Glucose 6 Phosphate dehydrogenase deficiency is the most common metabolic disorder of red blood cells. It is a genetic disorder in which there is deficiency of enzyme Glucose-6-phosphate dehydrogenase or G6PD in the blood. This enzyme plays a role in Hexose monophosphate pathway [a metabolic pathway involved in formation of precursor for the synthesis of nucleotides] and is critical in protecting red cell integrity.
Glucose 6 phosphate dehydrogenase deficiency results in premature breakdown of red cells resulting in hemolytic anemia.
Glucose 6 phosphate dehydrogenase deficiency is the most common human enzyme defect. Worldwide more than 400 million people are affected. It has a high prevalence in persons of African, Asian, and Mediterranean descent. Although global in distribution, it is seen mainly in black population of Africa, Nigeria, Sardinia, black Americans, Southeast Asia, China and Greece.
Because the disease has a high incidence in areas where malaria is endemic, Glucose 6 phosphate dehydrogenase deficiency is thought to confer protection against falciparum malaria. The exact cause of this protection is unknown.A plausible hypothesis is that the oxidative stress causes hemolysis of parasitised RBCs. Alternatively block in HMP pathway in RBCs may lead to inability of the parasite to synthesize nucleic acids resulting in inhibition of malarial parasite growth. In either case, G6PD deficient individuals have an evolutionary advantage which makes it easier for them to survive in malarial endemic environments.
Glucose 6 phosphate dehydrogenase deficiency is inherited as an X-linked recessive genetic disorder. Symptomatic patients are almost exclusively male, but female carriers can be clinically affected due to unfavorable lyonization [ where one of the two X chromosomes in each cell remains inactive throughout]
In people with glucose 6 dehydrogenase deficiency, hemolytic anemia is most often triggered by bacterial or viral infections or by certain drugs . Hemolytic anemia can also occur after eating fava beans or inhaling pollen from fava plants (a reaction called favism).
Glucose-6-dehydrogenase deficiency is also a significant cause of mild to severe jaundice in newborns. Many people with this disorder, however, never experience any signs or symptoms.
G6PD Enzyme and Variants
More than 200 structural variants of G6PD have been identified. They are distinguished by a variety of biochemical techniques including electrophoresis, heat stability studies and analysis of kinetic characteristics.
G6PD B : It is the normal wild type enzyme found in most Caucasians, Asians and majority of blacks. It has normal enzymatic activity and is not associated with hemolysis (class IV).
G6PD A+ : It is found in 20-30 % of blacks from Africa. It has normal enzymatic activity and does not cause hemolysis (class IV). It differs from G6PD B in having a much faster electrophoretic mobility.
G6PD A– : It is an unstable enzyme. Enzyme activity is normal in bone marrow cells and reticulocytes but decreases markedly in older RBCs. It is the most common variant associated with mild to moderate hemolysis (class III). It is found in 10-15% of Africans and is responsible for primaquine sensitivity in blacks.
G6PD Mediterranean: It is the most common abnormal variant found in Caucasians particularly those having origin in Mediterranean area. Activity of this enzyme is markedly reduced resulting in severe hemolysis (class II).
G6PD Canton: It is a variant seen in Asians. Its biochemical properties are similar to those of G6PD Mediterranean.
Pathophysiology of Glucose 6 Phosphate Dehydrogenase Deficiency
Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme in the HMP shunt pathway. G6PD converts glucose-6-phosphate into 6-phosphoglucono-δ-lactone and thereby reduces NADP to reduced NADP (i.e., NADPH). This NADPH in turn maintains glutathione in its reduced form ( i.e., GSSG), which acts as an intercellular reducing agent that protects cells against oxidative injury. The G6PD / NADPH pathway is the only source of reduced glutathione in red blood cells Coupling of HMP shunt and glutathione metabolism is responsible for protecting intracellular proteins from oxidative assault.
Exogenous factors like drugs, infections or certain foods like fava beans lead to oxidative stress within RBCs. People with G6PD deficiency cannot counter this stress resulting in denaturation of hemoglobin and resultant hemolytic anemia.
Classification of Glucose 6 Phosphate Dehydrogenase Deficiency
World Health Organization (WHO) classification of G6PD variants (based on degree of enzyme deficiency and severity of hemolysis)
- Class I: Very severe deficiency of enzyme activity (<10% of normal) with chronic hemolytic anemia
- Class II: Severe enzyme deficiency with intermittent hemolysis
- Class III: Moderate enzyme deficiency (10-60% of normal), hemolysis on exposure to drugs or infections
- Class IV: Normal enzyme activity, no anemia
- Class V: Increased enzyme activity, no clinical significance
Signs and symptoms of Glucose 6 Phosphate Dehydrogenase Deficiency
Most people with G6PD deficiency are asymptomatic but in some significant anemia occurs.
Four clinical entities are recognized:
Acute hemolytic anemia
Severe hemolysis occurs only after exposure to oxidants like drugs, particularly primaquine or some infection. After 2-4 days of oxidative injury, signs and symptoms of acute hemolysis like jaundice, pallor and dark urine are observed. In spite of continuous drug exposure, the episode is self-limiting and ends in about a week. This occurs because older RBCs get hemolysed leaving behind young RBCs which have a good G6PD activity.
Congenital chronic non spherocytic hemolytic anemia
In patients of class I variant, lifelong hemolysis occurs without exposure to drugs or infections. Anemia and jaundice are noted in the neonatal period. Jaundice may be severe enough to require exchange transfusion. In adults, there is usually mild continuous hemolysis with mild anemia and jaundice. Anemia and hemolysis may worsen due to drugs having oxidant properties or due to exposure to fava beans.
Acute hemolysis with hyperbilirubinemia ie jaundice may lead to kernicterus.
It is seen most commonly in children aged 1 to 5 years. Children are more susceptible than adults. An acute intravascular hemolysis occurs on ingestion of fava beans (Vicia fava, broad bean) within 5-24 hrs of intake. Headache, nausea, back pain, chills and fever occur followed by anemia, jaundice and blood in urine. Hemoglobin level may fall down severely and abruptly requiring blood transfusion. G6PD Mediterranean variant is most commonly implicated; so the disease is encountered mainly in Italy and Greece. Favism does not occur in all susceptible G6PD individuals. It also may not occur in the same person following repeated intake of beans suggesting the role of some other genetic factor possibly related to metabolism of fava bean oxidants.
Lab Tests for G6PD Deficiency
Development of anemia and jaundice after exposure to factors inducing oxidant stress (drugs, chemicals or infections) in a patient from a certain ethnic group should prompt an investigation for G6 PD deficiency.
- Complete blood cell count: Anemia of varying degree
- Reticulocyte count: is increased which indicates increased bone marrow response to anemia. In acute hemolysis, reticulocytosis appears 5 days after hemolysis, maximum level reaches between 7-10 days, after which the levels decline. In chronic hemolysis, count is usually between 10-15%
- Peripheral blood smear: Irregularly contracted RBCs (eccentrocytes with hemoglobin puddle to one side of RBCs) and bite cells are seen. Bite cells are abnormally shaped RBCs with one or more semicircular portions removed from the cell margin. These result from the removal of denatured hemoglobin (ie. Heinz bodies) by the spleen. Polychromasia, basophilic stippling, microspherocytes and nucleated RBCs may also be seen.
- Demonstration of Heinz bodies: Supra vital staining of peripheral blood film using brilliant cresyl blue may reveal Heinz bodies.
- Coombs test: negative because non immune mediated hemolysis occurs in G6PD deficiency.
- Total and unconjugated bilirubin : levels are increased due to increased RBC destruction
- Serum lactate dehydrogenase (LDH) level : increased
- Serum haptoglobin level : decreased
- Urine examination : for hematuria, hemoglobinuria and urinary hemosiderin
Specific tests to diagnose G6PD deficiency include
- Beutler fluorescent spot test: This is a semi-quantitative rapid fluorescent spot test. NADPH generated by G6PD present in red cells shows fluorescence under UV light. In G6PD deficient individuals, there is no formation of NADPH and hence no fluorescence. False negative results are obtained during hemolytic episodes because older RBCs deficient in enzyme are destroyed and young RBCS and reticulocytes have normal enzyme activity. Therefore, ideally the test should be done 2-3 weeks after an acute hemolytic episode.
- Methemoglobin reduction test: It is a cheaper test and can further be proceeded to methemoglobin elution test to detect female heterozygotes.
- Quantitative assay of G6PD activity: gives definite results. Similar to spot test, this test should not be done during acute hemolysis.
- Direct DNA testing and/or sequencing of the G6PD gene
All affected people should avoid ingestion of drugs and foods that cause hemolysis. Exchange transfusion in the first week of life is required to prevent kernicterus. Beyond the new born period, anemia is usually not severe enough to require regular blood transfusions. During aplastic crises, transfusion therapy may be required. Splenectomy can improve hemoglobin levels, but does not offer much benefit. Since vitamin E and selenium have antioxidant properties, they were proposed as therapeutic agents, but they have not been much useful in treating anemia.
List of Substances to be Avoided in G6PD Deficiency
Many substances are potentially harmful to people with G6PD deficiency. Variation in response to these substances makes individual predictions difficult. Following is partial list of substances to avoided.
- Antimalarial drugs – primaquine, pamaquine, and chloroquine
- Sulfonamides – sulfanilamide, sulfamethoxazole, and mafenide
- Methylene blue
- Rasburicase is also contraindicated in G6PD deficiency.
Henna has been known to cause hemolytic crisis in G6PD-deficient infants.
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