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Dr. Moyer discusses the latest information on laboratory diagnosis of hereditary hemochromatosis - the most common inherited disease in the United States. Use of a diagnostic testing algorithm for iron overload can identify patients who should undergo genetic testing. This approach can reduce the need for invasive liver biopsy testing. Proper diagnosis and early treatment can prevent the serious consequences of this disease.
Presenter: Thomas P. Moyer, PhD
Welcome to Mayo Medical Laboratories' Hot Topics. These presentations provide short discussions of current topics and may be helpful to you in your practice.
Our speaker for this program is Dr. Thomas Moyer, from the Division of Clinical Biochemistry & Immunology at Mayo Clinic. Dr. Moyer will discuss the latest information on laboratory diagnosis of hereditary hemochromatosis - the most common inherited disease in the United States. Use of a diagnostic testing algorithm for iron overload can identify patients who should undergo genetic testing. This approach can reduce the need for invasive liver biopsy testing. Proper diagnosis and early treatment can prevent the serious consequences of this disease.
Hereditary hemochromatosis is the most common inherited disease in the United States. It results in increased intestinal absorption of iron and excess iron deposition in liver, pancreas, heart, and other tissues. There is no normal physiologic process to facilitate elimination of iron that accumulates in hemochromatosis except by blood loss through pregnancy, menstruation, phlebotomy, or trauma.
Iron is absorbed from the gastrointestinal tract; typically 1‑2 mgs of iron are absorbed per day. Iron absorbed from the gastrointestinal tract enters the plasma pool. Since gastrointestinal nutrients are absorbed into the hepatic vein, the plasma pool contents cycle through the liver before entering the circulation. Bone marrow absorbs iron from the plasma pool to facilitate creation of erythrocytes. Approximately 20 mg of iron becomes integration into erythrocytes per day. Erythrocytes circulate for approximately 120 days; at the end of their lifetime, they are phagocytized by macrocytic cells; when these cells enter the liver, they are known as Kupfer cells.
In iron overload disease, daily iron uptake increases to 3 to 5 mg per day. The liver is one of the predominant storage sites for this excess iron. Bone marrow incorporates iron into erythrocytes at the usual rate of 20 mg per day.
Iron is taken up in the duodenum by the villa cell and the crypt cell. It is preserved in the liver in the hepatocyte and in the bile duct cell. It is eliminated by the kidney and the bowel.
This is a cross-section of the duodenum. The villus cells occupy the apical portion of the duodenal epithelium; they are primarily engaged in absorption. Crypt cells, on the other hand, occupy the cryptic portion of the duodenal epithelium and are primarily nutrient storage cells.
Iron is absorbed from the gut and transported to the blood by the duodenal villus cell. Let’s focus first on ferric iron in the lumen of the intestine. That ferric iron is reduced by the membrane bound protein ferrireductase to ferrous iron which is then transported across the cell membrane by divalent metal transporter 1, frequently referred to as DMT1. The transported iron enters the cellular iron pool. Now let’s focus back on the lumen of the intestine at heme bound iron. Heme iron is transported by the heme carrier protein which facilitates the uptake of ferrous iron into the cellular iron pool. The villus cell facilitates transport of ferrous iron into the blood capillary by an active iron transporter called ferroportin . Once ferrous iron reaches the blood , it undergoes oxidation by the membrane bound protein hephaestin to ferric iron which binds to the protein apotransferrin to form the circulating protein-bound iron complex called transferrin.
The duodenal crypt cell serves as a significant storage and regulatory cell for iron. The presence of ferric iron in blood plasma stimulates the formation of a complex involving beta-2 microglobulin, transferrin receptor-1 and the membrane bound HFE protein to facilitate the transport of iron by a process of endocytosis. Iron is also intercollated into endocytes via Transferrin Receptor 2. Endocytic iron is then transported by DMT1 into the cellular iron pool of the crypt cell. Iron is recirculated back into the blood capillary via the iron transporter, ferroportin. What ferrous iron is returned to the plasma pool is oxidized to ferric iron by membrane bound hephaestin. This iron cycling process is modulated by a circulating protein called hepciden created by the liver. Hepciden causes ferroportin to alter its position in the cell membrane, reducing transport of ferrous iron into the blood capillary.
Let’s now turn our focus to the liver. Depicted here is the detail of a liver lobule. Blood enters the lobule via the hepatic portal vein, it circulates adjacent to hepatocytes and exits the liver via the hepatic vein. During blood flow through the liver, hepatocytes absorb nutrients, minerals, carbohydrates, peptides, and proteins.
Let’s now zoom in to an hepatocyte. Focus your attention on the transferrin bound ferric iron in the central vein. Like the crypt cell, the hepatocyte facilitates iron absorption through formation of a complex of beta-2 microglobulin with membrane bound HFE and transferrin receptor-1 proteins. This complex facilitates the uptake of iron through endocytosis. There is a second iron uptake process that involves transferrin receptor‑2. The affinity of ferric iron for transferrin receptor‑1 is approximately 30 times higher than the affinity of ferric iron for transferrin receptor-2. Endocytosis of iron involving the complex of beta-2 microglobulin, HFE and Transferrin Receptor-1 stimulates the release of interleukin 2 which signals the nucleus to initiate transcription and translation of the protein hepciden or a juvenile protein called hemojuvulin. Hepatocytes also continuously release ceruloplasmin and transferrin into circulating blood plasma. Endocytic iron is transported into the cellular iron pool by DMT1. Iron is also taken up into the lysosome where it binds to apoferritin to create the prevalent iron storage complex called ferritin. When this complex is stored in the lysosome, it is referred to as hemosiderin. Iron in the cellular pool is recirculated back into the blood serum by the transporter ferroportin. Ferrous iron is converted to ferric iron by ceruloplasmin and that iron binds to transferrin where it circulates in the plasma pool. A small fraction of cellular iron is also transported into the bile duct by ferroportin where it is excreted into the bowel.
In hemochromatosis, the membrane bound HFE protein is altered in a way that inhibits the formation of the beta-2 microglobulin HFE transferrin receptor-1 complex. In hemochromatosis, the primary mode of iron uptake is through transferrin receptor-2 which still facilitates the endocytosis of iron. However, the absence of the beta-2 microglobulin HFE transferrin receptor-1 complex in the endocyte results in little or no release of IL-2 resulting in diminished translation and formation of hepciden. In this process, however, ceruloplasmin and transferrin synthesis remain intact.
Now let’s focus attention back to the duodenal crypt cell and the effect of reduced hepcidin synthesized by the liver. In the absence of hepciden, the ferroportin transporter is fully activated. And, like the hepatocyte, the membrane bound HFE protein in the crypt cell is modified which interferes with the formation of the beta‑2 microglobulin, HFE Protein, transferrin receptor-1 complex. But, iron is still taken up from the blood capillary by the transferrin receptor-2 process which facilitates uptake and storage of iron in the crypt cell. Because of the complete activation of ferroportin, iron is continuously recirculated into the blood capillary where it is oxidized by hephaestin to ferric iron to circulate as highly saturated transferrin. One of the early findings in hemochromatosis is increased plasma transferrin and increased transferrin bound iron.
Back in the liver, the presence of increased plasma transferrin also stimulates increased ferritin, another early indicator of iron overload.
The early stages of hemochromatosis are asymptomatic. Frequently, the disease is found by serendipitous evaluation of iron. Hemochromatosis is often suspected when transferrin iron saturation is > 45% and ferritin is > 1000 µg/L. The advanced stage of the disease is indicated when hepatic transaminase concentrations become elevated. Common symptoms of the disease are fatigue, non‑inflammatory osteoarthritis, hypogonadism, diabetes, and skin bronzing.
Hemochromatosis is frequently not evident until the third or fourth decade of life. It is observed more frequently in males than females because female menses reduces iron overload. Chronic alcohol ingestion or hepatitis C infection exacerbates hemochromatosis.
The syndrome of hemochromatosis includes hepatic cirrhosis and occasionally hepatocellular carcinoma. Cardiac muscle damage results in heart failure, dilated cardiomyopathy, and conduction disturbances. Pancreatic damage may result in diabetes, and skeletal changes result in osteoarthritis. Skin bronzing is often seen in late stages of disease.
The laboratory diagnosis involves serum testing for iron, total iron binding capacity and ferritin. In hemochromatosis, serum iron >300 µg/dL, total iron saturation > 45%, and ferritin > 1000 µg/L are common findings. Clinical guidelines suggest that workup for hemochromatosis is not indicated unless iron saturation exceeds 45%.
When evaluation of the hemochromatosis gene is indicated, the HFE gene locus on chromosome 6 is examined. Mutations of the gene for HFE Protein associated with hemochromatosis produce amino acid changes at position 282 where a cysteine is converted to tyrosine and at position 63 where a histidine is converted to asparagine. These are the two most common amino acid changes associated with hemochromatosis.
Hereditary hemochromatosis is associated with alterations in the HFE protein, identified by assessing the DNA sequences responsible for coding of the HFE protein changes of C282Y or H63D. More than 85% of patients with clinical hereditary hemochromatosis are either homozygous for the cysteine to tyrosine amino acid change or are compound heterozygous for both mutations at 282 and 63. Individuals who are heterozygous for the 282 or 63 amino acid changes are not at risk for developing hereditary hemochromatosis.
Alteration in the HFE Protein gene is an autosomal recessive process with carrier frequency of 1 in 10 in individuals of northern European decent. Approximately 0.4% of people with European ancestry carry the alleles associated with hereditary hemochromatosis, and 75% of these individuals will develop iron overload. Among these patients, 30% of males and 70% of the women will develop liver disease. 2-5% will develop diabetes.
Genetic testing for hemochromatosis is indicated if transferrin saturation and ferritin concentration are high. Patients who are either homozygous for C282Y or heterozygous for C282Y and H63D have hereditary hemochromatosis.
A diagnostic testing algorithm was proposed by Dr. David Brandhagen in 1999 which has become the standard of practice. This algorithm starts with a clinical suspicion of hemochromatosis which is followed by a transferrin iron saturation evaluation. If total iron binding capacity is less than 45%, the patient should not be considered for further workup of hereditary hemochromatosis. If TIBC is elevated, repeat transferrin and ferritin evaluation should be performed. If these are both elevated, causes of secondary iron overload such as viral hepatitis should be considered. However, if no secondary cause is identified, then evaluation for the HFE gene is indicated. If HFE gene testing is positive, the patient has hereditary hemochromatosis, and phlebotomy treatment should be started. However, in approximately 20% of patients with hereditary hemochromatosis, the HFE gene test will be negative. In that case, liver biopsy, histology, and hepatic iron concentration should be performed. If these findings are consistent with hemochromatosis, phlebotomy is indicated.
Gross examination of the liver from a patient with hereditary hemochromatosis reveals significant enlargement with mahogany color. This indicates significant iron content.
In hemochromatosis, the H&E stain will show significant cirrhosis, interstitial fibrosis, and regenerative nodules. Tissue staining with Prussian Blue will show iron localized in the bile ductuals.
Assuming serum testing and genetic testing have been completed and results are ambiguous, then liver tissue evaluation is indicated. The liver iron test is available. The hepatic iron index is considered diagnostic for hemochromatosis. A hepatic iron index < 1 µmol/year rules out hemochromatosis. A value > 2 µmol/year is diagnostic for hemochromatosis. Surgical pathology consult is also available for tissue histology.
Treatment for hereditary hemochromatosis involves phlebotomy. This reduces the iron concentration of the plasma pool, it prevents long-term consequences of iron overload, and patients treated with phlebotomy can expect to lead a normal life.
In summary, hereditary hemochromatosis is a common disease. Early treatment can prevent the serious sequelae associated with this disease. Laboratory testing for iron overload can identify patients who should undergo genetic testing. Genetic testing can reduce the need for invasive liver testing.