Heterozygosity for HLA-linked hemochromatosis as a likely cause of the hepatic siderosis associated with sporadic porphyria cutanea tarda.

Subnormal activity of hepatic uroporphyrinogen decarboxylase is responsible for the derangement of porphyrin biosynthesis in both sporadic and familial porphyria cutanea tarda, but the enzymatic defect is not clinically expressed in the absence of hepatic siderosis. The pedigree study described here offers support for the hypothesis that a single allele for HLA-linked hereditary hemochromatosis is responsible for the hepatic siderosis in sporadic porphyria cutanea tarda. A two-locus causation model for sporadic porphyria cutanea tarda might explain both the observed incidence of overt cases and the rarity of multiple affected individuals within a pedigree.