MAPK pathway-driven tumorigenesis, often induced by BRAFV600E, relies on epithelial dedifferentiation. However, how lineage differentiation events are reprogrammed remains unexplored. Here, we demonstrate that proteostatic reactivation of developmental factor, TBX3, accounts for BRAF/MAPK-mediated dedifferentiation and tumorigenesis. During embryonic development, BRAF/MAPK upregulates USP15 to stabilize TBX3, which orchestrates organogenesis by restraining differentiation. The USP15-TBX3 axis is reactivated during tumorigenesis, and Usp15 knockout prohibits BRAFV600E-driven tumor development in a Tbx3-dependent manner. Deleting Tbx3 or Usp15 leads to tumor redifferentiation, which parallels their overdifferentiation tendency during development, exemplified by disrupted thyroid folliculogenesis and elevated differentiation factors such as Tpo, Nis, Tg. The clinical relevance is highlighted in that both USP15 and TBX3 highly correlates with BRAFV600E signature and poor tumor prognosis. Thus, USP15 stabilized TBX3 represents a critical proteostatic mechanism downstream of BRAF/MAPK-directed developmental homeostasis and pathological transformation, supporting that tumorigenesis largely relies on epithelial dedifferentiation achieved via embryonic regulatory program reinitiation.

本研究表明，發育因子TBX3的蛋白抑制再激活，解釋了BRAF/ mapk介導的去分化和腫瘤發生。在胚胎發育過程中，BRAF/MAPK上調USP15以穩定TBX3, TBX3通過抑制分化來協調器官發生。Usp15 - tbx3軸在腫瘤發生過程中被重新激活，Usp15敲除以tbx3依賴的方式阻止BRAFV600E驅動的腫瘤發展。刪除Tbx3或Usp15會導致腫瘤再分化，這與它們在發育過程中的過度分化傾向相似，例如甲狀腺濾泡發生中斷和分化因子如Tpo、Nis、Tg升高。研究結果表明，USP15和TBX3均與BRAFV600E特征和腫瘤預后不良高度相關。因此，USP15穩定的TBX3代表了BRAF/ mapk導向的發育穩態和病理轉化下游的關鍵蛋白抑制機制，支持腫瘤發生主要依賴于通過胚胎調控程序重新啟動實現的上皮去分化。

采用PLA法驗證USP15和TBX3在K1細胞中的共定位。