Abstract LB-335: DHODH regulates transcriptional elongation during melanoma evolution: insights from zebrafish screening

Abstract

Abstract Melanoma cells are derived from the neural crest, a transient, multipotent tissue present only during embryogenesis. Using zebrafish models, we aimed to determine whether these embryonic neural crest gene programs would make therapeutic targets in melanoma. Transgenic zebrafish expressing BRAFV600E under the mitf promoter develop melanoma when crossed with p53 mutants. From the earliest stages of transformation, and throughout overt tumorigenesis, these tumors express multiple embryonic neural crest progenitor markers such as SOX10, EDNRB and MITF. Human melanomas were similar: 50–75% expressed progenitor markers, but only 15% expressed the differentiation marker DCT. We performed a chemical genetic screen in zebrafish embryos to identify suppressors of the neural crest gene program to determine their therapeutic utility. Of 2000 small molecules, we found one class, inhibitors of the enzyme DHODH, that led to a complete abrogation of neural crest development during embryogenesis. Treated animals were essentially albino, devoid of all derivative melanocytes that eventually give rise to melanoma. Developmentally, DHODH suppression exerts this effect by blocking transcriptional elongation of genes such as MITF, in a manner dependent upon the elongation factor SUPT5H. In melanoma cells, we measured transcriptional elongation after DHODH inhibition via RNA pol II chromatin immunprecipitation coupled with massively parallel sequencing (ChIP-Seq). Similar to embryonic development, inhibiting DHODH leads to a defect in transcriptional elongation at select loci. We then tested whether leflunomide, a DHODH inhibitor approved for rheumatoid arthritis, could block melanoma growth. Either alone, or in combination with the BRAFV600E inhibitor PLX4032, leflunomide led to a dramatic decrease in melanoma growth in vitro and in vivo. These studies suggest that modulating the neural crest identity of melanoma cells is a distinct therapeutic avenue that is likely to cooperate with inhibiting oncogenic mutations such as BRAFV600E. We plan to extend this work into the clinical trial setting, highlighting the rapidity with which discoveries in zebrafish can be translated into the clinic. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-335. doi:10.1158/1538-7445.AM2011-LB-335