Proteostatic reactivation of the developmental transcription factor TBX3 drives BRAF/MAPK-mediated tumorigenesis.

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. mRNA profiles of mouse thyroid control and Tbx3 knockout were generated by deep sequencing, in duplicate, using Illumina novaseq6000.由BRAF V600E突变（BRAFV600E）常诱导的丝裂原活化蛋白激酶（mitogen-activated protein kinase, MAPK）通路驱动的肿瘤发生，依赖于上皮去分化。然而，谱系分化事件如何被重编程仍未被阐明。本研究证实，发育因子TBX3的蛋白稳态重激活，是BRAF/MAPK介导的去分化与肿瘤发生的关键机制。在胚胎发育过程中，BRAF/MAPK可上调USP15以稳定TBX3，后者通过抑制分化调控器官发生。肿瘤发生过程中，USP15-TBX3轴被重新激活，而Usp15基因敲除可通过依赖Tbx3的方式阻断BRAFV600E驱动的肿瘤发展。敲除Tbx3或Usp15会导致肿瘤再分化，这与其在发育过程中的过度分化倾向一致，具体表现为甲状腺滤泡发生受损，以及Tpo、Nis、Tg等分化因子表达上调。临床相关性分析显示，USP15与TBX3的高表达均与BRAFV600E特征基因集及不良肿瘤预后显著相关。综上，USP15稳定TBX3这一过程，是BRAF/MAPK下游调控发育稳态与病理转化的关键蛋白稳态机制，支持肿瘤发生在很大程度上依赖于通过重新激活胚胎调控程序实现的上皮去分化。本研究通过Illumina NovaSeq 6000测序平台，对小鼠甲状腺对照样本与Tbx3敲除样本进行了两次生物学重复的深度测序，获得了其mRNA表达谱。