STRING protein interactions were used to highlight
STRING protein interactions were used to highlight enzymes directly associated with DHODH (Fig. 10) (Szklarczyk et al., 2015). The reported protein-protein associations are primarily within the de novo nucleotide synthesis pathways (CAD, carbamoyl phosphate synthetase 1, uridine monophosphate synthetase, and phosphoribosylglycinamide formyltransferase (de novo purine)) (Lane & Fan, 2015). Beyond de novo pathways, DHODH is reported to associate with the dihydropyrimidinase-like (DPYSL) family. DPYSL is also known as a collapsing response mediator protein (CRMP) family (Tan, Wahdan-Alaswad, Yan, Thiele, & Li, 2013). CRMP1 has been implicated in metastasis and differentiation (F. Tan, Thiele, & Li, 2014) acting as a tumor suppressor in prostate cancer and hindering the epithelial-mesenchymal transition (Cai et al., 2016). Additionally, CRMP may present as a potential cancer biomarker. In neuroblastoma, the activation of MYC selectively decreased the expression of CRMP3, but not CRMP1 or CRMP2. Conversely, siRNA knockdown of MYC increased expression of CRMP3, suggesting a unique role for CRMP3 as a biomarker for aggressive neuroblastoma (Gaetano, Matsuo, & Thiele, 1997; Tan et al., 2013). Beyond neuroblastoma, CRMP5 is reported as a biomarker for poor prognosis in osteosarcoma (Wang et al., 2017). However, the complete connection between these proteins, DHODH, and cancer is not fully understood.
Murine knockout studies, performed by the International Mouse Phenotyping Consortium, identified DHODH as a homozygous-lethal gene (Brown & Moore, 2012; Meehan et al., 2017) showing that at least one functional allele copy is necessary for survival. However, even heterozygous mutations in DHODH may be responsible for health defects, as mutations in DHODH were implicated as a cause of Miller syndrome, a genetic disorder characterized by facial and bone abnormalities (Ng et al., 2010). At least 11 different DHODH mutations have been identified (Ng et al., 2010). Some amino CDK4 inhibitor mg changes in patients with Miller\'s syndrome are R346W and G202A (Ng et al., 2010). These mutations do not have an impact on mitochondrial localization but were later observed to decrease protein stability (Fang et al., 2012). It is unclear how other mutations alter DHODH function in patients with Miller\'s syndrome. However, there is a clear implication that DHODH function is necessary for normal embryonic development. In fact, the DHODH inhibitor leflunomide is a known teratogen and its use is avoided during pregnancy (Hajdyla-Banas et al., 2009). In murine studies, treatment with leflunomide in pregnant mice caused significant malformations. These malformations did not occur when leflunomide was co-administered with uridine (Fukushima et al., 2007; Fukushima et al., 2009). For embryonic development, this data suggests that DHODH function is necessary to generate nucleotides for continued growth.
Select inhibitors of DHODH The importance of DHODH in cancer cells has resulted in many drug discovery campaigns. Various DHODH inhibitors belonging to different chemical classes have been reported and previously reviewed (Munier et al., 2013; Vyas & Ghate, 2011). Only select inhibitors targeting human DHODH, which were not previously reviewed, are discussed with the most notable inhibitors, brequinar and leflunomide/teriflunomide, discussed in detail.
Combination approaches to increase efficacy of DHODH inhibitors Results from the multitude of de novo pyrimidine inhibitors in anticancer clinical trials depict a mixed outlook for single agent DHODH inhibition. As previously discussed, brequinar produced minimal objective responses in clinical trials. Results were similar for all inhibitors of the de novo pyrimidine biosynthesis pathway evaluated in the clinic (e.g., PALA and pyrazofurin). However, a significant problem in targeting de novo pyrimidine biosynthesis is that cells may alternatively source nucleotides from the salvage pathway. Results from uridine rescue experiments have led to the hypothesis that a DHODH inhibitor may require concomitant administration of other agents to be successful (Peters, Sharma, Laurensse, & Pinedo, 1987; Schwartsmann et al., 1988).