High stem cell frequency in acute myeloid leukemia at diagnosis predicts high minimal residual disease and poor survival

Purpose: In CD34-positive acute myeloid leukemia (AML), the leukemia-initiating event originates from the CD34⁺CD38^- stem cell compartment. Survival of these cells after chemotherapy may lead to minimal residual disease (MRD) and subsequently to relapse. Therefore, the prognostic impact of stem cell frequency in CD34-positive AML was investigated. Experimental Design: First, the leukemogenic potential of unpurified CD34 ⁺CD38^- cells, present among other cells, was investigated in vivo using nonobese diabetic/severe combined immunodeficient mice transplantation experiments. Second, we analyzed whether the CD34 ⁺CD38^- compartment at diagnosis correlates with MRD frequency after chemotherapy and clinical outcome in 92 AML patients. Results: In vivo data showed that engraftment of AML blasts in nonobese diabetic/severe combined immunodeficient mice directly correlated with stem cell frequency of the graft. In patients, a high percentage of CD34⁺CD38^- stem cells at diagnosis significantly correlated with a high MRD frequency, especially after the third course of chemotherapy. Also, it directly correlated with poor survival. In contrast, total CD34⁺ percentage showed no such correlations. Conclusions: Both in vivo data, as well as the correlation studies, show that AML stem cell frequency at diagnosis offers a new prognostic factor. From our data, it is tempting to hypothesize that a large CD34 ⁺CD38^- population at diagnosis reflects a higher percentage of chemotherapy-resistant cells that will lead to the outgrowth of MRD, thereby affecting clinical outcome. Ultimately, future therapies should be directed toward malignant stem cells.