Project description:The stress of nucleotide pool reduction regulates transcription in neural crest and melanoma cells. To better understand the molecular response caused by nucleotide stress, we designed a chemical suppressor screen for leflunomide, an inhibitor of dihydroorate dehydrogenase. We found that alterations in the progesterone receptor (Pgr) activity suppressed the neural crest effects of leflunomide. To clarify the mechanism of action, we found that the RNA helicase protein, Ddx21, binds to Pgr, and loss of function of Ddx21 conferred resistance to nucleotide stress in zebrafish embryos. At the molecular level, nucleotide stress reduces DDX21 chromatin occupancy and thus, target gene expression. Together our results show that DDX21 is a transcriptional sensor and mediator of the nucleotide stress response.
Project description:The availability of nucleotides has a direct impact on transcription. The inhibition of dihydroorotate dehydrogenase (DHODH) with leflunomide impacts nucleotide pools by reducing pyrimidine levels. Leflunomide abrogates the effective transcription elongation of genes required for neural crest development and melanoma growth in vivo1. To define the mechanism of action, we undertook an in vivo chemical suppressor screen for restoration of neural crest after leflunomide treatment. Surprisingly, we found that alterations in progesterone and progesterone receptor (Pgr) signalling strongly suppressed leflunomide-mediated neural crest effects in zebrafish. In addition, progesterone bypasses the transcriptional elongation block resulting from Paf complex deficiency, rescuing neural crest defects in ctr9 morphant and paf1(alnz24) mutant embryos. Using proteomics, we found that Pgr binds the RNA helicase protein Ddx21. ddx21-deficient zebrafish show resistance to leflunomide-induced stress. At a molecular level, nucleotide depletion reduced the chromatin occupancy of DDX21 in human A375 melanoma cells. Nucleotide supplementation reversed the gene expression signature and DDX21 occupancy changes prompted by leflunomide. Together, our results show that DDX21 acts as a sensor and mediator of transcription during nucleotide stress.
