Inhibiting the pyrimidine biosynthesis enzyme DHODH triggers differentiation in multiple AML subtypes.
Major finding: Inhibiting the pyrimidine biosynthesis enzyme DHODH triggers differentiation in multiple AML subtypes.
Approach: A high-throughput screen shows that DHODH inhibitors overcome HOXA9-induced differentiation arrest.
Impact: DHODH inhibition may overcome leukemic differentiation blockade in the treatment of patients with AML.
Acute myeloid leukemia (AML) is characterized by the arrest of leukemic myeloblasts at an immature stage of development, highlighting the potential for therapies that induce differentiation. Although AMLs are genetically diverse, the majority exhibit upregulation of the HOXA9 transcription factor, which is normally downregulated during differentiation. In order to identify compounds that may promote differentiation, Sykes and colleagues used a cell-based model of HOXA9-induced differentiation blockade to screen for small molecules that induce differentiation. Of 330,000 small molecules tested, 12 induced myeloid differentiation in HOXA9-induced granulocyte-macrophage progenitor cell lines, including C07, which effectively triggered differentiation, but whose protein target was unknown. RNA sequencing and whole-exome sequencing of cells made resistant to C07 revealed that dihydroorotate dehydrogenase (DHODH), which catalyzes the conversion of dihydroorotate (DHO) to orotate in uridine monophosphate (UMP) synthesis, was overexpressed and amplified, suggesting that C07 induces differentiation via DHODH inhibition. Consistent with these findings, a previously developed DHODH inhibitor, brequinar (BRQ), also induced differentiation of HOXA9-dependent and -independent cells and human AML cell lines. BRQ was well tolerated in mice, and in xenograft models of AML, BRQ treatment slowed tumor growth, extended survival, and promoted cellular differentiation. BRQ also promoted the loss of leukemia-initiating cells, induced differentiation, and extended survival in an MLL–AF9 model of leukemia and induced differentiation in a FLT3–ITD-driven patient-derived xenograft. As expected, DHODH inhibition resulted in accumulation of DHO and depletion of downstream metabolites including uridine. Further, inhibition of a later step in uridine biosynthesis phenocopied the effects of DHODH inhibition on differentiation, indicating that inhibition of UMP synthesis promotes myeloid differentiation, which was also evident in vivo, as BRQ-treated mice had reduced levels of uridine. Although the underlying mechanism remains to be determined, these findings link pyrimidine synthesis to myeloid differentiation and suggest that DHODH may be effectively targeted to induce differentiation in multiple AML subtypes.