The porphyrias are metabolic disorders, each resulting from the deficiency of a specific enzyme in the heme biosynthetic pathway (Fig. 430-1 and Table 430-1).These enzyme deficiencies are inherited as autosomal dominant, autosomal recessive, or X-linked traits, with the exception of porphyria cutanea tarda (PCT), which usually is sporadic (Table 430-1). The porphyrias are classified as either hepatic or erythropoietic, depending on the primary site of overproduction and accumulation of their respective porphyrin precursors or porphyrins (Tables 430-1 and 430-2), although some have overlapping features. For example, PCT, the most common porphyria, is hepatic and presents with blistering cutaneous photosensitivity, which is typically characteristic of the erythropoietic porphyrias. The major manifestations of the acute hepatic porphyrias are neurologic, including neuropathic abdominal pain, peripheral motor neuropathy, and mental disturbances, with attacks often precipitated by dieting, certain drugs, and hormonal changes. While hepatic porphyrias are symptomatic primarily in adults, rare homozygous variants of the autosomal dominant hepatic porphyrias usually manifest clinically prior to puberty.
The human heme biosynthetic pathway indicating in linked boxes the enzyme that, when deficient, causes the respective porphyria. Hepatic porphyrias are shown in yellow boxes and erythropoietic porphyrias in pink boxes.
TABLE 430-1Human Porphyrias: Major Clinical and Laboratory Features ||Download (.pdf) TABLE 430-1 Human Porphyrias: Major Clinical and Laboratory Features
| || || ||Principal Symptoms: NV or CP+ ||Enzyme Activity % of Normal ||Increased Porphyrin Precursors and/or Porphyrins |
|Porphyria ||Deficient Enzyme ||Inheritance ||Erythrocytes ||Urine ||Stool |
|Hepatic Porphyrias |
|5-ALA dehydratase-deficient porphyria (ADP) ||ALA dehydratase ||AR ||NV ||~5 ||Zn-Protoporphyrin ||ALA, coproporphyrin III ||— |
|Acute intermittent porphyria (AIP) ||HMB synthase ||AD ||NV ||~50 ||— ||ALAa, PBG, uroporphyrin ||— |
|Porphyria cutanea tarda (PCT) ||URO decarboxylase ||AD ||CP ||~20 ||— ||Uroporphyrin, 7-carboxylate porphyrin ||Isocoproporphyrin |
|Hereditary coproporphyria (HCP) ||COPRO oxidase ||AD ||NV and CP ||~50 ||— ||ALA, PBG, coproporphyrin III ||Coproporphyrin III |
|Variegate porphyria (VP) ||PROTO oxidase ||AD ||NV and CP ||~50 ||— ||ALA, PBG, coproporphyrin III ||Coproporphyrin III, protoporphyrin |
|Erythropoietic Porphyrias |
|Congenital erythropoietic porphyria (CEP) ||URO-synthase ||AR ||CP ||1–5 ||Uroporphyrin I Coproporphyrin I ||Uroporphyrin Ib Coproporphyrin Ib ||Coproporphyrin I |
|Erythropoietic protoporphyria (EPP) ||Ferrochelatase ||ARa ||CP ||~20–30 ||Protoporphyrin ||— ||Protoporphyrin |
|X-linked protoporphyria (XLP) ||ALA synthase 2 ||XL ||CP ||>100c ||Protoporphyrin ||— ||Protoporphyrin |
TABLE 430-2Human Heme Biosynthetic Enzymes and Genes