blmp-1 is a Ras-cooperating Tumor Suppressor Gene in Caenorhabditis elegans

While hypermorphic Ras mutations are found in approximately 35% of colorectal, 45% of lung and 90% of pancreatic cancers, activated Ras alone is not sufficient to transform normal cells into cancerous tumors. Additional driver mutations or epigenetic alterations are required to cooperate with the activating Ras mutations in tumorigenesis. In C. elegans, worms homozygous for activated Ras mutations have 60 - 80% penetrant visible multivulval (Muv) phenotype. Several suppressors and enhancers of this phenotype have been shown to be orthologous to Ras-cooperating proto-oncogenes and tumor-suppressor genes, respectively. Because several epigenetic regulators are now targets of cancer treatments and many cancer genomes exhibit changes in epigenetic regulation, I sought to systematically identify chromatin regulators that cooperate with activated Ras using RNAi screening in C. elegans. After screening 215 C. elegans chromatin remodeling factors and 193 C. elegans orthologs of known driver genes from Tumor Sequencing Project, I identified several attenuators of Ras signaling. By generating double mutants, I was able to confirm that loss of PRDM1/Blimp1 homolog blmp-1 served as a strong enhancer of the let-60(n1046) Muv phenotype. In addition, blmp-1(tm548) mutant worms are not Muv. I chose to investigate one of these genes, called blmp-1, in great detail because of known roles of its mammalian ortholog PRDM1/Blimp1 in cancer. PRDM1/Blimp1 is a known tumor suppressor gene in activated-B cell diffuse large B-cell lymphoma (ABC-DLBCL). Whether PRDM1/Blimp1 cooperates with activated Ras is unknown. Therefore, I further sought to understand how BLMP-1 attenuates Ras signaling in C. elegans. blmp-1 mutants have mildly elevated Ras signaling in the vulval cells. In addition, blmp-1(RNAi) and null mutant blmp-1(tm548) are strong enhancers of let-60(n1046) Muv phenotype, resulting in nearly 100% penetrance Muv. blmp-1 expression is downregulated in response to vulval induction, and this downregulation is important for correct vulval induction. However, bypassing this downregulation reduced, but did not abolish vulval induction, suggesting that blmp-1 functions late in vulval development. In addition, I also identified distinct key regions within the blmp-1 promoter which are necessary and sufficient for the regulation of blmp-1 expression in the gut, seam cells, hypodermal cells and the vulva cells. In conclusion, this is the first characterization of blmp-1 as a putative Ras-cooperating tumor suppressor gene as blmp-1 enhances the activated Ras phenotype through a novel mechanism. These results may shed light into our understanding of ABC-DCLBL as a Ras pathway-driven cancer, and PRDM1/Blimp1 as a Ras-cooperating tumor suppressor genes.Ph.D