Minocycline Counteracts Ectopic Calcification in a Murine Model of Pseudoxanthoma Elasticum: A Proof-of-Concept Study.

Pseudoxanthoma elasticum (PXE) is an intractable Mendelian disease characterized by ectopic calcification in skin, eyes and blood vessels. Recently, increased activation of the DNA damage response (DDR) was shown to be involved in PXE pathogenesis, while the DDR/PARP1 inhibitor minocycline was found to attenuate aberrant mineralization in PXE cells and zebrafish. In this proof-of-concept study, we evaluated the anticalcifying properties of minocycline in Abcc6-/- mice, an established mammalian PXE model. Abcc6-/- mice received oral minocycline supplementation (40 mg/kg/day) from 12 to 36 weeks of age and were compared to untreated Abcc6-/- and Abcc6+/+ siblings. Ectopic calcification was evaluated using X-ray microtomography with three-dimensional reconstruction of calcium deposits in muzzle skin and Yasue's calcium staining. Immunohistochemistry for the key DDR marker H2AX was also performed. Following minocycline treatment, ectopic calcification in Abcc6-/- mice was significantly reduced (-43.4%, p < 0.0001) compared to untreated Abcc6-/- littermates. H2AX immunostaining revealed activation of the DDR at sites of aberrant mineralization in untreated Abcc6-/- animals. In conclusion, we validated the anticalcifying effect of minocycline in Abcc6-/- mice for the first time. Considering its favorable safety profile in humans and low cost as a generic drug, minocycline may be a promising therapeutic compound for PXE patients.

Vitamin D and Calcium Supplementation Accelerate Vascular Calcification in a Model of Pseudoxanthoma Elasticum.

Arterial calcification is a common feature of pseudoxanthoma elasticum (PXE), a disease characterized by ABCC6 mutations, inducing a deficiency in pyrophosphate, a key inhibitor of calcium phosphate crystallization in arteries. METHODS: we analyzed whether long-term exposure of Abcc6-/- mice (a murine model of PXE) to a mild vitamin D supplementation, with or without calcium, would impact the development of vascular calcification. Eight groups of mice (including Abcc6-/- and wild-type) received vitamin D supplementation every 2 weeks, a calcium-enriched diet alone (calcium in drinking water), both vitamin D supplementation and calcium-enriched diet, or a standard diet (controls) for 6 months. Aorta and kidney artery calcification was assessed by 3D-micro-computed tomography, Optical PhotoThermal IR (OPTIR) spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) and Yasue staining. RESULTS: at 6 months, although vitamin D and/or calcium did not significantly increase serum calcium levels, vitamin D and calcium supplementation significantly worsened aorta and renal artery calcification in Abcc6-/- mice. CONCLUSIONS: vitamin D and/or calcium supplementation accelerate vascular calcification in a murine model of PXE. These results sound a warning regarding the use of these supplementations in PXE patients and, to a larger extent, patients with low systemic pyrophosphate levels.

Oral supplementation of inorganic pyrophosphate in pseudoxanthoma elasticum.

Pseudoxanthoma elasticum (PXE; OMIM 264800) is a rare heritable multisystem disorder, characterized by ectopic mineralization affecting elastic fibres in the skin, eyes and the cardiovascular system. Skin findings often lead to early diagnosis of PXE, but currently, no specific treatment exists to counteract the progression of symptoms. PXE belongs to a group of Mendelian calcification disorders linked to pyrophosphate metabolism, which also includes generalized arterial calcification of infancy (GACI) and arterial calcification due to CD73 deficiency (ACDC). Inactivating mutations in ABCC6, ENPP1 and NT5E are the genetic cause of these diseases, respectively, and all of them result in reduced inorganic pyrophosphate (PPi ) concentration in the circulation. Although PPi is a strong inhibitor of ectopic calcification, oral supplementation therapy was initially not considered because of its low bioavailability. Our earlier work however demonstrated that orally administered pyrophosphate inhibits ectopic calcification in the animal models of PXE and GACI, and that orally given Na4 P2 O7 is absorbed in humans. Here, we report that gelatin-encapsulated Na2 H2 P2 O7  has similar absorption properties in healthy volunteers and people affected by PXE. The sodium-free K2 H2 P2 O7 form resulted in similar uptake in healthy volunteers and inhibited calcification in Abcc6-/- mice as effectively as its sodium counterpart. Novel pyrophosphate compounds showing higher bioavailability in mice were also identified. Our results provide an important step towards testing oral PPi in clinical trials in PXE, or potentially any condition accompanied by ectopic calcification including diabetes, chronic kidney disease or ageing.

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.

Pediatric Pseudo-pseudoxanthoma Elasticum Resulting From D-Penicillamine Treatment for Wilson Disease.

We describe a 14-year-old boy with Wilson disease (WD) who first developed pseudo-pseudoxanthoma elasticum (PPXE) after 4.5 years of treatment with D-penicillamine. Although previously reported cases have occurred in adults following at least a decade of high-dose D-penicillamine use, this case demonstrates that D-penicillamine-induced PPXE can present in children with shorter treatment courses. Upon this diagnosis, the patient was switched from D-penicillamine to trientine, with adequate cupriuresis and stabilization of the skin lesion. Prompt diagnosis and management of PPXE in children can limit systemic progression and prevent long-term complications.

Reversion of arterial calcification by elastin-targeted DTPA-HSA nanoparticles.

Generalized arterial calcification of infancy (GACI) and pseudoxanthoma elasticum (PXE) are characterized by pathologic calcifications in the media of large- and medium sized arteries. GACI is associated with biallelic mutations in ENPP1 in the majority of cases, whereas mutations in ABCC6 are known to cause PXE. Different treatment approaches including bisphosphonates and orally administered pyrophosphate (PPi) were investigated in recent years, but reversion of calcification could not be achieved. With this study, we pursued the idea of a combination of controlled drug delivery through nanoparticles and active targeting via antibody conjugation to develop a treatment for GACI and PXE. To establish a suitable drug delivery system, the chelating drug diethylenetriamine pentaacetic acid (DTPA) was conjugated to nanoparticles composed of human serum albumin (HSA) as biodegradable and non-toxic particle matrix. To accomplish an active targeting of the elastic fibers exposed through calcification of the affected areas, the nanoparticle surface was functionalized with an anti-elastin antibody. Cytotoxicity and cell interaction studies revealed favorable preconditions for the intended i.v. application. The chelating ability was evaluated in vitro and ex vivo on aortic ring culture isolated from two mouse models of GACI and PXE. The positive results led to the conclusion that the produced nanoparticles might be a promising therapy in the treatment of GACI and PXE.

IMPAIRED DARK ADAPTATION ASSOCIATED WITH A DISEASED BRUCH MEMBRANE IN PSEUDOXANTHOMA ELASTICUM.

PURPOSE: To characterize dark adaptation in patients with pseudoxanthoma elasticum, a systemic disease leading to calcification of elastic tissue including the Bruch membrane. METHODS: In this prospective case-control study, dark adaptation thresholds were measured using a Goldmann-Weekers dark adaptometer. Additional assessments included best-corrected visual acuity testing, contrast sensitivity, low luminance deficit, and vision-related quality of life. RESULTS: Dark adaptation thresholds were significantly higher, and adaptation periods were prolonged in patients with pseudoxanthoma elasticum (n = 35; 33 with 2 ABCC6 mutations) compared with controls (n = 35). The time to adapt 4 log units (20.6 ± 8.6 vs. 8.0 ± 1.3 minutes) and the mean dark adaptation threshold after 15 minutes (3.5 ± 1.1 vs. 1.8 ± 0.2 log units) were significantly different between patients and controls (both P < 0.001). Low luminance deficits (12.3 ± 6.4 vs. 6.1 ± 4.3 ETDRS letters), contrast sensitivity (1.4 ± 0.3 vs. 1.9 ± 0.1), and low luminance-related quality of life (LLQ score: 1,286 ± 355 vs. 2,167 ± 68) were also significantly worse in patients with pseudoxanthoma elasticum (all, P < 0.001). Two patients were treated with high-dose vitamin A which partially reversed impaired dark adaptation. CONCLUSION: Patients with pseudoxanthoma elasticum often have impaired dark adaptation. Positive effects of vitamin A supplementation may indicate restricted retinal access of vitamin A through the Bruch membrane as one possible underlying pathogenic factor.

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.

Pseudoxantoma elástico / Pseudoxanthoma elastic

Resumo O pseudoxantoma elástico é uma doença generalizada do tecido conjuntivo envolvendo a pele, olhos e sistema cardiovascular desencadeando a fragmentação e calcificação das fibras elásticas. Geralmente ocorre após a puberdade, as manifestações características são manchas pequenas, circunscritas, amareladas, localizadas no pescoço, axila e pregas inguinais. Estrias angioides na retina, tendência à hemorragia e insuficiência arterial são as complicações mais comuns. Esta doença pode ser herdada como autossômica dominante ou recessiva. O tratamento das manifestações oculares convencional é através da fototerapia a laser impedindo a ocorrência de hemorragias locais. Entretanto, novas abordagens terapêuticas estão sendo desenvolvidas como a utilização em longo prazo de drogas antiangiogênicas, as quais atuam inibindo a neovascularização ocular. Apesar de não ter ainda efetivamente substituído o tratamento original, pesquisas recentes já evidenciam benefícios da nova técnica. O objetivo deste estudo é relatar sobre o caso de uma paciente de 37 anos, portadora do pseudoxantoma elástico, com estrias angioides e hemorragia ocular, e o tratamento eficaz com a terapia antiangiogênica no ambulatório de oftalmologia em Nova Iguaçu, Rio de Janeiro.

Abstract The pseudoxanthoma elasticum is a generalized disease of the connective tissue involving the skin, eyes and cardiovascular system triggering the fragmentation and calcification of elastic fibers. Usually occurs after puberty, the manifestations characteristics are small spots, circumscribed, yellowish, located on the neck, axilla and inguinal folds. Angioid streaks in the retina, tendency to hemorrhage and arterial insufficiency are the most common complications. This disease can be inherited as autosomal dominant or recessive. The treatment of ocular manifestations is through the conventional phototherapy laser preventing the occurrence of local hemorrhages. However, new therapeutic approaches are being developed as the long-term use of drugs antiangiogenic, which act by inhibiting the ocular neovascularization. Despite not having yet effectively replaced the original treatment, recent research already show benefits of new technique. The objective of this study is to report on a case of a patient of 37 years, the carrier of the Pseudoxanthoma Elasticum, with angioid streaks and ocular hemorrhage, and the effective treatment with antiangiogenic therapy at the clinic of Ophthalmology in Nova Iguaçu, Rio de Janeiro.

Treatment of pseudoxanthoma elasticum-like papillary dermal elastolysis with nonablative fractional resurfacing laser resulting in clinical and histologic improvement in elastin and collagen.

OBJECTIVE: Pseudoxanthoma elasticum-like papillary dermal elastolysis (PXE-PDE) is a rare skin condition seen predominantly in elderly females. These asymptomatic lesions are brought to the dermatologist's attention due to patient's displeasure in their appearance. We report a case of a 28-year-old female with PXE-PDE on the right neck treated with nonablative fractional resurfacing (NAFR). CASE: The patient on examination had yellow-to-skin-colored papules that coalesced into a cobblestone-appearing plaque on her lateral neck. The area was treated with NAFR to improve texture and cosmesis. After three treatments, the patient noted at least a 50% improvement of appearance and texture in the affected skin. Adverse effects were mild without development of serious side effects, such as scarring and/or permanent dyspigmenation. CONCLUSION: Established therapies for elastin disorders like PXE-PDE have yielded unsatisfactory results by patient and clinician standards. Positive results with NAFR, illustrated in our PXE-PDE case, further provides a role for NAFR in elastin disorders.

Early arterial calcification does not correlate with bone loss in pseudoxanthoma elasticum.

BACKGROUND AND AIMS: Pseudoxanthoma elasticum (PXE; OMIM 264800, prevalence 1/25,000 to 1/50,000) is an autosomal recessive multisystem disease due to deficiency in ABCC6, an ATP-binding cassette, sub-family C transporter. The PXE phenotype is mainly characterized by progressive ectopic calcification of connective tissues (namely skin, retinal Bruch's membrane and peripheral arteries) but the impact of PXE on bone structure is currently unknown. The present study sought to investigate bone mineralization and its potential link with vascular calcification in a large cohort of PXE patients with inherited mutations of the ABCC6 gene. METHODS AND RESULTS: 96 patients (61 women) matching the PXE criteria participated in this study. Their clinical history and status and bone biological markers were collected. Bone mineral density (BMD) was measured by dual energy X-ray absorptiometry and expressed as T- and Z-scores. Osteoporotic fractures were identified by X-ray, and coronary (CAC) and lower limb arterial calcification (LLAC) scores were determined by CT scan. RESULTS: 44% of the women were menopausal. Osteopenia was disclosed in 46% (17 women) while 23% (9 women) exhibited osteoporosis, 3 with severe osteoporosis. Fractures of an osteoporotic nature were authenticated in 3 patients (1 woman). Markers of bone remodelling processes (CTX, BSAP and osteocalcin) were within the normal range for our laboratory standards. Severe vitamin D deficiency (<25nmol/L) was found in 15%, while 51% exhibited no vitamin D deficiency (vitamin D≥50nmol/L). LLAC and CAC scores were significantly higher in the patients with a low T- and/or Z-score, although this difference disappeared in multivariate analysis with age as a confounding factor. There was no significant difference in LLAC and CAC between PXE patients with and without osteoporotic fractures. There was no statistically significant association between BMD, LLAC and CAC and any of the bone remodelling factors. CONCLUSIONS: This is the first report on the bone mineralization process in PXE patients. Our data shows that PXE patients are not markedly prone to exaggerated bone demineralization and fracture risk, and prevalence of osteoporosis remains within the normal range for the general population. Furthermore, the relationships between LLAC, but not CAC, and BMD with age are similar to those observed in the general population. Therefore, despite its pivotal role in ectopic calcification, ABCC6 deficiency does not interfere with the bone-vascular axis. The lack of PXE-related disturbances between BMD and arterial calcification also supports vitamin D supplementation in PXE patients with vitamin D deficiency. ClinicalTrials.gov Identifier: NCT01446393.

Pyrophosphate Supplementation Prevents Chronic and Acute Calcification in ABCC6-Deficient Mice.

Soft tissue calcification occurs in several common acquired pathologies, such as diabetes and hypercholesterolemia, or can result from genetic disorders. ABCC6, a transmembrane transporter primarily expressed in liver and kidneys, initiates a molecular pathway inhibiting ectopic calcification. ABCC6 facilitates the cellular efflux of ATP, which is rapidly converted into pyrophosphate (PPi), a major calcification inhibitor. Heritable mutations in ABCC6 underlie the incurable calcification disorder pseudoxanthoma elasticum and some cases of generalized arterial calcification of infancy. Herein, we determined that the administration of PPi and the bisphosphonate etidronate to Abcc6-/- mice fully inhibited the acute dystrophic cardiac calcification phenotype, whereas alendronate had no significant effect. We also found that daily injection of PPi to Abcc6-/- mice over several months prevented the development of pseudoxanthoma elasticum-like spontaneous calcification, but failed to reverse already established lesions. Furthermore, we found that the expression of low amounts of the human ABCC6 in liver of transgenic Abcc6-/- mice, resulting in only a 27% increase in plasma PPi levels, led to a major reduction in acute and chronic calcification phenotypes. This proof-of-concept study shows that the development of both acute and chronic calcification associated with ABCC6 deficiency can be prevented by compensating PPi deficits, even partially. Our work indicates that PPi substitution represents a promising strategy to treat ABCC6-dependent calcification disorders.

Next-generation sequencing for mutation detection in heritable skin diseases: the paradigm of pseudoxanthoma elasticum.

Next-generation sequencing applied either to the entire genome or to a subset, such as a whole exome, has revolutionized the search for pathogenic mutations in heritable diseases, including genodermatoses. In this issue, Hosen et al. applied whole-exome sequencing to identify potential pathogenic mutations in four candidate genes associated with pseudoxanthoma elasticum, the prototype of ectopic mineralization disorders. The study highlights the advantages of this approach over traditional Sanger sequencing, including expedience and cost, but it also illustrates some of the challenges encountered in implementing this rapidly evolving technology.

The effects of bisphosphonates on ectopic soft tissue mineralization caused by mutations in the ABCC6 gene.

Pseudoxanthoma elasticum (PXE) and generalized arterial calcification of infancy (GACI) are heritable ectopic mineralization disorders. Most cases of PXE and many cases of GACI harbor mutations in the ABCC6 gene. There is no effective treatment for these disorders. We explored the potential efficacy of bisphosphonates to prevent ectopic calcification caused by ABCC6 mutations by feeding Abcc6(-/-) mice with diet containing etidronate disodium (ETD) or alendronate sodium trihydrate (AST) in quantities corresponding to 1x, 5x, or 12x of the doses used to treat osteoporosis in humans. The mice were placed on diet at 4 weeks of age, and the degree of mineralization was assessed at 12 weeks by quantitation of the calcium deposits in the dermal sheath of vibrissae, a progressive biomarker of the mineralization, by computerized morphometry of histopathologic sections and by direct chemical assay of calcium. We found that ETD, but not AST, at the 12x dosage, significantly reduced mineralization, suggesting that selected bisphosphonates may be helpful for prevention of mineral deposits in PXE and GACI caused by mutations in the ABCC6 gene, when combined with careful monitoring of efficacy and potential side-effects.

ABCC6-mediated ATP secretion by the liver is the main source of the mineralization inhibitor inorganic pyrophosphate in the systemic circulation-brief report.

OBJECTIVE: Mutations in ABCC6 underlie the ectopic mineralization disorder pseudoxanthoma elasticum (PXE) and some forms of generalized arterial calcification of infancy, both of which affect the cardiovascular system. Using cultured cells, we recently showed that ATP-binding cassette subfamily C member 6 (ABCC6) mediates the cellular release of ATP, which is extracellularly rapidly converted into AMP and the mineralization inhibitor inorganic pyrophosphate (PPi). The current study was performed to determine which tissues release ATP in an ABCC6-dependent manner in vivo, where released ATP is converted into AMP and PPi, and whether human PXE ptients have low plasma PPi concentrations. APPROACH AND RESULTS: Using cultured primary hepatocytes and in vivo liver perfusion experiments, we found that ABCC6 mediates the direct, sinusoidal, release of ATP from the liver. Outside hepatocytes, but still within the liver vasculature, released ATP is converted into AMP and PPi. The absence of functional ABCC6 in patients with PXE leads to strongly reduced plasma PPi concentrations. CONCLUSIONS: Hepatic ABCC6-mediated ATP release is the main source of circulating PPi, revealing an unanticipated role of the liver in systemic PPi homeostasis. Patients with PXE have a strongly reduced plasma PPi level, explaining their mineralization disorder. Our results indicate that systemic PPi is relatively stable and that PXE, generalized arterial calcification of infancy, and other ectopic mineralization disorders could be treated with PPi supplementation therapy.