Glucose-6-phosphate dehydrogenase deficiency

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Glucose-6-phosphate dehydrogenase deficiency
Classification and external resources
G6PD - 3D structure - PDB1qki.png
ICD-10 D55.0
ICD-9 282.2
OMIM 305900
DiseasesDB 5037
MedlinePlus 000528
eMedicine med/900
Patient UK Glucose-6-phosphate dehydrogenase deficiency
MeSH D005955

Glucose-6-phosphate dehydrogenase deficiency (G6PD deficiency) also known as favism (after the fava bean) is an X-linked recessive genetic condition that predisposes to hemolysis (spontaneous destruction of red blood cells) and resultant jaundice in response to a number of triggers, such as certain foods, illness, or medication. It is particularly common in people of Mediterranean and African origin. The condition is characterized by abnormally low levels of glucose-6-phosphate dehydrogenase, an enzyme involved in the pentose phosphate pathway that is especially important in the red blood cell. G6PD deficiency is the most common human enzyme defect.[1] There is no specific treatment, other than avoiding known triggers.

Carriers of the G6PD allele appear to be protected to some extent against malaria, and in some cases dominant males have shown complete immunity to the disease. This accounts for the persistence of the allele in certain populations in that it confers a selective advantage.[2]

Classification[edit]

The World Health Organization classifies G6PD genetic variants into five classes, the first three of which are deficiency states.[3]

  • Class I: Severe deficiency (<10% activity) with chronic (nonspherocytic) hemolytic anemia
  • Class II: Severe deficiency (<10% activity), with intermittent hemolysis
  • Class III: Mild deficiency (10-60% activity), hemolysis with stressors only
  • Class IV: Non-deficient variant, no clinical sequelae
  • Class V: Increased enzyme activity, no clinical sequelae

Signs and symptoms[edit]

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.[4] Favism is known to be more prevalent in infants and children, and G6PD genetic variant can influence chemical sensitivity.[5] Other than this, the specifics of the chemical relationship between favism and G6PD are not well understood.

6-phosphogluconate dehydrogenase deficiency (6PGD) has similar symptoms and is often mistaken for G6PD, as the effected enzyme is within the same pathway, however these diseases are not linked and can be found within the same patient.

Cause[edit]

Drug and Environmental triggers[edit]

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).[1][6][7] Henna has been known to cause haemolytic crisis in G6PD-deficient infants.[8]

Genetics[edit]

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).[9] These variants are believed to stem from a protective effect against Plasmodium falciparum and Plasmodium vivax malaria.[10]

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.[6] The following variants and mutations are well-known and described:

Table 1. Descriptive mutations and variants
Variants or mutations Gene Protein
Designation Short name Isoform
G6PD-Protein
OMIM-Code Type Subtype Position Position Structure change Function change
G6PD-A(+) Gd-A(+) G6PD A +305900.0001 Polymorphism nucleotide AG 376
(Exon 5)
126 AsparagineAspartic acid (ASN126ASP) No enzyme defect (variant)
G6PD-A(-) Gd-A(-) G6PD A +305900.0002 Substitution nucleotide GA 376
(Exon 5)
and
202
68
and
126
ValineMethionine (VAL68MET)
AsparagineAspartic acid (ASN126ASP)
G6PD-Mediterran Gd-Med G6PD B +305900.0006 Substitution nucleotide CT 563
(Exon 6)
188 SerinePhenylalanine (SER188PHE) Class II
G6PD-Canton Gd-Canton G6PD B +305900.0021 Substitution nucleotide GT 1376 459 ArginineLeucine (ARG459LEU) Class II
G6PD-Chatham Gd-Chatham G6PD +305900.0003 Substitution nucleotide GA 1003 335 AlanineThreonine (ALA335THR) Class II
G6PD-Cosenza Gd-Cosenza G6PD B +305900.0059 Substitution nucleotide GA 1376 459 ArginineProline (ARG459PRO) G6PD-activity <10%, thus high portion of patients.
G6PD-Mahidol Gd-Mahidol G6PD +305900.0005 Substitution nucleotide GA 487
(Exon 6)
163 GlycineSerine (GLY163SER) Class III
G6PD-Orissa Gd-Orissa G6PD +305900.0047 Substitution nucleotide 44 AlanineGlycine (ALA44GLY) NADP-binding place affected. Higher stability than other variants.
G6PD-Asahi Gd-Asahi G6PD A- +305900.0054 Substitution nucleotide (several) AG
±
GA
376
(Exon 5)
202
126
68
AsparagineAspartic acid (ASN126ASP)
ValineMethionine (VAL68MET)
Class III.

Pathophysiology[edit]

Pathology of G6PD deficiency.png

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 reduced form 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 buildup 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.[11]

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).

Diagnosis[edit]

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.

Treatment[edit]

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.[12]

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),[13] 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.

Epidemiology[edit]

G6PD deficiency is the most common human enzyme defect, being present in more than 400 million people worldwide.[14] In 2010, it resulted in about 4,000 deaths globally.[15] African, Middle Eastern and South Asian people are affected the most, including those who have these ancestries.[16] A side effect of this disease is that it confers protection against malaria,[17] 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.

Prognosis[edit]

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.[18]

History[edit]

In both legend and mythology, Favism has been known since antiquity. The priests of various Greek-Roman era cults were forbade to eat or even mention beans, and Pythagoras had a strict rule that to join the society of the Pythagoreans' they must swear off beans.[19] This ban was supposedly because beans somehow resembled the genitalia, 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.[20]

The modern understanding of the condition began with the analysis of patients who exhibited sensitivity to primaquine.[21] 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.[22]

References[edit]

  1. ^ a b Frank JE (October 2005). "Diagnosis and management of G6PD deficiency". Am Fam Physician 72 (7): 1277–82. PMID 16225031. 
  2. ^ Lewis, Ricki (1997). Human Genetics. Chicago, IL: Wm. C. Brown. pp. 247–248. ISBN 0-697-24030-4. 
  3. ^ WHO Working Group (1989). "Glucose-6-phosphate dehydrogenase deficiency.". Bulletin of the World Health Organization 67 (6): 601–11. PMID 2633878. 
  4. ^ Al-Ali, AK (1996). "Common G6PD variant from Saudi population and its prevalence" (PDF). Annals of Saudi Medicine 16 (6): 654–6. ISSN 0256-4947. PMID 17429252. Retrieved May 17, 2009. "This has led to the identification of one genetically determined common variant, 'G6PD Mediterranean.' The overall prevalence rate of this deficiency in these areas was determined to be in excess of 42%. None of the subjects studied displayed any sign of favism" 
  5. ^ Luzzatto, L. "GLUCOSE-6-PHOSPHATE DEHYDROGENASE DEFICIENCY." Advanced Medicine-Twelve: Proceedings of a Conference Held at the Royal College of Physicians of London, 11–14 February 1985. Vol. 21. Churchill Livingstone, 1986.
  6. ^ a b Warrell, David A.; Timothy M. Cox; John D. Firth; Edward J. Benz (2005). Oxford Textbook of Medicine, Volume Three. Oxford University Press. pp. 720–725. ISBN 0-19-857013-9. 
  7. ^ A comprehensive list of drugs and chemicals that are potentially harmful in G6PD deficiency can be found in Beutler E (December 1994). "G6PD deficiency". Blood 84 (11): 3613–36. PMID 7949118. .
  8. ^ Raupp P, Hassan JA, Varughese M, Kristiansson B (2001). "Henna causes life threatening haemolysis in glucose-6-phosphate dehydrogenase deficiency". Arch. Dis. Child. 85 (5): 411–2. doi:10.1136/adc.85.5.411. PMC 1718961. PMID 11668106. 
  9. ^ http://www.britannica.com/EBchecked/topic/202897/favism
  10. ^ Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson; Aster, Jon (2009-05-28). Robbins and Cotran Pathologic Basis of Disease, Professional Edition: Expert Consult - Online (Robbins Pathology) (Kindle Locations 33351-33354). Elsevier Health. Kindle Edition.
  11. ^ Gaskin RS, Estwick D, Peddi R (2001). "G6PD deficiency: its role in the high prevalence of hypertension and diabetes mellitus". Ethnicity & disease 11 (4): 749–54. PMID 11763298. 
  12. ^ Monga A, Makkar RP, Arora A, Mukhopadhyay S, Gupta AK (July 2003). "Case report: Acute hepatitis E infection with coexistent glucose-6-phosphate dehydrogenase deficiency". Can J Infect Dis 14 (4): 230–1. PMC 2094938. PMID 18159462. 
  13. ^ Hamilton JW, Jones FG, McMullin MF (August 2004). "Glucose-6-phosphate dehydrogenase Guadalajara – a case of chronic non-spherocytic haemolytic anaemia responding to splenectomy and the role of splenectomy in this disorder". Hematology 9 (4): 307–9. doi:10.1080/10245330410001714211. PMID 15621740. 
  14. ^ Cappellini MD, Fiorelli G (January 2008). "Glucose-6-phosphate dehydrogenase deficiency". Lancet 371 (9606): 64–74. doi:10.1016/S0140-6736(08)60073-2. PMID 18177777. 
  15. ^ Lozano, R (Dec 15, 2012). "Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010.". Lancet 380 (9859): 2095–128. doi:10.1016/S0140-6736(12)61728-0. PMID 23245604. 
  16. ^ G-6-PD FAQ section
  17. ^ Mehta A, Mason PJ, Vulliamy TJ (2000). "Glucose-6-phosphate dehydrogenase deficiency". Baillieres Best Pract. Res. Clin. Haematol. 13 (1): 21–38. PMID 10916676. 
  18. ^ thefreedictionary.com > glucose-6-phosphate dehydrogenase deficiency citing: Gale Encyclopedia of Medicine. Copyright 2008
  19. ^ Simoons, F.J. (1996-08-30). "8:". Plants of Life, Plants of Death. University of Wisconsin Press. p. 216. ISBN 0299159043. 
  20. ^ Rendall, Steven; Riedweg, Christoph (2005). Pythagoras: his life, teaching, and influence. Ithaca, N.Y: Cornell University Press. ISBN 0-8014-4240-0. 
  21. ^ Alving AS, Carson PE, Flanagan CL, Ickes CE (September 1956). "Enzymatic deficiency in primaquine-sensitive erythrocytes" (PDF). Science 124 (3220): 484–5. Bibcode:1956Sci...124..484C. doi:10.1126/science.124.3220.484-a. PMID 13360274. 
  22. ^ Beutler E (January 2008). "Glucose-6-phosphate dehydrogenase deficiency: a historical perspective". Blood 111 (1): 16–24. doi:10.1182/blood-2007-04-077412. PMID 18156501. 

External links[edit]