G6PD deficiency is the most common human enzyme defect.
Symptomatic patients are almost exclusively male, due to the X-linked pattern of inheritance, but female carriers can be clinically affected due to 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. Abnormal red blood cell breakdown (hemolysis) in G6PD deficiency can manifest in a number of ways:
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 G6PD deficiency, but none reported symptoms of favism, despite fava in the diet. 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.
All mutations that cause G6PD deficiency are found on the long arm of the X chromosome, on band Xq26. 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||valign="top"||Polymorphism nucleotide||A→G||376|
|126||Asparagine→Aspartic acid (ASN126ASP)||No enzyme defect (variant)|
|G6PD-A(-)||Gd-A(-)||G6PD A||valign="top"||Substitution nucleotide||G→A||376|
Asparagine→Aspartic acid (ASN126ASP)
|G6PD-Mediterran||Gd-Med||G6PD B||valign="top"||Substitution nucleotide||C→T||563|
|188||Serine→Phenylalanine (SER188PHE)||Class II|
|G6PD-Canton||Gd-Canton||G6PD A||valign="top"||Substitution nucleotide||G→T||1376||459||Arginine→Leucine (ARG459LEU)||Class II|
|G6PD-Chatham||Gd-Chatham||G6PD||valign="top"||Substitution nucleotide||G→A||1003||335||Alanine→Threonine (ALA335THR)||Class II|
|G6PD-Cosenza||Gd-Cosenza||G6PD B||valign="top"||Substitution nucleotide||G→A||1376||459||Arginine→Proline (ARG459PRO)||G6PD-activity <10%, thus high portion of patients.|
|163||Glycine→Serine (GLY163SER)||Class II|
|G6PD-Orissa||Gd-Orissa||G6PD||valign="top"||Substitution nucleotide||44||Alanine→Glycine (ALA44GLY)||NADP-binding place affected. Higher stability than other variants.|
|G6PD-Asahi||Gd-Asahi||G6PD A-||valign="top"||Substitution nucleotide (several)||A→G|
|Asparagine→Aspartic acid (ASN126ASP)|
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 an 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.
Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme in the pentose phosphate pathway (see image). 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/NAPDH/glutathione.
People with G6PD deficiency are therefore at risk of hemolytic anemia in states of oxidative stress. Oxidative stress can result from severe infection and from chemical exposure to medication and certain foods. Broad 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 causes a reduction of NADPH which is necessary 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 G6PD deficiency.
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).
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.
Some patients 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.
The modern understanding of the condition began with the analysis of patients who exhibited sensitivity to primaquine. The discovery of G6PD deficiency relieved 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 internal defect of erythrocytes.