KCTD5::PDGFRB Fusion Gene Leading to Clonal Hypereosinophilia With Cardiac Involvement in a Young Adult: A Case Report.

Hypereosinophilia may be the presenting feature of various conditions, including myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions (MLN-eo-TK). We report the case of a 22-year-old male presenting with chest pain and severe cardiac involvement in the setting of marked hypereosinophilia. Laboratory findings revealed marked leukocytosis with hypereosinophilia, mild anemia, elevated NT-proBNP, impaired renal function, and elevated vitamin B12 levels. Imaging showed cardiomegaly and severe mitral regurgitation. Eosinophilia persisted despite corticosteroid and antiparasitic therapy. Bone marrow analysis revealed hypercellularity with eosinophilia, and karyotyping identified a t(5, 16) chromosomal translocation. RNA sequencing revealed a novel KCTD5::PDGFRB fusion transcript, establishing the diagnosis of MLN-eo-TK with cardiac involvement. Imatinib (400 mg daily) induced complete hematological remission within 24 hours; however, severe mitral regurgitation persisted, requiring valve replacement. To our knowledge, this is the first reported case of MLN-eo-TK associated with a KCTD5::PDGFRB fusion. This case highlights the importance of comprehensive cytogenetic and molecular testing in unexplained hypereosinophilia, particularly in corticosteroid-refractory patients, and demonstrates a rapid and marked response to imatinib.