine.

Abstract

Acute myeloid leukemia (AML) is a highly heterogeneous malignancy characterized by the clonal expansion of myeloid stem and progenitor cells in the bone marrow, peripheral blood, and other tissues. AML results from the acquisition of gene mutations or chromosomal abnormalities that induce proliferation or block differentiation of hematopoietic progenitors. A combination of cytogenetic profiling and gene mutation analyses are essential for the proper diagnosis, classification, prognosis, and treatment of AML. In the present review, we provide a summary of genomic abnormalities in AML that have emerged as both markers of disease and therapeutic targets. We discuss the abnormalities of RARA, FLT3, BCL2, IDH1, and IDH2, their significance as therapeutic targets in AML, and how various mechanisms cause resistance to the currently FDA-approved inhibitors. We also discuss the limitations of current genomic approaches for producing a comprehensive picture of the activated signaling pathways at diagnosis or at relapse in AML patients, and how innovative technologies combining genomic and functional methods will improve the discovery of novel therapeutic targets in AML. The ultimate goal is to optimize a personalized medicine approach for AML patients and possibly those with other types of cancers.

Identification of variant APL translocations PRKAR1A-RARα and ZBTB16-RARα (PLZF-RARα) through the MI-ONCOSEQ platform

The cornerstone of management in patients with acute promyelocytic leukemia (APL) is early diagnosis and prompt initiation of treatment with an all-trans retinoic acid (ATRA)-based regimen. Identification of the t(15;17)(PML-RARA) chromosomal translocation through conventional cytogenetics fluorescence in-situ hybridization (FISH) or detection of the promyelocytic leukemia-retinoic acid receptor alpha (PML-RARα) fusion through RT-PCR represent the current standard of care for diagnosing APL. However, about 1-2% of patients with APL have a variant translocation involving other fusion partners with RARα besides PML. These patients present a unique diagnostic and clinical challenge in that conventional cytogenetics in addition to FISH and/or RT-PCR for PML-RARα may fail to identify these clinically relevant genetic lesions leading to an inappropriate diagnosis and treatment. We present two cases of patients who had APL with variant translocations whose bone marrow specimens were sent to the University of Michigan for enrollment in the MI-ONCOSEQ study (HUM00067928) after standard testing failed to identify PML-RARα or t(15;17) despite a phenotypic concern for this diagnosis. In these two patients, whole exome and transcriptome profiling via the MI-ONCOSEQ platform identified a PRKAR1A-RARα fusion in one patient and ZBTB16-RARα fusion in another patient. These cases illustrate the utility of whole exome and transcriptome profiling in diagnosing variant translocations in patients in whom there is a high clinical suspicion for APL based on hematopathology review.

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.

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.

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.

Alternative Splicing in the Nuclear Receptor Superfamily Expands Gene Function to Refine Endo-Xenobiotic Metabolism

The human genome encodes 48 nuclear receptor (NR) genes, whose translated products transform chemical signals from endo-xenobiotics into pleotropic RNA transcriptional profiles that refine drug metabolism. This review describes the remarkable diversification of the 48 human NR genes, which are potentially processed into over 1000 distinct mRNA transcripts by alternative splicing (AS). The average human NR expresses ∼21 transcripts per gene and is associated with ∼7000 single nucleotide polymorphisms (SNPs). However, the rate of SNP accumulation does not appear to drive the AS process, highlighting the resilience of NR genes to mutation. Here we summarize the altered tissue distribution/function of well characterized NR splice variants associated with human disease. We also describe a cassette exon visualization pictograph methodology for illustrating the location of modular, cassette exons in genes, which can be skipped in-frame, to facilitate the study of their functional relevance to both drug metabolism and NR evolution. We find cassette exons associated with all of the functional domains of NR genes including the DNA and ligand binding domains. The matrix of inclusion or exclusion for functional domain-encoding cassette exons is extensive and capable of significant alterations in cellular phenotypes that modulate endo-xenobiotic metabolism. Exon inclusion options are differentially distributed across NR subfamilies, suggesting group-specific conservation of resilient functionalities. A deeper understanding of this transcriptional plasticity expands our understanding of how chemical signals are refined and mediated by NR genes. This expanded view of the NR transcriptome informs new models of chemical toxicity, disease diagnostics, and precision-based approaches to personalized medicine. SIGNIFICANCE STATEMENT: This review explores the impact of alternative splicing (AS) on the human nuclear receptor (NR) superfamily and highlights the dramatic expansion of more than 1000 potential transcript variants from 48 individual genes. Xenobiotics are increasingly recognized for their ability to perturb gene splicing events, and here we explore the differential sensitivity of NR genes to AS and chemical exposure. Using the cassette exon visualization pictograph methodology, we have documented the conservation of splice-sensitive, modular, cassette exon domains among the 48 human NR genes, and we discuss how their differential expression profiles may augment cellular resilience to oxidative stress and fine-tune adaptive, metabolic responses to endo-xenobiotic exposure.

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.

Characterization of atypical acute promyelocytic leukaemia: Three cases report and literature review

RATIONALE

The vast majority of acute promyelocytic leukemia (APL) is characterized with a specific chromosomal translocation t (15, 17) (q22, q21), which fuses PML-RARα leading to a good response to all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). However, there are few cases of atypical APL, including PLZF-RARα, F1P1L1-RARα, STAT5b-RARα, et al. Neither PLZF-RARα nor STAT5b-RARα are sensitive to ATRA and ATO, and the prognosis is poor.

PATIENT CONCERNS

Here we have 3 cases (PLZF-RARα, n = 2; STAT5b-RARα, n = 1). Case A, A 53-year-old Chinese female had suffered ecchymosis in both legs for 3 days. Case B, A 44 years old male suffered pain from lower limbs and hip. Case C, 52-year-old male patient presented with fever for 3 weeks invalid to antibiotics and gingival bleeding for 1 week.

DIAGNOSES

With RT-PCR and karyotype, Case A is diagnosed with STAT5b-RARα-positive APL.Case B, C are diagnosed with PLZF-RARα-positive APL.

INTERVENTIONS

In case A, ATO, and ATRA were used for induction treatment. In Case B, ATO, and chemotherapy with DA were given in the first induction treatment. In Case C, ATRA, and ATO were used immediately, subsequently, chemotherapy was added with DA, ATRA, and CAG combination treatment, and medium-dose cytarabine with daunorubicin were given regularly.

OUTCOMES

In Case A, the patient refused the following treatment and discharged on day 25. In Case B, the patient got the disseminated intravascular coagulation (DIC).In Case C, the patient has survived for 7 months and remains CR.

LESSONS

Both STAT5b-RARα-positive APL and PLZF-RARα-positive APL appear to be resistant to both ATRA and ATO, so combined chemotherapy and allo-HSCT should be considered. Since the prognosis and long-term outcome are poor, more clinical trials, and researches should be taken.

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.

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.

The CD9+ CD11b- HLA-DR- immunophenotype can be used to diagnose acute promyelocytic leukemia

OBJECTIVE

To investigate the immunophenotypic characteristics of acute promyelocytic leukemia (APL) and explore the sensitivity and specificity of various antibody combinations for the timely and accurate diagnosis APL.

METHODS

A retrospective analysis was performed using morphological, immunological, genetic, and molecular biological data from 92 patients diagnosed with APL and 190 controls diagnosed with non-APL acute myeloid leukemia.

RESULTS

For APL diagnosis, the CD9/CD11b/human leukocyte antigen (HLA)-DR antibody combination had 85% sensitivity and 95% specificity, AUC = 0.85. However, the sensitivity and specificity were 39% and 92%, AUC = 0.65, respectively, for the HLA-DR/CD34/CD117 combination, and 80% and 80%, AUC = 0.80, respectively for the CD11b/HLA-DR combination. Significant differences were observed between the different antibody combinations.

CONCLUSIONS

The CD9/CD11b/HLA-DR antibody combination displays high sensitivity and specificity and can be used to diagnose APL.

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.

Diagnosis of variant RARA translocation using standard dual-color dual-fusion PML/RARA FISH probes: An illustrative report

Acute promyelocytic leukemia (APML) with variant RARa translocations comprises 1-2% of APML cases. However, the diagnosis of these cases is challenging as the routine practice includes fluorescence in situ hybridization (FISH) reverse transcription polymerase chain reaction targeting the PML and RARA genes to detect PML/RARA fusions. Here, we report a case highlighting the importance of atypical FISH signal patterns in standard dual-color dual-fusion PML/RARa FISH analysis complimented by karyotyping to detect these variant RARA translocations.

New variant of acute promyelocytic leukemia with IRF2BP2-RARA fusion

We present an acute promyelocytic leukemia (APL) patient with two subtypes of IRF2BP2–RARA, in which the IRF2BP2 gene showed completely new breakpoints. Bone marrow examination revealed morphologic features indicative of APL. However, promyelocytic leukemia–RARA fusion was not detected. A paired‐end mRNA sequencing followed by RT‐PCR and direct sequencing revealed two types of fusion transcripts between exon 1B of IRF2BP2 and exon 3 of RARA. The patient received all‐trans retinoic acid and conventional chemotherapy, but showed resistance. This is the second report of IRF2BP2 involvement in APL, and we describe various breakpoints for the IRF2BP2–RARA fusion gene.

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.

Acute Promyelocytic Leukemia: A Review and Discussion of Variant Translocations

The majority of patients with acute promyelocytic leukemia (APL) manifest the t(15;17)(q24.1;q21.2) translocation; however, a minor but significant proportion of patients with APL harbor complex, cryptic, or variant translocations, which typically involve RARA. With the exception of ZBTB16/RARA, these variants have similar morphologic and immunophenotypic features as classic APL. Study of the variant forms of APL not only gives insight into the pathogenesis of APL but also allows us to understand the mechanism of retinoid therapy. It is important to identify these cryptic and variant translocations because certain variants, including ZBTB16/RARA and STAT5B/RARA, are resistant to treatment with all-trans retinoic acid, arsenic trioxide, and anthracyclines.

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.

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.

Identification of a novel fusion gene, IRF2BP2-RARA, in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is characterized by the fusion of retinoic acid receptor alpha (RARA) with promyelocytic leukemia (PML) or, rarely, other gene partners. This report presents a patient with APL with a novel fusion between RARA and the interferon regulatory factor 2 binding protein 2 (IRF2BP2) genes. A bone marrow examination in a 19-year-old woman who presented with ecchymoses and epistaxis showed morphologic and immunophenotypic features consistent with APL. PML oncogenic domain antibody was positive. Results of fluorescence in situ hybridization, conventional cytogenetics, reverse transcription-polymerase chain reaction (RT-PCR), and oligonucleotide microarray for PML-RARA and common APL variant translocations were negative. Next-generation RNA-sequencing analysis followed by RT-PCR and direct sequencing revealed distinct breakpoints within IRF2BP2 exon 2 and RARA intron 2. The patient received all-trans retinoic acid, arsenic, and gemtuzumab ozogamicin, and achieved complete remission. However, the disease relapsed 10 months later, 2 months after consolidation therapy. This is the first report showing involvement of IRF2BP2 in APL, and it expands the list of novel RARA partners identified in APL.

A new transcriptional variant and small azurophilic granules in an acute promyelocytic leukemia case with NPM1/RARA fusion gene

We report here the first case of NPM1/RARA-positive acute promyelocytic leukemia (APL) preceded by myeloid sarcoma (MS) in the vertebra. A 52-year-old man was diagnosed with MS, as the tumor cells were positive for myeloperoxidase and CD68 but negative for CD163. After treatment with steroids and radiation, the size of the tumor was markedly reduced and peripheral blood count was normal. Bone marrow examination showed 89.2% consisted of unclassified promyelocytes characterized by round nuclei and abundant small azurophilic granules but no Auer rods. The results of chromosome analysis showed 46,XY,t(5;17)(q35;q12). Reverse-transcription polymerase chain reaction amplified the NPM1/RARA fusion transcripts derived from a combination of NPM1 exon 4 and RARA exon 5, or of NPM1 exon 1 and RARA exon 5; the latter of these has not been reported previously. Electron microscopic examination of the promyelocyte nuclei showed they were oval with mild nuclear chromatin condensation and small- to medium-sized nucleoli. Hematological and molecular complete remission was attained after induction therapy including all-trans retinoic acid. As MS was also diagnosed in two of the seven other reported cases of APL with NPM1/RARA, MS may occur more frequently in APL with NPM1/RARA than APL with PML/RARA.

Acute Myeloid Leukemia With Recurrent Cytogenetic Abnormalities

OBJECTIVES

Session 1 of the 2013 Society for Hematopathology/European Association for Hematopathology Workshop was devoted to the cases of acute myeloid leukemia (AML) with recurrent cytogenetic abnormalities.

METHODS

Based on World Health Organization 2008 criteria, seven specific translocations are defined as "recurrent" in AML. Of these seven, three are considered to be AML defining regardless of blast percentage. Workshop cases provided the opportunity to consider potential new AML-defining cytogenetic mutations, as well as other unique aspects of AML with cytogenetic abnormalities.

RESULTS

Most of the 38 cases submitted were acute promyelocytic leukemia (APL) with t(15;17)(q24.1;q21.1) and so-called variants (12 cases), AML with t(8;21)(q22;q22) (seven cases), AML with inv(3)(q11q26.2) (six cases), and AML with 11q23 translocations (five cases).

CONCLUSIONS

This review focuses on providing updated recommendations for the rapid diagnosis of APL, discussing the types and significance of variant RARA mutations in APL-like leukemias, and refining low-blast-count (oligoblastic) AML. In addition, the significance of unique morphologic, immunophenotypic, and genetic variations in AML defined by a recurrent cytogenetic abnormality is included.

Global characteristics of childhood acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) comprises approximately 5-10% of childhood acute myeloid leukemia (AML) cases in the US. While variation in this percentage among other populations was noted previously, global patterns of childhood APL have not been thoroughly characterized. In this comprehensive review of childhood APL, we examined its geographic pattern and the potential contribution of environmental factors to observed variation. In 142 studies (spanning >60 countries) identified, variation was apparent-de novo APL represented from 2% (Switzerland) to >50% (Nicaragua) of childhood AML in different geographic regions. Because a limited number of previous studies addressed specific environmental exposures that potentially underlie childhood APL development, we gathered 28 childhood cases of therapy-related APL, which exemplified associations between prior exposures to chemotherapeutic drugs/radiation and APL diagnosis. Future population-based studies examining childhood APL patterns and the potential association with specific environmental exposures and other risk factors are needed.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Extrinsic apoptosis is impeded by direct binding of the APL fusion protein NPM-RAR to TRADD

A subset of acute promyelocytic leukemia (APL) cases has been characterized by the t(5;17)(q35;q21) translocation variant, which fuses nucleophosmin (NPM) to retinoic acid receptor α (RARA). The resultant NPM-RAR fusion protein blocks myeloid differentiation and leads to a leukemic phenotype similar to that caused by the t(15;17)(q22;q21) PML-RAR fusion. The contribution of the N-terminal 117 amino acids of NPM contained within NPM-RAR has not been well studied. As a molecular chaperone, NPM interacts with a variety of proteins implicated in leukemogenesis. Therefore, a proteomic analysis was conducted to identify novel NPM-RAR-associated proteins. TNF receptor type I-associated DEATH domain protein (TRADD) was identified as a relevant binding partner for NPM-RAR. This interaction was validated by coprecipitation and colocalization analysis. Biologic assessment found that NPM-RAR expression impaired TNF-induced signaling through TRADD, blunting TNF-mediated activation of caspase-3 (CASP3) and caspase-8 (CASP8), to ultimately block apoptosis.

IMPLICATIONS

This study identifies a novel mechanism through which NPM-RAR affects leukemogenesis.

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.

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

The majority of acute promyelocytic leukemia (APL) cases are characterized by the PML-RARα fusion gene. Although the PML-RARα fusion gene can be detected in >98% of APL cases, RARα is also found to be fused with other partner genes, which are also related to all-trans retinoic acid (ATRA)-dependent transcriptional activity and cell differentiation. In this study, we identified a novel RARα fusion gene, TBLR1-RARα (GenBank KF589333), in a rare case of APL with a t(3;17)(q26;q21),t(7;17)(q11.2;q21) complex chromosomal rearrangement. To our knowledge, TBLR1-RARα is the 10th RARα chimeric gene that has been reported up to now. TBLR1-RARα contained the B-F domains of RARα and exhibited a distinct subcellular localization. It could form homodimers and also heterodimers with retinoid X receptor α. As a result, TBLR1-RARα exhibited diminished transcriptional activity by recruitment of more transcriptional corepressors compared with RARα. In the presence of pharmacologic doses of ATRA, TBLR1-RARα could be degraded, and its homodimerization was abrogated. Moreover, when treated with ATRA, TBLR1-RARα could mediate the dissociation and degradation of transcriptional corepressors, consequent transactivation of RARα target genes, and cell differentiation induction in a dose- and time-dependent manner.

RARA fusion genes in acute promyelocytic leukemia: a review

The t(15;17)(q24;q21), generating a PML-RARA fusion gene, is the hallmark of acute promyelocytic leukemia (APL). At present, eight other genes fusing with RARA have been identified. The resulting fusion proteins retain domains of the RARA protein allowing binding to retinoic acid response elements (RARE) and dimerization with the retinoid X receptor protein (RXRA). They participate in protein-protein interactions, associating with RXRA to form hetero-oligomeric complexes that can bind to RARE. They have a dominant-negative effect on wild-type RARA/RXRA transcriptional activity. Moreover, RARA fusion proteins can homodimerize, conferring the ability to regulate an expanded repertoire of genes normally not affected by RARA. RARA fusion proteins behave as potent transcriptional repressors of retinoic acid signalling, inducing a differentiation blockage at the promyelocyte stage which can be overcome with therapeutic doses of ATRA or arsenic trioxide. However, resistance to these two drugs is a major problem, which necessitates development of new therapies.

Retinoid receptor signaling and autophagy in acute promyelocytic leukemia

Retinoids are a family of signaling molecules derived from vitamin A with well established roles in cellular differentiation. Physiologically active retinoids mediate transcriptional effects on cells through interactions with retinoic acid (RARs) and retinoid-X (RXR) receptors. Chromosomal translocations involving the RARα gene, which lead to impaired retinoid signaling, are implicated in acute promyelocytic leukemia (APL). All-trans-retinoic acid (ATRA), alone and in combination with arsenic trioxide (ATO), restores differentiation in APL cells and promotes degradation of the abnormal oncogenic fusion protein through several proteolytic mechanisms. RARα fusion-protein elimination is emerging as critical to obtaining sustained remission and long-term cure in APL. Autophagy is a degradative cellular pathway involved in protein turnover. Both ATRA and ATO also induce autophagy in APL cells. Enhancing autophagy may therefore be of therapeutic benefit in resistant APL and could broaden the application of differentiation therapy to other cancers. Here we discuss retinoid signaling in hematopoiesis, leukemogenesis, and APL treatment. We highlight autophagy as a potential important regulator in anti-leukemic strategies.

Synergy against PML-RARa: targeting transcription, proteolysis, differentiation, and self-renewal in acute promyelocytic leukemia

Pandolfi et al. provide an in-depth discussion on the synergism between all-trans-retinoic acid and arsenic trioxide treatment and their mechanisms of action on acute promyelocytic leukemia.

Acute promyelocytic leukemia (APL) is a hematological malignancy driven by a chimeric oncoprotein containing the C terminus of the retinoic acid receptor-a (RARa) fused to an N-terminal partner, most commonly promyelocytic leukemia protein (PML). Mechanistically, PML-RARa acts as a transcriptional repressor of RARa and non-RARa target genes and antagonizes the formation and function of PML nuclear bodies that regulate numerous signaling pathways. The empirical discoveries that PML-RARa–associated APL is sensitive to both all-trans-retinoic acid (ATRA) and arsenic trioxide (ATO), and the subsequent understanding of the mechanisms of action of these drugs, have led to efforts to understand the contribution of molecular events to APL cell differentiation, leukemia-initiating cell (LIC) clearance, and disease eradication in vitro and in vivo. Critically, the mechanistic insights gleaned from these studies have resulted not only in a better understanding of APL itself, but also carry valuable lessons for other malignancies.

Role of arsenic trioxide in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a unique subtype of acute myeloid leukemia that is characterized by distinct clinical, morphological, cytogenetic, and molecular abnormalities. It is associated with a striking risk of early hemorrhagic death due to disseminated intravascular coagulation and hyperfibrinolysis. The prognosis of APL has improved dramatically following the introduction of all-trans retinoic acid (ATRA) and its combination with anthracycline-based chemotherapy during induction and consolidation. Patients with high-risk APL, defined by a white cell count >10 × 10(9)/L at diagnosis, also appear to benefit from the addition of intermediate- or high-dose cytarabine during consolidation. Arsenic trioxide (ATO) has proved to be even more effective than ATRA as a single agent, and is now routinely used for the treatment of the 20%-30% of patients who manifest disease relapse after initial treatment with ATRA and chemotherapy. ATO has a toxicity profile that differs considerably from that of both ATRA and cytotoxic chemotherapy, and accordingly presents its own specific challenges during treatment. Optimizing a strategy for the incorporation of ATO into initial therapy is currently the focus of several cooperative group trials, with an emphasis on minimizing or even eradicating the use of chemotherapy. ATRA plus ATO without chemotherapy appears to be adequate during induction and consolidation for patients with standard-risk APL, but triple therapy that includes limited anthracycline or gemtuzumab ozogamicin (GO) during induction is required for high-risk APL. Uncertainty still exists regarding the minimum amount of chemotherapy and number of consolidation cycles necessary, the optimal scheduling of ATO, and the potential utility of oral ATO administration. Although prolonged oral maintenance therapy is usually included in most current APL treatment protocols, its value remains controversial, and the superior anti-leukemic efficacy of ATO-based therapy may facilitate its elimination in the future.

Interaction with RXR is necessary for NPM-RAR-induced myeloid differentiation blockade

The t(5;17)(q35;q21) APL variant results in expression of a fusion protein linking the N-terminus of nucleophosmin (NPM) to the C-terminus of the retinoic acid receptor alpha (RAR). We have previously shown that NPM-RAR is capable of binding to DNA either as a homodimer or heterodimer with RXR. To determine the biological significance of NPM-RAR/RXR interaction, we developed two mutants of NPM-RAR that showed markedly diminished ability to bind RXR. U937 subclones expressing the NPM-RAR mutants showed significantly less inhibition of vitamin D3/TGFbeta-induced differentiation, compared with NPM-RAR. These results support the hypothesis that RXR interaction is necessary for NPM-RAR-mediated myeloid maturation arrest.