Expression Of Aldehyde Dehydrogenase Reflected Diverse Stem Cell Properties In Acute Myeloid Leukemia

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 ALDH^bright 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 (ALDH^bright 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 ALDH^bright 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 ALDH^bright cells (median 0.11%; range 0.004 – 1.77%; defined as ALDH-low AML), 17 patients had relatively numerous ALDH^bright cells (median 9.01; range 3.54 – 33.57%; defined as ALDH-numerous AML). In both groups, ALDH^bright cell populations were highly enriched for CD34⁺CD38^- cells. The ALDH^bright 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 ALDH^bright subset, whereas LSC were enriched in ALDH^dimCD34⁺CD38^- subset for patients with ALDH-low AML. For 17 patients with ALDH-numerous AML, the ALDH^bright 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 ALDH^brightCD34⁺CD38^- subset contained chemotherapy-resistant clones with repopulating ability. Furthermore, such ALDH^bright cells were characterized by a lower cell-cycle activity and an increased resistance to cytarabine in comparison with ALDH^dim 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 ALDH^bright cells. In patients with ALDH-numerous AML, the ALDH^bright 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.