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

A number of beta-thalassemia (ß-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 ß-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 ß-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 ß-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 ß-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.

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.

Low serum vitamin K in PXE results in defective carboxylation of mineralization inhibitors similar to the GGCX mutations in the PXE-like syndrome.

Soft-tissue mineralization is a tightly regulated process relying on the activity of systemic and tissue-specific inhibitors and promoters of calcium precipitation. Many of these, such as matrix gla protein (MGP) and osteocalcin (OC), need to undergo carboxylation to become active. This post-translational modification is catalyzed by the gammaglutamyl carboxylase GGCX and requires vitamin K (VK) as an essential co-factor. Recently, we described a novel phenotype characterized by aberrant mineralization of the elastic fibers resulting from mutations in GGCX. Because of the resemblance with pseudoxanthoma elasticum (PXE), a prototype disorder of elastic fiber mineralization, it was coined the PXE-like syndrome. As mutations in GGCX negatively affect protein carboxylation, it is likely that inactive inhibitors of calcification contribute to ectopic mineralization in PXE-like syndrome. Because of the remarkable similarities with PXE, we performed a comparative study of various forms of VK-dependent proteins in serum, plasma (using ELISA), and dermal tissues (using immunohistochemistry) of PXE-like and PXE patients using innovative, conformation-specific antibodies. Furthermore, we measured VK serum concentrations (using HPLC) in PXE-like and PXE samples to evaluate the VK status. In PXE-like patients, we noted an accumulation of uncarboxylated Gla proteins, MGP, and OC in plasma, serum, and in the dermis. Serum levels of VK were normal in these patients. In PXE patients, we found similar, although not identical results for the Gla proteins in the circulation and dermal tissue. However, the VK serum concentration in PXE patients was significantly decreased compared with controls. Our findings allow us to conclude that ectopic mineralization in the PXE-like syndrome and in PXE results from a deficient protein carboxylation of VK-dependent inhibitors of calcification. Although in PXE-like patients this is due to mutations in the GGCX gene, a deficiency of the carboxylation co-factor VK is at the basis of the decreased activity of calcification inhibitors in PXE.

Novel deletions causing pseudoxanthoma elasticum underscore the genomic instability of the ABCC6 region.

Mutations in ABCC6 cause pseudoxanthoma elasticum (PXE), a heritable disease that affects elastic fibers. Thus far, >200 mutations have been characterized by various PCR-based techniques (primarily direct sequencing), identifying up to 90% of PXE-causing alleles. This study wanted to assess the importance of deletions and insertions in the ABCC6 genomic region, which is known to have a high recombinational potential. To detect ABCC6 deletions/insertions, which can be missed by direct sequencing, multiplex ligation-dependent probe amplification (MLPA) was applied in PXE patients with an incomplete genotype. MLPA was performed in 35 PXE patients with at least one unidentified mutant allele after exonic sequencing and exclusion of the recurrent exon 23-29 deletion. Six multi-exon deletions and four single-exon deletions were detected. Using MLPA in addition to sequencing, we expanded the ABCC6 mutation spectrum with 9 novel deletions and characterized 25% of unidentified disease alleles. Our results further illustrate the instability of the ABCC6 genomic region and stress the importance of screening for deletions in the molecular diagnosis of PXE.

Pseudoxanthoma elasticum and familial hypercholesterolemia: a deleterious combination of cardiovascular risk factors.

BACKGROUND AND OBJECTIVE: Pseudoxanthoma Elasticum (PXE), an autosomal recessive disease due to mutations in ABCC6 gene, is characterised by fragmentation of elastic fibres with involvement of the cardiovascular system. We investigated a 60-year-old female with angina pectoris found to have PXE, associated with elevated plasma LDL-C suspected to be due to autosomal-co-dominant hypercholesterolemia. METHODS: ABCC6, LDLR, PCSK9 and exon 26 of APOB genes were re-sequenced. Cardiovascular involvement was assessed by coronary angiography, single-photon emission computed tomography (SPECT) and ultrasound examination. RESULTS AND CONCLUSIONS: The patient was a compound heterozygous for two ABCC6 mutations (p.S317R and p.R1141X) and heterozygous for a novel LDLR mutation (p.R574H). She had severe coronary stenosis and calcification of the arteries of the lower limbs. Treatment with ezetimibe/simvastatin 10/60mg/day, maintained over a 4.5-year period, reduced of LDL-C and the myocardial ischemic area. In PXE patients LDL-lowering treatment might contribute to delay macrovascular complications.

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.

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.

Exon 26-coded polypeptide: an isolated hydrophobic domain of human tropoelastin able to self-assemble in vitro.

Hydrophobic domains of human tropoelastin are able to aggregate in a variegated manner. Some aggregates have typical features of the whole protein while others show peculiar self-assembling profiles. Among these hydrophobic domains, an important role in the self-assembling properties of tropoelastin in vitro could be assigned to the peptide encoded by exon 26 of the human tropoelastin gene, that, although unstructured in solution, has great tendency to self-assemble in an ordered manner. The present report describes the aggregation properties of this hydrophobic domain of human tropoelastin analysed by different ultra-structural approaches. Transmission electron microscopy shows that the peptide is able to form different aggregation entities from short rods to very long and flexible fibers, depending on the temperature and on the incubation time. At a microm scale, very long fibers as well as fractal aggregation patterns were observed. Data show that the isolated domain encoded by exon 26 of the tropoelastin gene is able to aggregate in a manner very similar to the whole tropoelastin protein. The aggregation properties are due to the peculiar sequence of EX26, and not to its amino acid composition, as evidenced by the supramolecular analysis of a scrambled sequence of exon 26-coded domain of human tropoelastin, showing a quite different aggregation patterns. These findings confirm that specific sequences can play a driving role in the aggregation process of tropoelastin molecule, at least in vitro, and indicate exon 26-encoded domain among these sequences.

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-like phenotype with cutis laxa and multiple coagulation factor deficiency represents a separate genetic entity.

Data on six patients with a Pseudoxanthoma Elasticum (PXE)-like phenotype, characterized by excessive skin folding (resembling cutis laxa) and a deficiency of the vitamin K-dependent clotting factors (II, VII, IX, and X) are presented. A comparison is made between the clinical, ultrastructural, and molecular findings in these patients and those seen in classic PXE and cutis laxa, respectively. Clinical overlap with PXE is obvious from the skin manifestations of yellowish papules or leathery plaques with dot-like depressions at presentation, angioid streaks and/or ocular peau d'orange, and fragmentation and calcification of elastic fibers in the dermis. Important phenotypic differences with PXE include much more severe skin laxity with spreading toward the trunk and limbs with thick, leathery skin folds rather than confinement to flexural areas, and no decrease in visual acuity. Moreover, detailed electron microscopic analyses revealed that alterations of elastic fibers as well as their mineralization were slightly different from those in classic PXE. Molecular analysis revealed neither causal mutations in the ABCC6 gene (ATP-binding cassette subfamily C member 6), which is responsible for PXE, nor in VKORC1 (vitamin K 2,3 epoxide reductase), known to be involved in vitamin K-dependent factor deficiency. However, the GGCX gene (gamma-glutamyl carboxylase), encoding an enzyme important for gamma-carboxylation of gla-proteins, harbored mutations in six out of seven patients analyzed. These findings all support the hypothesis that the disorder indeed represents a separate clinical and genetic entity, the molecular background of which remains to be unraveled.

Oxidative stress in fibroblasts from patients with pseudoxanthoma elasticum: possible role in the pathogenesis of clinical manifestations.

Pseudoxanthoma elasticum (PXE) is a genetic disease characterized by calcification and fragmentation of elastic fibres of the skin, cardiovascular system and eye, caused by mutations of the ABCC6 gene, which encodes the membrane transporter MRP6. The pathogenesis of the lesions is unknown. Based on studies of similar clinical and histopathological damage present in haemolytic disorders, our working hypothesis is that PXE lesions may result from chronic oxidative stress occurring in PXE cells as a consequence of MRP6 deficiency. Our results show that PXE fibroblasts suffer from mild chronic oxidative stress due to the imbalance between production and degradation of oxidant species. The findings also show that this imbalance results, at least in part, from the loss of mitochondrial membrane potential (DeltaPsi(m)) with overproduction of H2O2. Whether mitochondrial dysfunction is the main factor responsible for the oxidative stress in PXE cells remains to be elucidated. However, mild chronic generalized oxidative stress could explain the great majority of structural and biochemical alterations already reported in PXE.

Identification of mineralized elastic fibers on wet samples by SEM.

A method is described that could be of potential use for the rapid ultrastructural identification of abnormal and fragmented elastic fibers in very small wet samples of dermal biopsies from patients affected by Pseudoxanthoma elasticum (PXE). Moreover, the method, which consists of the use of sealed capsules transparent to electrons, allows the rapid and accurate localization and detection of mineralized areas in PXE patients and of their ion composition by X-ray microanalysis. This methodology could be of great help in any tissue disorder, especially in connective tissue disorders, characterized by structural alterations associated with ion precipitation.

Dermal fibroblasts from pseudoxanthoma elasticum patients have raised MMP-2 degradative potential.

Cultured fibroblasts from the dermis of normal subjects and of Pseudoxanthoma elasticum (PXE) patients were analysed for enzyme activity, protein and mRNA expression of metalloproteases (MMP-2, MMP-3, MMP-9, MT1-MMP) and of their specific inhibitors (TIMP-1, TIMP-2 and TIMP-3). MMP-3, MMP-9 and TIMP-3 mRNAs and proteins failed to be detected in both the medium and the cell layer of both controls and PXE patients. MMP-2 mRNA was significantly more expressed in PXE than in control cell lines, whereas MT1-MMP, TIMP-1 and TIMP-2 mRNAs appeared unchanged. MMP-2 was significantly higher in the cell extracts from PXE fibroblasts than in control cells, whereas differences were negligible in the cell medium. Data suggest that PXE fibroblasts have an increased proteolytic potential, and that MMP-2 may actively contribute to connective tissue alterations in this genetic disorder.

Dissection of human tropoelastin: supramolecular organization of polypeptide sequences coded by particular exons.

Polypeptide sequences encoded by some exons of the human tropoelastin gene (EDP, elastin-derived peptide) have been analysed for their ability to coacervate and to self-assembly. The great majority of them were shown to form organized structures, but only a few were indeed able to coacervate. Negative staining and rotary shadowing transmission electron microscopy showed the polypeptides to adopt a variety of supramolecular organization, from filaments, as those typical of tropoelastin, to amyloid-like fibers. The results obtained gave significant insight to the possible roles played by specific polypeptide sequences of tropoelastin.

Heparan sulphate interacts with tropoelastin, with some tropoelastin peptides and is present in human dermis elastic fibers.

A number of reports point to the presence of proteoglycans and/or glycosaminoglycans within elastic fibers in normal and in pathological conditions. We present data that heparan sulphate (HS)-containing proteoglycans are associated with normal elastic fibers in human dermis and that isolated HS chains interact in vitro with recombinant tropoelastin and with peptides encoded by distinct exons of the human tropoelastin gene (EDPs). By immunocytochemistry, HS chains were identified as associated with the amorphous elastin component in the human dermis and remained associated with the residual elastin in the partially degenerated fibers of old subjects. HS appeared particularly concentrated in the mineralization front of elastic fibers in the dermis of patients affected by pseudoxanthoma elasticum (PXE). In in vitro experiments, HS induced substantial changes in the coacervation temperature and in the aggregation properties of recombinant tropoelastin and of synthetic peptides (EDPs) corresponding to sequences encoded by exons 18, 20, 24 and 30 of the human tropoelastin gene. In particular, HS modified the coacervation temperature and favoured the aggregation into ordered structures of tropoelastin molecules and of EDPs 18, 20 and 24, but not of EDP30. These data strongly indicate that HS-elastin interactions may play a role in tissue elastin fibrogenesis as well as modulating elastin stability with time and in diseases.

Supramolecular amyloid-like assembly of the polypeptide sequence coded by exon 30 of human tropoelastin.

Elastin is known to self-aggregate in twisted-rope filaments. However, an ultrastructural organization different from the fibrils typical of elastin, but rather similar to those shown by amyloid networks, is shown by the polypeptide sequence encoded by exon 30 of human tropoelastin. To better understand the molecular properties of this sequence to give amyloid fibers, we used CD, NMR, and FTIR (Fourier transform infrared spectroscopy) to identify the structural characteristics of the peptide. In this study, we have demonstrated, by FTIR, that antiparallel beta-sheet conformation is predominant in the exon 30 fibers. These physical-chemical studies were combined with transmission electron microscopy and atomic force microscopy to analyze the supramolecular structure of the self-assembled aggregate. These studies show the presence of fibrils that interact side-by-side probably originating from an extensive self-interaction of elemental cross beta-structures. Similar sequences, of the general type XGGZG(X, Z = V, L, A, I), are widely found in many proteins such as collagens IV and XVII, major prion protein precursor, amyloid beta A4 precursor protein-binding family, etc., thus suggesting that this sequence could be involved in contributing to the self-assembly of amyloid fibers even in other proteins.

The effect on rat thymocytes of the simultaneous in vivo exposure to 50-Hz electric and magnetic field and to continuous light.

Thymus plays an important role in the immune system and can be modulated by numerous environmental factors, including electromagnetic fields (EMF). The present study has been undertaken with the aim to investigate the role of long-term exposure to extremely low frequency electric and magnetic fields (ELF-EMF) on thymocytes of rats housed in a regular dark/light cycle or under continuous light. Male Sprague-Dawley rats, 2 months old, were exposed or sham exposed for 8 months to 50-Hz sinusoidal EMF at two levels of field strength (1 kV/m, 5 microT and 5 kV/m, 100 microT, respectively). Thymus from adult animals exhibits signs of gradual atrophy mainly due to collagen deposition and fat substitution. This physiological involution may be accelerated by continuous light exposure that induces a massive death of thymocytes. The concurrent exposure to continuous light and to ELF-EMF did not change significantly the rate of mitoses compared to sham-exposed rats, whereas the amount of cell death was significantly increased, also in comparison with animals exposed to EMF in a 12-h dark-light cycle. In conclusion, long-term exposure to ELF-EMF, in animals housed under continuous light, may reinforce the alterations due to a photic stress, suggesting that, in vivo, stress and ELF-EMF exposure can act in synergy determining a more rapid involution of the thymus and might be responsible for an increased susceptibility to the potentially hazardous effects of ELF-EMF.

ABCC6 mutations in Italian families affected by pseudoxanthoma elasticum (PXE).

Pseudoxanthoma elasticum (PXE) is a genetic disorder, characterized by cutaneous, ocular and cardiovascular clinical symptoms, caused by mutations in a gene (ABCC6) that encodes for MRP6 (Multidrug Resistance associated Protein 6), an ATP-binding cassette membrane transporter. The ABCC6 gene was sequenced in 38 unrelated PXE Italian families. The mutation detection rate was 82.9%. Mutant alleles occurred in homozygous, compound heterozygous and heterozygous forms, however the great majority of patients were compound heterozygotes. Twenty-three different mutations were identified, among which 11 were new. Fourteen were missense (61%); five were nonsense (22%); two were frameshift (8.5%) and two were putative splice site mutations (8.5%). The great majority of mutations were located from exon 24 to 30, exon 24 being the most affected. Among the others, exons 9 and 12 were particularly involved. Almost all mutations were located in the intracellular site of MRP6. A positive correlation was observed between patient's age and severity of the disorder, especially for eye alterations. The relevant heterogeneity in clinical manifestations between patients with identical ABCC6 mutations, even within the same family, seems to indicate that, apart from PXE causative mutations, other genes and/or metabolic pathways might influence the clinical expression of the disorder.

Extracutaneous ultrastructural alterations in pseudoxanthoma elasticum.

Pseudoxanthoma elasticum (PXE) is caused by mutations in the ABCC6 gene, encoding for the membrane transporter MRP6, whose physiological role is still unknown. PXE is characterized by skin, eye, and cardiovascular alterations mainly due to mineralization of elastic fibers. The ultrastructural alterations of a large number of tissues obtained at autopsy from 2 PXE patients were analyzed and compared to clarify the involvement of the various organs in PXE and to identify cell types responsible for clinical manifestations. Ultrastructural alterations typical of PXE were present in all organs examined and consisted mostly of fragmentation and mineralization of a number of elastic fibers, abnormalities of collagen fibril shape and size, and, less frequently, deposition of aggregates of matrix constituents in the extracellular space. The severity of alterations was more pronounced in the organs affected by the clinical manifestations of PXE. Interestingly, veins and arteries were similarly damaged, the adventitia and the perivascular connective tissue being the most affected areas. Therefore, alterations in PXE are systemic and affect all soft connective tissues, even in the absence of specific clinical manifestations. The localization of alterations suggests that fibroblasts and/or smooth muscle cells are very likely involved in the pathogenesis of the disorder. These findings may help in the diagnosis of PXE when clinical manifestations affect internal organs.