Li Xiao, Venugopalan Shankar R, Cao Huojun, Pinho Flavia O, Paine Michael L, Snead Malcolm L, Semina Elena V, Amendt Brad A
Department of Anatomy and Cell Biology and Craniofacial Anomalies Research Center, The University of Iowa, Iowa City, IA 52244, USA.
Hum Mol Genet. 2014 Jan 1;23(1):194-208. doi: 10.1093/hmg/ddt411. Epub 2013 Aug 23.
Patients with Axenfeld-Rieger Syndrome (ARS) present various dental abnormalities, including hypodontia, and enamel hypoplasia. ARS is genetically associated with mutations in the PITX2 gene, which encodes one of the earliest transcription factors to initiate tooth development. Thus, Pitx2 has long been considered as an upstream regulator of the transcriptional hierarchy in early tooth development. However, because Pitx2 is also a major regulator of later stages of tooth development, especially during amelogenesis, it is unclear how mutant forms cause ARS dental anomalies. In this report, we outline the transcriptional mechanism that is defective in ARS. We demonstrate that during normal tooth development Pitx2 activates Amelogenin (Amel) expression, whose product is required for enamel formation, and that this regulation is perturbed by missense PITX2 mutations found in ARS patients. We further show that Pitx2-mediated Amel activation is controlled by chromatin-associated factor Hmgn2, and that Hmgn2 prevents Pitx2 from efficiently binding to and activating the Amel promoter. Consistent with a physiological significance to this interaction, we show that K14-Hmgn2 transgenic mice display a severe loss of Amel expression on the labial side of the lower incisors, as well as enamel hypoplasia-consistent with the human ARS phenotype. Collectively, these findings define transcriptional mechanisms involved in normal tooth development and shed light on the molecular underpinnings of the enamel defect observed in ARS patients who carry PITX2 mutations. Moreover, our findings validate the etiology of the enamel defect in a novel mouse model of ARS.
阿克森费尔德-里格尔综合征（ARS）患者存在多种牙齿异常，包括牙齿发育不全和釉质发育不全。ARS在基因上与PITX2基因突变相关，该基因编码启动牙齿发育的最早转录因子之一。因此，长期以来Pitx2一直被认为是早期牙齿发育中转录层次的上游调节因子。然而，由于Pitx2也是牙齿发育后期阶段的主要调节因子，尤其是在釉质形成过程中，目前尚不清楚突变形式如何导致ARS牙齿异常。在本报告中，我们概述了ARS中存在缺陷的转录机制。我们证明，在正常牙齿发育过程中，Pitx2激活釉原蛋白（Amel）的表达，其产物是釉质形成所必需的，并且这种调节受到ARS患者中发现的错义PITX2突变的干扰。我们进一步表明，Pitx2介导的Amel激活受染色质相关因子Hmgn2的控制，并且Hmgn2阻止Pitx2有效结合并激活Amel启动子。与这种相互作用的生理意义一致，我们表明K14-Hmgn2转基因小鼠在下切牙唇侧显示出严重的Amel表达缺失，以及与人类ARS表型一致的釉质发育不全。总体而言，这些发现定义了正常牙齿发育所涉及的转录机制，并揭示了携带PITX2突变的ARS患者中观察到釉质缺陷的分子基础。此外，我们的发现验证了ARS新型小鼠模型中釉质缺陷病因。
