Structure of G6PD- The enzyme, Glucose-6-Phosphate Dehydrogenase, is comprised of a dimer or tetramer of identical polypeptide chains
–Each unit consists of 515 amino acids - The single G6PD locus in humans is located on the telomeric region of the long arm of the X-chromosome
–Females have two X chromosomes, hence two copies of G6PD, while males have only one X chromosome and one copy of G6PD
Function of G6PD –In Red Blood Cells (RBC), which lack nuclei, mitochondria, and other organelles, G6PD is particularly significant
–Catalyzes the oxidation of Glucose-6-Phosphate to 6-Phosphogluconolactone (Phosphogluconate)
–Only source of NADPH and GSH, necessary for the reduction of hydrogen peroxide
•Hydrogen Peroxide is a strong oxidant that will degrade the RBC and cause hemolysis if it is not reduced
Familial Genetics of G6PD–Females
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GdB GdB, Homozygous Normal; “Normal” GdB GdA-, Heterozygous; “Heterozygote” GdA-GdA-, Homozygous Deficient; “G6PD Deficient”
–Males
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GdB, Hemizygous Normal; “Normal” GdA-, Hemizygous Deficient; “G6PD Deficient"
Mendelian Transmission| | Males | | Females | GdB | Y | | GdB | GdB GdB | GdB | | GdMed | GdB GdMed | GdMed | | | Daughters | Sons |
G6PD Heterozygotes–G6PD Normal
–G6PD Deficient
Depending on which X chromosome was inactivated in the stem cell giving rise to the particular RBC
G6PD Variants | | Four most common variants out of 300+ known | | GdB | Normal Activity | All World Populations | | GdA | Normal Activity; Aspartic acid substituted for asparagine at position 126, Guanine for adenine at DNA position 376 | Africa (most common variant) | | GdA- | 8 - 20% Normal Activity; Methionine for Valine at position 67 and Aspartic Acid for Asparagine at position 126, Adenine for Guanine at position 202 and Guanine for Adenine at position 376 | Africa | | GdMed | < 5% Normal Activity; Phenylalanine for Serine at position 188; Thymine for Cytosine at position 563 | Iran, Iraq, India, Pakistan, Greece, Sardinia |
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G6PD Activity–GdB has 62 day half-life for decay of activity
•Sustains GSH levels for 100 to 120 day RBC life span –GdA- has normal activity when new, but the activity half-life is only 13 days
•Deficiency is due to instability of the enzyme –GdMed has greater instability with 8 day half-life
•New cells already have reduced activity, and mature
RBC have enzyme levels < 1% normal activity Symptoms of G6PD deficiency•G6PD deficiency is manifested as anemia, with RBCs being prematurely destroyed –RBCs are also extremely susceptible to oxidative stress
–Neonatal jaundice is a yellowish discoloration of the whites of the eyes, skin, and mucous membranes caused by deposition of bile salts in these tissues
•A severe form of this is a direct result of insufficient activity of the G6PD enzyme in the liver
–In some cases, the neonatal jaundice is severe enough to cause death or permanent neurologic damage (Beutler, 1994).
Symptoms of G6PD deficiency, 2•Outside areas where dietary components cause hemolytic crises, infection is the most common cause of hemolysis and anemia in subjects with G6PD deficiency –Oxidative metabolites produced by bacterial, viral, and rickettsial infections cause an anemic response
•Viral hepatitis, pneumonia, and typhoid fever are particularly likely to precipitate a hemolytic episode in G6PD deficient individuals
G6PD Hemolysis•Red blood cells will hemolyze or burst when the oxidant stress level becomes too high –Hemolysis occurs in G6PD deficient individuals due to the consumption of certain foods or drugs
•Substances that increase the oxidation of glutathione, thereby diminishing the available GSH for oxidation of peroxide, creating a potential for hemolysis
–Fava Beans contains vicine and convicine whose metabolites can cause a hemolytic crisis in GdMed individuals
–Many anti-malarial drugs, sulfonamides, sulfones and other drugs produce the same reaction in severely deficient individuals
–Can also cause the oxidation of hemoglobin, making it lose the ability to be a reversible oxygen carrier
Favism•The Fava Bean (Vicia faba) is a favored cultigen in areas where the GdMed allele is common –Vicine and convicine make up about 0.5% of the wet weight of the Fava bean
•These compounds metabolize to divicine and isouramil in the intestine
–These metabolites decrease RBC reduced glutathione (GSH)
•Increase the production of hydrogen peroxide and free radicals
–Creates a severe oxidant stress in G6PD deficient cells
Plasmodium in the RBC•Plasmodium protozoans preferentially attack immature RBC but P. falciparum can invade RBC of all ages –Plasmodium oxidizes RBC NADPH from the Pentose Phosphate pathway for its metabolism
•This results in a deficiency of RBC GSH, most severe in G6PD deficient individuals, leading to peroxide-induced hemolysis which curtails the development of Plasmodium
–After several cell cycles the Plasmodium can adapt to produce its own G6PD, reducing the adaptive benefit of G6PD deficiency
Fava Beans and Malaria•Recall that fava beans contain compounds that metabolize to powerful oxidants –In a cell that is oxidant-stressed by Plasmodium infection, the addition of another strong oxidant can lead to a rapid build-up of peroxide
–In vitro and in vivo (mouse) studies indicate a mild suppressant effect of divicine and isouramil on Plasmodium in G6PD normals
•This effect is even greater in G6PD deficient individuals
Case Study: GdMed and Favism•Fava bean cultivation is widespread, especially throughout the circum-Mediterranean region •There is substantial overlap between the cultivation of fava beans and the GdMed allele –Serious cases of hemolytic favism are described more than 2,000 years ago by Greeks
•About 1 in 12 cases of favism results in mortality
•Mostly affects children (up to 95% of cases)
•Why continue to cultivate fava beans? Nutrition and Fava Beans•Fava beans are only one of several legumes cultivated in the Mediterranean including chick peas, kidney beans, and lentils –Fava beans are a highly productive crop and produce a high yield of protein by dry weight
•However, kidney beans and chick peas are more efficient in terms of the ratio of weight of protein consumed to weight gained in growing individuals
•Lentils are as efficient as fava beans
–Continued use where Favism rates are high must be due to other factors
Responses to Favism•Mediterranean populations have developed several responses including food taboos, preparation techniques, and folk remedies –Highly susceptible groups including children and pregnant women are frequently forbidden to consume fava beans
–Drying, soaking, and removing the skins appear to reduce toxicity
–Increasing sugar consumption reduces the severity of an impending hemolytic crisis
Continued Cultivation•There are three lines of evidence that suggest continued cultivation of Fava Beans in the face of Favism is related to malaria –The association of divicine and isouramil with the suppression of Plasmodium growth
–The clinal association of fava beans cultivation and malaria
–The overlap of the peak fava bean harvest and consumption times with the peak Anopheles mosquito breeding season
Selection in Males•Males are G6PD deficient or Normal –Malaria alone:
•Select for an increase in GdMed because of resistance to Plasmodium in G6PD RBCs
–Combination of Malaria and Fava beans:
•Select against GdMed through favism and hemolytic anemia
–Cooking and preparation techniques may buffer the favism selection
–Other genes may also buffer favism
•Acid Phosphatase B and -Thallasemia reduce severity
•Decreased selection against GdB through increased resistance to Plasmodium from fava beans
Selection in Females•Malaria alone favors the heterozygotes –Selects against GdB/GdB, most susceptible genotype to Plasmodium
–Anemia selects against GdMed/GdMed, but they are resistant to severe malaria symptoms
–Heterozygotes (GdB/GdMed) are favored
•Increased resistance to malaria compared to GdB/GdB
•Less susceptible to hemolytic crises from diet or infection than GdMed/GdMed
•The balance is complicated by the random deactivation of an X chromosome in the cells producing the RBCs
–Heterozygotes will range widely from about 20% normal to about 80% normal RBCs, and the response to malaria and other hemolytic crises will vary accordingly
Selection in Females, 2•Combination of Malaria and Fava beans complicates selection –Fava beans intensify selection against GdMed/GdMed
•Favism and increased incidence of hemolytic anemia
–Cooking, preparation techniques, and other genes may buffer GdMed/GdMed from severe hemolytic crises
–Decreased selection against GdB/GdB
•Increased resistance to Plasmodium from fava beans
–Heterozygotes (GdB/GdMed) are still most fit
•The differential between GdB/GdMed and GdB/GdB resistance to malaria is reduced (selection is weaker)
•Selection differential may be stronger against GdMed/GdMed because of the increased incidence of hemolytic crises due to favism
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