Acute Differentiated Dendritic Cell Leukemia: A New Form of Pediatric Acute Myeloid Leukemia (AML) with MLL Translocations.

Background: Myelomonocytic precursors from acute or chronic leukemias can differentiate to dendritic cells in vitro , but leukemias with a dendritic cell immunophenotype are rare, have been reported mainly in adults, and their molecular pathogenesis is unknown. Dendritic cells are classified as Langherans, myeloid and lymphoid/plasmacytoid cells, but leukemias arising from dendritic cells are unclassified in the FAB system. We identified a new entity of pediatric acute myeloid leukemia (AML) presenting with morphologic and immunophenotypic features of mature dendritic cells, which is characterized by MLL gene translocation. Methods and Results: Standard methods were used to characterize the morphology, immunophenotype, karyotype and MLL translocations in 3 cases of pediatric AML. The patients included two boys and one girl diagnosed with AML between 1–6 years old. Their clinical histories and findings included fever, pallor, abdominal and joint pain, adenopathy, hepatosplenomegaly, normal WBC counts but anemia and thrombocytopenia. and no evidence of CNS disease. The bone marrow aspirates were hypocellular and replaced completely by large blasts with irregular nuclei, slightly basophilic cytoplasm, and prominent cytoplasmic projections. There were no cytoplasmatic granules or phagocytosis. Myeloperoxidase and alpha napthyl esterase reactions were negative, excluding FAB M5 AML, and the morphology was not consistent with any standard FAB morphologic diagnosis. The leukemic blasts in all three cases were CD83+, CD86+, CD116+, consistent with differentiated myeloid dendritic cells, and did not express CD34, CD56 or CD117. MLL translocations were identified in all 3 cases. In the first case FISH analysis showed t(10;11)(p12;q23) and RT-PCR identified and a ‘5-MLL-AF10-3’ fusion transcript. In the second case FISH analysis showed t(9;11)(p22;q23) and RT-PCR identified and a ‘5-MLL-AF9-3’ fusion transcript. In the remaining case, the MLL gene rearrangement was identified by Southern blot analysis and RT-PCR showed an MLL - AF 9 fusion transcript. The fusion transcripts in all 3 cases were in-frame. Remission induction was achieved with intensive chemotherapy, and all three patients have remained in durable remission for 30–60 months after hematopoietic stem cell transplantation. Conclusions. We have characterized a new pediatric AML entity with features of mature dendritic cells, MLL translocation and an apparently favorable prognosis. The in-frame MLL fusion transcripts suggest that chimeric MLL oncoproteins underlie its pathogenesis. The partner genes in all 3 cases were known partner genes of MLL that encode transcription factors. This study increases the spectrum of leukemias with MLL translocations. Comprehensive morphological, immunophenotypic, cytogenetic and molecular analyses are critical for this diagnosis, and will reveal its frequency and spectrum as additional cases are uncovered.