Enzyme Abnormalities

  • This disorder is inherited as an X-linked genetic disorder - primarily expressed in the male population
  • The LYON hypothesis states that:
    • In females, one of the two "X" chromosomes (see black or orange lines in figure below) is randomly inactivated - which one is an independent event in each cell.
    • This inactive chromosome appears as an unextended sex chromatin body (see ball in figure below)
    • Females heterozygous for G6PDH deficiency have two populations of cells (see figure below).
    • Affected males have all cells enzyme deficient
    Lyon Hypothesis
    Lyon Hypothesis

  • Red Blood Cells (RBC) have no nucleus so they rely upon the enzymes they obtained during their maturation in the bone marrow.
  • Approx. 10% of the RBC's glucose is used in the hexose monophosphate shunt (to produce reducing power via G6PDH)

    Hexose Monophosphate Shunt
    Hexose Monophosphate Shunt

  • If no G6PDH, then no NADPH so no GSH generated therefore no substrate for glutathione peroxidase and ultimately the build-up of hydrogen peroxide (H2O2) will lead to RBC membrane damage and oxidation of haemoglobin resulting in the appearance of Heinz bodies.
  • Heinz Bodies (HB) attach to the RBC membrane leading to increased RBC fragility. During passage of these cells through the spleen, macrophages lining the sinusoids will remove the HB producing bite cells (also seen with unstable haemoglobins and excess oxidant drugs). Destruction of the cells may take several passages - gradual "stripping" of the membrane can result in microspherocytes.

Glucose-6-phosphate dehydrogenase (G6PDH)

  • Some abnormal enzymes have a normal reactivity in reticulocytes but abnormal function in mature RBCs
  • There are many (approx 300) variants of G6PDH which may differ in their activity, stability and electrophoretic abnormality
    • G6PDH-B
      • Most common
      • 100% enzyme activity, normal mobility, t1/2 62days
    • G6PDH-A+
      • Normal severity, 60-85% enzyme activity, fast mobility
    • G6PDH-A0
      • Moderate - severe haemolysis after stress, 5-15% enzyme activity, fast mobility
      • Oxidant drugs (eg. sulphur containing-dapsone, anti-malarials, bactrim/septrim, sulphonamides, primaquinine), infection and diabetes (acid conditions) can precipitate haemolysis
      • Self-limiting as long as the body's ability to replace lost RBCs is not hampered
    • G6PDH-Mediterranean
      • Severe haemolysis after stress, not self-limiting
      • 0-7% enzyme activity, normal mobility, t1/2 of hours
      • A proportion are susceptible to favism (haemolysis after ingestion of fava beans)
    • G6PDH-Canton
      • Not self-limiting
    • Short t1/2

  • Clinical Symptoms After Stress

    • G6PDH Deficiency
      • Extravascular Haemolysis (EVH)
      • Pallor
      • Jaundice
    • Favism
      • Intravascular haemolysis (IVH)
      • Haemoglobinuria (If haematuria - a cell pellet will be visible after centrifugation)

    Blood Film

    • Haemoglobinization: Normochromic
    • Anisocytosis: ++ (Polychromatic macrocytes)
    • Poikilocytosis: +++ (Bite Cell, Blister Cell, Microspherocytes; Schistocytes and Acanthocytes in IVH)
    • Immature forms: +++ (Polychromasia, NRBCs)


    Haematology Profile

    • Haptoglobin: Dec in EVH, Inc in IVH
    • Osmotic Fragility Test: May increase in sever case due to membrane weaknesses and ion pump failures
    • Reticulocyte and Heinz Bodies: Present


    Confirmatory Test

    • Screening test for G6PDH presence and then function


    Treatment

    • Largely supportive (cure infection, remove drug treatment)
    • Majority of patients are asymptomatic and do not experience chronic haemolysis

Pyruvate Kinase (PK) Deficiency

      • 2nd most common enzyme disorder - still rare
      • Autosomal recessive inheritance
      • Variants of the enzyme are not well characterised
      • Clinical severity ranges from haemolysis in the neonate to mild symptoms in the adult
      • the Embden-Meyerhof (EM) pathway produces ATP that is used for maintenance of deformability and biconcavity through ATP driven pumps
      • The Rapoport-Luebering shunt of the EM p'way provide the RBC with 2,3-diphosphoglycerate (2,3-DPG) when stimulated by hypoxia
      • Without PK, the Na+/K+/Ca2+ shunt can not be maintained resulting in a dessicated RBC, producing acanthocytes (burr and echinocytes)

        Embden-Meyerhof, Part Pathway 1
      Embden-Meyerhof, Part Pathway 2
        Embden-Meyerhof, Part Pathway 3
       

      Anaerobic Glucose Metabolic Pathway
      (Embden-Meyerhof Pathway)

 

Blood Film

      • Haemoglobinization: Normochromic
      • Anisocytosis: +++ (Polychromatic macrocytes)
      • Poikilocytosis: ++ (Acanthocytes)
      • Immature forms: +++ (Polychromasia, NRBCs if severe)
 

Haematology Profile

      • Heinz Bodies: Absent
      • PK and haemolytic anaemia screening tests
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