Myeloid/Lymphoid Neoplasms Associated With Eosinophilia and Rearrangements of PDGFRA, PDGFRB, or FGFR1 or With PCM1-JAK2

World Health Organization and International Consensus Classification of eosinophilic disorders: 2024 update on diagnosis, risk stratification, and management

DISEASE OVERVIEW

The eosinophilias encompass a broad range of non-hematologic (secondary or reactive) and hematologic (primary or clonal) disorders with the potential for end-organ damage.

DIAGNOSIS

Hypereosinophilia (HE) has generally been defined as a peripheral blood eosinophil count greater than 1.5 × 109/L, and may be associated with tissue damage. After the exclusion of secondary causes of eosinophilia, diagnostic evaluation of primary eosinophilias relies on a combination of various tests. They include morphologic review of the blood and marrow, standard cytogenetics, fluorescence in situ hybridization, molecular testing and flow immunophenotyping to detect histopathologic or clonal evidence for an acute or chronic hematolymphoid neoplasm.

RISK STRATIFICATION

Disease prognosis relies on identifying the subtype of eosinophilia. After evaluation of secondary causes of eosinophilia, the 2022 World Health Organization and International Consensus Classification endorse a semi-molecular classification scheme of disease subtypes. This includes the major category "myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions" (MLN-eo-TK), and the MPN subtype, "chronic eosinophilic leukemia" (CEL). Lymphocyte-variant HE is an aberrant T-cell clone-driven reactive eosinophila, and idiopathic hypereosinophilic syndrome (HES) is a diagnosis of exclusion.

RISK-ADAPTED THERAPY

The goal of therapy is to mitigate eosinophil-mediated organ damage. For patients with milder forms of eosinophilia (e.g., <1.5 × 109/L) without symptoms or signs of organ involvement, a watch and wait approach with close follow-up may be undertaken. Identification of rearranged PDGFRA or PDGFRB is critical because of the exquisite responsiveness of these diseases to imatinib. Pemigatinib was recently approved for patients with relapsed or refractory FGFR1-rearranged neoplasms. Corticosteroids are first-line therapy for patients with lymphocyte-variant HE and HES. Hydroxyurea and interferon-α have demonstrated efficacy as initial treatment and in steroid-refractory cases of HES. Mepolizumab, an interleukin-5 (IL-5) antagonist monoclonal antibody, is approved by the U.S Food and Drug Administration for patients with idiopathic HES. Cytotoxic chemotherapy agents, and hematopoietic stem cell transplantation have been used for aggressive forms of HES and CEL, with outcomes reported for limited numbers of patients. Targeted therapies such as the IL-5 receptor antibody benralizumab, IL-5 monoclonal antibody depemokimab, and various tyrosine kinase inhibitors for MLN-eo-TK, are under active investigation.

Section E6.1-6.6 of the American College of Medical Genetics and Genomics (ACMG) Technical Laboratory Standards: Cytogenomic studies of acquired chromosomal abnormalities in neoplastic blood, bone marrow, and lymph nodes

Cytogenomic analyses of acquired clonal chromosomal abnormalities in neoplastic blood, bone marrow, and/or lymph nodes are instrumental in the clinical management of patients with hematologic neoplasms. Cytogenetic analyses assist in the diagnosis of such disorders and can provide important prognostic information. Furthermore, cytogenetic studies can provide crucial information regarding specific genetically defined subtypes of these neoplasms that may have targeted therapies. At time of relapse, cytogenetic analysis can confirm recurrence of the original neoplasm, detect clonal disease evolution, or uncover a new unrelated neoplastic process. This section deals specifically with the technical standards applicable to cytogenomic studies of acquired clonal chromosomal abnormalities in neoplastic blood, bone marrow, and/or lymph nodes. This updated Section E6.1-6.6 supersedes the previous Section E6 in Section E: Clinical Cytogenetics of the American College of Medical Genetics and Genomics Technical Standards for Clinical Genetics Laboratories.

Hematological Neoplasms with Eosinophilia

Eosinophils in peripheral blood account for 0.3-5% of leukocytes, which is equivalent to 0.05-0.5 × 109/L. A count above 0.5 × 109/L is considered to indicate eosinophilia, while a count equal to or above 1.5 × 109/L is defined as hypereosinophilia. In bone marrow aspirate, eosinophilia is considered when eosinophils make up more than 6% of the total nuclear cells. In daily clinical practice, the most common causes of reactive eosinophilia are non-hematologic, whether they are non-neoplastic (allergic diseases, drugs, infections, or immunological diseases) or neoplastic (solid tumors). Eosinophilia that is associated with a hematological malignancy may be reactive or secondary to the production of eosinophilopoietic cytokines, and this is mainly seen in lymphoid neoplasms (Hodgkin lymphoma, mature T-cell neoplasms, lymphocytic variant of hypereosinophilic syndrome, and B-acute lymphoblastic leukemia/lymphoma). Eosinophilia that is associated with a hematological malignancy may also be neoplastic or primary, derived from the malignant clone, usually in myeloid neoplasms or with its origin in stem cells (myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions, acute myeloid leukemia with core binding factor translocations, mastocytosis, myeloproliferative neoplasms, myelodysplastic/myeloproliferative neoplasms, and myelodysplastic neoplasms). There are no concrete data in standardized cytological and cytometric procedures that could predict whether eosinophilia is reactive or clonal. The verification is usually indirect, based on the categorization of the accompanying hematologic malignancy. This review focuses on the broad differential diagnosis of hematological malignancies with eosinophilia.

Expanding the spectrum of tyrosine kinase fusions in calcified chondroid mesenchymal neoplasms: Identification of a novel PDGFRA::USP8 gene fusion

Calcified chondroid mesenchymal neoplasms represent a distinct, and recently recognized, spectrum of tumors. To date most cases have been reported to be characterized by FN1 gene fusions involving multiple potential tyrosine kinase partners. Following incidental identification of a tumor morphologically corresponding to calcified chondroid mesenchymal neoplasm, but with a PDGFRA::USP8 gene fusion, we undertook a retrospective review to identify and characterize additional such cases. A total of four tumors were identified. Each was multilobulated and composed of polygonal-epithelioid-stellate cells with a background of chondroid matrix containing distinctive patterns of calcification. Targeted RNA sequencing revealed an identical PDGFRA (exon 22)::USP8 (exon 5) gene fusion in each case. Subsequent immunohistochemical staining confirmed the presence of PDGFRα overexpression. In summary, we report a series of four tumors within the morphologic spectrum of calcified chondroid mesenchymal neoplasms. In contrast to prior reports, these tumors harbored a novel PDGFRA::USP8 gene fusion, rather than FN1 rearrangement. Our findings expand the molecular diversity of these neoplasms, and suggest they are united through activation of protein kinases.

Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase fusion genes: A workshop report with focus on novel entities and a literature review including paediatric cases

Myeloid/lymphoid neoplasms with eosinophilia (M/LN-eo) and tyrosine kinase (TK) gene fusions are a rare group of haematopoietic neoplasms with a broad range of clinical and morphological presentations. Paediatric cases have increasingly been recognised. Importantly, not all appear as a chronic myeloid neoplasm and eosinophilia is not always present. In addition, standard cytogenetic and molecular methods may not be sufficient to diagnose M/LN-eo due to cytogenetically cryptic aberrations. Therefore, additional evaluation with fluorescence in-situ hybridisation and other molecular genetic techniques (array-based comparative genomic hybridisation, RNA sequencing) are recommended for the identification of specific TK gene fusions. M/LN-eo with JAK2 and FLT3-rearrangements and ETV6::ABL1 fusion were recently added as a formal member to this category in the International Consensus Classification (ICC) and the 5th edition of the WHO classification (WHO-HAEM5). In addition, other less common defined genetic alterations involving TK genes have been described. This study is an update on M/LN-eo with TK gene fusions with focus on novel entities, as illustrated by cases submitted to the Bone Marrow Workshop, organised by the European Bone Marrow Working Group (EBMWG) within the frame of the 21st European Association for Haematopathology congress (EAHP-SH) in Florence 2022. A literature review was performed including paediatric cases of M/LN-eo with TK gene fusions.

Clinical Validation of FusionPlex RNA Sequencing and Its Utility in the Diagnosis and Classification of Hematologic Neoplasms

Recurrent gene rearrangements result in gene fusions that encode chimeric proteins, driving the pathogenesis of many hematologic neoplasms. The fifth edition World Health Organization classification and International Consensus Classification 2022 include an expanding list of entities defined by such gene rearrangements. Therefore, sensitive and rapid methods are needed to identify a broad range of gene fusions for precise diagnosis and prognostication. In this study, we validated the FusionPlex Pan-Heme panel analysis using anchored multiplex PCR/targeted RNA next-generation sequencing for routine clinical testing. Furthermore, we assessed its utility in detecting gene fusions in myeloid and lymphoid neoplasms. The validation cohort of 61 cases demonstrated good concordance between the FusionPlex Pan-Heme panel and other methods, including chromosome analysis, fluorescence in situ hybridization, RT-PCR, and Sanger sequencing, with an analytic sensitivity and specificity of 95% and 100%, respectively. In an independent cohort of 28 patients indicated for FusionPlex testing, gene fusions were detected in 21 patients. The FusionPlex Pan-Heme panel analysis reliably detected fusion partners and patient-specific fusion sequences, allowing accurate classification of hematologic neoplasms and the discovery of new fusion partners, contributing to a better understanding of the pathogenesis of the diseases.

The role of flow cytometry in the classification of myeloid disorders

The World Health Organization classification (WHO-HAEM5) and the International Consensus Classification (ICC 2022) of myeloid neoplasms are based on the integration of clinical, morphologic, immunophenotypic, and genomic data. Flow cytometric immunophenotyping (FCIP) allows the identification, enumeration, and characterization of hematopoietic cells, and is therefore a powerful tool in the diagnosis, classification, and monitoring of hematological neoplasms. The vast majority of flow cytometry (FCM) studies in chronic myeloid neoplasms focus on its role in myelodysplastic neoplasms (MDS). FCM can also be helpful for the assessment of myeloproliferative neoplasms (MPN) and MDS/MPN, including the early detection of evolving myeloid or lymphoid blast crisis and the characterization of monocytic subsets. The classification of acute myeloid leukemia (AML) is primarily based on cytogenetic and molecular findings; however, FCIP is needed for subclassification of AML, not otherwise specified (NOS; ICC)/AML defined by differentiation (WHO-HAEM5). The main role of FCM in AML remains in making a rapid diagnosis and as a tool for measurable residual disease monitoring. Machine learning and artificial intelligence approaches can be used to analyze and classify FCM data. This article, based on an invited lecture at the 106th Annual Meeting of the German Society of Pathology in 2023, reviews the role of FCM in the classification of myeloid neoplasms, including recent publications on the application of artificial intelligence.

Myeloid/Lymphoid Neoplasms with ETV6::PDGFRB Fusion Gene: A Rare Case of Poor Response to Imatinib and Possible Transformation Mechanisms from Myeloid Neoplasms of Bone Marrow to T-Cell Lymphoblastic Lymphoma Invasion in Lymph Nodes

The ETV6::PDGFRB fusion gene is commonly reported in chronic myelomonocytic leukemia with eosinophilia, yet patients with ETV6::PDGFRB presenting myeloid and lymphoid neoplasms successively have not been reported. Here, we report the first case of a 35-year-old man with myeloid and lymphoid neoplasms harboring an ETV6::PDGFRB fusion gene who demonstrated poor response to imatinib. The patient was diagnosed with an ETV6::PDGFRB fusion gene myeloid neoplasm on initial diagnosis at our hospital. After 5 months of treatment with imatinib, he was diagnosed with T-cell lymphoblastic lymphoma. ETV6::PDGFRB turned negative after increasing the dose of imatinib, but enlarged superficial lymph nodes reappeared the following year. Notably, the patient exhibited a worse response to imatinib treatment. This study describes this rare case and speculates on a possible mechanism.

A novel subclonal rearrangement of the STRN3::PDGFRB gene in de novo acute myeloid leukemia with NPM1 mutation and its leukemogenic effects

Chromosome translocations in the 5q31-33 region are associated with a range of hematologic malignancies, some of which involve the platelet-derived growth factor receptor beta (PDGFRB) gene. We report a case of acute myeloid leukemia (AML) with a mutation in the NPM1 gene (NPM1-mut AML) and a subclonal gene rearrangement involving the PDGFRB gene. We identified a novel fusion gene, STRN3::PDGFRB, resulting from t(5;14) (q32;q12) chromosomal rearrangement. Sequential FISH confirmed that ~15% of leukemic cells carried the PDGFRB gene rearrangement, which suggests that STRN3::PDGFRB is a previously unreported fusion gene in a subclone. Reverse transcription PCR (RT-PCR) and Sanger sequencing confirmed that the fusion gene consisted of STRN3 exon 7 fused to PDGFRB exon 11, resulting in a chimeric protein containing the coiled-coil domain of striatin-3 and the transmembrane and intracellular tyrosine kinase domains of the PDGFRB. The new protein exhibited distinct cytoplasmic localization and had leukemogenic effects, as demonstrated by its ability to transform Ba/F3 cells to growth factor independence and cause a fatal myelodysplastic/myeloproliferative neoplasm (MDS/MPN)-like disease in mice, which then transformant to T-cell lymphoblastic lymphoma in secondary recipients. Ba/F3 cells expressing STRN3::PDGFRB or ETV6::PDGFRB were sensitive to tyrosine kinase inhibitors (TKIs) and selinexor, but in vitro experiments showed that the combination of imatinib and selinexor had a marked synergistic effect, although only the imatinib alone group could prolong the survival of T-cell blast transformation recipient mice. Our findings demonstrate the leukemogenic effects of the novel fusion gene and provide insights into the clone evolution of AML, which can be influenced by therapy selection. Furthermore, our results provide insight into the potential therapeutic options for patients with this type of mutation, as well as the need for careful consideration of treatment selection to prevent undesirable side effects.

Acute myeloid leukemia with LRRFIP1::FGFR1 rearrangement and a complex karyotype

We report a case of a 20-year-old man who presented with splenomegaly, hyperleukocytosis, anemia, and thrombocytopenia. A diagnosis of acute myeloid leukemia (AML) with LRRFIP1::FGFR1 rearrangement with complex karyotype was determined. Chromosome analysis showed a male karyotype: 46,XY,i(1)(q10),t(2;8)(q37;p11.2),der(5)t(1;5) (p22;q13)[17]46,XY[3]. Fluorescence in situ hybridization (FISH) analysis using the Cytocell FGFR1 break apart/amplification probe detected FGFR1 rearrangement with t(2:8) in 126/200 cells analyzed. Other FISH probes including 1p36/ 1q25 probes, del(5q) deletion probe, TLX3 break apart probe, and PDGFRB break apart probe were also utilized to confirm the other karyotypic abnormalities. Next-generation sequencing (NGS) SureSelectXT Custom DNA Target Somatic Detection detected RUNX1 gene mutation. NGS Archer FusionPlex (RNA) confirmed the LRRFIP1::FGFR1 rearrangement. This is the second reported case of AML with LRRFIP1::FGFR1 rearrangement and the first with a complex karyotype.

Guide to the Diagnosis of Myeloid Neoplasms: A Bone Marrow Pathology Group Approach

OBJECTIVES

The practicing pathologist is challenged by the ever-increasing diagnostic complexity of myeloid neoplasms. This guide is intended to provide a general roadmap from initial case detection, often triggered by complete blood count results with subsequent blood smear review, to final diagnosis.

METHODS

The integration of hematologic, morphologic, immunophenotypic, and genetic features into routine practice is standard of care. The requirement for molecular genetic testing has increased along with the complexity of test types, the utility of different testing modalities in identifying key gene mutations, and the sensitivity and turnaround time for various assays.

RESULTS

Classification systems for myeloid neoplasms have evolved to achieve the goal of providing a pathology diagnosis that enhances patient care, outcome prediction, and treatment options for individual patients and is formulated, endorsed, and adopted by hematologists/oncologists.

CONCLUSIONS

This guide provides diagnostic strategies for all myeloid neoplasm subtypes. Special considerations are provided for each category of testing and neoplasm category, along with classification information, genetic testing requirements, interpretation information, and case reporting recommendations based on the experience of 11 Bone Marrow Pathology Group members.

The multidisciplinary approach to eosinophilia

Eosinophilic granulocytes are normally present in low numbers in the bloodstream. Patients with an increased number of eosinophilic granulocytes in the differential count (eosinophilia) are common and can pose a clinical challenge because conditions with eosinophilia occur in all medical specialties. The diagnostic approach must be guided by a thorough medical history, supported by specific tests to guide individualized treatment. Neoplastic (primary) eosinophilia is identified by one of several unique acquired genetic causes. In contrast, reactive (secondary) eosinophilia is associated with a cytokine stimulus in a specific disease, while idiopathic eosinophilia is a diagnosis by exclusion. Rational treatment is disease-directed in secondary cases and has paved the way for targeted treatment against the driver in primary eosinophilia, whereas idiopathic cases are treated as needed by principles in eosinophilia originating from clonal drivers. The vast majority of patients are diagnosed with secondary eosinophilia and are managed by the relevant specialty-e.g., rheumatology, allergy, dermatology, gastroenterology, pulmonary medicine, hematology, or infectious disease. The overlap in symptoms and the risk of irreversible organ involvement in eosinophilia, irrespective of the cause, warrants that patients without a diagnostic clarification or who do not respond to adequate treatment should be referred to a multidisciplinary function anchored in a hematology department for evaluation. This review presents the pathophysiology, manifestations, differential diagnosis, diagnostic workup, and management of (adult) patients with eosinophilia. The purpose is to place eosinophilia in a clinical context, and therefore justify and inspire the establishment of a multidisciplinary team of experts from diagnostic and clinical specialties at the regional level to support the second opinion. The target patient population requires highly specialized laboratory analysis and therapy and occasionally has severe eosinophil-induced organ dysfunction. An added value of a centralized, clinical function is to serve as a platform for education and research to further improve the management of patients with eosinophilia. Primary and idiopathic eosinophilia are key topics in the review, which also address current research and discusses outstanding issues in the field.

Clinicopathologic characteristics, genetic features, and treatment options for acute lymphoblastic leukemia with JAK2 rearrangement-A 10-case study and literature review

JAK2 rearrangement (JAK2-R) in acute lymphoblastic leukemia (ALL) is rare and often categorized as B-ALL with BCR::ABL1-like features based on the World Health Organization classification. We report 10 patients with JAK2-R ALL, 9 males and 1 female, with a median age 40.5 years. Eight patients presented with marked leukocytosis (median WBC, 63 × 10 ⁹/L) and hypercellular (>95%) bone marrow with increased lymphoblasts (72%-95%). There was no evidence of bone marrow fibrosis or hypereosinophilia. Immunophenotypic analysis showed 9 B-cell and 1 T-cell neoplasms. Using fluorescence in situ hybridization (FISH) and RNA sequencing analysis, JAK2 partners were identified for 7 cases and included PCM1 (n=4), ETV6 (n=2) and BCR (n=1). All patients received upfront polychemotherapy. Additionally, 2 patients received ruxolitinib, 2 received allogeneic stem cell transplant, and 1 received CAR-T therapy. The 1- and 3-year overall survival rates were 55.6% and 22.2%, respectively. A literature review identified 24 B-ALL and 4 T-ALL cases with JAK2-R reported, including 16 males, 6 females and 6 gender not stated. Many JAK2 partner-genes were reported with the most common being PAX5 (n=7), ETV6 (n=4), BCR (n=3) and PCM1 (n=2). Survival data or 13 reported cases showed 1- and 3-year overall survival rates of 41.7% and 41.7%, respectively. In summary, JAK2-R ALL occurs more often in adult males, are mostly of B-cell lineage, and associated with an aggressive clinical course. Absence of eosinophilia and bone marrow fibrosis and no evidence of preexisting/concurrent JAK2-R myeloid neoplasms distinguish JAK2-R ALL from other myeloid/lymphoid neoplasms with eosinophilia and JAK2-R.

Clinical Response to Upfront Targeted Tyrosine Kinase Inhibitors among Patients with Myeloid/Lymphoid Neoplasms with Eosinophilia and Tyrosine Kinase Gene Fusion

INTRODUCTION

Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusion (MLN-TK) is an entity encompassed of a heterogeneous group of rare hematopoietic neoplasms that are driven by gene fusion involving PDGDRA/B, FGFR1, JAK2, FLT3 or ETV6::ABL1. Though patients presenting with chronic phase MLN-TK with PDGFRA fusion display a favorable outcome in response to upfront TK inhibitor (TKI) therapy, the outcomes of MLNs driven by other TK fusions are not well described. In this study, we aimed to critically analyze the treatment outcomes of patients with MLN-TK, focusing on the role of upfront TKIs in both chronic- and blast-phase diseases.

METHODS

The retrospective study included patients with confirmed MLN-TK from 3 centers and assessed demographic and clinical variables, treatment, and outcomes.

RESULTS

Forty-two patients with confirmed MLN-TK [PDGFRA (n = 22), PDGFRB (n = 4), FGFR1(n = 10), JAK2 (n = 2); and FLT3 (n = 3)] were included. Fifteen of 25 (60%) chronic-phased patients received upfront TKI therapy had a long-term remission. Nine of 16 (60%) blast-phase patients with upfront TKIs also achieved complete remission and remained alive at a median follow-up of 20 months. All 3 patients with blast phase disease who received upfront chemotherapy without positive response did not respond to subsequent TKI therapy, emphasizing the importance of initiating TKI therapy early. Upfront TKI therapy was associated with longer overall survival in univariate analyses (HR, 0.054 [95% CI, 0.007-0.42]) and multivariate analyses (HR, 0.03 [95% CI, 0.002-0.47]).

CONCLUSION

The outcomes of upfront TKI therapy are excellent for MLN-TK in both chronic and blast phases, regardless of gene abnormalities.

Case of cryptic TNIP1::PDGFRB rearrangement presenting with myelodysplastic syndrome achieved hematologic and cytogenetic remission with low-dose imatinib plus decitabine therapy

For a long time, FIP1L1::PDGFRA fusion seems to be the only cryptic rearrangement of myeloid/lymphoid neoplasm with tyrosine kinase gene fusions. Recently, with the wide application of RNA sequencing, more cryptic rearrangements of other TK genes have been identified, especially the PDGFRB. Here we report a case of myelodysplastic syndrome with severe thrombocytopenia. Conventional karyotype analysis revealed a t (5;19) (q33; p13.2) but no PDGFRB rearrangement was detected by the PDGFRB break-apart probe. The TNIP1::PDGFRB fusion was eventually found by RNA sequencing, leading us to treat with low-dose imatinib plus decitabine, and the patient achieved hematologic improvement and cytogenetic remission.

Updates on eosinophilic disorders

This review addresses changes and updates in eosinophilic disorders under the International Consensus Classification (ICC). The previous category of myeloid/lymphoid neoplasm with eosinophilia (M/LN-eo) and a specific gene rearrangement is changed to M/LN-eo with tyrosine kinase gene fusions to reflect the underlying genetic lesions. Two new members, M/LN-eo with ETV6::ABL1 fusion and M/LN-eo with various FLT3 fusions, have been added to the category; and M/LN-eo with PCM1::JAK2 and its genetic variants ETV6::JAK2 and BCR::JAK2 are recognized as a formal entity from their former provisional status. The updated understanding of the clinical and molecular genetic features of PDGFRA, PDGFRB and FGFR1 neoplasms is summarized. Clear guidance as to how to distinguish these fusion gene-associated disorders from the overlapping entities of Ph-like B-acute lymphoblastic leukemia (ALL), de novo T-ALL, and systemic mastocytosis is provided. Bone marrow morphology now constitutes one of the diagnostic criteria of chronic eosinophilic leukemia, NOS (CEL, NOS), and idiopathic hypereosinophilia/hypereosinophilic syndrome (HE/HES), facilitating the separation of a true myeloid neoplasm with characteristic eosinophilic proliferation from those of unknown etiology and not attributable to a myeloid neoplasm.

International Consensus Classification of Myeloid Neoplasms and Acute Leukemias: integrating morphologic, clinical, and genomic data

The classification of myeloid neoplasms and acute leukemias was last updated in 2016 within a collaboration between the World Health Organization (WHO), the Society for Hematopathology, and the European Association for Haematopathology. This collaboration was primarily based on input from a clinical advisory committees (CACs) composed of pathologists, hematologists, oncologists, geneticists, and bioinformaticians from around the world. The recent advances in our understanding of the biology of hematologic malignancies, the experience with the use of the 2016 WHO classification in clinical practice, and the results of clinical trials have indicated the need for further revising and updating the classification. As a continuation of this CAC-based process, the authors, a group with expertise in the clinical, pathologic, and genetic aspects of these disorders, developed the International Consensus Classification (ICC) of myeloid neoplasms and acute leukemias. Using a multiparameter approach, the main objective of the consensus process was the definition of real disease entities, including the introduction of new entities and refined criteria for existing diagnostic categories, based on accumulated data. The ICC is aimed at facilitating diagnosis and prognostication of these neoplasms, improving treatment of affected patients, and allowing the design of innovative clinical trials.

SOHO State of the Art Updates and Next Questions | Myeloid/Lymphoid Neoplasms with Eosinophilia and Gene Rearrangements: Diagnostic Pearls and Pitfalls

The myeloid and/or lymphoid neoplasms with eosinophilia and gene rearrangement (MLN-Eos) are a rare group of hematopoietic neoplasms with diverse and often perplexing presentations that can cause challenges, and even potential pitfalls, for the diagnostic pathologist. However, accurate diagnosis of this group of disorders is of the utmost importance, since the presence of specific gene rearrangements dictates targeted patient therapy. The goal of this review is to discuss the current literature, including emergence of novel molecular data, and equip pathologists and clinicians with morphologic and immunophenotypic clues for diagnosing this challenging group of hematopoietic neoplasms.

Case Report: Pediatric myeloid/lymphoid neoplasm with eosinophilia and PDGFRA rearrangement: The first case presenting as B-lymphoblastic lymphoma

According to the latest WHO classification of hematopoietic malignancies, myeloid and lymphoid neoplasms with eosinophilia and gene rearrangements include three specific rare diseases and one provisional entity. Myeloid/lymphoid neoplasms with platelet-derived growth factor receptor alpha (PDGFRA) rearrangements are the most frequent of these disorders and are usually present in adult males with a median age of the late 40s. Patients usually have chronic eosinophilic leukemia but can occasionally manifest as acute myeloid leukemia or extramedullary T- or B-lineage lymphoblastic lymphoma. We report a case of a previously healthy 2-year-old girl who presented with a right supraorbital swelling with no associated lymphadenopathy. Peripheral blood smear evaluation at initial presentation revealed microcytic hypochromic red blood cells and leukocytosis with marked eosinophilia, occasional myelocytes, and occasional blasts. Whole-body CT scans and PET scans revealed hypermetabolic potentially lymphomatous mass in the superior medial aspect of the right orbit in addition to splenomegaly but no evidence of hypermetabolic mediastinal, hilar, abdominal, or pelvic lymph nodes. Bone marrow aspirate and biopsy revealed hypercellular bone marrow with quantitatively decreased erythroid precursors and increased granulocytic precursors with 60% of the cells being eosinophilic cells in different stages of maturation. The diagnosis of myeloid neoplasm with eosinophilia and rearrangement of PDGFRA was made following confirmation by fluorescence in situ hybridization (FISH) test for FIP1L1-PDGFRA gene fusion. An incisional biopsy of the supraorbital mass revealed B-cell lymphoblastic lymphoma (B-LBL). FISH test for FIP1L1-PDGFRA gene fusion was positive in 70% of the cells studied. Thus, the final diagnosis was B-cell lymphoblastic lymphoma arising in the setting of myeloid/lymphoid neoplasm with eosinophilia and PDGFRA rearrangement. The patient was started on imatinib with concomitant therapy for B-LBL per the Children Oncology Group (COG) standard therapy for localized B-LBL and demonstrated a favorable outcome in the 2.5-year follow-up period. To our knowledge, this is the first pediatric case of myeloid/lymphoid neoplasm with PDGFRA rearrangement presenting with synchronous myeloproliferative disease and B-LBL. We present our diagnostic and management approach of this patient and review prior relevant pediatric cases of myeloid/lymphoid neoplasms with PDGFRA rearrangement.

World Health Organization-defined eosinophilic disorders: 2022 update on diagnosis, risk stratification, and management

DISEASE OVERVIEW

The eosinophilias encompass a broad range of nonhematologic (secondary or reactive) and hematologic (primary or clonal) disorders with potential for end-organ damage.

DIAGNOSIS

Hypereosinophilia (HE) has generally been defined as a peripheral blood eosinophil count greater than 1.5 × 10⁹ /L. After exclusion of secondary causes of eosinophilia, diagnostic evaluation of primary eosinophilias relies on morphologic review of the blood and marrow, standard cytogenetics, fluorescence in situ hybridization, next generation sequencing gene assays, and flow immunophenotyping to detect histopathologic or clonal evidence for an acute or chronic hematolymphoid neoplasm.

RISK STRATIFICATION

Disease prognosis relies on identifying the subtype of eosinophilia. After evaluation of secondary causes of eosinophilia, the 2016 World Health Organization endorses a semi-molecular classification scheme of disease subtypes. This includes the major category "myeloid/lymphoid neoplasms with eosinophilia and rearrangement of PDGFRA, PDGFRB, or FGFR1 or with PCM1-JAK2", and the myeloproliferative neoplasm subtype, "chronic eosinophilic leukemia, not otherwise specified" (CEL, NOS). Lymphocyte-variant HE is an aberrant T-cell clone-driven reactive eosinophila, and idiopathic hypereosinophilic syndrome (HES) is a diagnosis of exclusion.

RISK-ADAPTED THERAPY

The goal of therapy is to mitigate eosinophil-mediated organ damage. For patients with milder forms of eosinophilia (eg, < 1.5 × 10⁹ /L) without symptoms or signs of organ involvement, a watch and wait approach with close follow-up may be undertaken. Identification of rearranged PDGFRA or PDGFRB is critical because of the exquisite responsiveness of these diseases to imatinib. Corticosteroids are first-line therapy for patients with lymphocyte-variant HE and HES. Hydroxyurea and interferon-α have demonstrated efficacy as initial treatment and in steroid-refractory cases of HES. Mepolizumab, an interleukin-5 (IL-5) antagonist monoclonal antibody, was recently approved by the US Food and Drug Administration for patients with idiopathic HES. The use of the IL-5 receptor antibody benralizumab, as well as other targeted therapies such as JAK2 and FGFR1 inhibitors, is under active investigation.

A Case of Acute Myeloid Leukemia Harboring a Rare Three-Way Translocation t(5;7;7) Involving the PDGFRB Gene and Successfully Treated with Imatinib

Platelet-derived growth factor-beta (PDGFRB) gene maps for the receptor tyrosine kinase PDGRFβ. PDGFRB gene fusions have been implicated in multiple myeloid and lymphoid neoplasms and have shown exquisite sensitivity to tyrosine kinase inhibitors. We report a case of a 29-year-old male who presented with acute myeloid leukemia who was eventually found to harbor a unique three-way translocation t(5;7;7)(q33.2;q32;q11.2) involving the PDGFRB gene. The patient initially achieved a complete response after induction with daunorubicin and cytarabine, but when he returned for consolidation, his white cell count had increased, and he was found to have an underlying myeloproliferative neoplasm. He was given consolidation with high-dose cytarabine and imatinib with excellent response, and ultimately received a matched unrelated donor transplant. The patient remains in remission to this day more than eight years later.

Genomic and clinical findings in myeloid neoplasms with PDGFRB rearrangement

Platelet-derived growth factor receptor B (PDGFRB) gene rearrangements define a unique subgroup of myeloid and lymphoid neoplasms frequently associated with eosinophilia and characterized by high sensitivity to tyrosine kinase inhibition. To date, various PDGFRB/5q32 rearrangements, involving at least 40 fusion partners, have been reported. However, information on genomic and clinical features accompanying rearrangements of PDGFRB is still scarce. Here, we characterized a series of 14 cases with a myeloid neoplasm using cytogenetic, single nucleotide polymorphism array, and next-generation sequencing. We identified nine PDGFRB translocation partners, including the KAZN gene at 1p36.21 as a novel partner in a previously undescribed t(1;5)(p36;q33) chromosome change. In all cases, the PDGFRB recombination was the sole cytogenetic abnormality underlying the phenotype. Acquired somatic variants were mainly found in clinically aggressive diseases and involved epigenetic genes (TET2, DNMT3A, ASXL1), transcription factors (RUNX1 and CEBPA), and signaling modulators (HRAS). By using both cytogenetic and nested PCR monitoring to evaluate response to imatinib, we found that, in non-AML cases, a low dosage (100–200 mg) is sufficient to induce and maintain longstanding hematological, cytogenetic, and molecular remissions.

Diagnosis and Treatment of Myeloproliferative Neoplasms With PCM1-JAK2 Rearrangement: Case Report and Literature Review

Myeloproliferative neoplasm (MPN) with PCM1-JAK2 rearrangement is a rare disease with poor prognosis and lacks uniform treatment guidelines. Several studies confirmed the efficacy of ruxolitinib in hematological malignancies with PCM1-JAK2 fusion, but the efficacy is variable. Here, we report two patients diagnosed with MPN with PCM1-JAK2 fusion who were treated with ruxolitinib-based regimen, including the first case of ruxolitinib combined with pegylated interferon (Peg-IFN), and we conduct a literature review. We found that ruxolitinib combined with Peg-IFN is an effective treatment option in the case of poor efficacy of ruxolitinib monotherapy.

t(4;12)(q12;p13) ETV6-rearranged AML without eosinophilia does not involve PDGFRA: relevance for imatinib insensitivity

Apparent ETV6-PDGFRA fusions identified by FISH analysis in t(4;12)(q12;p13) AML should be confirmed by sequencing.

Sequence-confirmed ETV6-PDGFRA fusions have not been identified in patients with t(4;12)(q12;p13) AML without eosinophilia.

Acute myeloid leukemia (AML) with t(4;12)(q12;p13) translocation is rare and often associated with an aggressive clinical course and poor prognosis. Previous reports based on fluorescence in situ hybridization (FISH) analysis have suggested that ETV6::PDGFRA fusions are present in these patients, despite the absence of eosinophilia, which is typically found in other hematopoietic malignancies with PDGFRA-containing fusions. We first detected an ETV6-SCFD2 fusion by targeted RNA sequencing in a patient with t(4;12)(q12;p13) who had been diagnosed with an ETV6-PDGFRA fusion by FISH analysis but failed to respond to imatinib. We then retrospectively identified 4 additional patients with AML and t(4;12)(q12;p13) with apparent ETV6-PDGFRA fusions using chromosome and FISH analysis and applied targeted RNA sequencing to archival material. We again detected rearrangements between ETV6 and non-PDGFRA 4q12 genes, including SCFD2, CHIC2, and GSX2. None of the 3 patients who received imatinib based on the incorrect assumption of an ETV6-PDGFRA fusion responded. Our findings highlight the importance of using a sequencing-based assay to confirm the presence of targetable gene fusions, particularly in genomic regions, such as 4q12, with many clinically relevant genes that are too close to resolve by chromosome or FISH analysis. Finally, combining our data and review of the literature, we show that sequence-confirmed ETV6-PDGFRA fusions are typically found in eosinophilic disorders (3/3 cases), and patients with t(4;12)(q12;p13) without eosinophilia are found to have other 4q12 partners on sequencing (17/17 cases).

Gene Rearrangement Detection in Pediatric Leukemia

The detection of gene rearrangements in pediatric leukemia is an essential component of the work-up, with implications for accurate diagnosis, proper risk stratification, and therapeutic decisions, including the use of targeted therapies. The traditional methods of karyotype and fluorescence in situ hybridization are still valuable, but many new assays are also available, with different strengths and weaknesses. These assays include next-generation sequencing-based assays that have the potential for highly multiplexed and/or unbiased detection of rearrangements.

T-Cell Lymphoblastic Lymphoma Arising in the Setting of Myeloid/Lymphoid Neoplasms with Eosinophilia: LMO2 Immunohistochemistry as a Potentially Useful Diagnostic Marker

Simple Summary

Rarely, T-lymphoblastic lymphoma (T-LBL) may develop in the setting of myeloid/lymphoid neoplasms with eosinophilia. Given important therapeutic implications, it is crucial to identify T-LBL arising in this particular context. LIM domain only 2 (LMO2) is known to be overexpressed in almost all sporadic T-LBL and not in immature TdT-positive T-cells in the thymus and in indolent T-lymphoblastic proliferations. We retrospectively evaluated the clinical, morphological, immunohistochemical and molecular features of 11 cases of T-LBL occurring in the setting of myeloid/lymphoid neoplasms with eosinophilia and investigated the immunohistochemical expression of LMO2 in this setting of T-LBL. Interestingly, 9/11 cases were LMO2 negative, with only 2 cases showing partial expression. In our study, we would suggest that LMO2 immunostaining, as part of the diagnostic panel for T-LBL, may represent a useful marker to identify T-LBL developing in the context of myeloid/lymphoid neoplasms with eosinophilia.

Abstract

Background: Rarely, T-lymphoblastic lymphoma (T-LBL) may develop in the setting of myeloid/lymphoid neoplasms with eosinophilia (M/LNs-Eo), a group of diseases with gene fusion resulting in overexpression of an aberrant tyrosine kinase or cytokine receptor. The correct identification of this category has relevant therapeutic implications. LIM domain only 2 (LMO2) is overexpressed in most T-LBL, but not in immature TdT-positive T-cells in the thymus and in indolent T-lymphoblastic proliferations (iT-LBP). Methods and Results: We retrospectively evaluated 11 cases of T-LBL occurring in the context of M/LNs-Eo. Clinical, histological, immunohistochemical and molecular features were collected and LMO2 immunohistochemical staining was performed. The critical re-evaluation of these cases confirmed the diagnosis of T-LBL with morphological, immunohistochemical and molecular features consistent with T-LBL occurring in M/LNs-Eo. Interestingly, LMO2 immunohistochemical analysis was negative in 9/11 cases, whereas only 2 cases revealed a partial LMO2 expression with a moderate and low degree of intensity, respectively. Conclusions: LMO2 may represent a potentially useful marker to identify T-LBL developing in the context of M/LNs-Eo. In this setting, T-LBL shows LMO2 immunohistochemical profile overlapping with cortical thymocytes and iT-LBP, possibly reflecting different molecular patterns involved in the pathogenesis of T-LBL arising in the setting of M/LNs-Eo.