Hemochromatosis is a genetic disease that causes excessive iron to be deposited in the body’s parenchymal cells. Hemochromatosis causes symptoms like abdominal and arthritic pain, fatigue, abnormal heart beat, lower hormone production, and diabetes. Since parenchymal cells make up the functional tissue of organs like the brain and lungs, they are essential for our health and survival. An excess of iron in these cells due to hemochromatosis has been shown to shorten life span, so why is this disease so common?
The excess iron is deposited in the liver, pancreas, spleen, heart, and other organs, damaging them over time. Hemochromatosis results from a mutation of the HFE gene, which produces a major histocompatibility complex protein and is expressed in nearly all body tissues. There are two known mutations (C282Y and H63D) of HFE which cause an individual to develop hemochromatosis. On average, individuals with this genetic condition have a lower life expectancy than those without it. Despite the lower life expectancy associated with this disease, it is one of the most common hereditary diseases.
Because hemochromatosis leads to iron being deposited in the body, affected individuals are much less likely to develop anemia. Researchers originally thought that the high incidence of this genetic disorder was an adaptation in populations where anemia was likely to occur due to harsh environments. However, the incidence of hemochromatosis is significantly higher in individuals descended from Northern and Western Europe. Though the disease allele exists in other populations that fit the criteria of being a harsh environment, it is not as prevalent in these populations. This suggests that there was a strong and specific selection for hemochromatosis specifically in populations of Northern and Western European descent.
Though hemochromatosis leads to a general overload of iron in the body, macrophages in affected individuals have very low iron levels. As the level of iron in a macrophage decreases, its resistance to pathogens increases. This is probably because certain pathogens are limited by the availability of iron.
There are a number of catastrophic plagues that have been well-recorded in Europe. The best known is the Bubonic Plague, which was caused by a bacteria named Yersinia pestis. It is estimated that 25 million people died from this plague. Other European plagues were caused by the related organisms Yersinia pseudotuberculosis and Yersinia enterocolitica. These organisms operate more effectively when there is more iron available in their host. The global distribution of these three pathogens is also very similar to the global distribution of hemochromatosis. Some researchers hypothesize that having hemochromatosis increased resistance to these European plagues, which is why this allele is so common in these populations. The areas in which the plague caused the highest mortality also have the highest incidence of hemochromatosis alleles. Hemochromatosis also protects against pathogens other than the Yersinia pathogens. Salmonella typhi, which causes typhoid fever in humans, relies on iron being available in the macrophages of its host organism and it deterred by hemochromatosis.
It is clear that populations originating from Northern and Western Europe have a higher incidence of this genetic disorder. Hemochromatosis was probably selected for by environmental factors specific to this geographic area, such as a number of plagues caused by Yersinia pathogens. Luckily, hemochromatosis is easy to treat. Iron levels are reduced to healthy levels by bloodletting. This can be done by donating blood or having a doctor periodically remove blood from the body. With a doctor monitoring iron levels, hemochromatosis is a manageable condition that does not lower life span.
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