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，是由于无法确定前脑和/或中脸的中线而引起的。HPE与NODAL和Sonic hedgehog （SHH）通路组分的杂合突变有关，但临床表现变化很大，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修饰因子，遗传和/或环境相互作用。此外，在单个hpe相关基因中观察到许多不同的变异等位基因。因此，HPE的特点是缺乏明确的基因型-表型相关性和高遗传异质性。这些现象阻碍了对HPE病因的深入了解，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)必须从机制上识别和理解真正的致病突变与修饰因子之间的相互作用；2)必须分析特定的hpe相关遗传变异，以证明它们是功能缺陷并确定潜在的机制。这个建议解决了这两个问题。我们在小鼠身上建立了HPE模型。CDON编码一种多功能的辅助受体，通过多种途径促进信号传导，包括SHH途径。携带Cdon突变的小鼠在SHH信号传导中有很大程度的亚阈缺陷。这些小鼠对遗传或环境修饰因子诱导颅面和前脑HPE表型谱敏感，而这些修饰因子本身不足以产生HPE。在这些修饰因素中，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，除了在SHH信号传导中发挥作用外，

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.