Molecular and Developmental Analysis of Holoprosencephaly

描述(由申请人提供)：无前脑畸形(HPE)是一种常见的人类发育缺陷，原因是未能确定前脑和/或中线的中线。HPE与Node和Sonic Hedgehog(SHH)途径成分的杂合突变有关，但临床表现高度可变，许多突变携带者不受影响。因此，人们认为这种突变必须与更常见的基因和/或环境修饰因素相互作用。此外，在单个HPE相关基因中观察到了许多明显的变异等位基因。因此，HPE的特点是缺乏明确的基因-表型相关性和高度的遗传异质性。这些现象阻碍了对HPE病因学的深入了解，并使向突变携带者提供咨询变得困难。为了提供准确的信息，需要解决两个重要的问题：1)必须从机械上识别和理解真正的致病突变与分级外显性和表达能力的修饰物之间的相互作用；2)必须分析特定的HPE相关基因变体，以证明它们在功能上是缺陷的，并确定潜在的机制。这项建议解决了这两个问题。我们已经在老鼠身上建立了HPE的模型。CDON编码一种多功能的辅助受体，通过几条途径促进信号传递，包括SHH途径。携带Cdon突变的小鼠的SHH信号在很大程度上存在亚阈值缺陷。这些小鼠对遗传或环境修饰物诱导的一系列颅面部和前脑HPE表型很敏感，而这些遗传或环境修饰物本身不足以产生HPE。在这些修饰物中，有一种是在原肠形成过程中短暂的胎儿酒精暴露。新的发现表明，除了它在SHH信号中的作用外， CDON与结节受体复合体的组成成分相互作用。此外，结节通路活性的一过性抑制似乎是CDON丢失和胎儿酒精暴露之间协同作用的点。结节信号的调节代表了CDON的一个新的角色，补充了它在面部和前脑中线模式的SHH信号中的功能。我们还鉴定了人类HPE中功能丧失的CDON突变并对其进行了表征。SHH受体复合体的其他成分也在人类HPE中发生突变，包括ptch1和BOC，但关于这些受体变体是否以及如何在SHH信号转导中存在缺陷的信息很少。提出了以下目标：1)确定CDON和节点通路之间的遗传和机制相互作用；以及2)确定BOC和ptch1中HPE相关变体的功能和机制后果。拟议目标的成功完成将提供关于以下方面的重要信息：1)环境因素如何与易感突变相互作用，从而为预防行动提供基础；以及2)特定患者突变的致病性质。因此，这些研究有望帮助对其后代可能在遗传上易患HPE的个体进行咨询。

DESCRIPTION (provided by applicant): Holoprosencephaly (HPE) is a common human developmental defect caused by failure to define the midline of the forebrain and/or midface. HPE is associated with heterozygous mutations in NODAL and Sonic hedgehog (SHH) pathway components, but clinical presentation is highly variable, and many mutation carriers are unaffected. It is therefore thought that such mutations must interact with more common modifiers, genetic and/or environmental. Furthermore, many distinct variant alleles are observed for individual HPE-associated genes. HPE is therefore characterized by a lack of clear genotype-phenotype correlation and by high genetic heterogeneity. These phenomena have prevented a deep understanding of HPE etiology and made it difficult to provide counseling to mutation carriers. Two significant issues need to be addressed to provide accurate information: 1) interactions between bona fide pathogenic mutations and the modifiers that grade penetrance and expressivity must be identified and understood mechanistically; and 2) specific HPE-associated genetic variants must be analyzed to prove that they are functionally deficient and identify the underlying mechanism. This proposal addresses both issues. We have modeled HPE in mice. CDON encodes a multifunctional coreceptor that promotes signaling by several pathways, including the SHH pathway. Mice with a mutation in Cdon have a largely subthreshold defect in SHH signaling. These mice are sensitive to induction of a spectrum of craniofacial and forebrain HPE phenotypes by genetic or environmental modifiers that are, themselves, insufficient to produce HPE. Among these modifiers is transient fetal alcohol exposure during gastrulation. New findings indicate that, in addition to its role in SHH signaling, CDON interacts with components of the NODAL receptor complex. Furthermore, transient inhibition of NODAL pathway activity appears to be the point of synergistic interaction between loss of CDON and fetal alcohol exposure. Regulation of NODAL signaling represents a new role for CDON, complementing its function in SHH signaling during midline patterning of the face and forebrain. We have also identified and characterized loss-of-function CDON mutations in human HPE. Other components of the SHH receptor complex are also mutated in human HPE, including PTCH1 and BOC, but there is little information on whether and how these receptor variants are defective in SHH signaling. The following Aims are proposed: 1) to determine genetic and mechanistic interactions between CDON and the NODAL pathway; and 2) to determine the functional and mechanistic consequences of HPE-associated variants in BOC and PTCH1. The successful completion of the proposed aims will provide important information about: 1) how environmental factors interact with predisposing mutations, thus providing a basis for preventive action; and 2) the pathogenic nature of specific patient mutations. Therefore, these studies are expected to aid in the counseling of individuals whose offspring may be genetically predisposed to HPE.