Porphyria cutanea tarda: Literally, the late skin form of porphyria, a genetic photosensitive (light-sensitive) skin disease with onset in adult life with substances called uroporphyrins in the urine due to a deficiency of uroporphyrinogen decarboxylase (UROD), an enzyme required for the synthesis of heme (part of hemoglobin, the pigment in red blood cells that carries oxygen). The hallmarks of porphyria cutanea tarda (PCT) are blisters which become ulcerated in areas of the skin exposed to sunlight, especially on the face, ears and dorsum (back) of the hands. The affected areas of skin also tend to be fragile and show hyperpigmentation (excess pigment) and hypertrichosis (excess hair).
Porphyria cutanea tarda, the most common form of porphyria, comes in two clinical familial and a sporadic form:
- The familial form of porphyria cutanea tarda -- is inherited as an autosomal dominant trait with males and females affected in multiple generations. The enzyme UROD is reduced in all tissues.
- The sporadic form of porphyria cutanea tarda -- is more common. The enzyme UROD is only reduced in the liver. It appears sporadically in people with liver disease, as from alcoholism, and from exposure to agents such as estrogens.
Iron overload is frequently present in porphyria cutanea tarda and may be associated with varying degrees of damage of the liver.
A severe form of porphyria cutanea tarda, hepatoerythropoietic porphyria (HEP), has its onset in infancy with the accumulation of protoporphyrin in the red blood cells. The level of the enzyme UROD is very low in red cells.
The UROD gene has been mapped to 1p34. Mutations have been identified in the UROD gene, including DNA base substitutions and deletions. These mutations result in reduced activity of the enzyme. Some mutation result in porphyria cutanea tarda and others in the recessively inherited HEP. HEP is the homozygous form of familial porphyria cutanea tarda.