Disorders of iron metabolism, including hemochromatosis and other forms of iron overload; control of intestinal iron absorption; regulation of cellular iron-binding proteins
Iron is essential for all cells of the body, but excess iron can be harmful. Iron overload occurs most often in persons with hemochromatosis, a common autosomal recessive disorder characterized by excessive dietary iron absorption. If undetected and untreated, the condition leads to accumulation of toxic levels of iron in multiple organs, including the liver, pancreas, heart, joints, and anterior pituitary. Patients may develop a variety of complications, including diabetes mellitus, cardiac arrhythmias and failure, arthritis, hypogonadotrophic hypogonadism, cirrhosis of the liver and primary hepatocellular carcinoma. However, all these disease manifestations can be prevented if excess iron is removed by phlebotomy before organ damage occurs. The recent discovery of the hemochromatosis gene (HFE) has stimulated interest in genetic testing as a means for early identification of persons at risk who may benefit from such intervention. To evaluate this approach, we currently are conducting investigations of primary care patients at UCI (the HEIRS Study) and veterans at the Long Beach VA.
The HEIRS Study at UCI is designed to evaluate the prevalence, genetic and environmental determinants, and potential clinical, personal and societal correlates of iron overload and hemochromatosis in a multi-center, multi-ethnic, primary care based sample of 100,000 adults, 20,000 at each of five Field Centers in the U.S. and Canada. Participants are screened for common HFE mutations (C282Y and H63D) and for serum ferritin and transferrin saturation levels. At the UCI Field Center, we have enrolled a total of 20,398 persons, and we have performed further evaluation of those at risk for iron overload. Participants with confirmed iron overload having undergone phlebotomy therapy. For participants who have iron overload but do not posses the common hemochromatosis genotype, additional testing will be done to identify other, less common, HFE mutations. In addition, family studies are underway to identify mutations of candidate modifier genes involved in iron metabolism that may affect phenotypic expression.
The VA-sponsored study, titled Prevalence of Iron Overload and Frequency of the Hemochromatosis Gene, is designed to perform screening for iron overload and determine the frequency of the hemochromatosis gene among veterans receiving care at the VA Medical Center. In addition, this study is evaluating the proportional morbidity of hemochromatosis and iron overload in the VA setting. As in the HEIRS Study, participants in the VA study are screened for common HFE mutations (C282Y and H63D) and for transferrin saturation and serum ferritin levels. In addition, initial screening includes measurement of the circulating transferrin receptor (TfR) concentration. The TfR test, originally developed for the diagnosis of iron deficiency, has been helpful in quantifying body iron in veterans with iron deficiency anemia identified incidentally during the iron overload screening study. Statistical mixture modeling will be used to develop general guidelines for hemochromatosis screening regimens based on analysis of transferrin saturation, serum ferritin, and TfR levels. This approach will identify subpopulations with adequate, increased, or decreased body iron.
In a previous study, we examined the relationship between transferrin saturation and iron stores in the African-American and U.S. Caucasian Populations based on analysis of data from the Third National Health and Nutrition Examination Survey. The frequency distribution of transferrin saturation was evaluated using mixture-modeling techniques. We found that the distribution of transferrin saturation fit best to a trimodal pattern corresponding to a subpopulation of normal individuals and two subpopulations with successively higher transferrin saturation levels, consistent with the expected numbers of hemochromatosis heterozygotes and homozygotes, respectively. We now plan to extend this work by combining mixture-modeling techniques with HFE genotyping to further refine screening criteria, particularly with regard to the optimal transferrin saturation threshold for identification of affected individuals.
Regulation of Intestinal Iron Absorption:
The control of intestinal iron absorption is critical for maintaining body iron balance, and defective control of this process in patients with hemochromatosis ultimately leads to iron overload. To better understand the regulatory mechanism, we designed a mathematical compartment model of iron absorption for evaluation of altered intestinal mucosal iron transport kinetics. This physiologically-based model describes the three key intestinal mucosal iron transport steps: i. uptake of lumenal iron across the mucosal brush border, ii. storage in the form of ferritin within the enterocyte, or iii. baso-lateral transfer of iron to the systemic circulation. In our approach, a double isotope technique is used to trace the dynamics of both intestinal iron absorption and plasma iron kinetics simultaneously by administering oral and intravenous 59Fe and 55Fe, respectively. The rate constants for each of the steps of mucosal iron transport then are calculated by determining the best nonlinear least squares fit of the model to the data. Using this model, we found that increased baso-lateral iron transfer is the major determinant of excessive intestinal iron absorption in patients with hemochromatosis.
Another situation associated with increased intestinal iron absorption is an increase in ineffective erythropoiesis, which occurs in disorders such a thalassemia major and sideroblastic anemia. The increased erythropoiesis in these conditions can be mimicked experimentally in animals by induction of hemolytic anemia with phenylhydrazine (PHZ). Previous studies have demonstrated that iron absorption begins to increase within three to four days after PHZ administration. Using this approach in conjunction with our compartment model, we showed that increased iron absorption in beagle dogs with PHZ-induced hemolytic anemia was attributable primarily to an increase in brush border iron uptake. These findings indicate the existence of different mechanisms for increasing iron absorption under different circumstances.
The delay of several days between an erythropoietic stimulus (e.g., hemolysis) and the subsequent increase in intestinal iron absorption is commonly believed to represent the time required for signals from the body to program the immature crypt cells, and for these cells to then mature and migrate to the villus. Recent molecular data, however, suggest that signals from the body to alter iron absorption are mediated by circulating hepcidin and that this peptide has a direct effect on the mature villus enterocytes. To better understand this pathway, we have examined the delay in the absorptive response following stimulated erythropoiesis in rats using PHZ-induced hemolytic anemia. There was a delay of four days following the onset of hemolysis before an increase in iron absorption was observed. Hepatic hepcidin expression did not decrease until day three, reaching almost undetectable levels by days four and five. This coincided with the increase in expression of both divalent metal transporter 1 (DMT1, which is thought to transport iron across the brush border of the enterocyte) and Ireg1 (which transports iron across the baso-lateral membrane). These results suggest that the delayed increase in iron absorption following stimulated erythropoiesis is attributable to a lag in the hepcidin response rather than crypt programming and support a direct action of hepcidin on the mature villus enterocytes.
We have taken advantage of an inherited anemia of the mouse (hemoglobin deficit, or hbd) to gain insights into the factors regulating hepcidin expression. Hbd mice showed a significant anemia but, surprisingly, their iron absorption was not increased as it was in wild-type animals made anemic to a similar degree by dietary iron depletion. In wild-type mice, hepatic hepcidin levels were decreased, but in hbd animals a significant and unexpected increase was observed. The level of absorption was appropriate for the expression of hepcidin in each case, but in hbd mice did not reflect the degree of anemia. However, this apparent inappropriate regulation of hepcidin correlated with increased transferrin saturation and levels of diferric transferrin in the plasma, which in turn resulted from the reduced capacity of hbd animals to effectively use transferrin-bound iron. These data strengthen the proposal that diferric transferrin is a key indicator of body iron requirements.
Future studies will focus on understanding the mechanism of the hepcidin effect on mucosal iron transport at the level of the enterocyte under conditions of an increased erythropoietic stimulus and in other conditions associated with altered iron absorption, including chronic inflammatory disorders.
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Gordeuk, V.R., McLaren, G.D., and Samowitz, W. Etiologies, consequences and treatment of iron overload. Crit. Rev. Clin. Lab. Sci. 31:89-133, 1994.
Anderson, G.J., Murphy, T.L, Cowley, L., Evans, B.A., Halliday, J.W., and McLaren, G.D. Mapping the gene for sex-linked anemia: an inherited defect of intestinal iron absorption in the mouse. Genomics 48: 34-39, 1998.
McDonnell, S., Phatak, P.D., Felitti, V., Hover, A., and McLaren, G.D. Experiences with screening for hemochromatosis in primary care settings. Ann. Intern. Med. 129: 962-970, 1998.
Anderson, G.J. and McLaren, G.D. Genetic disorders of trace element metabolism. Chapter 12, in The Clinical Nutrition of the Essential Trace Elements and Minerals - The Guide for Health Professionals, Bogden, J.D. and Klevay, L.M. (Eds.), Totowa, Humana Press, 2000, pp. 201-226.
McLaren, C.E., Kambour, E.L., McLachlan, G.J., Lukaski, H.C., Li, X., Brittenham, G.M., and McLaren, G.D. Patient-specific analysis of sequential haematological data by multiple linear regression and mixture distribution modelling. Statist. Med. 19: 83-98, 2000.
McLaren, G.D. Extra-hepatic iron metabolism in hemochromatosis. Chapter 15, in Hemochromatosis: Genetics, Pathophysiology, Diagnosis and Treatment, Barton, J.C. and Edwards, C.Q. (Eds.), Cambridge, Cambridge University Press, 2000, pp. 163-169.
McLaren, G.D. Hereditary hemochromatosis and other iron-overload disorders. Chapter 5, in Manual of Clinical Hematology, 3rd edition, Mazza, J.J. (Ed.), Baltimore, Lippincott Williams & Wilkins, 2001, pp. 118-137.
McLaren, C.E., Li, K.-T., Gordeuk, V.R., Hasselblad, V., and McLaren, G.D. Relationship between transferrin saturation and iron stores in the African-American and U.S. Caucasian Populations: Analysis of data from the Third National Health and Nutrition Examination Survey. Blood 98: 2345-2351, 2001.
McLaren, G.D. and Gordeuk, V.R. Iron deficiency, in Conn's Current Therapy, Rakel, R.E. and Bope, E.T. (Eds.), New York, W.B. Saunders, 2002, pp. 355-359.
McLaren, C.E., Barton, J.C., Adams, P.C., Harris, E.L., Acton, R.T., Press, N., Reboussin, D.M., McLaren, G.D., Sholinsky, P., Walker, A.P., Gordeuk, V.R., Leiendecker-Foster, C., Dawkins, F.W., Eckfeldt, J.H., Mellen, B.G., Speechley, M., and Thomson, E., for the Hemochromatosis and Iron Overload Study Research Investigators. Hemochromatosis and Iron Overload Screening (HEIRS) Study Design for an Evaluation of 100,000 Primary Care-Based Adults. Am. J. Med. Sci. 325: 53-62, 2003.
Frazer, D.M., Inglis, H.R., Wilkins, S.J., Millard, K.N., Turner, T.M., McLaren, G.D., McKie, A.T., Vulpe, C.D., and Anderson, G.J. Delayed hepcidin response explains the lag period in iron absorption following a stimulus to increase erythropoiesis. Gut 53: 1509-1515, 2004.
Adams, P.C., Reboussin, D.M., Barton, J.C., McLaren, C.E., Eckfeldt, J.H., McLaren, G.D., Dawkins, F.W., Acton, R.T., Harris, E.L., Gordeuk, V.R., Leiendecker-Foster, C., Speechley, M., Snively, B.M., Holup, J.L., Thomson, E., and Sholinsky, P., for the Hemochromatosis and Iron Overload Screening (HEIRS) Study Research Investigators. Hemochromatosis and iron-overload screening in a racially diverse population. N. Engl. J. Med. 352: 1769-1778, 2005.
Means, R.T. and McLaren, G.D. Anemia of chronic disease in hematological disorders and oncology, Chapter 20 in Anemia of Chronic Disease, Weiss, G., Gordeuk, V.R., and Hershko, C. (Eds.), New York, Taylor & Francis, 2005, pp. 593-606.
Wilkins, S.J., Frazer, D.M., Millard, K.N., McLaren, G.D., and Anderson, G.J. Iron metabolism in the hemoglobin deficit mouse: correlation of diferric transferrin with hepcidin expression. Blood 107: 1659-1664, 2006.
Adams, P.C., Passmore, L., Chakrabarti, S., Reboussin, D.M., Acton, R.T., Barton, J. C., McLaren, G.D., Eckfeldt, J.H., Dawkins, F.W., Gordeuk, V.R., Harris, E.L., Leiendecker-Foster, C., Gossman, E., and Sholinsky, P., for the Hemochromatosis and Iron Overload Screening Study Research Investigators: Liver diseases in the Hemochromatosis and Iron Overload Screening Study. Clin. Gastroenterol. Hepatol. 4: 918-923, 2006.
Acton, R.T., Barton, J.C., Passmore, L.V., Adams, P.C., Speechley, M.R., Dawkins, F.W., Sholinsky, P., Reboussin, D.M., McLaren, G.D., Harris, E.L., Bent, T.C., Vogt, T.M., and Castro, O. Relationships of serum ferritin, transferrin saturation, and HFE mutations and self-reported diabetes mellitus in the Hemochromatosis and Iron Overload Screening (HEIRS) Study. Diabetes Care 29: 2084-2089, 2006.
Acton, R.T., Barton, J.C., Snively, B.M., McLaren, C.E., Adams, P.C., Harris, E.L., Speechley, M.R., McLaren, G.D., Dawkins, F.W., Leiendecker-Foster, C., Holup, J.L., and Balasubramanyam, A., for the Hemochromatosis and Iron Overload Screening Study Research Investigators. Geographic and racial/ethnic differences in HFE mutation frequencies in the Hemochromatosis and Iron Overload Screening (HEIRS) Study. Ethn. Dis. 16: 815-821, 2006.
McLaren, C.E., Li, K.-T., McLaren, G.D., Gordeuk, V.R., Snively, B.G., Reboussin, D.M., Barton, J.C., Acton, R.T., Dawkins, F.W., Harris, E.L., Eckfeldt, J.H., Moses, G.C., and Adams, P.C. Mixture models of phenotypic markers in population disease screening: the Hemochromatosis and Iron Overload Screening (HEIRS) Study. Transl. Res. 148: 196-206, 2006.
Steiner, M., Leiendecker-Foster, C., McLaren, G.D., Snively, B.M., McLaren, C.E., Adams, P.C., and Eckfeldt, J.H. HFE gene splice site mutation IVS5+1 G/A in North American Vietnamese with and without phenotypic evidence of iron overload. Transl. Res. 149: 92-95, 2007.
Harris, E.L., McLaren, C.E., Reboussin, D.M., Gordeuk, V.R., Barton, J. C., Acton R.T., McLaren, G.D., Vogt, T.M., Snively, B.M., Leiendecker-Foster, C., Holup. J.L., Passmore, L.V., Eckfeldt, J.H., Lin, E., and Adams, P.C. Serum ferritin and transferrin saturation in Asians and Pacific Islanders. Arch. Intern. Med. 167: 722-726, 2007.
Rivers, C.A., Barton, J.C., Gordeuk, V.R., Acton, R.T., Speechley, M.R., Snively, B.M., Leiendecker-Foster, C., Press, R.D., Adams, P.C., McLaren, G.D., Dawkins, F.W., McLaren, C.E., and Reboussin, D.M. Association of ferroportin Q248H polymorphism with elevated levels of serum ferritin in African Americans in the Hemochromatosis and Iron Overload Screening (HEIRS) Study. Blood Cells Mol. Dis. 38: 247-252, 2007.
Acton, R.T., Snively, B.M., Barton, J.C., McLaren, C.E., Adams, P.C., Rich, S.S., Eckfeldt, J.H., Press, R.D., Sholinsky, P., Leiendecker-Foster, C., McLaren, G.D., Speechley, M.R., Harris, E.L., Dawkins, F.W., and Gordeuk, V.R. A genome-wide linkage scan for iron phenotype quantitative trait loci: the HEIRS family study. Clin. Genet. 71: 518-529, 2007.
Dawkins, F.W., Gordeuk, V.R., Snively, B.M., Lovato, L., Barton, J.C., Ronald T. Acton, R.T., McLaren, G.D., Leiendecker-Foster, C., McLaren, C.E., Adams, P.C., Speechley, M., Harris, E.L., Jackson, S., and Thomson, E.J. African Americans at risk for increased iron stores or liver disease. Am. J. Med. 120: 734.e1-734.e9, 2007.
Adams, P.C., Reboussin, D.M., Press, R.D., Barton, J.C., Acton, R.T., Moses, G.C., Leiendecker-Foster, C., McLaren, G.D., Dawkins, F.W., Gordeuk, V.R., Lovato, L., and Eckfeldt, J.H. Biological variability of transferrin saturation and unsaturated iron binding capacity. Am. J. Med. 120: 999.e1-999.e7, 2007.
Barton, J.C., Acton, R.T., Leiendecker-Foster, C., Lovato, L., Adams, P.C., McLaren, G.D., Eckfeldt, J.H., McLaren, C.E., Reboussin, D.M., Gordeuk, V.R.., Speechley, M.R., Reiss, J.A., Press, R.D., and Dawkins, F.W. HFE C282Y homozygotes ages 25-29 years at HEIRS Study initial screening. Genet. Test. 11: 269-275, 2007.
Adams, P.C., Reboussin, D.M., Barton, J.C., Acton, R.T., Speechley, M.R., Leiendecker-Foster, C., Meenan, R., Passmore, L., McLaren, C.E., McLaren, G.D., and Gordeuk, V.R. Serial serum ferritin measurements in untreated HFE homozygotes in the Hemochromatosis and Iron Overload Screening (HEIRS) Study. Int. J. Lab. Hematol. 30: 300-305, 2008.
Acton, R.T., Barton, J.C., Passmore, L.V., Adams, P.C., McLaren, G.D., Leiendecker-Foster, C., Speechley, M.R., Harris, E.L., Castro, O., Reiss, J.A., Snively, B.M., Harrison, B.W., and McLaren, C.E. Accuracy of family history of hemochromatosis or iron overload: the Hemochromatosis and Iron Overload Screening study. Clin. Gastroenterol. Hepatol. 6: 934-938, 2008.
Gordeuk, V.R.: Reboussin, D.M., Barton, J.C., Acton, R.T., McLaren, G.D., McLaren, C.E., Harris E.L., Reiss, J.A., Adams, P.C., Speechley, M., Phatak, P.D., Sholinsky, P., Eckfeldt, J.H., Passmore, L., and Dawkins, F.W. Serum ferritin concentrations and body iron stores in a multicenter, multiethnic primary care population. Am. J. Hematology. 83: 618-626, 2008.
McLaren, G.D., McLaren, C.E., Adams, P.C., Barton, J. C., Reboussin, D.M., Gordeuk, V.R., Acton, R.T., Harris, E.L., Speechley, M.R., Sholinsky, P., Dawkins, F.W., Snively, B.M., Vogt, T.M., and Eckfeldt, J.H. Clinical manifestations of hemochromatosis in HFE C282Y homozygotes identified by screening. Can. J. Gastroenterol. 22: 923-930, 2008.
Adams, P.C., Pankow, J.S., Barton, J.C., Acton, R.T., Leiendecker-Foster, C., McLaren, G.D., Speechley, M., and Eckfeldt, J.H. HFE C282Y homozygosity is associated with lower total and LDL cholesterol: the Hemochromatosis and Iron Overload Screening (HEIRS) Study. Circulation: Cardiovasc. Genet. 2: 34-37, 2009.
Wang, X., Leiendecker-Foster, C., Acton, R.T., Barton, J.C., McLaren, C.E., McLaren, G.D., Gordeuk, V.R., and Eckfeldt, J.H. Heme carrier protein 1 (HCP1) genetic variants in the Hemochromatosis and Iron Overload Screening (HEIRS) Study participants. Blood Cells Mol. Dis. 42: 150-154, 2009.
Anderson, G.J., Frazer, D.M., and McLaren, G.D. Iron absorption and metabolism. Curr. Opin. Gastroenterol. 25: 129-135, 2009.
Khomenko, T., Szabo, S., Deng, X., Ishikawa, H., Anderson, G.J., and McLaren, G.D. Role of iron in the pathogenesis of cysteamine-induced duodenal ulceration in rats. Am. J. Physiol. Gastrointest. Liver Physiol. 296: G1277-G1286, 2009.
McLaren, G.D. and Gordeuk, V.R. Hereditary Hemochromatosis: Insights from the Hemochromatosis and Iron Overload Screening (HEIRS) Study. Hematology Am. Soc. Hematol. Educ. Program 2009:195-206.
McLaren, C.E., Barton, J.C., Eckfeldt, J.H., McLaren, G.D., Acton, R.T., Adams, P.C., Henkin, L.F., Gordeuk, V.R., Vulpe, C.D., Harris, E.L., Harrison, B.W., Reiss, J.A., and Snively, B.M. Heritability of serum iron measures in the Hemochromatosis and Iron overload Screening (HEIRS) Family Study. Am. J. Hematol. 85: 101-105, 2010.
Southwest Oncology Group
Iron Overload Diseases Association
American Society of Hematology
American Society of Clinical Oncology
American Federation for Medical Research
American Association for the Advancement of Science
Staff Physician-Hematology/Oncology Section
VA Long Beach Healthcare System 2000—curr
Chao Family Comprehensive Cancer Center