A case report and a literature review of Myeloid/Lymphoid Neoplasm with Eosinophilia and PCM1::JAK2 rearrangement representing as B-cell Acute Lymphoblastic Leukemia B-ALL

Myeloid/Lymphoid Neoplasms with Eosinophilia and Tyrosine Kinase Gene Fusions (MLN-eo-TK) represent a distinct and heterogeneous group of hematologic malignancies characterized by recurrent gene fusions involving tyrosine kinases, such as PDGFRA, PDGFRB, FGFR1, JAK2, FLT3, ETV::ABL1 and other partner genes/variants. Among these, gene rearrangements involving PCM1::JAK2 are rare and may present diagnostic challenges, particularly when manifesting as acute lymphoblastic leukemia (ALL). We describe a case of a patient who presented with B-cell acute lymphoblastic leukemia (B-ALL) with a JAK2 rearrangement. After the induction therapy, strong myeloid proliferation in bone marrow without evidence of residual lymphoblasts was observed and JAK2 rearrangement was recognized to be a consequence of translocation t(8;9)(p22;p24)] resulting in PCM1::JAK2 fusion. This finding indicated the presence of an underlying chronic myeloid/lymphoid neoplasm, meeting criteria for MLN-eo-TK. Following an inadequate response to standard chemotherapy, salvage regimens incorporating targeted agents (JAK2 and BCL-2 inhibitors) and allogeneic bone marrow transplantation were administered, all of which unfortunately resulted in short-lived clinical benefit. The case highlights the importance of distinguishing de novo lymphoid malignancies from MLN-eo-TK, especially when JAK2 rearrangements are detected. Recognition of the clonal myeloid component during or after lymphoid-directed therapy has important diagnostic and therapeutic implications, supporting the use of targeted JAK2 inhibition in addition to standard chemotherapy.