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

Elevated circulating levels of matrix metalloproteinases MMP-2 and MMP-9 in pseudoxanthoma elasticum patients

Pseudoxanthoma elasticum (PXE) is a rare disorder predominantly affecting the skin, the eyes, and the cardiovascular system. The disease is caused by mutations in the ABCC6 gene and characterized by ectopic calcification and extracellular matrix (ECM) alterations. Matrix metalloproteinases (MMPs) play a pivotal role in the process of ECM remodeling. In the present study, we investigated matrix metalloproteinases MMP-2 and MMP-9 in PXE patients compared to healthy controls. We analyzed the serum concentrations of MMP-2 and MMP-9 in a cohort of 69 German PXE patients and in 69 healthy, age-, and sex-matched control subjects using commercially available ELISA assays. We found elevated concentrations of both MMPs in the sera of PXE patients. MMP-2 levels were significantly higher in patients than controls (231 +/- 5.89 vs 202 +/- 5.17 ng/ml, p = 0.0002), as were MMP-9 levels (841 +/- 65.9 vs 350 +/- 30.8 ng/ml, p < 0.0001). Our findings point to an involvement of matrix metalloproteinases in PXE pathology. ECM remodeling in PXE is reflected by elevated levels of circulating MMP-2 and MMP-9. Those MMPs might, therefore, be applicable as serum markers for the matrix-degradative process in PXE.

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.

Complement factor H variant p.Y402H in pseudoxanthoma elasticum patients

Pseudoxanthoma elasticum (PXE) is a hereditary disorder predominantly affecting the eyes, the skin, and the vascular system. The subretinal neovascularization and retinal hemorrhages leading to the loss of central vision in PXE are similar to the process observed in age-related macular degeneration (AMD). The complement factor H (CFH) variant c.1277T > C (p.Y402H) is a recently discovered risk factor for AMD. The aim of this study was to analyze whether this CFH variant is a secondary genetic risk factor for PXE. Therefore, the genotypes of CFH c.1277T > C (p.Y402H) were determined in 189 German PXE patients and 189 age- and sex-matched controls. The allelic frequencies of the investigated variant did not differ between patients and controls. The frequencies were 33%, 56%, and 11% for wild-type, heterozygous, and homozygous genotypes in the PXE patients and 36%, 51%, and 13% in the control cohort, respectively. Further, no significant associations were identified when allele carriers were analyzed or after adjustment for sex, age, smoking, organ involvement, hypertension, or age at disease onset. No significant genotype-phenotype correlation was detected. In conclusion, our data reliably show that the CFH variant c.1277T > C (p.Y402H) is not a genetic risk factor for PXE.

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.

A clinical and histological study of dental defects in a 10-year-old girl with pseudoxanthoma elasticum and amelogenesis imperfecta

BACKGROUND

The prominent dental feature of a 10-year-old girl was severely hypoplastic enamel in permanent teeth.

CASE REPORT

Severe dental defects were detected in a 10-year-old female patient affected by pseudoxanthoma elasticum and amelogenesis imperfecta. An orthopantomographic examination revealed a reduction of enamel thickness on the crown of all erupted and unerupted teeth, agenesis of the maxillary right second premolar, delayed eruption of mandibular first premolars, and the presence of large calcifications in all tooth pulp chambers. A detailed histological analysis of permanent mandibular first molars showed that pulp calcifications presented a concentric laminate organization and merged to almost completely obliterate the pulp chamber. Osteodentine was visible all along the pulpal surface of the radicular dentine. Broad resorption areas were present in the outermost dentine at both coronal and radicular levels. Radicular resorption areas presented a typical rectangular form and were filled with acellular cementum. Cementum thickness was highly increased on the root surface. Apposition of cellular cementum-like tissue was also observed on the coronal dentine surface.

CONCLUSION

Before treating patients affected by amelogenesis imperfecta and/or pseudoxanthoma elasticum, paediatric dentists should be aware of the presence of pulp calcifications that add to the complexity of endodontic procedures.

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.

Elevated serum levels of intercellular adhesion molecule ICAM-1 in Pseudoxanthoma elasticum

BACKGROUND

Pseudoxanthoma elasticum (PXE, OMIM 177850 and 264800) is a rare heritable disorder predominantly affecting the skin, the eyes and the vascular system. The disease is caused by mutations in the ABCC6 gene and is characterized by calcification and extracellular matrix remodeling, including alterations of the vessel walls. Here, we investigated the cell adhesion molecules ICAM-1 in PXE patients.

METHODS

Soluble ICAM-1 was determined in 58 non-consanguineous PXE patients by quantitative sandwich enzyme immunoassay. The allelic frequencies of the ICAM-1 variant p.K469E were analyzed in patients and age- and sex-matched controls.

RESULTS

Soluble ICAM-1 levels were significantly elevated in male and female PXE patients (p<0.02 and p<0.001, respectively). In addition, the ICAM-1 concentration correlated with the ABCC6 gene status of the PXE patients. The ICAM variant p.K469E genotypes were not different in PXE patients and age- and sex-matched controls.

CONCLUSIONS

Our data show for the first time increased ICAM-1 concentrations in PXE patients, potentially due to the chronic oxidative stress and elevated protease activity followed by extracellular matrix remodeling which have been previously observed in PXE patients.

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.

SPP1 Promoter Polymorphisms: Identification of the First Modifier Gene for Pseudoxanthoma Elasticum

Background: Progressive calcification and fragmentation of elastic fibers are characteristic hallmarks of pseudoxanthoma elasticum (PXE), which is caused by mutations in ABCC6 encoding multidrug resistance–associated protein 6 (MRP6). Because of the great clinical variability of PXE, secondary genetic risk factors are suspected to exist. We investigated whether SPP1 (secreted phosphoprotein 1; previously OPN, osteopontin) promoter polymorphisms are associated with PXE.

Methods: We screened an ∼2-kb region spanning the theoretical promoter of the SPP1 gene for sequence variations by denaturing HPLC and direct sequencing in 93 PXE patients. Sequence variations with a prevalence &gt;5% were genotyped in 93 age- and sex-matched healthy controls. Statistical and haplotype association analyses were performed using Fisher exact test, PHASE v2.1.1, and Haploview 3.2.

Results: Mutational screening revealed 9 different sequence variations. Three SPP1 promoter polymorphisms (c.−1748A&gt;G, c.−155_156insG, and c.244_245insTG) were significantly more frequent in PXE patients than in 93 age- and sex-matched healthy controls (Pcorrected &lt; 0.05 each). The odds ratios (95% CI) for PXE among carriers of the 3 alleles were, respectively, 2.16 (1.34–3.48), 2.41 (1.51–3.82), and 1.97 (1.23–3.15). Haplotype analysis of 6 SPP1 promoter polymorphisms revealed 1 haplotype to be significantly reduced among PXE patients (Pcorrected = 0.035, odds ratio 1.80, 95% CI 1.19–2.71).

Conclusions: Polymorphisms in the SPP1 promoter are secondary genetic risk factors contributing to PXE susceptibility.

Pseudoxanthoma elasticum-like phenotypes: more diseases than one

The classic pseudoxanthoma elasticum (PXE) phenotype derives from mutations in ABCC6. PXE-like phenotypes have been observed in a number of disorders, with no evidence of mutations in ABCC6. Vanakker et al. report PXE-like skin findings in patients with mutations in GGCX critical for gamma-carboxylation of gla-proteins. This report expands the clinical spectrum of PXE-like conditions and also provides potential insights into the ectopic mineralization process.

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.

Pseudoxanthoma Elasticum: a Metabolic Disease?

Pseudoxanthoma elasticum (PXE), a pleiotropic multisystem disorder affecting the skin, eyes, and cardiovascular system, is caused by mutations in the ABCC6/MRP6 expressed primarily in the liver. A question has arisen regarding the pathomechanism of PXE, particularly the "metabolic" versus the "PXE cell" hypotheses. Le Saux et al. have now provided partial support for the notion that PXE is primarily a metabolic disorder.

Pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is an inherited disorder that is associated with accumulation of mineralized and fragmented elastic fibers in the skin, vessel walls, and Bruch's membrane. Clinically, patients exhibit characteristic lesions of the skin (soft, ivory-colored papules in a reticular pattern that predominantly affect the neck), the posterior segment of the eye (peau d'orange, angioid streaks, choroidal neovascularizations), and the cardiovascular system (peripheral arterial occlusive disease, coronary occlusion, gastrointestinal bleeding). There is no causal 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. The ABCC6 gene on chromosome 16p13.1 is associated with the disease. Mutations within the ABCC6 gene cause reduced or absent transmembraneous transport that leads to accumulation of substrate and calcification of elastic fibers. Although based on clinical features the diagnosis appears readily possible, variability in phenotypic expressions and the low prevalence may be responsible that the disease is underdiagnosed. This review covers current knowledge of PXE and presents therapeutic approaches.