Heterogeneity of leukemia stem cell candidates at diagnosis of acute myeloid leukemia and their clinical significance

Leukemia stem cell candidates (LSCC) can be enriched from patients with acute myeloid leukemia by high aldehyde dehydrogenase (ALDH) activity and CD34 expression. We have previously demonstrated the leukemia-initiating activity of ALDH^bright cells in xenograft transplantation models, as well as in vitro. Applying single-cell long-term culture-initiating cell assays, we have correlated the functional properties of individual cells within this LSCC population and the respective phenotypes. To define their biologic significance, we also analyzed the relationship between LSCC at diagnosis to long-term clinical outcomes. The median percentage of ALDH^bright cells among 101 acute myeloid leukemia patients was 0.51% (range, 0.01–12.90%). Single-cell long-term culture-initiating cell assays, followed by genetic analysis of the progeny cells, showed that the leukemia-initiating activity was found in the ALDH^bright/CD34^high subset and, to a lesser extent, in ALDH^bright/CD34^low or ALDH^bright/CD34⁻ subsets. Nevertheless, the frequency of ALDH^bright cells at diagnosis correlated significantly with the persistence of leukemia after induction chemotherapy (n = 84, Spearman R = 0.3261; p < 0.0025). In the multivariate model, frequency of ALDH^bright cells was the strongest prognostic marker (p = 0.0095) affecting overall survival (hazard ratio = 9.107). LSCC are heterogeneous and best reflected by ALDH activity. The frequency of ALDH^bright cells at diagnosis is a significant prognostic marker for acute myeloid leukemia.