Effects of Iron Overload
Total body iron overload occurs most often due either to hereditary hemochromatosis or to repeated transfusions in patients with severe anemia. Hereditary hemochromatosis is the more common of the two by far.
Hereditary haemochromatosis reflects a fractional increase in dietary iron absorption (Cox and Peters, 1978) (Cox and Peters, 1980) (Lynch et al., 1989). Tissue iron reaches dangerous levels after thirty or forty years. The gene responsible for hereditary haemochromatosis, HFE, resides on chromosome 6. Discovered in 1996, the gene encodes a protein that is homologous to the Class I HLA antigens (Feder, et al., 1996). The alteration in HFE protein that produces hereditary haemachromatosis involves the mutation of a cysteine to a leucine at position 282 (C282Y). People who have one copy of the mutant HFE gene are carriers who only rarely develop iron overload (usually in association with a second defect.) People with two copies of the mutant protein can develop iron overload and the myriad of problems that it can produce (see below). Nearly 90% of people who have hereditary haemochromatosis have the C282Y mutation in HFE (Nielson, et al., 1998).
Only recently have investigators gained insight into the mechanism by which the mutation in HFE alters cellular iron metabolism. Iron in the circulation is bound to the protein, transferrin, which maintains it in a non-toxic state. Cells contain receptors for transferrin on their plasma membranes which mediate cellular iron uptake. Transferrin receptors bind iron-transferrin complexes which are taken into endosomes. Iron is separated from transferrin in the endosome, and is shuttled into the interior of the cell. The iron-free transferrin (apotransferrin) is recycled into the circulation and is free to bind and transport additional iron atoms. The HFE protein associates with the transferrin receptor and prevents internalization of iron-transferrin complex into cells (Gross, et al., 1998). The HFE protein, in effect, acts as a brake on cellular iron uptake.
The C282Y mutation in HFE disrupts the folding of the protein (Lebron, et al., 1998). The mutant protein does not associate with the transferrin receptor and does not dampen iron uptake by cells. These insights do not fully explain the increase in gastrointestinal iron absorption, which is the root of hereditary hemochromatosis. They are, however, the first observations that mechanistically connect HFE and iron metabolism. Improved understanding of the complex process of intestinal iron absorption should surmount this shortcoming.
Hereditary hemachromatosis is remarkably common. About 10% to 12% of people of European background are heterozygous for the condition (Douabin, et al., 1999). The number of people who are homozygous for the condition approaches one in three hundred. This makes hereditary hemochromatosis one of the most prevalent genetic conditions in the world. Interestingly, the clinical expression of the disorder is less than frequency calculations predict. Variable penetrance, prerhaps related to secondary genetic or environemental conditions must influence clinical manifestations.
Transfusional Iron Overload
Transfusional iron overload occurs with severe, chronic anemias where patients survive for many years thanks to the transfusions. Conditions that fulfill this criteria include thalassemia major, myelodysplasia (including sideroblastic anemia), moderate aplastic anemia, and Diamond-Blackfan anemia. Iron accumulation with repeated transfusion reflects the retention of the heme iron from the transfused red cells after they become senscent and are destroyed. No physiological means of iron excretion exists. As a result, the element accumulates in all the body’s organs, and particularly in the liver.
With transfusional iron overload, the senescent red cells are destroyed by reticuloendothelial cells. The iron is deposited onto transferrin, the protein responsible for iron transport in the blood. From here, the iron is distributed to all body tissues. With transfusional iron overload, excess iron occurs both in the reticuloendothelial cells and parenchymal cells. In contrast, with hereditary hemochromatosis the iron is placed directly onto transferrin and from there moves to the tissues. The distinguishing feature between transfusional iron overload and hereditary hemochromatosis is the presence of large deposits of iron in the reticuloendothelial cells with the former. The pattern of organ injury is the same with the two.
Consequences of Iron Overload
The effect of iron overload on some organs, such as the skin, are trivial, while hemosiderotic harm to others, such as the liver, can be fatal (Bassett et al., 1986). Few notable symptoms precede advanced injury. Abdominal discomfort, lethargy, and fatigue are common but nonspecific complaints. Dyspnea with exertion and peripheral edema indicate significant cardiac compromise and reflect advanced iron loading.
As the major site of iron storage, the liver is a conspicuous victim of excess iron depositon (Bonkovsky, 1991). Mild to moderate hepatomegaly develops early, followed by shrinkage produced by fibrosis and cirrhosis (Conte et al., 1986) (Bassett et al., 1986). Hepatic tenderness occurs occasionally.
Hematoxylin and eosin staining reveals a brownish pigment in the hepatocytes which Perl’s Prussian blue staining unmasks as iron (Hultcrantz and Glaumann, 1982). Large amounts of iron are also deposited in Kupfer cells of patients with transfusional iron overload. Electron microscopy reveals substantial hemosiderin aggregates in addition to large quantities of ferritin.
As with many other conditions that injure the liver, hepatic damage secondary to excessive iron deposition produces fibrosis. With long standing hemochromatosis, micronodular cirrhosis can also develop. Hemosiderotic liver damage produces very little inflammation. Consequently, significant hepatic iron deposition and even fibrosis can occur with very little increase in the serum transaminase levels. Disturbances in liver synthetic function indicate advanced disease.
Congestive cardiomyopathy is the most common defect that occurs with iron overload, but other problems have been described including pericarditis, restrictive cardiomyopathy, and angina without coronary artery disease (Schellhammer et al., 1967) (Fitchett et al., 1980) (Sanyal et al., 1975) (Liu and Olivieri, 1994). A strong correlation exists between the cumulative number of blood transfusions and functional cardiac derangements in children with thalassemia (Wolfe et al., 1985) (Koren et al., 1987).
The physical examination is surprisingly benign even in patients with heavy cardiac iron deposition. Once evidence of cardiac failure appears, however, heart function rapidly deteriorates, often without response to medical intervention. Biventricular failure produces pulmonary congestion, peripheral edema, and hepatic engorgement. Vigorous iron extrication has reversed this potentially lethal complication on occasion (Rahko et al., 1986).
Iron deposition in the Bundle of His and the Purkinje system produces conduction defects (Buja and Roberts, 1971) (Olson et al., 1987). Sudden death is common in these patients, presumably due to arythmias. At one time, patients treated with the chelator desferrioxamine for transfusional iron overload received supplements of ascorbic acid in the range of 15 to 30 mg/kg per day to promote iron mobilization (O’Brien, 1977). Reports of sudden death prompted cessation of this practice (Nienhuis, 1981). At lower doses (2 to 4 mg/kg), ascorbic acid is a safe adjunct to chelation therapy in patients with transfusional iron overload.
Cardiac dysfunction can occur with very little tissue iron deposition. The total quantity of iron is less important than the unbound, or “toxic” iron subset. The concentration of unbound iron in tissues is extremely small, and virtually impossible to measure. This “toxic” iron is precisely the component bound and neutralized by iron chelators (in the case of desferrioxamine, the association constant is about 1032, see below). Therefore, cardiac damage is best prevented in patients with transfusional iron overload by maintaining a constant low level of chelator in the circulation (and consequently in the tissues, where the protection is rendered.) Chick cardiac myocytes in culture contract spontaneously. Iron salts added to the culture medium poison the cells and abrogate this function. Desferrioxamine chelates extracellular, and importantly intracellular iron, and restores myocyte contractility (Link G, et al. 1985).
Echocardiography in children and radionuclide ventriculography in adults are the most useful non-invasive diagnostic techniques. The echocardiographic abnormalities correlate roughly with the number of transfusions. Exercise radionuclide ventriculograms are particularly sensitive in the detection of cardiac dysfunction in patients with iron overload (Leon et al., 1979).
Dysfunction of the endocrine pancreas is common in patients with iron overload (Flynn et al., 1976). Some people develop overt diabetes mellitus requiring insulin therapy. The disturbances in carbohydrate metabolism are often more subtle, however. An oral glucose tolerance test often unmasks abnormal insulin production. Vigorous exorcism of the excess iron occasionally reverses the islet cell dysfunction (Bomford and Williams, 1976). Exocrine pancreatic function, in contrast, is usually well-preserved.
Pituitary dysfunction produces a plethora of endocrine disturbances (Costin et al., 1979). Reduced gonadotropin levels are common. When coupled with primary reductions in gonadal synthesis of sex steroids, this phenomenon delays sexual maturation in some children with transfusional iron overload. Secondary infertility is common (Schafer et al., 1981). Although Addison’s syndrome is uncommon with iron overload, production of ACTH is occasionally deficient. A metapyrone stimulation test shows delayed or diminished pituitary secretion of ACTH (Schafer et al., 1985).
Thyroid function is usually well-preserved in patients with iron overload. In contrast, parathyroid activity is frequently compromised. Functional hypoparathyroidism can be demonstrated in many patients by inducting hypocalcemia with an intravenous bolus of ethelyenediamine tetraacetic acid (EDTA) while monitoring the production of parathyroid hormone (Gertner et al., 1979).
Miscellaneous Abnormalities with Iron Overload
Hyperpigmentation is a nonspecific skin response to a variety of insults including excessive exposure to ultraviolet light (tanning), thermal injury, and drug eruptions. Cutaneous iron deposition damages the skin and enhances melanin production by melanocytes. Ultraviolet light exposure and iron are often synergistic in the induction of skin pigmentation, so that many patients tan very readily. Fair-skinned people who routinely tan poorly often never develop hyperpigmentation despite very large body iron burdens, highlighting the genetic contribution to skin pigmentation. In contrast, patients with moderate baseline pigmentation (for example, people of Mediterranean descent) frequently develop a striking almond-colored hue. With particularly heavy iron overload, visible iron deposits sometimes appear in the skin as a grayish discoloration.
Arthropathy, a common feature with hereditary hemochromatosis, is rare in patients with secondary iron overload (Mathews and Williams, 1987). The large joints, such as the hips are affected most commonly (Axford et al., 1991). Decades of iron deposition in articular cartilage in hereditary hemochromatosis is the presumed cause of this condition. Chondrocalcinosis is a late but characteristic feature of the arthropathy seen in hereditary hemochromatosis. Other troubling musculoskeletal problems include severe, recurrent cramps and disabling myalgias. Muscle biopsy shows iron deposits in the myocytes, but the pathophysiologic connection to the pain and cramps is unclear.
Bone disease, manifested as osteoporosis, is a significant problem in patients with thalassemia. Bone marrow expansion often thins the bone cortex, making these patients very susceptible to fractures. The etiology of the bony disorder in patients with thalassemia is unclear. One possible contributor is the desferrioxamine used to prevent iron overload. The chelator has a very high specificity for iron. It may, however, chelate a small amount of the calcium that is necessary for the production of new bone. Over the years, a very low rate of mineral scavenging from bone by desferrioxime could contribute to osetoporosis.
Pulmonary hypertension is a problem that has been widely recognized only recently in patients with iron overload. A number of reports have involved patients with thalassemia major or thalassemia intermedia with iron overload (Aessopos, et al., 1995) (Grisaru D,et al., 1990) (Koren, et al., 1987). No report exists of similar problems in people with iron overload from other causes, such as hereditary hemochromatosis. The combination of iron overload in the pulmonary tissues and high blood flow through the pulmonary vascular bed may be at fault. More work is needed to clarify these issues.
Iron overload and Opportunistic Infections
Withholding iron from potential pathogens is one strategy used in host defense (Weinberg, 1978). Transferrin’s extremely high affinity for iron, coupled with the fact that two-thirds of the iron binding sites of the protein normally are unoccupied, essentially eliminates free iron from plasma and extracellular tissues. Both transferrin and the structurally related protein, lactoferrin, are bacteriostatic in vitro for many bacteria (Bullen et al., 1971) (Reiter et al., 1975) (Lawrence 3d et al., 1977).
The very high transferrin saturations attained in patients with iron overload compromise the bacteriostatic properties of the protein. Iron sequestration is not a frontline defense against microbes. Therefore, iron overload does not produce the susceptibility to infection seen with defects in more central systems (e.g., chronic granulomatous disease.) Nonetheless, a number of infections, often with unusual organisms, have been reported in patients with iron overload (Abbott et al., 1986) (Brennan et al., 1983) (Bullen et al., 1991) (Capron et al., 1984):
Sideroblastic anemia often produces neutropenia or neutrophil dysfunction. Host defense is compromised even further in patients with sideroblastic anemia who develop secondary iron overload. Although aggressive antimicrobial therapy is often successful, some infections, such as the mucormycosis produced by Rhizopus oryazae, are almost uniformally fatal.
The iron chelator, desferrioxamine, has also been implicated in opportunistic infection with unusual organisms such as Rhizopus orayzae, the cause of mucormycosis, in some patients with iron overload (Boelaert et al., 1988) (Rex et al., 1988) (Daly et al., 1989). Streptomyces pilosissynthesizes this siderophore when grown in an iron-deficient environment. Desferrioxamine is released in the vicinity of these microbes, binds iron, and returns the element to the microorganisms to support growth and replication. Some pathogenic bacteria and fungi can utilize desferrioxamine-bound iron to promote their growth, thereby enhancing the risk of severe infection (Robins-Browne and Prpic, 1985).
The question of when to begin chelation therapy in a patient with transfusional hemochromatosis lacks a simple answer (Fargion et al., 1982). The decision must be carefully individualized. Serious infection in patients treated with desferrioxamine is uncommon, and the benefits of therapy to prevent iron-induced organ damage generally outweigh the risk of infectious complications.
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