| Iron overload | |
| Synonyms: | Haemochromatosis or Hemochromatosis |
| Field: | Hematology, gastroenterology/hepatology |
Iron overload (also known as haemochromatosis or hemochromatosis) is the abnormal and increased accumulation of total iron in the body, leading to organ damage.[1] The primary mechanism of organ damage is oxidative stress, as elevated intracellular iron levels increase free radical formation via the Fenton reaction. Iron overload is often primary (i.e. hereditary haemochromatosis) but may also be secondary to repeated blood transfusions (i.e. transfusional iron overload).[2] Iron deposition most commonly occurs in the liver, pancreas, skin, heart, and joints. People with iron overload classically present with the triad of liver cirrhosis, secondary diabetes mellitus, and bronze skin.[3] However, due to earlier detection nowadays, symptoms are often limited to general chronic malaise, arthralgia, and hepatomegaly.
Organs most commonly affected by hemochromatosis include the liver, heart, and endocrine glands.[4]
Hemochromatosis may present with the following clinical syndromes:
Hemochromatosis leading to secondary diabetes (through iron deposition in the insulin secreting beta cells of the pancreas), when combined with a bronzing or darkening of the skin, is sometimes known as "bronze diabetes".[9]
The term hemochromatosis was initially used to refer to what is now more specifically called hemochromatosis type 1 (or HFE-related hereditary hemochromatosis). Currently, hemochromatosis (without further specification) is mostly defined as iron overload with a hereditary or primary cause,[10] [11] or originating from a metabolic disorder.[12] However, the term is currently also used more broadly to refer to any form of iron overload, thus requiring specification of the cause, for example, hereditary hemochromatosis.
Hereditary hemochromatosis is an autosomal recessive disorder with estimated prevalence in the population of 1 in 200 among patients with European ancestry, with lower incidence in other ethnic groups.[13] Mutations of the HFE gene (hemostatic iron regulator) located on chromosome 6 (responsible for iron regulatory protein hepcidin production) are responsible for most cases of hereditary hemochromatosis; 95% of cases of hereditary hemochromatosis involve a mutation of this HFE gene.[1] Non-HFE hereditary hemochromatosis involves mutations in genes coding for the iron regulatory proteins hemojuvelin, transferrin receptor-2 and ferroportin.
Hereditary hemochromatosis is characterized by an accelerated rate of intestinal iron absorption and progressive iron deposition in various tissues. This typically begins to be expressed in the third to fifth decades of life, but may occur in children. The most common presentation is hepatic cirrhosis in combination with hypopituitarism, cardiomyopathy, diabetes, arthritis, or hyperpigmentation. Because of the severe sequelae of this disorder if left untreated, and recognizing that treatment is relatively simple, early diagnosis before symptoms or signs appear is important.[14] [15]
In general, the term hemosiderosis is used to indicate the pathological effect of iron accumulation in any given organ, which mainly occurs in the form of the iron-storage complex hemosiderin.[16] [17] Sometimes, the simpler term siderosis is used instead.
Other definitions distinguishing hemochromatosis or hemosiderosis that are occasionally used include:
The causes of hemochromatosis broken down into two subcategories: primary cases (hereditary or genetically determined) and less frequent secondary cases (acquired during life).[24] People of Northern European descent, including Celtic (Irish, Scottish, Welsh, Cornish, Breton etc.), English, and Scandinavian origin[25] have a particularly high incidence, with about 10% being carriers of the principal genetic variant, the C282Y mutation on the HFE gene, and 1% having the condition.[26]
The overwhelming majority depend on mutations of the HFE gene discovered in 1996, but since then others have been discovered and sometimes are grouped together as "non-classical hereditary hemochromatosis",[27] "non-HFE related hereditary hemochromatosis",[28] or "non-HFE hemochromatosis".[29]
| Description | Mutation | - | Hemochromatosis type 1 "classical" hemochromatosis | - | Hemochromatosis type 2A juvenile hemochromatosis | Haemojuvelin (HJV, also known as RGMc and HFE2) | - | Hemochromatosis type 2B juvenile hemochromatosis | hepcidin antimicrobial peptide (HAMP) or HFE2B | - | transferrin receptor-2 (TFR2 or HFE3) | - | ferroportin (SLC11A3/SLC40A1) | - | (unknown) | - | Acaeruloplasminaemia (very rare) | - | Congenital atransferrinaemia (very rare) | - | GRACILE syndrome (very rare) | BCS1L |
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Most types of hereditary hemochromatosis have autosomal recessive inheritance, while type 4 has autosomal dominant inheritance.[30]
Defects in iron metabolism, specifically involving the iron regulatory protein hepcidin are thought to play an integral role in the pathogenesis of hereditary hemochromatosis.[31]
Normally, hepcidin acts to reduce iron levels in the body by inhibiting intestinal iron absorption and inhibiting iron mobilization from stores in the bone marrow and liver. Iron is absorbed from the intestines (mostly in the duodenum) and transported across intestinal enterocytes or mobilized out of storage in liver hepatocytes or from macrophages in the bone marrow by the transmembrane ferroportin transporter. In response to elevated plasma iron levels, hepcidin inhibits the ferroportin transporter leading to decreased iron mobilization from stores and decreased intestinal iron absorption, thus functioning as a negative iron regulatory protein.
In hereditary hemochromatosis, mutations in the proteins involved in hepcidin production including HFE (hemostatic iron regulator), hemojuvelin and transferrin receptor 2 lead to a loss or decrease in hepcidin production, which subsequently leads to the loss of the inhibitory signal regulating iron absorption and mobilization and thus leads to iron overload. In very rare instances, mutations in ferroportin result in ferroportin resistance to hepcidin's negative regulatory effects, and continued intestinal iron absorption and mobilization despite inhibitory signaling from hepcidin. Approximately 95% of cases of hereditary hemochromatosis are due to mutations in the HFE gene.
The resulting iron overload causes iron to deposit in various sites throughout the body, especially the liver and joints, which coupled with oxidative stress leads to organ damage or joint damage and the pathological findings seen in hemochromatosis.
There are several methods available for diagnosing and monitoring iron overload.
Blood tests are usually the initial test if there is a clinical suspicion of iron overload. Serum ferritin testing is a low-cost, readily available, and minimally invasive method for assessing body iron stores. However ferritin levels may be elevated due to a variety of other causes including obesity, infection, inflammation (as an acute phase protein), chronic alcohol intake, liver disease, kidney disease, and cancer.[32] [33] In males and postmenopausal females, normal range of serum ferritin is between 12 and 300 ng/mL (670 pmol/L) .[34] [35] [36] In premenopausal females, normal range of serum ferritin is between 12 and 150 or 200 ng/mL (330 or 440 pmol/L). In those with hemochromatosis, the serum ferritin level correlates with the degree of iron overload. Ferritin levels are usually monitored serially in those with hemochromatosis to assess response to treatment.
Elevations in serum levels of the iron transporter protein transferrin saturation as well as increased red blood cell mean corpuscular volume and mean corpuscular hemoglobin concentration usually precede ferritin elevations in hemochromatosis. Transferrin saturation of greater than 45% combined with an elevated ferritin level is highly sensitive in diagnosing HFE hemochromatosis. Total iron binding capacity may be low in hemochromatosis, but can also be normal.[37]
General screening for hemochromatosis is not recommended, however first-degree relatives of those affected should be screened.[38]
Once iron overload has been established, HFE gene mutation genetic testing for hereditary causes of iron overload is indicated. The presence of HFE gene mutations in addition to iron overload confirms the clinical diagnosis of hereditary hemochromatosis. The alleles evaluated by HFE gene analysis are evident in ~80% of patients with hemochromatosis; a negative report for HFE gene does not rule out hemochromatosis.
Liver biopsy is the removal of small sample in order to be studied and can determine the cause of inflammation or cirrhosis. In someone with negative HFE gene testing, elevated iron status for no other obvious reason, and family history of liver disease, additional evaluation of liver iron concentration is indicated. In this case, diagnosis of hemochromatosis is based on biochemical analysis and histologic examination of a liver biopsy. Assessment of the hepatic iron index (HII) is considered the "gold standard" for diagnosis of hemochromatosis.
Magnetic resonance imaging (MRI) is used as a noninvasive method to estimate iron deposition levels in the liver and heart, which may aid in determining a response to treatment or prognosis. Liver elastography has limited utility in detecting liver fibrosis in hemochromatosis.
Phlebotomy, bloodletting or venesection is the mainstay of treatment in iron overload, consisting of regularly scheduled blood draws to remove red blood cells (and iron) from the body. Upon initial diagnosis of iron overload, the phlebotomies may be performed weekly or twice weekly, until iron levels are normalized. Once the serum ferritin and transferrin saturation are within the normal range, maintenance phlebotomies may be needed in some (depending upon the rate of reabsorption of iron), scheduled at varying frequencies to keep iron stores within normal range. A phlebotomy session typically draws between 450 and 500 mL of blood.[39] Routine phlebotomy may reverse liver fibrosis and alleviate some symptoms of hemochromatosis, but chronic arthritis is usually not responsive to treatment. In those with hemochromatosis; the blood drawn during phlebotomy is safe to be donated.[40]
Phlebotomy is associated with improved survival if it is initiated before the onset of cirrhosis or diabetes.
The human diet contains iron in two forms: heme iron and non-heme iron. Heme iron is usually found in red meat, whereas non-heme iron is found in plant based sources. Heme iron is the most easily absorbed form of iron. In those with hemochromatosis undergoing phlebotomy for treatment; restriction of dietary iron is not required.[41] [42] However, those who do restrict dietary iron usually require less blood needing to be phlebotomized (about 0.5–1.5 liters of blood less per year).[43] Vitamin C and iron supplementation should be avoided as vitamin C accelerates intestinal absorption of iron and mobilization of body iron stores. Raw seafood should be avoided because of increased risk of infections from iron loving pathogens such as Vibrio vulnificus.[44] Alcohol consumption should be avoided due to the risk of compounded liver damage with iron overload.
Medications are used for those unable to tolerate routine blood draws, there are chelating agents available for use.[45] The drug deferoxamine binds with iron in the bloodstream and enhances its elimination in urine and faeces. Typical treatment for chronic iron overload requires subcutaneous injection over a period of 8–12 hours daily. Two newer iron-chelating drugs that are licensed for use in patients receiving regular blood transfusions to treat thalassaemia (and, thus, who develop iron overload as a result) are deferasirox and deferiprone.[46] [47]
A minimally invasive approach to hereditary hemochromatosis treatment is the maintenance therapy with polymeric chelators.[48] [49] [50] These polymers or particles have a negligible or null systemic biological availability and they are designed to form stable complexes with Fe2+ and Fe3+ in the GIT and thus limiting their uptake and long-term accumulation. Although this method has only a limited efficacy, unlike small-molecular chelators, the approach has virtually no side effects in sub-chronic studies. Interestingly, the simultaneous chelation of Fe2+ and Fe3+ increases the treatment efficacy.
In general, provided there has been no liver damage, patients should expect a normal life expectancy if adequately treated by venesection. If the serum ferritin is greater than 1,000 μg/L at diagnosis there is a risk of liver damage and cirrhosis which may eventually shorten their life.[51] The presence of cirrhosis increases the risk of hepatocellular carcinoma.[52] Other risk factors for liver damage in hemochromatosis include alcohol use, diabetes, liver iron levels greater than 2,000 μmol/gram and increased aspartate transaminase levels.
The risk of death and liver fibrosis are elevated in males with HFE type hemochromatosis but not in females; this is thought to be due to a protective effect of menstruation and pregnancy seen in females as well as possible hormone-related differences in iron absorption.
HHC is most common in certain European populations (such as those of Irish or Scandinavian descent) and occurs in 0.6% of some unspecified population.[38] Men have a 24-fold increased rate of iron-overload disease compared with women.[38]
Diet and the environment are thought to have had large influence on the mutation of genes related to iron overload. Starting during the Mesolithic era, communities of people lived in an environment that was fairly sunny, warm and had the dry climates of the Middle East. Most humans who lived at that time were foragers and their diets consisted largely of wild plants, fish, and game. Archaeologists studying dental plaque have found evidence of tubers, nuts, plantains, grasses and other foods rich in iron. Over many generations, the human body became well-adapted to a high level of iron content in the diet.[53]
In the Neolithic era, significant changes are thought to have occurred in both the environment and diet. Some communities of foragers migrated north, leading to changes in lifestyle and environment, with a decrease in temperatures and a change in the landscape which the foragers then needed to adapt to. As people began to develop and advance their tools, they learned new ways of producing food, and farming also slowly developed. These changes would have led to serious stress on the body and a decrease in the consumption of iron-rich foods. This transition is a key factor in the mutation of genes, especially those that regulated dietary iron absorption. Iron, which makes up 70% of red blood cell composition, is a critical micronutrient for effective thermoregulation in the body.[54] Iron deficiency will lead to a drop in the core temperature. In the chilly and damp environments of Northern Europe, supplementary iron from food was necessary to keep temperatures regulated, however, without sufficient iron intake the human body would have started to store iron at higher rates than normal. In theory, the pressures caused by migrating north would have selected for a gene mutation that promoted greater absorption and storage of iron.[55]
Studies and surveys conducted to determine the frequencies of hemochromatosis help explain how the mutation migrated around the globe. In theory, the disease initially evolved from travelers migrating from the north. Surveys show a particular distribution pattern with large clusters and frequencies of gene mutations along the western European coastline.[56] This led the development of the "Viking Hypothesis".[57] Cluster locations and mapped patterns of this mutation correlate closely to the locations of Viking settlements in Europe established c.700 AD to c.1100 AD. The Vikings originally came from Norway, Sweden and Denmark. Viking ships made their way along the coastline of Europe in search of trade, riches, and land. Genetic studies suggest that the extremely high frequency patterns in some European countries are the result of migrations of Vikings and later Normans, indicating a genetic link between hereditary hemochromatosis and Viking ancestry.[58]
In 1865, Armand Trousseau (a French internist) was one of the first to describe many of the symptoms of a diabetic patient with cirrhosis of the liver and bronzed skin color. The term hemochromatosis was first used by German pathologist Friedrich Daniel von Recklinghausen in 1889 when he described an accumulation of iron in body tissues.[59]
Although it was known most of the 20th century that most cases of hemochromatosis were inherited, they were incorrectly assumed to depend on a single gene.[60]
In 1935 J.H. Sheldon, a British physician, described the link to iron metabolism for the first time as well as demonstrating its hereditary nature.
In 1996 Felder and colleagues identified the hemochromatosis gene, HFE gene. Felder found that the HFE gene has two main mutations, causing amino acid substitutions C282Y and H63D, which were the main cause of hereditary hemochromatosis.[59] [61] The next year the CDC and the National Human Genome Research Institute sponsored an examination of hemochromatosis following the discovery of the HFE gene, which helped lead to the population screenings and estimates that are still being used today.[62]