Separation of leukemic stem cells (LSC) and residual hematopoietic stem cells (HSC) from the same individual patient with acute myeloid leukemia (AML) is essential for a proper understanding of the leukemic driving mechanisms. We have studied the role of aldehyde dehydrogenase (ALDH) for this purpose and have defined the functional properties of ALDHbright cells in specific subgroups of AML.
We have examined the ALDH activity by flow cytometry in bone marrow samples (BM) from 14 healthy donors and 73 patients with de novo AML. The median frequency of cells with high ALDH activity (ALDHbright cells) in the healthy subjects was 1.92% with a range from 0.58 to 3.16%. For patients with AML, the median number of ALDHbright cells was 0.25% with a broad range from 0.004 to 33.57%. Whereas the majority of patients with AML (n = 56) had low frequencies of ALDHbright cells (median 0.11%; range 0.004 – 1.77%; defined as ALDH-low AML), 17 patients had relatively numerous ALDHbright cells (median 9.01; range 3.54 – 33.57%; defined as ALDH-numerous AML). In both groups, ALDHbright cell populations were highly enriched for CD34+CD38- cells. The ALDHbright cells derived from ALDH-low AML did not contain chromosomal and molecular aberrations characteristic of the original leukemia, and were able to induce multi-lineage hematopoiesis in NSG mouse models. Thus, genetically and functionally normal HSC could be successfully isolated in the ALDHbright subset, whereas LSC were enriched in ALDHdimCD34+CD38- subset for patients with ALDH-low AML. For 17 patients with ALDH-numerous AML, the ALDHbright subset was consistently contaminated with LSC. In clinical follow-ups, patients with ALDH-numerous AML showed resistance to induction chemotherapy and were characterized by a very poor long-term outcome that was comparable to patients with high-risk cytogenetic or molecular genetic markers. In four patients with ALDH-numerous AML we demonstrated that the ALDHbrightCD34+CD38- subset contained chemotherapy-resistant clones with repopulating ability. Furthermore, such ALDHbright cells were characterized by a lower cell-cycle activity and an increased resistance to cytarabine in comparison with ALDHdim blasts in in vitro assays.
Our data have provided evidence that LSC and residual HSC can be separated using ALDH in patients with low frequencies of ALDHbright cells. In patients with ALDH-numerous AML, the ALDHbright subset is associated with leukemic features both in vitro and in animal models. Thus our data demonstrated the feasibility of appropriate comparisons of LSC versus HSC from the same patient with specific subtypes of AML and the impact of LSC properties on clinical outcome.
Buss:Novartis: Travel support Other; Micromet/Amgen: Reimbursements for participation in a clinical study , Reimbursements for participation in a clinical study Other. Ho:Sanofi-Aventis: Consultancy, Honoraria, Membership on an entity’s Board of Directors or advisory committees; Genzyme: Consultancy, Honoraria, Membership on an entity’s Board of Directors or advisory committees.
Asterisk with author names denotes non-ASH members.