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|Glucose-6-phosphate dehydrogenase deficiency|
|Classification and external resources|
|Glucose-6-phosphate dehydrogenase deficiency|
|Classification and external resources|
Glucose-6-phosphate dehydrogenase deficiency (G6PD deficiency) is an X-linked recessive hereditary disease characterized by abnormally low levels of glucose-6-phosphate dehydrogenase, a metabolic enzyme involved in the pentose phosphate pathway, especially important in red blood cell metabolism. G6PD deficiency is the most common human enzyme defect. Individuals with the disease may exhibit nonimmune hemolytic anemia in response to a number of causes, most commonly infection or exposure to certain medications or fava beans. G6PD deficiency is closely linked to favism, a disorder characterized by a hemolytic reaction to consumption of fava or broad beans, with a name derived from the Italian name of the broad bean (fava). The name favism is sometimes used to refer to the enzyme deficiency as a whole, although this is misleading as not all people with G6PD deficiency or Favism will manifest physically observable symptoms to the consumption of broad beans.
Most individuals with G6PD deficiency are asymptomatic.
Symptomatic patients are almost exclusively male, due to the X-linked pattern of inheritance, but female carriers can be clinically affected due to unfavorable lyonization, where random inactivation of an X-chromosome in certain cells creates a population of G6PD-deficient red blood cells coexisting with normal red cells. A typical female with one affected X chromosome will show the deficiency in approximately half of her red blood cells. However, in rare cases, including double X deficiency, the ratio can be much more than half, making the individual almost as sensitive as a male.
Abnormal red blood cell breakdown (hemolysis) in G6PD deficiency can manifest in a number of ways, including the following:
Favism may be formally defined as a haemolytic response to the consumption of broad beans. All individuals with favism show G6PD deficiency. However, not all individuals with G6PD deficiency show favism. For example, in a small study of 757 Saudi men, more than 42% showed a variant of G6PD deficiency, but none displayed symptoms of favism. Favism is known to be more prevalent in infants and children, and G6PD genetic variant can influence chemical sensitivity. Other than this, the specifics of the chemical relationship between favism and G6PD are not well understood.
Many substances are potentially harmful to people with G6PD deficiency. Variation in response to these substances makes individual predictions difficult. Antimalarial drugs that can cause acute hemolysis in people with G6PD deficiency include primaquine, pamaquine, and chloroquine. There is evidence that other antimalarials may also exacerbate G6PD deficiency, but only at higher doses. Sulfonamides (such as sulfanilamide, sulfamethoxazole, and mafenide), thiazolesulfone, methylene blue, and naphthalene should also be avoided by people with G6PD deficiency as they antagonize folate synthesis, as should certain analgesics (such as aspirin, phenazopyridine, and acetanilide) and a few non-sulfa antibiotics (nalidixic acid, nitrofurantoin, isoniazid, dapsone, and furazolidone). Henna has been known to cause haemolytic crisis in G6PD-deficient infants.
Two variants (G6PD A− and G6PD Mediterranean) are the most common in human populations. G6PD A− has an occurrence of 10% of American blacks while G6PD Mediterranean is prevalent in the Middle East. The known distribution of the disease is largely limited to people of Mediterranean origins (Spaniards, Italians, Greeks, Armenians, and Jews). These variants are believed to stem from a protective effect against Plasmodium falciparum and Plasmodium vivax malaria.
All mutations that cause G6PD deficiency are found on the long arm of the X chromosome, on band Xq28. The G6PD gene spans some 18.5 kilobases. The following variants and mutations are well-known and described:
|Table 1. Descriptive mutations and variants|
|Variants or mutations||Gene||Protein|
|OMIM-Code||Type||Subtype||Position||Position||Structure change||Function change|
|G6PD-A(+)||Gd-A(+)||G6PD A||+305900.0001||Polymorphism nucleotide||A→G||376|
|126||Asparagine→Aspartic acid (ASN126ASP)||No enzyme defect (variant)|
|G6PD-A(-)||Gd-A(-)||G6PD A||+305900.0002||Substitution nucleotide||G→A||376|
Asparagine→Aspartic acid (ASN126ASP)
|G6PD-Mediterran||Gd-Med||G6PD B||+305900.0006||Substitution nucleotide||C→T||563|
|188||Serine→Phenylalanine (SER188PHE)||Class II|
|G6PD-Canton||Gd-Canton||G6PD B||+305900.0021||Substitution nucleotide||G→T||1376||459||Arginine→Leucine (ARG459LEU)||Class II|
|G6PD-Chatham||Gd-Chatham||G6PD||+305900.0003||Substitution nucleotide||G→A||1003||335||Alanine→Threonine (ALA335THR)||Class II|
|G6PD-Cosenza||Gd-Cosenza||G6PD B||+305900.0059||Substitution nucleotide||G→A||1376||459||Arginine→Proline (ARG459PRO)||G6PD-activity <10%, thus high portion of patients.|
|163||Glycine→Serine (GLY163SER)||Class III|
|G6PD-Orissa||Gd-Orissa||G6PD||+305900.0047||Substitution nucleotide||44||Alanine→Glycine (ALA44GLY)||NADP-binding place affected. Higher stability than other variants.|
|G6PD-Asahi||Gd-Asahi||G6PD A-||+305900.0054||Substitution nucleotide (several)||A→G|
|Asparagine→Aspartic acid (ASN126ASP)|
Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme in the pentose phosphate pathway (see image, also known as the HMP shunt pathway). G6PD converts glucose-6-phosphate into 6-phosphoglucono-δ-lactone and is the rate-limiting enzyme of this metabolic pathway that supplies reducing energy to cells by maintaining the level of the co-enzyme nicotinamide adenine dinucleotide phosphate (NADPH). The NADPH in turn maintains the supply of reduced glutathione in the cells that is used to mop up free radicals that cause oxidative damage.
The G6PD / NADPH pathway is the only source of reduced glutathione in red blood cells (erythrocytes). The role of red cells as oxygen carriers puts them at substantial risk of damage from oxidizing free radicals except for the protective effect of G6PD/NADPH/glutathione.
People with G6PD deficiency are therefore at risk of hemolytic anemia in states of oxidative stress. Oxidative stress can result from infection and from chemical exposure to medication and certain foods. Broad beans, e.g., fava beans, contain high levels of vicine, divicine, convicine and isouramil, all of which are oxidants.
When all remaining reduced glutathione is consumed, enzymes and other proteins (including hemoglobin) are subsequently damaged by the oxidants, leading to electrolyte imbalance, cross-bonding and protein deposition in the red cell membranes. Damaged red cells are phagocytosed and sequestered (taken out of circulation) in the spleen. The hemoglobin is metabolized to bilirubin (causing jaundice at high concentrations). The red cells rarely disintegrate in the circulation, so hemoglobin is rarely excreted directly by the kidney, but this can occur in severe cases, causing acute renal failure .
Deficiency of G6PD in the alternative pathway causes the build up of glucose and thus there is an increase of advanced glycation endproducts (AGE). The deficiency also reduces the amount of NADPH, which is required for the formation of nitric oxide (NO). The high prevalence of diabetes mellitus type 2 and hypertension in Afro-Caribbeans in the West could be directly related to the incidence of G6PD deficiency in those populations.
Although female carriers can have a mild form of G6PD deficiency (dependent on the degree of inactivation of the unaffected X chromosome – see lyonization), homozygous females have been described; in these females there is co-incidence of a rare immune disorder termed chronic granulomatous disease (CGD).
The diagnosis is generally suspected when patients from certain ethnic groups (see epidemiology) develop anemia, jaundice and symptoms of hemolysis after challenges from any of the above causes, especially when there is a positive family history.
Generally, tests will include:
When there are sufficient grounds to suspect G6PD, a direct test for G6PD is the "Beutler fluorescent spot test", which has largely replaced an older test (the Motulsky dye-decolouration test). Other possibilities are direct DNA testing and/or sequencing of the G6PD gene.
The Beutler fluorescent spot test is a rapid and inexpensive test that visually identifies NADPH produced by G6PD under ultraviolet light. When the blood spot does not fluoresce, the test is positive; it can be falsely negative in patients who are actively hemolysing. It can therefore only be done 2–3 weeks after a hemolytic episode.
When a macrophage in the spleen identifies a RBC with a Heinz body, it removes the precipitate and a small piece of the membrane, leading to characteristic "bite cells". However, if a large number of Heinz bodies are produced, as in the case of G6PD deficiency, some Heinz bodies will nonetheless be visible when viewing RBCs that have been stained with crystal violet. This easy and inexpensive test can lead to an initial presumption of G6PD deficiency, which can be confirmed with the other tests.
The most important measure is prevention – avoidance of the drugs and foods that cause hemolysis. Vaccination against some common pathogens (e.g. hepatitis A and hepatitis B) may prevent infection-induced attacks.
In the acute phase of hemolysis, blood transfusions might be necessary, or even dialysis in acute renal failure. Blood transfusion is an important symptomatic measure, as the transfused red cells are generally not G6PD deficient and will live a normal lifespan in the recipient's circulation. Those affected should avoid drugs such as aspirin.
Some patients may benefit from removal of the spleen (splenectomy), as this is an important site of red cell destruction. Folic acid should be used in any disorder featuring a high red cell turnover. Although vitamin E and selenium have antioxidant properties, their use does not decrease the severity of G6PD deficiency.
G6PD deficiency is the most common human enzyme defect, being present in more than 400 million people worldwide. In 2010 it result in about 4,000 deaths globally. African, Middle Eastern and South Asian people are affected the most, including those who have these ancestries. A side effect of this disease is that it confers protection against malaria, in particular the form of malaria caused by Plasmodium falciparum, the most deadly form of malaria. A similar relationship exists between malaria and sickle-cell disease. One theory to explain this is that cells infected with the Plasmodium parasite are cleared more rapidly by the spleen. This phenomenon might give G6PD deficiency carriers an evolutionary advantage by increasing their fitness in malarial endemic environments.
G6PD-deficient individuals do not appear to acquire any illnesses more frequently than other people, and may have less risk than other people for acquiring ischemic heart disease and cerebrovascular disease.
Favism has been known since antiquity. One strict rule of the Pythagoreans' was the avoidance of beans possibly due to their potential for Favism. There is building evidence that Favism was widely known in the Mediterranean 2,500 years ago as the prevalence of this deficiency is in those of Mediterranean descent including: Armenians, Greeks, Italians, Spaniards, and Jews. It is written that this ban was because beans resembled the genetalia, but it is possible that this was a philosophical or scientific matter, being that the belief was that beans and humans were created from the same material. 
The modern understanding of the condition began with the analysis of patients who exhibited sensitivity to primaquine. The discovery of G6PD deficiency relied heavily upon the testing of prisoner volunteers at Illinois State Penitentiary, although today such studies cannot be performed. When some prisoners were given the drug primaquine, some developed hemolytic anemia but others did not. After studying the mechanism through Cr51 testing, it was conclusively shown that the hemolytic effect of primaquine was due to an intrinsic defect of erythrocytes.