Holoprosencephaly (HPE) is a common birth defect of the forebrain and midface with both environmental (teratogenic) and genetic causes. Among the environmental factors is maternal alcohol consumption during early pregnancy. In addition, mutations in the Sonic hedgehog (Shh) signaling pathway have been identified in human HPE. The HPE phenotype of mutation carriers is highly variable, even within pedigrees, with the spectrum of effects ranging from no clinical manifestation, to facial """"""""microforms"""""""" of HPE, to full blown HPE, including cyclopia. The reason for this variability is unknown, but factors that may also be required include additional genetic influences or environmental exposures during fetal development. These possibilities are not mutually exclusive and gene-environment interactions are likely to be involved in an etiologically complex malady like HPE. In addition to clinical observations, animal models link fetal alcohol exposure to HPE, including a well-developed system in the mouse and a chick model associated with a disruption in Shh signaling. These observations raise important questions about the etiology of human HPE, including: 1) do teratogenic exposures worsen outcomes of fetuses with genetic predisposition to HPE; and 2) does genetic predisposition to HPE sensitize individuals to lower doses of teratogen than required to induce HPE in a """"""""normal"""""""" individual. This proposal addresses such questions experimentally for the first time. We have established a mouse genetic model of HPE, and it will be used to develop a new model of gene-environment interactions in the study of ethanol-induced HPE. Mice lacking the Shh pathway regulator, Cdo, display strain-specific severity of HPE phenotypes: mutants of the 129/Sv background show microforms of HPE at -50% penetrance, while mutants of the C57BI/6 background show severe forms at >80% penetrance.
The Specific Aim of this proposal is to assess the possibility of gene-environment interactions in the genesis of HPE, through in utero exposure of mice lacking Cdo to alcohol. It is hypothesized that exposure to alcohol will worsen the phenotypes of Cdo-/- embryos, such that those of the 129/Sv strain more closely resemble those of the C57BI/6 strain, while C57BI/6 embryos display the most severe HPE phenotypes, not normally observed with loss of Cdo, such as cyclopia. It is also hypothesized that exposure to alcohol will exaggerate the defect in Shh signaling seen in Cdo-/- embryos. These studies involve the development of a new model that could establish for the first time the relevance of gene-environment interactions between a defined mutation and a teratogen with wide use in the etiology of a common birth defect. Such information may have important public health impact and could ultimately aid in the counseling of HPE mutation carriers. ? ? ?
