Molecular genetics of disorders of haem biosynthesis

The potential role of cell penetrating peptides in the intracellular delivery of proteins for therapy of erythroid related disorders

The erythroid related disorders (ERDs) represent a large group of hematological diseases, which in most cases are attributed either to the deficiency or malfunction of biosynthetic enzymes or oxygen transport proteins. Current treatments for these disorders include histo-compatible erythrocyte transfusions or allogeneic hematopoietic stem cell (HSC) transplantation. Gene therapy delivered via suitable viral vectors or genetically modified HSCs have been under way. Protein Transduction Domain (PTD) technology has allowed the production and intracellular delivery of recombinant therapeutic proteins, bearing Cell Penetrating Peptides (CPPs), into a variety of mammalian cells. Remarkable progress in the field of protein transduction leads to the development of novel protein therapeutics (CPP-mediated PTs) for the treatment of monogenetic and/or metabolic disorders. The “concept” developed in this paper is the intracellular protein delivery made possible via the PTD technology as a novel therapeutic intervention for treatment of ERDs. This can be achieved via four stages including: (i) the production of genetically engineered human CPP-mediated PT of interest, since the corresponding native protein either is missing or is mutated in the erythroid progenitor cell (ErPCs) or mature erythrocytes of patients; (ii) isolation of target cells from the peripheral blood of the selected patients; (iii) ex vivo transduction of cells with the CPP-mediated PT of interest; and (iv) re-administration of the successfully transduced cells back into the same patients.

Neurological complications of acute intermittent porphyria

BACKGROUND

Acute intermittent porphyria (AIP) is an inherited disorder of heme biosynthesis, the clinical manifestations of which are incompletely understood. In this report, we describe 12 cases of AIP, focusing on the neurological manifestations.

METHODS

Twelve patients were diagnosed with AIP on the basis of characteristic clinical findings, erythrocyte porphobilinogen deaminase (PBGD) activity, and molecular genetics. Central and peripheral nervous system manifestations were noted, and electrophysiological and radiological studies performed. Potential precipitating factors were recorded.

RESULTS

Eleven PBGD gene mutations were identified in 12 patients. Nine patients experienced neurological symptoms involving the central nervous system (consciousness disturbance, n = 8; convulsion/seizure, n = 4; behavior change, n = 1), while 7 patients experienced peripheral neuropathies (motor paresis, n = 7; impairment of bulbar or respiratory function, n = 4). The electrophysiological and electroencephalographic findings were consistent with the neurological symptoms of AIP. Urinary PBG and δ-aminolevulinic acid levels were elevated in all patients. PBGD enzyme activity levels were below normal in all patients. Eight patients had documented exposure to porphyrogenic agents.

CONCLUSIONS

Our detailed description of a relatively large number of cases of AIP may help clinicians to recognize this often difficult-to-diagnose disorder.

[Acute intermittent porphyria and chronic transaminase elevation]

Acute intermittent porphyria is an autosomal dominant inherited disorder resulting from a deficiency of porphobilinogen deaminase activity, the third enzyme in the heme biosynthesis pathway. This disease is uncommon, although the prevalence is higher in asymptomatic heterozygotic carriers; however, this prevalence is difficult to establish because of the absence of symptoms. Although acute intermittent porphyria is a multisystemic disease, its most common form of presentation is abdominal pain and neurological or mental symptoms, which can sometimes be due to precipitating factors such as reduced energy intake, smoking, alcohol, some drugs, and stress. Diagnosis can be made by testing urinary porphobilinogen levels, with subsequent measurement of enzyme activity and DNA testing. Treatment is based on prevention of porphyria attacks by avoiding precipitating factors and early administration of intravenous glucose or hemin therapy. We present the case of a patient diagnosed with acute intermittent porphyria based on study of chronic mild alanine aminotransferase (ALT) elevation.

Proteasomal degradation regulates expression of porphobilinogen deaminase (PBGD) mutants of acute intermittent porphyria

Acute intermittent porphyria (AIP) is a neuropathic disease caused by a dominant inherited deficiency in porphobilinogen deaminase (PBGD). We investigated the expression and the degradation of the human PBGD-mutations G748A, G748C and 887insA following transfection into human SH-SY5Y neuroblastoma cells. Mutant proteins exhibited reduced protein expression compared to transfected wild-type (wt) PBGD as revealed by Western blotting. The transcription levels assessed by real-time PCR of these mutant species were identical to those of the wild type. Immuno-fluorescence microscopy revealed reduced cellular distribution of the mutated PBGDs in the cytosol and the nucleus in comparison to the wild-type PBGD. Enhanced cellular accumulation of the mutated and wild-type PBGDs was detected following inhibition of the proteasome by the inhibitors CLBL and hemin. Elevated expression of wt and mutated PBGD protein levels was either achieved by hemin or heme-arginate treatment. On the other hand, enhanced PBGD degradation was achieved by lead poisoning of ALAD in the SH-SY5Y cells concomitant with acceleration of proteasomal activity, most probably by ALAD participation in proteasomal regulation [G.G. Guo, M. Gu, J.D. Etlinger, 240-kDa proteasome inhibitor (CF-2) is identical to delta-aminolevulinic acid dehydratase. J Biol Chem 1994; 269:12399-402.] Our results suggest that the difference in expression between the wild-type and mutant proteins appears to be regulated on the level of protein degradation. In conclusion, we demonstrate that the PBGD cellular pool is controlled by the proteasome activity, which in turn is down regulated by hemin or up-regulated by Pb-ALAD.

Acute intermittent porphyria: biochemical and clinical analysis in the Argentinean population

Acute intermittent porphyria (AIP) is the most common type of hepatic acute porphyria. In this work, we have analyzed the biochemical data of all Argentinean AIP families studied in the Porphyrins and Porphyrias Research Centre (CIPYP). We have shown that: (i) the prevalence for this population is about 1:125,000; (ii) the disease is more frequent in women than in men (7:3); (iii) about 60% are latent carriers; (iv) 15% of patients with symptomatic AIP died during an acute attack; (v) the most important precipitating factors of acute attacks in our population were the ingestion of therapeutic drugs (25%), anesthetics in surgical interventions (25%) and infections (20%); (vi) the initial symptom in Argentinean AIP individuals is severe abdominal pain (100%), and it is often accompanied by constipation (37%), anorexia (37%) and tachycardia (30%); and (vii) the percentage of recurrence of the acute attacks is high (81%).

Detection of a R173W mutation in the porphobilinogen deaminase gene in the Nova Scotian "foreign Protestant" population with acute intermittent porphyria: a founder effect

OBJECTIVES

Acute intermittent porphyria (AIP) is caused by mutations in the porphobilinogen deaminase (PBGD) gene that disrupt the function of the enzyme. Many mutations that lead to decreased PBGD activity have been described. An Arg to Trp substitution at codon 173 (CGG-->TGG in exon 10) and designated R173W, which leads to a CRIM-negative phenotype, has been reported in Swedish, Finnish, Scottish, and South African kindreds, and in a Nova Scotian proband with fatal AIP. In this work, we investigated the presence of this mutation in a Nova Scotian patient population presenting with AIP.

DESIGN AND METHODS

Single-strand conformation polymorphism analysis and DNA sequencing by TA cloning and Sanger's dideoxy chain termination method, were used to confirm the maternal transmission of this mutation to the proband. The mutation also eliminates an Ncil (also Mspl) endonuclease restriction site, which allows for detection of the mutant allele by polymerase chain reaction amplification and restriction enzyme digestion.

RESULTS

The family of the Nova Scotian proband and four other AIP kindreds showed the presence of the same mutation. These five families are descendants of German, Swiss, and French immigrants historically known as the "Foreign Protestants," who were recruited to Nova Scotia in the 1750s.

CONCLUSION

In all these families, descent from one couple that settled in Nova Scotia in 1751 has been identified by genealogy research, consistent with a founder effect within this population. This is the first identified mutation in PBGD causing AIP that has been linked to a founder effect in descendants of an immigrant population to North America, and which could be traced to such a distant background, similar to the South African variegate porphyria mutation.

The little imitator--porphyria: a neuropsychiatric disorder

Three common subtypes of porphyria give rise to neuropsychiatric disorders; acute intermittent porphyria, variegate porphyria, and coproporphyria. The second two also give rise to cutaneous symptoms. Neurological or psychiatric symptoms occur in most acute attacks, and may mimic many other disorders. The diagnosis may be missed because it is not even considered or because of technical problems, such as sample collection and storage, and interpretation of results. A negative screening test does not exclude the diagnosis. Porphyria may be overrepresented in psychiatric populations, but the lack of control groups makes this uncertain. The management of patients with porphyria and psychiatric symptoms causes considerable problems. Three cases are described to illustrate some of these issues. Advances in molecular biology permit identification of patients and latent carriers in the family. Care to avoid relapses and improved treatments have reduced the mortality.

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.

Porphobilinogen deaminase deficiency in mice causes a neuropathy resembling that of human hepatic porphyria

Acute intermittent porphyria (AIP) is a human disease resulting from a dominantly inherited partial deficiency of the heme biosynthetic enzyme, porphobilinogen deaminase (PBGD). The frequency of the trait for AIP is 1/10,000 in most populations, but may be markedly higher (1/500) in psychiatric patients. The clinical expression of the disease is characterized by acute, life-threatening attacks of 'porphyric neuropathy' that include abdominal pain, motor and sensory neurological deficits and psychiatric symptoms. Attacks are frequently precipitated by drugs, alcohol and low caloric intake. Identical symptoms occur in other hepatic porphyrias. To study the pathogenesis of the neurologic symptoms of AIP we have generated Pbgd-deficient mice by gene targeting. These mice exhibit the typical biochemical characteristics of human AIP, notably, decreased hepatic Pbgd activity, increased delta-aminolevulinic acid synthase activity and massively increased urinary excretion of the heme precursor, delta-aminolevulinic acid after treatment with drugs such as phenobarbital. Behavioural tests reveal decreased motor function and histopathological findings include axonal neuropathy and neurologic muscle atrophy.

Acute intermittent porphyria: a single-base deletion and a nonsense mutation in the human hydroxymethylbilane synthase gene, predicting truncations of the enzyme polypeptide

Acute intermittent porphyria (AIP) is an autosomal-dominant inborn error of metabolism that results from the half-normal activity of the third enzyme in the heme biosynthetic pathway, hydroxymethylbilane synthase (HMB-synthase). AIP is an ecogenetic condition, since the life-threatening acute attacks are precipitated by various factors, including drugs, alcohol, fasting, and certain hormones. Biochemical diagnosis is problematic, and the identification of mutations in the HMB-synthase gene provides accurate detection of presymptomatic heterozygotes, permitting avoidance of the acute precipitating factors. By direct solid-phase sequencing, two mutations causing AIP were identified, an adenine deletion at position 629 in exon 11(629delA), which alters the reading frame and predicts premature truncation of the enzyme protein after amino acid 255, and a nonsense mutation in exon 12 (R225X). These mutations were confirmed by either restriction enzyme analysis or family studies of symptomatic patients, permitting accurate presymptomatic diagnosis of affected relatives.

Dual porphyria of coexisting variegata and cutanea tarda

While porphyria cutanea tarda and porphyria variegata are independent diseases, we report on seven rare cases with a coincidence of these two different porphyrias in one individuum. The mutual clinical symptom was a cutaneous photosensitivity, which is a major symptom in porphyria cutanea tarda and a facultative one in porphyria variegata. Additionally, five patients had also experienced episodes of acute abdominal pain, which were in three cases accompanied by neurological symptoms, thus offering evidence for an acute hepatic porphyria, such as porphyria variegata. Determination of urinary porphyrin metabolites revealed a porphyria cutanea tarda-like excretion pattern with an elevation of uroporphyrin (mean 1134 nmol/24 h, range 563-4052, normal < or = 30) and heptacarboxyporphyrin (mean 389 nmol/24 h, range 64-830, normal < or = 4). In all patients, however, urinary coproporphyrin was also increased, reaching levels too high for porphyria cutanea tarda but typical for porphyria variegata (mean 1788 nmol/24 h, range 142-4168, normal < or = 120). Fecal porphyrin excretion also resembled the variegate-type with a high concentration especially of protoporphyrin (mean 628 nmol/g dry weight, range 401-1018, normal < or = 151), accompanied by an increase of coproporphyrin (mean 194 nmol/g dry weight, range 75-409, normal < or = 37). The urinary porphyrin precursors 5-aminolaevulinic acid and porphobilinogen were markedly elevated only in one patient, who was in an acute porphyric phase at the time of investigation. The activity of uroporphyrinogen decarboxylase in erythrocytes was considerably decreased in six of our cases (33-64%) and slightly diminished in the other one (83% of normal activity).(ABSTRACT TRUNCATED AT 250 WORDS)

The diagnosis and follow-up of porphyria

This review details an approach to the biochemical diagnosis and follow-up of porphyria. We discuss the problems of diagnosis of both symptomatic patients suspected of porphyria and patients being investigated because of a family history of porphyria. High performance liquid chromatography plays a major role in the investigation of these patients. Molecular biology is emerging as a useful tool in further defining this group of diseases.

Porphobilinogen deaminase gene structure and molecular defects

Porphobilinogen deaminase (PBGD) is the third enzyme of the heme biosynthetic pathway. The half-normal activity of human PBGD causes acute intermittent porphyria (AIP), an autosomal dominant inherited disease. Two PBGD isoforms, one ubiquitous and one erythroid specific, are encoded by a single gene localized to chromosomal region 11q24.1-11q24.2. The 10-kb PBGD gene comprises 15 exons and two distinct promoters initiate the ubiquitous and the erythroid transcripts by alternative splicing. In AIP, diagnosis of asymptomatic heterozygotes is crucial to prevent occurrence of life-threatening acute attacks by avoiding known precipitating factors. Difficulties with the biochemical diagnosis could be overcome by the ability to identify the PBGD gene defects in AIP patients. Mutational analysis of the PBGD gene or the use of intragenic polymorphisms offer accurate identification of the gene carriers. To date, 58 mutations and 10 polymorphisms have been reported at the PBGD locus. The great heterogeneity of the mutations in AIP patients requires appropriate screening and diagnostic strategies to identify gene defects in AIP families.

A modified spectrophotometric assay for porphobilinogen deaminase: its application in the detection of both carriers and patients with acute intermittent porphyria

A spectrophotometric method for porphobilinogen deaminase assay in erythrocytes is described. This test is determinant for the definite diagnosis of acute intermittent porphyria. In the method described, delta-aminolevulinic acid is used as substrate. Mercaptoethanol and zinc ions are introduced to maintain delta-aminolevulinic acid dehydratase in optimal conditions and to guarantee the in vitro production of porphobilinogen. An incubation temperature of 45 degrees C leads to the production of uroporphyrins, which are measured spectrophotometrically at 405 nm, giving reproducible results. The assay can be performed easily in any clinical laboratory and is valuable for detecting both patients and carriers of acute intermittent porphyria.

Sideroblastic anaemia

The startling morphological abnormalities of sideroblastic anaemia contrasts our uncertainty about its cause. Studies are hampered by the fact that the abnormality resides in the dividing and differentiating erythroblast which is difficult to obtain pure and in large numbers, and in which many levels of metabolic control must coexist. Recent molecular biology approaches have confirmed abnormalities of erythroid delta-aminolaevulinic acid synthase as the cause of X-linked, pyridoxine-responsive sideroblastic anaemia and mitochondrial DNA deletions as the most common cause of congenital macrocytic sideroblastic anaemia. They have also identified a second X-linked sideroblastic anaemia locus linked to phosphoglycerate kinase and associated with ataxia. An association between sideroblastic anaemia and the use of an oral copper chelating agent has highlighted unexplained links between erythroid copper and iron metabolism. Management decisions in relation to pyridoxine treatment, iron reduction, family studies, genetic counselling and antenatal diagnosis have in recent years become of practical relevance to families with known cases of congenital sideroblastic anaemia and careful documentation of the clinical outcome of these cases and of other family members is invaluable. Parallel and integrated studies on the molecular biology of erythroid differentiation are revealing the range of possible controlling influences on erythroblasts and defining the circumstances for each, allowing studies on the cause of the most prevalent form of sideroblastic anaemia (the idiopathic acquired form) and those inherited forms that are not X-linked to be approached with a much clearer perspective.

Recessive inheritance of erythropoietic protoporphyria with liver failure

Erythropoietic protoporphyria is characterised by skin photosensitivity and deficiency of ferrochelatase; fatal liver disease occurs rarely. Transmission is considered to be dominant with incomplete penetrance. We investigated a family in which two siblings with erythropoietic protoporphyria developed hepatic failure that required transplantation. Their healthy parents had partial enzyme deficiency and were each heterozygous for a distinct mutation in a ferrochelatase gene. Both offspring were compound heterozygotes with ferrochelatase deficiency. Recessive transmission of protoporphyria predisposes to severe liver disease in this family. Patients with the recessive form of this disease may be at special risk of hepatic failure.

Molecular basis of acute intermittent porphyria: mutations and polymorphisms in the human hydroxymethylbilane synthase gene

Acute intermittent porphyria (AIP) is an autosomal dominant inborn error of metabolism that results from the half-normal activity of the third enzyme in the heme biosynthetic pathway, hydroxymethylbilane synthase (HMB-synthase). AIP is an ecogenetic condition, with life-threatening acute attacks precipitated by various factors including drugs, alcohol, fasting, and certain hormones. Biochemical diagnosis is problematic and the identification of mutations in the HMB-synthase gene provides accurate detection of presymptomatic heterozygotes, permitting avoidance of the acute precipitating factors. Two HMB-synthase isozymes are encoded by the HMB-synthase gene: one unique to erythroid cells and the other a housekeeping isozyme present in all cells. These two isozymes arise from a single gene by alternative splicing. The recent isolation of the cDNAs and entire genomic sequence encoding the HMB-synthase isozymes has facilitated the detection of diagnostically useful intragenic polymorphisms and disease-causing mutations. Of the 36 mutations identified to date, most caused the classic form of AIP. These mutations included small deletions and insertions, point mutations and RNA splice junction alterations and resulted in the half-normal activity of both the erythroid-specific and housekeeping isozymes. Most AIP mutations were private; however, certain mutations were frequently found in Dutch (R116W) and Swedish (W198X) AIP families. A variant form of AIP, in which patients have normal erythroid activity, but half-normal activity of the housekeeping isozyme, resulted from two mutations at the exon 1/intron 1 boundary, each altering splicing of the hepatic-specific transcript. In addition, 10 polymorphisms in the HMB-synthase gene have been identified that are useful for the diagnosis of presymptomatic AIP heterozygotes in families whose specific mutations have not been determined.