B-Raf^V600Eand thrombospondin-1 promote thyroid cancer progression

Although B-Raf^V600E is the most common somatic mutation in papillary thyroid carcinoma (PTC), how it induces tumor aggressiveness is not fully understood. Using gene set enrichment analysis and in vitro and in vivo functional studies, we identified and validated a B-Raf^V600E gene set signature associated with tumor progression in PTCs. An independent cohort of B-Raf^V600E-positive PTCs showed significantly higher expression levels of many extracellular matrix genes compared with controls. We performed extensive in vitro and in vivo validations on thrombospondin-1 (TSP-1), because it has been previously shown to be important in the regulation of tumor angiogenesis and metastasis and is present in abundance in tumor stroma. Knockdown of B-Raf^V600E resulted in TSP-1 down-regulation and a reduction of adhesion and migration/invasion of human thyroid cancer cells. Knockdown of TSP-1 resulted in a similar phenotype. B-Raf^V600E cells in which either B-Raf^V600E or TSP-1 were knocked down were implanted orthotopically into the thyroids of immunocompromised mice, resulting in significant reduction in tumor size and fewer pulmonary metastases from the primary carcinoma as compared with the control cells. Treatment of orthotopic thyroid tumors, initiated 1 week after tumor cell implantation with PLX4720, an orally available selective inhibitor of B-Raf^V600E, caused a significant tumor growth delay and decreased distant metastases, without evidence of toxicity. In conclusion, B-Raf^V600E plays an important role in PTC progression through genes (i.e., TSP-1) important in tumor invasion and metastasis. Testing of a patient's thyroid cancer for B-Raf^V600E will yield important information about potential tumor aggressiveness and also allow for future use of targeted therapies with selective B-Raf^V600E inhibitors, such as PLX4720.