KMT2A-rearranged infantile acute myeloid leukemia masquerading as juvenile myelomonocytic leukemia.

Mixed lineage leukemia [MLL; now known as lysine methyltransferase 2A (KMT2A)] rearrangement-positive acute myeloid leukemia (AML) and juvenile myelomonocytic leukemia (JMML) are distinct diseases, although age of susceptibility (infancy or early childhood) and abnormal monocytosis are common clinical features. Here, we report two cases of KMT2A-rearranged infantile AML masquerading as JMML at initial presentation. Both cases showed leukocytosis accompanied by atypical monocytosis. However, in both cases, leukemic blasts were absent at the initial examination. Thus, a diagnosis of JMML was suspected. However, initial cytogenetic analysis revealed that both cases had an 11q23 rearrangement, which is atypical in JMML. Eventually, due to the emergence of leukemic blasts and further cytogenetic studies, both cases were diagnosed with infantile AML with a KMT2A rearrangement. Although one patient remains in complete remission after the completion of AML appropriate chemotherapy, the other died of AML due to treatment failure. Our experience suggests that AML with KMT2A rearrangement should be considered for the differential diagnosis of infantile cases with atypical monocytosis suggestive of JMML. Cytogenetic studies, including fluorescence in situ hybridization analysis of KMT2A, may be helpful in distinguishing between AML with KMT2A rearrangement and JMML.