Title: PTEN regulates glutamine flux to pyrimidine synthesis and sensitivity to dihydroorotate dehydrogenase inhibition
Description: The importance of metabolism in tumor initiation and progression is becoming increasingly clear. Metabolic changes induced by oncogenic drivers of cancer contribute to tumor growth and are attractive targets for cancer treatment. Phosphatase and Tensin homolog deleted from chromosome ten (PTEN) is one of the most commonly mutated tumor suppressors in cancer and operates in multiple roles, rendering it a hub for understanding cancer biology and for developing targeted therapy. PTEN’s canonical function is its ability to antagonize the phosphoinositide 3-kinase (PI3K) pathway by dephosphorylating the lipid second messenger phosphatidylinositol (3,4,5) tri-phosphate (PIP3). This thesis focuses on the effects of PTEN loss on cellular metabolism, and the therapeutic vulnerability that stems from metabolic alterations. First, we discovered that loss of Pten in mouse embryonic fibroblasts (MEFs) increases cellular proliferation and the number of replication forks per cell, launching our investigation into metabolic pathways that may be altered to support increased growth.

Indeed, we found that Pten-/- cells exhibited a dependence on glutamine for their faster rate of growth, and that glutamine was channeled into the de novo synthesis of pyrimidines. The next chapter examined dihydroorotate dehydrogenase (DHODH), a rate limiting enzyme for pyrimidine ring synthesis in the de novo pyrimidine synthesis pathway. We found that PTEN-deficient primary cells and cancer cell lines were more sensitive to inhibition of DHODH than PTEN WT cells were, and that the growth inhibition could be rescued by metabolites downstream of DHODH. Furthermore, we found that xenografted human triple negative breast cancer tumors in mice could be diminished by treatment with leflunomide, a DHODH inhibitor. In the following chapter, we aimed to identify the mechanisms leading to cell death in PTEN mutant cells upon DHODH inhibition. We found that inherent defects in checkpoint regulation in PTEN-deficient cells were exacerbated by the stress of obstructed de novo pyrimidine synthesis, leading to a buildup of DNA damage at replication forks and ultimately chromosomal breaks.

This was instigated by AKT-mediated phosphorylation of TOPBP1 that caused inadequate ATR activation, as well as AKT-mediated phosphorylation and inactivation of CHK1. In sum, the findings of this thesis indicate that enhanced glutamine flux to de novo pyrimidine synthesis in PTEN mutant cells generates vulnerability to DHODH inhibition. The integration of altered glutamine regulation with PTEN’s effect on replication, DNA damage, and the checkpoint response manifests as synthetic lethality upon DHODH inhibition in cells with PTEN inactivation. Inhibition of DHODH could thus be a promising therapy for patients with PTEN mutant cancers.