Abnormal phenotype of in vitro dermal fibroblasts from patients with Pseudoxanthoma elasticum (PXE)

Fibroblasts' secretome from calcified and non-calcified dermis in Pseudoxanthoma elasticum differently contributes to elastin calcification

Pseudoxanthoma elasticum (PXE) is a rare disease characterized by ectopic calcification, however, despite the widely spread effect of pro/anti-calcifying systemic factors associated with this genetic metabolic condition, it is not known why elastic fibers in the same patient are mainly fragmented or highly mineralized in clinically unaffected (CUS) and affected (CAS) skin, respectively. Cellular morphology and secretome are investigated in vitro in CUS and CAS fibroblasts. Here we show that, compared to CUS, CAS fibroblasts exhibit: a) differently distributed and organized focal adhesions and stress fibers; b) modified cell-matrix interactions (i.e., collagen gel retraction); c) imbalance between matrix metalloproteinases and tissue inhibitor of metalloproteinases; d) differentially expressed pro- and anti-calcifying proteoglycans and elastic-fibers associated glycoproteins. These data emphasize that in the development of pathologic mineral deposition fibroblasts play an active role altering the stability of elastic fibers and of the extracellular matrix milieu creating a local microenvironment guiding the level of matrix remodeling at an extent that may lead to degradation (in CUS) or to degradation and calcification (in CAS) of the elastic component. In conclusion, this study contributes to a better understanding of the mechanisms of the mineral deposition that can be also associated with several inherited or age-related diseases (e.g., diabetes, atherosclerosis, chronic kidney diseases).

Matrix Metalloproteinases Contribute to the Calcification Phenotype in Pseudoxanthoma Elasticum

Ectopic calcification and dysregulated extracellular matrix remodeling are prominent hallmarks of the complex heterogenous pathobiochemistry of pseudoxanthoma elasticum (PXE). The disease arises from mutations in ABCC6, an ATP-binding cassette transporter expressed predominantly in the liver. Neither its substrate nor the mechanisms by which it contributes to PXE are completely understood. The fibroblasts isolated from PXE patients and Abcc6-/- mice were subjected to RNA sequencing. A group of matrix metalloproteinases (MMPs) clustering on human chromosome 11q21-23, respectively, murine chromosome 9, was found to be overexpressed. A real-time quantitative polymerase chain reaction, enzyme-linked immunosorbent assay and immunofluorescent staining confirmed these findings. The induction of calcification by CaCl2 resulted in the elevated expression of selected MMPs. On this basis, the influence of the MMP inhibitor Marimastat (BB-2516) on calcification was assessed. PXE fibroblasts (PXEFs) exhibited a pro-calcification phenotype basally. PXEF and normal human dermal fibroblasts responded with calcium deposit accumulation and the induced expression of osteopontin to the addition of Marimastat to the calcifying medium. The raised MMP expression in PXEFs and during cultivation with calcium indicates a correlation of ECM remodeling and ectopic calcification in PXE pathobiochemistry. We assume that MMPs make elastic fibers accessible to controlled, potentially osteopontin-dependent calcium deposition under calcifying conditions.

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.

Relationship Between Mitochondrial Structure and Bioenergetics in Pseudoxanthoma elasticum Dermal Fibroblasts

Pseudoxanthoma elasticum (PXE) is a genetic disease considered as a paradigm of ectopic mineralization disorders, being characterized by multisystem clinical manifestations due to progressive calcification of skin, eyes, and the cardiovascular system, resembling an age-related phenotype. Although fibroblasts do not express the pathogenic ABCC6 gene, nevertheless these cells are still under investigation because they regulate connective tissue homeostasis, generating the “arena” where cells and extracellular matrix components can promote pathologic calcification and where activation of pro-osteogenic factors can be associated to pathways involving mitochondrial metabolism. The aim of the present study was to integrate structural and bioenergenetic features to deeply investigate mitochondria from control and from PXE fibroblasts cultured in standard conditions and to explore the role of mitochondria in the development of the PXE fibroblasts’ pathologic phenotype. Proteomic, biochemical, and morphological data provide new evidence that in basal culture conditions (1) the protein profile of PXE mitochondria reveals a number of differentially expressed proteins, suggesting changes in redox balance, oxidative phosphorylation, and calcium homeostasis in addition to modified structure and organization, (2) measure of oxygen consumption indicates that the PXE mitochondria have a low ability to cope with a sudden increased need for ATP via oxidative phosphorylation, (3) mitochondrial membranes are highly polarized in PXE fibroblasts, and this condition contributes to increased reactive oxygen species levels, (4) ultrastructural alterations in PXE mitochondria are associated with functional changes, and (5) PXE fibroblasts exhibit a more abundant, branched, and interconnected mitochondrial network compared to control cells, indicating that fusion prevail over fission events. In summary, the present study demonstrates that mitochondria are modified in PXE fibroblasts. Since mitochondria are key players in the development of the aging process, fibroblasts cultured from aged individuals or aged in vitro are more prone to calcify, and in PXE, calcified tissues remind features of premature aging syndromes; it can be hypothesized that mitochondria represent a common link contributing to the development of ectopic calcification in aging and in diseases. Therefore, ameliorating mitochondrial functions and cell metabolism could open new strategies to positively regulate a number of signaling pathways associated to pathologic calcification.

Assessment of Inflammation and Calcification in Pseudoxanthoma Elasticum Arteries and Skin with 18F-FluroDeoxyGlucose and 18F-Sodium Fluoride Positron Emission Tomography/Computed Tomography Imaging: The GOCAPXE Trial

Background: Pseudoxanthoma elasticum (PXE) is an inherited metabolic disease characterized by elastic fiber fragmentation and ectopic calcification. There is growing evidence that vascular calcification is associated with inflammatory status and is enhanced by inflammatory cytokines. Since PXE has never been considered as an inflammatory condition, no incidence of chronic inflammation leading to calcification in PXE has been reported and should be investigated. In atherosclerosis and aortic stenosis, positron emission tomography combined with computed tomographic (PET-CT) imaging has demonstrated a correlation between inflammation and calcification. The purpose of this study was to assess skin/artery inflammation and calcification in PXE patients. Methods: 18F-FluroDeoxyGlucose (18F-FDG) and 18F-Sodium Fluoride (18F-NaF) PET-CT, CT-imaging and Pulse wave velocity (PWV) were used to determine skin/vascular inflammation, tissue calcification, arterial calcium score (CS) and stiffness, respectively. In addition, inorganic pyrophosphate, high-sensitive C-reactive protein and cytokines plasma levels were monitored. Results: In 23 PXE patients, assessment of inflammation revealed significant 18F-FDG uptake in diseased skin areas contrary to normal regions, and exclusively in the proximal aorta contrary to the popliteal arteries. There was no correlation between 18F-FDG uptake and PWV in the aortic wall. Assessment of calcification demonstrated significant 18F-NaF uptake in diseased skin regions and in the proximal aorta and femoral arteries. 18F-NaF wall uptake correlated with CS in the femoral arteries, and aortic wall PWV. Multivariate analysis indicated that aortic wall 18F-NaF uptake is associated with diastolic blood pressure. There was no significant correlation between 18F-FDG and 18F-NaF uptake in any of the artery walls. Conclusion: In the present cross-sectional study, inflammation and calcification were not correlated. PXE would appear to more closely resemble a chronic disease model of ectopic calcification than an inflammatory condition. To assess early ectopic calcification in PXE patients, 18F-NaF-PET-CT may be more relevant than CT imaging. It potentially constitutes a biomarker for disease-modifying anti-calcifying drug assessment in PXE.

Controlled release of 18-β-glycyrrhetic acid by nanodelivery systems increases cytotoxicity on oral carcinoma cell line

The topical treatment for oral mucosal diseases is often based on products optimized for dermatologic applications; consequently, a lower therapeutic effect may be present. 18-β-glycyrrhetic acid (GA) is extracted from Glycirrhiza glabra. The first aim of this study was to test the cytotoxicity of GA on PE/CA-PJ15 cells. The second aim was to propose and test two different delivery systems, i.e. nanoparticles and fibers, to guarantee a controlled release of GA in vitro. We used chitosan and poly(lactic-co-glycolic) acid based nanoparticles and polylactic acid fibers. We tested both delivery systems in vitro on PE/CA-PJ15 cells and on normal human gingival fibroblasts (HGFs). The morphology of GA-loaded nanoparticles (GA-NPs) and fibers (GA-FBs) was investigated by electron microscopy and dynamic light scattering; GA release kinetics was studied spectrophotometrically. MTT test was used to assess GA cytotoxicity on both cancer and normal cells. Cells were exposed to different concentrations of GA (20-500 μmol l^-1) administered as free GA (GA-f), and to GA-NPs or GA-FBs. ROS production was evaluated using dichlorodihydrofluorescein as a fluorescent probe. Regarding the cytotoxic effect of GA on PE/CA-PJ15 cells, the lowest TC₅₀ value was 200 μmol l^-1 when GA was added as GA-NPs. No cytotoxic effects were observed when GA was administered to HGFs. N-acetyl Cysteine reduced mortality induced by GA-f in PE/CA-PJ15 cells. The specific effect of GA on PE/CA-PJ15 cells is mainly due to the different sensitivity of cancer cells to ROS over-production; GA-NPs and GA-FBs formulations increase, in vitro, this toxic effect on oral cancer cells.

Mineralization by mesenchymal stromal cells is variously modulated depending on commercial platelet lysate preparations

BACKGROUND AIMS

Numerous cellular models have been developed to investigate calcification for regenerative medicine applications and for the identification of therapeutic targets in various complications associated with age-related diseases. However, results have often been contradictory due to specific culture conditions, cell type ontogeny and aging status. Human platelet lysate (hPL) has been recently investigated as valuable alternative to fetal bovine serum (FBS) in cell culture and bone regeneration. A parallel comparison of how all these multiple factors may converge to influence mineralization has yet to be reported.

METHODS

To compare mineralization of human mesenchymal cell types known to differ in extracellular matrix calcification potency, bone marrow-derived mesenchymal stromal cells and dermal fibroblasts from neonatal and adult donors, at both low and high passages, were investigated in an ex vivo experimental model by supplementing the osteogenic induction medium with FBS or with hPL. Four commercial hPL preparations were profiled by liquid chromatography/electrospray ionization quadrupole time-of-flight spectrometry, and mineralization was visualized by von Kossa staining and quantified by morphometric evaluations after 9, 14 and 21 days of culture.

RESULTS

Data demonstrate that (i) commercial hPL preparations differ according to mass spectra profiles, (ii) hPL variously influences mineral deposition depending on cell line and possibly on platelet product preparation methods, (iii) donor age modifies mineral deposition in the presence of the same hPL and (iv) reduced in vitro proliferative capacity affects osteogenic induction and response to hPL.

CONCLUSION

Despite the standardized procedures applied to obtain commercial hPL, this study highlights the divergent effects of different preparations and emphasizes the importance of cellular ontology, donor age and cell proliferative capacity to optimize the osteogenic induction capabilities of mesenchymal stromal cells and design more effective cell-based therapeutic protocols.

Characterization of dermal myofibroblast differentiation in pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is a rare hereditary disorder which is caused by ABCC6 (ATP-binding cassette subfamily C member 6) gene mutations. Characteristic hallmarks of PXE are progressive calcification and degradation of the elastic fibers in skin, cardiovascular system and ocular fundus. Since the underlying pathomechanisms of PXE remain unidentified, the aim of this study was to get new insights into PXE pathophysiology by characterizing dermal myofibroblast differentiation. Fibroblasts are the key cells of extracellular matrix (ECM) remodeling and, therefore, participate not only in physiological processes, such as calcification or wound healing, but also in pathologic events, such as fibrotization. We revealed that human dermal PXE fibroblasts possess exaggerated migration capability in wound healing and attenuated myofibroblast contractility in comparison to controls. Subsequent analyses reinforced these observations and indicated a diminished induction of the myofibroblast differentiation markers α-smooth muscle actin and xylosyltransferase-I as well as poor transforming growth factor-β1 responsiveness in PXE fibroblasts. In summary, we describe pathological deviations of dermal myofibroblast differentiation in PXE which might be mediated by aberrant supramolecular ECM organization. These results not only improve our insights into cellular PXE pathophysiology, but might also qualify us to interfere with ECM remodeling in the future.

Ectopic calcification in pseudoxanthoma elasticum responds to inhibition of tissue-nonspecific alkaline phosphatase

Biallelic mutations in ABCC6 cause pseudoxanthoma elasticum (PXE), a disease characterized by calcification in the skin, eyes, and blood vessels. The function of ATP-binding cassette C6 (ABCC6) and the pathogenesis of PXE remain unclear. We used mouse models and patient fibroblasts to demonstrate genetic interaction and shared biochemical and cellular mechanisms underlying ectopic calcification in PXE and related disorders caused by defined perturbations in extracellular adenosine 5'-triphosphate catabolism. Under osteogenic culture conditions, ABCC6 mutant cells calcified, suggesting a provoked cell-autonomous defect. Using a conditional Abcc6 knockout mouse model, we excluded the prevailing pathogenic hypothesis that singularly invokes failure of hepatic secretion of an endocrine inhibitor of calcification. Instead, deficiency of Abcc6 in both local and distant cells was necessary to achieve the early onset and penetrant ectopic calcification observed upon constitutive gene targeting. ABCC6 mutant cells additionally had increased expression and activity of tissue-nonspecific alkaline phosphatase (TNAP), an enzyme that degrades pyrophosphate, a major inhibitor of calcification. A selective and orally bioavailable TNAP inhibitor prevented calcification in ABCC6 mutant cells in vitro and attenuated both the development and progression of calcification in Abcc6-/- mice in vivo, without the deleterious effects on bone associated with other proposed treatment strategies.

Genetics of inherited cardiocutaneous syndromes: a review

The life of a human being originates as a single cell which, under the influence of certain factors, divides sequentially into multiple cells that subsequently become committed to develop and differentiate into the different structures and organs. Alterations occurring early on in the development process may lead to fetal demise in utero. Conversely, abnormalities at later stages may result in structural and/or functional abnormalities of varying severities. The cardiovascular system and skin share certain developmental and structural factors; therefore, it is not surprising to find several inherited syndromes with both cardiac and skin manifestations. Here, we will review the overlapping pathways in the development of the skin and heart, as well as the resulting syndromes. We will also highlight several cutaneous clues that may help physicians screen and uncover cardiac anomalies that may be otherwise hidden and result in sudden cardiac death.

Inhibition of Lon protease by triterpenoids alters mitochondria and is associated to cell death in human cancer cells

Mitochondrial Lon protease (Lon) regulates several mitochondrial functions, and is inhibited by the anticancer molecule triterpenoid 2-cyano-3, 12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO), or by its C-28 methyl ester derivative (CDDO-Me). To analyze the mechanism of action of triterpenoids, we investigated intramitochondrial reactive oxygen species (ROS), mitochondrial membrane potential, mitochondrial mass, mitochondrial dynamics and morphology, and Lon proteolytic activity in RKO human colon cancer cells, in HepG2 hepatocarcinoma cells and in MCF7 breast carcinoma cells. We found that CDDO and CDDO-Me are potent stressors for mitochondria in cancer cells, rather than normal non-transformed cells. In particular, they: i) cause depolarization; ii) increase mitochondrial ROS, iii) alter mitochondrial morphology and proteins involved in mitochondrial dynamics; iv) affect the levels of Lon and those of aconitase and human transcription factor A, which are targets of Lon activity; v) increase level of protein carbonyls in mitochondria; vi) lead to intrinsic apoptosis. The overexpression of Lon can rescue cells from cell death, providing an additional evidence on the role of Lon in conditions of excessive stress load.

Adhesion and Proliferation of Human Dermal Fibroblasts on Collagen Matrix

The purpose of this study was to evaluate adhesion and growth of human dermal fibroblasts on a 0.150 mm-thick matrix of reconstituted collagen isolated from horse tendon. Collagen was extracted and polymerized according to the standard procedures (Opocrin, Corlo, Modena, Italy). By light microscopy, the bottom surface of the matrix appeared linear and compact, whereas the superficial one was indented and less homogeneous. By scanning electron microscopy, the collagen fibrils had different diameters and the great majority of them was oriented parallel to the surface of the gel. By transmission electron microscopy, collagen fibrils showed the typical banding. Human dermal fibroblasts were seeded on the collagen matrix, previously equilibrated in growth medium. Fibroblast proliferation stopped in the second week and was always significantly lower than that of the same cell strain seeded on plastic and cultured in parallel. By light microscopy, after six days culture, cells formed a confluent multilayer on the surface of the gel. By scanning and transmission electron microscopy, fibroblasts appeared flat and adherent to the matrix. Contacts of cells among themselves and with the collagen fibrils were observed. Fibroblasts never moved into the collagen gel. In conclusion, human dermal fibroblasts can be grown in a three-dimensional matrix made by horse tendon that, on the other hand, seems to condition their proliferation rate.

Magnesium Modifies the Structural Features of Enzymatically Mineralized Collagen Gels Affecting the Retraction Capabilities of Human Dermal Fibroblasts Embedded within This 3D System

Mineralized collagen gels have been developed as in vitro models to better understand the mechanisms regulating the calcification process and the behavior of a variety of cell types. The vast majority of data are related to stem cells and to osteoblast-like cells, whereas little information is available for dermal fibroblasts, although these cells have been associated with ectopic calcification and consequently to a number of pathological conditions. Therefore, we developed and characterized an enzymatically mineralized collagen gel in which fibroblasts were encapsulated within the 3D structure. MgCl₂ was also added during gel polymerization, given its role as (i) modulator of ectopic calcification; (ii) component of biomaterials used for bone replacement; and (iii) constituent of pathological mineral deposits. Results demonstrate that, in a short time, an enzymatically mineralized collagen gel can be prepared in which mineral deposits and viable cells are homogeneously distributed. MgCl₂ is present in mineral deposits and significantly affects collagen fibril assembly and organization. Consequently, cell shape and the ability of fibroblasts to retract collagen gels were modified. The development of three-dimensional (3D) mineralized collagen matrices with both different structural features and mineral composition together with the use of fibroblasts, as a prototype of soft connective tissue mesenchymal cells, may pave new ways for the study of ectopic calcification.

Donor's age and replicative senescence favour the in-vitro mineralization potential of human fibroblasts

Aberrant mineralization of soft connective tissues (ectopic calcification) may occur as a frequent age-related complication. Still, it remains unclear the role of mesenchymal cell donor's age and of replicative senescence on ectopic calcification. Therefore, the ability of cells to deposit in-vitro hydroxyapatite crystals and the expression of progressive ankylosis protein homolog (ANKH), ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), tissue non specific alkaline phosphatase (TNAP) and osteopontin (OPN) have been evaluated in human dermal fibroblasts derived from neonatal (nHDF) and adult (aHDF) donors (ex-vivo ageing model) or at low and high cumulative population doublings (CPD) up to replicative senescence (in-vitro ageing model). This study demonstrates that: 1) replicative senescence favours hydroxyapatite formation in cultured fibroblasts; 2) donor's age acts as a major modulator of the mineralizing potential of HDF, since nHDF are less prone than aHDF to induce calcification; 3) donor's age and replicative senescence play in concert synergistically increasing the calcification process; 4) the ANKH+ENPP1/TNAP ratio, being crucial for pyrophosphate/inorganic phosphate balance, is greatly influenced by donor's age, as well as by replicative senescence, and regulates mineral deposition; 5) OPN is only modulated by replicative senescence.

Characterization of the cell penetrating properties of a human salivary proline-rich peptide

Saliva contains hundreds of small proline-rich peptides most of which derive from the post-translational and post-secretory processing of the acidic and basic salivary proline-rich proteins. Among these peptides we found that a 20 residue proline-rich peptide (p1932), commonly present in human saliva and patented for its antiviral activity, was internalized within cells of the oral mucosa. The cell-penetrating properties of p1932 have been studied in a primary gingival fibroblast cell line and in a squamous cancer cell line, and compared to its retro-inverso form. We observed by mass-spectrometry, flow cytometry and confocal microscopy that both peptides were internalized in the two cell lines on a time scale of minutes, being the natural form more efficient than the retro-inverso one. The cytosolic localization was dependent on the cell type: both peptide forms were able to localize within nuclei of tumoral cells, but not in the nuclei of gingival fibroblasts. The uptake was shown to be dependent on the culture conditions used: peptide internalization was indeed effective in a complete medium than in a serum-free one allowing the hypothesis that the internalization could be dependent on the cell cycle. Both peptides were internalized likely by a lipid raft-mediated endocytosis mechanism as suggested by the reduced uptake in the presence of methyl-ß-cyclodextrin. These results suggest that the natural peptide may play a role within the cells of the oral mucosa after its secretion and subsequent internalization. Furthermore, lack of cytotoxicity of both peptide forms highlights their possible application as novel drug delivery agents.

The ABCC6 Transporter as a Paradigm for Networking from an Orphan Disease to Complex Disorders

The knowledge on the genetic etiology of complex disorders largely results from the study of rare monogenic disorders. Often these common and rare diseases show phenotypic overlap, though monogenic diseases generally have a more extreme symptomatology. ABCC6, the gene responsible for pseudoxanthoma elasticum, an autosomal recessive ectopic mineralization disorder, can be considered a paradigm gene with relevance that reaches far beyond this enigmatic orphan disease. Indeed, common traits such as chronic kidney disease or cardiovascular disorders have been linked to the ABCC6 gene. While during the last decade the awareness of the wide ramifications of ABCC6 has increased significantly, the gene itself and the transmembrane transporter it encodes have not unveiled all of the mysteries that surround them. To gain more insights, multiple approaches are being used including next-generation sequencing, computational methods, and various "omics" technologies. Much effort is made to place the vast amount of data that is gathered in an integrated system-biological network; the involvement of ABCC6 in common disorders provides a good view on the wide implications and potential of such a network. In this review, we summarize the network approaches used to study ABCC6 and the role of this gene in several complex diseases.

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.

ABCC6- a new player in cellular cholesterol and lipoprotein metabolism?

BACKGROUND

Dysregulations in cholesterol and lipid metabolism have been linked to human diseases like hypercholesterolemia, atherosclerosis or the metabolic syndrome. Many ABC transporters are involved in trafficking of metabolites derived from these pathways. Pseudoxanthoma elasticum (PXE), an autosomal-recessive disease caused by ABCC6 mutations, is characterized by atherogenesis and soft tissue calcification.

METHODS

In this study we investigated the regulation of cholesterol biosynthesis in human dermal fibroblasts from PXE patients and healthy controls.

RESULTS

Gene expression analysis of 84 targets indicated dysregulations in cholesterol metabolism in PXE fibroblasts. Transcript levels of ABCC6 were strongly increased in lipoprotein-deficient serum (LPDS) and under serum starvation in healthy controls. For the first time, increased HMG CoA reductase activities were found in PXE fibroblasts. We further observed strongly elevated transcript and protein levels for the proprotein convertase subtilisin/kexin type 9 (PCSK9), as well as a significant reduction in APOE mRNA expression in PXE.

CONCLUSION

Increased cholesterol biosynthesis, elevated PCSK9 levels and reduced APOE mRNA expression newly found in PXE fibroblasts could enforce atherogenesis and cardiovascular risk in PXE patients. Moreover, the increase in ABCC6 expression accompanied by the induction of cholesterol biosynthesis supposes a functional role for ABCC6 in human lipoprotein and cholesterol homeostasis.

Fibroblasts from patients affected by Pseudoxanthoma elasticum exhibit an altered PPi metabolism and are more responsive to pro-calcifying stimuli

BACKGROUND

Pseudoxanthoma elasticum (PXE) is a genetic disorder characterized by progressive calcification of soft connective tissues. The pathogenesis is still hard to pin down. In PXE dermal fibroblasts, in addition to impaired carboxylation of the vitamin K-dependent inhibitor matrix Gla protein (MGP), we have also demonstrated an up-regulation of alkaline phosphatase activity. In the light of these data we have suggested that both calcium and phosphate metabolism might be locally altered, both pathways acting in synergy on the occurrence of matrix calcification.

OBJECTIVE

This study aims to better explore if cultured PXE fibroblasts, compared to control cells, exhibit a modified inorganic pyrophosphate (PPi) metabolism and are more responsive to pro-calcifying stimuli.

METHODS

Primary human dermal fibroblasts isolated from healthy individuals and from PXE patients were cultured for different time points in standard and in pro-calcifying media. The expression of ANKH/ANKH, ENPP1/PC1, ALPL/TNAP, SPP1/OPN was evaluated by qRT-PCR and Western blot, respectively. TNAP activity was measured by spectrophotometric analyses, whereas calcification was investigated by light and electron microscopy as well as by micro-analytical techniques.

RESULTS

In the presence of pro-calcifying stimuli, dermal fibroblasts alter their phenotype favouring matrix mineralization. In particular, ENPP1/PC1 and SPP1/OPN expression, as well as TNAP activity, was differently expressed in control and in PXE fibroblasts. Moreover, in pathologic cells the ratio between factors favouring and reducing PPi availability exhibits a more pronounced shift towards a pro-calcifying balance.

CONCLUSION

PXE fibroblasts are more susceptible to pro-calcifying stimuli and in these cells an altered PPi metabolism contributes to matrix calcification.

Changes in dermal fibroblasts from Abcc6(-/-) mice are present before and after the onset of ectopic tissue mineralization

Pseudoxanthoma elasticum (PXE), a rare genetic disease caused by mutations in the ABCC6 gene, is characterized by progressive calcification of elastic fibers in the skin, eyes and the cardiovascular system. The pathomechanisms of the mineralization is still obscure. Several hypotheses have been proposed, one of them suggesting a role for fibroblasts in controlling the amount and the quality of the calcified extracellular matrix. This hypothesis raises the question whether changes in mesenchymal cells are the cause and/or the consequences of the calcification process. In this study, fibroblasts were isolated and cultured from Abcc6^+/+ and Abcc6^−/− mice of different ages in order to investigate parameters known to be associated with the phenotype of fibroblasts from PXE patients. Results demonstrate few changes (Ank and Opn down-regulation) are already present before the occurrence of calcification. By contrast, a modification of other parameters (intracellular O₂⁻ content, Tnap activity and Bmp2 up-regulation) can be observed in Abcc6^−/− mice after the onset of tissue mineralization. These data suggest that in the Abcc6^−/− genotype, dermal fibroblasts actively contribute to changes that promote matrix calcification and that these cells can be further modulated with time by the calcified environment, thus contributing to the age-dependent progression of the disease.

Ectopic calcification in β-thalassemia patients is associated with increased oxidative stress and lower MGP carboxylation

A number of beta-thalassemia (β-thal) patients in the course of the disease exhibit ectopic calcification affecting skin, eyes and the cardiovascular system. Clinical and histopathological features have been described similar to those in pseudoxanthoma elasticum (PXE), although different genes are affected in the two diseases. Cultured dermal fibroblasts from β-thal patients with and without PXE-like clinical manifestations have been compared for parameters of redox balance and for the expression of proteins, which have been already associated with the pathologic mineralisation of soft connective tissues. Even though oxidative stress is a well-known condition of β-thal patients, our results indicate that the occurrence of mineralized elastin is associated with a more pronounced redox disequilibrium, as demonstrated by the intracellular increase of anion superoxide and of oxidized proteins and lipids. Moreover, fibroblasts from β-thal PXE-like patients are characterized by decreased availability of carboxylated matrix Gla protein (MGP), as well as by altered expression of proteins involved in the vitamin K-dependent carboxylation process. Results demonstrate that elastic fibre calcification is promoted when redox balance threshold levels are exceeded and the vitamin K-dependent carboxylation process is affected decreasing the activity of MGP, a well-known inhibitor of ectopic calcification. Furthermore, independently from the primary gene defect, these pathways are similarly involved in fibroblasts from PXE and from β-thal PXE-like patients as well as in other diseases leading to ectopic calcification, thus suggesting that can be used as markers of pathologic mineralisation.

Characterization of cardiovascular involvement in pseudoxanthoma elasticum families

OBJECTIVE

Pseudoxanthoma elasticum (PXE) is an autosomal recessive connective tissue disorder with involvement of the skin, the retina, and the cardiovascular system. Cardiovascular involvement is mainly characterized by mineralization and fragmentation of elastic fibers of blood vessels and premature atherosclerosis. We conducted an ultrasound study to investigate the cardiovascular phenotype and to propose recommendations for the management of patients with PXE and heterozygous ABCC6 mutation carriers.

APPROACH AND RESULTS

Thirty-two patients, 23 carriers, and 28 healthy volunteers underwent cardiac and vascular ultrasound studies. Cardiac imaging revealed left ventricular diastolic dysfunction in patients with PXE with a significantly prolonged deceleration time and lower septal early diastolic velocities of the mitral annulus compared with controls. Carriers also demonstrated significantly prolonged deceleration time. Carotid-to-femoral pulse wave velocity was significantly increased in patients with PXE when compared with carriers and controls. Vascular imaging revealed a high prevalence of peripheral artery disease in both patients and carriers and a significantly higher carotid intima-media thickness compared with controls.

CONCLUSIONS

The results of this study clearly demonstrate impaired left ventricular diastolic function, impairment of the elastic properties of the aorta, and a high prevalence of peripheral artery disease in patients with PXE. Carriers also seem to exhibit a cardiovascular phenotype with mainly mild diastolic dysfunction and accelerated atherosclerosis. Increased awareness for cardiovascular events in both patients and heterozygous carriers is warranted.

Fibroblast involvement in soft connective tissue calcification

Soft connective tissue calcification is not a passive process, but the consequence of metabolic changes of local mesenchymal cells that, depending on both genetic and environmental factors, alter the balance between pro- and anti-calcifying pathways. While the role of smooth muscle cells and pericytes in ectopic calcifications has been widely investigated, the involvement of fibroblasts is still elusive. Fibroblasts isolated from the dermis of pseudoxanthoma elasticum (PXE) patients and of patients exhibiting PXE-like clinical and histopathological findings offer an attractive model to investigate the mechanisms leading to the precipitation of mineral deposits within elastic fibers and to explore the influence of the genetic background and of the extracellular environment on fibroblast-associated calcifications, thus improving the knowledge on the role of mesenchymal cells on pathologic mineralization.

The vascular phenotype in Pseudoxanthoma elasticum and related disorders: contribution of a genetic disease to the understanding of vascular calcification

Vascular calcification is a complex and dynamic process occurring in various physiological conditions such as aging and exercise or in acquired metabolic disorders like diabetes or chronic renal insufficiency. Arterial calcifications are also observed in several genetic diseases revealing the important role of unbalanced or defective anti- or pro-calcifying factors. Pseudoxanthoma elasticum (PXE) is an inherited disease (OMIM 264800) characterized by elastic fiber fragmentation and calcification in various soft conjunctive tissues including the skin, eyes, and arterial media. The PXE disease results from mutations in the ABCC6 gene, encoding an ATP-binding cassette transporter primarily expressed in the liver, kidneys suggesting that it is a prototypic metabolic soft-tissue calcifying disease of genetic origin. The clinical expression of the PXE arterial disease is characterized by an increased risk for coronary (myocardial infarction), cerebral (aneurysm and stroke), and lower limb peripheral artery disease. However, the structural and functional changes in the arterial wall induced by PXE are still unexplained. The use of a recombinant mouse model inactivated for the Abcc6 gene is an important tool for the understanding of the PXE pathophysiology although the vascular impact in this model remains limited to date. Overlapping of the PXE phenotype with other inherited calcifying diseases could bring important informations to our comprehension of the PXE disease.

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.

The molecular and physiological roles of ABCC6: more than meets the eye

Abnormal mineralization occurs in the context of several common conditions, including advanced age, diabetes, hypercholesterolemia, chronic renal failure, and certain genetic conditions. Metabolic, mechanical, infectious, and inflammatory injuries promote ectopic mineralization through overlapping yet distinct molecular mechanisms of initiation and progression. The ABCC6 protein is an ATP-dependent transporter primarily found in the plasma membrane of hepatocytes. ABCC6 exports unknown substrates from the liver presumably for systemic circulation. ABCC6 deficiency is the primary cause for chronic and acute forms of ectopic mineralization described in diseases such as pseudoxanthoma elasticum (PXE), β-thalassemia, and generalized arterial calcification of infancy (GACI) in humans and dystrophic cardiac calcification (DCC) in mice. These pathologies are characterized by mineralization of cardiovascular, ocular, and dermal tissues. PXE and to an extent GACI are caused by inactivating ABCC6 mutations, whereas the mineralization associated with β-thalassemia patients derives from a liver-specific change in ABCC6 expression. DCC is an acquired phenotype resulting from cardiovascular insults (ischemic injury or hyperlipidemia) and secondary to ABCC6 insufficiency. Abcc6-deficient mice develop ectopic calcifications similar to both the human PXE and mouse DCC phenotypes. The precise molecular and cellular mechanism linking deficient hepatic ABCC6 function to distal ectopic mineral deposition is not understood and has captured the attention of many research groups. Our previously published work along with that of others show that ABCC6 influences other modulators of calcification and that it plays a much greater physiological role than originally thought.

Histopathology of pseudoxanthoma elasticum and related disorders: histological hallmarks and diagnostic clues

Among ectopic mineralization disorders, pseudoxanthoma elasticum (PXE)-a rare genodermatosis associated with ocular and cardiovascular manifestations-is considered a paradigm disease. The symptoms of PXE are the result of mineralization and fragmentation of elastic fibers, the exact pathophysiology of which is incompletely understood. Though molecular analysis of the causal gene, ABCC6, has a high mutation uptake, a skin biopsy has until now been considered the golden standard to confirm the clinical diagnosis. Although the histological hallmarks of PXE are rather specific, several other diseases-particularly those affecting the skin-can present with clinical and/or histological characteristics identical to or highly resemblant of PXE. In this paper, we will summarize the histopathological features of PXE together with those of disorders that are most frequently considered in the differential diagnosis of PXE.

Pseudoxanthoma elasticum: progress in diagnostics and research towards treatment : Summary of the 2010 PXE International Research Meeting

Pseudoxanthoma elasticum (PXE), a prototypic heritable disorder with ectopic mineralization, manifests with characteristic skin findings, ocular involvement, and cardiovascular problems. The classic forms of PXE are due to loss-of-function mutations in the ABCC6 gene, which encodes ABCC6, a putative transmembrane efflux transporter expressed primarily in the liver. While considerable progress has recently been made in understanding the molecular genetics and pathomechanisms of PXE, no effective or specific treatment is currently available for this disorder. PXE International, the premiere patient advocacy organization, organized a workshop in November 2010 to assess the current state of diagnostics and research to develop an agenda towards treatment of PXE. This overview summarizes the progress in PXE research, with emphasis on molecular therapies for this, currently intractable, disorder.

The abcc6a gene expression is required for normal zebrafish development

Pseudoxanthoma elasticum (PXE) is caused by mutations in the ABCC6 gene, which encodes a putative efflux transporter, ABCC6. The zebrafish (Danio rerio) has two ABCC6-related sequences. To study the function of abcc6 during zebrafish development, the mRNA expression levels were measured using RT-PCR and in situ hybridization. The abcc6a showed a relatively high level of expression at 5 days post-fertilization (d.p.f.) and the expression was specific to the Kupffer's vesicles. The abcc6b expression was evident at 6 hours post-fertilization (h.p.f.) and remained high up to 8 d.p.f., corresponding to embryonic kidney proximal tubules. Morpholinos were designed to both genes to prevent pre-mRNA splicing and block translation. Injection of the abcc6a morpholinos into 1-4 cell zebrafish embryos decreased gene expression by 54-81%, and induced a phenotype, pericardial edema and curled tail associated with death at around 8 d.p.f. Microinjecting zebrafish embryos with full-length mouse Abcc6 mRNA together with the morpholino completely rescued this phenotype. No phenotypic changes were observed when the abcc6b gene morpholino was injected into embryos with knock-down efficiency of 100%. These results suggest that abcc6a is an essential gene for normal zebrafish development and provide insight into the function of ABCC6, the gene mutated in PXE.

Pseudoxanthoma elasticum: molecular genetics and putative pathomechanisms

Pseudoxanthoma elasticum (PXE), a prototypic heritable disorder with ectopic mineralization, manifests with characteristic skin findings, ocular involvement and cardiovascular problems, with considerable morbidity and mortality. The classic forms of PXE are due to loss-of-function mutations in the ABCC6 gene, which encodes ABCC6, a transmembrane efflux transporter expressed primarily in the liver. Several lines of evidence suggest that PXE is a primary metabolic disorder, which in the absence of ABCC6 transporter activity, displays reduced plasma anti-mineralization capacity due to reduced fetuin-A and matrix gla-protein (MGP) levels. MGP requires to be activated by gamma-glutamyl carboxylation, a vitamin K-dependent reaction, to serve in an anti-mineralization role in the peripheral connective tissue cells. Although the molecules transported from the hepatocytes to circulation by ABCC6 in vivo remain unidentified, it has been hypothesized that a critical vitamin K derivative, such as reduced vitamin K conjugated with glutathione, is secreted to circulation physiologically, but not in the absence of ABCC6 transporter activity. As a result, activation of MGP by gamma-glutamyl carboxylase is diminished, allowing slow yet progressive mineralization of connective tissues characteristic of PXE. Understanding of the pathomechanistic details of PXE provides a basis for the development of targeted molecular therapies for this currently intractable disease.

Parabiotic heterogenetic pairing of Abcc6-/-/Rag1-/- mice and their wild-type counterparts halts ectopic mineralization in a murine model of pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE), a pleiotropic heritable disorder, is characterized by ectopic mineralization of the connective tissues. This disease is caused by mutations in the ABCC6 gene, which is expressed primarily in the baso-lateral surface of hepatocytes, and Abcc6(-/-) mice develop progressive mineralization mimicking human PXE. To investigate the hypothesis that PXE is a metabolic disorder, potentially caused by the absence of antimineralization factor(s) in circulation, we used parabiotic pairing, ie, surgical joining of two mice, to create a shared circulation between various Abcc6 genotypic mice. To prevent immune reaction between the parabiotic animals, all mice were bred to be Rag1(-/-). Shared circulation between the parabiotic animals was confirmed by Evans blue dye injection and by quantitative PCR of blood cell genotypes. Pairing of Abcc6(-/-) mice with their wild-type counterparts halted the connective tissue mineralization in the knockout mice. Homogenetic wild-type and heterozygous pairings serving as controls were phenotypically unaffected by parabiosis. Consequently, the observations on the parabiotic mice support the notion that PXE is a metabolic disease, potentially due to absence of systemic antimineralization factor(s). These observations suggest that reintroduction of the critical antimineralization factors into circulation could provide a potential treatment for this, currently intractable, disease.

Fibroblast protein profile analysis highlights the role of oxidative stress and vitamin K recycling in the pathogenesis of pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is a genetic disorder associated to mutations in the ABCC6 gene; however, the pathogenetic mechanisms leading to elastic fibre calcifications and to clinical manifestations are still unknown. Dermal fibroblasts, directly involved in the production of the extracellular milieu, have been isolated from healthy subjects and from patients affected by PXE, cultured in vitro and characterized for their ability to produce reactive oxygen species, for structural and functional properties of their cell membranes, for changes in their protein profile. Data demonstrate that oxidative stress has profound and endurable consequences on PXE fibroblast phenotype being responsible for: reduced levels of global DNA methylation, increased amount of carbonylated proteins and of lipid peroxidation products, altered structural properties of cell membranes, modified protein expression. Data shed new light on the pathogenetic pathways in PXE, by identifying a network of proteins affecting elastic fibre calcification through inefficient vitamin K recycling, and highlight the role of differentially expressed proteins as targets for validating the efficacy of future therapeutic strategies aiming to delay and/or revert the pathologic phenotype of PXE fibroblasts. Moreover, data open new perspectives for investigating PXE-like phenotypes in the absence of ABCC6 mutations.

Cytotoxic Evaluation of Elastomeric Dental Impression Materials on a Permanent Mouse Cell Line and on a Primary Human Gingival Fibroblast Culture

The need for clinically relevant in vitro tests of dental materials is widely recognized. Nearly all dental impression materials are introduced into the mouth just after mixing and allowed to set in contact with the oral tissues. Under these conditions, the materials may be toxic to cells or may sensitize the tissues. The aim of the present study is to evaluate the potential cytotoxicity of new preparations of elastomeric dental impression materials: A) four vinylpolysiloxanes: Elite H-D Putty and Elite H-D Light Body (Zhermack, Badia Polesine, Rovigo, Italy); Express Putty and Express Light Body (3M ESPE AG Seefeld, Germany) and B) two polyethers: Impregum Penta and Permadyne Penta L (3M ESPE AG Seefeld, Germany). The cytotoxicity of these impression materials were examined using two different cell lines: Balb/c 3T3 (permanent cell line) and human gingival fibroblasts (primary cell line) and their effects were studied by indirect and direct tests. The direct tests are performed by placing one sample of the impression materials in the centre of the Petri dishes at the time of the seeding of cells. The cell growth was evaluated at the 12th and 24th hours by cell number. The indirect tests were performed by incubating a square of 1 cm diameter impression material in 5 mL of medium at 37 °C for 24 hours (“eluates”). Subconfluent cultures are incubated with “eluates” for 24 hours. The MTT-formazan production is the method used for measuring the cell viability. The results indicate that: a) polyether materials are cytotoxic under both experimental conditions; b) among vinylpolysiloxanes, only Express Light Body (3M ESPE AG Seefeld, Germany) induces clear inhibition of cellular viability of Balb/c 3T3 evaluated by direct and indirect tests and c) the primary cell line is less sensitive to the toxic effect than the permanent cell line.

Pseudoxanthoma elasticum: genetics, clinical manifestations and therapeutic approaches

Pseudoxanthoma elasticum (PXE) is an inherited disorder that is associated with accumulation of mineralized and fragmented elastic fibers in the skin, vascular walls, and Bruch's membrane in the eye. Clinically, patients exhibit characteristic lesions of the posterior segment of the eye including peau d'orange, angioid streaks, and choroidal neovascularisations, of the skin including soft, ivory-colored papules in a reticular pattern that predominantly affect the neck and large flexor surfaces, and of the cardiovascular system with peripheral and coronary arterial occlusive disease as well as gastrointestinal bleedings. There is yet no definitive therapy. Recent studies suggest that PXE is inherited almost exclusively as an autosomal recessive trait. Its prevalence has been estimated to be 1:25,000-100,000. Very recently, the ABCC6 gene on chromosome 16p13.1 was found to be associated with the disease. Mutations within ABCC6 cause reduced or absent transmembraneous transport that leads to accumulation of extracellular material. Presumably, this mechanism causes calcification of elastic fibers. Despite the characteristic clinical features, the variability in phenotypic expressions, and the low prevalence may be responsible for the disease being underdiagnosed. This review compiles and summarizes current knowledge of PXE pathogenesis and clinical findings. Furthermore, different therapeutic strategies to treat retinal manifestations are discussed, including thermal laser coagulation, photodynamic therapy, and intravitreal injections of drugs inhibiting vascular endothelial growth factor.

Pseudoxanthoma elasticum: clinical phenotypes, molecular genetics and putative pathomechanisms

Pseudoxanthoma elasticum (PXE), a prototype of heritable multisystem disorders, is characterised by pathologic mineralisation of connective tissues, with primary clinical manifestations in the skin, eyes and the cardiovascular system. The causative gene was initially identified as ABCC6 which encodes an ABC transporter protein (ABCC6) expressed primarily in the liver and the kidneys. The critical role of ABCC6 in ectopic mineralisation has been confirmed by the development of Abcc6(-/-) knock-out mice which recapitulate the features of connective tissue mineralisation characteristic of PXE. Over 300 distinct loss-of-function mutations representative of over 1000 mutant alleles in ABCC6 have been identified by streamlined mutation detection strategies in this autosomal recessive disease. More recently, missense mutations in the GGCX gene, either in compound heterozygous state or digenic with a recurrent ABCC6 nonsense mutation (p.R1141X), have been identified in patients with PXE-like cutaneous findings and vitamin K-dependent coagulation factor deficiency. GGCX encodes a carboxylase which catalyses gamma-glutamyl carboxylation of coagulation factors as well as of matrix gla protein (MGP) which in fully carboxylated form serves as a systemic inhibitor of pathologic mineralisation. Collectively, these observations suggest the hypothesis that a consequence of loss-of-function mutations in the ABCC6 gene is the reduced vitamin K-dependent gamma-glutamyl carboxylation of MGP, with subsequent connective tissue mineralisation. Further progress in understanding the detailed pathomechanisms of PXE should provide novel strategies to counteract, and perhaps cure, this complex heritable disorder at the genome-environment interface.

Gene expression profiling of ABC transporters in dermal fibroblasts of pseudoxanthoma elasticum patients identifies new candidates involved in PXE pathogenesis

Mutations in the ABCC6 gene, encoding the multidrug resistance-associated protein 6 (MRP6), cause pseudoxanthoma elasticum (PXE). This heritable disorder leads to pathological alterations in connective tissues. The implication of MRP6 deficiency in PXE is still unknown. Moreover, nothing is known about a possible compensatory expression of other ATP binding-cassette (ABC) transporter proteins in MRP6-deficient cells. We investigated the gene expression profile of 47 ABC transporters in human dermal fibroblasts of healthy controls (n=2) and PXE patients (n=4) by TaqMan low-density array. The analysis revealed the expression of 37 ABC transporter genes in dermal fibroblasts. ABCC6 gene expression was not quantifiable in fibroblasts derived from PXE patients. Seven genes (ABCA6, ABCA9, ABCA10, ABCB5, ABCC2, ABCC9 and ABCD2) were induced, whereas the gene expression of one gene (ABCA3) was decreased, comparing controls and PXE patients (with at least twofold changes). We reanalyzed the gene expression of selected ABC transporters in a larger set of dermal fibroblasts from controls and PXE patients (n=6, each). Reanalysis showed high interindividual variability between samples, but confirmed the results obtained in the array analysis. The gene expression of ABC transporter genes, as well as lineage markers of PXE, was further examined after inhibition of ABCC6 gene expression by using specific small-interfering RNA. These experiments corroborated the observed gene expression alterations, most notably in the ABCA subclass (up to fourfold, P<0.05). We therefore conclude that MRP6-deficient dermal fibroblasts exhibit a distinct gene expression profile of ABCA transporters, potentially to compensate for MRP6 deficiency. Moreover, our results point to a function for ABCC6/MRP6 in sterol transport, as sterols are preferential regulators of ABCA transporter activity and expression. Further studies are now required to uncover the role of ABCA transporters in PXE.

Pseudoxanthoma elasticum is a metabolic disease

Pseudoxanthoma elasticum (PXE) is a pleiotropic multisystem disorder affecting skin, eyes, and the cardiovascular system with progressive pathological mineralization. It is caused by mutations in the ABCC6 gene expressed primarily in the liver and kidneys, and at very low levels, if at all, in tissues affected by PXE. A question has arisen regarding the pathomechanism of PXE, particularly the "metabolic" versus the "PXE cell" hypotheses. We examined a murine PXE model (Abcc6(-/-)) by transplanting muzzle skin from knockout (KO) and wild-type (WT) mice onto the back of WT and KO mice using mineralization of the connective tissue capsule surrounding the vibrissae as an early phenotypic biomarker. Grafting of WT mouse muzzle skin onto the back of KO mice resulted in mineralization of vibrissae, whereas grafting KO mouse muzzle skin onto WT mice did not. Thus, these findings implicate circulatory factors as a critical component of the mineralization process. This mouse grafting model supports the notion that PXE is a systemic metabolic disorder with secondary mineralization of connective tissues and that the mineralization process can be countered or even reversed by changes in the homeostatic milieu.

Parameters of oxidative stress are present in the circulation of PXE patients

Pseudoxanthoma elasticum (PXE) is an inherited disorder characterized by calcification of elastic fibres leading to dermatological and vascular alterations associated to premature aged features and to life threatening clinical manifestations. The severity of the disease is independent from the type of mutation in the ABCC6 gene, and it has been suggested that local and/or systemic factors may contribute to the occurrence of clinical phenotype. The redox balance in the circulation of 27 PXE patients and of 50 healthy subjects of comparable age was evaluated by measuring the advanced oxidation protein products (AOPP), the lipid peroxidation derivatives (LOOH), the circulating total antioxidant status (TAS), the thiol content and the extracellular superoxide dismutase activity (EC-SOD). Patients were diagnosed by clinical, ultrastructural and molecular findings. Compared to control subjects, PXE patients exhibited significantly lower antioxidant potential, namely circulating TAS and free thiol groups, and higher levels of parameters of oxidative damage, as LOOH and of AOPP, and of circulating EC-SOD activity. Interestingly, the ratio between oxidant and antioxidant parameters was significantly altered in PXE patients and related to various score indices. This study demonstrates, for the first time, that several parameters of oxidative stress are modified in the blood of PXE patients and that the redox balance is significantly altered compared to control subjects of comparable age. Therefore, in PXE patients the circulating impaired redox balance may contribute to the occurrence of several clinical manifestations in PXE patients, and/or to the severity of disease, thus opening new perspectives for their management.

Circulating P-, L- and E-selectins in pseudoxanthoma elasticum patients

OBJECTIVES

Pseudoxanthoma elasticum (PXE) is a hereditary disorder predominantly affecting the skin, retina and vascular system. The aim of this study was to measure cell adhesion molecules in PXE patients.

DESIGN AND METHODS

Soluble P-, E- and L-selectins were measured in 61 non-consanguineous PXE patients. The distribution of the variants E-selectin S128R and P-selectin T715R were determined.

RESULTS

P-selectin concentrations were significantly increased in male and female PXE patients. Furthermore, P-selectin levels correlated with the ABCC6 gene status of the PXE patients. Patients harboring two mutant ABCC6 alleles had 1.5-fold increased P-selectin concentrations in comparison to patients with at least one wild-type allele. E- and L-selectin levels were within normal range and the allelic frequencies of the investigated polymorphisms did not differ between patients and age- and sex-matched controls.

CONCLUSIONS

Our data show elevated P-selectin levels in PXE patients potentially due to oxidative stress and elevated protease activity in PXE.

Pseudoxanthoma elasticum: oxidative stress and antioxidant diet in a mouse model (Abcc6-/-)

Pseudoxanthoma elasticum (PXE), a multisystem disorder characterized by ectopic mineralization of soft connective tissues, is caused by mutations in the ABCC6 gene. The pathomechanistic details of the mineralization process are largely unknown, but oxidative stress has been suggested to play a role. In this study, we tested Abcc6(-/-) mice, which serve as a model of PXE, for markers of the oxidative stress in the liver and serum. The total antioxidant capacity as well as markers of protein oxidation and lipid peroxidation suggested the presence of chronic oxidative stress. Feeding these mice for 5 months with a diet supplemented with antioxidants (vitamins C and E, selenium, and N-acetylcysteine) countered the oxidative stress but did not modify the ectopic mineralization process. These results suggest that the Abcc6(-/-) mice suffer from chronic oxidative stress but this does not contribute to connective tissue mineralization, the hallmark of PXE.

Matrix Gla protein is involved in elastic fiber calcification in the dermis of pseudoxanthoma elasticum patients

Mature MGP (Matrix gamma-carboxyglutamic acid protein) is known to inhibit soft connective tissues calcification. We investigated its possible involvement in pseudoxanthoma elasticum (PXE), a genetic disorder whose clinical manifestations are due to mineralization of elastic fibers. PXE patients have lower serum concentration of total MGP compared to controls (P<0.001). Antibodies specific for the noncarboxylated (Glu-MGP) and for the gamma-carboxylated (Gla-MGP) forms of MGP were assayed on ultrathin sections of dermis from controls and PXE patients. Normal elastic fibers in controls and patients were slightly positive for both forms of MGP, whereas Gla-MGP was more abundant within control's than within patient's elastic fibers (P<0.001). In patients' calcified elastic fibers, Glu-MGP intensively colocalized with mineral precipitates, whereas Gla-MGP precisely localized at the mineralization front. Data suggest that MGP is present within elastic fibers and is associated with calcification of dermal elastic fibers in PXE. To investigate whether local cells produce MGP, dermal fibroblasts were cultured in vitro and MGP was assayed at mRNA and protein levels. In spite of very similar MGP mRNA expression, cells from PXE patients produced 30% less of Gla-MGP compared to controls. Data were confirmed by immunocytochemistry on ultrathin sections. Normal fibroblasts in vitro were positive for both forms of MGP. PXE fibroblasts were positive for Glu-MGP and only barely positive for Gla-MGP (P<0.001). In conclusion, MGP is involved in elastic fiber calcification in PXE. The lower ratio of Gla-MGP over Glu-MGP in pathological fibroblasts compared to controls suggests these cells may play an important role in the ectopic calcification in PXE.

Pseudoxanthoma elasticum: genetic variations in antioxidant genes are risk factors for early disease onset

BACKGROUND

Pseudoxanthoma elasticum (PXE) is a rare hereditary disorder characterized by progressive calcification and fragmentation of elastic fibers in connective tissues. PXE is caused by mutations in the ABCC6 gene, which encodes the membrane transporter multidrug resistance-associated protein 6. Chronic oxidative stress was recently suggested to play a crucial role in the pathogenesis of the disease. Our aim was to investigate the association of PXE with genetic variation in genes coding for antioxidant enzymes.

METHODS

We used restriction fragment length polymorphism and allele-specific PCR analyses to evaluate the distribution of single-nucleotide polymorphisms in the genes encoding catalase (CAT), superoxide dismutase 2 (SOD2), and glutathione peroxidase 1 (GPX1) in DNA samples from 117 German PXE patients and 117 healthy age- and sex-matched control individuals.

RESULTS

The investigated genetic variants had previously been shown to affect the activities of these antioxidant enzymes. We found a correlation between genotype and age of disease onset for polymorphisms in CAT (c.-262C>T), SOD2 (c.47C>T), and GPX1 (c.593C>T). Furthermore, the age of disease onset was inversely correlated with the number of mutated alleles, indicating a cumulative effect on the time of disease onset [mean (SD) age of 40.9 (13.6) years, 32.4 (16.3) years, and 25.7 (15.9) years for carriers of 0, 1-2, and >2 mutated alleles, respectively; P = 0.03].

CONCLUSION

Our findings demonstrate that increased oxidative stress due to activity-affecting polymorphisms in genes encoding antioxidant enzymes leads to earlier PXE onset.

Aberrant mineralization of connective tissues in a mouse model of pseudoxanthoma elasticum: systemic and local regulatory factors

Pseudoxanthoma elasticum (PXE) is caused by mutations in the ABCC6 gene, but the cellular and molecular events leading to aberrant mineralization of soft tissues are unknown. To characterize the mineralization process, we examined a PXE animal model, the Abcc6-/- mouse, with respect to specific proteins serving as inhibitors of mineralization. The levels of calcium and phosphate in serum of these mice were normal, but the Abcc6-/- serum had less ability to prevent the mineral deposition induced by inorganic phosphate in a cell culture system. Addition of fetuin-A to the culture system prevented the mineralization. The calcium x phosphate product was markedly elevated in the mineralized vibrissae of Abcc6-/- mice, an early biomarker of the mineralization process, consistent with histopathologic findings. Levels of fetuin-A were slightly decreased in Abcc6-/- serum, and positive immunostaining for matrix-gla-protein (MGP), fetuin-A, and ankylosis protein (Ank) as well as alkaline phosphatase activity were strongly associated with the mineralization process. In situ hybridization demonstrated that the genes for MGP and Ank were expressed locally in vibrissae, whereas fetuin-A was expressed highly in the liver. These data suggest that the deposition of the bone-associated proteins spatially coincides with mineralization and actively regulates this process locally and systemically.