Congenital erythropoietic porphyria: mutation update and correlations between genotype and phenotype

High quality genotype/phenotype analysis is a difficult issue in rare genetic diseases such as congenital erythropoietic porphyria (CEP) or Günther's disease, a heme biosynthesis defect due to uroporphyrinogen III synthase deficiency. The historical background and the main phenotypic features of the disease are depicted together with an update of published mutants and genotype/phenotype correlations. General rules concerning the prediction of disease severity are drawn as a guide for patient management and therapeutic choices. The phenotypic heterogeneity of the disease is presented in relation with a likely influence of modifying factors, either genetic or acquired.

Biochemical and molecular diagnosis of erythropoietic protoporphyria in an Ashkenazi Jewish family

Erythropoietic protoporphyria (EPP) is a rare hereditary disorder due to a partial deficiency of ferrochelatase (FECH). The genotype of EPP patients features a mutation on one allele of the FECH gene and a common hypomorphic FECH IVS3-48c on the other allele (M/c). The resulting enzyme activity in patients is ∼35% of that in normal individuals. Ferrochelatase deficiency results in the accumulation of protoporphyrin in the skin, which is responsible for the clinical symptom of cutaneous photosensitivity in patients. In this study, we report the identification of a novel FECH mutation delT23 in an 11-member EPP family of Jewish origin. Two EPP siblings shared an identical genotype of delT23/IVS3-48c (M/c). They were both photosensitive and showed highly increased erythrocyte protoporphyrin. The genotype of the patients' mother, who did not present with any EPP clinical symptoms, was delT23/IVS3-48t (M/t). The patients' father, an offspring of consanguineous parents, was homozygous IVS3-48 c/c. He exhibited a mild photosensitivity, and an increase of 4-fold in erythrocyte protoporphyrin. His FECH mRNA amount was 71% of that of genotype t/t. It is the first reported case of an individual with c/c genotype who exhibits both biochemical and clinical indications of EPP. These results suggest that IVS3-48c is a functional variant of ferrochelatase. The clinical symptoms and biochemical abnormalities in the patients' father could be the result of an interaction between genetic and environmental factors. In addition, the frequency of IVS3-48c in the Ashkenazi Jewish population was estimated at 8%, which is similar to that in the European populations.

Uroporphyrinogen III synthase knock-in mice have the human congenital erythropoietic porphyria phenotype, including the characteristic light-induced cutaneous lesions

Congenital erythropoietic porphyria (CEP), an autosomal recessive inborn error, results from the deficient but not absent activity of uroporphyrinogen III synthase (URO-synthase), the fourth enzyme in the heme biosynthetic pathway. The major clinical manifestations include severe anemia, erythrodontia, and disfiguring cutaneous involvement due to the accumulation of phototoxic porphyrin I isomers. Murine models of CEP could facilitate studies of disease pathogenesis and the evaluation of therapeutic endeavors. However, URO-synthase null mice were early embryonic lethals. Therefore, knock-in mice were generated with three missense mutations, C73R, V99A, and V99L, which had in vitro-expressed activities of 0.24%, 5.9%, and 14.8% of expressed wild-type activity, respectively. Homozygous mice for all three mutations were fetal lethals, except for mice homozygous for a spontaneous recombinant allele, V99A(T)/V99A(T), a head-to-tail concatemer of three V99A targeting constructs. Although V99A(T)/V99A(T) and C73R/V99A(T) mice had approximately 2% hepatic URO-synthase activity and normal hepatic microsomal heme and hemoprotein levels, they had 20% and 13% of wild-type activity in erythrocytes, respectively, which indicates that sufficient erythroid URO-synthase was present for fetal development and survival. Both murine genotypes showed marked porphyrin I isomer accumulation in erythrocytes, bone, tissues, and excreta and had fluorescent erythrodontia, hemolytic anemia with reticulocytosis and extramedullary erythropoiesis, and, notably, the characteristic light-induced cutaneous involvement. These mice provide insight into why CEP is an erythroid porphyria, and they should facilitate studies of the disease pathogenesis and therapeutic endeavors for CEP.

Two brothers with mild congenital erythropoietic porphyria due to a novel genotype

BACKGROUND

Congenital erythropoietic porphyria (CEP) is a rare autosomal recessive disease caused by the deficient activity of the heme biosynthetic enzyme, uroporphyrinogen III synthase (URO-synthase), and the accumulation of the nonphysiologic and phototoxic porphyrin I isomers. Clinical manifestations range from severe mutilation to mild erosions and blisters on sun-exposed areas. Evaluation of the URO-synthase mutation and residual enzyme activity has been correlated with the phenotypic expression of the disease.

OBSERVATIONS

We describe 16- and 4-year-old brothers with CEP with a mild phenotype due to a novel genotype, one allele having a promoter mutation (-76G-->A) and the other having an exonic missense mutation (G225S). The father and a 4-year-old fraternal twin brother were carriers of the -76G-->A mutation, whereas the mother and a 15-year-old brother were carriers of the G225S mutation. Previous in vitro expression studies demonstrated that the G225S mutation severely decreased URO-synthase activity to 1.2% of normal, whereas the promoter mutation decreased the activity to approximately 50% of wild type, accounting for the mild clinical phenotype.

CONCLUSION

The mild disease phenotype in these patients is a further example of the clinical heterogeneity seen in CEP and is additional proof that in vitro enzyme expression studies provide dependable genotype-phenotype correlations.

A knock-in mouse model of congenital erythropoietic porphyria

Congenital erythropoietic porphyria (CEP) is a recessive autosomal disorder characterized by a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. The severity of the disease, the lack of specific treatment except for allogeneic bone marrow transplantation, and the knowledge of the molecular lesions are strong arguments for gene therapy. An animal model of CEP has been designed to evaluate the feasibility of retroviral gene transfer in hematopoietic stem cells. We have previously demonstrated that the knockout of the Uros gene is lethal in mice (Uros(del) model). This work describes the achievement of a knock-in model, which reproduces a mutation of the UROS gene responsible for a severe UROS deficiency in humans (P248Q missense mutant). Homozygous mice display erythrodontia, moderate photosensitivity, hepatosplenomegaly, and hemolytic anemia. Uroporphyrin (99% type I isomer) accumulates in urine. Total porphyrins are increased in erythrocytes and feces, while Uros enzymatic activity is below 1% of the normal level in the different tissues analyzed. These pathological findings closely mimic the CEP disease in humans and demonstrate that the Uros(mut248) mouse represents a suitable model of the human disease for pathophysiological, pharmaceutical, and therapeutic purposes.

Congenital erythropoietic porphyria: identification and expression of eight novel mutations in the uroporphyrinogen III synthase gene

Mutations in the uroporphyrinogen III synthase (URO-synthase) gene cause congenital erythropoietic porphyria (CEP), an autosomal recessive inborn error of haem biosynthesis. Molecular analysis of the URO-synthase gene in seven unrelated CEP patients revealed eight novel mutations. These included four missense mutations (A69T, E81D, G188W and I219S), a deletion (21delG), two insertions (398insG and 672ins28) and one complex mutation (627del6ins39), as well as three previously reported mutations, C73R, T228M, and -86C-->A. When the four novel missense mutations were expressed in Escherichia coli, only E81D expressed significant enzymatic activity (30% of expressed wild-type activity), which was thermolabile. In addition, reverse transcription polymerase chain reaction studies demonstrated that E81D, which altered the penultimate nucleotide in exon 4, impaired splicing and caused about 85% exon 4 skipping. The identification and expression of these mutations provided genotype-phenotype correlations and further evidence of the molecular heterogeneity underlying this erythropoietic porphyria.

A genotype-phenotype correlation between null-allele mutations in the ferrochelatase gene and liver complication in patients with erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP), an inborn error of heme metabolism, causes in the majority of the patients only a symptom of photosensitivity. However, around 2% of the EPP sufferers develop liver complication in the form of liver cirrhosis and progressive liver failure. Mutations in the human ferrochelatase (FECH) gene causing EPP are highly heterogeneous and mostly family-specific. Actually, 62 FECH mutations have been published, 48 of them are "null allele" mutations inducing the formation of a truncated protein. The remaining 14 are missense mutations. In contrast to the null allele mutations, the latter lead to substitution of a single amino acid residue in the protein molecule and generate an enzyme that, although functionally impaired, is in its full length. In order to study the association between "null allele" mutation and liver complication, we combined our data with those in the literature. A total of 112 EPP patients were counted among 93 EPP families with a known FECH mutation. All 18 EPP patients who had severe liver complication carried a "null allele" mutation. In contrast, none of the 20 patients who carried a missense mutation had developed liver complication till the time of study (Fisher's exact test, p<0.05). High protoporphyrin blood concentration are considered to be a sign of an increased risk of liver disease. No correlation of protoporphyrin blood level with the type of mutation, was found, if patients with overt liver disease were excluded from the sample. Furthermore, no significant association of the liver complication with the location of the mutation within the FECH gene was found (Fisher exact test p = 0.46). These available data indicate a significant genotype-phenotype correlation between "null allele" mutation and protoporphyrin related liver disease in EPP. Although the risk for a EPP patient with a missense mutation to develop liver disease cannot be totally eliminated based on these data, it is comparably low.

Uroporphyrinogen III synthase erythroid promoter mutations in adjacent GATA1 and CP2 elements cause congenital erythropoietic porphyria

Congenital erythropoietic porphyria, an autosomal recessive inborn error of heme biosynthesis, results from the markedly deficient activity of uroporphyrinogen III synthase. Extensive mutation analyses of 40 unrelated patients only identified approximately 90% of mutant alleles. Sequencing the recently discovered erythroid-specific promoter in six patients with a single undefined allele identified four novel mutations clustered in a 20-bp region: (a) a -70T to C transition in a putative GATA-1 consensus binding element, (b) a -76G to A transition, (c) a -86C to A transversion in three unrelated patients, and (d) a -90C to A transversion in a putative CP2 binding motif. Also, a -224T to C polymorphism was present in approximately 4% of 200 unrelated Caucasian alleles. We inserted these mutant sequences into luciferase reporter constructs. When transfected into K562 erythroid cells, these constructs yielded 3 +/- 1, 54 +/- 3, 43 +/- 6, and 8 +/- 1%, respectively, of the reporter activity conferred by the wild-type promoter. Electrophoretic mobility shift assays indicated that the -70C mutation altered GATA1 binding, whereas the adjacent -76A mutation did not. Similarly, the -90C mutation altered CP2 binding, whereas the -86A mutation did not. Thus, these four pathogenic erythroid promoter mutations impaired erythroid-specific transcription, caused CEP, and identified functionally important GATA1 and CP2 transcriptional binding elements for erythroid-specific heme biosynthesis.

Molecular aspects of the inherited porphyrias

The porphyrias are diseases due to marked deficiencies of enzymes of the haem biosynthetic pathway (Fig. 1). Except for the first enzyme of the pathway, delta-aminolevulinate synthase (ALAS), deficiencies in seven other enzymes are associated with the various forms of porphyria (Fig. 2). Porphyrias can be classified as either hepatic or erythroid, depending on the major site of production of porphyrins or their precursors. The pathogenesis of all inherited porphyrias has now been defined at the molecular level, and it is clear that there is a great deal of genetic heterogeneity in each porphyria [1].

Hematologic aspects of the porphyrias

The porphyrias are disorders that can be inherited and acquired, in which the activities of the enzymes of the heme biosynthetic pathway are partially or almost totally deficient. There are 8 enzymes involved in the synthesis of heme, and, with the exception of the first enzyme, an enzymatic defect at every step leads to tissue accumulation and excessive excretion of porphyrins and/or their precursors, such as delta-aminolevulinic acid and porphobilinogen. Whereas heme, the final product of the biosynthetic pathway, is biologically important, porphyrins and their precursors are not only useless but also toxic. Porphyrias can be classified as either photosensitive or neurologic, depending on the type of symptoms, but some porphyrias cause both photosensitive and neurologic symptoms. Alternatively, they can be classified either hepatic or erythropoietic, depending on the principal site of expression of the specific enzymatic defect. The tissue-specific expression of porphyrias is largely due to the tissue-specific control of heme pathway gene expression, particularly at the level of delta-aminolevulinate synthase, the first and the rate-limiting enzyme of heme biosynthesis. In this chapter, hematologic aspects of the erythropoietic porphyrias will be described. The 3 major erythropoietic porphyrias are congenital erythropoietic porphyria (CEP), hepatoerythropoietic porphyria (HEP) and erythropoietic protoporphyria (EPP).

A novel stop codon mutation (X417L) of the ferrochelatase gene in bovine protoporphyria, a natural animal model of the human disease

Protoporphyria (PP) is caused by a deficiency of ferrochelatase (FC) activity, which catalyzes the final step in the heme biosynthesis pathway. Bovine are the only species other than man with naturally occurring PP. For expression of the PP phenotype, two copies of the mutated gene are necessary in bovine, whereas one copy is sufficient in humans. We report the first potential disease-causing mutation in the bovine FC gene. The coding region of FC was sequenced from the liver tissue of protoporphyric and normal bovine. A transversion was identified at nucleotide position 1250 which changed the stop codon to leucine (TGA-->TTA) in the protoporphyric FC sequence. As a consequence, the mutant protein is predicted to have an additional 27 amino acids. To screen other bovine for the G-->T transversion, cDNAs from liver tissue of clinically and biochemically normal, and from heterozygous and homozygous affected animals were used for allele-specific polymerase chain reaction. Three normal animals had only the G allele, five affected animals had only the T allele, and three heterozygous animals had both the G and T alleles. These results support our hypothesis that this mutation causes PP in bovine.

Immunological, enzymatic and biochemical studies of uroporphyrinogen III-synthase deficiency in 20 patients with congenital erythropoietic porphyria

Congenital erythropoietic porphyria (CEP), a rare autosomal recessive inborn error of heme biosynthesis, results from reduced activity of uroporphyrinogen III synthase (URO-III-S) leading to an excessive production and accumulation of porphyrins. Various clinical and biochemical observations point to a relationship between degree of disease expression and metabolic disturbance. We investigated 20 patients with early onset of clinical symptoms of CEP and, additionally, the relatives of six patients. CEP was confirmed by porphyrinemia and porphyrinuria with dominance of uroporphyrin and its isomer I. The investigation of the immunological nature of the defective URO-III-S gene from unrelated patients with unknown mutations was possible thanks to an antibody against the human enzyme. URO-III-S concentration in erythrocytes was determined by ELISA. No signal was achieved when assaying nonimmune serum by ELISA, whereas there was a positive reaction with the serum after immunisation. Furthermore, specificity of immune sera is demonstrated by immunoprecipitation of URO-III-S activity which caused a 33% reduction of enzyme activity. Normal levels of immunoreactive enzyme protein 100+/-10% of control (x +/- SD, n = 12) with a reduced specific activity 15+/-8.5% (x +/- SD, n = 12) were found in erythrocytes from all patients, with the exception of a girl with a remarkably high enzyme concentration of 149% of controls and a very low specific activity of 4%. In consequence, all patients had cross-reacting immunological material (CRIM)-positive mutations. CRIM-ratios varied between 3.2 and 24.5. The CRIM-positive nature of the gene defect indicated that the mutations altered the activity of URO-III-S. The different CRIM ratios implied the presence of various mutations, which is further evidence for the heterogeneity in the genetic defect found in CEP. URO-III-S activity was determined in erythrocyte lysates by a coupled enzyme assay. Erythrocyte URO-III-S activities of patients were reduced to 4-33% of the normal value (1.72+/-0.14 pkat/mg protein). An increase of urinary coproporphyrin isomer I (40-61%, norm = 17-31%) and a halved URO-III-S activity can serve as a biochemical test for asymptomatic heterozygous gene carriers of CEP.

Coexistence of deficiencies of uroporphyrinogen III synthase and decarboxylase in a patient with congenital erythropoietic porphyria and in his family

A hitherto undescribed dual deficiency of uroporphyrinogen III synthase and uroporphyrinogen decarboxylase was observed in the erythrocytes in a 14 year-old patient who had presented with congenital erythropoietic porphyria since early childhood. Whereas congenital erythropoietic porphyria was metabolically and clinically overt, a hereditary deficiency of uroporphyrinogen decarboxylase was confirmed by family study. The uroporphyrinogen III synthase activity of the propositus was decreased to 26% of the control while his asymptomatic family members had activities between 53-65% of the control. Additionally, the uroporphyrinogen decarboxylase activity was 55-66% of the control in the patient and his family. Family investigations have shown that the two disorders do not consistently segregate together. Although urinary porphyrin excretions of relatives were in the physiological range, the proportion of coproporphyrin isomer I showed a relative increase, which can serve as a biochemical indicator for heterozygous uroporphyrinogen III synthase gene carriers.

[A model of congenital erythropoietic porphyria for gene transfer in hematopoietic cells]

CEP is a rare disease inherited as an autosomal recessive trait and characterized by an overproduction and accumulation of porphyrins in the bone-marrow. Because the predominant site of metabolic expression of the disease is the erythropoietic system, bone marrow transplantation represents a curative treatment for patients with severe phenotypes. This treatment can be considered in severe cases when the disease appears in the first few years of life. When bone marrow transplantation is not possible, gene therapy by transplantation of genetically modified hematopoietic cells is an attractive alternative for the future. In this report, we present the restoration of enzymatic activity and the metabolic correction of deficient cells in vitro after transduction with retroviral vectors. The future availability of a mouse model of the disease will permit ex vivo gene therapy experiments on the entire animal.

Congenital erythropoietic porphyria: clinical, biochemical, and enzymatic profile of a severely affected infant

Blistering of light-exposed skin, pink-stained fluorescing diapers, and fluorescing peripheral erythrocytes led to diagnosis of congenital porphyria in an infant born to consanguineous parents. Although massive coproporphyrinuria and coproporphyrinemia initially suggested a coproporphyrinogen oxidase deficiency disorder, excess porphyrins were chiefly of the isomer I series, implicating a uroporphyrinogen III synthase defect. Congenital erythropoietic porphyria was confirmed by demonstration of a profound defect in the activity of the infant's uroporphyrinogen III synthase (4% of the mean value for nine normal controls) and in both parents at approximately 50% of the mean normal activity. Coinheritance of gene defects for either hereditary coproporphyria or erythropoietic protoporphyria in addition to those for congenital erythropoietic porphyria was excluded by demonstrating normal activities of both coproporphyrinogen oxidase and ferrochelatase in the infant. The complicated perinatal and postnatal clinical course and biochemical and enzyme assay data for the infant and his parents are described.

A novel point mutation in congenital erythropoietic porphyria in two members of Japanese family

The molecular basis of the uroporphyrinogen III synthase (UROIIIS) deficiency was investigated in two members of a Japanese family. This defect in heme biosynthesis is responsible for a rare autosomal recessive disease: congenital erythropoietic porphyria (CEP) or Gnther's disease. The first patient was homoallelic for a novel missense mutation: a T to C transition of nucleotide 634 that predicted a serine to proline substitution at residue 212 (S212P). The second patient appeared heteroallelic, carrying the same missense mutation and a nonsense mutation: a C to T change at nucleotide 745, resulting in a premature stop at codon 249, instead of a glutamine (Q249X). The corresponding mutated proteins were expressed in Escherichia coli and no residual activity was observed. A family study was also performed to determine the carrier status.

A systematic analysis of the mutations of the uroporphyrinogen III synthase gene in congenital erythropoietic porphyria

Congenital erythropoietic porphyria (CEP) or Günther's disease is an inborn error of heme biosynthesis, transmitted as an autosomal recessive trait and characterized by a profound deficiency of uroporphyrinogen III synthase activity (UROIIIS). The molecular defects observed in CEP are mainly heterogeneous, except for one missense mutation, C73R (Cys to Arg substitution at codon 73) which represents nearly 40% of the disease alleles. A convenient strategy was designed to establish a rapid diagnosis at the genetic level in samples from patients with CEP. As a first step, the most frequent mutation is searched for by restriction analysis from genomic. DNA amplified by PCR. Next, the nine coding exons and intron-exon boundaries are sequenced from genomic DNA. As an alternative, the mutation can be determined by sequencing the UROIIIS cDNA of the patient, using the RT-PCR technique on RNAs when a lymphoblastoid cell line can be established. Finally, for each new mutation in UROIIIS coding sequence, the corresponding mutant protein is expressed in Escherichia coli, in order to demonstrate the pathological significance of the mutation. This work describes the analysis of UROIIIS gene mutations in 10 new families with CEP and summarizes the data from 20 unrelated families studied in our laboratory. Three new missense mutations of UROIIIS coding sequence (H173Y, Q187P and P248Q) have been observed together with 8 known mutations. The significance of three intronic base changes (476 -31 T-->C; 562 -4 A-->T; 562 -23 A-->G) is discussed. In 6 alleles out of 40 (15%), the mutation remains undetermined.

[Congenital erythropoietic porphyria. Apropos of a fatal case in the neonatal period due to acute hemolysis with hepatic failure]

BACKGROUND

Congenital erythropoietic porphyria, an autosomal recessive disease, is characterized by deficiency of uroporphyrinogen III synthase. Clinical variability of the disease is related to the different mutations found in the patients.

CASE REPORT

A newborn suffered one hour after birth from jaundice and polypnea with acute hemolysis. Severe cutaneous photosensitivity occurred after phototherapy. Congenital erythropoietic porphyria was suspected because of reddish-colored urine and confirmed by porphyrin analyses. The baby died one month later due to severe hemolytic anemia with hepatic failure. Uroporphyrinogen III synthase activity was decreased by 99% in bone marrow cells and established lymphoblastoid cells from the patient. Molecular biology studies demonstrated the presence of the Cys 73-->Arg substitution at the homozygous state in the patient.

CONCLUSION

This mutation, the most frequently found in this disease, is responsible for a severe phenotype. Molecular characterization provides genotype/phenotype correlations in this porphyria and allows to clarify unusual cases of porphyrias.

Congenital erythropoietic porphyria: identification and expression of 10 mutations in the uroporphyrinogen III synthase gene

To investigate the molecular basis of the phenotypic heterogeneity in congenital erythropoietic porphyria, the mutations in the uroporphyrinogen III synthase gene from unrelated patients were determined. Six missense (L4F, Y19C, V82F, V99A, A104V, and G225S), a nonsense (Q249X), a frameshift (633insA), and two splicing mutations (IVS2+1 and IVS9 delta A + 4) were identified. When L4F, Y19C, V82F, V99A, A104V, 633insA, G225S, and Q249X were expressed in Escherichia coli, only the V82F, V99A, and A104V alleles expressed residual enzymatic activity. Of note, the V82F mutation, which occurs adjacent to the 5' donor site of intron 4, resulted in approximately 54% aberrantly spliced transcripts with exon 4 deleted. Thus, this novel exonic single-base substitution caused two lesions, a missense mutation and an aberrantly spliced transcript. Of the splicing mutations, the IVS2+1 allele produced a single transcript with exon 2 deleted, whereas the IVS9 delta A+4 allele was alternatively spliced, approximately 26% being normal transcripts and the remainder with exon 9 deleted. The amount of residual activity expressed by each allele provided a basis to correlate genotype with disease severity, thereby permitting genotype/phenotype predictions in this clinically heterogeneous disease.

Correction of the enzyme defect in cultured congenital erythropoietic porphyria disease cells by retrovirus-mediated gene transfer

Congenital erythropoietic porphyria (CEP) is a genetic disease characterized by an overproduction and accumulation of porphyrins in bone marrow. The enzyme defect concerns uroporphyrinogen III synthase (UROIIIS), the fourth enzyme of the heme biosynthetic pathway. It is the most severe porphyria and the treatment is largely symptomatic: gene therapy would represent a great therapeutic improvement. As a step toward the development of an effective gene therapy, we have constructed two retroviral vectors, LUSN and pMFG-US (with and without the selectable marker Neo), containing a full-length human cDNA for UROIIIS. Recombinant retroviruses were obtained by transfection of the LUSN or pMFG-US plasmid into the amphotropic packaging cell line psi CRIP. For each construct, three different producing clones were selected for their high titer (LUSN) or for their ability to express the message at a high level (pMFG-US). In vitro amplification of genomic DNA from target tissue demonstrated the presence of vector sequences. Murine fibroblasts infected in vitro expressed the human enzyme efficiently, as indicated by RNA and enzymatic studies. Retroviral-mediated gene transfer was then used to introduce the UROIIIS cDNA into human deficient cells. Enzyme activity was increased from 2% (deficient fibroblasts) to 121-274% of the normal value for the different clones. Transduced cells selected with G418 presented an 18-fold increase in enzyme activity compared to the normal cells. Furthermore, high gene transfer rate into peripheral blood progenitor cells (PBPB) was documented by in vitro amplification (PCR). These results demonstrate the potential usefulness of somatic gene therapy for the treatment of CEP.

Molecular defects of uroporphyrinogen decarboxylase in a patient with mild hepatoerythropoietic porphyria

The molecular defect of uroporphyrinogen decarboxylase (UROD) was examined in a patient with mild hepatoerythropoietic porphyria. To elucidate the UROD defect, we cloned UROD cDNAs from EBV-transformed lymphoblastoid cells of the proband using reverse transcriptase-polymerase chain reaction. Nucleotide sequence analysis of the cloned UROD cDNAs revealed two separate missense mutations, each occurring in a separate allele. One mutation was a Val134-->Gln transition, and was due to three sequential point mutations (T417G418T419-->CCA); the other mutation was a His220-->Pro transition (A677-->C). UROD phenotype studies demonstrated that the TGT-->CCA mutation was inherited from the father, and the A-->C mutation was inherited from the mother. In contrast to the null activity previously described for a mutant UROD from a patient with familial porphyria cutanea tarda, these mutant URODs had subnormal but substantial enzyme activities, when expressed in Chinese hamster ovary cells. This is the first demonstration of a mutation caused by three sequential base substitutions.

Screening for ferrochelatase mutations: molecular heterogeneity of erythropoietic protoporphyria

The DNA of 21 patients from 19 unrelated families with erythropoietic protoporphyria (EPP) were screened for the 6 ferrochelatase point mutations so far described. The mutation previously described by us (A >> T transversion at position -3 of the donor site of intron 10, causing exon 10 skipping) was detected in two additional unrelated EPP patients: in these patients, cDNA lacking exon 10 was also detected. The mutation described by Nakahashi et al. as responsible for exon 2 skipping (C >> T transition at position -23 of the acceptor site of intron 1), although also observed in some normal individuals, was invariably observed in all EPP patients tested and may thus play some role in the pathogenesis of EPP. Thus, it does not appear that this mutation is the primary mechanism underlying exon 2 skipping. None of the other four previously described mutations were detected. These data demonstrate the heterogeneity of the ferrochelatase locus and of the genetic defect in EPP.