Pseudoxanthoma elasticum: molecular genetics and putative pathomechanisms

The pathogenic c.1171A>G (p.Arg391Gly) and c.2359G>A (p.Val787Ile) ABCC6 variants display incomplete penetrance causing pseudoxanthoma elasticum in a subset of individuals

ABCC6 promotes ATP efflux from hepatocytes to bloodstream. ATP is metabolized to pyrophosphate, an inhibitor of ectopic calcification. Pathogenic variants of ABCC6 cause pseudoxanthoma elasticum, a highly variable recessive ectopic calcification disorder. Incomplete penetrance may initiate disease heterogeneity, hence symptoms may not, or differently manifest in carriers. Here, we investigated whether incomplete penetrance is a source of heterogeneity in pseudoxanthoma elasticum. By integrating clinical and genetic data of 589 patients, we created the largest European cohort. Based on allele frequency alterations, we identified two incomplete penetrant pathogenic variants, c.2359G>A (p.Val787Ile) and c.1171A>G (p.Arg391Gly), with 6.5% and 2% penetrance, respectively. However, when penetrant, the c.1171A>G (p.Arg391Gly) manifested a clinically unaltered severity. After applying in silico and in vitro characterization, we suggest that incomplete penetrant variants are only deleterious if a yet unknown interacting partner of ABCC6 is mutated simultaneously. The low penetrance of these variants should be contemplated in genetic counseling.

Inherited connective tissue diseases highlight macromolecular network interdependences in skin extracellular matrix: a histomorphometric study

Mutation of just a single extracellular matrix protein, a receptor or enzyme involved in connective tissue metabolism is sufficient to cause systemic pathologies and failure of tissues that are subjected to strong mechanical stresses. Skin histological and computerized image analyses can provide a good qualitative and quantitative indication of these inherited connective tissue diseases. In this study, skin biopsies from young (10 to 25 years) and middle-aged patients (26 to 50 years) suffering from Ehlers-Danlos syndromes (EDS), Marfan syndrome (MS) or pseudoxanthoma elasticum (PXE) were studied after specific staining of both the collagen and elastic networks. Findings from the histomorphometric analyses conducted on skin sections of the patients with EDS, MS and PXE were compared to skin sections of healthy subjects from the same age groups. Our results show that both the collagen and the elastic networks were affected in all the studied pathological cases, but that the adverse changes to the elastic network in older patients were distinct from the physiological changes observed during aging process for healthy subjects. This degenerative process may be explained by an added phenomenon involving a general connective tissue proteolysis.

Pulmonary affection of patients with Pseudoxanthoma elasticum: Long-term development and genotype-phenotype-correlation

Pseudoxanthoma elasticum (PXE) is a rare, heritable disease caused by various, mainly recessively transmitted mutations in the ABCC6 gene. Due to calcification of soft connective tissue phenotypic hallmarks are progressive loss of vision, alternation of the skin and early onset atherosclerosis. Beside these main features patients also suffer from impaired alveolar diffusion. The present study focused on impaired lung functioning based on a large cohort of patients with PXE, its long-term development, and genotype-phenotype correlation. Retrospectively, 98 patients and 45 controls were enrolled. All patients underwent body plethysmography and carbon monoxide diffusion testing. Of 35 patients three or more body plethysmographic records were available for long-term analysis. For genotype-phenotype analysis ABCC6 genotypes were grouped as two missense, mixed, or two nonsense mutations. Patients with PXE showed significantly reduced vital capacity (p < 0.05), diffusion capacity (p < 0.01), and diffusion transfer coefficient (p < 0.05). Over a mean period of 38 months diffusion capacity (p < 0.05) and diffusion transfer coefficient (p < 0.01) dropped significantly whereas lung volumes remained unchanged. Genotype-phenotype correlation revealed no connection between gene variants and lung functioning. In conclusion, PXE is accompanied by progressive reduction of alveolar diffusion indicating progressive alterations of lung tissue. Genotype-phenotype correlation with genotypes sorted as missense and nonsense mutations do not explain impaired lung functioning.

Development of the BioHybrid Assay: Combining Primary Human Vascular Smooth Muscle Cells and Blood to Measure Vascular Calcification Propensity

BACKGROUND

Vascular calcification is an active process that increases cardiovascular disease (CVD) risk. There is still no consensus on an appropriate biomarker for vascular calcification. We reasoned that the biomarker for vascular calcification is the collection of all blood components that can be sensed and integrated into a calcification response by human vascular smooth muscle cells (hVSMCs).

METHODS

We developed a new cell-based high-content assay, the BioHybrid assay, to measure in vitro calcification. The BioHybrid assay was compared with the o-Cresolphthalein assay and the T50 assay. Serum and plasma were derived from different cohort studies including chronic kidney disease (CKD) stages III, IV, V and VD (on dialysis), pseudoxanthoma elasticum (PXE) and other cardiovascular diseases including serum from participants with mild and extensive coronary artery calcification (CAC). hVSMCs were exposed to serum and plasma samples, and in vitro calcification was measured using AlexaFluor®-546 tagged fetuin-A as calcification sensor.

RESULTS

The BioHybrid assay measured the kinetics of calcification in contrast to the endpoint o-Cresolphthalein assay. The BioHybrid assay was more sensitive to pick up differences in calcification propensity than the T50 assay as determined by measuring control as well as pre- and post-dialysis serum samples of CKD patients. The BioHybrid response increased with CKD severity. Further, the BioHybrid assay discriminated between calcification propensity of individuals with a high CAC index and individuals with a low CAC index. Patients with PXE had an increased calcification response in the BioHybrid assay as compared to both spouse and control plasma samples. Finally, vitamin K1 supplementation showed lower in vitro calcification, reflecting changes in delta Agatston scores. Lower progression within the BioHybrid and on Agatston scores was accompanied by lower dephosphorylated-uncarboxylated matrix Gla protein levels.

CONCLUSION

The BioHybrid assay is a novel approach to determine the vascular calcification propensity of an individual and thus may add to personalised risk assessment for CVD.

Mutagenic Analysis of the Putative ABCC6 Substrate-Binding Cavity Using a New Homology Model

Inactivating mutations in ABCC6 underlie the rare hereditary mineralization disorder pseudoxanthoma elasticum. ABCC6 is an ATP-binding cassette (ABC) integral membrane protein that mediates the release of ATP from hepatocytes into the bloodstream. The released ATP is extracellularly converted into pyrophosphate, a key mineralization inhibitor. Although ABCC6 is firmly linked to cellular ATP release, the molecular details of ABCC6-mediated ATP release remain elusive. Most of the currently available data support the hypothesis that ABCC6 is an ATP-dependent ATP efflux pump, an un-precedented function for an ABC transporter. This hypothesis implies the presence of an ATP-binding site in the substrate-binding cavity of ABCC6. We performed an extensive mutagenesis study using a new homology model based on recently published structures of its close homolog, bovine Abcc1, to characterize the substrate-binding cavity of ABCC6. Leukotriene C4 (LTC₄), is a high-affinity substrate of ABCC1. We mutagenized fourteen amino acid residues in the rat ortholog of ABCC6, rAbcc6, that corresponded to the residues in ABCC1 found in the LTC₄ binding cavity. Our functional characterization revealed that most of the amino acids in rAbcc6 corresponding to those found in the LTC₄ binding pocket in bovine Abcc1 are not critical for ATP efflux. We conclude that the putative ATP binding site in the substrate-binding cavity of ABCC6/rAbcc6 is distinct from the bovine Abcc1 LTC₄-binding site.

Rare Modifier Variants Alter the Severity of Cardiovascular Disease in Pseudoxanthoma Elasticum: Identification of Novel Candidate Modifier Genes and Disease Pathways Through Mixture of Effects Analysis

Introduction: Pseudoxanthoma elasticum (PXE), an ectopic mineralization disorder caused by pathogenic ABCC6 variants, is characterized by skin, ocular and cardiovascular (CV) symptoms. Due to striking phenotypic variability without genotype-phenotype correlations, modifier genes are thought to play a role in disease variability. In this study, we evaluated the collective modifying effect of rare variants on the cardiovascular phenotype of PXE.

Materials and Methods: Mixed effects of rare variants were assessed by Whole Exome Sequencing in 11 PXE patients with an extreme CV phenotype (mild/severe). Statistical analysis (SKAT-O and C-alpha testing) was performed to identify new modifier genes for the CV PXE phenotype and enrichment analysis for genes significantly associated with the severe cohort was used to evaluate pathway and gene ontology features.

Results Respectively 16 (SKAT-O) and 74 (C-alpha) genes were significantly associated to the severe cohort. Top significant genes could be stratified in 3 groups–calcium homeostasis, association with vascular disease and induction of apoptosis. Comparative analysis of both analyses led to prioritization of four genes (NLRP1, SELE, TRPV1, and CSF1R), all signaling through IL-1B.

Conclusion This study explored for the first time the cumulative effect of rare variants on the severity of cardiovascular disease in PXE, leading to a panel of novel candidate modifier genes and disease pathways. Though further validation is essential, this panel may aid in risk stratification and genetic counseling of PXE patients and will help to gain new insights in the PXE pathophysiology.

A New Zebrafish Model for Pseudoxanthoma Elasticum

Calcification of various tissues is a significant health issue associated with aging, cancer and autoimmune diseases. There are both environmental and genetic factors behind this phenomenon and understanding them is essential for the development of efficient therapeutic approaches. Pseudoxanthoma elasticum (PXE) is a rare genetic disease, a prototype for calcification disorders, resulting from the dysfunction of ABCC6, a transport protein found in the membranes of cells. It is identified by excess calcification in a variety of tissues (e.g., eyes, skin, arteries) and currently it has no cure, known treatments target the symptoms only. Preclinical studies of PXE have been successful in mice, proving the usefulness of animal models for the study of the disease. Here, we present a new zebrafish (Danio rerio) model for PXE. By resolving some ambiguous assemblies in the zebrafish genome, we show that there are two functional and one non-functional paralogs for ABCC6 in zebrafish (abcc6a, abcc6b.1, and abcc6b.2, respectively). We created single and double mutants for the functional paralogs and characterized their calcification defects with a combination of techniques. Zebrafish deficient in abcc6a show defects in their vertebral calcification and also display ectopic calcification foci in their soft tissues. Our results also suggest that the impairment of abcc6b.1 does not affect this biological process.

Elastic fibers during aging and disease

Elastic fibers are essential constituents of the extracellular matrix of higher vertebrates and endow several tissues and organs including lungs, skin and blood vessels with elasticity and resilience. During the human lifespan, elastic fibers are exposed to a variety of enzymatic, chemical and biophysical influences, and accumulate damage due to their low turnover. Aging of elastin and elastic fibers involves enzymatic degradation, oxidative damage, glycation, calcification, aspartic acid racemization, binding of lipids and lipid peroxidation products, carbamylation and mechanical fatigue. These processes can trigger an impairment or loss of elastic fiber function and are associated with severe pathologies. There are different inherited or acquired pathological conditions, which influence the structure and function of elastic fibers and microfibrils predominantly in the cardiorespiratory system and skin. Inherited elastic-fiber pathologies have a direct or indirect impact on elastic-fiber formation due to mutations in the fibrillin genes (fibrillinopathies), in the elastin gene (elastinopathies) or in genes encoding proteins that are associated with microfibrils or elastic fibers. Acquired elastic-fiber pathologies appear age-related or as a result of multiple factors impairing tissue homeostasis. This review gives an overview on the fate of elastic fibers over the human lifespan in health and disease.

Mesopic and Scotopic Light Sensitivity and Its Microstructural Correlates in Pseudoxanthoma Elasticum

Importance

Correlates for Bruch membrane alterations are needed for interventional trials targeting the Bruch membrane in pseudoxanthoma elasticum (PXE).

Objectives

To quantify mesopic and scotopic light sensitivity and identify its microstructural correlates associated with a diseased Bruch membrane in patients with PXE.

Design, Setting, and Participants

A prospective, single-center, cross-sectional case-control study was conducted at a tertiary referral center from January 31, 2018, to February 20, 2020. Twenty-two eyes of 22 patients with PXE and 40 eyes of 40 healthy individuals were included. Data analysis was completed March 15, 2020.

Exposures

Mesopic and dark-adapted 2-color fundus-controlled perimetry (microperimetry) and multimodal retinal imaging including spectral-domain optical coherence tomography (SD-OCT) and OCT angiography were performed. Perimetry thresholds were analyzed using mixed models, and structure-function correlation with SD-OCT data was performed using machine learning.

Main Outcomes and Measures

Observed dark-adapted cyan sensitivity loss as measure of rod photoreceptor dysfunction, as well as mean absolute error between predicted and observed retinal sensitivity to assess the accuracy of structure-function correlation.

Results

Of the 22 patients with PXE included in this study, 15 were women (68%); median age was 56.5 years (interquartile range, 50.4-61.2). These patients exhibited mesopic (estimate, 5.13 dB; 95% CI, 2.89-7.38 dB), dark-adapted cyan (estimate, 9.08 dB; 95% CI, 6.34-11.82 dB), and dark-adapted red (estimate, 7.05 dB; 95% CI, 4.83-9.27 dB) sensitivity losses. This sensitivity loss was also evident in 9 eyes with nonneovascular PXE (mesopic: estimate, 3.21 dB; 95% CI, 1.28-5.14 dB; dark-adapted cyan: 5.93 dB; 95% CI, 3.59-8.27 dB; and dark-adapted red testing: 4.84 dB; 95% CI, 2.88-6.80 dB), showing a distinct centrifugal pattern of sensitivity loss with preserved function toward the periphery. Retinal function could be predicted from microstructure with high accuracy (mean absolute errors, of 4.91 dB for mesopic, 5.44 dB for dark-adapted cyan, and 4.99 dB for dark-adapted red). The machine learning-based analysis highlighted an association of a thinned inner retina and putative separation of the pigment-epithelium-photoreceptor complex with sensitivity loss.

Conclusions and Relevance

In this study, among 22 patients with PXE, those with and without choroidal neovascularization exhibited reductions of retinal sensitivity being most pronounced in dark-adapted cyan testing. This finding suggests that pathologic characteristics of this Bruch membrane disease may be dominated by rod photoreceptor degeneration and/or dysfunction. A putative pigment-epithelium-photoreceptor separation may further impair rod function, while inner retinal abnormalities appear to be correlated with overall dysfunction.

Macular choroidal thickness in patients with pseudoxanthoma elasticum measured by enhanced-depth imaging spectral-domain optical coherence tomography

BACKGROUND/OBJECTIVES

To analyze macular choroidal thickness in patients with pseudoxanthoma elasticum (PXE) by enhanced depth imaging optical coherence tomography (EDI-OCT).

SUBJECTS/METHODS

This is a prospective cross-sectional study. Sixty-eight eyes of 34 patients with PXE and 68 normal eyes of 34 controls were included to study the macular area with enhanced depth imaging spectral-domain optical coherence tomography (EDI-OCT). Eyes with PXE were classified in three groups: those without choroidal neovascularization (CNV) or chorioretinal macular atrophy macular (Group 1); those with active CNV (Group 2) and those with macular atrophy secondary to inactive CNV (Group 3).

RESULTS

Mean subfoveal choroidal thickness (CT) was 266.70 ± 46.93 µm in control group, 304.24 ± 65.52 µm in group 1, 198.55 ± 66.33 µm in group 2, and 119.45 ± 63.89 µm in group 3 (p = 0.00). Comparison between PXE subgroups showed that subfoveal CT was significantly decreased in group 2 and 3 compared to group 1 (p < 0.0001 for both groups). The CT in the different quadrants (superior, inferior, temporal and nasal) was significantly thinner in group 3, followed by group 2 and 1 in ascendant order. Group 1 showed significant increased thickness compared to the other groups.

CONCLUSION

To the best of our knowledge, this is the first report suggesting thicker macular choroid in patients with PXE without active or inactive CNV than in normal eyes. Initial changes in Bruch membrane (MB) and choroid, in addition to the increased oxidative stress, would lead to hyperpermeability of the choroid and alterations of the barrier BM-RPE causing a thick choroid in early stages.

Pseudoxanthoma elasticum – also a microvascular disease

Summary: Background: Pseudoxanthoma elasticum (PXE) is a heritable recessive disease characterized by calcification and fragmentation of soft connective tissue. Besides progressive loss of vision, alternations of the skin, and early-onset atherosclerosis different reports have suggested a microvascular manifestation of PXE and restrictive lung disease. Aim of this study was to elaborate a specific pattern of capillary alterations in PXE as well as to contemplate a possible connection to restrictive lung disease. Patients and methods: 53 consecutive patients with PXE and 26 controls were studied. All patients underwent nailfold capillaroscopy, body plethysmography, capillary blood gas analysis, and venous puncture to assess titer of autoantibodies. Results: PXE was associated with highly pathological alterations of capillaries compared to control. Atypical capillaries, such as ramifications and bushy forms, as well as dilatations varied at highest significance (p < .001). This effect was mirrored by perivascular edema, density and tortuous capillaries. Titer of anti-nuclear autoantibodies were not elevated in patients with PXE. Further analysis revealed negative correlation between vital capacity and presence of atypical capillaries. Conclusions: This study firstly describes the pattern of nailfold capillaries in PXE. Capillaries are highly pathological and consist of ramifications and bushy forms as well as dilatations. Frequently, tortuous capillaries, pericapillary edema and reduced denseness of capillary loops occur. Frequency of atypical capillaries is negatively correlated with vital capacity which can be interpreted as further lead on restrictive lung disease.

Pseudoxanthoma Elasticum, Kidney Stones and Pyrophosphate: From a Rare Disease to Urolithiasis and Vascular Calcifications

Pseudoxanthoma elasticum is a rare disease mainly due to ABCC6 gene mutations and characterized by ectopic biomineralization and fragmentation of elastic fibers resulting in skin, cardiovascular and retinal calcifications. It has been recently described that pyrophosphate (a calcification inhibitor) deficiency could be the main cause of ectopic calcifications in this disease and in other genetic disorders associated to mutations of ENPP1 or CD73. Patients affected by Pseudoxanthoma Elasticum seem also prone to develop kidney stones originating from papillary calcifications named Randall’s plaque, and to a lesser extent may be affected by nephrocalcinosis. In this narrative review, we summarize some recent discoveries relative to the pathophysiology of this mendelian disease responsible for both cardiovascular and renal papillary calcifications, and we discuss the potential implications of pyrophosphate deficiency as a promoter of vascular calcifications in kidney stone formers and in patients affected by chronic kidney disease.

Imaging Review of Common and Rare Causes of Stroke in Children

Vascular injury is increasingly recognized as an important cause of mortality and morbidity in children (29 days to 18 years of age). Since vascular brain injury in children appears to be less common than in adults, the index of suspicion for vascular brain injury is usually lower. In this review article, we describe frequent and rare conditions underlying pediatric stroke including cardioembolic, viral, autoimmune, post-traumatic, and genetic etiologies. Furthermore, we provide a neuroimaging correlate for clinical mimics of pediatric stroke. This review highlights the role of multimodal noninvasive neuroimaging in the early diagnosis of pediatric stroke, providing a problem-solving approach to the differential diagnosis for the neuroradiologist, emergency room physician, and neurologist.

Ectopic mineralization of cartilage and collagen-rich tendons and ligaments in Enpp1asj-2J mice

Generalized arterial calcification of infancy (GACI), an autosomal recessive disorder caused by mutations in the ENPP1 gene, manifests with extensive mineralization of the cardiovascular system. A spontaneous asj-2J mutant mouse has been characterized as a model for GACI. Previous studies focused on phenotypic characterization of skin and vascular tissues. This study further examined the ectopic mineralization phenotype of cartilage, collagen-rich tendons and ligaments in this mouse model. The mice were placed on either control diet or the ‘acceleration diet’ for up to 12 weeks of age. Soft connective tissues, such as ear (elastic cartilage) and trachea (hyaline cartilage), were processed for standard histology. Assessment of ectopic mineralization in articular cartilage and fibrocartilage as well as tendons and ligaments which are attached to long bones were performed using a novel cryo-histological method without decalcification. These analyses demonstrated ectopic mineralization in cartilages as well as tendons and ligaments in the homozygous asj-2J mice at 12 weeks of age, with the presence of immature osteophytes displaying alkaline phosphatase and tartrate-resistant acid phosphatase activities as early as at 6 weeks of age. Alkaline phosphatase activity was significantly increased in asj-2J mouse serum as compared to wild type mice, indicating increased bone formation rate in these mice. Together, these data highlight the key role of ENPP1 in regulating calcification of both soft and skeletal tissues.

The ABCC6 Transporter: A New Player in Biomineralization

Pseudoxanthoma elasticum (PXE) is an inherited metabolic disease with autosomal recessive inheritance caused by mutations in the ABCC6 gene. Since the first description of the disease in 1896, alleging a disease involving the elastic fibers, the concept evolved with the further discoveries of the pivotal role of ectopic mineralization that is preponderant in the elastin-rich tissues of the skin, eyes and blood vessel walls. After discovery of the causative gene of the disease in 2000, the function of the ABCC6 protein remains elusive. More than 300 mutations have been now reported and the concept of a dermal disease has progressively evolved toward a metabolic disorder resulting from the remote effects caused by lack of a circulating anti-mineralization factor. Very recently, evidence has accumulated that this anti-mineralizing factor is inorganic pyrophosphate (PPi). This leads to decreased PPi/Pi (inorganic phosphate) ratio that results from the lack of extracellular ATP release by hepatocytes and probably renal cells harboring the mutant ABCC6 protein. However, the mechanism by which ABCC6 dysfunction causes diminished ATP release remains an enigma. Studies of other ABC transporters, such as ABCC7 or ABCC1 could help our understanding of what ABCC6 exact function is. Data and a hypothesis on the possible roles of ABCC6 in acquired metabolic diseases are also discussed.

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.

Mutation spectrum in the ABCC6 gene and genotype-phenotype correlations in a French cohort with pseudoxanthoma elasticum

PURPOSE

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disorder caused by variants in the ABCC6 gene. Ectopic mineralization of connective tissues leads to skin, eye, and cardiovascular manifestations with considerable phenotypic variability of unknown cause. We aimed to identify genotype-phenotype correlations in PXE.

METHODS

A molecular analysis was performed on 458 French PXE probands clinically evaluated using the Phenodex score (PS). Variant topographic analysis and genotype-phenotype correlation analysis were performed according to the number and type of identified variants.

RESULTS

Complete molecular analysis of 306 cases allowed the identification of 538 mutational events (88% detection rate) with 142 distinct variants, of which 66 were novel. Missense variant distribution was specific to some regions and residues of ABCC6. For the 220 cases with a complete PS, there was a higher prevalence of eye features in Caucasian patients (P = 0.03) and more severe eye and vascular phenotype in patients with loss-of-function variants (P = 0.02 and 0.05, respectively). Nephrolithiases and strokes, absent from the PS, were prevalent features of the disorder (11 and 10%, respectively).

CONCLUSION

We propose an updated PS including renal and neurological features and adaptation of follow-up according to the genetic and ethnic status of PXE-affected patients.Genet Med advance online publication 19 January 2017.

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.

Arterial calcification due to CD73 deficiency (ACDC): imaging manifestations of ectopic mineralization

Arterial calcification due to CD73 deficiency (ACDC) is a recently identified rare and debilitating adult-onset disorder caused by autosomal recessive NT5E gene mutations. ACDC is characterized by progressive and painful arterial calcifications primarily affecting the lower extremities, as well as calcifications affecting small joint capsules of the hands and feet. In this case report, the authors provide clinical follow-up for one of the first individuals identified by the National Institutes of Health (NIH) as having ACDC, focusing mainly on the imaging manifestations of periarticular joint mineralization, which are bilateral but slightly asymmetric, bulky up to the levels of the metacarpophalangeal and metatarsophalangeal joints, but smaller and more capsular in distribution at the proximal and distal interphalangeal joints, without erosive change or intra-articular mineralization. Differential considerations for similar appearing joint mineralization are provided.

Approach to the management of pregnancy in patients with pseudoxanthoma elasticum: a review

Management of pregnancy in patients with rare diseases is often guided by incomplete knowledge because of a lack of high-quality case control studies or single-centre experience. Pseudoxanthoma elasticum (PXE) is an autosomal recessive metabolic disorder that results in calcification of elastic fibres of the skin, retina, and arteries, leading to skin lesions, eventual central visual loss, and potential arterial insufficiency in most patients. It is due to mutations in ABCC6, which encodes the eponymous membrane transport protein. We review the literature on pregnancy in PXE, including the effects of the diseases on pregnancy and its complications, the effect of PXE on the foetus, and the effects of pregnancy on PXE, and conclude that in the majority of pregnancies in women with PXE, the outcome for mother, baby, and the disease is uneventful. We also provide recommendations for managing pregnancy in PXE.

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

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

Mouse genome-wide association study identifies polymorphisms on chromosomes 4, 11, and 15 for age-related cardiac fibrosis

Dystrophic cardiac calcinosis (DCC), also called epicardial and myocardial fibrosis and mineralization, has been detected in mice of a number of laboratory inbred strains, most commonly C3H/HeJ and DBA/2J. In previous mouse breeding studies between these DCC susceptible and the DCC-resistant strain C57BL/6J, 4 genetic loci harboring genes involved in DCC inheritance were identified and subsequently termed Dyscalc loci 1 through 4. Here, we report susceptibility to cardiac fibrosis, a sub-phenotype of DCC, at 12 and 20 months of age and close to natural death in a survey of 28 inbred mouse strains. Eight strains showed cardiac fibrosis with highest frequency and severity in the moribund mice. Using genotype and phenotype information of the 28 investigated strains, we performed genome-wide association studies (GWAS) and identified the most significant associations on chromosome (Chr) 15 at 72 million base pairs (Mb) (P < 10(-13)) and Chr 4 at 122 Mb (P < 10(-11)) and 134 Mb (P < 10(-7)). At the Chr 15 locus, Col22a1 and Kcnk9 were identified. Both have been reported to be morphologically and functionally important in the heart muscle. The strongest Chr 4 associations were located approximately 6 Mb away from the Dyscalc 2 quantitative trait locus peak within the boundaries of the Extl1 gene and in close proximity to the Trim63 and Cap1 genes. In addition, a single-nucleotide polymorphism association was found on chromosome 11. This study provides evidence for more than the previously reported 4 genetic loci determining cardiac fibrosis and DCC. The study also highlights the power of GWAS in the mouse for dissecting complex genetic traits.

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

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

L-carnitine affects osteoblast differentiation in NIH3T3 fibroblasts by the IGF-1/PI3K/Akt signalling pathway

Fibroblasts in soft tissues are one of the progenitors of ectopic calcification. Our previous experiment found that the serum concentrations of small metabolite L-carnitine (LC) decreased in an ectopic calcification animal model, indicating LC is a potential calcification or mineralization inhibitor. In this study, we investigated the effect of LC on NIH3T3 fibroblast osteoblast differentiation, and explored its possible molecular mechanisms. Two concentrations of LC (10 μM and 100 μM) were added in Pi-induced NIH3T3 fibroblasts, cell proliferation was compared by MTT assays, osteoblast differentiation was evaluated by ALP activity, mineralized nodules formation, calcium deposition, and expressions of the osteogenic marker genes. Our results indicated that 10 μM LC increased the proliferation of NIH3T3 cells, but 100 μM LC slightly inhibited cell proliferation. 100 μM LC inhibits NIH3T3 differentiation as evidenced by decreases in ALP activity, mineralized nodule formation, calcium deposition, and down-regulation of the osteogenic marker genes ALP, Runx2 and OCN, meanwhile 10 μM of LC exerts an opposite effect that promotes NIH3T3 osteogenesis. Mechanistically, 100 μM LC significantly inhibits IGF-1/PI3K/Akt signalling, while 10 μM LC slightly activates this pathway. Our study suggests that a decease in LC level might contribute to the development of ectopic calcification in fibroblasts by affecting IGF-1/PI3K/Akt, and addition of LC may benefit patients with ectopic calcification.

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

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

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

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

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

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

Genetic heterogeneity of pseudoxanthoma elasticum: the Chinese signature profile of ABCC6 and ENPP1 mutations

Pseudoxanthoma elasticum (PXE), an autosomal recessive disorder characterized by ectopic mineralization, is caused by mutations in the ABCC6 gene. We examined clinically 29 Chinese PXE patients from unrelated families, so far the largest cohort of Asian PXE patients. In a subset of 22 patients, we sequenced ABCC6 and another candidate gene, ENPP1, and conducted pathogenicity analyses for each variant. We identified a total of 17 distinct mutations in ABCC6, 15 of them being, to our knowledge, previously unreported, including 5 frameshift and 10 missense variants. In addition, a missense mutation in combination with a recurrent nonsense mutation in ENPP1 was discovered in a pediatric PXE case. No cases with p.R1141X or del23-29 mutations, common in Caucasian patient populations, were identified. The 10 missense mutations in ABCC6 were expressed in the mouse liver via hydrodynamic tail-vein injections. One mutant protein showed cytoplasmic accumulation indicating abnormal subcellular trafficking, while the other nine mutants showed correct plasma membrane location. These nine mutations were further investigated for their pathogenicity using a recently developed zebrafish mRNA rescue assay. Minimal rescue of the morpholino-induced phenotype was achieved with eight of the nine mutant human ABCC6 mRNAs tested, implying pathogenicity. This study demonstrates that the Chinese PXE population harbors unique ABCC6 mutations. These genetic data have implications for allele-specific therapy currently being developed for PXE.

Vascular calcification in chronic kidney disease: an update

Cardiovascular calcification is both a risk factor and contributor to morbidity and mortality. Patients with chronic kidney disease (and/or diabetes) exhibit accelerated calcification of the intima, media, heart valves and likely the myocardium as well as the rare condition of calcific uraemic arteriolopathy (calciphylaxis). Pathomechanistically, an imbalance of promoters (e.g. calcium and phosphate) and inhibitors (e.g. fetuin-A and matrix Gla protein) is central in the development of calcification. Next to biochemical and proteinacous alterations, cellular processes are also involved in the pathogenesis. Vascular smooth muscle cells undergo osteochondrogenesis, excrete vesicles and show signs of senescence. Therapeutically, measures to prevent the initiation of calcification by correcting the imbalance of promoters and inhibitors appear to be essential. In contrast to prevention, therapeutic regression of cardiovascular calcification in humans has been rarely reported. Measures to enhance secondary prevention in patients with established cardiovascular calcifications are currently being tested in clinical trials.

ABCC6 deficiency is associated with activation of BMP signaling in liver and kidney

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ABCC6 deficiency stimulates BMP signaling in multiple organs.

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ABCC6 deficiency causes tissue-specific induction of BMP-related genes.

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Tissue-specific targeting of BMP signaling may be needed in ABCC deficiency.

Mutations in ABCC6 (ATP-binding cassette, subfamily C, member 6), an orphan transporter expressed in the liver, are the cause of pseudoxanthoma elasticum. Since ABCC6 was reported to affect matrix Gla protein (MGP), an inhibitor of bone morphogenetic proteins (BMPs), we studied BMP signaling and expression in various tissues of mice with and without functional ABCC. Enhanced BMP signaling was found in all examined tissues in the absence of ABCC6. Despite this, the expression of particular BMP proteins varied widely between tissues. Interestingly, the expression of most BMP proteins in the liver moved in the opposite direction to the same BMP proteins in kidneys in response to ABCC6 alterations. Thus, ABCC6 deficiency stimulates BMP signaling by acting on the expression of multiple BMPs.

Copy number variation in the ATP-binding cassette transporter ABCC6 gene and ABCC6 pseudogenes in patients with pseudoxanthoma elasticum

Single mutations in the ATP-binding cassette transporter (ABCC6) gene (OMIM 603234) are known to cause the rare autosomal recessive disease pseudoxanthoma elasticum (PXE). Recently, we have found that copy number variations (CNVs) in pseudogenes of the ABCC6 gene are quite common. The aim of this study was to investigate the frequency and possible contribution of CNV in ABCC6 and its pseudogenes in PXE. Genomic DNA from 212 PXE individuals were examined for copy number by pyrosequencing and quantitative polymerase chain reaction (PCR) and compared with healthy individuals. The frequency of PXE individuals with any CNV was higher than in healthy individuals. The majority of variation comprised known and possibly new deletions in the ABCC6 gene and duplications of the ABCC6P1 and ABCC6P2 genes. ABCC6 deletions and ABCC6P2 duplications were not observed in 142 healthy individuals. In conclusion, by pyrosequencing and quantitative PCR, we were able to detect known and possibly new deletions in the ABCC6 gene that may have caused the PXE phenotype. Pyrosequencing may be used in PXE patients who have obtained incomplete genotype from conventional techniques. The frequency of ABCC6P2 pseudogene duplication was more common in PXE patients than healthy individuals and may affect the PXE phenotype.

Large-scaled metabolic profiling of human dermal fibroblasts derived from pseudoxanthoma elasticum patients and healthy controls

Mutations in the ABC transporter ABCC6 were recently identified as cause of Pseudoxanthoma elasticum (PXE), a rare genetic disorder characterized by progressive mineralization of elastic fibers. We used an untargeted metabolic approach to identify biochemical differences between human dermal fibroblasts from healthy controls and PXE patients in an attempt to find a link between ABCC6 deficiency, cellular metabolic alterations and disease pathogenesis. 358 compounds were identified by mass spectrometry covering lipids, amino acids, peptides, carbohydrates, nucleotides, vitamins and cofactors, xenobiotics and energy metabolites. We found substantial differences in glycerophospholipid composition, leucine dipeptides, and polypeptides as well as alterations in pantothenate and guanine metabolism to be significantly associated with PXE pathogenesis. These findings can be linked to extracellular matrix remodeling and increased oxidative stress, which reflect characteristic hallmarks of PXE. Our study could facilitate a better understanding of biochemical pathways involved in soft tissue mineralization.

Acute transient myopia with shallowing of the anterior chamber induced by sulfamethoxazole in a patient with pseudoxanthoma elasticum

PURPOSE

To report a case of acute transient myopia with anterior chamber shallowing induced by sulfamethoxazole in a patient with pseudoxanthoma elasticum (PXE).

DESIGN

Observational case report.

METHODS

A case report of a 45-year-old woman who presented with bilateral acute myopia, anterior chamber shallowing, and intraocular hypertension induced by sulfamethoxazole and was found to have PXE. Initial and follow-up examination findings were reviewed.

RESULTS

On first examination, bilateral myopic shift of 4.25 D, bilateral narrowed angles, and ocular hypertension (36 mm Hg right eye and 38 mm Hg left eye) were found. Pentacam images documented the anterior displacement of the iris-lens diaphragm. Undilated fundus examination disclosed bilateral angioid streaks radiating from the papilla. Several redundant skin folds on the neck and axillae were found on external examination. With sulfamethoxazole discontinuation and administration of topical intraocular pressure-lowering drops, there was complete clinical resolution within 1 week. The diagnosis of PXE was confirmed by biopsy of the skin lesions.

CONCLUSIONS

Acute myopia with angle narrowing is an extremely rare sulfamethoxazole side effect, and its relationship, if any, with PXE is unknown. As far as we know, this is the first reported case of PXE presenting with bilateral angle narrowing induced by sulfamethoxazole.

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.

Paediatric pseudoxanthoma elasticum with cardiovascular involvement

BACKGROUND

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

PSEUDOXANTHOMA ELASTICUM: DIAGNOSTIC FEATURES, CLASSIFICATION, AND TREATMENT OPTIONS

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Retinitis pigmentosa, cutis laxa, and pseudoxanthoma elasticum-like skin manifestations associated with GGCX mutations

Gamma-glutamyl carboxylase (GGCX) mutations have been reported in patients with a pseudoxanthoma elasticum (PXE)-like phenotype, loose redundant skin, and multiple vitamin K-dependent coagulation factor deficiencies. We report on the clinical findings and molecular results in 13 affected members of two families who had a uniform phenotype consisting of (PXE)-like skin manifestations in the neck and trunk, loose sagging skin of the trunk and upper limbs, and retinitis pigmentosa confirmed by electroretinographies in 10 affected individuals. There were no coagulation abnormalities. Molecular investigations of the ATP-binding cassette subfamily C member 6 did not yield causative mutations. All 13 affected family members were found to be homozygous for the splice-site mutation c.373+3G>T in the GGCX gene. All tested parents were heterozygous for the mutation, and healthy siblings were either heterozygous or had the wild type. We suggest that the present patients represent a hitherto unreported phenotype associated with GGCX mutations. Digenic inheritance has been suggested to explain the variability in phenotype in GGCX mutation carriers. Consequently, the present phenotype may not be explained only by the GGCX mutations only but may be influenced by variants in other genes or epigenetic and environmental factors.

Mutations in the ABCC6 gene as a cause of generalized arterial calcification of infancy: genotypic overlap with pseudoxanthoma elasticum

Generalized arterial calcification of infancy (GACI) is an autosomal recessive disorder characterized by congenital calcification of large- and medium-sized arteries, associated with early myocardial infarction, heart failure, and stroke, and premature death. Most cases of GACI are caused by mutations in the ENPP1 gene. We first studied two siblings with GACI from a non-consanguineous family without mutations in the ENPP1 gene. To search for disease-causing mutations, we identified genomic regions shared between the two affected siblings but not their unaffected parents or brother. The ABCC6 gene, which is mutated in pseudoxanthoma elasticum (PXE), resided within a small region of homozygosity shared by the affected siblings. Sequence analysis of ABCC6 revealed that the two affected siblings were homozygous for the missense mutation p.R1314W. Subsequently, ABCC6 mutations were identified in five additional GACI families with normal ENPP1 sequences. Genetic mutations in ABCC6 in patients with PXE are associated with ectopic tissue mineralization in the skin and arterial blood vessels. Thus, our findings provide additional evidence that the ABCC6 gene product inhibits calcification under physiologic conditions and confirm a second locus for GACI. In addition, our study emphasizes the potential utility of shared homozygosity mapping to identify genetic causes of inherited disorders.

Mouse models for pseudoxanthoma elasticum: genetic and dietary modulation of the ectopic mineralization phenotypes

Pseudoxanthoma elasticum (PXE), a heritable ectopic mineralization disorder, is caused by mutations in the ABCC6 gene. Null mice (Abcc6^−/−) recapitulate the genetic, histopathologic and ultrastructural features of PXE, and they demonstrate early and progressive mineralization of vibrissae dermal sheath, which serves as a biomarker of the overall mineralization process. Recently, as part of a mouse aging study at The Jackson Laboratory, 31 inbred mouse strains were necropsied, and two of them, KK/HlJ and 129S1/SvImJ, were noted to have vibrissae dermal mineralization similar to Abcc6^−/− mice. These two strains were shown to harbor a single nucleotide polymorphism (rs32756904) in the Abcc6 gene, which resulted in out-of-frame splicing and marked reduction in ABCC6 protein expression in the liver of these mice. The same polymorphism is present in two additional mouse strains, DBA/2J and C3H/HeJ, with similar reduction in Abcc6 protein levels, yet these mice did not demonstrate tissue mineralization when kept on standard rodent diet. However, all four mouse strains, when placed on experimental diet enriched in phosphate and low in magnesium, developed extensive ectopic mineralization. These results indicate that the genetic background of mice and the mineral composition of their diet can profoundly modulate the ectopic mineralization process predicated on mutations in the Abcc6 gene. These mice provide novel model systems to study the pathomechanisms and the reasons for strain background on phenotypic variability of PXE.