Complement-mediated haemolysis and the role of blood transfusion in paroxysmal nocturnal haemoglobinuria.

Abstract

Paroxysmal nocturnal haemoglobinuria (PNH) is a rare form of acquired chronic haemolytic anaemia with unique characteristics. From a clinical point of view the classical triad is that of intravascular haemolysis, thrombosis and cytopenias1,2. From the point of view of pathogenesis, PNH can be regarded as a non-malignant clonal disorder, in which a mutation of the X-linked PIG-A gene in a haematopoietic stem cell affects the its mature progeny in the peripheral blood, including erythrocytes, leucocytes and platelets3: this progeny is, therefore, referred to as a PNH clone4. In PNH patients the PNH clone co-exists with qualitatively normal blood cells, although in many cases these are a minority5. PNH has no gender predilection and an estimated population frequency of between 1 and 10 per million. Molecular basis of paroxysmal nocturnal haemoglobinuria The first biochemical abnormality to be demonstrated in red cells from patients with PNH was a deficiency of the membrane protein acetylcholinesterase6. Subsequent studies showed that several more membrane proteins were missing or markedly decreased in the blood cells belonging to the PNH clone. From the serological point of view it is interesting that one of these (decay accelerating factor, see below) bears the Cromer blood groups7. In the 1980s it emerged that these deficient membrane proteins have one important feature in common: they are anchored to the cell surface through a glycosyl phosphatidyl inositol (GPI) molecule8,9. This suggested that the underlying biochemical abnormality would be in the GPI biosynthetic pathway, and it was indeed pinpointed at the step at which N-acetylglucosamine (NAcGlcN) is transferred onto phoshatidyl inositol10. Through expression cloning in a GPI-deficient cell line Taroh Kinoshita’s group identified the PIG-A (phosphatidyl inositol glycan complementation group A) gene as being defective in PNH cells11, and PIG-A somatic mutations were indeed identified in PNH patients12. PIG-A maps to the short arm of the X chromosome (band p22.1)13. The PIG-A product is a 484 amino acid protein which, with three other proteins, forms NAcGlcN transferase, the enzyme that catalyses the first step of the biosynthesis of the GPI anchor. Almost all the somatic mutations of the PIG-A gene identified so far in PNH patients are either frame-shift or nonsense mutations, only two are large deletions14,15. It should be noted that one of these inactivating mutations is sufficient to give rise on its own to the PNH phenotype because PIG-A is X-linked: as a result, only one allele is present in men, and only one allele is active in somatic cells in women. Very recently a PNH patient has been reported who had no PIG-A mutation: instead, she had a germ-line mutation of another gene (PIG-T) encoding a GPI biosynthetic enzyme, and a somatic mutation of the other PIG-T allele16. This patient provides a powerful confirmation of the notion that the critical biochemical failure in the PNH clone is in biosynthesis of the GPI anchor; in terms of frequency, this is likely to remain an exceptional situation17.