Acute hepatic and erythropoietic porphyrias: from ALA synthases 1 and 2 to new molecular bases and treatments

Exploring current and emerging therapies for porphyrias

Porphyrias are rare, mostly inherited disorders resulting from altered activity of specific enzymes in the haem synthesis pathway that lead to accumulation of pathway intermediates. Photocutaneous symptoms occur when excess amounts of photoreactive porphyrins circulate in the blood to the skin, whereas increases in potentially neurotoxic porphyrin precursors are associated with neurovisceral symptoms. Current therapies are suboptimal and their mechanisms are not well established. As described here, emerging therapies address underlying disease mechanisms by introducing a gene, RNA or other specific molecule with the potential to cure or slow progression of the disease. Recent progress in nanotechnology and nanoscience, particularly regarding particle design and formulation, is expanding disease targets. More secure and efficient drug delivery systems have extended our toolbox for transferring specific molecules, especially into hepatocytes, and led to proof-of-concept studies in animal models. Repurposing existing drugs as molecular chaperones or haem synthesis inhibitors is also promising. This review summarizes key examples of these emerging therapeutic approaches and their application for hepatic and erythropoietic porphyrias.

Acute intermittent porphyria: a disease with low penetrance and high heterogeneity

Acute intermittent porphyria (AIP) is caused by mutations in the gene encoding hydroxymethylbilane synthase (HMBS), a key enzyme in the heme biosynthesis pathway. AIP is an autosomal dominant disorder characterized by low penetrance and a highly heterogenous clinical presentation. The estimated prevalence of AIP is 5-10 cases per 100,000 persons, with acute attacks manifesting in less than 1% of the at-risk population. This low frequency of attacks suggests significant roles for oligogenic inheritance and environmental factors in the pathogenesis of the disease. In recent years, identification of several modifier genes has advanced our understanding of the factors influencing AIP penetrance and disease severity. This review summarizes these factors including the impact of specific HMBS mutations, oligogenic inheritance, mitochondrial DNA copy number, age, sex, the influence of sex hormones, and the role of environmental factors. Further studies into the etiology of AIP disease penetrance should inform pathogenesis, potentially allowing for the development of more precise diagnostic and therapeutic approaches.

Update on the Porphyrias

The porphyrias are a group of rare diseases, each resulting from a defect in a different enzymatic step of the heme biosynthetic pathway. They can be broadly divided into two categories, hepatic and erythropoietic porphyrias, depending on the primary site of accumulation of heme intermediates. These disorders are multisystemic with variable symptoms that can be encountered by physicians in any specialty. Here, we review the porphyrias and describe their clinical presentation, diagnosis, and management. We discuss novel therapies that are approved or in development. Early diagnosis is key for the appropriate management and prevention of long-term complications in these rare disorders.

Recent Insights into the Pathogenesis of Acute Porphyria Attacks and Increasing Hepatic PBGD as an Etiological Treatment

Rare diseases, especially monogenic diseases, which usually affect a single target protein, have attracted growing interest in drug research by encouraging pharmaceutical companies to design and develop therapeutic products to be tested in the clinical arena. Acute intermittent porphyria (AIP) is one of these rare diseases. AIP is characterized by haploinsufficiency in the third enzyme of the heme biosynthesis pathway. Identification of the liver as the target organ and a detailed molecular characterization have enabled the development and approval of several therapies to manage this disease, such as glucose infusions, heme replenishment, and, more recently, an siRNA strategy that aims to down-regulate the key limiting enzyme of heme synthesis. Given the involvement of hepatic hemoproteins in essential metabolic functions, important questions regarding energy supply, antioxidant and detoxifying responses, and glucose homeostasis remain to be elucidated. This review reports recent insights into the pathogenesis of acute attacks and provides an update on emerging treatments aimed at increasing the activity of the deficient enzyme in the liver and restoring the physiological regulation of the pathway. While further studies are needed to optimize gene therapy vectors or large-scale production of liver-targeted PBGD proteins, effective protection of PBGD mRNA against the acute attacks has already been successfully confirmed in mice and large animals, and mRNA transfer technology is being tested in several clinical trials for metabolic diseases.

Recognized and Emerging Features of Erythropoietic and X-Linked Protoporphyria

Erythropoietic protoporphyria (EPP) and X-linked protoporphyria (XLP) are inherited disorders resulting from defects in two different enzymes of the heme biosynthetic pathway, i.e., ferrochelatase (FECH) and delta-aminolevulinic acid synthase-2 (ALAS2), respectively. The ubiquitous FECH catalyzes the insertion of iron into the protoporphyrin ring to generate the final product, heme. After hemoglobinization, FECH can utilize other metals like zinc to bind the remainder of the protoporphyrin molecules, leading to the formation of zinc protoporphyrin. Therefore, FECH deficiency in EPP limits the formation of both heme and zinc protoporphyrin molecules. The erythroid-specific ALAS2 catalyses the synthesis of delta-aminolevulinic acid (ALA), from the union of glycine and succinyl-coenzyme A, in the first step of the pathway in the erythron. In XLP, ALAS2 activity increases, resulting in the amplified formation of ALA, and iron becomes the rate-limiting factor for heme synthesis in the erythroid tissue. Both EPP and XLP lead to the systemic accumulation of protoporphyrin IX (PPIX) in blood, erythrocytes, and tissues causing the major symptom of cutaneous photosensitivity and several other less recognized signs that need to be considered. Although significant advances have been made in our understanding of EPP and XLP in recent years, a complete understanding of the factors governing the variability in clinical expression and the severity (progression) of the disease remains elusive. The present review provides an overview of both well-established facts and the latest findings regarding these rare diseases.

Porphyrias in the Age of Targeted Therapies

The porphyrias are a group of eight rare genetic disorders, each caused by the deficiency of one of the enzymes in the heme biosynthetic pathway, resulting in the excess accumulation of heme precursors and porphyrins. Depending on the tissue site as well as the chemical characteristics of the accumulating substances, the clinical features of different porphyrias vary substantially. Heme precursors are neurotoxic, and their accumulation results in acute hepatic porphyria, while porphyrins are photoactive, and excess amounts cause cutaneous porphyrias, which present with photosensitivity. These disorders are clinically heterogeneous but can result in severe clinical manifestations, long-term complications and a significantly diminished quality of life. Medical management consists mostly of the avoidance of triggering factors and symptomatic treatment. With an improved understanding of the underlying pathophysiology and disease mechanisms, new treatment approaches have become available, which address the underlying defects at a molecular or cellular level, and promise significant improvement, symptom prevention and more effective treatment of acute and chronic disease manifestations.

In silico insights into potential gut microbial modulation of NAD+ metabolism and longevity

Recent evidence has prompted the notion of gut-microbial signatures as an indirect marker of aging and aging-associated decline in humans. However, the underlying host-symbiont molecular interactions contributing to these signatures remain poorly understood. In this study, we address this gap using cheminformatic analyses to elucidate potential gut microbial metabolites that may perturb the longevity-associated NAD+ metabolic network. In silico ADMET, KEGG interaction analysis, molecular docking, molecular dynamics simulation, and molecular mechanics calculation predict a large number of safe and bioavailable microbial metabolites to be direct and/or indirect activators of NAD+-dependent sirtuin proteins. Our simulation results suggest dihydropteroate, phenylpyruvic acid, indole-3-propionic acid, phenyllactic acid, all-trans-retinoic acid, and multiple deoxy-, methyl-, and cyclic nucleotides from intestinal microbiota as the best-performing regulators of NAD+ metabolism. Retracing these molecules to their source microorganisms also suggest commensal Escherichia, Bacteroides, Bifidobacteria, and Lactobacilli to be associated with the highest number of pro-longevity metabolites. These findings from our early-stage study, therefore, provide an informatics-based context for previous evidence in the area and grant novel insights for future clinical investigation intersecting anti-aging drug discovery, probiotics, and gut microbial signatures.

Muscle atrophy induced by overexpression of ALAS2 is related to muscle mitochondrial dysfunction

BACKGROUND

ALAS2 (delta-aminolevulinate synthase 2) is one of the two isoenzymes catalyzing the synthesis of delta-aminolevulinic acid (ALA), which is the first precursor of heme synthesis. ALAS2-overexpressing transgenic mice (Tg mice) showed syndrome of porphyria, a series of diseases related to the heme anabolism deficiency. Tg mice showed an obvious decrease in muscle size. Muscle atrophy results from a decrease in protein synthesis and an increase in protein degradation, which ultimately leads to a decrease in myofiber size due to loss of contractile proteins, organelles, nuclei, and cytoplasm.

METHODS

The forelimb muscle grip strength of age-matched ALAS-2 transgenic mice (Tg mice) and wild-type mice (WT mice) were measured with an automated grip strength meter. The activities of serum LDH and CK-MB were measured by Modular DPP. The histology of skeletal muscle (quadriceps femoris and gastrocnemius) was observed by hematoxylin and eosin (HE) staining, immunohistochemistry, and transmission electron microscope. Real-time PCR was used to detect mtDNA content and UCP3 mRNA expression. Evans blue dye staining was used to detect the membrane damage of the muscle fiber. Single skeletal muscle fiber diameter was measured by single-fiber analyses. Muscle adenosine triphosphate (ATP) levels were detected by a luminometric assay with an ATP assay kit.

RESULTS

Compared with WT mice, the strength of forelimb muscle and mass of gastrocnemius were decreased in Tg mice. The activities of serum CK-MB and LDH, the number of central nuclei fibers, and Evans blue positive fibers were more than those in WT mice, while the diameter of single fibers was smaller, which were associated with suppressed expression levels of MHC, myoD1, dystrophin, atrogin1, and MuRF1. Re-expression of eMyHC was only showed in the quadriceps of Tg mice, but not in WT mice. Muscle mitochondria in Tg mice showed dysfunction with descented ATP production and mtDNA content, downregulated UCP3 mRNA expression, and swelling of mitochondria.

CONCLUSION

ALAS2 overexpressing-transgenic mice (Tg mice) showed muscle dystrophy, which was associated with decreased atrogin-1 and MuRF-1, and closely related to mitochondrial dysfunction.

Identification of novel UROS mutations in a patient with congenital erythropoietic porphyria and efficient treatment by phlebotomy

Congenital erythropoietic porphyria (CEP) is an autosomal recessive disorder of the heme biosynthetic pathway that is characterized by uroporphyrinogen III synthase (UROS) deficiency and the accumulation of non-physiological isomer I porphyrins. These phototoxic metabolites predominantly produced by the erythron result in ineffective erythropoiesis, chronic hemolysis and splenomegaly, but they also disseminate in tissues causing bullous photosensitivity to UV light and skin fragility that may progress to scarring with photo mutilation. Therapeutic management is currently limited to supportive care and bone marrow transplantation is reserved for the most severe cases. We describe here a 26-year-old women previously diagnosed with CEP harbouring two novel UROS gene mutations whose pathogenic mechanism was investigated by extensive molecular analysis. Clinical features included disabling hypertrichosis and skin photosensitivity without hemolysis. The first and rate-limiting 5-aminolevulinate synthase 2 (ALAS2) enzyme controls heme synthesis and porphyrin production in erythroid cells, while iron availability modulates its expression through a post-transcriptional mechanism. We performed iterative phlebotomies over 26 months to induce iron depletion in the patient and investigated the effectiveness and tolerance of this cost-effective approach. We observed a progressive decrease in plasma ferritin and urinary porphyrins upon treatment without inducing anemia. The patient reported improved quality of life and photosensitivity. Our data confirm recent reports highlighting the benefit of iron restriction on the disease phenotype through a reduction in porphyrin accumulation. This new strategy may represent an efficient and well-tolerated treatment for CEP patients with skin involvement and limited hematological component if iron restriction is carefully monitored.

Recent advances in the epidemiology and genetics of acute intermittent porphyria

Acute intermittent porphyria (AIP) is a dominant inherited disorder with a low penetrance that is caused by mutations in the gene coding for hydroxymethylbilane synthase (HMBS). Information about the epidemiology and molecular genetic features of this rare disorder is crucial to clinical research, and particularly to the evaluation of new treatments. Variations in the prevalence and penetrance of AIP in various studies may due to the different inclusion criteria and methods of assessment. Here, the prevalence and penetrance of AIP are analyzed systematically, and the genetic traits of different populations and findings regarding the genotype-phenotype correlation are summarized. In addition, quite a few studies have indicated that AIP susceptibility was affected by other factors, such as modifying genes. Findings regarding possible modifying genes are documented here, helping to reveal the pathogenesis of and treatments for AIP. The status of research on AIP in China reveals the lack of epidemiological and genetic studies of the Chinese population, a situation that needs to be promptly remedied.

Kidney transplantation improves the clinical outcomes of Acute Intermittent Porphyria

BACKGROUND

Acute Intermittent Porphyria (AIP) is a rare inherited autosomal dominant disorder of heme biosynthesis. Porphyria-associated kidney disease occurs in more than 50% of the patients with AIP, and end stage renal disease (ESRD) can be a devastating complication for AIP patients. The outcomes of AIP patients after kidney transplantation are poorly known.

METHODS

We examined the outcomes of 11 individuals with AIP, identified as kidney transplant recipients in the French Porphyria Center Registry.

RESULTS

AIP had been diagnosed on average 19 years before the diagnosis of ESRD except for one patient in whom the diagnosis of AIP had been made 5 years after the initiation of dialysis. Median follow-up after transplantation was 9 years. A patient died 2 months after transplantation from a cardiac arrest and a patient who received a donation after cardiac death experienced a primary non-function. No rejection episode and no noticeable adverse event occurred after transplantation. Serum creatinine was on average 117 μmol/l, and proteinuria <0.5 g/l in all patients at last follow up. All usually prescribed drugs after transplantation are authorized except for trimethoprim/sulfamethoxazole. Critically, acute porphyria attacks almost disappeared after kidney transplantation, and skin lesions resolved in all patients.

CONCLUSION

Kidney transplantation is the treatment of choice for AIP patients with ESRD and dramatically reduces the disease activity.

5-Aminolevulinate synthase catalysis: The catcher in heme biosynthesis

5-Aminolevulinate (ALA) synthase (ALAS), a homodimeric pyridoxal-5'-phosphate (PLP)-dependent enzyme, catalyzes the first step of heme biosynthesis in metazoa, fungi and α-proteobacteria. In this review, we focus on the advances made in unraveling the mechanism of the ALAS-catalyzed reaction during the past decade. The interplay between the PLP cofactor and the protein moiety determines and modulates the multi-intermediate reaction cycle of ALAS, which involves the decarboxylative condensation of two substrates, glycine and succinyl-CoA. Substrate binding and catalysis are rapid, and product (ALA) release dominates the overall ALAS kinetic mechanism. Interconversion between a catalytically incompetent, open conformation and a catalytically competent, closed conformation is linked to ALAS catalysis. Reversion to the open conformation, coincident with ALA dissociation, defines the slowest step of the reaction cycle. These findings were further substantiated by introducing seven mutations in the16-amino acid loop that gates the active site, yielding an ALAS variant with a greatly increased rate of catalytic turnover and heightened specificity constants for both substrates. Recently, molecular dynamics (MD) simulation analysis of various dimeric ALAS forms revealed that the seven active site loop mutations caused the proteins to adopt different conformations. In particular, the emergence of a β-strand in the mutated loop, which interacted with two preexisting β-strands to form an anti-parallel three-stranded β-sheet, conferred the murine heptavariant with a more stable open conformation and prompted faster product release than wild-type mALAS2. Moreover, the dynamics of the mALAS2 active site loop anti-correlated with that of the 35 amino acid C-terminal sequence. This led us to propose that this C-terminal extension, which is absent in prokaryotic ALASs, finely tunes mammalian ALAS activity. Based on the above results, we extend our previous proposal to include that discovery of a ligand inducing the mammalian C-terminal extension to fold offers a good prospect for the development of a new drug for X-linked protoporphyria and/or other porphyrias associated with enhanced ALAS activity and/or porphyrin accumulation.

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.

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.

Current and innovative emerging therapies for porphyrias with hepatic involvement

Porphyrias are rare inherited disorders caused by specific enzyme dysfunctions in the haem synthesis pathway, which result in abnormal accumulation of specific pathway intermediates. The symptoms depend upon the chemical characteristics of these substances. Porphyrins are photoreactive and cause photocutaneous lesions on sunlight-exposed areas, whereas accumulation of porphyrin precursors is related to acute neurovisceral attacks. Current therapies are suboptimal and mostly address symptoms rather than underlying disease mechanisms. Advances in the understanding of the molecular bases and pathogenesis of porphyrias have paved the way for the development of new therapeutic strategies. In this Clinical Trial Watch we summarise the basic principles of these emerging approaches and what is currently known about their application to porphyrias of hepatic origin or with hepatic involvement.

Iron Hack - A symposium/hackathon focused on porphyrias, Friedreich's ataxia, and other rare iron-related diseases

Background: Basic and clinical scientific research at the University of South Florida (USF) have intersected to support a multi-faceted approach around a common focus on rare iron-related diseases. We proposed a modified version of the National Center for Biotechnology Information’s (NCBI) Hackathon-model to take full advantage of local expertise in building “Iron Hack”, a rare disease-focused hackathon. As the collaborative, problem-solving nature of hackathons tends to attract participants of highly-diverse backgrounds, organizers facilitated a symposium on rare iron-related diseases, specifically porphyrias and Friedreich’s ataxia, pitched at general audiences.

Methods: The hackathon was structured to begin each day with presentations by expert clinicians, genetic counselors, researchers focused on molecular and cellular biology, public health/global health, genetics/genomics, computational biology, bioinformatics, biomolecular science, bioengineering, and computer science, as well as guest speakers from the American Porphyria Foundation (APF) and Friedreich’s Ataxia Research Alliance (FARA) to inform participants as to the human impact of these diseases.

Results: As a result of this hackathon, we developed resources that are relevant not only to these specific disease-models, but also to other rare diseases and general bioinformatics problems. Within two and a half days, “Iron Hack” participants successfully built collaborative projects to visualize data, build databases, improve rare disease diagnosis, and study rare-disease inheritance.

Conclusions: The purpose of this manuscript is to demonstrate the utility of a hackathon model to generate prototypes of generalizable tools for a given disease and train clinicians and data scientists to interact more effectively.

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.

Human health in relation to exposure to solar ultraviolet radiation under changing stratospheric ozone and climate

The Montreal Protocol has limited increases in the UV-B (280-315 nm) radiation reaching the Earth's surface as a result of depletion of stratospheric ozone. Nevertheless, the incidence of skin cancers continues to increase in most light-skinned populations, probably due mainly to risky sun exposure behaviour. In locations with strong sun protection programs of long duration, incidence is now reducing in younger age groups. Changes in the epidemiology of UV-induced eye diseases are less clear, due to a lack of data. Exposure to UV radiation plays a role in the development of cataracts, pterygium and possibly age-related macular degeneration; these are major causes of visual impairment world-wide. Photodermatoses and phototoxic reactions to drugs are not uncommon; management of the latter includes recognition of the risks by the prescribing physician. Exposure to UV radiation has benefits for health through the production of vitamin D in the skin and modulation of immune function. The latter has benefits for skin diseases such as psoriasis and possibly for systemic autoimmune diseases such as multiple sclerosis. The health risks of sun exposure can be mitigated through appropriate sun protection, such as clothing with both good UV-blocking characteristics and adequate skin coverage, sunglasses, shade, and sunscreen. New sunscreen preparations provide protection against a broader spectrum of solar radiation, but it is not clear that this has benefits for health. Gaps in knowledge make it difficult to derive evidence-based sun protection advice that balances the risks and benefits of sun exposure.

Characterization and origin of heme precursors in amniotic fluid: lessons from normal and pathological pregnancies

BACKGROUND

Heme is the prosthetic group of numerous proteins involved in vital processes such as oxygen transport, oxidative stress, and energetic mitochondrial metabolism. Free heme also plays a significant role at early stages of development and in cell differentiation processes. The metabolism of heme by the fetal placenta unit is not well-established in humans.

METHODS

In a retrospective study, we measured heme precursors in the amniotic fluid (AF) of 51 healthy women, and 10 AF samples from pregnancies with either upper or lower intestinal atresia or ileus were also analyzed.

RESULTS

We showed that the porphyrin precursors aminolevulinic acid, porphobilinogen, and protoporphyrin IX are present at the limit of detection in the AF. Total porphyrin levels decreased progressively from week 13 to week 33 (p < 0.01). Interestingly, uroporphyrin, initially detected as traces, increased with maturation, in contrast to coproporphyrin. Uro- and coproporphyrins were type I immature isomers (>90%), suggesting a lack of maturity in the fetal compartment of the heme pathway. Finally, the differential analysis of AF from normal and pathological pregnancies demonstrated the predominant hepatic origin of fetal porphyrins excreted in the AF.

CONCLUSION

This study gives the first insight into heme metabolism in the AF during normal and pathological pregnancies.

Anti-Correlation between the Dynamics of the Active Site Loop and C-Terminal Tail in Relation to the Homodimer Asymmetry of the Mouse Erythroid 5-Aminolevulinate Synthase

Biosynthesis of heme represents a complex process that involves multiple stages controlled by different enzymes. The first of these proteins is a pyridoxal 5′-phosphate (PLP)-dependent homodimeric enzyme, 5-aminolevulinate synthase (ALAS), that catalyzes the rate-limiting step in heme biosynthesis, the condensation of glycine with succinyl-CoA. Genetic mutations in human erythroid-specific ALAS (ALAS2) are associated with two inherited blood disorders, X-linked sideroblastic anemia (XLSA) and X-linked protoporphyria (XLPP). XLSA is caused by diminished ALAS2 activity leading to decreased ALA and heme syntheses and ultimately ineffective erythropoiesis, whereas XLPP results from “gain-of-function” ALAS2 mutations and consequent overproduction of protoporphyrin IX and increase in Zn²⁺-protoporphyrin levels. All XLPP-linked mutations affect the intrinsically disordered C-terminal tail of ALAS2. Our earlier molecular dynamics (MD) simulation-based analysis showed that the activity of ALAS2 could be regulated by the conformational flexibility of the active site loop whose structural features and dynamics could be changed due to mutations. We also revealed that the dynamic behavior of the two protomers of the ALAS2 dimer differed. However, how the structural dynamics of ALAS2 active site loop and C-terminal tail dynamics are related to each other and contribute to the homodimer asymmetry remained unanswered questions. In this study, we used bioinformatics and computational biology tools to evaluate the role(s) of the C-terminal tail dynamics in the structure and conformational dynamics of the murine ALAS2 homodimer active site loop. To assess the structural correlation between these two regions, we analyzed their structural displacements and determined their degree of correlation. Here, we report that the dynamics of ALAS2 active site loop is anti-correlated with the dynamics of the C-terminal tail and that this anti-correlation can represent a molecular basis for the functional and dynamic asymmetry of the ALAS2 homodimer.

Porphyria and kidney diseases

The kidneys, after the bone marrow and liver, are third in terms of the amounts of haem synthesized daily. Haem is incorporated into haemoproteins that are critical to renal physiology. In turn, disturbances in haem metabolism interfere with renal physiology and are tightly interrelated with kidney diseases. Acute intermittent porphyria causes kidney injury, whereas medical situations associated with end-stage renal disease, such as porphyrin accumulation, iron overload and hepatitis C, participate in the inhibition of uroporphyrinogen decarboxylase and predispose the individual to porphyria cutanea tarda. Even if some of these interactions have been known for a long time, the clinical situations associated with these interrelations have strikingly evolved over time with the advent of new therapeutic strategies for dialysis therapy and a better understanding of the pathophysiological mechanisms of porphyria-associated kidney disease. Physicians should be aware of these interactions. The aim of this review is to summarize the complex interactions between kidney physiology and pathology in the settings of porphyria and to emphasize their often-underestimated importance.