ALDH Genes and Reactive Aldehydes Play Important Roles in HSCs and Leukemia and May Be Exploited to Treat AML

ALDH1A1 is expressed at high levels in normal HSCs and we previously reported that its function might involve metabolism of compounds termed reactive aldehydes. We also reported that loss of ALDH1A1 led to a compensatory increase in a related isoform, ALDH3A1, which also metabolizes reactive aldehydes. Double knockouts for both ALDH1A1/3A1 accumulate reactive aldehydes, which appear to impact a number of cellular processes including signal transduction and gene expression. As reactive aldehydes also cause DNA damage, we hypothesized that excess accumulation of reactive aldehydes may predispose to leukemic transformation of HSCs. In support of this, we found that ALDH1A1/3A1 double knockout HSCs readily form acute leukemia following transduction with a NUP98-HOXA10 fusion gene, which rarely causes leukemic transformation in wild type HSCs. Furthermore, in human AML, frequent absence of ALDH1A1 and the universal absence of ALDH3A1 was observed. A human AML cell line, Kasumi-1, was found to be ALDH1A1/3A1 deficient and to have high levels of intracellular reactive aldehydes. In addition, Kasumi-1 was highly sensitive to DNA damage and cell death following exposure to exogenous 4-HNE, a prototypic reactive aldehyde. In contrast, normal CD34+ HSCs were relatively resistant to 4-HNE. Based on these observations, we further hypothesized that treatment of ALDH1A1/3A1 deficient AMLs with clinically relevant compounds that further increase intracellular 4-HNE levels would selectively eliminate AML while sparing normal CD34+ HSCs. To test this, Kasumi-1 were exposed to a series of compounds including the pro-oxidant Arsenic tri-oxide (ATO), the sesquiterpene lactone parthenolide (PTL) and 4-HC, the active metabolite of cyclophosphamide (Cy) and a substrate of ALDH1A1. All increased intracellular 4-HNE levels and DNA damage. Exposure to combinations of 4-HC, ATO and PTL induced high levels of cell death in Kasumi-1. In contrast, Kasumi-1 cells engineered to express ALDH1A1 through lentiviral gene transfer and normal CD34+ HSCs were relatively resistant to several of these treatments. Primary ALDH1A1/3A1- AMLs were also relatively sensitive to treatment with these same compounds.

In conclusion, ALDHs and reactive aldehydes may play important roles in HSCs and leukemia and exploitation of their biology may lead to novel therapies for AML and possibly other cancers. As an initial application of this treatment strategy, we are developing a clinical trial to treat patients with relapsed/refractory ALDH1A1/3A1 deficient AML with Cy/ATO.