28 - Heme Biosynthesis and the Porphyrias

Summary

The porphyrias are metabolic disorders, each resulting from the deficiency of a specific enzyme in the heme biosynthetic pathway (Figure 28.1; Table 28.1). These enzyme deficiencies are inherited as autosomal dominant or recessive traits, with the exception of porphyria cutanea tarda (PCT), which usually is sporadic. The porphyrias are classified as either hepatic or erythropoietic depending on the primary site of overproduction and accumulation of porphyrin precursors or porphyrins (Table 28.2) although some have overlapping features. The hepatic porphyrias are characterized by overproduction and initial accumulation of porphyrin precursors and/or porphyrins primarily in the liver, whereas in the erythropoietic porphyrias, overproduction and initial accumulation of the pathway intermediates occur primarily in bone marrow erythroid cells.

The major manifestations of the acute hepatic porphyrias, which typically present after puberty, are neurologic, including neuropathic abdominal pain, neuropathy, and mental disturbances. The neurologic involvement appears to be the result of hepatic production of a neurotoxic substance, as liver transplantation prevented further occurrences in a patient who had frequent attacks of acute intermittent porphyria (AIP) [1]. Steroid hormones, drugs, and nutrition influence the hepatic production of porphyrin precursors and porphyrins, thereby precipitating or increasing the severity of some hepatic porphyrias. Rare homozygous variants of the autosomal dominant hepatic porphyrias have been identified and usually manifest clinically before puberty. The symptoms in these patients are usually more severe and occur earlier than those of patients with the respective autosomal dominant porphyria (see below) [2].