Examination of Ferrochelatase Mutations That Cause Erythropoietic Protoporphyria

Nrf2 Activation Ameliorates Hepatotoxicity Induced by a Heme Synthesis Inhibitor

Transcription factor Nrf2 protects hepatocytes against various toxicants by upregulating cytoprotective genes. The heme synthesis inhibitor 3, 5-diethoxycarbonyl-1, 4-dihydrocollidine (DDC) leads to liver injury around the portal vein, unlike other groups of toxicants that cause hemorrhage and necrosis in the centrilobular area. To examine whether and how Nrf2 protects livers from the injury, we fed DDC to Nrf2 knockout (Nrf2KO), wild-type (WT), Keap1flox/flox (Keap1-knockdown; Keap1KD), and liver-specific Keap1 knockout (Keap1-Alb) mice, as these lines of mice exhibit stepwise increases in Nrf2 protein expression levels. Liver-specific Keap1::Nrf2 double-knockout (Keap1::Nrf2-Alb) mice were also exploited to examine the contribution of Nrf2. Two weeks after DDC feeding, Keap1-Alb mice were fully recovered from body weight loss, but the WT and Nrf2KO mice were not. The liver-to-body-weight ratio of Keap1-Alb mice was significantly larger than that of WT and Nrf2KO mice. Two indicators of hepatotoxicity, alanine aminotransferase and bilirubin in plasma, were both elevated in WT mice, but downregulated in Keap1-Alb mice after the DDC-feeding. DDC-induced porphyrin accumulation was reduced in the livers of Keap1-Alb and Keap1KD mice compared with that of WT mice. When assessed by the Nqo1 level, Nrf2 expression was further enhanced by DDC in Keap1-Alb mice, suggesting that DDC may have a Keap1 independent potential to activate Nrf2. Genetic activation of Nrf2 in Keap1-Alb mice increased the extracellular excretion of porphyrins, but contrary to our expectation, hepatic damages in Nrf2KO mice appeared to be similar to that of WT mice. Based on these observations, we conclude that Nrf2 activation protects livers against DDC-elicited hepatotoxicity.

Strong correlation of ferrochelatase enzymatic activity with Mitoferrin-1 mRNA in lymphoblasts of patients with protoporphyria

Accumulation of protoporphyrin IX (PPIX) and Zn-PPIX, are the clinical hallmarks of protoporphyria. Phenotypic expression of protoporphyria is due to decreased activity of ferrochelatase (FECH) or to increased activity of aminolevulinic acid synthase (ALAS) in red blood cells. Other genetic defects have been shown to contribute to disease severity including loss of function mutations in the mitochondrial AAA-ATPase, CLPX and mutations in the Iron-responsive element binding protein 2 (IRP2), in mice. It is clear that multiple paths lead to a common phenotype of excess plasma PPIX that causes a phototoxic reaction on sun exposed areas. In this study we examined the association between mitochondrial iron acquisition and utilization with activity of FECH. Our data show that there is a metabolic link between the activity FECH and levels of MFRN1 mRNA. We examined the correlation between FECH activity and MFRN1 mRNA in cell lines established from patients with the classical protoporphyria, porphyria due to defects in ALAS2 mutations. Our data confirm MFRN1 message levels positively correlated with FECH enzymatic activity in all cell types.

Cutaneous porphyrias part I: epidemiology, pathogenesis, presentation, diagnosis, and histopathology

The porphyrias are a group of disorders characterized by defects in the heme biosynthesis pathway. Many present with skin findings including photosensitivity, bullae, hypertrichosis, and scarring. Systemic symptoms may include abdominal pain, neuropsychiatric changes, anemia, and liver disease. With advances in DNA analysis, researchers are discovering the underlying genetic causes of the porphyrias, enabling family members to be tested for genetic mutations. Here we present a comprehensive review of porphyria focusing on those with cutaneous manifestations. In Part I, we have included the epidemiology, pathogenesis, presentation, diagnosis, and histopathology. Treatment and management options will be discussed in Part II.

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.

Structure and function of enzymes in heme biosynthesis

Tetrapyrroles like hemes, chlorophylls, and cobalamin are complex macrocycles which play essential roles in almost all living organisms. Heme serves as prosthetic group of many proteins involved in fundamental biological processes like respiration, photosynthesis, and the metabolism and transport of oxygen. Further, enzymes such as catalases, peroxidases, or cytochromes P450 rely on heme as essential cofactors. Heme is synthesized in most organisms via a highly conserved biosynthetic route. In humans, defects in heme biosynthesis lead to severe metabolic disorders called porphyrias. The elucidation of the 3D structures for all heme biosynthetic enzymes over the last decade provided new insights into their function and elucidated the structural basis of many known diseases. In terms of structure and function several rather unique proteins were revealed such as the V-shaped glutamyl-tRNA reductase, the dipyrromethane cofactor containing porphobilinogen deaminase, or the "Radical SAM enzyme" coproporphyrinogen III dehydrogenase. This review summarizes the current understanding of the structure-function relationship for all heme biosynthetic enzymes and their potential interactions in the cell.

The ins and outs of iron homeostasis

Iron is an essential element that is toxic when it accumulates in excess. Intricate regulatory mechanisms have evolved to maintain iron homeostasis within cells and between different tissues of complex organisms. This review discusses the proteins involved in iron transport and storage and their regulation in health and disease.

Molecular studies of liver disease in erythropoietic protoporphyria

GOALS

The goal of this study was to define molecular determinants of liver disease in erythropoietic protoporphyria (EPP).

BACKGROUND

EPP is a genetic disorder in which deficient ferrochelatase activity causes excessive production of protoporphyrin, which is excreted in bile. Some patients develop liver disease that necessitates transplantation.

STUDY

Ferrochelatase gene analysis was done in 25 families with EPP to identify mutations and a polymorphism (IVS3-48c) that causes low gene expression. Expression of multiple hepatic genes was also examined by DNA microarray analysis in patients who had liver transplantation to identify genes with altered regulation.

RESULTS

Heterozygous ferrochelatase mutations were found in 43 individuals. In 94% of 31 symptomatic patients, 15 of whom had liver disease, the polymorphism was also present in the nonmutant allele. Explanted liver of patients who had transplantation showed significant change in expression of several genes involved in wound healing, organic anion transport, and oxidative stress.

CONCLUSIONS

Patients with EPP who develop liver disease usually have a mutation in one ferrochelatase allele that alters enzyme function, together with a polymorphism in the nonmutant allele that causes low gene expression. This results in significant increase in the hepatobiliary excretion of protoporphyrin, which can damage the liver through both cholestatic injury and oxidative stress.

Hypermethylation of the wild-type ferrochelatase allele is closely associated with severe liver complication in a family with erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP) is an inherited disorder of heme biosynthesis caused by cellular decreases in ferrochelatase (FECH) activity. Clinical expression of this disorder usually requires coinheritance of a mutant FECH allele and a normal FECH allele expressed at a low level. In this study, we investigated the methylation status of a normal, but poorly expressed, FECH gene in a single Japanese family with EPP. In this family, the proband died from liver failure, whereas the mother and sister exhibited overt EPP with mild liver dysfunction. A splicing mutation (IVS9+1g-->a) in the FECH gene, which produces a mutant FECH transcript lacking exon 9, was detected in the maternal allele of the proband and his sister. All subjects, including the father, who did not exhibit EPP, possessed the IVS3-48c/c genotype. This allele increases the proportion of aberrantly spliced mRNA, resulting in reduced FECH activity. Normal FECH transcripts were, however, detected in the mother and sister, but not in the proband. The CpG sites in the region from bases -78 to -31 were partially methylated in the proband and his father, but not in his mother or sister. Additionally, CpG methylation within this region reduced transcription of the FECH gene. These results suggest that whereas the combination of a maternal IVS9+1a allele and a paternal IVS3-48c allele results in overt EPP, CpG methylation of the FECH gene promoter, likely inherited from the father, increases the severity of EPP, leading to fatal liver failure, as seen in the proband.

An exon 10 deletion in the mouse ferrochelatase gene has a dominant-negative effect and causes mild protoporphyria

Protoporphyria is generally inherited as an autosomal dominant disorder. The enzymatic defect of protoporphyria is a deficiency in ferrochelatase, which chelates iron and protoporphyrin IX to form heme. Patients with protoporphyria have decreased ferrochelatase activities that range from 5% to 30% of normal caused by heterogeneous mutations in the ferrochelatase gene. The molecular mechanism by which the ferrochelatase activity is decreased to less than an expected 50% is unresolved. In this study, we assessed the effect of a ferrochelatase exon 10 deletion, a common mutation in human protoporphyria, introduced into the mouse by gene targeting. F1 crosses produced (+/+), (+/-), and (-/-) mice at a ratio of 1:2:0; (-/-) embryos were detected at 3.5 days postcoitus, consistent with embryonic lethality for the homozygous mutant genotype. Heterozygotes demonstrated equivalent levels of wild-type and mutant ferrochelatase messenger RNAs and 2 immunoreactive proteins that corresponded to the full-length and an exon 10-deleted ferrochelatase protein. Ferrochelatase activities in the heterozygotes were an average of 37% of normal, and protoporphyrin levels were elevated in erythrocytes and bile. Heterozygous mice exhibited skin photosensitivity but no liver disease. These results lend support for a dominant-negative effect of a mutant allele on ferrochelatase activity in patients with protoporphyria.

Targeted disruption of the mouse ferrochelatase gene producing an exon 10 deletion

Protoporphyria is a disease characterized by a deficiency in ferrochelatase, the terminal enzyme in the heme biosynthetic pathway, which catalyzes the chelation of iron and protoporphyrin to form heme. Clinical symptoms arise from an accumulation of protoporphyrin behind the partial enzyme block and include photosensitivity and sometimes hepatobiliary disease. Protoporphyria is described as an dominant disease, yet patients exhibit decreased ferrochelatase activities of 15-30% of normal, not 50% as might be expected. Missense, nonsense, and splicing mutations have been identified in ferrochelatase cDNA from protoporphyric patients. In this study we introduce an exon 10 deletion, an analogous mutation to that described in some protoporphyric patients, into the mouse embryonic stem (ES) cell genome via homologous recombination. Targeted ES cells were confirmed by Southern blot analysis. Expression of wild-type and exon 10-deleted mRNA was demonstrated by reverse transcriptase-polymerase chain reaction (RT-PCR) and cDNA sequencing. Ferrochelatase levels were analyzed by immunoblotting. Ferrochelatase activity was measured by the chelation of zinc and mesoporphyrin, and by the decrease in protoporphyrin accumulation after adding delta-aminolevulinic acid. In the exon 10 +/- ES cells there is expression of both wild-type and exon 10-deleted mRNA, a 50% decrease in cross-reactive material with an anti-ferrochelatase antibody, and an approximate 50% decrease in ferrochelatase activity compared to wild-type ES cells. Therefore, an exon 10 deletion alone is insufficient to decrease ferrochelatase activity to the levels in protoporphyric patients. This suggests that requirement of an additional mutation to decrease the expression of the wild-type allele.

Cloning and characterization of Gallus and Xenopus ferrochelatases: presence of the [2Fe-2S] cluster in nonmammalian ferrochelatase

Ferrochelatase (EC 4.99.1.1) catalyzes the insertion of ferrous iron into protoporphyrin IX to form protoheme IX. This membrane-bound enzyme has been cloned from a variety of bacteria, plants, mammals, and yeast. Interestingly, only in mammals has the enzyme been found to contain a [2Fe-2S] cluster. Since the presence of this feature only in mammals would have significant evolutionary implications and because there have been no nonmammalian animal ferrochelatases cloned, expressed, and characterized, we report here the cloning and characterization of ferrochelatase from chicken (Gallus gallus) and an amphibian (Xenopus laevis). The cDNAs for both of these ferrochelatases were cloned by complementation of an Escherichia coli DeltahemH strain. The expressed and purified enzymes were characterized biochemically and both were found to contain [2Fe-2S] clusters. These clusters have spectral characteristics essentially identical to those of human ferrochelatase, although their EPR spectra are recognizably distinct from the human one. The [2Fe-2S] clusters of both chicken and amphibian ferrochelatases are readily destroyed by NO. Sequence analysis of the 3' UTR of both chicken and amphibian cDNAs show that while both have poly(A) tails neither have a consensus polyadenylation signal. The 5' UTR of Xenopus as isolated contained 135 bp and possesses no identifiable stem-loop structure.

Evidence that the fourth ligand to the [2Fe-2S] cluster in animal ferrochelatase is a cysteine. Characterization of the enzyme from Drosophila melanogaster

In a previous study, site-directed mutagenesis experiments identified three of the four ligands to the [2Fe-2S] cluster in animal ferrochelatase as conserved cysteines in the COOH-terminal extension, Cys-403, Cys-406, and Cys-411 in human ferrochelatase (Crouse, B. R., Sellers, V. M., Finnegan, M. G., Dailey, H. A. & Johnson, M. K. (1996) Biochemistry 35, 16222-16229). The nature of the fourth ligand was left unresolved, and spectroscopic studies raised the possibility of one noncysteinyl, oxygenic ligand. In this work, we report two lines of evidence that strongly suggest the fourth ligand is a cysteine residue. Cysteine at position 196 in human recombinant ferrochelatase when changed to a serine results in an inactive enzyme that is lacking the [2Fe-2S] cluster. Furthermore, whole cell EPR studies demonstrate that in the C196S mutant the cluster fails to assemble. Additionally, the cloning and expression of Drosophila melanogaster ferrochelatase has allowed the identification, by EPR and UV-visible spectroscopy, of a [2Fe-2S]2+ cluster with properties analogous to those of animal ferrochelatases. The observation that Drosophila ferrochelatase contains only four conserved cysteines at positions 196, 403, 406, and 411, is in accord with the proposal that these residues function as cluster ligands. In the case of the ferrochelatase iron-sulfur cluster ligands, NH2-Cys-X206-Cys-X2-Cys-X4-Cys-COOH, the position distant from other ligands may lead to a spatial positioning of the cluster near the enzyme active site or at the interface of two domains, thereby explaining the loss of enzyme activity that accompanies cluster degradation and reinforcing the idea that the cluster functions as a regulatory switch.