Agenesis of the corpus callosum: lessons from humans and mice.

BACKGROUND AND
PURPOSE: The corpus callosum serves as a bridge to associate fibres between the two cerebral hemispheres. In placental mammals, this commissure provides for higher order neurological advantages. The molecular pathways involved in the development and pathogenesis of accallosal defects are sparse. The article reviews the current progress of studies undertaken to discern the embryological and genetic basis of the development of the corpus callosum. SOURCES OF DATA: The literature, including from sources such as MEDLINE and OMIM, were subjected to searches for articles reporting findings on corpus callosum development in humans and mice. PRINCIPAL
FINDINGS: At least forty-six malformation syndromes and metabolic disorders have been reported in patients with complete agenesis or hypoplasia (dysgenesis) of the corpus callosum. Thirteen of these syndromes have an unknown mode of genetic inheritance, and the remaining syndromes and metabolic disorders exhibit either autosomal or X-linked inheritance among affected families. The use of patients with accallosal defects have identified mutations in at least thirty genes of the human genome, and therefore with roles implicated in the development of the corpus callosum. Patients with chromosome aberrations have been useful in defining regions on chromosomes that contain candidate genes for the development of the corpus callosum. At least eighteen different human chromosomes with numerical and/or structural aberrations have been reported in patients with acallosal defects. The mouse is an excellent model to study the structural and genetic factors that influence the development of the corpus callosum, with many similarities evident in humans. Spontaneous development of acallosal defects has been reported in at least seventeen mouse strains. Furthermore, with the use of Genetically Engineered Mice, a minimum of 15 candidate callosal agenesis genes were modeled in order to provide insightful knowledge of the molecular-structural parameters required for development of the corpus callosum. Of these mice, six had complete true agenesis of the corpus callosum, five had either true agenesis or hypoplasia of the corpus callosum, and four had hypoplasia of the corpus callosum.
CONCLUSIONS: The molecular mechanisms implicated in the pathogenesis of agenesis or hypoplasia of the corpus callosum are at the verge of discovery, and are challenged by the complexity of many genes involved. Despite these barriers, findings from a complementary human-mouse model system have been helpful in understanding the genetic (molecular) causes of accallosal defects, a fascinating phenotype for over a century.