Definition:

Paroxysmal nocturnal hemoglobinuria (PNH) is an uncommon intravascular hemolytic anemia that results from the clonal expansion of hematopoietic stem cells harboring somatic mutations in an X-linked gene, termed PIG-A. PIG-A is required for the biosynthesis of a lipid moiety, glycosylphosphatidylinositol (GPI), that attaches dozens of different proteins to the cell surface. PIG-A mutations block GPI anchor biosynthesis, resulting in a deficiency or absence of all GPI-anchored proteins on the cell surface. This deficiency on erythrocytes leads to intravascular hemolysis since certain GPI anchored proteins normally function as complement regulators.