[KMT2A-rearranged acute lymphoblastic leukemia with lineage switch following relapse: a case report and literature review].

To enhance the understanding of lineage switch (LS), we retrospectively analyzed the clinical data of a patient with KMT2A-rearranged acute lymphoblastic leukemia (ALL) who underwent a LS at relapse and reviewed the relevant literature. The patient, a 23-year-old woman, was initially diagnosed with ALL (KMT2A::AFF1-positive). She experienced a relapse during consolidation chemotherapy and achieved a second remission following chemotherapy containing blinatumomab, subsequently bridging to allogeneic stem cell transplantation. Approximately 2 months after transplantation, extensive extramedullary relapse occurred. Immunohistochemical analysis of the extramedullary mass revealed a partial loss of CD19 expression and emergence of MPO expression, whereas the KMT2A rearrangement was retained. After receiving CD19-CAR-T therapy, the patient experienced another extramedullary relapse, at which time complete loss of CD19 expression and positivity for MPO were observed, leading to a diagnosis of myeloid sarcoma. Ultimately, the family opted to discontinue treatment and the patient was discharged to receive care locally. Following antigen-targeted immunotherapy, LS, as a potential mechanism of drug resistance and adaptation, warrants increased clinical attention.