G6PD Deficiency

Blood film in G6PD deficiency with acute hemolysis after an oxidant stress. Some of the cells show loss of cytoplasm with separation of remaining hemoglobin from the cell membrane (‘blister’ cells). There are also numerous contracted and deeply staining cells. Supravital staining (as for reticulocytes) showed the presence of Heinz bodies.

• Glucose‐6‐phosphate dehydrogenase (G6PD) functions to reduce nicotinamide adenine dinucleotide phosphate (NADP). It is the only source of NADPH which is needed for the production of reduced glutathione; a deficiency renders the red cell susceptible to oxidant stress.
  • Oxidation of the sulfhydryl groups on hemoglobin leads to the formation of denatured globin or sulfhemoglobin. The latter form insoluble masses which attach to the red cell membrane by disulfide bridges, and these are known as Heinz bodies.
  • The end result of these changes is the production of rigid, nondeformable erythrocytes that are susceptible to stagnation and destruction by reticuloendothelial macrophages in the spleen and liver.
  • Both extravascular and intravascular hemolysis occur in G6PD-deficient individuals.
• As red cells age, the activity of G6PD declines. The normal enzyme (G6PD B) has an in vivo half-life of 62 days. Despite this loss of enzyme activity, normal old RBC contain sufficient G6PD activity to generate NADPH and thereby sustain GSH levels in the face of oxidant stress. In contrast, the G6PD variants associated with hemolysis are unstable and have much shorter half-lives. The activity of G6PD A− in reticulocytes is normal, but it declines rapidly thereafter with a half-life of only 13 days.
• The inheritance is sex‐linked, affecting males, and carried by females who show approximately half the normal red cell G6PD values.
  • The female heterozygotes have an advantage of resistance to Falciparum malaria.
  • Dependent upon the degree of lyonization and the degree to which the abnormal G6PD variant is expressed, the mean RBC enzyme activity in females may be normal, moderately reduced, or grossly deficient. A female with 50% normal G6PD activity has 50% normal red cells and 50% G6PD-deficient red cells. The G6PD-deficient cells in females, however, are as vulnerable to hemolysis as are enzyme-deficient RBC in males.
• The degree of deficiency varies with ethnic group, often being mild (10–60% of normal activity) in black African people, more severe in Middle‐Eastern and South‐East Asian people, and most severe in Mediterranean people (<10% of normal activity). Severe deficiency occurs occasionally in white people.

Classification

• The World Health Organization (WHO) has classified G6PD variants on the magnitude of the enzyme deficiency and also the severity of hemolysis.
  • Class I variants have very severe enzyme deficiency (<10%-20% of normal) and have chronic hemolytic anemia.
  • Class II variants also have severe enzyme deficiency (<10% of normal), but there is usually only intermittent hemolysis.
  • Class III variants have moderate enzyme deficiency (10%-60% of normal) with intermittent hemolysis usually associated with infection or drugs.
  • Class IV variants have no enzyme deficiency or hemolysis.
  • Class V variants are those in which enzyme activity is increased.
  • Variants in the last two groups, although of much interest to biologists, geneticists, and anthropologists, are of no major clinical significance.
• The normal wild-type enzyme, G6PD B, is found in most Caucasians, Asians, and a majority of blacks. It has normal catalytic activity and is not associated with hemolysis (class IV).
• G6PD A+ is found in 20% to 30% of blacks from Africa. It has normal catalytic properties and does not cause hemolysis (class IV). It differs from G6PD B in that it has a much faster electrophoretic mobility (the letters A and B refer to relative electrophoretic mobilities).
• G6PD A− is the enzyme responsible for primaquine sensitivity in blacks, and it is the most common variant associated with mild to moderate hemolysis (class III).
  • This G6PD variant is found in 10% to 15% of African Americans, and with similar frequencies in western and central Africa. It has an electrophoretic mobility identical to that of G6PD A+. The + and − denote enzyme activity.
• G6PD Mediterranean is a common abnormal variant found in people whose origins are in the Mediterranean area, but also in the Mid-East and India. The electrophoretic mobility of G6PD Mediterranean is identical to that of G6PD B, but its catalytic activity is markedly reduced, and hemolysis can be severe (class II).
• In China, there are at least 21 variants causing G6PD deficiency.
  • The three most common are G6PD Canton (1376G>T), G6PD Kaiping (1388G>A), and G6PD Gaohe (95A>G) (also applies to Taiwan). G6PD Canton and G6PD Gahoe are mainly regarded as WHO class II variants, whereas G6PD Kaiping is considered a WHO class III variant. These three variants account for over 70% of G6PD deficiency cases in China.
  • The most common G6PD mutation in Southeast Asia is G6PD Mahidol (487G>A), a class III variant. Of interest, although India borders China, none of the Chinese G6PD variants are found in India, and the most common type is G6PD Mediterranean (563C>T).

Clinical Presentation

• Neonatal jaundice.
• Acute hemolytic anemia in response to oxidant stress, e.g. drugs, fava beans or infections.
  • The acute hemolytic anemia is caused by rapidly developing intravascular hemolysis with hemoglobinuria.
  • The anemia may be self‐limiting as new young red cells are made with near normal enzyme levels.
• The common denominator of these drugs is their interaction with hemoglobin and oxygen, thus accelerating the intracellular formation of H2O2 and other oxidizing radicals.
• Fava beans (Vica fava) contain an oxidant chemical, divicine. Unlike other agents capable of inducing hemolysis, the fava bean is toxic for only some G6PD-deficient individuals. Class II variants, such as G6PD Mediterranean and in some of the Asian G6PD variants. Africans and African Americans with G6PD deficiency are much less susceptible.
• A variety of infectious agents has been implicated: Salmonella, Escherichia coli, β-hemolytic streptococci, and rickettsiae. Hemolysis is particularly prominent in G6PD-deficient individuals with viral hepatitis.
• Rarely, a congenital non‐spherocytic hemolytic anemia.

Diagnosis