PGAP2 and Elevated circulating alkaline phosphatase concentration: The top ranking connecting concept is PGAP2. PGAP2 defects cause Hyperphosphatasia with mental retardation syndrome 3 and are thought to be related to the function of PGAP2 in the maturation of glycosylphosphatidylinositol (GPI) modifications to proteins that anchor them in the plasma membrane [22,23].