Mutation of which of the following gene is most impoant in paroxysmal nocturnal hemoglobinuria?

Paroxysmal Nocturnal Hemoglobinuria (PNH): PNH is an acquired chronic Hemolytic Anemia characterized by persistent intravascular hemolysis subject to recurrent exacerbations. In addition to hemolysis, there is often pancytopenia and a distinct tendency to venous thrombosis. This triad makes PNH a truly unique clinical condition.Hemolysis in PNH is mainly intravascular and is due to an intrinsic abnormality of the red cell, which makes it exquisitely sensitive to activated C, whether it is activated through the alternative pathway or through an antigen-antibody reaction. The former mechanism is mainly responsible for chronic hemolysis in PNH; the latter explains why the hemolysis can be dramatically exacerbated in the course of a viral or bacterial infection. Hypersusceptibility to C is due to deficiency of several protective membrane proteins, of which CD59 is the most impoant because it hinders the inseion into the membrane of C9 polymers. The molecular basis for the deficiency of these proteins has been pinpointed not to a defect in any of the respective genes, but rather to the shoage of a unique glycolipid molecule, GPI, which, through a peptide bond, anchors these proteins to the surface membrane of cells. The shoage of GPI is due in turn to a mutation in an X-linked gene, called PIG-A, required for an early step in GPI biosynthesis.Ref: Harrison&;s Principles of Internal Medicine; 19th edition; Chapter 129 Hemolytic Anemias and Anemia Due to Acute Blood Loss Page no: 660