Haploinsufficiency for Core Exon Junction Complex Components Disrupts Embryonic Neurogenesis and Causes p53-Mediated Microcephaly

The exon junction complex (EJC) is an RNA binding complex comprised of the core components Magoh, Rbm8a, and Eif4a3. Human mutations in EJC components cause neurodevelopmental pathologies. Further, mice heterozygous for either Magoh or Rbm8a exhibit aberrant neurogenesis and microcephaly. Yet despite the requirement of these genes for neurodevelopment, the pathogenic mechanisms linking EJC dysfunction to microcephaly remain poorly understood. Here we employ mouse genetics, transcriptomic and proteomic analyses to demonstrate that haploinsufficiency for each of the 3 core EJC components causes microcephaly via converging regulation of p53 signaling. Using a new conditional allele, we first show that Eif4a3 haploinsufficiency phenocopies aberrant neurogenesis and microcephaly of Magoh and Rbm8a mutant mice. Transcriptomic and proteomic analyses of embryonic brains at the onset of neurogenesis identifies common pathways altered in each of the 3 EJC mutants, including ribosome, proteasome, and p53 signaling components. We further demonstrate all 3 mutants exhibit defective splicing of RNA regulatory proteins, implying an EJC dependent RNA regulatory network that fine-tunes gene expression. Finally, we show that genetic ablation of one downstream pathway, p53, significantly rescues microcephaly of all 3 EJC mutants. This implicates p53 activation as a major node of neurodevelopmental pathogenesis following EJC impairment. Altogether our study reveals new mechanisms to help explain how EJC mutations influence neurogenesis and underlie neurodevelopmental disease.

外显子连接复合物（exon junction complex, EJC）是一类由核心组分Magoh、Rbm8a及Eif4a3构成的RNA结合复合物。人类EJC组分发生突变可引发神经发育病理异常。进一步研究表明，携带Magoh或Rbm8a杂合突变的小鼠会表现出神经发生异常与小头畸形。尽管这些基因对神经发育不可或缺，但将EJC功能异常与小头畸形关联的致病机制仍有待阐明。本研究借助小鼠遗传学、转录组学与蛋白质组学分析，证实3种核心EJC组分的单倍剂量不足均通过调控p53信号通路引发小头畸形。我们利用新型条件性等位基因，首先证实Eif4a3单倍剂量不足可重现Magoh与Rbm8a突变小鼠的神经发生异常及小头畸形表型。在神经发生起始阶段对胚胎脑组织开展转录组与蛋白质组分析，发现3种EJC突变小鼠均存在共同的通路异常，包括核糖体、蛋白酶体及p53信号通路相关组分。我们进一步证实，3种突变小鼠均存在RNA调控蛋白的剪接缺陷，这提示存在依赖于EJC的RNA调控网络，可精细调控基因表达。最终我们发现，敲除下游通路之一的p53可显著挽救3种EJC突变小鼠的小头畸形表型。这表明p53激活是EJC功能受损后神经发育致病的核心节点。综上，本研究揭示了EJC突变影响神经发生、进而介导神经发育疾病的全新机制。