A novel missense mutation in exon 4 of the human coproporphyrinogen oxidase gene in two patients with 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.

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.

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.

DGGE analysis of the coproporphyrinogen oxidase gene: two new mutations in DNA from Danish patients with hereditary coproporphyria

The knowledge of at least 21 different mutations and several polymorphisms in the coproporphyrinogen oxidase (CPO) gene demonstrates that the molecular basis of hereditary coproporphyria is heterogeneous. We developed a DGGE-based assay for the analysis of exons 2 to 7, including 14-96 nucleotides of the flanking intronic sequences of the CPO gene. To render it suitable for the clinical diagnostic laboratory, we designed the assay to allow use of identical PCR conditions and the same DGGE gel for analyses of all the regions. Using this assay, and subsequent sequencing of gene regions containing interallelic variations, two novel mutations in the CPO gene were identified: a missense mutation (607G-->A), leading to the substitution of an alanine with a threonine, and a nonsense mutation (1281G-->A), giving rise to a stop codon 28 codons upstream to the wild-type stop codon.

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.