Mitochondrial Priming and Anti-Apoptotic Dependencies in Aging and Diseased Bone Marrow

Abstract

This thesis explores two questions; how aging of the hematopoietic stem cell (HSC) may contribute to the increase in hematological malignancies with age and secondly, how hematological malignancies can be better treated when they arise. At the intersection of both of these questions is the mitochondrial apoptotic pathway and the tool, BH3 profiling, which measures the mitochondrial priming and anti-apoptotic dependencies within normal and malignant cells. Mitochondrial priming and anti-apoptotic dependencies are measured by assessing the sensitivity of cellular mitochondria to standardized amounts of BH3 peptides derived from the BH3 domains of pro-apoptotic proteins. In our first study, we use BH3 profiling to identify anti-apoptotic dependencies to direct treatment of the anti-apoptotic BCL-2 inhibitor ABT-199 in acute myeloid leukemia (AML). We found that AML blasts are often dependent on the pro-survival protein BCL-2 and the mitochondrial dependence on BCL-2 measured by BH3 profiling correlated with cellular sensitivity to ABT-199. These pre-clinical results showed that ABT-199 was functioning on-target at the mitochondria and that BH3 profiling could be used to identify patients who would be most sensitive to BCL-2 inhibition. Importantly, these results directly led to a Phase II clinical trial of ABT-199 in relapsed/refractory AML patients and BH3 profiling was integrated into the study to test its efficacy as a predictive biomarker. In the clinical trial, we showed that BH3 profiling correlated well with clinical response. Secondly, in our studies of aging in the HSC compartment, we used BH3 profiling to explore how mitochondrial priming is altered during normal hematopoietic differentiation and during differentiation of HSCs isolated from young and old mice. We found that HSCs are less primed than more differentiated progenitor cells and that old HSCs are even less primed than young HSCs which correlates with decreased sensitivity to apoptotic stimuli of old HSCs. These studies expand upon the biological understanding of functional defects of aged HSCs, and showed for the first time in a clinical setting that BH3 profiling may be used successfully to direct treatment of BH3 mimetics in the clinic.