Three novel mutations in the coproporphyrinogen oxidase gene

Heme biosynthesis and the porphyrias

Porphyrias, is a general term for a group of metabolic diseases that are genetic in nature. In each specific porphyria the activity of specific enzymes in the heme biosynthetic pathway is defective and leads to accumulation of pathway intermediates. Phenotypically, each disease leads to either neurologic and/or photocutaneous symptoms based on the metabolic intermediate that accumulates. In each porphyria the distinct patterns of these substances in plasma, erythrocytes, urine and feces are the basis for diagnostically defining the metabolic defect underlying the clinical observations. Porphyrias may also be classified as either erythropoietic or hepatic, depending on the principal site of accumulation of pathway intermediates. The erythropoietic porphyrias are congenital erythropoietic porphyria (CEP), and erythropoietic protoporphyria (EPP). The acute hepatic porphyrias include ALA dehydratase deficiency porphyria, acute intermittent porphyria (AIP), hereditary coproporphyria (HCP) and variegate porphyria (VP). Porphyria cutanea tarda (PCT) is the only porphyria that has both genetic and/or environmental factors that lead to reduced activity of uroporphyrinogen decarboxylase in the liver. Each of the 8 enzymes in the heme biosynthetic pathway have been associated with a specific porphyria (Table 1). Mutations affecting the erythroid form of ALA synthase (ALAS2) are most commonly associated with X-linked sideroblastic anemia, however, gain-of-function mutations of ALAS2 have also been associated with a variant form of EPP. This overview does not describe the full clinical spectrum of the porphyrias, but is meant to be an overview of the biochemical steps that are required to make heme in both erythroid and non-erythroid cells.

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.

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.

Biochemical and genetic characterization of four cases of hereditary coproporphyria in Spain

We report a biochemical and genetic characterization of four cases of hereditary coproporphyria (HCP) in Spain. All patients showed a typical HCP porphyrin excretion pattern with a high concentration of coproporphyrins in feces and inverted I:III isomer ratio. The porphyrin precursors in urine were found elevated in two patients who showed acute symptoms. The analysis of the CPO gene showed that three cases harboured novel mutations: V135A (404T>C; exon 1); L214R (641T>G; exon 2); and P249R (746C>G; exon 3) and in the fourth, a previously described R426X mutation in exon 6.

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.

The long, but not the short, presequence of human coproporphyrinogen oxidase is essential for its import and sorting to mitochondria

Nucleotide sequence analyses of human coproporphyrinogen oxidase (CPO) have shown two possible translation initiation eodons (TICs) in the region upstream from the region coding the mature polypeptide. These TICs lead to the precursor polypeptides of CPO (long and short pCPOs), which have 110 and 10 amino acids (long and short) presequences, respectively. To determine which TIC is the real one, import reactions to mitochondria of the two pCPOs synthesized using in vitro transcription-translation system were analyzed. The results showed that the long pCPO was imported into mitochondria and sorted properly to the intermembrane space, whereas the short pCPO was not. We also found several miss-readings in the reported nucleotide sequence of mouse CPO gene. This correction revealed that mouse pCPO has also a longer presequence, which are very similar to the long presequence of human. These results indicate that the long, but not short, presequence is the real one.

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.

Function and structure of rat hepatic coproporphyrinogen oxidase

Rat hepatic coproporphyrinogen oxidase, the sixth enzyme in the heme biosynthetic pathway, was purified 1340-fold with a yield of 39.7%. To obtain the soluble enzyme, different methods were applied to disrupt mitochondria, with sonication giving the highest yield (85%). The minimum catalytic form of enzyme was a dimer with a molecular mass of 77 +/- 4 kDa. The existence of aggregated forms was possible since in fractions of gel filtration elution activity was observed with higher molecular mass. We determined a Stokes radius of 36.3 A, a sedimentation coefficient (S20,w) of 5.06 S, and frictional ratio of 1.29, suggesting a nearly globular shape of the protein. Regardless of the type of salt, high ionic strength inhibits the enzyme, probably modifying its native structure. Experiments with amino acid modifiers showed that histidine, arginine, and tryptophan are involved in the catalytic process. Non-ionic detergents and phospholipids activated the enzyme, probably because they reproduce its natural hydrophobic environment. The present study describes a simple method for the purification of rat liver coproporphyrinogen oxidase, introducing for the first time data on the structure and function of the protein in a tissue often used as a laboratory model in biological studies, and contributing to the study of human hereditary coproporphyria.

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%).

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 porphyrias and alcohol]

Alcohol has an porphyrinogenic action and can cause a disturbance of porphyrin metabolism in healthy people as well as lead to a biochemical and clinical manifestation of acute and chronic hepatic porphyrias, especially acute intermittent porphyria and porphyria cutanea tarda. After excessive consumption of alcohol a temporary, clinically asymptomatic secondary hepatic coproporphyrinuria in man can be observed, which can become persistent in cases of alcohol-induced liver damage. Nowadays alcohol-liver-porphyrinuria syndrome is the first to be mentioned in secondary hepatic disturbances of porphyrin metabolism. In people with a genetic lack of uroporphyrinogen-decarboxylase alcohol is able to transform an asymptomatic coproporphyrinuria into a chronic hepatic porphyria or porphyria cutanea tarda. From experimental and clinical studies the conclusion can be drawn that alcohol inhibits the enzymes delta-aminolevulinic-acid-dehydratase (synonym: porphobilinogen-synthase), uroporphyrinogen-decarboxylase and coproporphyrinogen-oxidase and induces delta-aminolevulinic-acid-synthase in the liver. Abstinence of alcohol is a therapeutically and prophylactically important measurement in all types of hepatic porphyrias. For clinical experience follows that in cases with chronic consumption of alcohol, fatty liver, alcohol induced hepatitis and liver cirrhosis porphyrin studies in urine should be made to notice a hepatic porphyria in the latent phase very early. When dealing with abdominal and cutaneous symptoms in clinical context with consumption of alcohol one has to exclude hepatic porphyria differential diagnostically.

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.