A molecular defect in coproporphyrinogen oxidase gene causing harderoporphyria, a variant form of hereditary coproporphyria

Exploring current and emerging therapies for porphyrias

Porphyrias are rare, mostly inherited disorders resulting from altered activity of specific enzymes in the haem synthesis pathway that lead to accumulation of pathway intermediates. Photocutaneous symptoms occur when excess amounts of photoreactive porphyrins circulate in the blood to the skin, whereas increases in potentially neurotoxic porphyrin precursors are associated with neurovisceral symptoms. Current therapies are suboptimal and their mechanisms are not well established. As described here, emerging therapies address underlying disease mechanisms by introducing a gene, RNA or other specific molecule with the potential to cure or slow progression of the disease. Recent progress in nanotechnology and nanoscience, particularly regarding particle design and formulation, is expanding disease targets. More secure and efficient drug delivery systems have extended our toolbox for transferring specific molecules, especially into hepatocytes, and led to proof-of-concept studies in animal models. Repurposing existing drugs as molecular chaperones or haem synthesis inhibitors is also promising. This review summarizes key examples of these emerging therapeutic approaches and their application for hepatic and erythropoietic porphyrias.

mRNA-based therapy in a rabbit model of variegate porphyria offers new insights into the pathogenesis of acute attacks

Variegate porphyria (VP) results from haploinsufficiency of protoporphyrinogen oxidase (PPOX), the seventh enzyme in the heme synthesis pathway. There is no VP model that recapitulates the clinical manifestations of acute attacks. Combined administrations of 2-allyl-2-isopropylacetamide and rifampicin in rabbits halved hepatic PPOX activity, resulting in increased accumulation of a potentially neurotoxic heme precursor, lipid peroxidation, inflammation, and hepatocyte cytoplasmic stress. Rabbits also showed hypertension, motor impairment, reduced activity of critical mitochondrial hemoprotein functions, and altered glucose homeostasis. Hemin treatment only resulted in a slight drop in heme precursor accumulation but further increased hepatic heme catabolism, inflammation, and cytoplasmic stress. Hemin replenishment did protect against hypertension, but it failed to restore action potentials in the sciatic nerve or glucose homeostasis. Systemic porphobilinogen deaminase (PBGD) mRNA administration increased hepatic PBGD activity, the third enzyme of the pathway, and rapidly normalized serum and urine porphyrin precursor levels. All features studied were improved, including those related to critical hemoprotein functions. In conclusion, the VP model recapitulates the biochemical characteristics and some clinical manifestations associated with severe acute attacks in humans. Systemic PBGD mRNA provided successful protection against the acute attack, indicating that PBGD, and not PPOX, was the critical enzyme for hepatic heme synthesis in VP rabbits.

Laboratory Diagnosis of Porphyria

Porphyrias are a group of diseases that are clinically and genetically heterogeneous and originate mostly from inherited dysfunctions of specific enzymes involved in heme biosynthesis. Such dysfunctions result in the excessive production and excretion of the intermediates of the heme biosynthesis pathway in the blood, urine, or feces, and these intermediates are responsible for specific clinical presentations. Porphyrias continue to be underdiagnosed, although laboratory diagnosis based on the measurement of metabolites could be utilized to support clinical suspicion in all symptomatic patients. Moreover, the measurement of enzymatic activities along with a molecular analysis may confirm the diagnosis and are, therefore, crucial for identifying pre-symptomatic carriers. The present review provides an overview of the laboratory assays used most commonly for establishing the diagnosis of porphyria. This would assist the clinicians in prescribing appropriate diagnostic testing and interpreting the testing results.

Cystathionine-γ-lyase (CSE) deficiency increases erythropoiesis and promotes mitochondrial electron transport via the upregulation of coproporphyrinogen III oxidase and consequent stimulation of heme biosynthesis

BACKGROUND

Hydrogen sulfide (H₂S) is an endogenous gasotransmitter produced by mammalian cells. The current study investigated the potential role of H₂S in the regulation of heme biosynthesis using mice deficient in cystathionine gamma-lyase (CSE), one of the three major mammalian H₂S-producing enzymes.

METHODS

Wild-type and global CSE^-/- mice, as well as mitochondria prepared from their liver were used. In vivo, arterial and venous blood gases were measured, and survival of the mice to severe global hypoxia was monitored. Ex vivo, expression of various heme biosynthetic enzymes including coproporphyrinogen oxidase (CPOX) was measured, and mitochondrial function was evaluated using Extracellular Flux Analysis. Urine samples were collected to measure the oxidized porphyrinogen intermediates. The in vivo/ex vivo studies were complemented with mitochondrial bioenergetic studies in hepatocytes in vitro. Moreover, the potential effect of H₂S on the CPOX promoter was studied in cells expressing a CPOX promoter construct system.

RESULTS

The main findings are as follows: (1) CSE^-/- mice exhibit elevated red blood cell counts and red blood cell mean corpuscular volumes compared to wild-type mice; (2) these changes are associated with elevated plasma and liver heme levels and (3) these alterations are likely due to an induction of CPOX (the sixth enzyme involved in heme biosynthesis) in CSE^-/- mice. (4) Based on in vitro promoter data the promoter activation of CPOX is directly influenced by H₂S, the product of CSE. With respect to the potential functional relevance of these findings, (5) the increased circulating red blood cell numbers do not correspond to any detectable alterations in blood gas parameters under resting conditions, (6) nor do they affect the hypoxia tolerance of the animals in an acute severe hypoxia model. However, there may be a functional interaction between the CSE system and the CPOX system in terms of mitochondrial bioenergetics: (7) CSE^-/- hepatocytes and mitochondria isolated from them exhibit increased oxidative phosphorylation parameters, and (8) this increase is partially blunted after CPOX silencing. Although heme is essential for the biosynthesis of mitochondrial electron chain complexes, and CPOX is required for heme biosynthesis, (9) the observed functional mitochondrial alterations are not associated with detectable changes in mitochondrial electron transport chain protein expression.

CONCLUSIONS

The CSE system regulates the expression of CPOX and consequent heme synthesis. These effects in turn, do not influence global oxygen transport parameters, but may regulate mitochondrial electron transport.

Harderoporphyria: Case of lifelong photosensitivity associated with compound heterozygous coproporphyrinogen oxidase (CPOX) mutations

A 78-year-old man with a history of neonatal anemia and jaundice and life-long photosensitivity was found to have harderoporphyria, as evidenced by increased porphyrins in urine, plasma, erythrocytes and feces including large amounts of harderoporphyrin in feces and erythrocytes. Two previously undescribed coproporphyrinogen oxidase (CPOX) mutations were identified, including a deletion of four amino acids in a region of the enzyme mutated in 7 of the 8 previously reported cases. This case increases the molecular heterogeneity of this rare porphyria, and illustrates that it should be considered as a cause of chronic photosensitivity and porphyrin elevation at any age.

Molecular analysis of 19 Spanish patients with mixed porphyrias

Porphyrias are rare diseases caused by alterations in the heme biosynthetic pathway. Depending on the afected enzyme, porphyrin precursors or porphyrins are overproduced, causing acute neurovisceral attacks or dermal photosensitivity, respectively. Hereditary Coproporphyria (HCP) and Variegate Porphyria (VP) are mixed porphyrias since they can present acute and/or cutaneous symptoms. These diseases are caused by a deficiency of coproporphyrinogen oxidase (CPOX) in HCP, and protoporphyrinogen oxidase (PPOX) in VP. Herein, we studied nineteen unrelated Spanish patients with mixed porphyrias. The diagnosis of either, HCP or VP was made on the basis of clinical symptoms, biochemical findings and the identification of the mutation responsible in the CPOX or PPOX genes. Two patients presented both acute and cutaneous symptoms. In most patients, the biochemical data allowed the diagnosis. Among eleven patients with HCP, ten CPOX mutations were identified, including six novel ones: two frameshift (c.32delG and c.1102delC), two nonsense (p.Cys239Ter and p.Tyr365Ter), one missense (p.Trp275Arg) and one amino acid deletion (p.Gly336del). Moreover, seven previously described PPOX mutations were identified in eight patients with VP. The impacts of CPOX mutations p.Trp275Arg and p.Gly336del, were evaluated using prediction softwares and their functional consequences were studied in a prokaryotic expression system. Both alterations were predicted as deleterious by in silico analysis. Aditionally, when these alleles were expressed in E. coli, only p.Trp275Arg retained some residual activity. These results emphasize the usefulness of integrated the biochemical tests and molecular studies in the diagnosis. Furthermore, they extend knowledge on the molecular heterogeneity of mixed porphyrias in Spain.

In silico multiple-targets identification for heme detoxification in the human malaria parasite Plasmodium falciparum

Detoxification of hemoglobin byproducts or free heme is an essential step and considered potential targets for anti-malaria drug development. However, most of anti-malaria drugs are no longer effective due to the emergence and spread of the drug resistant malaria parasites. Therefore, it is an urgent need to identify potential new targets and even for target combinations for effective malaria drug design. In this work, we reconstructed the metabolic networks of Plasmodium falciparum and human red blood cells for the simulation of steady mass and flux flows of the parasite's metabolites under the blood environment by flux balance analysis (FBA). The integrated model, namely iPF-RBC-713, was then adjusted into two stage-specific metabolic models, which first was for the pathological stage metabolic model of the parasite when invaded the red blood cell without any treatment and second was for the treatment stage of the parasite when a drug acted by inhibiting the hemozoin formation and caused high production rate of heme toxicity. The process of identifying target combinations consisted of two main steps. Firstly, the optimal fluxes of reactions in both the pathological and treatment stages were computed and compared to determine the change of fluxes. Corresponding enzymes of the reactions with zero fluxes in the treatment stage but non-zero fluxes in the pathological stage were predicted as a preliminary list of potential targets in inhibiting heme detoxification. Secondly, the combinations of all possible targets listed in the first step were examined to search for the best promising target combinations resulting in more effective inhibition of the detoxification to kill the malaria parasites. Finally, twenty-three enzymes were identified as a preliminary list of candidate targets which mostly were in pyruvate metabolism and citrate cycle. The optimal set of multiple targets for blocking the detoxification was a set of heme ligase, adenosine transporter, myo-inositol 1-phosphate synthase, ferrodoxim reductase-like protein and guanine transporter. In conclusion, the method has shown an effective and efficient way to identify target combinations which are obviously useful in the development of novel antimalarial drug combinations.

Harderoporphyria due to homozygosity for coproporphyrinogen oxidase missense mutation H327R

Hereditary coproporphyria (HCP) is an autosomal dominant acute hepatic porphyria due to the half-normal activity of the heme biosynthetic enzyme, coproporphyrinogen oxidase (CPOX). The enzyme catalyzes the step-wise oxidative decarboxylation of the heme precursor, coproporphyrinogen III, to protoporphyrinogen IX via a tricarboxylic intermediate, harderoporphyrinogen. In autosomal dominant HCP, the deficient enzymatic activity results primarily in the accumulation of coproporphyrin III. To date, only a few homozygous HCP patients have been described, most having Harderoporphyria, a rare variant due to specific CPOX mutations that alter enzyme residues D400-K404, most patients described to date having at least one K404E allele. Here, we describe a Turkish male infant, the product of a consanguineous union, who presented with the Harderoporphyria phenotype including neonatal hyperbilirubinemia, hemolytic anemia, hepatosplenomegaly, and skin lesions when exposed to UV light. He was homoallelic for the CPOX missense mutation, c.980A>G (p.H327R), and had massively increased urinary uroporphyrins I and III (9,250 and 2,910 μM, respectively) and coproporphyrins I and III (895 and 19,400 μM, respectively). The patient expired at 5 months of age from an apparent acute neurologic porphyric attack. Structural studies predicted that p.H327R interacts with residue W399 in the CPOX active site, thereby accounting for the Harderoporphyria phenotype.

Role of aspartate 400, arginine 262, and arginine 401 in the catalytic mechanism of human coproporphyrinogen oxidase

Coproporphyrinogen oxidase (CPO) is the sixth enzyme in the heme biosynthetic pathway, catalyzing two sequential oxidative decarboxylations of propionate moieties on coproporphyrinogen-III forming protoporphyrinogen-IX through a monovinyl intermediate, harderoporphyrinogen. Site-directed mutagenesis studies were carried out on three invariant amino acids, aspartate 400, arginine 262, and arginine 401, to determine residue contribution to substrate binding and/or catalysis by human recombinant CPO. Kinetic analyses were performed on mutant enzymes incubated with three substrates, coproporphyrinogen-III, harderoporphyrinogen, or mesoporphyrinogen-VI, in order to determine catalytic ability to perform the first and/or second oxidative decarboxylation. When Asp400 was mutated to alanine no divinyl product was detected, but the production of a small amount of monovinyl product suggested the K(m) value for coproporphyrinogen-III did not change significantly compared to the wild-type enzyme. Upon mutation of Arg262 to alanine, CPO was again a poor catalyst for the production of a divinyl product, with a catalytic efficiency <0.01% compared to wild-type, including a 15-fold higher K(m) for coproporphyrinogen-III. The efficiency of divinyl product formation for mutant enzyme Arg401Ala was approximately 3% compared to wild-type CPO, with a threefold increase in the K(m) value for coproporphyrinogen-III. These data suggest Asp400, Arg262, and Arg401 are active site amino acids critical for substrate binding and/or catalysis. Possible roles for arginine 262 and 401 include coordination of carboxylate groups of coproporphyrinogen-III, while aspartate 400 may initiate deprotonation of substrate, resulting in an oxidative decarboxylation.

Biosynthesis of heme in mammals

Most iron in mammalian systems is routed to mitochondria to serve as a substrate for ferrochelatase. Ferrochelatase inserts iron into protoporphyrin IX to form heme which is incorporated into hemoglobin and cytochromes, the dominant hemoproteins in mammals. Tissue-specific regulatory features characterize the heme biosynthetic pathway. In erythroid cells, regulation is mediated by erythroid-specific transcription factors and the availability of iron as Fe/S clusters. In non-erythroid cells the pathway is regulated by heme-mediated feedback inhibition. All of the enzymes in the heme biosynthetic pathway have been crystallized and the crystal structures have permitted detailed analyses of enzyme mechanisms. All of the genes encoding the heme biosynthetic enzymes have been cloned and mutations of these genes are responsible for a group of human disorders designated the porphyrias and for X-linked sideroblastic anemia. The biochemistry, structural biology and the mechanisms of tissue-specific regulation are presented in this review along with the key features of the porphyric disorders.

Characterization of the heme synthesis enzyme coproporphyrinogen oxidase (CPO) in zebrafish erythrogenesis

Hemoglobin consists of heme and globin proteins and is essential for oxygen transport in all vertebrates. Although biochemical features of heme synthesis enzymes have been well characterized, the function of these enzymes in early embryogenesis is not fully understood. We found that the sixth heme synthesis enzyme, coproporphyrinogen oxidase (CPO), is predominantly expressed in the intermediate cell mass (ICM) that is a major site of zebrafish primitive hematopoiesis. Knockdown of zebrafish CPO using anti-sense morpholinos (CPO-MO) leads to a significant suppression of hemoglobin production without apparent reduction of blood cells. Injection of human CPO RNA, but not a mutant CPO RNA that is similar to a mutant responsible for a hereditary coproporphyria (HCP), restores hemoglobin production in the CPO-MO-injected embryos. Furthermore, expression of CPO in the ICM is severely suppressed in both vlad tepes/gata1 mutants and in biklf-MO-injected embryos. In contrast, over-expression of biklf and gata1 significantly induces ectopic CPO expression. The function of CPO in heme biosynthesis is apparently conserved between zebrafish and human, suggesting that CPO-MO-injected zebrafish embryos might be a useful in vivo assay system to measure the biological activity of human CPO mutations.

Structural basis of hereditary coproporphyria

Hereditary coproporphyria is an autosomal dominant disorder resulting from the half-normal activity of coproporphyrinogen oxidase (CPO), a mitochondrial enzyme catalyzing the antepenultimate step in heme biosynthesis. The mechanism by which CPO catalyzes oxidative decarboxylation, in an extraordinary metal- and cofactor-independent manner, is poorly understood. Here, we report the crystal structure of human CPO at 1.58-A resolution. The structure reveals a previously uncharacterized tertiary topology comprising an unusually flat seven-stranded beta-sheet sandwiched by alpha-helices. In the biologically active dimer (K(D) = 5 x 10(-7) M), one monomer rotates relative to the second by approximately 40 degrees to create an intersubunit interface in close proximity to two independent enzymatic sites. The unexpected finding of citrate at the active site allows us to assign Ser-244, His-258, Asn-260, Arg-262, Asp-282, and Arg-332 as residues mediating substrate recognition and decarboxylation. We favor a mechanism in which oxygen serves as the immediate electron acceptor, and a substrate radical or a carbanion with substantial radical character participates in catalysis. Although several mutations in the CPO gene have been described, the molecular basis for how these alterations diminish enzyme activity is unknown. We show that deletion of residues (392-418) encoded by exon six disrupts dimerization. Conversely, harderoporphyria-causing K404E mutation precludes a type I beta-turn from retaining the substrate for the second decarboxylation cycle. Together, these findings resolve several questions regarding CPO catalysis and provide insights into hereditary coproporphyria.

Mutations in human CPO gene predict clinical expression of either hepatic hereditary coproporphyria or erythropoietic harderoporphyria

Hereditary coproporphyria (HCP), an autosomal dominant acute hepatic porphyria, results from mutations in the gene that encodes coproporphyrinogen III oxidase (CPO). HCP (heterozygous or rarely homozygous) patients present with an acute neurovisceral crisis, sometimes associated with skin lesions. Four patients (two families) have been reported with a clinically distinct variant form of HCP. In such patients, the presence of a specific mutation (K404E) on both alleles or associated with a null allele, produces a unifying syndrome in which hematological disorders predominate: 'harderoporphyria'. Here, we report the fifth case (from a third family) with harderoporphyria. In addition, we show that harderoporphyric patients exhibit iron overload secondary to dyserythropoiesis. To investigate the molecular basis of this peculiar phenotype, we first studied the secondary structure of the human CPO by a predictive method, the hydrophobic cluster analysis (HCA) which allowed us to focus on a region of the enzyme. We then expressed mutant enzymes for each amino acid of the region of interest, as well as all missense mutations reported so far in HCP patients and evaluated the amount of harderoporphyrin in each mutant. Our results strongly suggest that only a few missense mutations, restricted to five amino acids encoded by exon 6, may accumulate significant amounts of harderoporphyrin: D400-K404. Moreover, all other type of mutations or missense mutations mapped elsewhere throughout the CPO gene, lead to coproporphyrin accumulation and subsequently typical HCP. Our findings, reinforced by recent crystallographic results of yeast CPO, shed new light on the genetic predisposition to HCP. It represents a first monogenic metabolic disorder where clinical expression of overt disease is dependent upon the location and type of mutation, resulting either in acute hepatic or in erythropoietic porphyria.

Crystal structure of the oxygen-dependant coproporphyrinogen oxidase (Hem13p) of Saccharomyces cerevisiae

Coproporphyrinogen oxidase (CPO) is an essential enzyme that catalyzes the sixth step of the heme biosynthetic pathway. Unusually for heme biosynthetic enzymes, CPO exists in two evolutionarily and mechanistically distinct families, with eukaryotes and some prokaryotes employing members of the highly conserved oxygen-dependent CPO family. Here, we report the crystal structure of the oxygen-dependent CPO from Saccharomyces cerevisiae (Hem13p), which was determined by optimized sulfur anomalous scattering and refined to a resolution of 2.0 A. The protein adopts a novel structure that is quite different from predicted models and features a central flat seven-stranded anti-parallel sheet that is flanked by helices. The dimeric assembly, which is seen in different crystal forms, is formed by packing of helices and a short isolated strand that forms a beta-ladder with its counterpart in the partner subunit. The deep active-site cleft is lined by conserved residues and has been captured in open and closed conformations in two different crystal forms. A substratesized cavity is completely buried in the closed conformation by the approximately 8-A movement of a helix that forms a lid over the active site. The structure therefore suggests residues that likely play critical roles in catalysis and explains the deleterious effect of many of the mutations associated with the disease hereditary coproporphyria.

Identification of monovinyl tripropionic acid porphyrins and metabolites from faeces of patients with hereditary coproporphyria by high-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry

Harderoporphyrin (2-vinyl-4,6,7-tripropionic acid porphyrin) and its metabolites in faeces of patients with hereditary coproporphyria (HCP) have been separated and characterized by high-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry (HPLC/ESI-Q-TOFMS/MS). The metabolites identified were 2-ethyl-4,6,7-tripropionic acid porphyrin, 2-hydro-4,6,7-tripropionic acid porphyrin, 2-methoxyethyl-4,6,7-tripropionic acid porphyrin and 2-acetyl-4,6,7-tripropionic acid porphyrin. Isomers of harderoporphyrin derived from isomerization of harderoporphyrinogen were also detected.

Porphyric neuropathy

The hepatic porphyrias are a group of rare metabolic disorders characterized by enzymatic defects in the biosynthesis of heme, a metalloporphyrin that is the principal product of porphyrin metabolism. The hepatic porphyrias are genetically transmitted as autosomal-dominant disorders with variable expression that produce a particularly severe form of neuropathy. Most medical students readily recognize acute attacks of porphyria when the classic triad of abdominal pain, psychosis, and neuropathy is present. Yet, porphyric neuropathy is a source of confusion in practice, and patients with porphyria rarely receive the correct diagnosis early in the course of the illness. Porphyric neuropathy is manifest by symptoms, signs, and cerebrospinal fluid abnormalities resembling acute Guillain-Barré syndrome. However, accompanying psychological features, a proximal predilection of asymmetric weakness, and electrodiagnostic findings indicative of an axonal polyradiculopathy or neuronopathy all suggest the diagnosis of porphyria. Confirmation of the diagnosis depends on use of appropriate laboratory studies. The underlying pathophysiology of porphyric neuropathy has not been established, but it may be related to direct neurotoxicity of elevated levels of delta-aminolevulinic acid. The severity of the neuropathy and the availability of potential treatments, including avoidance of provocative factors, make identification important.

Neonatal-onset hereditary coproporphyria with male pseudohermaphrodism

The appearance of hereditary coproporphyria (HCP) before puberty is very rare, and all reported cases of early-onset HCP have been in the homozygous or the compound heterozygous state. Some have been identified as harderoporphyria, which is a rare erythropoietic variant form of HCP. These conditions can be differentiated by molecular analysis because the gene abnormality responsible for harderoporphyria seems to be unique (K404E). Early-onset HCP, not harderoporphyria, is reported with a gene mutation in the heterozygous state and male pseudohermaphrodism. It was shown that adrenal gland hypofunction resulted in male pseudohermaphrodism. This case demonstrates the possibility that abnormalities of steroid metabolism influence porphyria.

Characterization of mutations in the CPO gene in British patients demonstrates absence of genotype-phenotype correlation and identifies relationship between hereditary coproporphyria and harderoporphyria

Hereditary coproporphyria (HCP) is the least common of the autosomal dominant acute hepatic porphyrias. It results from mutations in the CPO gene that encodes the mitochondrial enzyme, coproporphyrinogen oxidase. A few patients have also been reported who are homoallellic or heteroallelic for CPO mutations and are clinically distinct from those with HCP. In such patients the presence of a specific mutation (K404E) on one or both alleles produces a neonatal hemolytic anemia that is known as "harderoporphyria"; mutations on both alleles elsewhere in the gene give rise to the "homozygous" variant of HCP. The molecular relationship between these disorders and HCP has not been defined. We describe the molecular investigation and clinical features of 17 unrelated British patients with HCP. Ten novel and four previously reported CPO mutations, together with three previously unrecognized single-nucleotide polymorphisms, were identified in 15 of the 17 patients. HCP is more heterogeneous than other acute porphyrias, with all but one mutation being restricted to a single family, with a predominance of missense mutations (10 missense, 2 nonsense, 1 frameshift, and 1 splice site). Of the four known mutations, one (R331W) has previously been reported to cause disease only in homozygotes. Heterologous expression of another mutation (R401W) demonstrated functional properties similar to those of the K404E harderoporphyria mutation. In all patients, clinical presentation was uniform, in spite of the wide range (1%-64%) of residual coproporphyrinogen oxidase activity, as determined by heterologous expression. Our findings add substantially to knowledge of the molecular epidemiology of HCP, show that single copies of CPO mutations that are known or predicted to cause "homozygous" HCP or harderoporphyria can produce typical HCP in adults, and demonstrate that the severity of the phenotype does not correlate with the degree of inactivation by mutation of coproporphyrinogen oxidase.

Mutations in the iron-sulfur cluster ligands of the human ferrochelatase lead to erythropoietic protoporphyria

Ferrochelatase (FECH; EC 4.99.1.1) catalyzes the terminal step of the heme biosynthetic pathway. Defects in the human FECHgene may lead to erythropoietic protoporphyria (EPP), a rare inherited disorder characterized by diminished FECH activity with protoporphyrin overproduction and subsequent skin photosensitivity and in rare cases liver failure. Inheritance of EPP appeared to be autosomal dominant with possible modulation by low expression of the wild-type FECH allele. Animal FECHs have been demonstrated to be [2Fe-2S] cluster-containing proteins. Although enzymatic activity and stability of the protein appear to be dependent on the presence of the [2Fe-2S] cluster, the physiologic role of the iron-sulfur center remains to be unequivocally established. Three of the 4 [2Fe-2S] cluster-coordinating cysteines (ie, C403, C406, and C411 in the human enzyme) are located within the C-terminal domain. In this study 5 new mutations are identified in patients with EPP. Three of the point mutations, in 3 patients, resulted in FECH variants with 2 of the [2Fe-2S] cluster cysteines substituted with tyrosine, serine, and glycine (ie, C406Y, C406S, and C411G) and with undetectable enzymatic activity. Further, one of the patients exhibited a triple point mutation (T1224→A, C1225→T, and T1231→G) leading to the N408K/P409S/C411G variant. This finding is entirely novel and has not been reported in EPP. The mutations of the codons for 2 of the [2Fe-2S] cluster ligands in patients with EPP supports the importance of the iron-sulfur center for the proper functioning of mammalian FECH and, in at least humans, its absence has a direct clinical impact.

Mutations in the iron-sulfur cluster ligands of the human ferrochelatase lead to erythropoietic protoporphyria

Ferrochelatase (FECH; EC 4.99.1.1) catalyzes the terminal step of the heme biosynthetic pathway. Defects in the human FECHgene may lead to erythropoietic protoporphyria (EPP), a rare inherited disorder characterized by diminished FECH activity with protoporphyrin overproduction and subsequent skin photosensitivity and in rare cases liver failure. Inheritance of EPP appeared to be autosomal dominant with possible modulation by low expression of the wild-type FECH allele. Animal FECHs have been demonstrated to be [2Fe-2S] cluster-containing proteins. Although enzymatic activity and stability of the protein appear to be dependent on the presence of the [2Fe-2S] cluster, the physiologic role of the iron-sulfur center remains to be unequivocally established. Three of the 4 [2Fe-2S] cluster-coordinating cysteines (ie, C403, C406, and C411 in the human enzyme) are located within the C-terminal domain. In this study 5 new mutations are identified in patients with EPP. Three of the point mutations, in 3 patients, resulted in FECH variants with 2 of the [2Fe-2S] cluster cysteines substituted with tyrosine, serine, and glycine (ie, C406Y, C406S, and C411G) and with undetectable enzymatic activity. Further, one of the patients exhibited a triple point mutation (T1224→A, C1225→T, and T1231→G) leading to the N408K/P409S/C411G variant. This finding is entirely novel and has not been reported in EPP. The mutations of the codons for 2 of the [2Fe-2S] cluster ligands in patients with EPP supports the importance of the iron-sulfur center for the proper functioning of mammalian FECH and, in at least humans, its absence has a direct clinical impact.

Hereditary coproporphyria in Germany: clinical-biochemical studies in 53 patients

OBJECTIVES

To describe the biochemical and clinical features in hereditary coproporphyria (HCP).

DESIGN AND METHOD

Within the last 20 years, we investigated 53 patients (male:female = 1:2.5; age = 8-86 years) suffering from HCP. We describe the characteristic levels of urine, and fecal porphyrins and their precursors in hereditary coproporphyria and present the clinical features. Especially, we measured the coproporphyrin isomers I and III.

RESULTS AND CONCLUSION

The group of hereditary coproporphyria patients exhibited a significantly higher (p<0.0001) excretion of urinary porphyrin precursors, delta-aminolevulinic acid (median = 84 micromol/24 h) and porphobilinogen (median = 39 micromol/24 h), as compared to controls (delta-aminolevulinic acid: 22 micromol/24 h, porphobilinogen: 3 micromol/24 h; median, n = 20). The median of coproporphyrin in urine (1315 nmol/24 h) and feces (1855 nmol/g) were enhanced 12- and 168-fold, as compared to healthy subjects (urinary coproporphyrin: 106 nmol/24 h, fecal coproporphyrin: 11 nmol/g; median, n = 20). During therapy on one female patient, with IV application of heme arginate, a considerable decline of porphyrin precursors and porphyrin excretion was observed. The examination of urinary and fecal coproporphyrin isomers I and III revealed an excessive elevation of the coproporphyrin isomer III of 87% in urine and 94% in feces, respectively (normal: urinary isomer III = 69-83% and fecal isomer III = 25-40%). In feces the increase of isomer III caused an inversion of the physiologic coproporphyrin isomer III:I ratio that could be recognized in all various stages in hereditary coproporphyria and in children. Acute attacks of hereditary coproporphyria are accompanied by an acute polysymptomatic clinical syndrome, and this is associated with high levels of urinary porphyrin precursors. On review of our patients, the highest percentage had abdominal pain (89%), followed by neurologic (33%), psychiatric (28%), cardiovascular (25%), and skin symptoms (14%).

Hereditary coproporphyria: exon screening by heteroduplex analysis detects three novel mutations in the coproporphyrinogen oxidase gene

Hereditary coproporphyria is a dominantly inherited disorder of porphyrin metabolism caused by a partial deficiency of coproporphyrinogen oxidase, the sixth enzyme in the heme synthetic pathway. We investigated the molecular basis of hereditary coproporphyria in three unrelated patients, amplifying each exon of the coproporphyrinogen oxidase gene and performing heteroduplex analysis to look for mutations. Unique heteroduplex patterns were noted in exons 2, 3, and 6. Sequencing revealed different mutations in each patient: a G-->A point mutation encoding a glutamic acid to lysine substitution at codon 101 (E101K), a C-->T point mutation encoding a proline to serine substitution at codon 149 (P149S), and a one base-pair insertion in exon 6 (968insT). No other mutations were found on sequencing the remaining exons and their intron-exon junctions. The two point mutations affect amino acids that are conserved in all species studied to date. The one base-pair insertion in exon 6 is the first frameshift mutation to be described in the coproporphyrinogen oxidase gene. This study adds three new mutations to those that have been previously reported, and all have been restricted to single families. These results indicate that hereditary coproporphyria is a genetically heterogeneous disease.

Systematic analysis of coproporphyrinogen oxidase gene defects in hereditary coproporphyria and mutation update

Hereditary coproporphyria (HC) is an acute hepatic porphyria with autosomal dominant inheritance caused by deficient activity of coproporphyrinogen III oxidase (CPO). Clinical manifestations of the disease are characterized by acute attacks of neurological dysfunction often precipitated by drugs, fasting, cyclical hormonal changes, or infectious diseases. Skin photosensitivity may also be present. The seven exons, the exon/intron boundaries and part of 3' noncoding sequence of the CPO gene were systematically analyzed by an exon-by-exon denaturing gradient gel electrophoresis (DGGE) strategy followed by direct sequencing in seven unrelated heterozygous HC patients from France, Holland, and Czech Republic. Seven novel mutations and two new polymorphisms were detected. Among these mutations: two are missense (G197W, W427R), two are nonsense (Q306X, Q385X), two are small deletions (662de14bp; 1168del3bp removing a glycine at position 390), and one is a splicing mutation (IVS1-15c-->g) which creates a new acceptor splice site. The pathological significance of the point mutations G197W, W427R, and the in-frame deletion 390delGly were assessed by their respective expression in a prokaryotic system using site-directed mutagenesis. These mutations resulted in the absence or a dramatic decrease of CPO activity. The two polymorphisms were localized in noncoding part of the gene: 1) a C/G polymorphism in the promotor region, 142 bp upstream from the transcriptional initiation site (-142C/G), and 2) a 6 bp deletion polymorphism in the 3' noncoding part of the CPO gene, 574 bp downstream of the last base of the normal termination codon (+574 delATTCTT). Five intragenic dimorphisms are now well characterized and the high degree of allelic heterogeneity in HC is demonstrated with seven new different mutations making a total of nineteen CPO gene defects reported so far.

[Hepatic porphyria]

INTRODUCTION

This review is aimed at presenting classification and diagnosis criteria of hepatic porphyrias and at proposing guidelines for diagnosis and management of these diseases.

CURRENT KNOWLEDGE AND KEY POINTS

Porphyrias are inherited disorders: each type of porphyria is the result of a specific decrease in the activity of one of the enzymes of heme biosynthesis. Porphyrias are presently classified as erythropoietic or hepatic, depending on the primary organ in which excess production of porphyrins or precursors takes place. From 1970 to 1998, there have been important advances in the understanding of these diseases: specific enzyme deficiencies have been demonstrated, and genes have been isolated and located. These advances have been followed rapidly by identification of mutations.

PERSPECTIVES AND PROJECTS

Treatment of acute attacks by hematin completely changed the disease prognosis. Relationships between porphyria cutanea tarda and hepatitis C virus or hemochromatosis have also been clarified. However, several important issues are still not solved: for instance, pathogenesis of neuronal dysfunction that produces the acute attacks is poorly understood. Differences related to susceptibility to develop acute attacks are not known.

Molecular characterization of homozygous variegate porphyria

Variegate porphyria (VP) is a low penetrance, autosomal dominant disorder that results from partial deficiency of protoporphyrinogen oxidase (PPOX) activity caused by mutation in the PPOX gene. The rare homozygous variant of VP is characterized by severe PPOX deficiency, onset of photosensitization by porphyrins in early childhood, skeletal abnormalities of the hand and, less constantly, short stature, mental retardation and convulsions. We have identified PPOX mutations on both alleles of five of the 11 unrelated patients with homozygous VP reported to date. Two patients were homoallelic for missense mutations (D349A and A433P), while three were heteroallelic. Functional analysis by prokaryotic expression showed that the D349A and A433P and one missense mutation in each of the three heteroallelic patients (G358R in two patients and A219KANA) preserved some PPOX activity (9.5-25% of wild-type). Mutations on the other allele of the heteroallelic patients abolished or markedly decreased activity. There was no relation between genotype assessed by functional analysis and the presence or severity of non-cutaneous manifestations. The mutations were absent from 104 unrelated patients with autosomal dominant VP. Our findings define the molecular pathology of homozygous VP and suggest that mild PPOX mutations occur in the general population but have very low or no clinical penetrance in heterozygotes.

Mutations in the ferrochelatase gene of four Spanish patients with erythropoietic protoporphyria

Erythropoietic protoporphyria is a hereditary disorder of porphyrin metabolism caused by mutations in the ferrochelatase gene. Ferrochelatase catalyzes the chelation of ferrous iron into protoporphyrin IX to form heme. Mutation analysis was performed in four Spanish erythropoietic protoporphyria families resulting in the identification of four different mutations in the ferrochelatase gene. Two of them were novel mutations, a missense mutation (1157 A-->C, H386P) and a frameshift mutation (843delC) found in two Spanish families, respectively. The third and the forth Spanish patients carried already published ferrochelatase gene mutations, a nonsense mutation (343C-->T, R115X) and a missense mutation (557T-->C, I186T), respectively. The newly described frameshift mutation (843delC) predicted formation of an abrupt mRNA. The deleterious effect of His386 to Pro substitution as a result of mutation 1157 A-->C on the ferrochelatase activity was investigated by expressing the mutant ferrochelatase in Escherichia coli. The mutant ferrochelatase exhibited only 0.8% of the wild-type ferrochelatase activity. Prediction of the secondary structure of ferrochelatase suggested that the H386P mutation disrupted the original alpha-helical structure by way of introducing a turn, a rather drastic structural change of the enzyme sufficient to cause activity loss.

Systematic analysis of molecular defects in the ferrochelatase gene from patients with erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP; MIM 177000) is an inherited disorder caused by partial deficiency of ferrochelatase (FECH), the last enzyme in the heme biosynthetic pathway. In EPP patients, the FECH deficiency causes accumulation of free protoporphyrin in the erythron, associated with a painful skin photosensitivity. In rare cases, the massive accumulation of protoporphyrin in hepatocytes may lead to a rapidly progressive liver failure. The mode of inheritance in EPP is complex and can be either autosomal dominant with low clinical penetrance, as it is in most cases, or autosomal recessive. To acquire an in-depth knowledge of the genetic basis of EPP, we conducted a systematic mutation analysis of the FECH gene, following a procedure that combines the exon-by-exon denaturing-gradient-gel-electrophoresis screening of the FECH genomic DNA and direct sequencing. Twenty different mutations, 15 of which are newly described here, have been characterized in 26 of 29 EPP patients of Swiss and French origin. All the EPP patients, including those with liver complications, were heterozygous for the mutations identified in the FECH gene. The deleterious effect of all missense mutations has been assessed by bacterial expression of the respective FECH cDNAs generated by site-directed mutagenesis. Mutations leading to a null allele were a common feature among three EPP pedigrees with liver complications. Our systematic molecular study has resulted in a significant enlargement of the mutation repertoire in the FECH gene and has shed new light on the hereditary behavior of EPP.

Neonatal Hemolytic Anemia Due to Inherited Harderoporphyria: Clinical Characteristics and Molecular Basis

Porphyrias, a group of inborn errors of heme synthesis, are classified as hepatic or erythropoietic according to clinical data and the main site of expression of the specific enzymatic defect. Hereditary coproporphyria (HC) is an acute hepatic porphyria with autosomal dominant inheritance caused by deficient activity of coproporphyrinogen III oxidase (COX). Typical clinical manifestations of the disease are acute attacks of neurological dysfunction; skin photosensitivity may also be present. We report a variant form of HC characterized by a unifying syndrome in which hematologic disorders predominate: harderoporphyria. Harderoporphyric patients exhibit jaundice, severe chronic hemolytic anemia of early onset associated with hepatosplenomegaly, and skin photosensitivity. Neither abdominal pain nor neuropsychiatric symptoms are observed. COX activity is markedly decreased. In a first harderoporphyric family, with three affected siblings, a homozygous K404E mutation has been previously characterized. In the present study, molecular investigations in a second family with neonatal hemolytic anemia and harderoporphyria revealed two heterozygous point mutations in the COX gene. One allele bore the missense mutation K404E previously described. The second allele bore an A→G transition at the third position of the donor splice site in intron 6. This new COX gene mutation resulted in exon 6 skipping and the absence of functional protein production. In contrast with other COX gene defects that produce the classical hepatic porphyria presentation, our data suggest that the K404E substitution (either in the homozygous or compound heterozygous state associated with a mutation leading to the absence of functional mRNA or protein) is responsible for the specific hematologic clinical manifestations of harderoporphyria.

Hepatic porphyrias in children

Clinically overt hepatic porphyria is uncommon in children. The autosomal dominant acute hepatic porphyrias, acute intermittent porphyria (AIP), variegate porphyria (VP) and hereditary coproporphyria (HCP), are rarely present before puberty. Identification of asymptomatic children who have inherited these disorders is an important aspect of the management of the disease in their families and requires either enzymatic or DNA methods. Homozygous variants of AIP, VP and HCP usually present in early childhood and have phenotypes of variable severity. Mutational analysis is currently elucidating the relationship between these disorders and their autosomal dominant counterparts. 5-Aminolaevulinate dehydratase deficiency porphyria is a rare, autosomal recessive acute porphyria that may present at any age. Two cutaneous hepatic porphyrias are seen in children. Porphyria cutanea tarda (PCT), although mainly an adult disease, has been reported in young children with the autosomal dominant (type II) form of the disorder. Hepatoerythropoietic porphyria usually develops before the age of 2 years; patients are homo- or heteroallelic for uroporphyrinogen decarboxylase mutations, at least one of which is known to cause type II PCT.

Review: molecular pathogenesis of hepatic acute porphyrias

The molecular cloning of cDNA and genes encoding enzymes of the haem biosynthetic pathway have permitted the genetic defects underlying acute intermittent porphyria (AIP) and hereditary coproporphyria to be unravelled. In AIP, many different gene abnormalities have been documented since 1989. The prevalence of specific defective alleles among AIP families depends on which human population is studied. Founder effects are likely to account for a high frequency of a single mutation in Finland and, to a lesser extent, in Holland, while many other mutations have only been found once, each of them in a single family. In hereditary coproporphyria several different mutations have already been identified since 1994, suggesting that a large allelic heterogeneity also exists. The search for mutations in variegate porphyria has just started since the recent publication of the human cDNA sequence. Direct detection of the mutations using DNA analysis brings a growing contribution to the detection of asymptomatic carriers among relatives of porphyric patients and will, therefore, improve the prevention of acute attacks.

Family study of acute intermittent porphyria and hereditary coproporphyria in Niigata and Akita Prefectures, Japan

Simple screening tests, urinary porphobilinogen (PBG) for acute intermittent porphyria (AIP) and fecal coproporphyrin for hereditary coproporphyria (HCP), were performed in a family study of AIP and HCP. Urinary PBG was positive in 93 of 211 members of 10 AIP families, but was negative in 568 of 572 controls. Fecal coproporphyrin was positive in 54 of 108 members of 10 HCP families, but was negative in 188 controls. A dominant inheritance was assumed by a chi-square test and Weinberg segregation ratio. Worsening factors around puberty were suggested by the onset age and cumulative percentage of genetically loaded cases. Sex-related expression of symptoms was also inferred by a higher incidence of both porphyrias in females than in males. Fitness and penetrance of both porphyrias were good. An l-triiodothyronine loading test was the most useful for the detection of masked carriers of AIP. In conclusion, AIP and HCP in Japan show a dominant inheritance with sex-related metabolic and clinical manifestations.