ETV6 /FLT3 fusion in a mixed-phenotype acute leukemia arising in lymph nodes in a patient with myeloproliferative neoplasm with eosinophilia

The ETV6/FLT3 fusion gene has been recently reported in association with myeloproliferative neoplasm with eosinophilia (MPN-Eo) and peripheral T cell lymphoma. Favorable clinical response to a targeted FLT3 tyrosine kinase inhibitor (FLT3 TKI) was noted. Here we report a novel phenotype associated with ETV6/FLT3 rearrangement, in a 38-year-old female with presentation of progressive lymphadenopathy. Lymph node biopsy showed mixed-phenotype acute leukemia (MPAL) with expression of T cell and myeloid markers. Bone marrow morphology was consistent with MPN-Eo and no evidence of MPAL. Karyotype analysis revealed 46, XX, t(12;13)(p13;q12). ETV6/FLT3 fusion was demonstrated by fluorescence in situ hybridization. This is the first report of ETV6/FLT3 rearrangement showing a phenotype of extramedullary T/myeloid MPAL arising in the setting of MPN-Eo. We suggest addition of this entity to the WHO category of “myeloid/lymphoid neoplasms with eosinophilia,” particularly given the possibility of clinical response to FLT3 TKI in MPAL, a disease usually associated with poor prognosis.