Porphyria. From Sir Walter Raleigh to molecular biology

The porphyrias are a group of disorders caused by deficient activity of the enzymes responsible for the biosynthesis of haem. The skin is one of the major organs involved in most of these diseases because the porphyrins which accumulate are phototoxic. The common cutaneous porphyrias are variegate porphyria, porphyria cutanea tarda, congenital erythropoietic porphyria and erythropoietic protoporphyria, each caused by a different enzyme deficiency causing a distinctive pattern of porphyrin accumulation and typical clinical features. The genes encoding these enzymes have all been cloned recently, enabling the genetic defects underlying these disorders to be elucidated. The factors triggering sporadic porphyria cutanea tarda in predisposed individuals are now becoming clear: hepatic iron overload is required to induce the hepatic enzyme defect and many patients are haemochromatosis gene carriers. Hepatitis B, C, and HIV virus infection also contribute to disease expression. In erythropoietic protoporphyria, up to 5% of patients develop liver failure. It is now clear that some of these patients suffer from a different recessively transmitted form of the disease: this finding may make it possible to identify these patients at an earlier stage. Gene therapy holds particular promise as a future therapy and has successfully been used to correct enzyme defects in vitro. Bone marrow transplantation has also been tried in patients with congenital erythropoietic porphyria. The joints are not involved by porphyria. However, some non-steroidal inflammatory drugs prescribed by rheumatologists have phototoxic properties similar to uroporphyrin. These drugs cause a syndrome clinically and histologically indistinguishable from porphyria cutanea tarda which is known as pseudoporphyria.