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Paroxysmal Nocturnal Hemoglobinuria (PNH), Flow Cytometry

Paroxysmal nocturnal hemoglobinemia (PNH) is a hematopoietic stem cell disorder that arises from a mutation of the phosphatidylinositol glycan-class A (PIG-A) gene, which is located on the X chromosome. The abnormal gene occurs in somatic cells, especially hematopoietic stem cells, but not in germ cells, making PNH an acquired disorder. So far, PIG-A gene mutations have not been detected in somatic cells outside of the hematopoietic system. Approximately 174 somatic mutations in the PIG-A gene have been identified. The initial mutagenic event remains unknown.

The PIG-A gene is required for biosynthesis of glycosylphosphatidylinositol (GPI), which is a glycolipd that anchors approximately 30 different proteins to cell membranes. A partial list GPI-linked proteins includes CD14, CD16, CD24, CD55, CD56, CD58, CD59, C8-binding protein, alkaline phosphatase, acetylcholine esterase, and a variety of high frequency human blood group antigens. In PNH, erythrocytes, platelets, granulocytes, and monocytes have a partial or total lack of these GPI-anchored proteins.

Development of clinical disease requires the combination of a hypoplastic bone marrow, somatic mutation restricted to the PIG-Agene in hematopoietic stem cells, and clonal expansion of the mutant stem cells. Clinical features can vary greatly from patient to patient during the course of the disease. There are three main pathophysiologic features:

  • Intravascular hemolysis
  • Venous thrombosis
  • Bone marrow aplasia

Hemolysis is a consequence of abnormal erythrocyte sensitivity to complement-mediated lysis. PNH erythrocytes are abnormally sensitive to complement because they are deficient in two complement regulatory proteins, decay accelerating factor (DAF, CD55) and membrane inhibitor of reactive lysis (MIRL, CD59). DAF regulates the early portion of complement activation including C3-C5 while MIRL inhibits the incorporation of C9 into the membrane attack complex. The absence of CD59 is primarily responsible for red cell hemolysis and may contribute to thrombosis.

Deficiency of CD59 on platelets is also thought to play a role in thrombosis. CD59 deficient platelets are more readily activated by complement, leading to greater procoagulant activity and thrombin generation. Venous thrombosis often occurs in unusual anatomic locations such as mesenteric, hepatic (Budd Chiari Syndrome) and splenic veins. Thrombosis, which is often recurrent and resistant to treatment, is the major cause of death in Western patients with PNH while pancytopenia is the major manifestation of PNH in younger and Asian patients.

The International PNH Group recommends that patients with the following conditions should be tested for PNH:

  • Coombs-negative acquired hemolytic anemia
  • Hemoglobinuria
  • Venous thrombosis at atypical sites
  • Dysphagia with elevated LDH
  • Aplastic Anemia (AA)
  • Myelodysplasia (MDS)

The frequent association of PNH clones in patients with AA supports the theory that a hypocellular aplastic marrow may be conducive to the development of PNH. Recent data showed that small PNH clones can be detected in a relatively high percentage of cases of aplastic anemia and myelodysplastic syndrome.

Flow cytometry analysis of peripheral blood is the current gold standard for laboratory detection of PNH. Flow cytometric immunophenotyping is performed to detect the presence or absence of GPI-linked proteins on granulocytes, monocytes, and erythrocytes, using fluorescently labeled monoclonal antibodies or FLAER. FLAER is a fluorescently labeled variant of aerolysin that binds directly to the GPI anchor and can be used to evaluate the expression of the GPI linkage. Individuals with PNH have decreased or absent expression of CD14 on monocytes, CD 16 on neutrophils and NK cells, CD24 on neutrophils, CD59 on red blood cells and FLAER on neutrophils and monocytes. The Ham’s and sugar water tests are now considered obsolete.

Eculizumab, is a recombinant humanized monoclonal antibody directed against C5. The drug shows a high affinity to C5 and when bound remains until the complex of drug-complement is removed from circulation. Blocking complement at this step prevents the genesis of the membrane attack complex and inhibits the release of the inflammatory mediator C5a.

Transfusion may be necessary during episodes of severe hemolysis. Patients with PNH should be transfused with ABO-specific red blood cells (RBCs), which do not need to be washed. If, ABO specific red cells are not available, group O red blood cells should be washed prior to tranfusion. Red cells should also be leukocyte reduced because recipient antibodies to granulocyte antigens can trigger hemolytic episodes in PNH.

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