Protoporphyrin IX-induced phototoxicity: Mechanisms and therapeutics.

Protoporphyrin IX (PPIX) is an intermediate in the heme biosynthesis pathway. Abnormal accumulation of PPIX due to certain pathological conditions such as erythropoietic protoporphyria and X-linked protoporphyria causes painful phototoxic reactions of the skin, which can significantly impact daily life. Endothelial cells in the skin have been proposed as the primary target for PPIX-induced phototoxicity through light-triggered generation of reactive oxygen species. Current approaches for the management of PPIX-induced phototoxicity include opaque clothing, sunscreens, phototherapy, blood therapy, antioxidants, bone marrow transplantation, and drugs that increase skin pigmentation. In this review, we discuss the present understanding of PPIX-induced phototoxicity including PPIX production and disposition, conditions that lead to PPIX accumulation, symptoms and individual differences, mechanisms, and therapeutics.

In ferrochelatase-deficient protoporphyria patients, ALAS2 expression is enhanced and erythrocytic protoporphyrin concentration correlates with iron availability.

The activity of the erythroid-specific isoenzyme of 5-aminolevulinic acid synthase (ALAS2), the first and rate-limiting enzyme in heme biosynthesis, is down-regulated during iron-deficiency. Ferrochelatase (FECH), the last enzyme of this pathway, inserts iron into protoporphyrin IX (PPIX) to form heme. Patients with erythropoietic protoporphyria (EPP), an inherited deficiency in FECH, often show signs of iron deficiency in addition to phototoxicity which is caused by PPIX accumulation. However, iron supplementation often leads to exacerbation of phototoxicity. We report three EPP patients who had reduced erythrocytic PPIX concentrations when they were iron-deficient and their microcytic and hypochromic anemia deteriorated. Additionally, we found a significant increase in the amount of ALAS2 mRNA and protein among EPP patients. To verify the connection between FECH deficiency and ALAS2 over-expression, we inhibited FECH in cultured cells and found a subsequent increase in ALAS2 mRNA. We conclude that the primary deficiency in ferrochelatase leads to a secondary increase in ALAS2 expression. The combined action of these two enzymes within the heme biosynthetic pathway contributes to the accumulation of PPIX. Furthermore, we hypothesize that EPP patients may benefit from a mild iron deficiency since it would limit PPIX production by restricting ALAS2 over-expression.

Abnormal mitoferrin-1 expression in patients with erythropoietic protoporphyria.

OBJECTIVE: Most patients with erythropoietic protoporphyria have deficient ferrochelatase (FECH) activity due to changes in FECH DNA. We evaluated seven patients with erythropoietic protoporphyria phenotype in whom abnormalities of FECH DNA were not found by conventional analysis. The major focus was mitoferrin-1 (MFRN1), the mitochondrial transporter of Fe used for heme formation by FECH and for 2Fe2S cluster synthesis, which is critical to FECH activity/stability. MATERIALS AND METHODS: Four patients had a deletion in ALAS2 that causes enzyme gain-of-function, resulting in increased formation of protoporphyrin; one had a heterozygous major deletion in FECH DNA. All had an abnormal transcript of MFRN1 in messenger RNA extracted from blood leukocytes and/or liver tissue. The abnormal transcript contained an insert of intron 2 that had a stop codon. The consequences of abnormal MFRN1 expression were examined using zebrafish and yeast MFRN-deficient strains and cultured lymphoblasts from the patients. RESULTS: Abnormal human MFRN1 complementary DNA showed loss-of-function in zebrafish and yeast mutants, whereas normal human MFRN1 complementary DNA rescued both. Using cultured lymphoblasts, quantitative reverse transcription polymerase chain reaction showed increased formation of abnormal transcript that was accompanied by decreased formation of normal transcript and reduced FECH activity in patients compared to normal lines. A positive correlation coefficient (0.75) was found between FECH activity and normal MFRN1 messenger RNA in lymphoblasts. However, no obvious cause for increased formation of abnormal transcript was identified in MFRN1 exons and splice junctions. CONCLUSIONS: Abnormal MFRN1 expression can contribute to erythropoietic protoporphyria phenotype in some patients, probably by causing a reduction in FECH activity.