Restricting dietary magnesium accelerates ectopic connective tissue mineralization in a mouse model of pseudoxanthoma elasticum (Abcc6(-/-) )

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.

The Abcc6a Knockout Zebrafish Model as a Novel Tool for Drug Screening for Pseudoxanthoma Elasticum

Pseudoxanthoma elasticum (PXE) is a multisystem ectopic mineralization disorder caused by pathogenic variants in the ABCC6 gene. Though complications of the disease can be treated, PXE itself remains currently intractable. A strategy for rapid and cost-effective discovery of therapeutic drugs would be to perform chemical compound screening using zebrafish, but this approach remains to be validated for PXE. In this paper, we validate a stable CRISPR/Cas9 abcc6a knockout zebrafish model–which has spinal column hypermineralization as its primary phenotypic feature–as a model system for compound screening in ectopic mineralization. We evaluated the anti-mineralization potential of five compounds, which had (anecdotal) positive effects reported in Abcc6 knockout mice and/or PXE patients. Abcc6a knockout zebrafish larvae were treated from 3 to 10 days post-fertilization with vitamin K1, sodium thiosulfate, etidronate, alendronate or magnesium citrate and compared to matching controls. Following alizarin red S staining, alterations in notochord sheath mineralization were semiquantified and found to largely congrue with the originally reported outcomes. Our results demonstrate that the use of this abcc6a knockout zebrafish model is a validated and promising strategy for drug discovery against ectopic mineralization.

Abcc6 Null Mice-a Model for Mineralization Disorder PXE Shows Vertebral Osteopenia Without Enhanced Intervertebral Disc Calcification With Aging

Chronic low back pain is a highly prevalent health condition intricately linked to intervertebral disc degeneration. One of the prominent features of disc degeneration that is commonly observed with aging is dystrophic calcification. ATP-binding cassette sub-family C member 6 (ABCC6), a presumed ATP efflux transporter, is a key regulator of systemic levels of the mineralization inhibitor pyrophosphate (PPi). Mutations in ABCC6 result in pseudoxanthoma elasticum (PXE), a progressive human metabolic disorder characterized by mineralization of the skin and elastic tissues. The implications of ABCC6 loss-of-function on pathological mineralization of structures in the spine, however, are unknown. Using the Abcc6 -/- mouse model of PXE, we investigated age-dependent changes in the vertebral bone and intervertebral disc. Abcc6 -/- mice exhibited diminished trabecular bone quality parameters at 7 months, which remained significantly lower than the wild-type mice at 18 months of age. Abcc6 -/- vertebrae showed increased TRAP staining along with decreased TNAP staining, suggesting an enhanced bone resorption as well as decreased bone formation. Surprisingly, however, loss of ABCC6 resulted only in a mild, aging disc phenotype without evidence of dystrophic mineralization. Finally, we tested the utility of oral K3Citrate to treat the vertebral phenotype since it is shown to regulate hydroxyapatite mechanical behavior. The treatment resulted in inhibition of the osteoclastic response and an early improvement in mechanical properties of the bone underscoring the promise of potassium citrate as a therapeutic agent. Our data suggest that although ectopic mineralization is tightly regulated in the disc, loss of ABCC6 compromises vertebral bone quality and dysregulates osteoblast-osteoclast coupling.

Therapy of Pseudoxanthoma Elasticum: Current Knowledge and Future Perspectives

Pseudoxanthoma elasticum (PXE) is a rare, genetic, metabolic disease with an estimated prevalence of between 1 per 25,000 and 56,000. Its main hallmarks are characteristic skin lesions, development of choroidal neovascularization, and early-onset arterial calcification accompanied by a severe reduction in quality-of-life. Underlying the pathology are recessively transmitted pathogenic variants of the ABCC6 gene, which results in a deficiency of ABCC6 protein. This results in reduced levels of peripheral pyrophosphate, a strong inhibitor of peripheral calcification, but also dysregulation of blood lipids. Although various treatment options have emerged during the last 20 years, many are either already outdated or not yet ready to be applied generally. Clinical physicians often are left stranded while patients suffer from the consequences of outdated therapies, or feel unrecognized by their attending doctors who may feel uncertain about using new therapeutic approaches or not even know about them. In this review, we summarize the broad spectrum of treatment options for PXE, focusing on currently available clinical options, the latest research and development, and future perspectives.

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.

Phylogeny and chemistry of biological mineral transport

Three physiologically mineralizing tissues - teeth, cartilage and bone - have critical common elements and important evolutionary relationships. Phylogenetically the most ancient densely mineralized tissue is teeth. In jawless fishes without skeletons, tooth formation included epithelial transport of phosphates, a process echoed later in bone physiology. Cartilage and mineralized cartilage are skeletal elements separate from bone, but with metabolic features common to bone. Cartilage mineralization is coordinated with high expression of tissue nonspecific alkaline phosphatase and PHOSPHO1 to harvest available phosphate esters and support mineralization of collagen secreted locally. Mineralization in true bone results from stochastic nucleation of hydroxyapatite crystals within the cross-linked collagen fibrils. Mineral accumulation in dense collagen is, at least in major part, mediated by amorphous aggregates - often called Posner clusters - of calcium and phosphate that are small enough to diffuse into collagen fibrils. Mineral accumulation in membrane vesicles is widely suggested, but does not correlate with a definitive stage of mineralization. Conversely mineral deposition at non-physiologic sites where calcium and phosphate are adequate has been shown to be regulated in large part by pyrophosphate. All of these elements are present in vertebrate bone metabolism. A key biological element of bone formation is an epithelial-like cellular organization which allows control of phosphate, calcium and pH during mineralization.

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.

The biology of vascular calcification

Vascular calcification (VC), characterized by different mineral deposits (i.e., carbonate apatite, whitlockite and hydroxyapatite) accumulating in blood vessels and valves, represents a relevant pathological process for the aging population and a life-threatening complication in acquired and in genetic diseases. Similarly to bone remodeling, VC is an actively regulated process in which many cells and molecules play a pivotal role. This review aims at: (i) describing the role of resident and circulating cells, of the extracellular environment and of positive and negative factors in driving the mineralization process; (ii) detailing the types of VC (i.e., intimal, medial and cardiac valve calcification); (iii) analyzing rare genetic diseases underlining the importance of altered pyrophosphate-dependent regulatory mechanisms; (iv) providing therapeutic options and perspectives.

Extraskeletal Calcifications in Hutchinson-Gilford Progeria Syndrome

PURPOSE

Children with Hutchinson-Gilford progeria syndrome (HGPS), a rare premature aging disease, exhibit extraskeletal calcifications detected by radiographic analysis and on physical examination. The aim of this study was to describe the natural history and pathophysiology of these abnormal calcifications in HGPS, and to determine whether medications and/or supplements tested in clinical trials alter their development.

METHODS

Children from two successive clinical trials administering 1) lonafarnib (n = 26) and 2) lonafarnib + pravastatin + zoledronic acid (n = 37) were studied at baseline (pre-therapy), one year on therapy, and at end-of-therapy (3.3-4.3 years after the baseline visit). Calcium supplementation (oral calcium carbonate) was administered during the first year of the second trial and was subsequently discontinued. Information on calcifications was obtained from physical examinations, radiographs, and serum and urinary biochemical measures. The mineral content of two skin-derived calcifications was determined by x-ray diffraction.

RESULTS

Extraskeletal calcifications were detected radiographically in 12/39 (31%) patients at baseline. The odds of exhibiting calcifications increased with age (p = 0.045). The odds were unaffected by receipt of lonafarnib, pravastatin, and zoledronate therapies. However, administration of calcium carbonate supplementation, in conjunction with all three therapeutic agents, significantly increased the odds of developing calcifications (p = 0.009), with the odds plateauing after the supplement's discontinuation. Composition analysis of calcinosis cutis showed hydroxyapatite similar to bone. Although serum calcium, phosphorus, and parathyroid hormone (PTH) were within normal limits at baseline and on-therapy, PTH increased significantly after lonafarnib initiation (p < 0.001). Both the urinary calcium/creatinine ratio and tubular reabsorption of phosphate (TRP) were elevated at baseline in 22/39 (56%) and 31/37 (84%) evaluable patients, respectively, with no significant changes while on-therapy. The mean calcium × phosphorus product (Ca × Pi) was within normal limits, but plasma magnesium decreased over both clinical trials. Fibroblast growth factor 23 (FGF23) was lower compared to age-matched controls (p = 0.03).

CONCLUSIONS

Extraskeletal calcifications increased with age in children with HGPS and were composed of hydroxyapatite. The urinary calcium/creatinine ratio and TRP were elevated for age while FGF23 was decreased. Magnesium decreased and PTH increased after lonafarnib therapy which may alter the ability to mobilize calcium. These findings demonstrate that children with HGPS with normal renal function and an unremarkable Ca × Pi develop extraskeletal calcifications by an unidentified mechanism that may involve decreased plasma magnesium and FGF23. Calcium carbonate accelerated their development and is, therefore, not recommended for routine supplementation in these children.

Quantitative Trait Locus and Integrative Genomics Revealed Candidate Modifier Genes for Ectopic Mineralization in Mouse Models of Pseudoxanthoma Elasticum

Pseudoxanthoma elasticum, a prototype of heritable multisystem ectopic mineralization disorders, is caused by mutations in the ABCC6 gene encoding a putative efflux transporter, ABCC6. The phenotypic spectrum of pseudoxanthoma elasticum varies, and the correlation between genotype and phenotype has not been established. To identify genetic modifiers, we performed quantitative trait locus analysis in inbred mouse strains that carry the same hypomorphic allele in Abcc6 yet with highly variable ectopic mineralization phenotypes of pseudoxanthoma elasticum. Abcc6 was confirmed as a major determinant for ectopic mineralization in multiple tissues. Integrative analysis using functional genomics tools that included GeneWeaver, String, and Mouse Genome Informatics identified a total of nine additional candidate modifier genes that could influence the organ-specific ectopic mineralization phenotypes. Integration of the candidate genes into the existing ectopic mineralization gene network expands the current knowledge on the complexity of the network that, as a whole, governs ectopic mineralization in soft connective tissues.

Aging-associated perifollicular changes and calcium deposition in poodles

BACKGROUND

It is commonly accepted that canine dystrophic mineralization of the hair follicle glassy membrane can be seen in hyperadrenocorticism and as a senile change in poodles. Pathology textbooks define this change as deposition of calcium salts in the form of basophilic, amorphous, granular material along collagen fibrils.

HYPOTHESIS/OBJECTIVES

The aim was to evaluate whether the incidence of the lesion is specific to poodles and if it is always associated with calcium deposition.

ANIMALS

One hundred and forty-seven dogs divided into three groups: (i) 91 normal poodles; (ii) 40 dogs of other breeds; and (iii) 16 dogs with clinical/histopathological diagnosis of hyperadrenocorticism.

METHODS AND MATERIALS

Retrospective study; with haematoxylin and eosin, and von Kossa staining.

RESULTS

Our findings demonstrate that perifollicular changes of the hair follicle glassy membrane of poodles are not always associated with calcium deposition.

CONCLUSIONS AND CLINICAL IMPORTANCE

A specific staining is needed to identify true calcium deposition when performing histological examination of the skin of poodles.

PXE, a Mysterious Inborn Error Clarified

Ever since Garrod deduced the existence of inborn errors in 1901, a vast array of metabolic diseases has been identified and characterized in molecular terms. In 2018 it is difficult to imagine that there is any uncharted backyard left in the metabolic disease landscape. Nevertheless, it took until 2013 to identify the cause of a relatively frequent inborn error, pseudoxanthoma elasticum (PXE), a disorder resulting in aberrant calcification. The mechanism found was not only biochemically interesting but also points to possible new treatments for PXE, a disease that has remained untreatable. In this review we sketch the tortuous road that led to the biochemical understanding of PXE and to new ideas for treatment. We also discuss some of the controversies still haunting the field.

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.

Living inside the box: environmental effects on mouse models of human disease

The impact of the laboratory environment on animal models of human disease, particularly the mouse, has recently come under intense scrutiny regarding both the reproducibility of such environments and their ability to accurately recapitulate elements of human environmental conditions. One common objection to the use of mice in highly controlled facilities is that humans live in much more diverse and stressful environments, which affects the expression and characteristics of disease phenotypes. In this Special Article, we review some of the known effects of the laboratory environment on mouse phenotypes and compare them with environmental effects on humans that modify phenotypes or, in some cases, have driven genetic adaptation. We conclude that the 'boxes' inhabited by mice and humans have much in common, but that, when attempting to tease out the effects of environment on phenotype, a controlled and, importantly, well-characterized environment is essential.

Magnesium as a Calcification Inhibitor

Vascular calcification (VC) is associated with elevated cardiovascular mortality rates in patients with CKD. Recent clinical studies of patients with advanced CKD have observed an association between low serum magnesium (Mg) levels on one hand and elevated VC and cardiovascular mortality on the other. These findings have stimulated interest in understanding Mg's impact on CKD in general and the associated VC in particular. In vitro and preclinical in vivo data indicate that Mg has the potential to protect vascular smooth muscle cells against calcification via several different molecular mechanisms. Accordingly, data from pilot interventional studies in the clinic suggest that oral Mg supplementation reduces VC in patients with CKD. The present review provides an overview of our current understanding of the impact of Mg on the development of VC in patients with CKD.

Elevated dietary magnesium during pregnancy and postnatal life prevents ectopic mineralization in Enpp1asj mice, a model for generalized arterial calcification of infancy

Generalized arterial calcification of infancy (GACI) is an autosomal recessive disorder caused by mutations in the ENPP1 gene. It is characterized by mineralization of the arterial blood vessels, often diagnosed prenatally, and associated with death in early childhood. There is no effective treatment for this devastating disorder. We previously characterized the Enpp1^asjmutant mouse as a model of GACI, and we have now explored the effect of elevated dietary magnesium (five-fold) in pregnant mothers and continuing for the first 14 weeks of postnatal life. The mothers were kept on either control diet or experimental diet supplemented with magnesium. Upon weaning at 4 weeks of age the pups were placed either on control diet or high magnesium diet. The degree of mineralization was assessed at 14 weeks of age by histopathology and a chemical calcium assay in muzzle skin, kidney and aorta. Mice placed on high magnesium diet showed little, if any, evidence of mineralization when their corresponding mothers were also placed on diet enriched with magnesium during pregnancy and nursing. The reduced ectopic mineralization in these mice was accompanied by increased calcium and magnesium content in the urine, suggesting that magnesium competes calcium-phosphate binding thereby preventing the mineral deposition. These results have implications for dietary management of pregnancies in which the fetus is suspected of having GACI. Moreover, augmenting a diet with high magnesium may be beneficial for other ectopic mineralization diseases, including nephrocalcinosis.

Magnesium prevents vascular calcification in vitro by inhibition of hydroxyapatite crystal formation

Magnesium has been shown to effectively prevent vascular calcification associated with chronic kidney disease. Magnesium has been hypothesized to prevent the upregulation of osteoblastic genes that potentially drives calcification. However, extracellular effects of magnesium on hydroxyapatite formation are largely neglected. This study investigated the effects of magnesium on intracellular changes associated with transdifferentiation and extracellular crystal formation. Bovine vascular smooth muscle cells were calcified using β-glycerophosphate. Transcriptional analysis, alkaline phosphatase activity and detection of apoptosis were used to identify transdifferentiation. Using X-ray diffraction and energy dispersive spectroscopy extracellular crystal composition was investigated. Magnesium prevented calcification in vascular smooth muscle cells. β-glycerophosphate increased expression of osteopontin but no other genes related to calcification. Alkaline phosphatase activity was stable and apoptosis was only detected after calcification independent of magnesium. Blocking of the magnesium channel TRPM7 using 2-APB did not abrogate the protective effects of magnesium. Magnesium prevented the formation of hydroxyapatite, which formed extensively during β-glycerophosphate treatment. Magnesium reduced calcium and phosphate fractions of 68% and 41% extracellular crystals, respectively, without affecting the fraction of magnesium. This study demonstrates that magnesium inhibits hydroxyapatite formation in the extracellular space, thereby preventing calcification of vascular smooth muscle cells.

Inherited Arterial Calcification Syndromes: Etiologies and Treatment Concepts

PURPOSE OF REVIEW

We give an update on the etiology and potential treatment options of rare inherited monogenic disorders associated with arterial calcification and calcific cardiac valve disease.

RECENT FINDINGS

Genetic studies of rare inherited syndromes have identified key regulators of ectopic calcification. Based on the pathogenic principles causing the diseases, these can be classified into three groups: (1) disorders of an increased extracellular inorganic phosphate/inorganic pyrophosphate ratio (generalized arterial calcification of infancy, pseudoxanthoma elasticum, arterial calcification and distal joint calcification, progeria, idiopathic basal ganglia calcification, and hyperphosphatemic familial tumoral calcinosis; (2) interferonopathies (Singleton-Merten syndrome); and (3) others, including Keutel syndrome and Gaucher disease type IIIC. Although some of the identified causative mechanisms are not easy to target for treatment, it has become clear that a disturbed serum phosphate/pyrophosphate ratio is a major force triggering arterial and cardiac valve calcification. Further studies will focus on targeting the phosphate/pyrophosphate ratio to effectively prevent and treat these calcific disease phenotypes.

Magnesium Counteracts Vascular Calcification: Passive Interference or Active Modulation?

Over the last decade, an increasing number of studies report a close relationship between serum magnesium concentration and cardiovascular disease risk in the general population. In end-stage renal disease, an association was found between serum magnesium and survival. Hypomagnesemia was identified as a strong predictor for cardiovascular disease in these patients. A substantial body of in vitro and in vivo studies has identified a protective role for magnesium in vascular calcification. However, the precise mechanisms and its contribution to cardiovascular protection remain unclear. There are currently 2 leading hypotheses: first, magnesium may bind phosphate and delay calcium phosphate crystal growth in the circulation, thereby passively interfering with calcium phosphate deposition in the vessel wall. Second, magnesium may regulate vascular smooth muscle cell transdifferentiation toward an osteogenic phenotype by active cellular modulation of factors associated with calcification. Here, the data supporting these major hypotheses are reviewed. The literature supports both a passive inorganic phosphate-buffering role reducing hydroxyapatite formation and an active cell-mediated role, directly targeting vascular smooth muscle transdifferentiation. However, current evidence relies on basic experimental designs that are often insufficient to delineate the underlying mechanisms. The field requires more advanced experimental design, including determination of intracellular magnesium concentrations and the identification of the molecular players that regulate magnesium concentrations in vascular smooth muscle cells.

Plasma PPi Deficiency Is the Major, but Not the Exclusive, Cause of Ectopic Mineralization in an Abcc6-/- Mouse Model of PXE

Pseudoxanthoma elasticum (PXE), a prototype of heritable ectopic mineralization disorders, is caused in most cases by inactivating mutations in the ABCC6 gene. It was recently discovered that absence of ABCC6-mediated adenosine triphosphate release from the liver and consequently reduced plasma inorganic pyrophosphate (PPi) levels underlie PXE. This study examined whether reduced levels of circulating PPi, an antimineralization factor, is the sole mechanism of PXE. The Abcc6^-/- and Enpp1^asj mice were crossed with transgenic mice expressing human ENPP1, an ectonucleotidase that generates PPi from adenosine triphosphate. We generated Abcc6^-/- and Enpp1^asj mice, either wild-type or hemizygous for human ENPP1. Plasma levels of PPi and the degree of ectopic mineralization were determined. Overexpression of human ENPP1 in Enpp1^asj mice normalized plasma PPi levels to that of wild-type mice and, consequently, completely prevented ectopic mineralization. These changes were accompanied by restoration of their bone microarchitecture. In contrast, although significantly reduced mineralization was noted in Abcc6^-/- mice expressing human ENPP1, small mineralization foci were still evident despite increased plasma PPi levels. These results suggest that PPi is the major mediator of ectopic mineralization in PXE, but there might be an alternative, as yet unknown mechanism, independent of PPi, by which ABCC6 prevents ectopic mineralization under physiologic conditions.

Insights into Pathomechanisms and Treatment Development in Heritable Ectopic Mineralization Disorders: Summary of the PXE International Biennial Research Symposium-2016

Pseudoxanthoma elasticum is a prototype of heritable ectopic mineralization disorders, with phenotypic overlap with generalized arterial calcification of infancy and arterial calcification due to CD73 deficiency. Recent observations have suggested that the reduced inorganic pyrophosphate/phosphate ratio is the cause of soft connective tissue mineralization in these disorders. PXE International, a patient advocacy organization, supports research in part by sponsoring biennial research symposia on these disorders; the latest meeting was held in September 2016 at Thomas Jefferson University, Philadelphia. This report summarizes the progress in pseudoxanthoma elasticum and other ectopic mineralization disorders, as presented in the symposium, with focus on translational aspects of precision medicine toward improved diagnostics and treatment development for these currently intractable disorders.

Abcc6 Knockout Rat Model Highlights the Role of Liver in PPi Homeostasis in Pseudoxanthoma Elasticum

Pseudoxanthoma elasticum, a heritable ectopic mineralization disorder, is caused by mutations in the ABCC6 gene primarily expressed in the liver and the kidneys. The fundamental question on pathogenesis of pseudoxanthoma elasticum, whether lack of ABCC6 expression in liver or kidney is the primary site of molecular pathology in peripheral tissues, has not been addressed. We generated a series of Abcc6^-/- rats as models of pseudoxanthoma elasticum depicting ectopic mineralization in the skin, eyes, and the arterial blood vessels. Plasma inorganic pyrophosphate (PPi) level was reduced (<30%) in the Abcc6^-/- rats leading to a lowered PPi/inorganic phosphate plasma ratio. In situ liver and kidney perfusions were performed to determine the relative contribution of these organs to PPi levels in circulation. PPi levels in the perfusates both in the liver and kidney of Abcc6^-/- rats were significantly reduced, but the PPi levels in the liver perfusates of wild-type rats were 10-fold higher than that in the kidney perfusates. These observations suggest a critical role of hepatic ABCC6 in contributing to plasma PPi levels, identifying liver as a target of molecular correction to counteract ectopic mineralization in pseudoxanthoma elasticum.

Mineral content of the maternal diet influences ectopic mineralization in offspring of Abcc6(-/-) mice

Ectopic mineralization disorders inflicting the connective tissues display a spectrum of severity, some developing in utero and being diagnosed by prenatal ultrasound. This study was designed to test the hypothesis that the mineral content of maternal diet can influence the mineralization in the offspring. Pregnant Abcc6(-/-) mice, on 2 different strain backgrounds, were maintained either on normal rodent diet or on "acceleration diet," rich in phosphate and low in magnesium, which has been previously shown to enhance the mineralization processes. The offspring were examined for mineralization by histopathology of various tissues and quantitated by chemical assay of calcium. The ectopic mineralization in the dermal sheath of vibrissae, a progressive biomarker of the overall mineralization, was readily detectable at the age of 4 weeks in the pups whose mothers were on the acceleration diet, while no evidence of mineralization was noted in those on normal diet. The mineralization of the vibrissae progressively increased when examined at 12 weeks of age. There was a significant reduction in urinary calcium and significant increase in urinary phosphorus concentrations both at 4 and 12 weeks of age in mice on the acceleration diet as compared to those on control diet. The results demonstrate that the mineral content of the maternal diet can influence ectopic mineralization in the offspring of mice genetically predisposed to ectopic mineralization (Abcc6(-/-)). These observations have implications for dietary management of pregnancies in which the fetus is diagnosed by prenatal ultrasound to have an ectopic mineralization disorder.

Research Progress in Pseudoxanthoma Elasticum and Related Ectopic Mineralization Disorders

Heritable ectopic mineralization disorders represent a phenotypically diverse group of conditions characterized by deposition of calcium phosphate complexes in soft connective tissues. The prototype of such conditions is pseudoxanthoma elasticum, and related conditions with overlapping clinical features include generalized arterial calcification of infancy and arterial calcification due to CD73 deficiency. Molecular genetic investigations have revealed mutations in the genes physiologically involved in generation of inorganic pyrophosphate and inorganic phosphate, and the findings suggest a unifying pathomechanism relating to reduced inorganic pyrophosphate/inorganic phosphate ratio. This hypothesis is based on the notion that inorganic pyrophosphate serves as a powerful inhibitor of mineralization, whereas inorganic phosphate is a promineralization factor, and an appropriate inorganic pyrophosphate/inorganic phosphate ratio is critical for prevention of ectopic mineralization under homeostatic conditions.

Juxta-articular joint-capsule mineralization in CD73 deficient mice: similarities to patients with NT5E mutations

Arterial calcification due to CD73 deficiency (ACDC), an autosomal recessive disorder, manifests with extensive mineralization of the lower-extremity arteries as well as of hand and foot joint-capsules. This disease is caused by mutations in the NT5E gene which encodes CD73, a membrane-bound ecto-5'-nucleotidase hydrolyzing 5'-AMP into adenosine and Pi. To gain insight into the pathophysiologic details of ACDC, we have characterized a Nt5e(-/-) knock out mouse (Nt5e(tm1Jgsc)) deficient in CD73. These mice, when maintained on appropriate strain background, demonstrated stiffening of the joints and micro CT revealed distinct changes in the thoracic skeletal structure with evidence of mineralization at the costochondral junctions. Mineralization was also noted in the juxta-articular spaces of the lower extremities as well as of ligaments and capsules adjacent to the bony structures. No evidence of vascular mineralization was noted either by CT or by microdissection of arteries in the thoracic area or in lower extremities. The Nt5e(-/-) mutant mice demonstrated significantly increased Pi levels in the serum and significantly reduced PPi concentration in the heparinized plasma, resulting in markedly increased Pi/PPi ratio, thus creating a pro-mineralization environment. In conclusion, the Nt5e(-/-) targeted mutant mice recapitulate some, but not all, features of ACDC and serve as a model system to study pharmacologic interventions for ectopic mineralization. Collectively, this mouse model deficient in CD73, with other targeted mutant mice with vascular mineralization, attests to the presence of a complex pro-mineralization/anti-mineralization network that under physiologic homeostatic conditions prevents ectopic tissue mineralization.

Pseudoxanthoma elasticum and skin: Clinical manifestations, histopathology, pathomechanism, perspectives of treatment

Pseudoxantoma elasticum (PXE), also known as Groenblad-Strandberg syndrome, is a rare heritable disease with an estimated prevalence of 1:50,000 in the general population. PXE is considered a prototype of multisystem ectopic mineralization disorders and it is characterized by aberrant mineralization of soft connective tissue with degeneration of the elastic fibers, involving primarily the eyes, the cardiovascular system, and the skin. Cutaneous lesions consist of small, asymptomatic, yellowish papules or larger coalescent plaques, typically located on the neck and the flexural areas. PXE is caused by mutations in the ABCC6 (ATP-binding cassette subfamily C member 6) gene that encodes a transmembrane ATP binding efflux transporter, normally expressed in the liver and the kidney; however, the exact mechanism of ectopic mineralization remains largely unknown. The histological examination of cutaneous lesions, revealing accumulation of pleomorphic elastic structures in middermis, is essential for the definitive diagnosis of PXE, excluding PXE-like conditions. PXE is currently an intractable disease; although the cutaneous findings primarily present a cosmetic problem, they signify the risk for development of ocular and cardiovascular complications associated with considerable morbidity and mortality. The purpose of this review is to present a comprehensive overview of this rare form of hereditary connective tissue disorders, focus on the pathogenesis, the clinical manifestation, and the differential diagnosis of PXE. Emphasis is also placed on the management of cutaneous lesions and treatment perspectives of PXE.

Magnesium and cardiovascular complications of chronic kidney disease

Cardiovascular complications are the leading cause of death in patients with chronic kidney disease (CKD). Abundant experimental evidence suggests a physiological role of magnesium in cardiovascular function, and clinical evidence suggests a role of the cation in cardiovascular disease in the general population. The role of magnesium in CKD-mineral and bone disorder, and in particular its impact on cardiovascular morbidity and mortality in patients with CKD, is however not well understood. Experimental studies have shown that magnesium inhibits vascular calcification, both by direct effects on the vessel wall and by indirect, systemic effects. Moreover, an increasing number of epidemiologic studies in patients with CKD have shown associations of serum magnesium levels with intermediate and hard outcomes, including vascular calcification, cardiovascular events and mortality. Intervention trials in these patients conducted to date have had small sample sizes and have been limited to the study of surrogate parameters, such as arterial stiffness, vascular calcification and atherosclerosis. Randomized controlled trials are clearly needed to determine the effects of magnesium supplementation on hard outcomes in patients with CKD.

Spontaneous asj-2J mutant mouse as a model for generalized arterial calcification of infancy: a large deletion/insertion mutation in the Enpp1 gene

Generalized arterial calcification of infancy (GACI), an autosomal recessive disorder caused by mutations in the ENPP1 gene, manifests with extensive mineralization of the cardiovascular system. The affected individuals in most cases die within the first year of life, and there is currently no effective treatment for this disorder. In this study, we characterized a spontaneous mutant mouse, asj-2J, as a model for GACI. These mice were identified as part of a phenotypic deviant search in a large-scale production colony of BALB/cJ mice at The Jackson Laboratory. They demonstrated a characteristic gait due to stiffening of the joints, with phenotypic similarity to a previously characterized asj (“ages with stiffened joints”) mouse, caused by a missense mutation in the Enpp1 gene. Complementation testing indicated that asj-2J and asj were allelic. PCR-based mutation detection strategy revealed in asj-2J mice a large, 40,035 bp, deletion spanning from intron 1 to the 3′-untranslated region of the Enpp1 gene, coupled with a 74 bp insertion. This was accompanied with a significant reduction in the plasma PP_i concentration and reduced PP_i/P_i ratio. As a consequence, extensive aberrant mineralization affecting the arterial vasculature, a number of internal organs, and the dermal sheath of vibrissae, a progressive biomarker of the ectopic mineralization process, was demonstrated by a combination of micro computed tomography, histopathology with calcium-specific stains, and direct chemical assay of calcium. Comparison of the asj and asj-2J mice demonstrated that the latter ones, particularly when placed on an acceleration diet high in phosphate and low in magnesium, had more extensive mineralization. Thus, the asj-2J mouse serves as a novel model for GACI, a currently intractable disorder.

Paediatric pseudoxanthoma elasticum with cardiovascular involvement

BACKGROUND

Pseudoxanthoma elasticum (PXE) is characterized by aberrant mineralization of connective tissues, causing considerable morbidity and mortality. The disease is typically of late onset, the skin manifestations first being noted in the teens or later. Another aberrant mineralization disorder, generalized arterial calcification of infancy (GACI), is present at birth and can demonstrate a phenotypic overlap with PXE.

OBJECTIVES

A patient with PXE was noted to have skin findings as early as at 6 years of age, with cardiovascular involvement. The purpose of this study was to examine the genetic basis of this phenotypic presentation in the spectrum of PXE/GACI.

METHODS

The patient's genotype was studied by sequencing ABCC6 and ENPP1, genes known to be associated with PXE and/or GACI.

RESULTS

Screening of the ABCC6 gene revealed two pathogenetic mutations, p.R1141X and g.del23-29. Analysis of the ENPP1 gene failed to demonstrate the presence of mutations.

CONCLUSIONS

This study demonstrates the presence of cutaneous findings of PXE in an 8-year-old paediatric patient, with cardiovascular involvement, illustrating the phenotypic spectrum of PXE.

Genetic modulation of nephrocalcinosis in mouse models of ectopic mineralization: the Abcc6(tm1Jfk) and Enpp1(asj) mutant mice

Ectopic mineralization of renal tissues in nephrocalcinosis is a complex, multifactorial process. The purpose of this study was to examine the role of genetic modulation and the role of diet in nephrocalcinosis using two established mouse models of ectopic mineralization, Abcc6(tm1Jfk) and Enpp1(asj) mice, which serve as models for pseudoxanthoma elasticum and generalized arterial calcification of infancy, two heritable disorders, respectively. These mutant mice, when on standard rodent diet, develop nephrocalcinosis only at a very late age. In contrast, when placed on an 'acceleration diet' composed of increased phosphate and reduced magnesium content, they showed extensive mineralization of the kidneys affecting primarily the medullary tubules as well as arcuate and renal arteries, as examined by histopathology and quantitated by chemical assay for calcium. Mineralization could also be detected noninvasively by micro computed tomography. Whereas the heterozygous mice did not develop nephrocalcinosis, compound heterozygous mice carrying both mutant alleles, Abcc6(tm1Jfk/+) and Enpp1(+/asj), developed ectopic mineralization similar to that noted in homozygous mice for either gene, indicating that deletion of one Abcc6 allele along with Enpp1 haploinsufficiency resulted in renal mineralization. Thus, synergistic genetic defects in the complex mineralization/antimineralization network can profoundly modulate the degree of ectopic mineralization in nephrocalcinosis.

PSEUDOXANTHOMA ELASTICUM: DIAGNOSTIC FEATURES, CLASSIFICATION, AND TREATMENT OPTIONS

INTRODUCTION

Pseudoxanthoma elasticum (PXE), a multisystem orphan disease, clinically affects the skin, the eyes, and the cardiovascular system with considerable morbidity and mortality. The clinical manifestations reflect the underlying pathology consisting of ectopic mineralization of peripheral connective tissues.

AREAS COVERED

The diagnostic criteria of PXE include characteristic clinical findings, together with histopathology of accumulation of pleiomorphic elastic structures in the dermis with progressive mineralization, and the presence of mutations in the ABCC6 gene. PXE-like cutaneous changes can also be encountered in other ectopic mineralization disorders, including generalized arterial calcification of infancy (GACI) caused by mutations in the ENPP1 gene. In some cases, overlapping clinical features of PXE/GACI, associated with mutations either in ABCC6 or ENPP1, have been noted. PXE demonstrates considerable inter- and intrafamilial heterogeneity, and consequently, accurate diagnosis is required for appropriate classification with prognostic implications. There is no effective and specific treatment for the systemic manifestations of PXE, but effective therapies to counteract the ocular complications are in current clinical use.

EXPERT OPINION

A number of observations in the murine model, the Abcc6^-/- mouse, have indicated that the mineral composition of diet, particularly the magnesium content, can influence the severity of the mineralization phenotype. These observations suggest that appropriate dietary interventions, coupled with lifestyle modifications, including smoking cessation, might alleviate the symptoms and improve the quality of life of individuals affected with this, currently intractable, orphan disease.

Ectopic mineralization disorders of the extracellular matrix of connective tissue: molecular genetics and pathomechanisms of aberrant calcification

Ectopic mineralization of connective tissues is a complex process leading to deposition of calcium phosphate complexes in the extracellular matrix, particularly affecting the skin and the arterial blood vessels and common in age-associated disorders. A number of initiating and contributing metabolic and environmental factors are linked to aberrant mineralization in these diseases, making the identification of precise pathomechanistic pathways exceedingly difficult. However, there has been significant recent progress in understanding the ectopic mineralization processes through study of heritable single-gene disorders, which have allowed identification of discrete pathways and contributing factors leading to aberrant connective tissue mineralization. These studies have provided support for the concept of an intricate mineralization/anti-mineralization network present in peripheral connective tissues, providing a perspective to development of pharmacologic approaches to limit the phenotypic consequences of ectopic mineralization. This overview summarizes the current knowledge of ectopic heritable mineralization disorders, with accompanying animal models, focusing on pseudoxanthoma elasticum and generalized arterial calcification of infancy, two autosomal recessive diseases manifesting with extensive connective tissue mineralization in the skin and the cardiovascular system.

Mineralization/anti-mineralization networks in the skin and vascular connective tissues

Ectopic mineralization has been linked to several common clinical conditions with considerable morbidity and mortality. The mineralization processes, both metastatic and dystrophic, affect the skin and vascular connective tissues. There are several contributing metabolic and environmental factors that make uncovering of the precise pathomechanisms of these acquired disorders exceedingly difficult. Several relatively rare heritable disorders share phenotypic manifestations similar to those in common conditions, and, consequently, they serve as genetically controlled model systems to study the details of the mineralization process in peripheral tissues. This overview will highlight diseases with mineral deposition in the skin and vascular connective tissues, as exemplified by familial tumoral calcinosis, pseudoxanthoma elasticum, generalized arterial calcification of infancy, and arterial calcification due to CD73 deficiency. These diseases, and their corresponding mouse models, provide insight into the pathomechanisms of soft tissue mineralization and point to the existence of intricate mineralization/anti-mineralization networks in these tissues. This information is critical for understanding the pathomechanistic details of different mineralization disorders, and it has provided the perspective to develop pharmacological approaches to counteract the consequences of ectopic mineralization.

Mutant Enpp1asj mice as a model for generalized arterial calcification of infancy

Generalized arterial calcification of infancy (GACI), an autosomal recessive disorder, is characterized by early mineralization of blood vessels, often diagnosed by prenatal ultrasound and usually resulting in demise during the first year of life. It is caused in most cases by mutations in the ENPP1 gene, encoding an enzyme that hydrolyzes ATP to AMP and inorganic pyrophosphate, the latter being a powerful anti-mineralization factor. Recently, a novel mouse phenotype was recognized as a result of ENU mutagenesis – those mice developed stiffening of the joints, hence the mutant mouse was named ‘ages with stiffened joints’ (asj). These mice harbor a missense mutation, p.V246D, in the Enpp1 gene. Here we demonstrate that the mutant ENPP1 protein is largely absent in the liver of asj mice, and the lack of enzymatic activity results in reduced inorganic pyrophosphate (PP_i) levels in the plasma, accompanied by extensive mineralization of a number of tissues, including arterial blood vessels. The progress of mineralization is highly dependent on the mineral composition of the diet, with significant shortening of the lifespan on a diet enriched in phosphorus and low in magnesium. These results suggest that the asj mouse can serve as an animal model for GACI.

Warfarin accelerates ectopic mineralization in Abcc6(-/-) mice: clinical relevance to pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is a multisystem ectopic mineralization disorder caused by mutations in the ABCC6 gene. Warfarin, a commonly used anticoagulant, is associated with increased mineralization of the arterial blood vessels and cardiac valves. We hypothesized that warfarin may accelerate ectopic tissue mineralization in PXE, with clinical consequences. To test this hypothesis, we developed a model in which Abcc6(-/-) mice, which recapitulate features of PXE, were fed a diet supplemented with warfarin and vitamin K1. Warfarin action was confirmed by significantly increased serum levels of oxidized vitamin K. For mice placed on a warfarin-containing diet, quantitative chemical and morphometric analyses revealed massive accumulation of mineral deposits in a number of tissues. Mice fed a warfarin-containing diet were also shown to have abundant uncarboxylated form of matrix Gla protein, which allowed progressive tissue mineralization to ensue. To explore the clinical relevance of these findings, 1747 patients with PXE from the approximately 4000 patients in the PXE International database were surveyed about the use of warfarin. Of the 539 respondents, 2.6% reported past or present use of warfarin. Based on the prevalence of PXE (approximately 1:50,000), thousands of patients with PXE worldwide may be at risk for worsening of PXE as a result of warfarin therapy.

Is classical pseudoxanthoma elasticum a consequence of hepatic 'intoxication' due to ABCC6 substrate accumulation in the liver?

Pseudoxanthoma elasticum (PXE) is a serious genetic disorder with ectopic mineralization affecting the skin, the eye and the cardiovascular system. The disease is predominantly caused by mutations in the transmembrane ABC protein ABCC6, a putative small substrate transporter. Interestingly, ABCC6 seems virtually absent in the affected organs, whereas a high expression is seen in hepatocytes. This and further published experimental evidence indicate that PXE is a systemic, metabolic liver disease where circulatory changes affect the peripheral mineralization process. Owing to the well-characterized transport of organic substrates by related ABC proteins, it has been proposed that PXE is caused by impaired export of an antimineralization compound to the blood. The authors here present an alternative hypothesis that explains ectopic mineralization in PXE as a consequence of hepatic accumulation of ABCC6 substrate(s) that via gene-regulating effects leads to altered hepatic secretion and activation of antimineralization/anticalcification proteins such as fetuin-A and Gla proteins.