Prevalence of the 281 (Gly----Glu) mutation in hepatoerythropoietic porphyria and porphyria cutanea tarda

Molecular heterogeneity of familial porphyria cutanea tarda in Spain: characterization of 10 novel mutations in the UROD gene

BACKGROUND

Porphyria cutanea tarda (PCT) results from decreased hepatic uroporphyrinogen decarboxylase (UROD) activity. In the majority of patients, the disease is sporadic (S-PCT or type I) and the enzyme deficiency is limited to the liver. Familial PCT (F-PCT or type II) is observed in 20-30% of patients in whom mutations on one allele of the UROD gene reduce UROD activity by approximately 50% in all tissues. Another variant of PCT (type III) is characterized by family history of the disease although it is biochemically indistinguishable from S-PCT.

OBJECTIVES

To investigate the molecular basis of PCT in Spain and to compare enzymatic and molecular analysis for the identification of patients with F-PCT.

METHODS

Erythrocyte UROD activity measurement and mutation analysis of the UROD gene were carried out in a cohort of 61 unrelated Spanish patients with PCT and 50 control individuals. Furthermore, each novel missense mutation identified was characterized by prokaryotic expression studies.

RESULTS

Of these 61 patients, 40 (66%) were classified as having S-PCT, 16 (26%) as having F-PCT and five (8%) as having type III PCT. Discordant results between enzymatic and molecular analysis were observed in two patients with F-PCT. In total, 14 distinct mutations were found, including 10 novel mutations: five missense, one nonsense, three deletions and an insertion. Prokaryotic expression of the novel missense mutations demonstrated that each results in decreased enzyme activity or stability.

CONCLUSIONS

These results confirm the high degree of molecular heterogeneity of F-PCT in Spain and emphasize the usefulness of molecular genetic analysis to distinguish between F-PCT and S-PCT.

Uroporphyrinogen decarboxylase gene mutations in Danish patients with porphyria cutanea tarda

Decreased uroporphyrinogen decarboxylase (UROD) activity is a characteristic feature of the most common of the porphyrias, porphyria cutanea tarda (PCT). A subgroup of the clinically overt PCT cases is associated with mutations in the gene encoding UROD and inherited as an autosomal-dominant trait. In this study, DNAs from 53 Danish PCT patients were subjected to genetic analysis for UROD mutations using denaturing gradient gel electrophoresis. Eleven genetic variations, seven of which are possible disease causing, were identified. All but one of these mutations were previously unknown, lending further support to the assumption that PCT is a heteroallelic disease. Only 11% of the examined patients were previously recognized as familial PCT cases. However, possible disease-related UROD mutations were identified in 24% of the examined patients, indicating that genetic analysis of PCT patients may improve differentiation between familial and sporadic PCT cases.

Screening for mutations in the uroporphyrinogen decarboxylase gene using denaturing gradient gel electrophoresis. Identification and characterization of six novel mutations associated with familial PCT

The two porphyrias, familial porphyria cutanea tarda (fPCT) and hepatoerythropoietic porphyria (HEP), are associated with mutations in the gene encoding the enzyme uroporphyrinogen decarboxylase (UROD). Several mutations, most of which are private, have been identified in HEP and fPCT patients, confirming the heterogeneity of the underlying genetic defects of these diseases. We have established a denaturing gradient gel electrophoresis (DGGE) assay for mutation detection in the UROD gene, enabling the simultaneous screening for known and unknown mutations. The established assay has proved able to detect the underlying UROD mutation in 10 previously characterized DNA samples as well as a new mutation in each of six previously unexamined PCT patients. The six novel UROD mutations comprise three missense mutations (M01T, F229L, and M324T), two splice mutations (IVS3-2A-->T and IVS5-2A-->G) leading to exon skipping, and a 2-bp deletion (415-416delTA) resulting in a frameshift and the introduction of a premature stop codon. Heterologous expression and enzymatic studies of the mutant proteins demonstrate that the three mutations leading to shortening or truncation of the UROD protein have no residual catalytic activity, whereas the two missense mutants retained some residual activity. Furthermore, the missense mutants exhibited a considerable increase in thermolability. The six new mutations bring to a total of 29 the number of disease-related mutations in the UROD gene. The DGGE assay presented greatly improves the genetic diagnosis of fPCT and HEP, thereby facilitating the detection of familial UROD deficient patients as well as the discrimination between familial and sporadic PCT cases.

A Huntington disease-like neurodegenerative disorder maps to chromosome 20p

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder characterized by motor disturbance, cognitive loss, and psychiatric manifestations. The disease is associated with a CAG trinucleotide-repeat expansion in the Huntington gene (IT15) on chromosome 4p16.3. One family with a history of HD was referred to us initially for predictive testing using linkage analysis. However, the chromosome 4p region was completely excluded by polymorphic markers, and later no CAG-repeat expansion in the HD gene was detected. To map the disease trait segregating in this family, whole-genome screening with highly polymorphic dinucleotide-, trinucleotide-, and tetranucleotide-repeat DNA markers was performed. A positive LOD score of 3.01 was obtained for the marker D20S482 on chromosome 20p, by two-point LOD-score analysis with the MLINK program. Haplotype analysis indicated that the gene responsible for the disease is likely located in a 2.7-cM region between the markers D20S193 and D20S895. Candidate genes from the mapping region were screened for mutations.

Familial porphyria cutanea tarda: characterization of seven novel uroporphyrinogen decarboxylase mutations and frequency of common hemochromatosis alleles

Familial porphyria cutanea tarda (f-PCT) results from the half-normal activity of uroporphyrinogen decarboxylase (URO-D). Heterozygotes for this autosomal dominant trait are predisposed to photosensitive cutaneous lesions by various ecogenic factors, including iron overload and alcohol abuse. The 3.6-kb URO-D gene was completely sequenced, and a long-range PCR method was developed to amplify the entire gene for mutation analysis. Four missense mutations (M165R, L195F, N304K, and R332H), a microinsertion (g10insA), a deletion (g645Delta1053), and a novel exonic splicing defect (E314E) were identified. Expression of the L195F, N304K, and R332H polypeptides revealed significant residual activity, whereas reverse transcription-PCR and sequencing demonstrated that the E314E lesion caused abnormal splicing and exon 9 skipping. Haplotyping indicated that three of the four families with the g10insA mutation were unrelated, indicating that these microinsertions resulted from independent mutational events. Screening of nine f-PCT probands revealed that 44% were heterozygous or homozygous for the common hemochromatosis mutations, which suggests that iron overload may predispose to clinical expression. However, there was no clear correlation between f-PCT disease severity and the URO-D and/or hemochromatosis genotypes. These studies doubled the number of known f-PCT mutations, demonstrated that marked genetic heterogeneity underlies f-PCT, and permitted presymptomatic molecular diagnosis and counseling in these families to enable family members to avoid disease-precipitating factors.

Crystal structure of human uroporphyrinogen decarboxylase

Uroporphyrinogen decarboxylase (URO-D) catalyzes the fifth step in the heme biosynthetic pathway, converting uroporphyrinogen to coproporphyrinogen by decarboxylating the four acetate side chains of the substrate. This activity is essential in all organisms, and subnormal activity of URO-D leads to the most common form of porphyria in humans, porphyria cutanea tarda (PCT). We have determined the crystal structure of recombinant human URO-D at 1.60 A resolution. The 40.8 kDa protein is comprised of a single domain containing a (beta/alpha)8-barrel with a deep active site cleft formed by loops at the C-terminal ends of the barrel strands. Many conserved residues cluster at this cleft, including the invariant side chains of Arg37, Arg41 and His339, which probably function in substrate binding, and Asp86, Tyr164 and Ser219, which may function in either binding or catalysis. URO-D is a dimer in solution (Kd = 0.1 microM), and this dimer also appears to be formed in the crystal. Assembly of the dimer juxtaposes the active site clefts of the monomers, suggesting a functionally important interaction between the catalytic centers.

The porphyrias: a brief overview based on 25 years of experience (1969-1994) by the Department of Dermatology of the Hospital Clinic and Faculty of Medicine of Barcelona, Spain

The porphyrias are uncommon diseases caused by enzymatic deficiencies in the heme pathway. In the 25 year period 1969-1994, the Department of Dermatology of the Hospital Clinic of Barcelona has been able to study 793 cases of porphyria (724 cases of PCT, 27 of EPP, 26 of PV, 5 of CEP, 5 of HEP, 5 of AIP, 1 of HCP). Homozygous expression of an enzymatic deficiency in the heme pathway produces severe disease. Commonly, clinical manifestations appear in the homozygous state (the autosomal recessive porphyrias). However, homozygous forms of autosomal dominant porphyrias may occur exceptionally. Moreover, there are cutaneous porphyrias whose clinical manifestations do not permit dermatologists to classify them clearly into one of the well-defined syndromes. These uncommon and atypical forms are difficult to recognize without biochemical and enzyme studies. The porphyrias have a wide clinico-biochemical spectrum, including a large proportion of well defined diseases. Nevertheless, atypical forms occur and may be difficult to evaluate. It is important to note the genetic heterogeneicity of porphyrias, which accounts for the varying phenotypic expression.

Neurologic disease in a child with hepatoerythropoietic porphyria

Hepatoerythropoietic porphyria (HEP) is a rare, autosomal recessive disorder due to deficient uroporphyrinogen decarboxylase enzyme activity. Patients exhibit photosensitivity, red urine, hypertrichosis, and characteristic serum and urine porphyrin profiles. Two siblings had the classic clinical and biochemical findings of HEP. The older patient developed a left-sided hemiparesis accompanied by an abnormal brain magnetic resonance imaging study. Although central nervous system abnormalities are a common feature of other hepatic porphyrias, they have not been previously documented in association with HEP.

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.

Analysis of uroporphyrinogen decarboxylase complementary DNAs in sporadic porphyria cutanea tarda

BACKGROUND

Sporadic porphyria cutanea tarda (S-PCT) has been considered an acquired disease because of the generation of liver-specific inhibitors of uroporphyrinogen decarboxylase (URO-D) activity. Several families have been described with S-PCT in multiple generations, raising the possibility of an inherited basis for the disease. To determine if S-PCT is associated with mutant URO-Ds that might be sensitive to liver-specific inhibitors, a molecular analysis of genomic and hepatocellular URO-Ds was undertaken.

METHODS

Total RNA from lymphoid cell lines from three unrelated patients with S-PCT and poly A+ RNA from liver biopsy samples from two additional patients was used as a template for single-stranded cDNA synthesis, and URO-D sequences were amplified and sequenced. DNA prepared from peripheral blood leukocytes was used as a template to polymerase chain reaction (PCR) amplify the promoter region of the URO-D gene. Sequencing of PCR products was performed completely in both directions by the chain termination method using a variety of custom oligonucleotide primers.

RESULTS

Ten URO-D alleles were sequenced, and no mutations were found. The promoter region of the URO-D gene was also normal.

CONCLUSIONS

It is concluded that S-PCT is not due to mutations at the URO-D locus. If inherited factors are involved, other loci must be affected.

Hepatoerythropoietic porphyria in a woman with short stature and deformed hands

A 23-year-old woman from Honduras was diagnosed to have hepatoerythropoietic porphyria. She had photosensitive skin of early onset, hypertrichosis, and severe scleroderma-like lesions of the hands. Erythrocyte uroporphyrinogen decarboxylase activity was reduced to about 10% of the normal activity.

Ionizing radiation and genetic risks. I. Epidemiological, population genetic, biochemical and molecular aspects of Mendelian diseases

This paper reviews the currently available information on naturally occurring Mendelian diseases in man; it is aimed at providing a background and framework for discussion of experimental data on radiation-induced mutations (papers II and III) and for the estimation of the risk of Mendelian disease in human populations exposed to ionizing radiation (paper IV). Current consensus estimates indicate that a total of about 125 per 10(4) livebirths are directly affected by one or another naturally occurring Mendelian disease (autosomal dominants, 95/10(4); X-linked ones, 5/10(4); and autosomal recessives, 25/10(4). These estimates are conservative and take into account conditions which are very rare and for which prevalence estimates are unavailable. Most, although not all, of the recognized "common" dominants have onset in adult ages while most sex-linked and autosomal recessives have onset at birth or in childhood. Autosomal dominant and X-linked diseases (i.e., the responsible mutant alleles) presumed to be maintained in the population due to a balance between mutation and selection are the ones which may be expected to increase in frequency as a result of radiation exposures. Viewed from this standpoint, the above assumption seems safe only for a small proportion of such diseases; for the remainder, there is no easy way to discriminate between different mechanisms that may be responsible or to rigorously exclude some in favor of some others. Mutations in genes that code for enzymic proteins are more often recessive in contrast to those that code for non-enzymic proteins, which are more often dominant. At the molecular level, with recessives, a wide variety of changes is possible and these include specific types of point mutations, small and large intragenic deletions, multilocus deletions and rearrangements. In the case of dominants, however, the kinds of recoverable point mutations and deletion-type changes are less extensive because of functional constraints. The mutational potential of genes varies, depending on the gene, its size, sequence content and arrangement, location and its normal functions, and can be grouped into three groups: those in which only point mutations have been found to occur, those in which only deletions or other gross changes have been recovered and those in which both kinds of changes are known. Molecular data are available for about 75 Mendelian conditions and these suggest that in approximately 50% of them, the changes categorized to date are point mutations and in the remainder, intragenic deletions or other gross changes; there does not seem to be any fundamental difference between dominants and recessives with respect to the underlying molecular defect.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of a new mutation responsible for hepatoerythropoietic porphyria

A deficiency in the activity of uroporphyrinogen decarboxylase (URO-D), the fifth enzyme of the haem biosynthetic pathway, is found in two hereditary diseases, familial porphyria cutanea tarda (PCT) and hepatoerythropoietic porphyria (HEP). Little is known about the genetic relationship between those two diseases and it has been postulated that HEP is the homozygous form of PCT. A URO-D cDNA was cloned from an HEP patient and the comparison between the mutant and the wild-type sequences showed a single base difference within the coding sequence leading to the replacement of a glutamic acid by a lysine at codon 167 of the mutant protein. This replacement produced a protein which is rapidly degraded in the presence of cell lysate. On the basis of hybridization of synthetic oligomers to amplified genomic DNA, we demonstrated that this patient is homozygous for this single base mutation. In order to look for any relationship between HEP and PCT, we tested six unrelated patients with familial PCT and could not detect the codon 167 mutation in any of them. These results indicate an heterogeneity in the mutations responsible for the PCT and HEP phenotypes.

Uroporphyrinogen decarboxylase: a splice site mutation causes the deletion of exon 6 in multiple families with porphyria cutanea tarda

Uroporphyrinogen decarboxylase (URO-D) is a cytosolic heme-biosynthetic enzyme that converts uroporphyrinogen to coproporphyrinogen. Defects at the uroporphyrinogen decarboxylase locus cause the human genetic disease familial porphyria cutanea tarda. A splice site mutation has been found in a pedigree with familial porphyria cutanea tarda that causes exon 6 to be deleted from the mRNA. The intron/exon junctions on either side of exon 6 fall between codons, so the resulting protein is shorter than the normal protein, missing only the amino acids coded by exon 6. The shortened protein lacks catalytic activity, is rapidly degraded when exposed to human lymphocyte lysates, and is not detectable by Western blot analysis in lymphocyte lysates derived from affected individuals. The mutation was detected in five of 22 unrelated familial porphyria cutanea tarda pedigrees tested, so it appears to be common. This is the first splice site mutation to be found at the URO-D locus, and the first mutation that causes familial porphyria cutanea tarda to be found in more than one pedigree.

Enzyme heterogeneity in the porphyrias

The use of immunological methods for measuring enzyme mass has identified several varieties of porphyria in which the reduction of porphyrin enzyme activity is not accompanied by a corresponding change in the enzyme mass. Currently, acute intermittent porphyria and hepatoerythropoietic porphyria have exhibited this phenomenon. In porphyria cutanea tarda, it has recently been shown that the pattern of enzyme deficiency in erythrocytic and nonerythrocytic tissues does not strictly follow the inheritance pattern (familial and sporadic) previously described. Also, contrary to previous dogma, some cases of type 1 porphyria cutanea tarda appear to be positive for cross reactive immunological material (CRIM).

Homozygous variegate porphyria: a case report

Homozygous variegate porphyria is described in a 14-year-old girl with a unique clinical presentation of photosensitivity from the second year of life, mental retardation, clinodactyly, and normal growth rate. The erythrocyte protoporphyrin concentration was raised with the protoporphyrin being predominantly zinc-chelated, which appears to be characteristic for all homozygous hepatic porphyrias. Protoporphyrinogen oxidase activity in lymphoblasts was decreased in both patient and parents despite the latter having normal porphyrin excretion.

Genetics and pathogenesis of human uroporphyrinogen decarboxylase defects

Two types of human porphyria, porphyria cutanea tarda (PCT) and hepatoerythropoietic porphyria (HEP), result from partial deficiency of uroporphyrinogen decarboxylase (UROD). About 20% of patients with PCT have a 50% decrease in UROD concentration in all tissues that is inherited as an autosomal dominant trait with low penetrance (type II PCT). Both this condition and its postulated homozygous counterpart, HEP, show genetic heterogeneity. Identification of a form of familial PCT in which the activity and concentration of erythrocyte UROD is normal, as in type I or sporadic PCT, suggests than an autosomal gene, not necessarily at the UROD locus, may be important in determining the onset of type I PCT. Clinically overt PCT results from a liver-specific process that causes reversible inactivation of UROD and which may be iron dependent. The predisposition to develop PCT in response to common hepatotoxic agents and other acquired factors may be determined by interaction between genes that control the concentration of active UROD in cells and genes that facilitate the inactivation process.

Applications of molecular genetics to gastrointestinal and liver diseases. II. Clinical relevance

The use of DNA probes within or near disease genes is becoming increasingly important in clinical medicine. Probes are available for prenatal and carrier diagnosis for several of the more than 100 genetic diseases of the gastrointestinal tract and liver. These include familial adenomatous polyposis, haemochromatosis, cystic fibrosis, alpha 1-antitrypsin deficiency and the hereditary porphyrias. This review uses examples drawn from such diseases to show the relevance of these approaches to the clinician.

Applications of molecular genetics to gastrointestinal and liver diseases. I. Technical approaches

Recent developments in recombinant DNA techniques have allowed an understanding of the molecular genetics of many diseases, some affecting the gastrointestinal tract and liver. DNA probes which detect sequences within or near disease genes can be selected by direct approaches, if the gene product or primary gene function is known, or by indirect methods when the chromosomal location is known. Such probes have resulted in extensive family studies which can now define risks to family members of developing a genetic disease. The development of the polymerase chain reaction will also be of considerable use in clinical genetics and in the diagnosis of some infectious diseases. The techniques are summarized and examples of their use are given. A glossary of terms is also provided.