<i>Cited1</i> Deficiency Suppresses Intestinal Tumorigenesis

Conditional deletion of Apc in the murine intestine alters crypt-villus architecture and function. This process is accompanied by multiple changes in gene expression, including upregulation of Cited1, whose role in colorectal carcinogenesis is unknown. Here we explore the relevance of Cited1 to intestinal tumorigenesis. We crossed Cited1 null mice with ApcMin/+ and AhCre+Apcfl/fl mice and determined the impact of Cited1 deficiency on tumour growth/initiation including tumour multiplicity, cell proliferation, apoptosis and the transcriptome. We show that Cited1 is up-regulated in both human and murine tumours, and that constitutive deficiency of Cited1 increases survival in ApcMin/+ mice from 230.5 to 515 days. However, paradoxically, Cited1 deficiency accentuated nearly all aspects of the immediate phenotype 4 days after conditional deletion of Apc, including an increase in cell death and enhanced perturbation of differentiation, including of the stem cell compartment. Transcriptome analysis revealed multiple pathway changes, including p53, PI3K and Wnt. The activation of Wnt through Cited1 deficiency correlated with increased transcription of β-catenin and increased levels of dephosphorylated β-catenin. Hence, immediately following deletion of Apc, Cited1 normally restrains the Wnt pathway at the level of β-catenin. Thus deficiency of Cited1 leads to hyper-activation of Wnt signaling and an exaggerated Wnt phenotype including elevated cell death. Cited1 deficiency decreases intestinal tumourigenesis in ApcMin/+ mice and impacts upon a number of oncogenic signaling pathways, including Wnt. This restraint imposed by Cited1 is consistent with a requirement for Cited1 to constrain Wnt activity to a level commensurate with optimal adenoma formation and maintenance, and provides one mechanism for tumour repression in the absence of Cited1.在小鼠肠道中条件性敲除Apc会改变隐窝-绒毛结构与功能。该过程伴随多项基因表达改变，包括Cited1的上调，而Cited1在结直肠癌发生中的作用尚不明确。本研究探讨Cited1与肠道肿瘤发生的相关性。我们将Cited1组成型敲除小鼠与ApcMin/+及AhCre+Apcfl/fl小鼠进行杂交，分析Cited1缺陷对肿瘤生长/起始的影响，包括肿瘤多发数目、细胞增殖、细胞凋亡及转录组特征。研究发现，Cited1在人类与小鼠肿瘤中均呈上调表达；且Cited1组成型缺陷可将ApcMin/+小鼠的生存期从230.5天延长至515天。但反常的是，在Apc条件性敲除4天后，Cited1缺陷会加剧几乎所有即时表型，包括细胞死亡增加、分化扰动增强（涵盖干细胞区室）。转录组分析显示存在多条通路的改变，包括p53、PI3K及Wnt通路。通过Cited1缺陷激活Wnt通路，与β-连环蛋白（β-catenin）的转录上调及去磷酸化β-连环蛋白水平升高相关。因此，在Apc缺失后的早期阶段，Cited1通常会在β-连环蛋白层面抑制Wnt通路。由此，Cited1缺陷会导致Wnt信号通路过度激活，并引发包括细胞死亡升高在内的过度Wnt表型。Cited1缺陷可降低ApcMin/+小鼠的肠道肿瘤发生风险，并影响包括Wnt在内的多条致癌信号通路。Cited1所施加的这种抑制作用，与Cited1需要将Wnt活性维持在适配腺瘤最佳形成与维持的水平这一需求相符，同时也为Cited1缺失时的肿瘤抑制机制提供了一种解释。