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C H A P T E R 31

PAROXYSMAL NOCTURNAL HEMOGLOBINURIA

Robert A. Brodsky

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal hematopoi- enzyme, phosphatidylinositol-specific phospholipase C (PIPLC).
etic stem cell disorder that has fascinated hematologists for more than PIPLC cleaved the phosphate from phosphatidylinositol and left the
a century because of its protean clinical manifestations and captivat- enzyme with full activity after its release, suggesting that the protein
1
ing pathophysiology (MIM 300818). One of the earliest descriptions structure was unperturbed. This fundamental observation led to the
of PNH was by Dr. Paul Strübing, who in 1882 described a 29-year- discovery of dozens of GPI-anchored proteins.
old cartwright who presented with fatigue, abdominal pain, and The GPI anchor consists of a highly conserved glycan core
severe nocturnal paroxysms of hemoglobinuria that were exacerbated (ethanoloamine-P-6Manα1-2Manα1-6Manα1-4GlcN) linked to
by excess alcohol, physical exertion, and iron salts. Strübing deduced the 6-position of the D-myo-inositol ring of phosphatidylinositol
that the hemolysis was occurring intravascularly as the patient’s (Fig. 31.1). The anchor is synthesized in the endoplasmic reticulum
plasma turned red following severe attacks of hemoglobinuria. membrane and involves more than 10 reactions and more than 30
Decades later his prescient deduction was confirmed. Later reports different genes. The first step in GPI anchor biosynthesis is the
1a
by Marchiafava and Micheli led to the eponym, Marchiafava- transfer of N-acetylglucosamine (GlcNAc) from uridine diphosphate-
1b
Micheli syndrome, but it was Enneking, in 1925, who introduced GlcNAc to phosphatidylinositol (PI) to yield GlcNAc-PI. This step
the term paroxysmal nocturnal hemoglobinuria. is catalyzed by GlcNAc-PI α1-6 GlcNAc transferase, an enzyme
1c
In 1937, Thomas Ham found that PNH erythrocytes were whose subunits are encoded by seven different genes: PIGA, PIGC,
hemolyzed when incubated with normal, acidified serum. This PIGH, GPI1, PIGY, PIGP, and DPM2. In the second step, GlcNAc-PI
seminal discovery resulted in the first diagnostic test for PNH, the is deacetylated by the gene product of PIGL to form glucosamine
acidified serum or Ham test. The cell lysis following acidified serum (GlcN)-PI. GPI anchor assembly continues in the endoplasmic
appeared to be complement dependent because heat inactivation reticulum with acylation of the inositol and stepwise addition of
abrogated the reaction; however, it was not until 1954, with the mannosyl and phosphoethanolamine residues. The preassembled GPI
discovery of the alternative pathway of complement activation, that is linked to nascent proteins that contain a C-terminal GPI-
complement was formally proven to cause the hemolysis of PNH red attachment signal peptide, displacing it in a transamidase reaction.
cells. Following the emergence of specific diagnostic tests, additional The GPI-anchored protein then transits the secretory pathway to
disease manifestations such as venous thrombosis, bone marrow reach its final destination at the plasma membrane in compartments
failure, and development of myelodysplastic syndromes (MDS) known as lipid rafts. If the GPI anchor is not attached to the protein,
and acute leukemia were associated with PNH. These nonerythroid it is degraded intracellularly, probably in lysosomes.
manifestations of the disease foreshadowed the discovery that PNH Given the numerous gene products involved in GPI anchor
results from the clonal expansion of a mutated hematopoietic assembly, it seemed improbable that PNH would be the consequence
stem cell. of a single genetic mutation. However, after intense scrutiny of this
In the 1980s, roughly 100 years after Strübing’s initial description pathway, it became apparent that in virtually all PNH cases, the
of the disease, it was discovered that PNH cells display a global defect can be attributed to mutations in the PIGA gene, whose
deficiency in a group of proteins affixed to the cell surface by a product is essential for the first step of GPI anchor biosynthesis.
glycosylphosphatidylinositol (GPI) anchor. Interestingly, several of Later it was determined that the PIGA gene is on the X chro-
the missing proteins (e.g., CD55 and CD59) are important comple- mosome and that its product is part of a complex that transfers
ment regulatory proteins. A few years later, the genetic mutation N-acetylglucosamine to phosphatidylinositol to form GlcNAc-PI.
phosphatidylinositol-glycan complementation class A (PIGA) respon- Thus a single “hit” will generate a PNH phenotype because males
2
sible for the GPI-anchor protein deficiency was discovered, and most have only one X chromosome, and in females one X chromosome is
recently, a humanized monoclonal antibody that inhibits terminal inactivated through lyonization. Conceivably a mutation in any one
complement activation has been shown to ameliorate hemolysis and of the genes in this pathway would cause the disease; however, other
3
disease symptoms in PNH patients. Although the pathophysiology genes involved in GPI anchor biosynthesis are located on autosomes.
of many of PNH’s clinical manifestations are now understood, the Inactivating mutations in these genes would have to occur on both
mechanism of thrombosis, the mechanism of clonal dominance, and alleles to produce the PNH phenotype. Rare cases of PNH caused
4
the close association with aplastic anemia continue to be areas of by mutations other than PIGA have been described. In one example
intense investigation. PNH is an extremely rare condition; however, the disease was caused by a compound heterozygous mutation in the
the risk for developing PNH in patients with acquired aplastic anemia PIGT gene. In addition, rare cases of congenital CD59 deficiency
is 20% to 30%. In addition, more than half of patients with acquired have been shown to produce a PNH-like phenotype. These patients
aplastic anemia harbor a small to moderate PNH population at have chronic hemolytic anemia and a propensity for thrombosis. In
diagnosis. contrast to PNH, patients with germline CD59 deficiency present
with a relapsing immune-mediated peripheral neuropathy. Congenital
PIGA mutations resulting in an absence of GPI-anchored proteins are
PATHOPHYSIOLOGY embryonic lethal; however, germline hypomorphic PIGA mutations
5,6
have now been described. The hypomorphic PIGA mutations cause
The Glycosylphosphatidylinositol Anchor a syndrome known as multiple congenital abnormalities-hypotonia-
seizure syndrome 2 (MCAHS2; MIM 300868). MCAHS2 patients
Covalent linkage to GPI is an important means of anchoring many present with severe intellectual disability, dysmorphic facial
cell-surface glycoproteins to the cell membrane. Alkaline phosphatase features, seizures, and early death. Red cells from these patients
was the first GPI-anchor protein recognized after it was discovered tend to have little to no GPI anchor deficiency and hence no
that cell surface alkaline phosphatase could be removed by a bacterial hemolysis.

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