Paroxysmal nocturnal hemoglobinuria: an acquired X-linked genetic disease with somatic-cell mosaicism

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Paroxysmal nocturnal hemoglobinuria (PNH) is a severe hemolytic anemia caused by an intrinsic abnormality of the red blood cells that makes them exceedingly susceptible to the lytic action of activated complement (C). This abnormality results from a mutation in the PIG-A gene on Xp22. Given that the mutation is not inherited but is somatically acquired by a hematopoietic stem cell, it creates two populations of blood cells: normal cells and PNH cells. The clinical expression of PNH depends on the relative and absolute expansion of the PNH cell population, which probably depends, in turn, on a paradoxical growth advantage conferred to it by the existence in the patients of an autoimmune process that exerts negative selection against the ‘normal’ hematopoietic stem cells.

Introduction

PNH is the first known example of an acquired genetic X-linked disorder. From the clinical point of view, PNH is characterized by a triad of hemolytic anemia, venous thrombosis and blood cytopenias [1]. Recently, recommendations on management of the disorder have been published [2]. From the genetic point of view, PNH is caused by a somatic mutation; it is never inherited. From the cellular point of view, the clinical picture of PNH develops when the somatic mutation takes place in a hematopoietic stem cell. At the molecular level, the mutation is in the phosphatidylinositolglycan class A (PIG-A) gene, which maps to Xp22; the mutation causes severe or total loss of function of the gene itself.

In this review, I focus on the mechanism by which PIG-A mutant clones (i.e. PNH clones) expand; and, in keeping with the theme of this issue, I discuss the significant fact that PIG-A is on the X chromosome.

PIG-A encodes one of the subunits of N-acetylglucosamine phosphatidylinositol transferase, the enzyme required for an early step of the glycan phosphatidylinositol (GPI) anchor biosynthesis. When this enzyme is defective, GPI will be absent or in short supply, thus preventing the presence on the cell membrane of any of the proteins that require this anchor [1].

Given that the PIG-A mutation in PNH patients occurs in a hematopoietic stem cell, deficiency of GPI-linked proteins will affect all types of blood cells, and this will have a pleiotropic effect (Figure 1). The most dramatic consequences are seen in red blood cells, where two of these proteins, CD55 and CD59, are responsible for controlling the activity of plasma complement (C). Specifically, when C becomes activated — with consequent assembly of the C5–C8 complex — deficiency of CD59 will fail to prevent insertion of C9 into the red cell membrane [3], which results ultimately in the destruction of the red blood cell itself: this explains the occurrence of massive intravascular hemolysis any time C is activated in a PNH patient [4]. Although this mechanism is well documented in vitro, it has been difficult to confirm in a model system in vivo, because the Pig-A knockout in mouse is embryonic lethal [5]. Subsequently, by using a conditional knockout approach, mice with large numbers of PNH red cells in their blood have been obtained [6, 7, 8], and intravascular hemolysis has been demonstrated in these animals. At the same time, evidence was obtained that not all of the hemolysis was intravascular, indicating that at least in the mouse PNH red blood cells might be destroyed selectively by some other mechanism [9].

Section snippets

Significance of X linkage of the PIG-A gene

The biosynthesis of the GPI anchor is a complex process that requires more than ten enzymatic steps [10, 11]. However, no case of PNH has ever been shown to result from a mutation in any gene other than PIG-A; the simple explanation for this interesting fact is that the other genes involved are autosomal. Given that PNH is an acquired disease caused by a somatic mutation, and given that the mutation must produce loss of function in order to give the disease, both alleles would need to mutate

Advances in the pathogenesis of PNH

It has been suspected for a long time that a somatic mutation in the PIG-A gene is necessary but not sufficient to cause PNH [12]. We now know that PIG-A mutations are present in normal people [13, 14], and that PNH as a clinical disease emerges only if a PNH clone expands (see a recent review [15]).

Advances on the clinical front

Clinical management of PNH remains a major challenge, particularly with respect to controlling hemolysis. In a pre-clinical model, a single-chain antibody fragment specific for the red cell antigen TER-119, coupled to CD48, has ‘provided’ the latter molecule to GPI-deficient mouse red cells, inhibiting C-mediated hemolysis in vivo [40]. A modified CD59 molecule has been attached to GPI− (CD59−) human red blood cells and has protected them from C-mediated lysis in vitro.

By far the most important

Conclusion

In contemporary biology and pathology, the phrase somatic mutation is strongly associated with neoplasia, but PNH is not a leukemia; indeed, it has emerged as a prototype example of a non-malignant disease arising through a somatic mutation. To make it more interesting, the mutant gene maps to the X chromosome, and the development of PNH is strictly contingent on the haploid nature of X-linked genes subject to X-chromosome inactivation. The expansion of the PNH clone, without which PNH would

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

I thank all my collaborators with whom I have shared enthusiasm for understanding PNH, and the care of PNH patients over the past 35 years in Ibadan, Nigeria; London, UK; New York, USA; Napoli, Genova and Firenze, Italy; and particularly Dr Rosario Notaro for our work together over the past 10 years.

References (42)

  • A.M. Risitano et al.

    Large granular lymphocyte (LGL)-like clonal expansions in paroxysmal nocturnal hemoglobinuria (PNH) patients

    Leukemia

    (2005)
  • E.C. Howe et al.

    Killer immunoglobulin-like receptor genotype in immune-mediated bone marrow failure syndromes

    Exp Hematol

    (2005)
  • N. Hanaoka et al.

    Immunoselection by natural killer cells of PIGA mutant cells missing stress-inducible ULBP

    Blood

    (2006)
  • W. Barcellini et al.

    Increased resistance of PIG-A bone marrow progenitors to tumor necrosis factor α and interferon γ: possible implications for the in vivo dominance of paroxysmal nocturnal hemoglobinuria clones

    Haematologica

    (2004)
  • G. Chen et al.

    Differential gene expression in hematopoietic progenitors from paroxysmal nocturnal hemoglobinuria patients reveals an apoptosis/immune response in ‘normal’ phenotype cells

    Leukemia

    (2005)
  • P. Boccuni et al.

    Glycosyl phosphatidylinositol (GPI)-anchored molecules and the pathogenesis of paroxysmal nocturnal hemoglobinuria

    Crc Cr Rev Oncol-Hem

    (2000)
  • J.P. Maciejewski et al.

    Analysis of the expression of glycosylphosphatidylinositol anchored proteins on platelets from patients with paroxysmal nocturnal hemoglobinuria

    Thromb Res

    (1996)
  • M. Grunewald et al.

    The platelet function defect of paroxysmal nocturnal haemoglobinuria

    Platelets

    (2004)
  • P. Hillmen et al.

    Effect of eculizumab on hemolysis and transfusion requirements in patients with paroxysmal nocturnal hemoglobinuria

    New Engl J Med

    (2004)
  • L. Luzzatto et al.

    Paroxysmal nocturnal hemoglobinuria

  • S. Meri et al.

    Human protectin (CD59), an 18,000–20,000 MW complement lysis restricting factor, inhibits C5b-8 catalysed insertion into lipid bilayers

    Immunology

    (1990)
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