Title: Mitochondrial Priming Determines Chemotherapeutic Response in Acute Myeloid Leukemia
Description: Gain- and loss-of-function studies of the BCL-2 family of proteins have shown that they can impact chemotherapeutic sensitivity. However, cells contain myriad anti-apoptotic and pro-apoptotic BCL-2 family members making it difficult to predict cell fate decisions based on the initial conditions of these proteins. BH3 profiling is a tool that measures mitochondrial priming, the readiness of a cell to die through the intrinsic (or mitochondrial) apoptotic pathway. Priming is due to the cumulative effect of the BCL-2 family of proteins that act as the gate keepers of the mitochondrial apoptotic pathway. Priming is measured by determining the sensitivity of mitochondria to perturbation by peptides derived from the BH3 domains of pro-apoptotic proteins. Using BH3 profiling, we now have a functional readout that can quantify priming and assess its contribution to drug sensitivity.

Here we show that priming affects the sensitivity of acute myeloid leukemia (AML) cell lines to various standard chemotherapeutics, especially topoisomerase II inhibitors. Priming predicts clinical response to conventional induction chemotherapy as well as the long term maintenance of remission in AML patients. Interestingly, the priming of normal hematopoietic stem cells (HSCs) sits at the boundary line between the priming of cured and refractory patient AML. This HSC priming likely defines the therapeutic index since AML that are lower primed than HSCs are often refractory and cannot be cured without transplantation. Additionally, our BH3 profiles revealed that AML cells are more sensitive to BCL-2 antagonism than normal HSCs, which are primarily dependent on MCL-1. Indeed, we were able to kill primary refractory AML cells in vitro with the BCL-2 antagonist ABT-737 at doses that left HSCs unharmed.

Cumulatively, these findings show that priming is a major mechanistic determinant of AML response in vitro and in the clinic to standard induction chemotherapy. With the ability to predict outcome, BH3 profiling may offer physicians and patients a promising tool for treatment decision-making.