Hemochromatosis is charaterized by the excessive accumulation of body iron, most of which is deposited in parenchymal organs such as the liver and pancreas. Because humans do not have a major excretory pathway for iron hemochromatosis results either from a genetic defect causing excessive iron absorption or as a consequence of parenteral administration of iron (usually in the form of transfusions). Genetic hemochromatosis, also called hereditary hemochromatosis is a homozygous recessive heritable disorder. Acquired forms of hemochromatosis with known sources of excess iron are called secondary hemochromatosis.
The total body iron pool ranges from 2 to 6 gm in normal elders about 0.5 gm is stored in the liver, 98% of which is in hepatocytes. In genetic hemochromatosis, total iron accumulation may exceed 50 gm more than one third of which accumulates in the liver. The following features characterize this disease:
Fully developed cases exhibit (micronodular cirrhosis- all patients; (2)diabetes mellitus- 75% to 80% and (3) skin pigmentation – 75%to 80% of cases.
(4)n accumulation is lifelong; symptoms usually first appear in the fifth to sixth decades of life.
(5)hemochromatosis gene is located on the short arm of chromosome.
(6) close to the HLA gene locus. This gene called HLA-H encode a novel HLA class I-like molecule that in some uncharacterized manner influences iron absorption. The most common mutation is a cystein to tyrosine substitution at amino acid 282, which inactivates this 343-amino acid protein; other alleles have also been identified. HLA-H is in linkage disequlibrium with HLA-A3 thus accounting for the association of this haplotype with genetic hemochromatosis. Males predominate with slightly earlier clinical presentation , partly because physiologic iron loss delays iron accumulation in women.
In white populations of northern European extraction the gene frequency has been estimated at approximately 6%. The frequency for homozygosity is 0.45% and 11% for heterozygosity, making genetic hemochromatosis one of the most common inborn errors of metabolism.
It may be recalled that the total body content of iron is tightly regulated whereby the limited daily losses of iron are matched by gastrointestinal absorption. In genetic hemochromatosis, there is a primary defect in the intestinal absorption of dietary iron leading to net iron accumulation of 0.5 to 1.0 gm/year. The disease manifests itself typically after 20gm of storage iron has accumulated. The exact mechanism by which a defect in HLA-H engenders excessive iron absorption is not yet known. Regulated transfer of iron from intestinal mucosal cells to plasma may be involved , since HLA-H is expressed on the mucosal cell surface and interacts with circulating β2- microglobulin develop a syndrome resembling genetic hemochromatosis.
Excessive iron appears to be directly toxic to host tissues by the following mechanisms: (1) lipid peroxidation through iron-catalized free radical reactions: (2) stimulation of collagen formation and (3) direct interactions of iron with DNA leading to lethal injury or predisposition to hepatocellular carcinoma. Whatever the deleterious effects or iron, they are reversible in cells not fatally injured and removal of excess iron during therapy promotes recovery of tissue function.
The most common causes of secondary hemochromatosis are the hemolytic anemias associated with ineffective erythropoiesis, In thiese disorders the excess iron may result not only from transfusions but also from increased intestinal absorption. Transfusions alone as in aplastic anemias lead to systemic hemosiderosis in which parenchymal organ injury tends to occur only in extreme cases. Alcholic cirrhosis is often associated with a modest increase in staiable iron within liver cells. However this represents alchohol induced redistribution of iron since total body iron is not significantly increased. A rather unusual form of iron overload resembling genetic hemochromatosis occurs in sub-Saharan Africa, the result of ingesting large quantities of alcholic beverages fermented in iron utensils. Home brewing in steel drums continues to this day, and a genetic susceptibility in this population has been implicated.
Genetic hemochromatosis is more often a disease of males and rarely becomes evident befoe age 40 years. The principal manifestations include hepatomegaly, abdominal pain, skin pigmentation, deraged glucose homestasis or frank diabetes mellitus due to destruction of pancreatic islets, cardiac dysfunction and atypical arthritis. In some patients, the presenting compliant is hypogonadism. The classic triad of pigment cirrhosis with hepatomegaly, skin pigmentation, and diabetes mellitus may not develop until late in the course of the disease. Death may result from cirrhosis or cardiac disease. A significant cause of death is hepatocellular carcinoma; the risk is 200 fold greater than in the general population and treatment for iron overload does not remove the risk for this aggressive neoplasm.
Fortunately genetic hemochromatosis can be diagnosed long before irreversible tissue damage has occurred. Screening involves demonstration of high levels of serum iron and ferritin exclusion of secondary causes of iron overload, HLA gene molecular analysis and liver biopsy if indicated. Screening of family members of probands is important. Heterozygotes for genetic hemochromatosis also accumulate excessive iron, but not to the degree required to cause significant tissue damage. Homozygotes may be identified before onset of clinical disease. Patients with genetic hemochromatosis diagnosed in the subclinical precirrhotic stage and treated by regular phlebotomy have normal life expectancy.
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