Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions (M/LN-eo-TK) such as PDGFRA, PDGFRB, FGFR1, JAK2, FLT3 rearrangement and ETV6::ABL1 fusion include rare and heterogeneous clinical-pathological entities with some similarities, not always associated with peripheral eosinophilia. Accurate diagnosis and demonstration of the specific genetic substrate have important implications since target therapy is possibly available. Herein we report two cases showing different bone marrow features and clinical presentation. Recognition of eosinophilic granuloblasts prompted genetic analysis that showed PDGFRB (case 1) and PDGFRA (case 2) gene rearrangement. Diagnosis of M/LN-eo-TK may be challenging. Pathologists may be the first professionals to suspect the disorder and should be aware of the therapeutic implication. Accurate BOM marrow evaluation with a panel of immunohistochemical reactions, and specific molecular analyses are required for proper diagnosis.

A novel fusion gene NRF1::PDGFRA was identified in a patient with myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions (MLN-TK), harboring the chromosome abnormality t(4;7)(q12;q32). This represents the first reported case of the NRF1::PDGFRA fusion gene, and the ninth PDGFRA-associated fusion gene identified in MLN-TK. The fusion event led to the constitutive activation of the PDGFRA kinase, resulting in uncontrolled eosinophil proliferation and potentially contributing to the occurrence of cerebral infarction. Our study indicates treatment with low-dose imatinib effectively alleviates the symptoms associated with NRF1::PDGFRA gene fusion.

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. 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. 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. 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.

In the 2022, WHO and ICC classifications, myeloid/lymphoid neoplasms with eosinophilia (M/LN-eo) and tyrosine kinase gene fusions represent rare hematologic malignancies driven by rearrangements of PDGFRA, PDGFRB, FGFR1, JAK2, FLT3, and ETV6::ABL1 fusion. Eosinophilia is the most constant finding, whereas the clinicopathological features are quite heterogeneous, presenting as Chronic eosinophilic leukemia (CEL) NOS, myelodysplastic/myeloproliferative neoplasm (MDS/MPN), MDS, MPN, systemic mastocytosis (SM), T or B cell acute lymphoblastic leukemia/lymphoblastic lymphoma (ALL/LBL), acute myeloid leukemia (AML), blastic phase of MPN, or mixed phenotype acute leukemia (MPAL). Extramedullary involvement at diagnosis or during progression is common. Here, we report a very unusual case of myeloid/lymphoid neoplasm with ETV6::FLT3 fusion with a nodal presentation without associated eosinophilia. Our case draws attention to diagnostic pitfalls in these rare entities.