What is the pathophysiology of paroxysmal nocturnal hemoglobinuria (PNH)?

Updated: May 20, 2021
  • Author: Emmanuel C Besa, MD; Chief Editor: Sara J Grethlein, MD, FACP  more...
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Answer

The common denominator in the disease, a biochemical defect, appears to be a genetic mutation leading to the inability to synthesize the glycosyl-phosphatidylinositol (GPI) anchor that binds these proteins to cell membranes. [5, 6, 7] The corresponding gene PIGA (phosphatidylinositol glycan class A) in the X chromosome can have several mutations, from deletions to point mutations. [8]

Due to its location on the X chromosome, and X inactivation in female somatic cells, only one mutation is required in either males or females to abolish the expression of GPI-linked proteins. Most type II PNH cells (total lack of GPI-linked protein) are due to a frame shift mutation occurring in the early hematopoietic progenitor cells, resulting in the same mutation in all blood cell lines.

The essential group of membrane proteins that are lacking in all hematopoietic cells in PNH are called complement-regulating surface proteins, including the decay-accelerating factor (DAF), or CD55 [9] ; homologous restriction factor (HRF), or C8 binding protein; and membrane inhibitor of reactive lysis (MIRL), or CD59. [10] All of these proteins interact with complement proteins, particularly C3b and C4b, dissociate the convertase complexes of the classic and alternative pathways, and halt the amplification of the activation process.

The absence of these regulating proteins results in uncontrolled amplification of the complement system. This leads to intravascular destruction of the RBC membrane, to varying degrees. See the image below.

In paroxysmal nocturnal hemoglobinuria (PNH), the absence of anchor proteins that bind complement-regulating proteins (eg, CD55, CD59) to the surface of red blood cells (RBCs) leaves these RBCs susceptible to destruction by the complement membrane attack complex (MAC). Image courtesy of Haematologica. 2010 April;95(4).

In paroxysmal nocturnal hemoglobinuria (PNH), the In paroxysmal nocturnal hemoglobinuria (PNH), the absence of anchor proteins that bind complement-regulating proteins (eg, CD55, CD59) to the surface of red blood cells (RBCs) leaves these RBCs susceptible to destruction by the complement membrane attack complex (MAC). Image courtesy of Haematologica. 2010 April;95(4).

Breakdown of RBC membranes by complement leads to the release of hemoglobin into the circulation. Hemoglobin is bound to haptoglobin for efficient clearance from the circulation. After saturating the haptoglobin, free hemoglobin circulates and binds irreversibly with nitric oxide (NO), depleting NO levels in peripheral blood.


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