Identification of CD34+ and CD34− leukemia-initiating cells in MLL-rearranged human acute lymphoblastic leukemia

Abstract

Translocation of the mixed-lineage leukemia (MLL) gene with AF4, AF9, or ENL results in acute leukemia with both lymphoid and myeloid involvement. We characterized leukemia-initiating cells (LICs) in primary infant MLL-rearranged leukemia using a xenotransplantation model. In MLL-AF4 patients, CD34⁺CD38⁺CD19⁺ and CD34⁻CD19⁺ cells initiated leukemia, and in MLL-AF9 patients, CD34⁻CD19⁺ cells were LICs. In MLL-ENL patients, either CD34⁺ or CD34⁻ cells were LICs, depending on the pattern of CD34 expression. In contrast, in patients with these MLL translocations, CD34⁺CD38⁻CD19⁻CD33⁻ cells were enriched for normal hematopoietic stem cells (HSCs) with in vivo long-term multilineage hematopoietic repopulation capacity. Although LICs developed leukemic cells with clonal immunoglobulin heavy-chain (IGH) rearrangement in vivo, CD34⁺CD38⁻CD19⁻CD33⁻ cells repopulated recipient bone marrow and spleen with B cells, showing broad polyclonal IGH rearrangement and recipient thymus with CD4⁺ single positive (SP), CD8⁺ SP, and CD4⁺CD8⁺ double-positive (DP) T cells. Global gene expression profiling revealed that CD9, CD32, and CD24 were over-represented in MLL-AF4, MLL-AF9, and MLL-ENL LICs compared with normal HSCs. In patient samples, these molecules were expressed in CD34⁺CD38⁺ and CD34⁻ LICs but not in CD34⁺CD38⁻CD19⁻CD33⁻ HSCs. Identification of LICs and LIC-specific molecules in primary human MLL-rearranged acute lymphoblastic leukemia may lead to improved therapeutic strategies for MLL-rearranged leukemia.