Acute hepatic porphyrias: Recommendations for diagnosis and management with real-world examples.

Acute hepatic porphyria (AHP) is a group of four rare inherited diseases, each resulting from a deficiency in a distinct enzyme in the heme biosynthetic pathway. Characterized by acute neurovisceral symptoms that may mimic other medical and psychiatric conditions, lack of recognition of the disease often leads to a delay in diagnosis and initiation of effective treatment. Biochemical testing for pathway intermediates that accumulate when the disease is active forms the basis for screening and establishing a diagnosis. Subsequent genetic analysis identifies the pathogenic variant, supporting screening of family members and genetic counseling. Management of AHP involves avoidance of known exogenous and hormonal triggers, symptomatic treatment, and prevention of recurrent attacks. Here we describe six case studies from our own real-world experience to highlight current recommendations and challenges associated with the diagnosis and long-term management of the disease.

Small interfering RNA: Discovery, pharmacology and clinical development-An introductory review.

Post-transcriptional gene silencing targets and degrades mRNA transcripts, silencing the expression of specific genes. RNA interference technology, using synthetic structurally well-defined short double-stranded RNA (small interfering RNA [siRNA]), has advanced rapidly in recent years. This introductory review describes the utility of siRNA, by exploring the underpinning biology, pharmacology, recent advances and clinical developments, alongside potential limitations and ongoing challenges. Mediated by the RNA-induced silencing complex, siRNAs bind to specific complementary mRNAs, which are subsequently degraded. siRNA therapy offers advantages over other therapeutic approaches, including ability of specifically designed siRNAs to potentially target any mRNA and improved patient adherence through infrequent administration associated with a very long duration of action. Key pharmacokinetic and pharmacodynamic challenges include targeted administration, poor tissue penetration, nuclease inactivation, rapid renal elimination, immune activation and off-target effects. These have been overcome by chemical modification of siRNA and/or by utilising a range of delivery systems, increasing bioavailability and stability to allow successful clinical translation. Patisiran (hereditary transthyretin-mediated amyloidosis) was the first licensed siRNA, followed by givosiran (acute hepatic porphyria), lumasiran (primary hyperoxaluria type 1) and inclisiran (familial hypercholesterolaemia), which all use N-acetylgalactosamine (GalNAc) linkage for effective liver-directed delivery. Others are currently under development for indications varying from rare genetic diseases to common chronic non-communicable diseases (hypertension, cancer). Technological advances are paving the way for broader clinical use. Ongoing challenges remain in targeting organs beyond the liver and reaching special sites (e.g., brain). By overcoming these barriers, siRNA therapy has the potential to substantially widen its therapeutic impact.

Prevalence of Undiagnosed Acute Hepatic Porphyria in Cyclic Vomiting Syndrome and Overlap in Clinical Symptoms.

BACKGROUND AND AIMS: Acute hepatic porphyria (AHP) presents with nausea and vomiting and can mimic cyclic vomiting syndrome (CVS). The prevalence of AHP in CVS and overlap in clinical symptomatology is not known. We thus sought to determine the prevalence of pathogenic variants for AHP and characterize symptom overlap between CVS and AHP. METHODS: We conducted a cross-sectional study of 234 CVS patients using Rome criteria. Patients were eligible for AHP genetic testing if they had recurrent episodes of severe, diffuse abdominal pain with ≥ 2 of the following-peripheral nervous system (muscle weakness/aching, numbness, tingling), central nervous system (confusion, anxiety, seizures, hallucinations), autonomic nervous system (hyponatremia, tachycardia, hypertension, constipation) symptoms, red/brownish urine, or blistering skin lesions on sun-exposed areas. A family history of AHP or elevated urinary porphobilinogen (PBG)/aminolaevulinic acid (ALA) were also criteria for genetic testing and was performed using a 4-gene panel. RESULTS: Mean age was 38.7 ± 14.5 years, 180 (76.9%) were female and 200 (85.5%) were Caucasian. During a CVS attack, 173 (92%) reported abdominal pain, 166 (87.2%) had peripheral nervous system, 164 (86.8%) had central nervous system and 173 (92) % had autonomic symptoms. Ninety-one eligible patients completed genetic testing. None were positive for AHP but two had variants of uncertain significance (VUS) in the HMBS gene. CONCLUSIONS: There is a high prevalence of non-gastrointestinal symptoms in CVS, like AHP, which is important for clinicians to recognize. AHP was not detected in this study and larger studies are warranted to ascertain its prevalence.

ALAD Inhibition by Porphobilinogen Rationalizes the Accumulation of δ-Aminolevulinate in Acute Porphyrias.

Patients with major forms of acute hepatic porphyria present acute neurological attacks with overproduction of porphobilinogen (PBG) and δ-aminolevulinic acid (ALA). Even if ALA is considered the most likely agent inducing the acute symptoms, the mechanism of its accumulation has not been experimentally demonstrated. In the most frequent form, acute intermittent porphyria (AIP), inherited gene mutations induce a deficiency in PBG deaminase; thus, accumulation of the substrate PBG is biochemically obligated but not that of ALA. A similar scenario is observed in other forms of acute hepatic porphyria (i.e., porphyria variegate, VP) in which PBG deaminase is inhibited by metabolic intermediates. Here, we have investigated the molecular basis of δ-aminolevulinate accumulation using in vitro fluxomics monitored by NMR spectroscopy and other biophysical techniques. Our results show that porphobilinogen, the natural product of δ-aminolevulinate deaminase, effectively inhibits its anabolic enzyme at abnormally low concentrations. Structurally, this high affinity can be explained by the interactions that porphobilinogen generates with the active site, most of them shared with the substrate. Enzymatically, our flux analysis of an altered heme pathway demonstrates that a minimum accumulation of porphobilinogen will immediately trigger the accumulation of δ-aminolevulinate, a long-lasting observation in patients suffering from acute porphyrias.

Spotlight on Givosiran as a Treatment Option for Adults with Acute Hepatic Porphyria: Design, Development, and Place in Therapy.

Small interfering ribonucleic acids [siRNAs] are short ribonucleic acid (RNA) fragments cleaved from double-stranded RNA molecules that target and bind to specific sequences on messenger RNA (mRNA), leading to their destruction. Therefore, the siRNA down-regulates the formation of selected mRNAs and their protein products. Givosiran is one such siRNA that uses this mechanism to treat acute hepatic porphyrias. Acute hepatic porphyrias are a group of rare, inherited metabolic disorders, characterized by acute potentially life-threatening attacks as well as chronic symptoms with a negative impact on quality of life. It has four types, each associated with distinct enzyme defects in the heme biosynthesis pathway in the liver. By targeting the expression of hepatic 5-aminolevulinic acid [ALA] synthase-1 [ALAS1], givosiran can down-regulate levels of toxic metabolites, leading to biochemical and clinical improvement. Givosiran selectively targets hepatocytes due to its linkage to N-acetylgalactosamine (GalNac) leading to its selective uptake via asialoglycoprotein receptors (ASGPR). We provide an up-to-date literature review regarding givosiran in the context of a clinical overview of the porphyrias, an overview of siRNAs for therapy of human disorders, the design and development of givosiran, key clinical trial results of givosiran for prevention of acute porphyric attacks, emerging concerns regarding chronic use of givosiran, and the overall management of acute hepatic porphyrias. These insights are important not only for the management of acute hepatic porphyrias but also for the emerging field of siRNAs and their role in novel therapies for various diseases.

Givosiran for the treatment of acute hepatic porphyria.

INTRODUCTION: Acute hepatic porphyrias (AHPs) are a family of rare inherited disorders characterized by enzyme dysfunctions in the hepatic pathway of heme biosynthesis. In AHPs, accumulation of the neurotoxic porphyrin precursors delta-aminolevulinic acid and porphobilinogen, caused by enhanced activity of hepatic aminolevulinate synthase 1 (ALAS1), is associated with acute, potentially life-threatening neurovisceral attacks. Symptoms during and between attacks dramatically reduce patients' quality of life (QoL). Givosiran is the first mRNA-targeted treatment for AHPs, silencing ALAS1 expression. AREAS COVERED: For givosiran, this review summarizes its chemistry, mechanism of action, pharmacokinetics, pharmacodynamics, safety, preclinical and clinical data in AHP, postmarketing surveillance, and regulatory status. A literature search of public and internal databases was performed, bibliographies of retrieved articles were manually searched to identify additional studies of relevance, and information was also provided by Alnylam Pharmaceuticals. EXPERT OPINION: Givosiran is a small interfering RNA (siRNA) therapeutic that reduces hepatic activity of ALAS1 and decreases accumulation of neurotoxic porphyrin precursors in patients with AHPs, ultimately reducing the number of acute attacks and improving symptoms and QoL between attacks. As AHPs are lifelong diseases, long-term safety data are needed for givosiran as an siRNA-based therapy.

ABCB6 polymorphisms are not overly represented in patients with porphyria.

The Mendelian inheritance pattern of acute intermittent porphyria, hereditary coproporphyria, and variegate porphyria is autosomal dominant, but the clinical phenotype is heterogeneous. Within the general population, penetrance is low, but among first-degree relatives of a symptomatic proband, penetrance is higher. These observations suggest that genetic factors, in addition to mutation of the specific enzyme of the biosynthetic pathway of heme, contribute to the clinical phenotype. Recent studies by others suggested that the genotype of the transporter protein ABCB6 contribute to the porphyria phenotype. Identifying the molecule(s) that are transported by ABCB6 has been problematic and has led to uncertainty with respect to how or if variants/mutants contribute to phenotypic heterogeneity. Knockout mouse models of Abcb6 have not provided a direction for investigation as homozygous knockout animals do not have a discrete phenotype. To address the proposed link between ABC6 genotype and porphyria phenotype, a large cohort of patients with acute hepatic porphyria and erythropoietic protoporphyria was analyzed. Our studies showed that ABCB6 genotype did not correlate with disease severity. Therefore, genotyping of ABCB6 in patients with acute hepatic porphyria and erythropoietic protoporphyria is not warranted.

Clinical, biochemical, and genetic characterization of acute hepatic porphyrias in a cohort of Argentine patients.

BACKGROUND: Acute Hepatic Porphyrias (AHPs) are characterized by an acute neuroabdominal syndrome including both neuropsychiatric symptoms and neurodegenerative changes. Two main hypotheses explain the pathogenesis of nervous system dysfunction: (a) the ROS generation by autooxidation of 5-aminolevulinic acid accumulated in liver and brain; (b) liver heme deficiency and in neural tissues that generate an oxidative status, a component of the neurodegenerative process. METHODS: We review results obtained from Acute Intermittent Porphyria (AIP) and Variegate Porphyria (VP) families studied at clinical, biochemical, and molecular level at the CIPYP in Argentina. The relationship between the porphyric attack and oxidative stress was also evaluated in AHP patients and controls, to identify a marker of neurological dysfunction. RESULTS: We studied 116 AIP families and 30 VP families, 609 and 132 individuals, respectively. Genotype/phenotype relation was studied. Oxidative stress parameters and plasma homocysteine levels were measured in 20 healthy volunteers, 22 AIP and 12 VP individuals. CONCLUSION: No significant difference in oxidative stress parameters and homocysteine levels between the analyzed groups were found.

Givosiran to treat acute porphyria.

Porphyrias are a family of rare diseases chiefly due to inborn errors of heme biosynthesis. The porphyrias are generally characterized either by the main site of overproduction of heme precursors (hepatic or erythropoietic) or the main clinical manifestations (acute or cutaneous). The regulation of 5- (or δ)-aminolevulinic acid synthase 1 (ALAS1) plays a key role in the pathway of normal hepatic heme synthesis, providing insight into the pathophysiologic mechanisms and potential therapeutic targets for the treatment of the porphyrias. Givosiran (Givlaari; Alnylam Pharmaceuticals) is an ALAS1-directed small interfering RNA (siRNA) which has been developed for the treatment of acute hepatic porphyria (AHP). It was first approved in 2019 by the U.S. Food and Drug Administration (FDA) for the treatment of adult patients with AHP, and it received also approval in the E.U. in 2020 for the treatment of AHP in adults and adolescents aged 12 years and older.

5-Aminolevulinate dehydratase porphyria: Update on hepatic 5-aminolevulinic acid synthase induction and long-term response to hemin.

BACKGROUND: 5-Aminolevulinic acid dehydratase (ALAD) porphyria (ADP) is an ultrarare autosomal recessive disease, with only eight documented cases, all of whom were males. Although classified as an acute hepatic porphyria (AHP), induction of the rate limiting hepatic enzyme 5-aminolevulinic acid synthase-1 (ALAS1) has not been demonstrated, and the marrow may also contribute excess 5-aminolevulinic acid (ALA). Two patients have died and reported follow up for the others is limited, so the natural history of this disease is poorly understood and treatment experience limited. METHODS: We report new molecular findings and update the clinical course and treatment of the sixth reported ADP patient, now 31 years old and the only known case in the Americas, and review published data regarding genotype-phenotype correlation and treatment. RESULTS: Circulating hepatic 5-aminolevulinic acid synthase-1 (ALAS1) mRNA was elevated in this case, as in other AHPs. Gain of function mutation of erythroid specific ALAS2 - an X-linked modifying gene in some other porphyrias - was not found. Seven reported ADP cases had compound heterozygous ALAD mutations resulting in very low residual ALAD activity and symptoms early in life or adolescence. One adult with a germline ALAD mutant allele developed ADP in association with a clonal myeloproliferative disorder, polycythemia vera. CONCLUSIONS: Elevation in circulating hepatic ALAS1 and response to treatment with hemin indicate that the liver is an important source of excess ALA in ADP, although the marrow may also contribute. Intravenous hemin was effective in most reported cases for treatment and prevention of acute attacks of neurological symptoms.

Use of Generalized Additive Model to Detect the Threshold of δ-Aminolevulinic Acid Dehydratase Activity Reduced by Lead Exposure.

BACKGROUND: Lead inhibits the enzymes in heme biosynthesis, mainly reducing δ-aminolevulinic acid dehydratase (ALAD) activity, which could be an available biomarker. The aim of this study was to detect the threshold of δ-aminolevulinic acid dehydratase activity reduced by lead exposure. METHODS: We collected data on 121 lead workers and 117 non-exposed workers when annual health examinations were performed. ALAD activity was determined by the standardized method of the European Community. ALAD G177C (rs1800435) genotyping was conducted using the polymerase chain reaction and restricted fragment length polymorphism (PCR-RFLP) method. In order to find a threshold effect, we used generalized additive models (GAMs) and scatter plots with smoothing curves, in addition to multiple regression methods. RESULTS: There were 229 ALAD1-1 homozygotes and 9 ALAD1-2 heterozygotes identified, and no ALAD2-2 homozygotes. Lead workers had significantly lower ALAD activity than non-exposed workers (41.6 ± 22.1 vs. 63.3 ± 14.0 U/L, p < 0.001). The results of multiple regressions showed that the blood lead level (BLL) was an important factor inversely associated with ALAD activity. The possible threshold of BLL affecting ALAD activity was around 5 µg/dL. CONCLUSIONS: ALAD activity was inhibited by blood lead at a possible threshold of 5 µg/dL, which suggests that ALAD activity could be used as an indicator for lead exposure regulation.

Hypothesis: Metabolic targeting of 5-aminolevulinate synthase by tryptophan and inhibitors of heme utilisation by tryptophan 2,3-dioxygenase as potential therapies of acute hepatic porphyrias.

Metabolic targeting of liver 5-aminolevulinate synthase (5-ALAS) by inhibition of heme utilisation by tryptophan (Trp) 2,3-dioxygenase (TDO) or the use of tryptophan is proposed as a therapy of acute hepatic porphyrias. 5-ALAS, the rate-limiting enzyme of heme biosynthesis, is under negative feedback control by a small regulatory heme pool in the hepatic cytosol. Acute porphyric attacks, precipitated by fasting, certain hormones and some drugs, involve induction of 5-ALAS secondarily to depletion of the above pool, and the resultant elevation of 5-ALA levels initiates the abdominal and neurological symptoms of attacks. By utilising the regulatory heme, cytosolic TDO undermines the feedback control, thus allowing 5-ALAS induction to occur, e.g. upon glucocorticoid induction of TDO during fasting (starvation) and exogenous glucocorticoid administration. Currently, glucose therapy is the preferred strategy for reversing moderate attacks induced by fasting (calorie restriction), with more severe attacks being treated by intravenous heme preparations. Reversal of fasting-induced attacks by glucose is explained by the previously demonstrated reversal of increased heme utilisation by TDO. Inhibitors of this utilisation are therefore potential therapeutic targets in acute attacks and also for maintenance of a symptomless state. Existing TDO inhibitors other than glucose include allopurinol, nicotinamide and recently developed potent inhibitors such as LM10 used in cancer therapy. Based on studies in rats, the hypothesis predicts that the safety or otherwise of drugs in the hepatic porphyrias is determined by their ability to inhibit TDO utilisation of heme under basal conditions or after glucocorticoid induction or heme activation of TDO, in parallel with reciprocal changes in 5-ALAS induction. Tryptophan is also proposed as a potential therapy of acute attacks either alone or as an adjunct to the recently proposed 5-ALAS1 gene silencing. Trp increases heme biosynthesis by enhancing 5-ALA dehydratase activity and, based on a Trp-5-ALA model presented herein, Trp offers several advantages over heme therapy, namely rapid conversion of 5-ALA into heme, a greatly enhanced heme availability, a near complete inhibition of 5-ALAS induction, assumed rapid clearance of 5-ALA and hence accelerated resolution of symptoms of attacks, and finally provision of the neuroprotective metabolite kynurenic acid to neutralise the neurological symptoms. The hypothesis also addresses heme regulation in species lacking the TDO free apoenzyme and its glucocorticoid induction mechanism and proposes detailed assessment of heme biosynthesis in these species. Detailed proposals for testing the hypothesis are presented.

Acute hepatic porphyrias: Current diagnosis & management.

Each of the four acute hepatic porphyrias is due to mutation of an enzyme in the heme biosynthetic pathway. The accumulation of pathway intermediates that occur most notably when these diseases are active is the basis for screening and establishing a biochemical diagnosis of these rare disorders. Measurement of enzyme activities and especially DNA testing also are important for diagnosis. Suspicion of the diagnosis and specific testing, particularly measurement of urinary porphobilinogen, are often delayed because the symptoms are nonspecific, even when severe. Urinary porphyrins are also measured, but their elevation is much less specific. If porphobilinogen is elevated, second line testing will establish the type of acute porphyria. DNA testing identifies the familial mutation and enables screening of family members. Management includes removal of triggering factors whenever possible. Intravenous hemin is the most effective treatment for acute attacks. Carbohydrate loading is sometimes used for mild attacks. Cyclic attacks, if frequent, can be prevented by a GnRH analogue. Frequent noncyclic attacks are sometime preventable by scheduled (e.g. weekly) hemin infusions. Long term complications may include chronic pain, renal impairment and liver cancer. Other treatments, including RNA interference, are under development.

A next-generation-sequencing panel for mutational analysis of dominant acute hepatic porphyrias.

Molecular diagnosis of autosomal dominant acute hepatic porphyrias (AHPs) plays an important role in the management of these disorders. To introduce next generation sequencing (NGS) to the porphyria diagnosis, we designed a panel that contained four genes, ALAS1, HMBS, CPOX and PPOX for mutational analysis of acute intermittent porphyria (AIP), hereditary coproporphyria (HCP) and variegate porphyria (VP). To validate the AHP panel, 30 samples with known pathogenic variants as determined by Sanger sequencing, were analyzed using the Ion PGM™. Among them, nine have so far not been reported. The pathogenic variants were identified and annotated manually in IGV by three individuals who were blinded to the Sanger results. The AHP panel consists of 95 amplicons that covers 92% of the coding region of the four genes. Of the 95 amplicons, 93 had an average read-depth of >500 reads. In 29 of the 30 tested samples, pathogenic variants were correctly identified and annotated. The number of reads from the mutated alleles were approximately 50% of the total. The annotation of a 22-bp duplication with NGS differed from that of Sanger by one nucleotide. NGS showed an advantage in allelic discrimination over Sanger sequencing and was also able to detect a known somatic variant in the HMBS gene. The AHP panel will be applied in the initial diagnosis of new patients. Any sequence variations with a frequency of ≥10% will be confirmed by Sanger sequencing. The cost-effectiveness of a NGS approach for AHP in a diagnostic laboratory needs to be further assessed.

International Porphyria Molecular Diagnostic Collaborative: an evidence-based database of verified pathogenic and benign variants for the porphyrias.

With the advent of precision and genomic medicine, a critical issue is whether a disease gene variant is pathogenic or benign. Such is the case for the three autosomal dominant acute hepatic porphyrias (AHPs), including acute intermittent porphyria, hereditary coproporphyria, and variegate porphyria, each resulting from the half-normal enzymatic activities of hydroxymethylbilane synthase, coproporphyrinogen oxidase, and protoporphyrinogen oxidase, respectively. To date, there is no public database that documents the likely pathogenicity of variants causing the porphyrias, and more specifically, the AHPs with biochemically and clinically verified information. Therefore, an international collaborative with the European Porphyria Network and the National Institutes of Health/National Center for Advancing Translational Sciences/National Institute of Diabetes and Digestive and Kidney Diseases (NIH/NCATS/NIDDK)-sponsored Porphyrias Consortium of porphyria diagnostic experts is establishing an online database that will collate biochemical and clinical evidence verifying the pathogenicity of the published and newly identified variants in the AHP-causing genes. The overall goal of the International Porphyria Molecular Diagnostic Collaborative is to determine the pathogenic and benign variants for all eight porphyrias. Here we describe the overall objectives and the initial efforts to validate pathogenic and benign variants in the respective heme biosynthetic genes causing the AHPs.

Pathogenesis and clinical features of the acute hepatic porphyrias (AHPs).

The acute hepatic porphyrias include four disorders: acute intermittent porphyria [AIP], hereditary coproporphyria [HCP], variegate porphyria [VP], and the rare porphyria due to severe deficiency of ALA dehydratase [ADP]. In the USA, AIP is the most severe and most often symptomatic. AIP, HCP, and VP are due to autosomal dominant genetic abnormalities, in which missense, nonsense, or other mutations of genes of normal hepatic heme biosynthesis, in concert with other environmental, nutritional, hormonal and genetic factors, may lead to a critical deficiency of heme, the end-product of the pathway, in a small but critical 'regulatory pool' within hepatocytes. This deficiency leads to de-repression of the first and normally rate-controlling enzyme of the heme synthetic pathway, delta- or 5-aminolevulinic acid [ALA] synthase-1, and thus to marked up-regulation of this key enzyme and to marked hepatic overproduction of ALA. In addition, except for ADP, there is marked overproduction as well of porphobilinogen [PBG], the intermediate immediately downstream of ALA in the synthetic chain, and, especially in HCP and VP, also porphyrinogens and porphyrins farther down the pathway. The major clinical features of the acute porphyrias are attacks of severe neuropathic-type pain. Pain is felt first and foremost in the abdomen but may also occur in the back, chest, and extremities. Attacks are more common in women than in men [ratio of about 4:1], often accompanied by nausea, vomiting, constipation, tachycardia, and arterial hypertension. Hyponatremia may also occur. Some patients also describe chronic symptoms of pain, anxiety, insomnia, and others.

Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management.

Erythropoietic Protoporphyria (EPP) and X-linked Protoporphyria (XLP) are rare, genetic photodermatoses resulting from defects in enzymes of the heme-biosynthetic pathway. EPP results from the partial deficiency of ferrochelatase, and XLP results from gain-of-function mutations in erythroid specific ALAS2. Both disorders result in the accumulation of erythrocyte protoporphyrin, which is released in the plasma and taken up by the liver and vascular endothelium. The accumulated protoporphyrin is activated by sunlight exposure, generating singlet oxygen radical reactions leading to tissue damage and excruciating pain. About 2-5% of patients develop clinically significant liver dysfunction due to protoporphyrin deposition in bile and/or hepatocytes which can advance to cholestatic liver failure requiring transplantation. Clinically these patients present with acute, severe, non-blistering phototoxicity within minutes of sun-exposure. Anemia is seen in about 47% of patients and about 27% of patients will develop abnormal serum aminotransferases. The diagnosis of EPP and XLP is made by detection of markedly increased erythrocyte protoporphyrin levels with a predominance of metal-free protoporphyrin. Genetic testing by sequencing the FECH or ALAS2 gene confirms the diagnosis. Treatment is limited to sun-protection and there are no currently available FDA-approved therapies for these disorders. Afamelanotide, a synthetic analogue of α-melanocyte stimulating hormone was found to increase pain-free sun exposure and improve quality of life in adults with EPP. It has been approved for use in the European Union since 2014 and is not available in the U.S. In addition to the development of effective therapeutics, future studies are needed to establish the role of iron and the risks related to the development of hepatopathy in these patients.

Murine models of the human porphyrias: Contributions toward understanding disease pathogenesis and the development of new therapies.

Mouse models of the human porphyrias have proven useful for investigations of disease pathogenesis and to facilitate the development of new therapeutic approaches. To date, mouse models have been generated for all major porphyrias, with the exception of X-linked protoporphyria (XLP) and the ultra rare 5-aminolevulinic acid dehydratase deficient porphyria (ADP). Mouse models have been generated for the three autosomal dominant acute hepatic porphyrias, acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), and variegate porphyria (VP). The AIP mice, in particular, provide a useful investigative model as they have been shown to have acute biochemical attacks when induced with the prototypic porphyrinogenic drug, phenobarbital. In addition to providing important insights into the disease pathogenesis of the neurological impairment in AIP, these mice have been valuable for preclinical evaluation of liver-targeted gene therapy and RNAi-mediated approaches. Mice with severe HMBS deficiency, which clinically and biochemically mimic the early-onset homozygous dominant AIP (HD-AIP) patients, have been generated and were used to elucidate the striking phenotypic differences between AIP and HD-AIP. Mice modeling the hepatocutaneous porphyria, porphyria cutanea tarda (PCT), made possible the identification of the iron-dependent inhibitory mechanism of uroporphyrinogen decarboxylase (UROD) that leads to symptomatic PCT. Mouse models for the two autosomal recessive erythropoietic porphyrias, congenital erythropoietic porphyria (CEP) and erythropoeitic protoporphyria (EPP), recapitulate many of the clinical and biochemical features of the severe human diseases and have been particularly useful for evaluation of bone marrow transplantation and hematopoietic stem cell (HSC)-based gene therapy approaches. The EPP mice have also provided valuable insights into the underlying pathogenesis of EPP-induced liver damage and anemia.

Recent advances on porphyria genetics: Inheritance, penetrance & molecular heterogeneity, including new modifying/causative genes.

The inborn errors of heme biosynthesis, the Porphyrias, include eight major disorders resulting from loss-of-function (LOF) or gain-of-function (GOF) mutations in eight of the nine heme biosynthetic genes. The major sites of heme biosynthesis are the liver and erythron, and the underlying pathophysiology of each of these disorders depends on the unique biochemistry, cell biology, and genetic mechanisms in these tissues. The porphyrias are classified into three major categories: 1) the acute hepatic porphyrias (AHPs), including Acute Intermittent Porphyria (AIP), Hereditary Coproporphyria (HCP), Variegate Porphyria (VP), and 5-Aminolevlulinic Acid Dehydratase Deficient Porphyria (ADP); 2) a hepatic cutaneous porphyria, Porphyria Cutanea Tarda (PCT); and 3) the cutaneous erythropoietic porphyrias, Congenital Erythropoietic Porphyria (CEP), Erythropoietic Protoporphyria (EPP), and X-Linked Protoporphyria (XLP). Their modes of inheritance include autosomal dominant with markedly decreased penetrance (AIP, VP, and HCP), autosomal recessive (ADP, CEP, and EPP), or X-linked (XLP), as well as an acquired sporadic form (PCT). There are severe homozygous dominant forms of the three AHPs. For each porphyria, its phenotype, inheritance pattern, unique genetic principles, and molecular genetic heterogeneity are presented. To date, >1000 mutations in the heme biosynthetic genes causing their respective porphyrias have been reported, including low expression alleles and genotype/phenotype correlations that predict severity for certain porphyrias. The tissue-specific regulation of heme biosynthesis and the unique genetic mechanisms for each porphyria are highlighted.

Diffuse 18F-FDG Avidity in Liver Associated With X-Linked Protoporphyria on PET/CT.

An F-FDG PET/CT was performed on a 43-year-old woman with photosensitive skin rash, abnormal liver function, and pancytopenia, which demonstrated prominent hepatomegaly, splenomegaly, and diffuse liver F-FDG avidity. The liver biopsy revealed intrahepatic cholestasis with biliary fibrosis resulting from the deposition of protoporphyrin. X-linked erythroid-specific 5-aminolevulinate synthase gene analysis proved the diagnosis of X-linked protoporphyria.