Iron Metabolism


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Iron Metabolism

  1. 1. Iron Homeostasis Rekha Parameswaran, M.D Division of Hematology
  2. 2. Iron Homeostasis • Iron deficiency world’s most common cause of anemia • Body conserves iron such that we have no physiologic methods of iron excretion • No metabolism • Iron moves through different compartments • Regulated at level of absorption
  3. 3. Enterocyte: Transfer Erythroid Precursor: Use Hepatocyte: Storage Macrophage: Recycling Iron Compartments Iolascon A, et al. Semin Hematol. 2009;46:358-370.
  4. 4. Hepcidin Master Regulator of Iron Homeostasis • Negative regulator of iron release • Expression induced by iron in the liver • Coded by the HAMP gene (chromosome 19) • Hepcidin over-expression leads to iron-deficient anaemia and hepcidin knock-out to iron overload • Hepcidin inhibits duodenal iron absorption and macrophage iron release • Mechanism of action: binds to ferroportin, which is then internalized and degraded leading to blockage of iron export • Regulation of hepcidin itself poorly understood
  5. 5. • Expression in GIT limited to cells in deep crypts in proximity to site of iron absorption. • HFE protein associated with TfR, acts to modulate uptake of Tf-bound iron into crypt cells. • Along with hepcidin, acts as iron sensor. • Hereditary haemochromatosis with HFE gene mutation - reduced association with TfR1- increase uptake of iron into cells. HFE(High Iron) protein
  6. 6. • Ferroportin-1 in basal portion of placental syncytiotrophoblasts, basolateral surface of duodenal enterocytes, macrophages, hepatocytes. • Upregulated by amount of available iron, downregulated through interaction with hepcidin. Ferroportin
  7. 7. Hemojuvelin • Highly expressed in skeletal muscle and heart, and to a lesser extent in the liver • Hemojuvelin is a positive regulator of hepcidin, the central iron regulatory hormone • Lack of hemojuvelin leads to lower levels of hepcidin which results in iron overload states
  8. 8. Interplay of Key Proteins in Iron Homeostasis Fleming RE, Bacon BR. N Engl J Med 2005;352:1741-1744.
  9. 9. Body Iron Regulation by Hepcidin 11 Body iron decrease lowers hepcidin synthesis in the liver 33 Duodenal absorption of iron increases 44 Splenic iron is released into the circulation 55 Iron concentration in plasma increases, leading to restoration of iron balance Hepcidin deficiency targets the duodenum and spleen 22 Iron DeficiencyIron Deficiency Hepcidin 11 22 33 44 IronIron Ganz T, et al. Am J Physiol Gastrointest Liver Physiol. 2006;290:G199-G203. 55
  10. 10. Types 1, 2, and 3 Hemochromatosis— Quantitative Hepcidin Deficiency 11 HFE or non-HFE mutations decrease hepcidin synthesis in the liver 33 Duodenal absorption of iron increases 44 Splenic iron is released into the circulation 55 Iron concentration in plasma strongly increases Hepcidin deficiency targets the duodenum and spleen 22 HFEHFE (type1)(type1) or non-or non-HFEHFE (type 2 or 3)(type 2 or 3) mutationsmutations Hepcidin 11 22 33 44 IronIron 55 66 66 Increased plasma iron produces parenchymal iron deposition Brissot P, et al. Blood Rev. 2008;22:195-210.
  11. 11. Blood = Iron atom Pathophysiology Iron Overload For Ferroportin Disease (type 4 HC) and Aceruloplasminaemia Deficiency of Cellular Iron Export Ferroportin Disease* Aceruloplasminaemia Mutated ferroportin Macrophagic iron excess due to deficient iron export (kupffer cell siderosis shown in 3) Fe2+ Fe3+ Cp . Transferrin Mutated ceruloplasmin (Cp) Mutation leads to absence of ferroxidase activity (needed for iron uptake by transferrin) Excessive ferroportin degradation leads to decreased cellular export of iron Blood *Valid for form A. In form B (resistance to hepcidin) mechanism of iron excess (corresponding to inactive hepcidin) is similar to type 1, 2, or 3 HC. This leads to intracellular retention of iron 1 2 1 2 1 1 2 2 3 43 4 In both diseases plasma iron concentration is normal or low
  12. 12. Anemia of Inflammation/Chronic Disease —Hepcidin Overproduction 11 Inflammation increases hepcidin synthesis in the liver 33 Duodenal absorption of iron decreases 44 Splenic iron released into circulation is decreased Hepcidin increase targets the duodenum and spleen 22 InflammationInflammation Hepcidin 11 22 33 44 IronIron 66 55 66 Plasma iron concentration decreases, leading to anemia Brissot P, et al. Blood Rev. 2008;22:195-210. 55 Overloaded macrophages 55
  13. 13. Iron-Refractory Iron-Deficiency Anemia (IRIDA)—Hepcidin Overproduction 11 Matriptase-2 mutations increase hepcidin synthesis in the liver 33 Duodenal absorption of iron decreases 44 Splenic iron released into circulation is decreased Hepcidin increase targets the duodenum and spleen 22 Hepcidin 11 22 33 44 IronIron 55 55 Plasma iron concentration decreases, leading to anemia Matriptase-2Matriptase-2 (TMPRSS6)(TMPRSS6) mutationsmutations Finberg KE, et al. Nat Genet. 2008;40:569-571.
  14. 14. Primary Iron-Overload Disorders Pietrangelo A. N Engl J Med 2004;350:2383-2397.
  15. 15. Inheritance Genetic transmission of HC • Autosomal recessive • Exception: ferroportin disease (Dominant transmission)
  16. 16. HFE hemochromatosis • HFE-haemochromatosis (type 1) • HFE gene closed in 1996 – autosomal recessive – 10% of Caucasians of Northern European descent are heterozygotes for C282Y or H63D mutations – 5 in 1000 people are homozygous for the most penetrant form i.e. C282Y mutation – Compound heterozygosity less penetrant – Overall penetrance remains debated and estimated to be about 1% or less
  17. 17. Epidemiology—Penetrance Incomplete for HFE-HC–Phenotypic Variability (5-Scale Grading) Tf Sat (transferrin saturation) = >45%; ferritin = >300 µg/L (male), >200 µg/L (female). Quality of life symptoms = asthenia, impotence, arthropathy Life-threatening symptoms = cirrhosis, diabetes, cardiomyopathy. Tf Sat Ferritin Tf Sat 0 1 2 PRECLINICAL 3 4 Ferritin Quality of life Tf Sat Life Ferritin Quality of life Tf Sat CLINICAL Brissot P, et al, Hematology, Jan 2006:36,
  18. 18. Epidemiology—Penetrance Modifying factors • Acquired – Diet – Menses – Pregnancies – Blood loss/blood donation • Genetic – Polymorphism or mutations of other genes related to iron metabolism
  19. 19. Arthropathy Skin Pigmentation Clinical and Biochemical Features • Clinical syndromes – Asthenia, arthropathy, osteopaenia, skin pigmentation, impotence, diabetes, hepatomegaly, cardiac symptoms8 • Biochemical parameter – Hyperferritinaemia = >300 µg/L in men, >200 µg/L in women • Confounding factors – Alcoholism – Polymetabolic syndrome – Inflammation – Hepatitis
  20. 20. Imaging • Magnetic resonance imaging (MRI) – Hyposignal (T2 weighted MRI) provides hepatic iron concentration • Benefits of MRI – Accurate, noninvasive strategy that most often eliminates the need for liver biopsy in diagnosing iron overload
  21. 21. Caucasian ? Elevated transferrin >45% Yes C282Y/C282Y ? Type 1 HC No Yes No Genetic test Genetic test if < 30 years old Haemojuvelin (Type 2A HC) Hepcidin (Type 2B HC) Ferroportin (Type 4B HC) TfR2 (Type 3 HC) Diagnosis to Prove Genetic Origin— Elevated TF
  22. 22. Diagnosis to Prove Genetic Origin— Normal or Low TF Normal or low transferrin (<45%) Plasma ceruloplasmin level Aceruloplasminaemia If anaemia & neurologic symptoms 0 (or low)Normal Genetic test Ferroportin? (Type 4A HC) Yes No
  23. 23. Diagnosis of HFE (Type 1)- Hemochromatosis • Five successive steps: • Clinical features assessment • Evaluate foran increased transferrin saturation : the earliest biochemicalparameter tobe increased • HFE gene testing. Typically, a homozygous C282Y mutation The diagnosisof type1 hemochromatosis is then established, and no furtherinvestigationsare required to confirm the diagnosis. • Quantify iron overload.plasmaferritin level (N< 300 µg/L in men, < 200 µg/Lin women). • Stage the phenotypicmanifestations of HFE hemochromatosis:This is important inorder to define the most appropriate measuresfor both treatmentand follow-up.
  24. 24. HFE-Related Hereditary Hemochromatosis, a Multistep, Multifactorial Iron-Overload Disorder. Pietrangelo A. N Engl J Med 2004;350:2383-2397.
  25. 25. Diagnostic Workup of Suspected Adult-Onset Hereditary Hemochromatosis. Pietrangelo A. N Engl J Med 2004;350:2383-2397.
  26. 26. Treatment—Venesection Therapy • Treatment of choice for HC related to hepcidin deficiency (types 1, 2, and 3 HC) or inactivity (type 4B HC) • Revisited guidelines (for type 1 HC) • Start: grade 2 (ferritin level >300 µg/L for men, >200 µg/L for women) • Induction phase: 7 mL/kg body weight (<550 mL) weekly until ferritin = 50 µg/L1 • Maintenance phase: every 1–4 months until ferritin ≤50 µg/L (lifetime regimen thereafter)1
  27. 27. Results/Contraindications for Venesection Therapy • Results in types 1, 2, 3, and 4B – Good for asthenia, skin pigmentation, liver disease, cardiac function – Moderate for arthropathy (which may worsen) and diabetes – Poor for cirrhosis (risk of hepatocellular carcinoma) • Ferroportin disease (type 4A HC) – Poorly tolerated: risk of anemia Note: Life expectancy is normal if treatment starts before cirrhosis and insulin-dependent diabetes
  28. 28. Family Screening • HFE-HC (type 1) • Whatever the grading of the C282Y/C282Y proband, should evaluate first-degree relatives (18 years or older) for C282Y mutation + serum iron markers (transferrin saturation, ferritin) C282Y = 0 or C282Y = 1* No special follow-up *C282Y/wild-type C282Y = 2† Grading Venesections if grade ≥ 2† C282Y/C282Y
  29. 29. Family Screening • Types 2 and 3 HC (juvenile HC and TfR2 HC) – Similar procedure: search for identity using the proband mutation profile coupled with evaluation of individual’s biochemical iron status • Type 4 (A and B) HC (ferroportin disease) – The screening approach is different because of dominant transmission; hyperferritinaemia (corresponding to ferroportin mutation) would exist in 50% of siblings and offspring
  30. 30. Genetic Forms of Iron Deficiency Anaemia • Mutations in the gene encoding DMT1 • Mutations in the gene encoding glutaredoxin 5 • Hypotransferrinaemia or atransferrinaemia • Deficiency of ceruloplasmin • IRIDA (Iron-Refractory, Iron-Deficiency Anaemia)
  31. 31. Differential Diagnosis of Microcytic Anaemia • Thalassaemia syndromes • Certain haemoglobinopathies (Hb C) • True (classical) iron deficiency secondary to blood loss, iron-poor diet, increased iron needs, Helicobacter pylori infection or gastric pathology • Anaemia of chronic inflammatory diseases • Certain forms of sideroblastic anaemia • Genetic forms of iron deficiency anaemia
  32. 32. Genes Involved in Hereditary Iron Deficiency Anaemia Protein (Gene Symbol) Chromosome Protein Function Disease Caused by Mutations DMT1 (SLC11A2)1 121 Transmembrane iron transporter1 Autosomal recessive hypochromic, microcytic anaemia with hepatic iron overload1 Glutaredoxin 5 (GLRX5)1 141 Participates in iron-sulfur cluster biogenesis1 Anaemia with iron overload and sideroblasts1 Transferrin (TF)1 31 Plasma iron binding protein; ligand for TFR1 & TFR21 Iron deficiency anaemia with tissue iron overload1 Ceruloplasmin (CP)1 31 Plasma ferroxidase1 Mild iron deficiency anaemia associated with iron accumulation in the liver and brain1 Matriptase -2 (TMPRSS6)1 222 Regulates hepcidin expression by an unknown mechanism1 IRIDA1 With permission from Andrews NC. Blood. 2008;112:219-230. 1. Andrews NC. Blood. 2008;112:219-230. 2. Finberg KE, et al. Nat Genet. 2008;40:569-571.
  33. 33. IRIDA (Iron-Refractory, Iron- Deficiency Anaemia) The Mask Mouse “…a chronically iron-deficient mouse with an unusual pattern of hair loss over the trunk but not the head (the mask phenotype) due to a homozygous recessive genetic mutation. Mask mice were shown to express inappropriately high levels of hepcidin mRNA in the liver, even when fed an iron-deficient diet. Using positional cloning techniques, Dr. Beutler’s group was able to ascribe the mask phenotype to a splicing error in the TMPRSS6 gene, which encodes a membrane-bound serine protease.”1,2 1. Coghill JM, Ma A. Available at: 2. Du X, et al. Science. 2008;320:1088-1092.
  34. 34. Mutation in TMPRSS6 (Matriptase-2), Suppressor of Hepcidin Gene Expression, in Familial Iron Deficiency Anaemia Photograph: With permission from Du X. Science. 2008;320:1088-1092. Graphic (top right): With permission from Ramsay AJ, et al. Front Biosci. 2008;13:569-579. Graphic (bottom): With permission from Finberg KE, et al. Nat Genet. 2008;40:569-571.
  35. 35. IRIDA (Iron-Refractory, Iron- Deficiency Anaemia) Generation of mutant mice TMPRSS6-/- provided evidence that matriptase-2 is an essential regulator of iron homeostasis. In fact, in mice as well as in humans, mutations in the TMPRSS6-/- gene lead to severe iron deficiency anaemia. This state is characterized by reduced ferroportin expression (shown in the mice model) and both animals and humans have high hepcidin levels. Folgueras AR, et al. Blood. 2008 Jun 3. [Epub ahead of print] PMID: 18523150.
  36. 36. IRIDA (Iron-Refractory, Iron- Deficiency Anaemia) Case-ReportsPatient 11 Patient 22 Patient 33 Patient 43 Patient 53 Patient 63 Patient 73 Patient 83 Patient 93 Age (y) 14 18 6 13mo. 17mo. 11 7 3 15mo. Hb (g/L) 86 100 88 92 70 82 75 97 79 MCV (fL) 54.3 61 58 65 49 56 49 61 53 IRON STATUS Fe (µmol/L) 2.44 2.96 Ferritin (µg/L) 9 53 Transferrin saturation (%) 6.52 sTfR (mg/L) 5 2 10 5 3 4 4 2 pHepcidin (ng/mL) 70 133.5 1. Guillem F, Blood 2008 Jul 2. [Epub ahead of print] PMID: 18596229. 2. Melis MA, et al. Haematologica. 2008 Jul 4. [Epub ahead of print] PMID: 18603562. 3. Finberg KE, et al. Nat Genet. 2008 May;40(5):569-71.
  37. 37. IRIDA (Iron-Refractory, Iron- Deficiency Anaemia) Case-Reports Treatment • Oral iron administration is ineffective • Response to parenteral iron administration is partial • Anaemia becomes less severe in adulthood as a consequence of the greater availability of the limited amount of available iron to erythropoiesis Melis MA, et al. Haematologica. 2008 Jul 4. [Epub ahead of print] PMID: 18603562.
  38. 38. Diagnostic Approach—Genetic Forms of Iron Deficiency Anaemia • Look for consanguinity (recessive transmission) • Is the anaemia associated with massive iron accumulation? If yes, then do detailed evaluation of CNS to distinguish between aceruloplasmia (CNS damage present) and the other forms with iron overload • If presence of basophilic stippling and double erythroid population in peripheral blood, then do bone marrow aspiration to look for the presence of ring sideroblasts
  39. 39. Diagnostic Approach—Genetic Form of Iron Deficiency Anaemia • Was the anaemia detected at birth or early in life? If yes, then look for a mutation in the DMT1 or the TMPRSS6 gene • Does the patient have microcytosis with high serum iron values? If yes, then a DMT1 mutation is highly suspected • If no, then look for a TMPRSS6 mutation. Hepcidin levels are also very helpful as hepcidin is normal or increased in the TMPRSS6 mutation despite low serum iron
  40. 40. Main Biologic and Clinical Differences in Genetic Forms of Iron Deficiency AnaemiaDMT1 Glutaredoxin 5 Atransferrinaemia Acerulo- plasminaemia TMPRSS6 (matriptase-2) Age at diagnosis At birth Usually midlife Late onset provided some transferrin is present Late onset with moderate anaemia 18–24 mo Liver iron overload Yes Yes Yes Yes No Brain damage No No No Yes No Serum iron High High Low Low Low Transferrin saturation High High High or non- measureable Low Low Ringed sideroblasts No Yes No No No Hepcidin levels Low Not yet measured Not yet measured Not yet measured High for low iron values Ferritin Low or normal High High High Normal
  41. 41. Conclusions • Iron deficiency anaemia is an acquired disease with an estimated 3 billion people affected and represents a major public health problem worldwide1 • Recent advances in iron metabolism led to the recognition of new entities of iron deficiency anaemia in nonbleeding and “high cost diet” nourished individuals • Apparently rare, these genetic forms of iron deficiency anaemia should be recognized by haematologists, as they are refractory to classical oral or intravenous iron administration 1. Andrews NC. Blood. 2008;112:219-230.