Porphyria cutanea tarda
Porphyria cutanea tarda is the most common type of porphyria. The disorder results from low levels of the enzyme responsible for the fifth step in heme production. Heme is a vital molecule for all of the body's organs. It is a component of hemoglobin, the molecule that carries oxygen in the blood. Porphyria cutanea tarda is a subtype of porphyria. more...
When signs and symptoms occur, they usually begin in adulthood and result from the skin becoming overly sensitive to sunlight. Areas of skin exposed to the sun develop severe blistering, scarring, changes in pigmentation, and increased hair growth. Exposed skin becomes fragile and is easily damaged. People with porphyria cutanea tarda also have increased iron levels in the liver. They face a higher risk of developing abnormal liver function and liver cancer. The signs and symptoms of this condition are triggered by nongenetic factors such as alcohol abuse, excess iron, certain hormones, and viral infections.
This type of porphyria occurs in an estimated 1 in 25,000 people, including both inherited and sporadic (noninherited) cases. An estimated 80 % of porphyria cutanea tarda cases are sporadic. The exact frequency is not clear because many people with the condition never experience symptoms.
Inherited mutations in the UROD gene cause about 20 % of cases. (The other 80 % of cases do not have mutations in UROD, and are classified as sporadic.) UROD makes an enzyme called uroporphyrinogen decarboxylase, which is critical to the chemical process that leads to heme production. The activity of this enzyme is usually reduced by 50 % in all tissues in people with the inherited form of the condition.
Nongenetic factors such as alcohol abuse, excess iron, and others listed above can increase the demand for heme and the enzymes required to make heme. The combination of this increased demand and reduced activity of uroporphyrinogen decarboxylase disrupts heme production and allows byproducts of the process to accumulate in the body, triggering the signs and symptoms of porphyria cutanea tarda.
The HFE gene makes a protein that helps cells regulate the absorption of iron from the digestive tract and into the cells of the body. Certain mutations in the HFE gene cause hemochromatosis (an iron overload disorder). People who have these mutations are also at an increased risk of developing porphyria cutanea tarda.
In the 20% of cases where porphyria cutanea tarda is inherited, it is inherited in an autosomal dominant pattern, which means one copy of the altered gene is sufficient to decrease enzyme activity and cause the signs and symptoms of the disorder.
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