[Inherited GPI deficiencies:a new disease with intellectual disability and epilepsy]

Glycosylphosphatidylinositol (GPI) is a glycolipid, which anchors 150 or more types of proteins to the cell surface. There are at least 26 genes involved in the biosynthesis and transport of GPI-anchored proteins (GPI-APs). Many inherited GPI deficiencies (IGDs) have been recently found using whole-exome sequencing. Patients with IGD have only a partial deficiency because complete GPI deficiency causes embryonic death. The major symptoms of IGDs include intellectual disability, epilepsy, coarse facial features, and multiple organ anomalies. These symptoms vary in severity depending upon the degree of the defect and/or position in the pathway of the affected gene. We clarified a mechanism of hyperphosphatasia, which is characterized by elevated release of tissue-nonspecific alkaline phosphatase. Hyperphosphatasia is observed in some patients with IGDs, such as hyperphosphatasia mental retardation syndrome or Mabry syndrome, caused by mutations in genes in the later stage of GPI biosynthesis. The possibility of IGD should be considered in patients with seizures and intellectual disability. The presence of hyperphosphatasia is strong evidence of IGD. Flow cytometric analysis of GPI-APs on granulocytes is also useful for the detection of IGD.