Valproic acid induces differentiation and transient tumor regression, but spares leukemia-initiating activity in mouse models of APL

Aberrant histone acetylation was physiopathologically associated with the development of acute myeloid leukemias (AMLs). Reversal of histone deacetylation by histone deacetylase inhibitor (HDACis) activates a cell death program that allows tumor regression in mouse models of AMLs. We have used several models of PML-RARA-driven acute promyelocytic leukemias (APLs) to analyze the in vivo effects of valproic acid, a well-characterized HDACis. Valproic acid (VPA)-induced rapid tumor regression and sharply prolonged survival. However, discontinuation of treatment was associated to an immediate relapse. In vivo, as well as ex vivo, VPA-induced terminal granulocytic differentiation. Yet, despite full differentiation, leukemia-initiating cell (LIC) activity was actually enhanced by VPA treatment. In contrast to all-trans retinoic acid (ATRA) or arsenic, VPA did not degrade PML-RARA. However, in combination with ATRA, VPA synergized for PML-RARA degradation and LIC eradication in vivo. Our studies indicate that VPA triggers differentiation, but spares LIC activity, further uncouple differentiation from APL clearance and stress the importance of PML-RARA degradation in APL cure.