Somatic mutations in TBX3 promote hepatic clonal expansion by accelerating VLDL secretion

We've previously demonstrated that hepatocytes with Tbx3 deletions have increased cellular fitness and clonally expand during metabolic-dysfunction associated steatotic liver disease (MASLD) development. TBX3 is a transcription factor that is critical for embryonic development, but has never been studied within the context of MASLD. Here, we show that somatic mutations in TBX3 are found in human patients with MASLD. Liver specific deletion of Tbx3 in mice protects against diet induced MASLD through upregulating VLDL-TG particle secretion. TBX3 transcriptionally suppresses the conventional secretory pathway and cholesterol biosynthesis. Hdlbp is a direct target of TBX3 that is responsible for altered VLDL secretion. In contrast to WT TBX3, TBX3 harboring somatic mutations found in humans failed to suppress VLDL secretion in vivo. In conclusion, mutations in TBX3 promote clonal expansion during MASLD development through increased lipid disposal, demonstrating clonal fitness can benefit the liver even at the cost of hyperlipidemia. Overall design: High throughput sequencing of DNA from the CUT&RUN protocol of stable H2.35 mouse hepatocyte cell line overexpressing a V5 tagged mouse Tbx3 compared to an IgG isotype control (n=3) and ChIP-sequencing of human TBX3 from mouse livers overexpressing Ty1 tagged TBX3 via AAV compared to an IgG isotype control (n=4).

本团队此前已证实，Tbx3敲除的肝细胞在代谢功能障碍相关性脂肪性肝病（metabolic-dysfunction associated steatotic liver disease, MASLD）发生过程中，细胞适应性增强并发生克隆扩增。TBX3是一种对胚胎发育至关重要的转录因子，但此前尚未在MASLD的研究背景中得到探究。
本研究证实，MASLD患者体内可检测到TBX3的体细胞突变。小鼠肝脏特异性敲除Tbx3可通过上调极低密度脂蛋白-甘油三酯（VLDL-TG）颗粒分泌，抵御饮食诱导的MASLD。TBX3可在转录层面抑制经典分泌通路与胆固醇生物合成过程。Hdlbp是TBX3的直接靶基因，其参与调控VLDL分泌的改变。与野生型（wild type, WT）TBX3不同，携带人类体细胞突变的TBX3无法在体内抑制VLDL分泌。
综上，TBX3突变可通过增强脂质清除能力，促进MASLD发生过程中的克隆扩增，这表明克隆适应性可在以高脂血症为代价的情况下，为肝脏带来益处。
实验整体设计：采用CUT&RUN技术对过表达V5标签小鼠Tbx3的稳定H2.35小鼠肝细胞系的基因组DNA进行高通量测序，并以IgG同型对照作为空白对照（n=3）；同时对通过腺相关病毒（adeno-associated virus, AAV）在小鼠肝脏中过表达Ty1标签人源TBX3的样本进行染色质免疫共沉淀测序（ChIP-sequencing），同样以IgG同型对照作为空白对照（n=4）。