Alteration of Extracellular Nucleotide Metabolism in Pseudoxanthoma Elasticum

Histological Findings in the Eyes of Abcc6 Knockout Rat Model of Pseudoxanthoma Elasticum

Purpose

To describe the ocular findings of murine pseudoxanthoma elasticum (PXE) models with ATP-binding cassette subfamily C member 6 (Abcc6) gene knockout.

Methods

This experiment was conducted in four Abcc6-/- rats and compared with six wild-type Abcc6+/+ control rats. The animals underwent necropsy at 6 months of age. Histological examination of the eyes was performed.

Results

Histological examination of eight eyes from four Abcc6-/- rats revealed multiple nodular foci of calcification in the uvea, sclera, and conjunctiva, focally in perivascular distribution, as well as linear and nodular calcification of Bruch's membrane. Calcific foci were not associated with inflammation in the knockout rats. There was no evidence of calcification in control eyes.

Discussion

The Abcc6-/- rat model shows that PXE can affect multiple ocular tissues beyond the calcification in Bruch's membrane noted in human eyes. Nodular calcific foci probably correspond to comet lesions seen in patients with PXE. The presence of ectopic calcium without inflammation distinguishes it from inflammatory calcium deposition in atherosclerosis. Further studies are needed to determine why PXE does not cause inflammatory infiltration.

Translational Relevance

The Abcc6-/- murine model may be suitable for studying ocular PXE pathophysiology and ectopic calcification and developing effective therapies.

Bone Marrow-Derived ABCC6 Is an Essential Regulator of Ectopic Calcification In Pseudoxanthoma Elasticum

Physiological calcification of soft tissues is a common occurrence in aging and various acquired and inherited disorders. ABCC6 sequence variations cause the calcification phenotype of pseudoxanthoma elasticum (PXE) as well as some cases of generalized arterial calcification of infancy, which is otherwise caused by defective ENPP1. ABCC6 is primarily expressed in the liver, which has given the impression that the liver is central to the pathophysiology of PXE/generalized arterial calcification of infancy. The emergence of inflammation as a contributor to the calcification in PXE suggested that peripheral tissues play a larger role than expected. In this study, we investigated whether bone marrow-derived ABCC6 contributes to the calcification in PXE. In Abcc6‒/‒ mice, we observed prevalent mineralization in several lymph nodes and surrounding connective tissues and an extensive network of lymphatic vessels within vibrissae, a calcified tissue in Abcc6‒/‒ mice. Furthermore, we found evidence of lymphangiogenesis in patients with PXE and mouse skin, suggesting an inflammatory process. Finally, restoring wild-type bone marrow in Abcc6‒/‒ mice produced a significant reduction of calcification, suggesting that the liver alone is not sufficient to fully inhibit mineralization. With evidence that ABCC6 is expressed in lymphocytes, we suggest that the adaptative immune system and inflammation largely contribute to the calcification in PXE/generalized arterial calcification of infancy.

The Purinergic Nature of Pseudoxanthoma Elasticum

Pseudoxanthoma Elasticum (PXE) is an inherited disease characterized by elastic fiber calcification in the eyes, the skin and the cardiovascular system. PXE results from mutations in ABCC6 that encodes an ABC transporter primarily expressed in the liver and kidneys. It took nearly 15 years after identifying the gene to better understand the etiology of PXE. ABCC6 function facilitates the efflux of ATP, which is sequentially hydrolyzed by the ectonucleotidases ENPP1 and CD73 into pyrophosphate (PPi) and adenosine, both inhibitors of calcification. PXE, together with General Arterial Calcification of Infancy (GACI caused by ENPP1 mutations) as well as Calcification of Joints and Arteries (CALJA caused by NT5E/CD73 mutations), forms a disease continuum with overlapping phenotypes and shares steps of the same molecular pathway. The explanation of these phenotypes place ABCC6 as an upstream regulator of a purinergic pathway (ABCC6 → ENPP1 → CD73 → TNAP) that notably inhibits mineralization by maintaining a physiological Pi/PPi ratio in connective tissues. Based on a review of the literature and our recent experimental data, we suggest that PXE (and GACI/CALJA) be considered as an authentic "purinergic disease". In this article, we recapitulate the pathobiology of PXE and review molecular and physiological data showing that, beyond PPi deficiency and ectopic calcification, PXE is associated with wide and complex alterations of purinergic systems. Finally, we speculate on the future prospects regarding purinergic signaling and other aspects of this disease.

Current Pharmacology and Modulation of the Purinergic System in Takotsubo Syndrome Triggered by Cytokine Storm

Studies show that with the COVID-19 pandemic, the world's population went through multiple stress and anxiety factors, generating serious psychological problems, in addition, the virus also caused damage and physical stress to those contaminated. In this way, the intense emotional experiences and stressful effects on the body caused by SARS-CoV-2 are capable of triggering the excessive release of catecholamines in the body. Thus, the framework of Takotsubo Syndrome is characterized by myocardial dysfunction as a response of cardiac receptors to the spillage of such hormones in an unregulated way in the human body. The purinergic system plays a central role in this process, as it actively participates in actions responsible for the syndromic cascade, such as the stress generated by the cytokine storm triggered by the virus and the stimulation of deregulated catecholamine release. Therefore, further pharmacological studies on the role of purines in this pathology should be developed in order to avoid the evolution of the syndrome and to modulate its P1 and P2 receptors aiming at developing means of reversing or treating the Takotsubo Syndrome.

Multi-ancestry genome-wide study identifies effector genes and druggable pathways for coronary artery calcification

Coronary artery calcification (CAC), a measure of subclinical atherosclerosis, predicts future symptomatic coronary artery disease (CAD). Identifying genetic risk factors for CAC may point to new therapeutic avenues for prevention. Currently, there are only four known risk loci for CAC identified from genome-wide association studies (GWAS) in the general population. Here we conducted the largest multi-ancestry GWAS meta-analysis of CAC to date, which comprised 26,909 individuals of European ancestry and 8,867 individuals of African ancestry. We identified 11 independent risk loci, of which eight were new for CAC and five had not been reported for CAD. These new CAC loci are related to bone mineralization, phosphate catabolism and hormone metabolic pathways. Several new loci harbor candidate causal genes supported by multiple lines of functional evidence and are regulators of smooth muscle cell-mediated calcification ex vivo and in vitro. Together, these findings help refine the genetic architecture of CAC and extend our understanding of the biological and potential druggable pathways underlying CAC.

Lansoprazole Increases Inorganic Pyrophosphate in Patients with Pseudoxanthoma Elasticum: A Double-Blind, Randomized, Placebo-Controlled Crossover Trial

Pseudoxanthoma elasticum (PXE) is characterized by low levels of inorganic pyrophosphate (PPi) and a high activity of tissue-nonspecific alkaline phosphatase (TNAP). Lansoprazole is a partial inhibitor of TNAP. The aim was to investigate whether lansoprazole increases plasma PPi levels in subjects with PXE. We conducted a 2 × 2 randomized, double-blind, placebo-controlled crossover trial in patients with PXE. Patients were allocated 30 mg/day of lansoprazole or a placebo in two sequences of 8 weeks. The primary outcome was the differences in plasma PPi levels between the placebo and lansoprazole phases. 29 patients were included in the study. There were eight drop-outs due to the pandemic lockdown after the first visit and one due to gastric intolerance, so twenty patients completed the trial. A generalized linear mixed model was used to evaluate the effect of lansoprazole. Overall, lansoprazole increased plasma PPi levels from 0.34 ± 0.10 µM to 0.41 ± 0.16 µM (p = 0.0302), with no statistically significant changes in TNAP activity. There were no important adverse events. 30 mg/day of lansoprazole was able to significantly increase plasma PPi in patients with PXE; despite this, the study should be replicated with a large number of participants in a multicenter trial, with a clinical end point as the primary outcome.

Inorganic Pyrophosphate Plasma Levels Are Decreased in Pseudoxanthoma Elasticum Patients and Heterozygous Carriers but Do Not Correlate with the Genotype or Phenotype

Pseudoxanthoma elasticum (PXE) is a rare ectopic calcification disorder affecting soft connective tissues that is caused by biallelic ABCC6 mutations. While the underlying pathomechanisms are incompletely understood, reduced circulatory levels of inorganic pyrophosphate (PPi)-a potent mineralization inhibitor-have been reported in PXE patients and were suggested to be useful as a disease biomarker. In this study, we explored the relation between PPi, the ABCC6 genotype and the PXE phenotype. For this, we optimized and validated a PPi measurement protocol with internal calibration that can be used in a clinical setting. An analysis of 78 PXE patients, 69 heterozygous carriers and 14 control samples revealed significant differences in the measured PPi levels between all three cohorts, although there was overlap between all groups. PXE patients had a ±50% reduction in PPi levels compared to controls. Similarly, we found a ±28% reduction in carriers. PPi levels were found to correlate with age in PXE patients and carriers, independent of the ABCC6 genotype. No correlations were found between PPi levels and the Phenodex scores. Our results suggest that other factors besides PPi are at play in ectopic mineralization, which limits the use of PPi as a predictive biomarker for severity and disease progression.

Extracellular ATP: A powerful inflammatory mediator in the central nervous system

Nucleotides play a crucial role in extracellular signaling across species boundaries. All the three kingdoms of life (Bacteria, Archea and Eukariota) are responsive to extracellular ATP (eATP) and many release this and other nucleotides. Thus, eATP fulfills different functions, many related to danger-sensing or avoidance reactions. Basically all living organisms have evolved sensors for eATP and other nucleotides with very different affinity and selectivity, thus conferring a remarkable plasticity to this signaling system. Likewise, different intracellular transduction systems were associated during evolution to different receptors for eATP. In mammalian evolution, control of intracellular ATP (iATP) and eATP homeostasis has been closely intertwined with that of Ca²⁺, whether in the extracellular milieu or in the cytoplasm, establishing an inverse reciprocal relationship, i.e. high extracellular Ca²⁺ levels are associated to negligible eATP, while low intracellular Ca²⁺ levels are associated to high eATP concentrations. This inverse relationship is crucial for the messenger functions of both molecules. Extracellular ATP is sensed by specific plasma membrane receptors of widely different affinity named P2 receptors (P2Rs) of which 17 subtypes are known. This confers a remarkable plasticity to P2R signaling. The central nervous system (CNS) is a privileged site for purinergic signaling as all brain cell types express P2Rs. Accruing evidence suggests that eATP, in addition to participating in synaptic transmission, also plays a crucial homeostatic role by fine tuning microglia, astroglia and oligodendroglia responses. Drugs modulating the eATP concentration in the CNS are likely to be the new frontier in the therapy of neuroinflammation. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

A new enzymatic assay to quantify inorganic pyrophosphate in plasma

Inorganic pyrophosphate (PPi) is a crucial extracellular mineralization regulator. Low plasma PPi concentrations underlie the soft tissue calcification present in several rare hereditary mineralization disorders as well as in more common conditions like chronic kidney disease and diabetes. Even though deregulated plasma PPi homeostasis is known to be linked to multiple human diseases, there is currently no reliable assay for its quantification. We here describe a PPi assay that employs the enzyme ATP sulfurylase to convert PPi into ATP. Generated ATP is subsequently quantified by firefly luciferase-based bioluminescence. An internal ATP standard was used to correct for sample-specific interference by matrix compounds on firefly luciferase activity. The assay was validated and shows excellent precision (< 3.5%) and accuracy (93-106%) of PPi spiked into human plasma samples. We found that of several anticoagulants tested only EDTA effectively blocked conversion of ATP into PPi in plasma after blood collection. Moreover, filtration over a 300,000-Da molecular weight cut-off membrane reduced variability of plasma PPi and removed ATP present in a membrane-enclosed compartment, possibly platelets. Applied to plasma samples of wild-type and Abcc6-/- rats, an animal model with established low circulating levels of PPi, the new assay showed lower variability than the assay that was previously in routine use in our laboratory. In conclusion, we here report a new and robust assay to determine PPi concentrations in plasma, which outperforms currently available assays because of its high sensitivity, precision, and accuracy.

Lifelong impact of ENPP1 Deficiency and the early onset form of ABCC6 Deficiency from patient or caregiver perspective

The ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) and ATP-binding cassette subfamily C member 6 (ABCC6) proteins play a prominent role in inhibiting ectopic calcification and arterial stenosis. Patients with ENPP1 Deficiency or infant onset ABCC6 Deficiency often present with pathological calcification, narrowed blood vessels, multiorgan dysfunction and high infant mortality. The heterogenous presentation and progression is well documented. Our objective was to characterize how these morbidities lead to burden of illness and poor quality of life across ages from the patient/caregiver perspective. Patients/caregivers were interviewed via phone using Institutional Review Board–approved questionnaires. Patient-reported outcomes were collected via validated instruments. Thirty-one caregivers and 7 patients participated: infant onset ABCC6 Deficiency, n = 6 (infants/children); ENPP1 Deficiency, n = 32 (13 infants, 12 children, 7 adults). ENPP1 and ABCC6-deficient children aged <8 years and aged 8–18 years reported poor school functioning (0.69 vs 0.72 effect size, respectively) and poor physical health (0.88 vs 1, respectively). In the total ENPP1 cohort, 72% (23/32) reported bone/joint pain and/or mobility/fatigue issues. Three of seven ENPP1-deficient adults reported moderate to severe pain (>4), as measured by the Brief Pain Inventory (BPI), that interfered with daily activities despite pain medication. Top reported burdens for caregivers of infants with ABCC6/ENPP1 Deficiencies included heart-related issues and hospitalizations. Treatment/medications, and hearing loss were the highest burdens reported by caregivers/families of the pediatric ENPP1 Deficiency cohort, whereas adults reported bone/joint pain and mobility impairment as the greatest burdens. Individuals with ENPP1 Deficiency or infant onset ABCC6 Deficiency experience lifelong morbidity causing substantial physical and emotional burden to patients/caregivers.

Ectopic Calcification and Hypophosphatemic Rickets: Natural History of ENPP1 and ABCC6 Deficiencies

Generalized arterial calcification of infancy (GACI) is a rare disorder caused by ENPP1 or ABCC6 variants. GACI is characterized by low pyrophosphate, arterial calcification, and high mortality during the first year of life, but the natural course and possible differences between the causative genes remain unknown. In all, 247 individual records for patients with GACI (from birth to 58.3 years of age) across 19 countries were reviewed. Overall mortality was 54.7% (13.4% in utero or stillborn), with a 50.4% probability of death before the age of 6 months (critical period). Contrary to previous publications, we found that bisphosphonate treatment had no survival benefit based on a start-time matched analysis and inconclusive results when initiated within 2 weeks of birth. Despite a similar prevalence of GACI phenotypes between ENPP1 and ABCC6 deficiencies, including arterial calcification (77.2% and 89.5%, respectively), organ calcification (65.8% and 84.2%, respectively), and cardiovascular complications (58.4% and 78.9%, respectively), mortality was higher for ENPP1 versus ABCC6 variants (40.5% versus 10.5%, respectively; p = 0.0157). Higher prevalence of rickets was reported in 70.8% of surviving affected individuals with ENPP1 compared with that of ABCC6 (11.8%; p = 0.0001). Eleven affected individuals presenting with rickets and without a GACI diagnosis, termed autosomal recessive hypophosphatemic rickets type 2 (ARHR2), all had confirmed ENPP1 variants. Approximately 70% of these patients demonstrated evidence of ectopic calcification or complications similar to those seen in individuals with GACI, which shows that ARHR2 is not a distinct condition from GACI but represents part of the spectrum of ENPP1 deficiency. Overall, this study identified an early mortality risk in GACI patients despite attempts to treat with bisphosphonates, high prevalence of rickets almost exclusive to ENPP1 deficiency, and a spectrum of heterogenous calcification and multiple organ complications with both ENPP1 and ABCC6 variants, which suggests an overlapping pathology. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR). This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Cutaneous and Vascular Deposits of 18F-NaF by PET/CT in the Follow-Up of Patients with Pseudoxanthoma Elasticum

Active microcalcification of elastic fibers is a hallmark of pseudoxanthoma elasticum and it can be measured with the assessment of deposition of 18F-NaF using a PET/CT scan at the skin and vascular levels. It is not known whether this deposition changes over time in absence of specific therapy. We repeated in two years a PET/CT scan using 18F-NaF as a radiopharmaceutical in patients with the disease and compared the deposition at skin and vessel. Furthermore, calcium score values at the vessel wall were also assessed. Main results indicate in the vessel walls that calcification progressed in each patient; by contrast, the active microcalcification, measured and target-to-background ratio showed reduced active deposition. By contrast, at skin levels (neck and axillae) the uptake of the pharmaceutical remains unchanged. In conclusion, because calcification in the arterial wall is not specific for pseudoxanthoma elasticum condition, the measurement of the deposition of 18F-NaF in the neck might be potentially used as a surrogate marker in future trials for the disease.

Pseudoxanthoma elasticum overlaps hereditary spastic paraplegia type 56

PURPOSE

Pseudoxanthoma elasticum (PXE) is a recessive disorder involving skin, eyes and arteries, mainly caused by ABCC6 pathogenic variants. However, almost one fifth of patients remain genetically unsolved despite extensive genetic screening of ABCC6, as illustrated in a large French PXE series of 220 cases. We searched for new PXE gene(s) to solve the ABCC6-negative patients.

METHODS

First, family-based exome sequencing was performed, in one ABCC6-negative PXE patient with additional neurological features, and her relatives. CYP2U1, involved in hereditary spastic paraplegia type 56 (SPG56), was selected based on this complex phenotype, and the presence of two candidate variants. Second, CYP2U1 sequencing was performed in a retrospective series of 46 additional ABCC6-negative PXE probands. Third, six additional SPG56 patients were evaluated for PXE skin and eye phenotype. Additionally, plasma pyrophosphate dosage and functional analyses were performed in some of these patients.

RESULTS

6.4% of ABCC6-negative PXE patients (n = 3) harboured biallelic pathogenic variants in CYP2U1. PXE skin lesions with histological confirmation, eye lesions including maculopathy or angioid streaks, and various neurological symptoms were present. CYP2U1 missense variants were confirmed to impair protein function. Plasma pyrophosphate levels were normal. Two SPG56 patients (33%) presented some phenotypic overlap with PXE.

CONCLUSION

CYP2U1 pathogenic variants are found in unsolved PXE patients with neurological findings, including spastic paraplegia, expanding the SPG56 phenotype and highlighting its overlap with PXE. The pathophysiology of ABCC6 and CYP2U1 should be explored to explain their respective role and potential interaction in ectopic mineralization.

ABCC6, Pyrophosphate and Ectopic Calcification: Therapeutic Solutions

Pathological (ectopic) mineralization of soft tissues occurs during aging, in several common conditions such as diabetes, hypercholesterolemia, and renal failure and in certain genetic disorders. Pseudoxanthoma elasticum (PXE), a multi-organ disease affecting dermal, ocular, and cardiovascular tissues, is a model for ectopic mineralization disorders. ABCC6 dysfunction is the primary cause of PXE, but also some cases of generalized arterial calcification of infancy (GACI). ABCC6 deficiency in mice underlies an inducible dystrophic cardiac calcification phenotype (DCC). These calcification diseases are part of a spectrum of mineralization disorders that also includes Calcification of Joints and Arteries (CALJA). Since the identification of ABCC6 as the “PXE gene” and the development of several animal models (mice, rat, and zebrafish), there has been significant progress in our understanding of the molecular genetics, the clinical phenotypes, and pathogenesis of these diseases, which share similarities with more common conditions with abnormal calcification. ABCC6 facilitates the cellular efflux of ATP, which is rapidly converted into inorganic pyrophosphate (PPi) and adenosine by the ectonucleotidases NPP1 and CD73 (NT5E). PPi is a potent endogenous inhibitor of calcification, whereas adenosine indirectly contributes to calcification inhibition by suppressing the synthesis of tissue non-specific alkaline phosphatase (TNAP). At present, therapies only exist to alleviate symptoms for both PXE and GACI; however, extensive studies have resulted in several novel approaches to treating PXE and GACI. This review seeks to summarize the role of ABCC6 in ectopic calcification in PXE and other calcification disorders, and discuss therapeutic strategies targeting various proteins in the pathway (ABCC6, NPP1, and TNAP) and direct inhibition of calcification via supplementation by various compounds.

ABCC6 deficiency promotes dyslipidemia and atherosclerosis

ABCC6 deficiency promotes ectopic calcification; however, circumstantial evidence suggested that ABCC6 may also influence atherosclerosis. The present study addressed the role of ABCC6 in atherosclerosis using Ldlr-/- mice and pseudoxanthoma elasticum (PXE) patients. Mice lacking the Abcc6 and Ldlr genes were fed an atherogenic diet for 16 weeks before intimal calcification, aortic plaque formation and lipoprotein profile were evaluated. Cholesterol efflux and the expression of several inflammation, atherosclerosis and cholesterol homeostasis-related genes were also determined in murine liver and bone marrow-derived macrophages. Furthermore, we examined plasma lipoproteins, vascular calcification, carotid intima-media thickness and atherosclerosis in a cohort of PXE patients with ABCC6 mutations and compared results to dysmetabolic subjects with increased cardiovascular risk. We found that ABCC6 deficiency causes changes in lipoproteins, with decreased HDL cholesterol in both mice and humans, and induces atherosclerosis. However, we found that the absence of ABCC6 does not influence overall vascular mineralization induced with atherosclerosis. Decreased cholesterol efflux from macrophage cells and other molecular changes such as increased pro-inflammation seen in both humans and mice are likely contributors for the phenotype. However, it is likely that other cellular and/or molecular mechanisms are involved. Our study showed a novel physiological role for ABCC6, influencing plasma lipoproteins and atherosclerosis in a haploinsufficient manner, with significant penetrance.

Therapeutics Development for Pseudoxanthoma Elasticum and Related Ectopic Mineralization Disorders: Update 2020

Pseudoxanthoma elasticum (PXE), the prototype of heritable ectopic mineralization disorders, manifests with deposition of calcium hydroxyapatite crystals in the skin, eyes and arterial blood vessels. This autosomal recessive disorder, due to mutations in ABCC6, is usually diagnosed around the second decade of life. In the spectrum of heritable ectopic mineralization disorders are also generalized arterial calcification of infancy (GACI), with extremely severe arterial calcification diagnosed by prenatal ultrasound or perinatally, and arterial calcification due to CD73 deficiency (ACDC) manifesting with arterial and juxta-articular mineralization in the elderly; the latter disorders are caused by mutations in ENPP1 and NT5E, respectively. The unifying pathomechanistic feature in these three conditions is reduced plasma levels of inorganic pyrophosphate (PPi), a powerful endogenous inhibitor of ectopic mineralization. Several on-going attempts to develop treatments for these conditions, either with the goal to normalize PPi plasma levels or by means of preventing calcium hydroxyapatite deposition independent of PPi, are in advanced preclinical levels or in early clinical trials. This overview summarizes the prospects of treatment development for ectopic mineralization disorders, with PXE, GACI and ACDC as the target diseases, from the 2020 vantage point.

Adenosine metabolism in the vascular system

The concept of extracellular purinergic signaling was first proposed by Geoffrey Burnstock in the early 1970s. Since then, extracellular ATP and its metabolites ADP and adenosine have attracted an enormous amount of attention in terms of their involvement in a wide range of immunomodulatory, thromboregulatory, angiogenic, vasoactive and other pathophysiological activities in different organs and tissues, including the vascular system. In addition to significant progress in understanding the properties of nucleotide- and adenosine-selective receptors, recent studies have begun to uncover the complexity of regulatory mechanisms governing the duration and magnitude of the purinergic signaling cascade. This knowledge has led to the development of new paradigms in understanding the entire purinome by taking into account the multitude of signaling and metabolic pathways involved in biological effects of ATP and adenosine and compartmentalization of the adenosine system. Along with the "canonical route" of ATP breakdown to adenosine via sequential ecto-nucleoside triphosphate diphosphohydrolase-1 (NTPDase1/CD39) and ecto-5'-nucleotidase/CD73 activities, it has now become clear that purine metabolism is the result of concerted effort between ATP release, its metabolism through redundant nucleotide-inactivating and counteracting ATP-regenerating ectoenzymatic pathways, as well as cellular nucleoside uptake and phosphorylation of adenosine to ATP through complex phosphotransfer reactions. In this review I provide an overview of key enzymes involved in adenosine metabolic network, with special emphasis on the emerging roles of purine-converting ectoenzymes as novel targets for cancer and vascular therapies.

From membrane to mineralization: the curious case of the ABCC6 transporter

ATP-binding cassette subfamily C member 6 gene/protein (ABCC6) is an ATP-dependent transmembrane transporter predominantly expressed in the liver and the kidney. ABCC6 first came to attention in human medicine when it was discovered in 2000 that mutations in its encoding gene, ABCC6, caused the autosomal recessive multisystemic mineralization disease pseudoxanthoma elasticum (PXE). Since then, the physiological and pathological roles of ABCC6 have been the subject of intense research. In the last 20 years, significant findings have clarified ABCC6 structure as well as its physiological role in mineralization homeostasis in humans and animal models. Yet, several facets of ABCC6 biology remain currently incompletely understood, ranging from the precise nature of its substrate(s) to the increasingly complex molecular genetics. Nonetheless, advances in our understanding of pathophysiological mechanisms causing mineralization lead to several treatment options being suggested or already tested in pilot clinical trials for ABCC6 deficiency. This review highlights current knowledge of ABCC6 and the challenges ahead, particularly the attempts to translate basic science into clinical practice.

Hereditary Disorders of Cardiovascular Calcification

Arterial calcification is a common phenomenon in the elderly, in patients with atherosclerosis or renal failure and in diabetes. However, when present in very young individuals, it is likely to be associated with an underlying hereditary disorder of arterial calcification. Here, we present an overview of the few monogenic disorders presenting with early-onset cardiovascular calcification. These disorders can be classified according to the function of the respective disease gene into (1) disorders caused by an altered purine and phosphate/pyrophosphate metabolism, (2) interferonopathies, and (3) Gaucher disease. The finding of arterial calcification in early life should alert the clinician and prompt further genetic work-up to define the underlying genetic defect, to establish the correct diagnosis, and to enable appropriate therapy.

Skin and Arterial Wall Deposits of 18F-NaF and Severity of Disease in Patients with Pseudoxanthoma Elasticum

Pseudoxanthoma elasticum (PXE) is a genetic disease characterized by the calcification of elastin fibers. Our aim was to quantify vascular calcification in the arteries and the deposition of 18F-sodium-fluoride (18F-NaF) in the skin and vessel walls with positron emission tomography/computed tomography. This was an observational study including 18 patients with PXE. Vascular calcification was measured in Agatston units, and deposition in the skin and vessel walls was shown using target-to-background ratio (TBR). Severity of the disease was scored by Phenodex. We found higher vascular calcification in the popliteal, femoral, and aortic arch vessels compared to other vascular regions; however, the uptake of radiotracer was the highest in the aorta and femoral arteries. In the skin, the highest uptake was observed in the neck and the axillae. There was no significant association between 18F-NaF deposition in the arteries or skin and the global Phenodex score. In contrast, the Phenodex score was significantly associated in univariate analyses with the averaged vascular calcium score (p < 0.01). In the neck, patients with higher skin Phenodex scores exhibited higher radiotracer uptake. As a conclusion, because vascular calcification is physiological, our data suggested that the detection of cutaneous (neck) 18F-NaF deposits might serve to monitor the calcification process in the short-term for patients with PXE.

Nonlinear optical microscopy is a novel tool for the analysis of cutaneous alterations in pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE, OMIM 264800) is a rare autosomal recessive disorder with ectopic mineralization and fragmentation of elastin fibers. It is caused by mutations of the ABCC6 gene that leads to decreased serum levels of inorganic pyrophosphate (PPi) anti-mineralization factor. The occurrence of severe complications among PXE patients highlights the importance of early diagnosis so that prompt multidisciplinary care can be provided to patients. We aimed to examine dermal connective tissue with nonlinear optical (NLO) techniques, as collagen emits second-harmonic generation (SHG) signal, while elastin can be excited by two-photon excitation fluorescence (TPF). We performed molecular genetic analysis, ophthalmological and cardiovascular assessment, plasma PPi measurement, conventional histopathological examination, and ex vivo SHG and TPF imaging in five patients with PXE and five age- and gender-matched healthy controls. Pathological mutations including one new variant were found in the ABCC6 gene in all PXE patients and their plasma PPi level was significantly lower compared with controls. Degradation and mineralization of elastin fibers and extensive calcium deposition in the mid-dermis was visualized and quantified together with the alterations of the collagen structure in PXE. Our data suggests that NLO provides high-resolution imaging of the specific histopathological features of PXE-affected skin. In vivo NLO may be a promising tool in the assessment of PXE, promoting early diagnosis and follow-up.

Plasma inorganic pyrophosphate and alkaline phosphatase in patients with pseudoxanthoma elasticum

Background

Inorganic pyrophosphate (PPi) plays a major role inhibiting dystrophic calcification. The aim was to analyze levels of PPi in patients having pseudoxanthoma elasticum (PXE), and controls as well as the enzymes who regulate the PPi plasma concentration.

Methods

We collected fasting blood samples from PXE patients and age- and sex-matched controls in ethylenediamine tetraacetic acid (EDTA) and citrate-theophylline-adenosine-dipyridamole (CTAD) containing tubes. We measured PPi, ENPP1 mass and activity, alkaline phosphatase (AP) and tissue non-specific alkaline phosphatase (TNAP), CD73 and Human Platelet Factor-4 (CXCL4).

Results

PPi in EDTA and CTAD samples were lower in PXE subjects than in controls (1.11±0.26 vs. 1.43±0.41 µM/L and 0.35±0.15 vs. 0.61±0.18 µM/L respectively, P<0.05). TNAP and liver TNAP activities were also higher in PXE than in controls (80.3±27.0 vs. 63.3±16.4 UI/L and 25.6±14.9 vs. 12.9±9.2 UI/L respectively, P<0.05). ENPP1 mass and activity as well as CD73 were almost identical. There was a weak but significant inverse correlation between TNAP activity and PPi levels (Pearson correlation -0.379, P<0.05) in both groups.

Conclusions

High TNAP activity seems to contribute to low plasma levels of PPi in subjects with PXE, reinforcing the idea that pharmacological reduction of TNAP activity may help to reduce dystrophic calcification in PXE patients.

Cellular signaling in pseudoxanthoma elasticum: an update

Pseudoxanthoma elasticum is an autosomal recessive genodermatosis with variable expression, due to mutations in the ABCC6 or ENPP1 gene. It is characterized by elastic fiber mineralization and fragmentation, resulting in skin, eye and cardiovascular symptoms. Significant advances have been made in the last 20 years with respect to the phenotypic characterization and pathophysiological mechanisms leading to elastic fiber mineralization. Nonetheless, the substrates of the ABCC6 transporter - the main cause of PXE - remain currently unknown. Though the precise mechanisms linking the ABCC6 transporter to mineralization of the extracellular matrix are unclear, several studies have looked into the cellular consequences of ABCC6 deficiency in PXE patients and/or animal models. In this paper, we compile the evidence on cellular signaling in PXE, which seems to revolve mainly around TGF-βs, BMPs and inorganic pyrophosphate signaling cascades. Where conflicting results or fragmented data are present, we address these with novel signaling data. This way, we aim to better understand the up- and down-stream signaling of TGF-βs and BMPs in PXE and we demonstrate that ANKH deficiency can be an additional mechanism contributing to decreased serum PPi levels in PXE patients.

Dietary Pyrophosphate Modulates Calcification in a Mouse Model of Pseudoxanthoma Elasticum: Implication for Treatment of Patients

Pseudoxanthoma elasticum is a heritable disease caused by ABCC6 deficiency. Patients develop ectopic calcification in skin, eyes, and vascular tissues. ABCC6, primarily found in liver and kidneys, mediates the cellular efflux of ATP, which is rapidly converted into inorganic pyrophosphate (PPi), a potent inhibitor of calcification. Pseudoxanthoma elasticum patients and Abcc6^-/- mice display reduced PPi levels in plasma and peripheral tissues. Pseudoxanthoma elasticum is currently incurable, although some palliative treatments exist. In recent years, we have successfully developed therapeutic methodologies to compensate the PPi deficit in animal models and humans. Here, we inadvertently discovered that modulating dietary PPi can also be an effective approach to reducing calcification in Abcc6^-/- mice. Our findings were prompted by a change in institutional rodent diet. The new chow was enriched in PPi, which increased plasma PPi, and significantly reduced mineralization in Abcc6^-/- mice. We also found that dietary PPi is readily absorbed in humans. Our results suggest that the consumption of food naturally or artificially enriched in PPi represents a possible intervention to mitigate calcification progression in pseudoxanthoma elasticum, that dietary preferences of patients may explain pseudoxanthoma elasticum heterogeneous manifestations, and that animal chow has the potential to influence data reproducibility.

Pseudoxanthoma Elasticum as a Paradigm of Heritable Ectopic Mineralization Disorders: Pathomechanisms and Treatment Development

Ectopic mineralization is a global problem and leading cause of morbidity and mortality. The pathomechanisms of ectopic mineralization are poorly understood. Recent studies on heritable ectopic mineralization disorders with defined gene defects have been helpful in elucidation of the mechanisms of ectopic mineralization in general. The prototype of such disorders is pseudoxanthoma elasticum (PXE), a late-onset, slowly progressing disorder with multisystem clinical manifestations. Other conditions include generalized arterial calcification of infancy (GACI), characterized by severe, early-onset mineralization of the cardiovascular system, often with early postnatal demise. In addition, arterial calcification due to CD73 deficiency (ACDC) occurs late in life, mostly affecting arteries in the lower extremities in elderly individuals. These three conditions, PXE, GACI, and ACDC, caused by mutations in ABCC6, ENPP1, and NT5E, respectively, are characterized by reduced levels of inorganic pyrophosphate (PPi) in plasma. Because PPi is a powerful antimineralization factor, it has been postulated that reduced PPi is a major determinant for ectopic mineralization in these conditions. These and related observations on complementary mechanisms of ectopic mineralization have resulted in development of potential treatment modalities for PXE, including administration of bisphosphonates, stable PPi analogs with antimineralization activity. It is conceivable that efficient treatments may soon become available for heritable ectopic mineralization disorders with application to common calcification disorders.

Inhibition of Tissue-Nonspecific Alkaline Phosphatase Attenuates Ectopic Mineralization in the Abcc6-/- Mouse Model of PXE but Not in the Enpp1 Mutant Mouse Models of GACI

Pseudoxanthoma elasticum (PXE), a prototype of heritable ectopic mineralization disorders, is caused by mutations in the ABCC6 gene encoding a putative efflux transporter ABCC6. It was recently shown that the absence of ABCC6-mediated adenosine triphosphate release from the liver and, consequently, reduced inorganic pyrophosphate levels underlie the pathogenesis of PXE. Given that tissue-nonspecific alkaline phosphatase (TNAP), encoded by ALPL, is the enzyme responsible for degrading inorganic pyrophosphate, we hypothesized that reducing TNAP levels either by genetic or pharmacological means would lead to amelioration of the ectopic mineralization phenotype in the Abcc6^-/- mouse model of PXE. Thus, we bred Abcc6^-/- mice to heterozygous Alpl^+/- mice that display approximately 50% plasma TNAP activity. The Abcc6^-/-Alpl^+/- double-mutant mice showed 52% reduction of mineralization in the muzzle skin compared with the Abcc6^-/-Alpl^+/+ mice. Subsequently, oral administration of SBI-425, a small molecule inhibitor of TNAP, resulted in 61% reduction of plasma TNAP activity and 58% reduction of mineralization in the muzzle skin of Abcc6^-/- mice. By contrast, SBI-425 treatment of Enpp1 mutant mice, another model of ectopic mineralization associated with reduced inorganic pyrophosphate, failed to reduce muzzle skin mineralization. These results suggest that inhibition of TNAP might provide a promising treatment strategy for PXE, a currently intractable disease.