PIGA and paroxysmal nocturnal hemoglobinuria: Pathogenic variants in PIGV typically cause hyperphosphatasia mental retardation syndrome when present in homozygous or compound heterozygous states.[19] Defects in GPI-anchor biosynthesis are implicated in various human diseases; in particular, mutations in PIGA[32] and, as was recently discovered, PIGV[34] mutations have been found to be causative of paroxysmal nocturnal hemoglobinuria.