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Porphyrias are metabolic diseases caused by abnormalities of the 8 enzymes in the heme biosynthetic pathway. All but one arise from mutation of a pathway enzyme; the exception is porphyria cutanea tarda (PCT), which develops as an acquired deficiency of the fifth enzyme in the pathway with or without a mutation. Porphyrias are classified as hepatic or erythropoietic based on whether heme precursors first accumulate in liver or bone marrow, the tissues that most actively synthesize heme. Porphyrias are categorized clinically based on their clinical features as either cutaneous or acute (Table 124-1). Cutaneous porphyrias are due to overproduction and accumulation of photosensitizing porphyrins. Most, as exemplified by PCT, cause chronic blistering and scarring on sun-exposed areas of skin, whereas protoporphyrias produce an acute, severe, and mostly nonblistering reaction to light, often leaving few if any chronic skin changes. Acute porphyrias are characterized by neurologic symptoms and elevated levels of the porphyrin precursors, δ-aminolevulinic acid (ALA) and porphobilinogen (PBG). Porphyrins also accumulate in acute porphyrias, and sometimes achieve levels in plasma sufficient to cause cutaneous blistering, as exemplified especially by variegate porphyria (VP). In some porphyrias, damage to other organs, such as liver and kidney, may also occur. Symptoms, signs, and histologic findings caused by porphyrias are nonspecific, whereas patterns of porphyrins and porphyrin precursors enable specific diagnoses and treatments (Table 124-2).

Table 124-1Human Porphyrias: Specific Enzymes Affected by Mutations, Modes of Inheritance, Classification, and Major Types of Clinical Features
Table 124-2Major Biochemical Abnormalities (in Bold) and Other Differentiating Measurements in the Human Porphyriasa

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