Aldehyde Dehydrogenase Expression in Primitive Human Hematopoietic Progenitor Cells with Side Population Characteristic and in Samples from Patients with Acute Myeloid Leukemia.

High activity of the detoxifying enzyme aldehyde dehydrogenase (ALDH) has been attributed to primitive hematopoietic stem cells in the murine and human system. ALDH activity can be measured by a functional assay employing BODIPY-aminoacetaldehyde (BAAA) as a fluorescent intracellular substrate. High ALDH activity within hematopoietic stem cells is associated with CD34 and AC133 surface expression.

Given the co-expression of these markers in acute myeloid leukemia (AML), we sought to investigate whether ALDH-activity can be detected in early blood stem cell disorders and in primitive hematopoietic progenitor cells with the side population phenotype. Employing a commercial Aldefluor^™ -kit thirteen peripheral blood and bone marrow samples from patients with AML were stained for ALDH activity. Four samples exhibited a clearly distinctive positive staining pattern when compared to the inhibitor control, i.e. 16–91% of AML blast cells were considered as ALDH positive. The staining pattern was similar to marrow samples from healthy donors or peripheral blood sample controls from G-CSF-mobilised healthy stem cell donors.

In addition we investigated ALDH activity in primitive bone marrow side population (SP) cells from healthy individuals. Although the ALDH substrate BAAA is actively extruded out of cells and especially SP stem cells by an ABC-transporter-activity, we established a co-staining method for Hoechst 33342 and BAAA by inclusion of specific ABC-transporter inhibitors.

Both SP cells and ALDH-positive cells appeared as a clear distinctive population with some overlap between both populations. Quantitative RT-PCR of different sorted stem cell samples confirmed that SP cells had higher ABCG2-expression than ALDH positive cells and ABCG2-expression was higher in ALDH positive cells compared to the total cell fraction or granulocytes.

We conclude that ALDH is expressed in some but not all AML blast cells. This is in accordance with the cytogenetic heterogeneity of this disease and may have therapeutic implications because of the detoxifying activity of the ALDH enzyme. In addition ALDH activity can also be found on a minor population of very primitive marrow progenitor cells. Whether high ALDH activity is a feature of the leukemic stem cell requires further investigation.