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Cancer biology in the zebrafish

Leonard Ira Zon

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National Institutes of Health (NIH)
Cancer comprises a set of devastating diseases that cause significant morbidity and mortality. My laboratory has utilized the zebrafish as a model for studying cancer. Our zebrafish melanoma model was constructed by overexpression of the human BRAFV600 allele (mutated in about 70% of human melanomas) in combination with p53 deficiency: we developed an expression cloning strategy to examine the effect of specific genes on melanoma formation. In one example, we found the driver gene on human chromosome 1 that is amplified in 30 percent of melanoma. SETDB1, an H3K9 trimethylase, enhanced tumorigenesis in our melanoma model. SETDB1 is an epigenetic regulator that suppresses gene expression and promotes melanoma formation and invasion. We have since successfully undertaken a large-scale overexpression screen for other chromatin factors that accelerate melanoma in vivo. Here we propose to study the mechanism of action for SATB2, along with its family member SATB1 (SATB1 protein expression correlates to the stage of human melanoma), and HDGF. We will purify the SATB2 and HDGF protein complexes in human melanoma cells, and study components of the complex by gene knockdown with morpholinos and by overexpression in zebrafish. Our laboratory has also recently documented that leflunomide (LEF), known to inactivate DHODH (an enzyme required for pyrimidine synthesis), is capable of suppressing melanoma formation and, in combination with a BRAF inhibitor, lead to substantial reduction of melanoma growth. We have rapidly proceeded to a clinical trial that tests the combination therapy of a BRAF inhibitor and LEF as a new therapy for metastatic melanoma. The mechanism that underlies LEF-associated suppression of melanoma involves a block of transcriptional elongation of neural crest genes. Our study of the regulation of transcriptional pausing in melanoma cells revealed the involvement of the HEXIM complex, which is composed of RNA and proteins, and which sequesters the kinase P-TEFb that phosphorylates POLII to allow transcriptional elongation to occur. Inactivation of the HEXIM complex rescues the effects of LEF treatment of zebrafish embryos. Informatics studies have uncovered that HEXIM is substantially downregulated in a variety of melanomas. Preliminary data suggest that HEXIM overexpression can suppress melanoma formation in our model. We plan to express a constitutively active and dominant negative HEXIM mutant and evaluate the effect on melanoma formation. We have also initiated a chemical screen to find factors that alter transcriptional pausing in melanoma. Chemicals that rescue LEF could be helpful in determining the mechanism of action by which a DHODH inhibitor leads to a transcription elongation defect. We anticipate that results of the proposed experiments will have a broad impact on our understanding of the basic biology of melanoma, including epigenetic and transcriptional mechanisms, and will rapidly translate to new clinical trials on patients with melanoma.

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