Contribution of a common single-nucleotide polymorphism to the genetic predisposition for erythropoietic protoporphyria

Erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP) is an inherited disorder of the haem metabolic pathway characterised by accumulation of protoporphyrin in blood, erythrocytes and tissues, and cutaneous manifestations of photosensitivity. EPP has been reported worldwide, with prevalence between 1:75,000 and 1:200,000. It usually manifests in early infancy upon the first sun exposures. EPP is characterised by cutaneous manifestations of acute painful photosensitivity with erythema and oedema, sometimes with petechiae, together with stinging and burning sensations upon exposure to sunlight, without blisters. These episodes have a variable severity depending on the exposure duration and may result in chronic permanent lesions on exposed skin. As protoporphyrin is a lipophilic molecule that is excreted by the liver, EPP patients are at risk of cholelithiasis with obstructive episodes, and chronic liver disease that might evolve to rapid acute liver failure. In most patients, EPP results from a partial deficiency of the last enzyme of the haem biosynthetic pathway, ferrochelatase, EC 4.99.1.1/FECH (encoded by the FECH gene). EPP appears to be inherited as an autosomal dominant disease, the clinical expression of which is modulated by the presence of the hypomorphic FECH IVS3-48C allele trans, but recessive inheritance with two mutated FECH alleles has also been described. In about 2% of patients, overt disease was recently shown to be caused by gain-of-function mutations in the erythroid-specific aminolevulinic acid synthase 2 (ALAS2/ALAS, EC 2.3.1.27) gene and named X-linked dominant protoporphyria. Diagnosis is established by finding increased levels of protoporphyrin in plasma and red blood cells, and detection of a plasma fluorescence peak at 634 nm. Investigations for hepatic involvement, ferrochelatase activity level, genetic analysis (FECH mutations, presence of the hypomorphic FECH IVS3-48C allele trans and ALAS2 mutations) and family studies are advisable. Differential diagnosis includes phototoxic drug reactions, hydroa vacciniforme, solar urticaria, contact dermatitis, angio-oedema and, in some cases, other types of porphyria. Management includes avoidance of exposure to light, reduction of protoporphyrin levels and prevention of progression of possible liver disease to liver failure. As the major risk in EPP patients is liver disease, a regular follow-up of hepatic involvement is essential. Sequential hepatic and bone marrow transplantation should be considered as a suitable treatment for most severe cases of EPP with hepatic involvement. EPP is a lifelong disorder whose prognosis depends on the evolution of the hepatic disease. However, photosensitivity may have a significant impact on quality of life of EPP patients.

Characterization of human septic sera induced gene expression modulation in human myocytes

To gain a better understanding of the gene expression changes that occurs during sepsis, we have performed a cDNA microarray study utilizing a tissue culture model that mimics human sepsis. This study utilized an in vitro model of cultured human fetal cardiac myocytes treated with 10% sera from septic patients or 10% sera from healthy volunteers. A 1700 cDNA expression microarray was used to compare the transcription profile from human cardiac myocytes treated with septic sera vs normal sera. Septic sera treatment of myocytes resulted in the down-regulation of 178 genes and the up-regulation of 4 genes. Our data indicate that septic sera induced cell cycle, metabolic, transcription factor and apoptotic gene expression changes in human myocytes. Identification and characterization of gene expression changes that occur during sepsis may lead to the development of novel therapeutics and diagnostics.

Automated imaging of circulating fluorocytes for the diagnosis of erythropoietic protoporphyria: a pilot study for population screening

Objectives

To improve the traditional fresh blood film method to a high-throughput analysis of the presence of circulating fluorescent red cells (fluorocytes) in erythropoietic protoporphyria (EPP) using an automated imaging system.

Methods

Based on the autofluorescence of protoporphyrin, we used an automatic image acquisition platform for examining fluorocytes in peripheral blood with minimal sample preparation. The image acquisition is easy-to-use under automated operations of excitation, focusing, detection and data analysis. Quality image and semi-quantitative fluorescence measurement of fluorocytes can be generated in a single step. For high-throughput analysis, the platform can image more than 200 96-well micro-plates, i.e. 19200 samples, in approximately 10 hours. Importantly, the reagent cost of analysis is negligible.

Results

In this pilot study, three EPP patients were diagnosed and 4000 normal individuals were screened for EPP by this method. Our results showed that the method can distinguish the overt case and asymptomatic carriers. It gives reliable evidence for rapid EPP screening.

Conclusion

This automated imaging system provides multiple advantages that improve the traditional fresh blood film method as a more effective diagnostic tool and facilitates population screening for EPP. As fluorocytes are present in the umbilical cord blood of EPP patients, this high-throughput method can be potentially used for newborn screening of EPP.

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.

Dominant versus recessive: molecular mechanisms in metabolic disease

Inborn errors of metabolism used to be regarded as simple monogenic traits, but a closer look at how different alleles of a gene determine different phenotypes shows that the molecular mechanisms in the individual case are often complicated. Most metabolic disorders represent a spectrum of phenotypes from normal via attenuated to severe (and sometimes prenatally fatal), and disease manifestation is often influenced by other specific genetic or exogenous factors. The terms 'dominant' or 'recessive' relate to the functional consequences of differing alleles in the (compound) heterozygous individual; the terms are irrelevant for homozygous individuals and inappropriate for X-linked disorders. Mutations affecting the same amino acid residue may be associated with different inheritance patterns. True dominant inheritance in metabolism is rare; it may be found e.g. in tightly regulated biosynthetic pathways or when minor changes in metabolite concentrations have a functional effect. Some disorders such as erythropoietic protoporphyria show pseudodominant inheritance due to prevalent loss-of-function polymorphisms in the general population and are better acknowledged as recessive traits. The term 'variable expressivity' is not helpful with regard to autosomal recessive disorders when variable phenotypes are explained by different mutations in the respective gene. Clonal unmasking of a heterozygous mutation through somatic loss of the second allele, the main pathomechanism in inherited tumour predisposition syndromes, is rare in metabolic disorders, but focal congenital hyperinsulinism is a notable exception. Somatic mosaicism for an OTC gene mutation is given as an example of an apparently heterozygous mutation pattern in a boy with an X-linked disease.

Identifying modifier genes of monogenic disease: strategies and difficulties

Substantial clinical variability is observed in many Mendelian diseases, so that patients with the same mutation may develop a very severe form of disease, a mild form or show no symptoms at all. Among the factors that may explain these differences in disease expression are modifier genes. In this paper, we review the different strategies that can be used to identify modifier genes and explain their advantages and limitations. We focus mainly on the statistical aspects but illustrate our points with a variety of examples from the literature.

Seasonal palmar keratoderma in erythropoietic protoporphyria indicates autosomal recessive inheritance

Erythropoietic protoporphyria (EPP) is an inherited disorder that results from partial deficiency of ferrochelatase (FECH). It is characterized clinically by acute photosensitivity and, in 2% of patients, liver disease. Inheritance is usually autosomal dominant with low penetrance but is recessive in about 4% of families. A cross-sectional study of 223 patients with EPP in the United Kingdom identified six individuals with palmar keratoderma. We now show that these and three additional patients, from six families, have an inherited subtype of EPP which is characterized by seasonal palmar keratoderma, relatively low erythrocyte protoporphyrin concentrations, and recessive inheritance. No patient had evidence of liver dysfunction; four patients had neurological abnormalities. Patients were hetero- or homoallelic for nine different FECH mutations; four of which were previously unreported. Prokaryotic expression predicted that FECH activities were 2.7-25% (mean 10.6%) of normal. Neither mutation type nor FECH activity provided an explanation for the unusual phenotype. Our findings show that palmar keratoderma is a clinical indicator of recessive EPP, identify a phenotype that occurs in 38% of reported families with recessive EPP that to our knowledge is previously unreported, and suggest that patients with this phenotype may carry a lower risk of liver disease than other patients with recessive EPP.

C-terminal deletions in the ALAS2 gene lead to gain of function and cause X-linked dominant protoporphyria without anemia or iron overload

All reported mutations in ALAS2, which encodes the rate-regulating enzyme of erythroid heme biosynthesis, cause X-linked sideroblastic anemia. We describe eight families with ALAS2 deletions, either c.1706-1709 delAGTG (p.E569GfsX24) or c.1699-1700 delAT (p.M567EfsX2), resulting in frameshifts that lead to replacement or deletion of the 19-20 C-terminal residues of the enzyme. Prokaryotic expression studies show that both mutations markedly increase ALAS2 activity. These gain-of-function mutations cause a previously unrecognized form of porphyria, X-linked dominant protoporphyria, characterized biochemically by a high proportion of zinc-protoporphyrin in erythrocytes, in which a mismatch between protoporphyrin production and the heme requirement of differentiating erythroid cells leads to overproduction of protoporphyrin in amounts sufficient to cause photosensitivity and liver disease.

The diagnosis and management of erythropoietic protoporphyria

Porphyrias are a group of metabolic disorders resulting from enzymatic defects in the heme biosynthetic pathway. Erythropoietic protoporphyria is thought to be the second most common porphyria seen in clinical practice. It is, however, commonly under-recognized and can lead to both cutaneous manifestations as well as derangement in hepatic function in a minority of patients. This review summarizes the current understanding of this disorder. Different treatment options are discussed with the goal of preventing liver damage. The roles of liver and bone marrow transplantation are also addressed.

Erythropoietic protoporphyria without skin symptoms-you do not always see what they feel

Erythropoietic protoporphyria (EPP) is an inherited disorder of the porphyrin metabolism that often remains undiagnosed in children. We report on a 4-year-old girl who had been suffering for 1 year from recurrent painful crises affecting her hands, feet, and nose following sun exposure. Objective skin lesions were absent until the age of 6. Porphyrin analysis revealed elevated free erythrocyte protoporphyrin (FEP) levels confirming the diagnosis of EPP. This illustrates that skin lesions might be completely absent in children affected with EPP, a fact that has only been reported once previously. Because EPP can manifest with few and unspecific cutaneous symptoms or no skin lesions at all, like in this patient, the diagnosis of EPP might be delayed or missed. EPP should be excluded in all photosensitive children, especially when discomfort is disproportionate to the extent of the cutaneous lesions. The clinic, pathophysiology, diagnosis, complications, and therapy of EPP are discussed.

Making sense of the porphyrias

Patients with cutaneous porphyrias can be worrying for dermatologists. The diseases are rare enough to be unfamiliar, are associated with internal diseases, can have genetic implications, and are associated with incomprehensible biochemical pathways. In this review, I will try to explain why porphyrias occur, why they present as they do in the clinic, and provide a checklist for treating patients with porphyria.

Identification of a ferrochelatase mutation in a Chinese family with erythropoietic protoporphyria

BACKGROUND/AIMS

Erythropoietic protoporphyria (EPP) is a rare autosomal dominant disorder of heme biosynthesis characterized by a partial decrease in ferrochelatase (FECH) activity leading to excessive accumulation of protoporphyrin. While a majority of EPP patients only exhibit photosensitivity, a small percentage of patients also develop liver complications and need liver transplantation.

METHODS

In this study, we have sequenced the ferrochelatase gene of a Chinese EPP patient who suffered from EPP-related liver complications.

RESULTS

A nonsense mutation in exon 4, 343C>T, introducing a premature stop codon at position arginine 115, was identified in the proband as well as her symptomatic mother and brother, but was absent in her father. All the family members with overt photosensitivity also carried the low-expressed allele IVS3-48c, whose prevalence in the Chinese Han population was determined to be 41.35% and which was also functional in producing an aberrant 63 bp insertion.

CONCLUSION

We describe the first FECH mutation identified in the Chinese Han population and report a high frequency of the hypomorphic IVS3-48c allele in China.

Erythropoiesis and iron metabolism in dominant erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP) results from deficiency of ferrochelatase (FECH). Accumulation of protoporphyrin IX causes life-long acute photosensitivity. Microcytic anemia occurs in 20% to 60% of patients. We investigated 178 patients with dominant EPP confirmed by molecular analysis. Erythropoiesis was impaired in all patients; all had a downward shift in hemoglobin (Hb), and the mean decreased in males by 12 g/L (1.2 g/dL). By World Health Organization criteria, 48% of women and 33% of men were anemic. Iron stores, assessed by serum ferritin (sFn), were decreased by two-thirds, but normal serum soluble transferrin receptor-1 and iron concentrations suggested that erythropoiesis was not limited by iron supply. FECH deficiency in EPP appears to lead to a steady state in which decreased erythropoiesis is matched by reduced iron absorption and supply. This response may in part be mediated by protoporphyrin, but we found no correlation between erythrocyte protoporphyrin and Hb, sFn, total iron-binding capacity, or transferrin saturation.

Gene Dosage Analysis Identifies Large Deletions of the FECH Gene in 10% of Families with Erythropoietic Protoporphyria

Erythropoietic protoporphyria (EPP) is an inherited cutaneous porphyria characterized by partial deficiency of ferrochelatase (FECH), accumulation of protoporphyrin IX in erythrocytes, skin, and liver, and acute photosensitivity. Genetic counseling in EPP requires identification of FECH mutations, but current sequencing-based procedures fail to detect mutations in about one in six families. We have used gene dosage analysis by quantitative PCR to identify large deletions of the FECH gene in 19 (58%) of 33 unrelated UK patients with EPP in whom mutations could not be detected by sequencing. Seven deletions were identified, six of which were previously unreported. Breakpoints were identified for six deletions (c.1-7887-IVS1+2425insTTCA; c.1-9629-IVS1+2437; IVS2-1987-IVS4+352del; c.768-IVS7+244del; IVS7+2784-IVS9+108del; IVS6+2350-TGA+95del). Five breakpoints were in intronic repeat sequences (AluSc, AluSq, AluSx, L1MC4). The remaining deletion (Del Ex3-4) is likely to be a large insertion-deletion. Combining quantitative PCR with routine sequencing increased the sensitivity of mutation detection in 189 unrelated UK patients with EPP from 83% (95% CI: 76-87%) to 93% (CI: 88-96%) (P=0.003). Our findings show that large deletions of the FECH gene are an important cause of EPP. Gene dosage analysis should be incorporated into routine procedures for mutation detection in EPP.

Late presentation of erythropoietic protoporphyria: case report and genetic analysis of family members

Erythropoietic protoporphyria (EPP) is an inherited disorder of haem biosynthesis caused by decreased activity of the enzyme ferrochelatase (FECH), which catalyses the insertion of iron into protoporphyrin, the last step in haem biosynthesis. Development of clinically overt EPP usually requires inheritance of a severe FECH mutation trans to a low-expression FECH variant (FECH IVS3-48C), which is present in 13% of the U.K. population. Reduced FECH activity leads to accumulation of protoporphyrin in various tissues. An excess amount of free protoporphyrin in the skin causes photosensitivity. EPP usually presents in early childhood or infancy, with painful burning and pruritus within minutes of light exposure. Onset of symptoms in adults is rare and often associated with acquired somatic mutation of the FECH gene secondary to haematological malignancy. Here we describe a patient with EPP, in whom the presenting clinical symptom, night-time itch, did not appear until middle age and who had an asymptomatic sister with the same FECH genotype.

Heterogeneity of mutations in the ferrochelatase gene in Italian patients with erythropoietic protoporphyria

Mutations and a low-expressed allele IVS3-48c (in trans to the mutation) of the ferrochelatase (FECH) gene are responsible for erythropoietic protoporphyria (EPP) which is characterized clinically by cutaneous photosensitivity. In this study of 15 Italian EPP families, we identified 10 different FECH gene mutations, six of them were novel mutations (Q32X; IVS2-2, a-->g; IVS3-67, g-->a; 488-501del 14bp; IVS5-3, c-->t and 757-761delAGAAG). Four were known mutations (213insT; R115X; S264L and 899-900delTG). Real-time PCR quantification of FECH mRNA from a patient carrying mutation IVS-67, g-->a revealed a 51% reduction in the normal FECH transcript. This mutation generated an abnormal mRNA species with a 63-bp intron 3 retention--the same mRNA species which the low-expressed FECH allele IVS3-48c also produces. In addition, a double intronic deletion (IVS8-61 del ag /IVS9-58 del a) was found in some patients as well as their healthy relatives of this cohort. Sequencing of 50 Italian control subjects revealed an 11% prevalence of the double deletion. However, the double intronic deletion is not a disease-causing mutation as evidenced by a normal amount of FECH mRNA measured from a non-porphyric individual who carried the double deletion. Another interesting finding of this study was a 1% prevalence of IVS3-48c allele in the white Italian population, comparing to 11% in the French population. Further epidemiological studies are needed to test whether the 1% frequency of the low-expressed IVS3-48c allele is associated with a low prevalence of EPP in the white Italian population.

A common variant in combination with a nonsense mutation in a member of the thioredoxin family causes primary ciliary dyskinesia

Thioredoxins belong to a large family of enzymatic proteins that function as general protein disulfide reductases, therefore participating in several cellular processes via redox-mediated reactions. So far, none of the 18 members of this family has been involved in human pathology. Here we identified TXNDC3, which encodes a thioredoxin-nucleoside diphosphate kinase, as a gene implicated in primary ciliary dyskinesia (PCD), a genetic condition characterized by chronic respiratory tract infections, left-right asymmetry randomization, and male infertility. We show that the disease, which segregates as a recessive trait, results from the unusual combination of the following two transallelic defects: a nonsense mutation and a common intronic variant found in 1% of control chromosomes. This variant affects the ratio of two physiological TXNDC3 transcripts: the full-length isoform and a novel isoform, TXNDC3d7, carrying an in-frame deletion of exon 7. In vivo and in vitro expression data unveiled the physiological importance of TXNDC3d7 (whose expression was reduced in the patient) and the corresponding protein that was shown to bind microtubules. PCD is known to result from defects of the axoneme, an organelle common to respiratory cilia, embryonic nodal cilia, and sperm flagella, containing dynein arms, with, to date, the implication of genes encoding dynein proteins. Our findings, which identify a another class of molecules involved in PCD, disclose the key role of TXNDC3 in ciliary function; they also point to an unusual mechanism underlying a Mendelian disorder, which is an SNP-induced modification of the ratio of two physiological isoforms generated by alternative splicing.

Increased plasma transferrin, altered body iron distribution, and microcytic hypochromic anemia in ferrochelatase-deficient mice

Patients with deficiency in ferrochelatase (FECH), the last enzyme of the heme biosynthetic pathway, experience a painful type of skin photosensitivity called erythropoietic protoporphyria (EPP), which is caused by the excessive production of protoporphyrin IX (PPIX) by erythrocytes. Controversial results have been reported regarding hematologic status and iron status of patients with EPP. We thoroughly explored these parameters in Fechm1Pas mutant mice of 3 different genetic backgrounds. FECH deficiency induced microcytic hypochromic anemia without ringed sideroblasts, little or no hemolysis, and no erythroid hyperplasia. Serum iron, ferritin, hepcidin mRNA, and Dcytb levels were normal. The homozygous Fechm1Pas mutant involved no tissue iron deficiency but showed a clear-cut redistribution of iron stores from peripheral tissues to the spleen, with a concomitant 2- to 3-fold increase in transferrin expression at the mRNA and the protein levels. Erythrocyte PPIX levels strongly correlated with serum transferrin levels. At all stages of differentiation in our study, transferrin receptor expression in bone marrow erythroid cells in Fechm1Pas was normal in mutant mice but not in patients with iron-deficiency anemia. Based on these observations, we suggest that oral iron therapy is not the therapy of choice for patients with EPP and that the PPIX–liver transferrin pathway plays a role in the orchestration of iron distribution between peripheral iron stores, the spleen, and the bone marrow.

Erythropoietic protoporphyria in the U.K.: clinical features and effect on quality of life

BACKGROUND

Erythropoietic protoporphyria (EPP) is a rare inherited photodermatosis that causes lifelong painful photosensitivity. Neither its full clinical spectrum nor its impact on quality of life (QoL) has been investigated in a large cohort of patients.

OBJECTIVES

To document the clinical features of EPP and its impact on QoL in a high proportion of all patients with EPP resident in the U.K.

METHODS

Patients with EPP were identified from U.K. clinical databases and assessed by the same clinical investigator over a 7-month period using a standardized proforma and validated adult (Dermatology Life Quality Index, DLQI) and children's (Children's Dermatology Life Quality Index, CDLQI) QoL questionnaires.

RESULTS

Three hundred and eighty-nine living patients with EPP were identified, of whom 223 [114 females, 109 males; median age 34 years (range: 5-87), from 193 families] were investigated. Total erythrocyte porphyrin (TEP) was higher in males (median: 25.3 micromol L1) than females (median: 19.3 micromol L1). The median ages at onset and diagnosis were 1 and 12 years, respectively. Median times for onset of symptoms after sun exposure, onset of signs (oedema, erythema) and resolution of symptoms were 20 min, 6 h and 3 days, respectively. Most patients reported absence of protection by glass (92%), priming (85%), exacerbation by wind (68%), no family history of photosensitivity (56%), no symptoms during winter (56%) and had chronic skin lesions (79%). Symptoms changed little with age but improved during pregnancy in 47% of gravid women. Most patients used protective clothing and a sunscreen; 28% were taking beta-carotene and a further 56% had taken it; 29% were not under regular medical care. Two patients (1%) had liver failure and 8% reported gallstone disease. QoL was markedly impaired, with scores similar to those in severe dermatological disease (mean DLQI score 14.0, n = 176; mean CDLQI score 12.8, n = 44), indicating a large effect on patients' lives. DLQI scores correlated weakly with TEP (rs = 0.228; P = 0.002) and time to onset of symptoms (rs = -0.233; P = 0.002) but not with age at onset.

CONCLUSIONS

EPP is a persistent, severely painful, socially disabling disease with a marked impact on QoL. Its diagnosis is often overlooked. None of TEP, age at onset nor time to onset of symptoms is a useful predictor of impaired QoL in individual patients.