Characterization of pseudoxanthoma elasticum-like lesions in the skin of patients with beta-thalassemia

Pseudoxanthoma elasticum - Genetics, pathophysiology, and clinical presentation

Pseudoxanthoma elasticum (PXE) is an autosomal-recessively inherited multisystem disease. Mutations in the ABCC6-gene are causative, coding for a transmembrane transporter mainly expressed in hepatocytes, which promotes the efflux of adenosine triphosphate (ATP). This results in low levels of plasma inorganic pyrophosphate (PPi), a critical anti-mineralization factor. The clinical phenotype of PXE is characterized by the effects of elastic fiber calcification in the skin, the cardiovascular system, and the eyes. In the eyes, calcification of Bruch's membrane results in clinically visible lesions, including peau d'orange, angioid streaks, and comet tail lesions. Frequently, patients must be treated for secondary macular neovascularization. No effective therapy is available for treating the cause of PXE, but several promising approaches are emerging. Finding appropriate outcome measures remains a significant challenge for clinical trials in this slowly progressive disease. This review article provides an in-depth summary of the current understanding of PXE and its multi-systemic manifestations. The article offers a detailed overview of the ocular manifestations, including their morphological and functional consequences, as well as potential complications. Lastly, previous and future clinical trials of causative treatments for PXE are discussed.

Mitochondrial Dysfunction and Oxidative Stress in Hereditary Ectopic Calcification Diseases

Ectopic calcification (EC) is characterized by an abnormal deposition of calcium phosphate crystals in soft tissues such as blood vessels, skin, and brain parenchyma. EC contributes to significant morbidity and mortality and is considered a major health problem for which no effective treatments currently exist. In recent years, growing emphasis has been placed on the role of mitochondrial dysfunction and oxidative stress in the pathogenesis of EC. Impaired mitochondrial respiration and increased levels of reactive oxygen species can be directly linked to key molecular pathways involved in EC such as adenosine triphosphate homeostasis, DNA damage signaling, and apoptosis. While EC is mainly encountered in common diseases such as diabetes mellitus and chronic kidney disease, studies in rare hereditary EC disorders such as pseudoxanthoma elasticum or Hutchinson-Gilford progeria syndrome have been instrumental in identifying the precise etiopathogenetic mechanisms leading to EC. In this narrative review, we describe the current state of the art regarding the role of mitochondrial dysfunction and oxidative stress in hereditary EC diseases. In-depth knowledge of aberrant mitochondrial metabolism and its local and systemic consequences will benefit the research into novel therapies for both rare and common EC disorders.

ABCC6, Pyrophosphate and Ectopic Calcification: Therapeutic Solutions

Pathological (ectopic) mineralization of soft tissues occurs during aging, in several common conditions such as diabetes, hypercholesterolemia, and renal failure and in certain genetic disorders. Pseudoxanthoma elasticum (PXE), a multi-organ disease affecting dermal, ocular, and cardiovascular tissues, is a model for ectopic mineralization disorders. ABCC6 dysfunction is the primary cause of PXE, but also some cases of generalized arterial calcification of infancy (GACI). ABCC6 deficiency in mice underlies an inducible dystrophic cardiac calcification phenotype (DCC). These calcification diseases are part of a spectrum of mineralization disorders that also includes Calcification of Joints and Arteries (CALJA). Since the identification of ABCC6 as the “PXE gene” and the development of several animal models (mice, rat, and zebrafish), there has been significant progress in our understanding of the molecular genetics, the clinical phenotypes, and pathogenesis of these diseases, which share similarities with more common conditions with abnormal calcification. ABCC6 facilitates the cellular efflux of ATP, which is rapidly converted into inorganic pyrophosphate (PPi) and adenosine by the ectonucleotidases NPP1 and CD73 (NT5E). PPi is a potent endogenous inhibitor of calcification, whereas adenosine indirectly contributes to calcification inhibition by suppressing the synthesis of tissue non-specific alkaline phosphatase (TNAP). At present, therapies only exist to alleviate symptoms for both PXE and GACI; however, extensive studies have resulted in several novel approaches to treating PXE and GACI. This review seeks to summarize the role of ABCC6 in ectopic calcification in PXE and other calcification disorders, and discuss therapeutic strategies targeting various proteins in the pathway (ABCC6, NPP1, and TNAP) and direct inhibition of calcification via supplementation by various compounds.

Diagnosing Calciphylaxis: A Review With Emphasis on Histopathology

Calciphylaxis is a cutaneous vasculopathy with high morbidity and mortality characterized by vascular intimal fibrosis, calcification, stenosis, thrombosis, and eventual tissue death due to ischemia. Histopathologic diagnosis is often difficult, frequently necessitating multiple tissues samples due to lack of specific histopathologic features and subtle changes on biopsies of early lesions. In this study, we review the reported clinical and histopathologic features of calciphylaxis, correlating them with relevant imaging, ancillary studies, and pathophysiology. Although many histopathologic changes seen in calciphylaxis are also reported in other conditions (eg, Mönckeberg sclerosis, lupus panniculitis, pancreatic panniculitis, and peripheral artery disease), calcification of subcutaneous small vessels, thrombosis and/or ischemic changes, pseudoxanthoma elasticum-like changes in the subcutis, and perieccrine calcification may serve as helpful clues. von Kossa and Alizarin red stains can assist in the identification of subtle calcification. Netlike calcification of the affected blood vessels on imaging further supports the diagnosis. Studies into the pathophysiology of calciphylaxis are ongoing and will hopefully facilitate the development of additional diagnostic adjuncts to increase sensitivity and specificity for the diagnosis of this disease.

From membrane to mineralization: the curious case of the ABCC6 transporter

ATP-binding cassette subfamily C member 6 gene/protein (ABCC6) is an ATP-dependent transmembrane transporter predominantly expressed in the liver and the kidney. ABCC6 first came to attention in human medicine when it was discovered in 2000 that mutations in its encoding gene, ABCC6, caused the autosomal recessive multisystemic mineralization disease pseudoxanthoma elasticum (PXE). Since then, the physiological and pathological roles of ABCC6 have been the subject of intense research. In the last 20 years, significant findings have clarified ABCC6 structure as well as its physiological role in mineralization homeostasis in humans and animal models. Yet, several facets of ABCC6 biology remain currently incompletely understood, ranging from the precise nature of its substrate(s) to the increasingly complex molecular genetics. Nonetheless, advances in our understanding of pathophysiological mechanisms causing mineralization lead to several treatment options being suggested or already tested in pilot clinical trials for ABCC6 deficiency. This review highlights current knowledge of ABCC6 and the challenges ahead, particularly the attempts to translate basic science into clinical practice.

Cutaneous polyarteritis nodosa and concurrent pseudoxanthoma elasticum–like phenotype: A case report

We report a unique case of a patient presenting with histologically confirmed pseudoxanthoma elasticum–like phenotype and cutaneous polyarteritis nodosa. Cardiac, gastroenterological, and ophthalmologic evaluations were within normal limits. Genetic evaluation was pertinent for absent ABCC6, ENPP1, and GGCX mutations and a normal array comparative genomic hybridization. Extensive workup revealed skin-limited cutaneous polyarteritis nodosa, and further genetic testing for ADA2 deficiency was negative. The cutaneous polyarteritis nodosa lesions had an excellent response to hydroxychloroquine and methotrexate. Pseudoxanthoma elasticum and polyarteritis nodosa are relatively uncommon, and our patient is among the first reported cases presenting with both pseudoxanthoma elasticum–like and polyarteritis nodosa. Furthermore, this case emphasizes the importance of a thorough cutaneous exam, as the patient had the lesions consistent with pseudoxanthoma elasticum–like since childhood and had previously gone undiagnosed.

What's New in Dermatopathology: Inflammatory Dermatoses

Inflammatory skin diseases encompass a vast array of conditions. The field continues to expand and evolve with resurgence of conditions, through newly recognized medication adverse effects, and via more detailed descriptions of known dermatoses. The importance of clinicopathologic correlation and an up to date knowledge of dermatologic conditions cannot be overstated. This review focuses on an array of recent important developments in the histologic diagnosis of inflammatory conditions that affect the skin.

Rheumatological complications of beta-thalassaemia: an overview

Beta-thalassaemia, an autosomal recessive haemoglobinopathy, ranks among the most frequent monogenetic diseases globally. The severe form of the disease, beta-thalassaemia major, is accompanied by progressive involvement of multiple organ systems as a result of the disease pathophysiology as well as iron overload from blood transfusions on a regular basis. Some of the manifestations might also be caused by medications used to manage iron overload. The purpose of this review is to highlight the rheumatological complications of beta-thalassaemia, which include musculoskeletal manifestations, such as arthritis and arthropathies, joint effusions, osteoporosis, bone fractures and myalgias, in addition to CTDs, such as pseudoxanthoma elasticum. Rheumatologists are strongly encouraged to take part in a multidisciplinary approach to the management of this debilitating disease.

Pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is a genetic metabolic disease with autosomal recessive inheritance caused by mutations in the ABCC6 gene. The lack of functional ABCC6 protein leads to ectopic mineralization that is most apparent in the elastic tissues of the skin, eyes and blood vessels. The clinical prevalence of PXE has been estimated at between 1 per 100,000 and 1 per 25,000, with slight female predominance. The first clinical sign of PXE is almost always small yellow papules on the nape and sides of the neck and in flexural areas. The papules coalesce, and the skin becomes loose and wrinkled. The mid-dermal elastic fibers are short, fragmented, clumped and calcified. Dystrophic calcification of Bruch's membrane, revealed by angioid streaks, may trigger choroidal neovascularization and, ultimately, loss of central vision and blindness in late-stage disease. Lesions in small and medium-sized artery walls may result in intermittent claudication and peripheral artery disease. Cardiac complications (myocardial infarction, angina pectoris) are thought to be relatively rare but merit thorough investigation. Ischemic strokes have been reported. PXE is a metabolic disease in which circulating levels of an anti-mineralization factor are low. There is good evidence to suggest that the factor is inorganic pyrophosphate (PPi), and that the circulating low levels of PPi and decreased PPi/Pi ratio result from the lack of ATP release by hepatocytes harboring the mutant ABCC6 protein. However, the substrate(s) bound, transported or modulated by the ABCC6 protein remain unknown. More than 300 sequence variants of the ABCC6 gene have been identified. There is no cure for PXE; the main symptomatic treatments are vascular endothelial growth factor inhibitor therapy (for ophthalmic manifestations), lifestyle, lipid-lowering and dietary measures (for reducing vascular risk factors), and vascular surgery (for severe cardiovascular manifestations). Future treatment options may include gene therapy/editing and pharmacologic chaperone therapy.

Visual Impairment in Pseudoxanthoma Elasticum: A Survey of 53 Patients from Turkey

Purpose

To estimate the age at onset and severity of ophthalmologic aspects of pseudoxanthoma elasticum (PXE).

Methods

Medical records of 53 patients with PXE were evaluated. Findings at initial presentation, including refraction, best-corrected visual acuity (BCVA), and biomicroscopic and funduscopic examination were explored.

Results

The mean age of the patients at time of presentation was 43 years. The male-to-female ratio was 2.8:1. The mean age at occurrence of the first visual symptoms and retinal complications associated with choroidal neovascularization (CNV) was 42 years. Active subfoveal CNV or choroidal scar formation at the time of presentation was observed in 80 eyes (75%) of 49 patients (mean age 43 years). Twenty-seven patients (51%) (mean age 43 years) had BCVA ≤20/200 in at least one eye. Ten patients (19%) had BCVA ≤20/200 in both eyes (mean age 48 years). Twenty patients (41%) presenting with subfoveal chorioretinal scar or an active subfoveal CNV reported a spontaneous decline in visual acuity before age 40.

Conclusions

Subfoveal CNV was a frequent finding responsible for the poor visual acuity of patients with PXE. Its formation was not strictly age-dependent, and in patients under 40 years of age could be underestimated. More trials are needed to determine clinical recommendations for follow-up of young patients with PXE.

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

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

Pseudoxanthoma elasticum-like change adjacent to a benign adnexal neoplasm: a histopathologic reaction pattern

The progressive calcification and fragmentation of elastic fibers, primarily affecting the skin, are histopathologic features typically associated with pseudoxanthoma elasticum (PXE), pseudo-pseudo PXE, or even acquired PXE. We present a case of a 65-year-old woman, with a known history of breast carcinoma, status after wide excision (T1N0Mx) in 2010, who presented with a 0.6 × 0.5-cm, nontender pink nodule in the right inferior axilla of 1 week duration on the same side as her mastectomy. Histopathologically, the lesion revealed a circumscribed, nonencapsulated bland dermal tumor with poroid and clear cells and foci of ductal differentiation-features diagnostic for hidradenoma. Of note, immediately adjacent to the hidradenoma, the specimen also exhibited short, frayed elastic tissue fibers (positive for EVG and Von Kossa stains)-features reminiscent of PXE. With this case, we expand the spectrum of reaction patterns observed in the skin to include a PXE-like change, a reaction pattern that, to the best of our knowledge, has not been previously reported in association with a benign cutaneous tumor.

Efficiency of exome sequencing for the molecular diagnosis of pseudoxanthoma elasticum

The molecular etiology of pseudoxanthoma elasticum (PXE), an autosomal recessive connective tissue disorder, has become increasingly complex as not only mutations in ATP-binding cassette family C member 6 (ABCC6) but also ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) and gamma-glutamyl carboxylase (GGCX) can cause resembling phenotypes. Identification of modifier genes, such as vascular endothelial growth factor A, has further contributed to the molecular heterogeneity of PXE. In such heterogeneous diseases, next-generation sequencing (NGS) allows to perform mutation screening of several genes in a single reaction. We explored whole-exome sequencing (WES) as an efficient diagnostic tool to identify the causal mutations in ABCC6, GGCX, ENPP1, and vitamin K epoxide reductase complex, subunit 1 (VKORC1) in 16 PXE patients. WES identified a causal ABCC6 mutation in 30 out of 32 alleles and one GGCX mutation, whereas no causal mutations in ENPP1 or VKORC1 were detected. Exomes with insufficient reads (⩽20 depth) for the four genes and patients with single mutations were further evaluated by Sanger sequencing (SS), but no additional mutations were found. The potential of WES compared with targeted NGS is the ease to examine target genes and the opportunity to search for novel genes when targeted analysis is negative. Together with low cost, rapid and less laborious workflow, we conclude that WES complemented with SS can provide a tiered approach to molecular diagnostics of PXE.

Perturbation of specific pro-mineralizing signalling pathways in human and murine pseudoxanthoma elasticum

Background

Pseudoxanthoma elasticum (PXE) is characterized by skin (papular lesions), ocular (subretinal neovascularisation) and cardiovascular manifestations (peripheral artery disease), due to mineralization and fragmentation of elastic fibres in the extracellular matrix (ECM). Caused by mutations in the ABCC6 gene, the mechanisms underlying this disease remain unknown. The knowledge on the molecular background of soft tissue mineralization largely comes from insights in vascular calcification, with involvement of the osteoinductive Transforming Growth Factor beta (TGFβ) family (TGFβ1-3 and Bone Morphogenetic Proteins [BMP]), together with ectonucleotides (ENPP1), Wnt signalling and a variety of local and systemic calcification inhibitors. In this study, we have investigated the relevance of the signalling pathways described in vascular soft tissue mineralization in the PXE knock-out mouse model and in PXE patients.

Methods

The role of the pro-osteogenic pathways BMP2-SMADs-RUNX2, TGFβ-SMAD2/3 and Wnt-MSX2, apoptosis and ER stress was evaluated using immunohistochemistry, mRNA expression profiling and immune-co-staining in dermal tissues and fibroblast cultures of PXE patients and the eyes and whiskers of the PXE knock-out mouse. Apoptosis was further evaluated by TUNEL staining and siRNA mediated gene knockdown. ALPL activity in PXE fibroblasts was studied using ALPL stains.

Results

We demonstrate the upregulation of the BMP2-SMADs-RUNX2 and TGFβ-2-SMAD2/3 pathway, co-localizing with the mineralization sites, and the involvement of MSX2-canonical Wnt signalling. Further, we show that apoptosis is also involved in PXE with activation of Caspases and BCL-2. In contrast to vascular calcification, neither the other BMPs and TGFβs nor endoplasmic reticulum stress pathways seem to be perturbed in PXE.

Conclusions

Our study shows that we cannot simply extrapolate knowledge on cell signalling in vascular soft tissue calcification to a multisystem ectopic mineralisation disease as PXE. Contrary, we demonstrate a specific set of perturbed signalling pathways in PXE patients and the knock-out mouse model. Based on our findings and previously reported data, we propose a preliminary cell model of ECM calcification in PXE.

The spectrum of ocular alterations in patients with β-thalassemia syndromes suggests a pathology similar to pseudoxanthoma elasticum

PURPOSE

To determine the prevalence and spectrum of ocular fundus abnormalities in patients with β-thalassemia and to investigate risk factors for their development.

DESIGN

Cross-sectional, observational study.

PARTICIPANTS

A total of 255 patients with β-thalassemia major (TM) and β-thalassemia intermedia (TI) were consecutively recruited and investigated.

METHODS

Patients underwent best correct visual acuity, indirect ophthalmoscopy, and fundus photography, including fundus autofluorescence (FAF) and near-infrared reflectance imaging using a confocal scanning laser ophthalmoscope (cSLO). Hematologic parameters were determined, including mean ferritin levels, aspartate amino transferase, alanine amino transferase, calcium, pre-transfusion hemoglobin, history of splenectomy, and liver iron concentration. Factors associated with the ocular phenotype were assessed using logistic regression.

MAIN OUTCOME MEASURES

Ocular phenotype as determined by clinical examination and used multimodal imaging.

RESULTS

A total of 153 patients (60.0%) affected by TM and 102 patients (40.0%) affected by TI participated, of whom 216 (84.7%) were receiving iron-chelating therapy. Ocular fundus abnormalities characteristic of pseudoxanthoma elasticum (PXE) were detected by cSLO in 70 of 255 patients (27.8%) and included peau d'orange (19.6%), angioid streaks (12.9%), pattern dystrophy-like changes (7.5%), and optic disc drusen (2.0%). Pseudoxanthoma elasticum-like changes were more frequent in patients with TI (P<0.001). Patients with PXE-like fundus changes were older than patients without these fundus changes (P<0.001). In both patients with TI and TM, age (P = 0.001) and splenectomy (P = 0.001) had the strongest association with presence of PXE-like fundus changes in multivariate analyses. A total of 43 of 255 patients (16.9%) showed increased retinal vascular tortuosity independently of the PXE-like fundus changes, which was associated with aspartate amino transferase (P = 0.036), hemoglobin (P = 0.008), and ferritin levels (P = 0.005).

CONCLUSIONS

Pseudoxanthoma elasticum-like fundus changes are a frequent finding in patients with β-thalassemia. In TI, these changes increase with duration or severity of the disease. This particular ocular phenotype suggests an ocular pathology similar to PXE. Retinal vascular tortuosity may be an additional disease manifestation independent of the PXE-like syndrome. Patients with long-standing disease requiring iron-chelating treatment and a history of splenectomy need regular ophthalmic checkups because they are at risk of developing PXE-like fundus changes and potentially of subsequent choroidal neovascularization.

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

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

Pseudoxanthoma elasticum: genetic diagnostic markers

Pseudoxanthoma elasticum (PXE), an autosomal recessive disorder with considerable phenotypic variability, mainly affects the eyes, skin and cardiovascular system, and is characterized by ectopic mineralization of elastic fibers of connective tissues. Since the identification of the ABCC6 gene (ATP-binding cassette family C member 6), which encodes a putative transmembrane transporter (ABCC6), as the site of mutations responsible for PXE, a number of researchers have disclosed mutations spanning the entire gene. An important advance in the ability to identify mutations has been the identification of two closely related pseudogenes and identifying sequence differences between the coding gene and the pseudogenes allowing accurate sequencing. In this review, the mutation spectrum in PXE is summarized and a strategy to optimize mutation detection in this difficult disorder is outlined.

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.

Is classical pseudoxanthoma elasticum a consequence of hepatic 'intoxication' due to ABCC6 substrate accumulation in the liver?

Pseudoxanthoma elasticum (PXE) is a serious genetic disorder with ectopic mineralization affecting the skin, the eye and the cardiovascular system. The disease is predominantly caused by mutations in the transmembrane ABC protein ABCC6, a putative small substrate transporter. Interestingly, ABCC6 seems virtually absent in the affected organs, whereas a high expression is seen in hepatocytes. This and further published experimental evidence indicate that PXE is a systemic, metabolic liver disease where circulatory changes affect the peripheral mineralization process. Owing to the well-characterized transport of organic substrates by related ABC proteins, it has been proposed that PXE is caused by impaired export of an antimineralization compound to the blood. The authors here present an alternative hypothesis that explains ectopic mineralization in PXE as a consequence of hepatic accumulation of ABCC6 substrate(s) that via gene-regulating effects leads to altered hepatic secretion and activation of antimineralization/anticalcification proteins such as fetuin-A and Gla proteins.

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

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

Cutaneous features of pseudoxanthoma elasticum in a patient with generalized arterial calcification of infancy due to a homozygous missense mutation in the ENPP1 gene

BACKGROUND

Pseudoxanthoma elasticum (PXE) manifests with cutaneous lesions consisting of yellowish papules coalescing into plaques of inelastic skin. Histopathology demonstrates accumulation of pleiomorphic elastic structures with progressive mineralization. The classic form of PXE is caused by mutations in the ABCC6 gene.

OBJECTIVES

A 2-year-old patient with PXE of the neck, inguinal folds and lower abdomen, and with extensive tissue mineralization, was evaluated for the underlying mutations in candidate genes known to be involved in ectopic mineralization disorders.

METHODS

The patient's genotype was studied by sequencing ABCC6, MGP and ENPP1 genes, encoding proteins which harbour mutations in ectopic mineralization disorders.

RESULTS

No pathogenetic mutations were found in the ABCC6 or MGP genes. Sequencing of ENPP1 disclosed a homozygous missense mutation, p.Y513C, associated with generalized arterial calcification of infancy.

CONCLUSIONS

This study demonstrates the presence of the cutaneous features of PXE in a genetically distinct disease, generalized arterial calcification of infancy, and thus expands the spectrum of PXE-related disorders.

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

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

A mouse model of β-thalassemia shows a liver-specific down-regulation of Abcc6 expression

β-Thalassemia and pseudoxanthoma elasticum (PXE) are distinct genetic disorders. Yet, a dystrophic mineralization phenotype similar to PXE has frequently been associated with β-thalassemia or sickle cell anemia patients of Mediterranean descent. These calcifications are clinically and structurally identical to inherited PXE. As we previously excluded the presence of PXE-causing mutations in the ABCC6 gene of β-thalassemia patients with PXE manifestations, we hypothesized that a molecular mechanism independent of gene mutations either altered the ABCC6 gene expression or disrupted the biologic properties of its product in the liver or kidneys, which are the tissues with the highest levels of expression. To test this possibility, we investigated Abcc6 synthesis in the liver and kidneys of a β-thalassemia mouse model (Hbb(th3/+)). We found a progressive liver-specific down-regulation of the Abcc6 gene expression and protein levels by quantitative PCR, Western blotting, and immunofluorescence. The levels of Abcc6 protein decreased significantly at 6 months of age and stabilized at 10 months and older ages at ∼25% of the wild-type protein levels. We studied the transcriptional regulation of the Abcc6 gene in wild-type and Hbb(th3/+) mice, and we identified the erythroid transcription factor NF-E2 as the main cause of the transcriptional down-regulation using transcription factor arrays and chromatin immunoprecipitation. The Hbb(th3/+) mice did not develop spontaneous calcification as seen in the Abcc6(-/-) mice probably because the Abcc6 protein decrease occurred late in life and was probably insufficient to promote mineralization in the Hbb(th3/+) mouse C57BL/6J genetic background. Nevertheless, our result suggested that a similar decrease of ABCC6 expression occurs in the liver of β-thalassemia patients and may be responsible for their frequent PXE-like manifestations.

Pseudoxanthoma elasticum, the paradigm of heritable ectopic mineralization disorders - can diet help?

Pseudoxanthoma elasticum (PXE) is a heritable multi-system disorder manifesting with characteristic cutaneous lesions, associated with ocular findings and cardiovascular involvement. The skin lesions, yellowish papules which coalesce into plaques of inelastic and leathery skin, demonstrate by histopathologic and ultrastructural examinations ectopic mineralization of dermal connective tissues, primarily the elastic structures. PXE is inherited in an autosomal recessive fashion due to mutations in the ABCC6 gene. Significant insights into the pathogenesis of PXE have been recently obtained from observations on the Abcc6(-/-) knockout mouse which mimics the genetic, histopathologic and ultrastructural features of PXE. This mouse model has provided a platform to test various treatment modalities to counteract the mineralization phenotypes. One of the intriguing findings emanating from these studies is that supplementation of the mouse diet with magnesium, at levels that are ∼5-fold higher than those in control diet, completely inhibits the development of tissue mineralization. These and related observations suggest that changes in the diet might counteract the progression of PXE and improve the quality of life of patients with this, currently intractable, disease.

Pseudoxanthoma elasticum, ocular manifestations, complications and treatment

Pseudoxanthoma elasticum (PXE), also known as Groenblad syndrome, is an inherited disorder characterised by mineralisation and fragmentation of elastic fibres in a number of organs including the skin, eyes and arterial blood vessels. The clinical manifestations of PXE centre on three major organ systems: skin, cardiovascular system and the eyes. This review focuses on the ocular manifestations of pseudoxanthoma elasticum, namely, peau d'orange, angioid streaks and choroidal neovascularisation, the clinical course of patients, the diagnostic approaches and current therapeutic strategies, such as laser photocoagulation whether transpupillary thermotherapy or photodynamic therapy, macular translocation surgery and anti-vascular endothelial growth factor treatment.

Pseudoxanthoma elasticum: molecular genetics and putative pathomechanisms

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

Skin diseases in patients with beta-thalassemia major

BACKGROUND

Beta-thalassemia major affects multiple organs and is associated with considerable morbidity and mortality. The goal of this analysis was to document the frequency of skin diseases among patients with beta-thalassemia major.

METHODS

A sample of 78 patients with beta-thalassemia major was recruited and interviewed at the thalassemia clinic between April and June 2008. A dermatologist completed a skin examination of each patient and recorded any skin disease present. The patients' laboratory results and treatment modalities were recorded from the charts.

RESULTS

Sixty-five (83.3%) of the 78 patients examined had a diagnosed skin disease: pruritus (37.2%), xerosis (34.6%), scars (24.4%), ephelides (23.1%), skin irritation/erythema caused by deferoxamine pump (12.8%), idiopathic guttate hypomelanosis (6.4%), pityriasis alba (6.4%), tinea infections (5.1%), verruca vulgaris (5.1%), urticaria (3.8%), hyperhidrosis, contact dermatitis, and acne (2.6%), necrobiosis lipoidica (1.3%), melasma (1.3%), and others (14.1%). Pruritus and xerosis increased in frequency with age. The mean serum ferritin level was higher in patients with xerosis than in those without (5607.7 +/- 2997.5 ng/mL and 4285.2 +/- 4732.4 ng/mL, respectively) (P1/4 0.005). Moreover, xerosis was observed significantly more frequently inpatients using deferoxamine and deferiprone than in those using deferasirox (P1/4 0.047, P1/4 0.027, and P1/4 0.273, respectively).

CONCLUSION

Skin diseases, especially pruritus and xerosis, are observed highly frequently inpatients with beta-thalassemia major. Treatment is needed for these patients who already have other significant morbidities.

The mineralization phenotype in Abcc6 ( -/- ) mice is affected by Ggcx gene deficiency and genetic background--a model for pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disorder characterized by ectopic mineralization of connective tissues and shows considerable intra- and inter-familial phenotypic variability. PXE is caused by mutations in the ABCC6 gene, and targeted ablation of Abcc6 in mouse recapitulates PXE. In this study, we examined the hypothesis that the GGCX gene encoding gamma-glutamyl carboxylase may interfere with the mineralization process in Abcc6 ( -/- ) mice. Thus, Abcc6 ( -/- ) and Ggcx (+/-) mice were generated on 129S1;C57 and 129S1;129X1;C57 genetic backgrounds, respectively, and backcrossed with C57BL/6J for five generations. Thus, these strains differ by the 129X1 contribution to the background of the mice. We then generated Abcc6 ( -/- ) ;Ggcx (+/+) and Abcc6 ( -/- ) ;Ggcx (+/-) mice by crossing Abcc6 ( -/- ) and Ggcx (+/-) mice. The degree of mineralization of connective capsule of vibrissae, a biomarker of the mineralization process in PXE, was evaluated by computerized morphometric analysis and quantified colorimetrically by calcium and phosphate levels in tissues. The mineralization of the vibrissae in Abcc6 ( -/- ) mice takes place at approximately 5-6 weeks of age and is significantly enhanced at 3 months of age in comparison to wild-type mice (>10-fold, p < 0.001). However, the onset of mineralization in Abcc6 ( -/- ) ;Ggcx (+/+) mice was delayed until between 3 and 4 months of age, suggesting that the genetic background plays a role in modifying the mineralization process. The mineralization in the Abcc6 ( -/- ) ;Ggcx (+/- ) mice was accelerated in comparison with age-matched Abcc6 ( -/- ) ;Ggcx (+/+) mice, with approximately 3-fold difference at 3, 4, and 9 months of age (p < 0.01). The mineralization process was also accelerated in these mice by a special custom-designed diet with mineral modifications. These findings suggest a role for both the GGCX gene and the genetic background as well as dietary factors in modulating the phenotypic severity of PXE caused by loss-of-function mutations in ABCC6.

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.

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.

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.

Loss of vision associated with angioid streaks in beta-thalassemia intermedia

An acquired diffuse elastic tissue defect that resembles inherited pseudoxanthoma elasticum (PXE) has been noticed with a significant age-related frequency in hemoglobin disorders, especially beta-thalassemia and has been held responsible for a number of complications observed in these cases, some of which are quite severe. We report here two patients with beta-thalassemia intermedia, who presented with severe visual acuity impairment associated with angioid streaks, the typical ocular manifestation of PXE.

Pseudoxanthoma elasticum: biopsy of clinically normal skin in the investigation of patients with angioid streaks

BACKGROUND

Pseudoxanthoma elasticum (PXE) is a genetic disorder characterized by fragmentation and calcification of elastic fibres with resultant pathological changes in the dermis, Bruch's membrane and blood vessels. Defects in Bruch's membrane produce angioid streaks on the retina but this appearance is not pathognomonic of PXE. Biopsy of clinically normal skin or scar tissue in patients with angioid streaks may show the histological features of PXE.

OBJECTIVES

To test the hypothesis that biopsy of clinically normal skin is a useful investigation in patients with angioid streaks.

METHODS

This prospective study investigated 18 consecutive patients with angioid streaks. Each patient underwent a full dermatological examination and was investigated for diseases known to be associated with angioid streaks. Axillary skin biopsies were taken from 14 consenting patients.

RESULTS

Typical PXE was found in 11 patients. No other diseases associated with angioid streaks were identified. Five patients had angioid streaks in the absence of systemic disease. Two patients had nondiagnostic dermatological features which were not clarified by histology. Two of the 11 patients with PXE showed histological evidence of PXE from clinically normal axillary skin. However, in both cases flexural skin elsewhere showed the typical clinical and histological features of PXE.

CONCLUSIONS

This study demonstrates the association between angioid streaks and PXE. However, it does not support the hypothesis that biopsy of normal-looking skin is helpful in the investigation of adult patients with angioid streaks.

Nephrogenic fibrosing dermopathy and calciphylaxis with pseudoxanthoma elasticum-like changes

Nephrogenic fibrosing dermopathy (NFD) and calciphylaxis are rare conditions that are associated with chronic kidney disease. Histopathologic changes, including dystrophic dermal calcification, often in association with elastic fibers have been observed in NFD and calciphylaxis. A pattern of dermal elastic fiber calcification that mimics pseudoxanthoma elasticum (PXE) has been previously reported as an incidental finding in the setting of calciphylaxis. Despite a shared association with renal disease and abnormal calcium deposits, however, NFD and calciphylaxis are discrete pathologic processes with distinct clinical and histopathologic features. Criteria for each are reviewed through case presentation of a patient meeting the clinical and histopathologic criteria for both NFD and calciphylaxis with histologic features mimicking PXE.

Augmentation de l’hémoglobine A2 au cours du pseudoxanthome élastique

BACKGROUND

Pseudoxanthoma elasticum (PXE) is normally associated with mutations in the ABCC6 gene. A PXE phenotype without mutations in ABCC6 has been described in Greek and Italian patients presenting with beta thalassemia. We attempted to determine the incidence of beta thalassemia in a cohort of French patients with PXE.

PATIENTS AND METHODS

Fifty patients with PXE were included in the study. Laboratory examinations comprised hemoglobin electrophoresis, ABCC6 gene study and in some studies: mutation analysis, beta-globin gene.

RESULTS

No cases of beta thalassemia were diagnosed in this cohort of French patients with PXE. However, 20% of the latter exhibited a significant but isolated (i.e. without microcytic anemia) increase of hemoglobin A2 (HbA2). Statistical comparisons showed no difference in terms of geographical origin or severity of PXE between patients with high levels of HbA2 and those with normal levels of HbA2 other than the extent of cutaneous involvement. Study of the beta-globin gene displayed mutations only in the two patients with the highest recorded levels of HbA2. ABCC6 + beta-globin digenism was ruled out of the pathogenesis of PXE.

DISCUSSION

The PXE phenotype seen in some patients with beta thalassemia appears to be associated with epigenetic modification of ABCC6 transcription and depends specifically on the beta globin locus. Isolated increase in HbA2 is probably a laboratory marker for PXE. Here again, a functional epigenetic reaction between ABCC6 and the beta-globin locus was suspected. However, these reciprocal interactions are clearly unequal since the change in ABCC6 transcription occurring during the course of beta thalassaemia is responsible for a PXE phenotype while increased HbA2 during the course of PXE has no clinical consequences.

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.

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.

Pseudoxanthoma elasticum: a clinical, pathophysiological and genetic update including 11 novel ABCC6 mutations

Pseudoxanthoma elasticum (PXE) is an inherited systemic disease of connective tissue primarily affecting the skin, retina, and cardiovascular system. It is characterised pathologically by elastic fibre mineralisation and fragmentation (so called "elastorrhexia"), and clinically by high heterogeneity in age of onset and the extent and severity of organ system involvement. PXE was recently associated with mutations in the ABCC6 (ATP binding cassette subtype C number 6) gene. At least one ABCC6 mutation is found in about 80% of patients. These mutations are identifiable in most of the 31 ABCC6 exons and consist of missense, nonsense, frameshift mutations, or large deletions. No correlation between the nature or location of the mutations and phenotype severity has yet been established. Recent findings support exclusive recessive inheritance. The proposed prevalence of PXE is 1/25,000, but this is probably an underestimate. ABCC6 encodes the protein ABCC6 (also known as MRP6), a member of the large ATP dependent transmembrane transporter family that is expressed predominantly in the liver and kidneys, and only to a lesser extent in tissues affected by PXE. The physiological substrates of ABCC6 remain to be determined, but the current hypothesis is that PXE should be considered to be a metabolic disease with undetermined circulating molecules interacting with the synthesis, turnover, or maintenance of elastic fibres.