Characterization of Mutations in the CPO Gene in British Patients Demonstrates Absence of Genotype-Phenotype Correlation and Identifies Relationship between Hereditary Coproporphyria and Harderoporphyria

Abstract

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.

Two novel mutations and coexistence of the 991C.T and the 1339C.T mutation on a single allele in the coproporphyrinogen oxidase gene in Swedish patients with hereditary coproporphyria

“…Three previously reported polymorphisms, 814A/C (exon 4), 880G/A (exon 4), and 990G/A (exon 5) (Fujita et al 1994;Lamoril et al 2001;Martasek et al 1994b;Petersen et al 2000;Sassa et al 1997) were identified among the investigated individuals. The frequency of these alleles in the control individuals were 0.87A/0.13C (814), 0.94G/ 0.06A (880), and 0.72G/0.28A (990).…”

“…In family A, no relatives were available for investigation. In the probands of families C, D, and E, the two previously reported mutations 991CϾT (R331W, exon 5) (Lamoril et al 2001;Martasek et al 1994b) and 1339CϾT (R447C, exon 7) (Lamoril et al 2001) were shown to coexist. Investigation of the members of these three families revealed that both these mutations are located on one allele, and that they segregate with the HCP diagnosis (Fig.…”

“…Until now, a total of 34 different mutations had been identified in the CPO gene of patients suffering from HCP in heterozygous or homozygous form (Doss et al 1999;Lamoril et al 2001;Petersen et al 2000;Sassa et al 1997; Human Gene Mutation Database). In the present study, DGGE analysis and sequencing identified two novel and two previously reported mutations in five of nine investigated Swedish HCP families.…”

“…Several HCP-coupled mutations have been identified in the CPO gene (Doss et al 1999;Lamoril et al 2001;Petersen et al 2000;Sassa et al 1997, and Human Gene Mutation Database). The mutations identified so far are, with a few exceptions, restricted to single families, making HCP a genetically heterogeneous disease.…”

Diagnosing Acute Porphyrias

“…Previously, determination of the diagnostic accuracy of metabolite and enzyme tests for acute porphyria has been difficult because of the absence of any method for defining groups of affected and unaffected individuals with 100% accuracy. The introduction of tests for disease-specific mutations has helped to solve this problem (5)(6)(7). A recent report in this Journal used this approach to evaluate investigations for AIP (8 ), and in this issue Hift et al (9 ) do the same for VP, capitalizing on its high prevalence in South Africa and the availability of a simple DNA test for the predominant R59W mutation.…”