ETV6-FLT3-positive myeloid/lymphoid neoplasm with eosinophilia presenting in an infant: an entity distinct from JMML

Myeloid/lymphoid neoplasm with eosinophilia (MLN-Eo) is a World Health Organization (WHO) established category of hematologic malignancies primarily arising in adults. We discuss an 8-month-old infant who presented with clinical features similar to those of juvenile myelomonocytic leukemia (JMML) but who was diagnosed with MLN-Eo driven by an ETV6-FLT3 fusion. Results of patient-derived leukemia ex vivo studies demonstrated increased sensitivity to type I FLT3 inhibitors as compared with type II inhibitors. Treatment with the type I inhibitor gilteritinib resulted in complete immunophenotypic and cytogenetic remission. This patient subsequently underwent a hematopoietic stem cell transplant and remains in complete remission 1 year later. This is the youngest patient reported with an ETV6-FLT3 fusion and adds to the mounting reports of FLT3-rearranged MLN-Eo, supporting its addition to the WHO classification. Furthermore, this case highlights the clinical utility of ex vivo drug testing of targeted therapies.

Automated summary

MLN-Eo is a WHO-established category of hematologic malignancies, and we report a case of an 8-month-old infant diagnosed with ETV6-FLT3 driven MLN-Eo who achieved complete remission after treatment. This case highlights the clinical utility of ex vivo drug testing in targeted therapies.