Severe skeletal toxicity from protracted etidronate therapy for generalized arterial calcification of infancy

Diagnosis, treatment, and management of rickets: a position statement from the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology

Rickets results from impaired mineralization of growing bone due to alterations in calcium and phosphate homeostasis. Clinical signs of rickets are related to the age of the patient, the duration of the disease, and the underlying disorder. The most common signs of rickets are swelling of the wrists, knees or ankles, bowing of the legs (knock-knees, outward bowing, or both) and inability to walk. However, clinical features alone cannot differentiate between the various forms of rickets. Rickets includes a heterogeneous group of acquired and inherited diseases. Nutritional rickets is due to a deficiency of vitamin D, dietary calcium or phosphate. Mutations in genes responsible for vitamin D metabolism or function, the production or breakdown of fibroblast growth factor 23, renal phosphate regulation, or bone mineralization can lead to the hereditary form of rickets. This position paper reviews the relevant literature and presents the expertise of the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology (SIEDP). The aim of this document is to provide practical guidance to specialists and healthcare professionals on the main criteria for diagnosis, treatment, and management of patients with rickets. The various forms of rickets are discussed, and detailed references for the discussion of each form are provided. Algorithms to guide the diagnostic approach and recommendations to manage patients with rare forms of hereditary rickets are proposed.

ENPP1 in Blood and Bone: Skeletal and Soft Tissue Diseases Induced by ENPP1 Deficiency

The enzyme ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) codes for a type 2 transmembrane glycoprotein that hydrolyzes extracellular ATP to generate pyrophosphate (PPi) and adenosine monophosphate, thereby contributing to downstream purinergic signaling pathways. The clinical phenotypes induced by ENPP1 deficiency are seemingly contradictory and include early-onset osteoporosis in middle-aged adults and life-threatening vascular calcifications in the large arteries of infants with generalized arterial calcification of infancy. The progressive overmineralization of soft tissue and concurrent undermineralization of skeleton also occur in the general medical population, where it is referred to as paradoxical mineralization to highlight the confusing pathophysiology. This review summarizes the clinical presentation and pathophysiology of paradoxical mineralization unveiled by ENPP1 deficiency and the bench-to-bedside development of a novel ENPP1 biologics designed to treat mineralization disorders in the rare disease and general medical population.

Clinical and Molecular Characterization of a Patient with Generalized Arterial Calcification of Infancy Caused by Rare ABCC6 Mutation

Generalized arterial calcification of infancy (GACI) is a rare autosomal-recessive disease characterized by extensive arterial calcification in infancy, with clinical manifestations such as arterial stenoses and heart failure. The ENPP1 inactivation mutation has been identified as a potential defect in most of the cases of GACI, while mutations in ABCC6 are demonstrated in patients who are genotyped as pseudoxanthoma elasticum and only limited cases of GACI are reported. Whole-exome sequencing was applied for the detection of pathogenic variants. Copy-number variants of pathogenic genes were also evaluated through a bioinformatic process and were further validated by real-time quantitative PCR. In this report, we described the clinical information and treatment of a patient with extensive arterial calcification. We have identified the underlying cause as biallelic mutations in ABCC6 (NM_00117: exon30, c.4223_4227dupAGCTC p.(Leu1410Serfs*56)) and a unique exonic deletion that spans from the first to the fourth exons of ABCC6 (chr16:16313388-16330869)). This discovery was made by utilizing a combined genetic testing approach. With the review of previously reported GACI patients with ABCC6 mutation, our work contributed to enriching the mutation spectrum of GACI and providing further information on this rare form of inherited disorder.

Musculoskeletal pain and muscular weakness as the main symptoms of adult hypophosphatasia in a Spanish cohort: clinical characterization and identification of a new ALPL gene variant

INTRODUCTION

Hypophosphatasia (HPP) is a rare inherited disorder, caused by mutations in the alkaline phosphatase (ALPL) gene, which encodes for the tissue non-specific alkaline phosphatase (TNSALP) isoform of alkaline phosphatase (ALP). Adult HPP is one of the mild forms that presents with unspecific signs such as osteopenia, osteomalacia and muscle involvement. Our purpose was to identify and characterize possibly misdiagnosed adult HPP patients at a clinical and biochemical level.

MATERIAL AND METHODS

At the laboratory of Miguel Servet University Hospital we retrospectively reviewed serum ALP levels in adults over a 48-month period. The clinical records of individuals with consistently low ALP levels were reviewed to exclude secondary causes. Those with persistent hypophosphatasemia were screened for symptoms of HPP. The study participants were evaluated at biochemical and genetic levels.

RESULTS

We identified 705 ALP determinations (out of 384,000 processed) in 589 patients below the reference range (30 U/l). Only 21 patients with clinical signs and symptoms of HPP were selected for genetic testing. Finally, only 12 patients participated in the study, 83.3% of whom (10/12) harbored a pathogenic or likely pathogenic variant in a heterozygous state. The major symptoms of our cohort were the presence of musculoskeletal pain (100% of patients) and muscular weakness (83.3% patients).

CONCLUSION

Mild HPP patients presenting with diffuse symptoms such as musculoskeletal pain may be undiagnosed or misdiagnosed as osteoporosis patients by routine diagnosis. It is important to identify these individuals, to avoid inappropriate treatment with antiresorptive drugs.

Severe early-onset manifestations of generalized arterial calcification of infancy (mimicking severe coarctation of the aorta) with ABCC6 gene variant - Case report and literature review

Introduction

Generalized arterial calcification of infancy (GACI) is a rare cause of infantile heart failure and systemic hypertension with a poor prognosis, characterized by extensive calcification and proliferation of the intimal layer of large and medium sized arteries.

Case report

We present the first case report of successful surgical treatment of severe aortic arch obstruction by calcified plaques mimicking severe coarctation of the aorta and the outcome (of bisphosphonate therapy) in a newborn with GACI. Furthermore, we report the identification of a variant in ATP Binding Cassette Subfamily C, Member 6 (ABCC6) gene, possibly associated with severe early-onset manifestations of GACI.

Conclusion

This case report highlights the importance of considering GACI in an infant with heart failure, systemic hypertension, and evidence of increased echogenicity of the arterial vessels. We noted the favorable outcome in improving the aortic calcification in our patient after surgical treatment and bisphosphonates therapy. Early diagnosis and treatment improve the long-term prognosis. A better understanding of this rare genetic disease could lead to new therapeutic strategies.

Longitudinal assessment of vascular calcification in generalized arterial calcification of infancy

BACKGROUND

Generalized arterial calcification of infancy (GACI), also known as idiopathic infantile arterial calcification, is a very uncommon genetic disorder characterized by calcifications and stenoses of large- and medium-size arteries that can lead to end-organ damage.

OBJECTIVE

To describe changes in imaging findings in 10 children with GACI at a single institution from 2010 to 2021.

MATERIALS AND METHODS

In this retrospective study we reviewed initial and follow-up body imaging in children with genetic confirmation of GACI at our hospital. All initial images were analyzed for the presence and distribution of arterial calcifications, stenoses and wall thickening/irregularity within the chest, abdomen and pelvis. We compared available follow-up studies to the initial imaging findings. We extracted clinical information including prenatal and postnatal treatment from the children's medical records.

RESULTS

We evaluated 10 children (five boys) with a diagnosis of GACI. Median age at first body imaging was 8 days (range: 1 day to 5 years). Six children were identified prenatally and four postnatally. Postnatal presentation included cardiac failure, seizures and hypertension. Images in newborns (n = 8) most commonly showed diffuse arterial calcifications (6/8; 75%), while stenoses were less common (2/8; 25%) during this period. Two children were diagnosed after the neonatal period - one in infancy and one during childhood. In total, half the children (5/10; 50%) had arterial stenoses - three cases visualized at first imaging and two identified on follow-up images during infancy. Stenoses had completely resolved in one child (1/5; 20%) at last follow-up. Eight children received prenatal or postnatal treatment or both. All children who received both prenatal and postnatal treatment (n = 4) had completely resolved calcifications at last follow-up.

CONCLUSION

Children with GACI might have characteristic vascular calcifications at birth that raise the suspicion of this disease. Arterial calcifications decrease or disappear spontaneously or after treatment, but arterial stenoses usually persist. Calcifications and arterial stenoses can be easily identified and followed with non-contrast CT and CT angiography.

Inorganic Pyrophosphate Deficiency Syndromes and Potential Treatments for Pathologic Tissue Calcification

Pathologic soft tissue calcification can occur in both genetic and acquired clinical conditions, causing significant morbidity and mortality. Although the pathomechanisms of pathologic calcification are poorly understood, major progress has been made in recent years in defining the underlying genetic defects in Mendelian disorders of ectopic calcification. This review presents an overview of the pathophysiology of five monogenic disorders of pathologic calcification: pseudoxanthoma elasticum, generalized arterial calcification of infancy, arterial calcification due to deficiency of CD73, ankylosis, and progeria. These hereditary disorders, caused by mutations in genes encoding ATP binding cassette subfamily C member 6, ectonucleotide pyrophosphatase/phosphodiesterase 1, CD73, progressive ankylosis protein, and lamin A/C proteins, respectively, are inorganic pyrophosphate (PPi) deficiency syndromes with reduced circulating levels of PPi, the principal physiologic inhibitor of calcium hydroxyapatite deposition in soft connective tissues. In addition to genetic diseases, PPi deficiency has been encountered in acquired clinical conditions accompanied by pathologic calcification. Because specific and effective treatments are lacking for pathologic calcification, the unifying finding of PPi deficiency suggests that PPi-targeted therapies may be beneficial to counteract pathologic soft tissue calcification in both genetic and acquired diseases.

Case Report: A Novel Genetic Mutation Causes Idiopathic Infantile Arterial Calcification in Preterm Infants

Idiopathic infantile arterial calcification (IIAC), also known as generalized arterial calcification of infancy (GACI), is a heritable ectopic mineralization disorder that results in diffuse arterial calcifications and or stenosis, which are attributed to mutations in the ENPP1 gene. In this case study, we report the development of IIAC in a 2-month-old male preterm infant. The patient presented with severe hypertension and seizures, which revealed diffused calcifications and c.130C > T and c.1112A > T mutations in the ENPP1 gene. With biphosphonate, antihypertensive, and control epilepsy therapy, his blood pressure was maintained at 110–120/50–60 mmHg. Intellectual motor development retardation was anticipated in this patient. To the best of our knowledge, this is the first case in which a novel c.130C > T mutation in the ENPP1 gene has been identified, and the administration of bisphosphonates to patients with IIAC has been assessed.

Endocrinology of bone mineralization: an update

Throughout the world, millions of people suffer from fragilizing osteopathies such as osteomalacia and osteoporosis.Osteomalacia is a rare disorder, corresponding to mineralization abnormalities in adult bone, as opposed to rickets in children. Renal phosphate loss and hypophosphatasia are the main causes of vitamin-resistant osteomalacia. Diagnosis is based on clinical history, phosphocalcic metabolism assessment and, if necessary, molecular characterization, and must be rapid in order to initiate the most appropriate treatment and consider new treatments such as burosumab if necessary.Osteoporosis is characterized by reduced bone mass and strength, which increases the risk of fragility fracture. Fracture-related burden is expected to increase over the coming decades linked to the aging of population and a treatment gap. In order to reduce this treatment gap, it is important to develop two strategies: improvement of screening and of treatment. Systematic screening using the FRAX® fracture risk assessment tool could be useful to increase anti-osteoporosis medical treatment and reduce fracture rates. The question of treatment sequencing in osteoporosis is another challenge, notably after denosumab cessation, complicated by a decrease in bone mineral density and increased risk of fracture. New treatments are also available, including romosozumab, a humanized monoclonal antibody which promotes bone formation and inhibits bone resorption by inhibiting sclerostin. Romosozumab is approved in several countries, including France, for treating severe osteoporosis in postmenopausal women at high risk of fracture and free of cardiovascular comorbidity.Endocrinologists need to be aware of these fragilizing osteopathies in order to improve both diagnosis and treatment.

Autosomal recessive hypophosphatemic rickets type 2 (ARHR2) due to ENPP1-deficiency

Awareness for hypophosphatemic rickets has increased in the last years, based on the availability of specific medical treatments. Autosomal recessive hypophosphatemic rickets type 2 (ARHR2) is a rare form of hypophosphatemic rickets, which is known to develop in survivors of generalized arterial calcification of infancy (GACI). Both disorders are based on a deficiency of ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) and present with a high clinical variability and a lack of a phenotype-genotype association. ARHR2 is characterized by phosphate wasting due to elevated fibroblast growth factor 23 (FGF23) levels and might represent a response of the organism to minimize ectopic calcification in individuals with ENPP1-deficiency. This report reviews the recent clinical and preclinical data on this ultra-rare disease in childhood.

Genetic heterogeneity of heritable ectopic mineralization disorders in a large international cohort

PURPOSE

Heritable ectopic mineralization disorders comprise a group of conditions with a broad range of clinical manifestations in nonskeletal connective tissues. We report the genetic findings from a large international cohort of 478 patients afflicted with ectopic mineralization.

METHODS

Sequence variations were identified using a next-generation sequencing panel consisting of 29 genes reported in association with ectopic mineralization. The pathogenicity of select splicing and missense variants was analyzed in experimental systems in vitro and in vivo.

RESULTS

A total of 872 variants of unknown significance as well as likely pathogenic and pathogenic variants were disclosed in 25 genes. A total of 159 distinct variants were identified in 425 patients in ABCC6, the gene responsible for pseudoxanthoma elasticum, a heritable multisystem ectopic mineralization disorder. The interpretation of variant pathogenicity relying on bioinformatic predictions did not provide a consensus. Our in vitro and in vivo functional assessment of 14 ABCC6 variants highlighted this dilemma and provided unambiguous interpretations to their pathogenicity.

CONCLUSION

The results expand the ABCC6 variant repertoire, shed new light on the genetic heterogeneity of heritable ectopic mineralization disorders, and provide evidence that functional characterization in appropriate experimental systems is necessary to determine the pathogenicity of genetic variants.

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.

Coalescing expansile skeletal disease: Delineation of an extraordinary osteopathy involving the IFITM5 mutation of osteogenesis imperfecta type V

In 2003, we briefly reported the remarkable osteopathy of a 12-year-old boy who at age two months began fracturing his limbs with subsequent hyperplastic callus formation and expansion and fusion of appendicular bones. By age ten years he had coalesced his lumbosacral spine, pelvis, femurs, and leg and foot bones as a single structure. Computed tomography of expanded bone revealed a thin cortical shell, diminished irregular trabeculae, and cystic areas. Histopathology featured foci of woven bone, densely packed osteocytes, cartilage, fibrovascular tissue, and massive fat deposition in the marrow space lacking hematogenous precursor cells. Bone turnover markers indicated accelerated remodeling and the few radiographically assessable appendicular bones improved during brief adherence to alendronate therapy. Following puberty, serum multiplex biomarker profiling confirmed accelerated bone turnover. At age 23 years, macrospecimens from leg amputation revealed ossification along capsular tissue together with hyaline cartilage degeneration. Concurrently, the life-long course of this same disorder was delineated in an unrelated woman until her death at age 51 years. Both patients demonstrated the radiographic hallmarks and harbored the heterozygous point mutation (c.-14C>T) in the 5'-UTR of IFITM5 associated with osteogenesis imperfecta type V (OI-V). Herein, we detail the clinical, radiological, histopathological, biochemical, and molecular findings and discuss the etiology and pathogenesis of this extraordinary osteopathy that we call coalescing expansile skeletal disease.

An update on vascular calcification and potential therapeutics

Pathological calcification is a major cause of cardiovascular morbidities primarily in population with chronic kidney disease (CKD), end stage renal diseases (ERSD) and metabolic disorders. Investigators have accepted the fact that vascular calcification is not a passive process but a highly complex, cell mediated, active process in patients with cardiovascular disease (CVD) resulting from, metabolic insults of bone fragility, diabetes, hypertension, dyslipidemia and atherosclerosis. Over the years, studies have revealed various mechanisms of vascular calcification like induction of bone formation, apoptosis, alteration in Ca-P balance and loss of inhibition. Novel clinical studies targeting cellular mechanisms of calcification provide promising and potential avenues for drug development. The interventions include phosphate binders, sodium thiosulphate, vitamin K, calcimimetics, vitamin D, bisphosphonates, Myoinositol hexaphosphate (IP6), Denosumab and TNAP inhibitors. Concurrently investigators are also working towards reversing or curing pathological calcification. This review focuses on the relationship of vascular calcification to clinical diseases, regulators and factors causing calcification including genetics which have been identified. At present, there is lack of any significant preventive measures for calcifications and hence this review explores further possibilities for drug development and treatment modalities.

Therapeutics Development for Pseudoxanthoma Elasticum and Related Ectopic Mineralization Disorders: Update 2020

Pseudoxanthoma elasticum (PXE), the prototype of heritable ectopic mineralization disorders, manifests with deposition of calcium hydroxyapatite crystals in the skin, eyes and arterial blood vessels. This autosomal recessive disorder, due to mutations in ABCC6, is usually diagnosed around the second decade of life. In the spectrum of heritable ectopic mineralization disorders are also generalized arterial calcification of infancy (GACI), with extremely severe arterial calcification diagnosed by prenatal ultrasound or perinatally, and arterial calcification due to CD73 deficiency (ACDC) manifesting with arterial and juxta-articular mineralization in the elderly; the latter disorders are caused by mutations in ENPP1 and NT5E, respectively. The unifying pathomechanistic feature in these three conditions is reduced plasma levels of inorganic pyrophosphate (PPi), a powerful endogenous inhibitor of ectopic mineralization. Several on-going attempts to develop treatments for these conditions, either with the goal to normalize PPi plasma levels or by means of preventing calcium hydroxyapatite deposition independent of PPi, are in advanced preclinical levels or in early clinical trials. This overview summarizes the prospects of treatment development for ectopic mineralization disorders, with PXE, GACI and ACDC as the target diseases, from the 2020 vantage point.

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.

Kaposiform lymphangiomatosis treated with multimodal therapy improves coagulopathy and reduces blood angiopoietin-2 levels

Kaposiform lymphangiomatosis (KLA) is a rare, life-threatening congenital lymphatic malformation. Diagnosis is often delayed due to complex indistinct symptoms. Blood angiopoietin-2 (ANG2) levels are elevated in KLA and may be useful as a biomarker to monitor disease status. We report a 7-year-old male child with easy bruising, inguinal swelling, and consumptive coagulopathy, diagnosed with KLA. A multimodal treatment regimen of prednisone, sirolimus, vincristine, and adjunctive zoledronate was used. Plasma ANG2 levels were highly elevated at diagnosis but decreased during treatment. The patient showed significant clinical improvement over a 38-month period and normalization of ANG2 levels correlated with resolution of the coagulopathy.

Hypophosphatasia in adolescents and adults: overview of diagnosis and treatment

This article provides an overview of the current knowledge on hypophosphatasia-a rare genetic disease of very variable presentation and severity-with a special focus on adolescents and adults. It summarizes the available information on the many known mutations of tissue-nonspecific alkaline phosphatase (TNSALP), the epidemiology and clinical presentation of the disease in adolescents and adults, and the essential diagnostic clues. The last section reviews the therapeutic approaches, including recent reports on enzyme replacement therapy (EnzRT).

Generalized Arterial Calcification of Infancy: New Insights, Controversies, and Approach to Management

PURPOSE OF REVIEW

This review summarizes current understanding of generalized arterial calcification of infancy (GACI), emphasizing pathophysiology, clinical presentation, and approaches and controversies in management.

RECENT FINDINGS

Identification of causative ENPP1 mutations revealed that GACI arises from deficiencies in inorganic pyrophosphate (leading to calcifications) and adenosine monophosphate (leading to intimal proliferation). Identification of genotypic and phenotypic overlap with pseudoxanthoma elasticum and autosomal recessive hypophosphatemic rickets further advanced understanding of GACI as a complex, multisystemic disease. Clinical data is limited to small, retrospective samples; it is therefore unknown whether commonly used medications, such as bisphosphonates and hypophosphatemia treatment, are therapeutic or potentially harmful. ENPP1-Fc replacement represents a promising approach warranting further study. Knowledge gaps in natural history place clinicians at high risk of assigning causality to interventions that are correlated with changes in clinical status. There is thus a critical need for improved natural history studies to develop and test targeted therapies.

Skeletal abnormalities secondary to antenatal etidronate treatment for suspected generalised arterial calcification of infancy

Background

Generalised arterial calcification of infancy (GACI) is a rare disorder characterised by the deposition of hydroxyapatite crystals within the vessel walls. It is associated with a high mortality rate. Bisphosphonates have been used with some success in the treatment of GACI. However, there is a paucity of data on the antenatal use of bisphosphonates for GACI. In this paper, we report development of the skeletal changes suggestive of hypophosphatasia (HPP) in an infant with GACI, whose mother was treated with etidronate during pregnancy.

Case report

A Caucasian infant boy had a suspected antenatal diagnosis of GACI based on the findings suggestive of calcification of the annulus of the tricuspid valve and wall of the right ventricular (RV) outflow tract and main pulmonary artery on foetal echocardiography and the genetic analysis which showed a pathogenic heterozygous mutation in ABCC6. Based on these findings, mother was started on etidronate treatment from 26 weeks of gestation. A healthy male baby was delivered at 38 weeks of gestation. Initial postnatal echocardiogram on day 1 of life was normal with good biventricular function; subtle changes suggestive of microcalcifications were detected on the CT angiography. Serum calcium, phosphate, alkaline phosphatase and renal profile were normal. Further, the serum inorganic pyrophosphate (PPi) level was significantly low. Skeletal changes suggestive of HPP were seen on the radiographs. The baby developed cardiac dysfunction on day 4 of life with evidence of ischaemic changes on electrocardiogram (ECG).Treatment with etidronate was started in view of probable evolving coronary calcifications. Despite treatment with cardiac supportive measures and bisphosphonate, he succumbed to death in the third week of life.

Discussion

We believe, this is the first report of skeletal changes suggestive of HPP, arising secondary to antenatal etidronate (first generation bisphosphonate) used for the treatment of suspected GACI due to a heterozygous ABCC6 mutation.

Hyperphosphatemia with low FGF7 and normal FGF23 and sFRP4 levels in the circulation characterizes pediatric hypophosphatasia

Hypophosphatasia (HPP) is the inborn-error-of-metabolism caused by loss-of-function mutation(s) of the ALPL gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). TNSALP in healthy individuals is on cell surfaces richly in bone, liver, and kidney. Thus, TNSALP natural substrates accumulate extracellularly in HPP, including inorganic pyrophosphate (PPi), a potent inhibitor of hydroxyapatite crystal formation and growth. Superabundance of extracellular PPi (ePPi) in HPP impairs mineralization of bones and teeth, often leading to rickets during childhood and osteomalacia in adult life and to tooth loss at any age. HPP's remarkably broad-ranging severity is largely explained by nearly four hundred typically missense mutations throughout the ALPL gene that are transmitted as an autosomal dominant or autosomal recessive trait. In the clinical laboratory, the biochemical hallmark of HPP is low serum ALP activity (hypophosphatasemia). However, our experience indicates that hyperphosphatemia from increased renal reclamation of filtered inorganic phosphate (Pi) is also common. Herein, from our prospective single-center study, we document throughout the clinical spectrum of non-lethal pediatric HPP that hyperphosphatemia reflects increased renal tubular threshold maximum for phosphorus adjusted for the glomerular filtration rate (TmP/GFR). To explore its pathogenesis, we studied mineral metabolism and quantitated circulating levels of three phosphatonins [fibroblast growth factor 23 (FGF23), secreted frizzled-related protein 4 (sFRP4), and fibroblast growth factor 7 (FGF7)] in 41 pediatric patients with HPP, 73 with X-linked hypophosphatemia (XLH), and 15 healthy pediatric control (CTR) subjects. The HPP and XLH cohorts had normal serum total and ionized calcium and parathyroid hormone levels (Ps > 0.10) and uncompromised glomerular filtration. In XLH, serum FGF23 was characteristically elevated (P < 0.0001) and despite hypophosphatemia sFRP4 was normal (P > 0.4) while FGF7 was low (P < 0.0001). In HPP, despite hyperphosphatemia serum FGF23 and sFRP4 were normal (Ps > 0.8) while FGF7 was low (P < 0.0001). Subsequently, in rats, we confirmed that FGF7 is phosphaturic. Thus, hyperphosphatemia in non-lethal pediatric HPP is associated with phosphatonin insufficiency together with, as we discuss, ePPi excess and diminished renal TNSALP activity.

The biology of vascular calcification

Vascular calcification (VC), characterized by different mineral deposits (i.e., carbonate apatite, whitlockite and hydroxyapatite) accumulating in blood vessels and valves, represents a relevant pathological process for the aging population and a life-threatening complication in acquired and in genetic diseases. Similarly to bone remodeling, VC is an actively regulated process in which many cells and molecules play a pivotal role. This review aims at: (i) describing the role of resident and circulating cells, of the extracellular environment and of positive and negative factors in driving the mineralization process; (ii) detailing the types of VC (i.e., intimal, medial and cardiac valve calcification); (iii) analyzing rare genetic diseases underlining the importance of altered pyrophosphate-dependent regulatory mechanisms; (iv) providing therapeutic options and perspectives.

Update on the management of hypophosphatasia

Hypophosphatasia is a rare inherited disease caused by a loss of function mutations in the gene that codes for the tissue-nonspecific alkaline phosphatase enzyme. It is autosomally inherited and at least 388 different genetic defects have been identified. The clinical presentation is variable from a severe perinatal form, that is fatal if untreated, to adult-onset disease. This review covers the pathophysiology, diagnosis and current management option including the recently licensed enzyme replacement therapy asfotase alfa.

No vascular calcification on cardiac computed tomography spanning asfotase alfa treatment for an elderly woman with hypophosphatasia

Hypophosphatasia (HPP) is the inborn-error-of-metabolism characterized enzymatically by insufficient activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP) and caused by either mono- or bi-allelic loss-of-function mutation(s) of the gene ALPL that encodes this cell surface phosphomonoester phosphohydrolase. In HPP, the natural substrates of TNSALP accumulate extracellularly and include inorganic pyrophosphate (PPi), a potent inhibitor of biomineralization. This PPi excess leads to rickets or osteomalacia in all but the most mild "odonto" form of the disease. Adults with HPP understandably often also manifest calcium PPi dihydrate deposition, whereas enthesopathy and calcific periarthritis from hydroxyapatite (HA) crystal deposition can seem paradoxical in face of the defective skeletal mineralization. In 2015, asfotase alfa (AA), a HA-targeted TNSALP, was approved multinationally as an enzyme replacement therapy for HPP. AA hydrolyzes extracellular PPi (ePPi) and in HPP enables HA crystals to grow and mineralize skeletal matrix. In direct contrast to HPP, deficiency of ePPi characterizes the inborn-errors-of-metabolism generalized arterial calcification of infancy (GACI) and pseudoxanthoma elasticum (PXE). In GACI and PXE, deficiency of ePPi leads to ectopic mineralization including vascular calcification (VC). Therefore, in HPP, ectopic mineralization including VC could hypothetically result from, or be exacerbated by, the persistently high circulating TNSALP activity that occurs during AA treatment. Herein, using a routine computed tomography (CT) method to quantitate coronary artery calcium, we found no ectopic mineralization in the heart of an elderly woman with HPP before or after 8 months of AA treatment. Subsequently, investigational high-resolution peripheral quantitative CT and dual-energy X-ray absorptiometry showed absence of peripheral artery and aortic calcium after further AA treatment. Investigation of additional adults with HPP could reveal if the superabundance of ePPi protects against VC, and whether long-term AA therapy causes or exacerbates any ectopic mineralization.

ENPP1 in the Regulation of Mineralization and Beyond

ENPP1 is well known for its role in regulating skeletal and soft tissue mineralization. It primarily exerts its function through the generation of pyrophosphate, a key inhibitor of hydroxyapatite formation. Several previous studies have suggested that ENPP1 also contributes to a range of human diseases including diabetes, cancer, cardiovascular disease, and osteoarthritis. In this review, we summarize the pathological roles of ENPP1 in mineralization and these soft tissue disorders. We also discuss the underlying mechanisms through which ENPP1 exerts its pathological effects. A fuller understanding of the pathways through which ENPP1 acts may help to develop novel therapeutic strategies for these commonly diagnosed morbidities.

Asfotase alfa for infants and young children with hypophosphatasia: 7 year outcomes of a single-arm, open-label, phase 2 extension trial

BACKGROUND

Our previous phase 2, open-label study of 11 infants and young children with life-threatening perinatal or infantile hypophosphatasia showed 1 year safety and efficacy of asfotase alfa, an enzyme replacement therapy. We aimed to report the long-term outcomes over approximately 7 years of treatment.

METHODS

We did a prespecified, end of study, 7 year follow-up of our single-arm, open-label, phase 2 trial in which children aged 3 years or younger with life-threatening perinatal or infantile hypophosphatasia were recruited from ten hospitals (six in the USA, two in the UK, one in Canada, and one in the United Arab Emirates). Patients received asfotase alfa (1 mg/kg three times per week subcutaneously, adjusted to 3 mg/kg three times per week if required) for up to 7 years (primary treatment period plus extension phase) or until the product became commercially available; dosage adjustments were made at each visit according to changes in the patient's weight. The primary objectives of this extension study were to assess the long-term tolerability of asfotase alfa, defined as the number of patients with one or more treatment-emergent adverse events, and skeletal manifestations associated with hypophosphatasia, evaluated using the Radiographic Global Impression of Change (RGI-C) scale (-3 indicating severe worsening, and +3 complete or near-complete healing). Respiratory support, growth, and cognitive and motor functions were also evaluated. All efficacy and safety analyses were done in all patients who received any asfotase alfa (full-analysis population). This study and extension phase are registered with ClinicalTrials.gov, number NCT01205152, and EudraCT, number 2009-009369-32.

FINDINGS

11 participants were recruited between Oct 6, 2008, and Dec 4, 2009. Ten patients completed a 6 month treatment period and entered the extension phase; nine received asfotase alfa for at least 6 years and completed the study, with four being treated for more than 7 years. Skeletal healing was sustained over 7 years of treatment; all evaluable patients had RGI-C scores of at least +2 at year 6 (n=9; median score +2·0 [range 2·0-3·0]) and year 7 (n=7; median score +2·3 [2·0-3·0]). No patient who completed the study required respiratory support after year 4. Weight Z scores improved to within normal range from year 3 to study end; length or height Z scores improved but remained below normal. Age-equivalent scores on gross motor, fine motor, and cognitive subscales of the Bayley Scales of Infant and Toddler Development also improved. All 11 patients had at least one treatment-emergent adverse event. The most common adverse events were pyrexia (eight [73%] of 11 patients), upper respiratory tract infection (eight [73%]), craniosynostosis (seven [64%]), and pneumonia (seven [64%]). Serious adverse events related to asfotase alfa occurred in three (27%) patients (severe chronic hepatitis; moderate immediate post-injection reaction; and severe craniosynostosis with severe conductive deafness).

INTERPRETATION

Patients with perinatal or infantile hypophosphatasia treated with asfotase alfa for up to 7 years showed early, sustained improvements in skeletal mineralisation. Respiratory function, growth, and cognitive and motor function also improved, and asfotase alfa was generally well tolerated.

FUNDING

Alexion Pharmaceuticals, Inc.

[Persistent hypertension for two months in a preterm infant]

A boy aged 2 months (born at 36 weeks of gestation) was admitted due to cough and dyspnea. After admission, he was found to have persistent hypertension, proteinuria, and persistent convulsion, and imaging examination showed extensive calcification of the aorta and major branches and stenosis of local lumens of the abdominal aorta and the right renal artery with increased blood flow velocity. The boy was admitted during the neonatal period due to wet lung and pulmonary arterial hypertension and was found to have hypertension and proteinuria. High-throughput whole-exome sequencing was performed and found two compound heterozygous mutations in the ENPP1 gene from his parents, c.130C>T (p.Q44X) and c.1112A>T (p.Y371F). c.130C>T was a nonsense mutation, which could cause partial deletion of protein from 44 amino acids, and was defined as a primary pathogenic mutation. c.1112A>T was a missense mutation which had been reported as a pathogenic mutation associated with idiopathic infantile arterial calcification (IIAC). Therefore, he was diagnosed with IIAC. He was given phosphonate drugs, antihypertensive drugs, anticonvulsion treatment, and respiratory support. Blood pressure was maintained at the upper limit of normal value. There was no deterioration of arterial calcification. It is concluded that IIAC should be considered for infants with persistent hypertension and extensive vascular calcification, and imaging and genetic examinations should be performed as early as possible to make a confirmed diagnosis.

ENPP1-Fc prevents neointima formation in generalized arterial calcification of infancy through the generation of AMP

Generalized arterial calcification of infancy (GACI) is associated with widespread arterial calcification and stenoses and is caused by mutations in ENPP1. ENPP1 encodes for ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), which cleaves ATP to generate inorganic pyrophosphate (PP_i) and adenosine monophosphate (AMP) extracellularly. The current study was designed to define the prevalence of arterial stenoses in GACI individuals and to identify the mechanism through which ENPP1 deficiency causes intimal proliferation. Furthermore, we aimed to effectively prevent and treat neointima formation in an animal model of GACI through the systemic administration of recombinant human (rh)ENPP1-Fc protein. Based on a literature review, we report that arterial stenoses are present in at least 72.4% of GACI cases. We evaluated the effect of rhENPP1-Fc on ENPP1-silenced human vascular smooth muscle cells (VSMCs) and on induced intimal proliferation in Enpp1-deficient ttw/ttw mice treated with carotid ligation. We demonstrate that silencing ENPP1 in VSMCs resulted in a tenfold increase in proliferation relative to that of cells transfected with negative control siRNA. The addition of rhENPP1-Fc, AMP or adenosine restored the silenced ENPP1-associated proliferation. In contrast, neither PP_i nor etidronate, a current off-label treatment for GACI, had an effect on VSMC proliferation. Furthermore, subcutaneous rhENPP1-Fc protein replacement was effective in preventing and treating intimal hyperplasia induced by carotid ligation in an animal model of GACI. We conclude that ENPP1 inhibits neointima formation by generating  AMP. RhENPP1-Fc may serve as an approach for the effective prevention and treatment of arterial stenoses in GACI.

A protein replacement therapy may prove useful in tackling calcification and narrowing of the arteries in babies with a severe genetic disorder. Generalized Arterial Calcification of Infancy (GACI) is a rare condition in which infants’ arteries become calcified and their blood vessels internally scarred. It often leads to congestive heart failure. The ENPP1 gene encodes a protein that is crucial to preventing excess calcium build-up in the body. Mutations in the ENPP1 gene lead to GACI, but no therapies for the condition exist. Now, Frank Rutsch at Muenster University Children’s Hospital in Germany and co-workers have shown that administering a protein replacement can inhibit blood vessel scarring and arterial clogging in GACI mice models and in human stem cell cultures. The protein replacement boosts production of a key metabolic molecule called adenosine monophosphate.

ENPP1 enzyme replacement therapy improves blood pressure and cardiovascular function in a mouse model of generalized arterial calcification of infancy

Generalized arterial calcification of infancy (GACI) is a rare, life-threatening disorder caused by loss-of-function mutations in the gene encoding ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1), which normally hydrolyzes extracellular ATP into AMP and pyrophosphate (PP_i). The disease is characterized by extensive arterial calcification and stenosis of large- and medium-sized vessels, leading to vascular-related complications of hypertension and heart failure. There is currently no effective treatment available, but bisphosphonates – nonhydrolyzable PP_i analogs – are being used off-label to reduce arterial calcification, although this has no reported impact on the hypertension and cardiac dysfunction features of GACI. In this study, the efficacy of a recombinant human ENPP1 protein therapeutic (rhENPP1) was tested in Enpp1^asj-2J homozygous mice (Asj-2J or Asj-2J hom), a model previously described to show extensive mineralization in the arterial vasculature, similar to GACI patients. In a disease prevention study, Asj-2J mice treated with rhENPP1 for 3 weeks showed >95% reduction in aorta calcification. Terminal hemodynamics and echocardiography imaging of Asj-2J mice also revealed that a 6-week rhENPP1 treatment normalized elevated arterial and left ventricular pressure, which translated into significant improvements in myocardial compliance, contractility, heart workload and global cardiovascular efficiency. This study suggests that ENPP1 enzyme replacement therapy could be a more effective GACI therapeutic than bisphosphonates, treating not just the vascular calcification, but also the hypertension that eventually leads to cardiac failure in GACI patients.

Summary: ENPP1 enzyme replacement therapy can have important implications for generalized arterial calcification of infancy by treating both vascular calcification and hypertension, which are the leading causes of cardiac failure and mortality in patients.

Hypophosphatasia

We review here clinical, pathophysiological, diagnostic, genetic and molecular aspects of Hypophosphatasia (HPP), a rare inherited metabolic disorder. The clinical presentation is a continuum ranging from a prenatal lethal form with no skeletal mineralization to a mild form with late adult onset presenting with nonpathognomonic symptoms. The prevalence of severe forms is low, whereas less severe forms are more frequently observed. The disease is caused by loss-of-function mutations in the ALPL gene encoding the Tissue Nonspecific Alkaline Phosphatase (TNSALP), a central regulator of mineralization. Severe forms are recessively inherited, whereas moderate forms are either recessively or dominantly inherited, and the more severe the disease is, the more often it is subject to recessive inheritance. The diagnosis is based on a constantly low alkaline phosphatase (AP) activity in serum and genetic testing that identifies ALPL mutations. More than 340 mutations have been identified and are responsible for the extraordinary clinical heterogeneity. A clear but imperfect genotype-phenotype correlation has been observed, suggesting that other genetic or environmental factors modulate the phenotype. Enzyme replacement therapy is now available for HPP, and other approaches, such as gene therapy, are currently being investigated.

Elevated dietary magnesium during pregnancy and postnatal life prevents ectopic mineralization in Enpp1asj mice, a model for generalized arterial calcification of infancy

Generalized arterial calcification of infancy (GACI) is an autosomal recessive disorder caused by mutations in the ENPP1 gene. It is characterized by mineralization of the arterial blood vessels, often diagnosed prenatally, and associated with death in early childhood. There is no effective treatment for this devastating disorder. We previously characterized the Enpp1^asjmutant mouse as a model of GACI, and we have now explored the effect of elevated dietary magnesium (five-fold) in pregnant mothers and continuing for the first 14 weeks of postnatal life. The mothers were kept on either control diet or experimental diet supplemented with magnesium. Upon weaning at 4 weeks of age the pups were placed either on control diet or high magnesium diet. The degree of mineralization was assessed at 14 weeks of age by histopathology and a chemical calcium assay in muzzle skin, kidney and aorta. Mice placed on high magnesium diet showed little, if any, evidence of mineralization when their corresponding mothers were also placed on diet enriched with magnesium during pregnancy and nursing. The reduced ectopic mineralization in these mice was accompanied by increased calcium and magnesium content in the urine, suggesting that magnesium competes calcium-phosphate binding thereby preventing the mineral deposition. These results have implications for dietary management of pregnancies in which the fetus is suspected of having GACI. Moreover, augmenting a diet with high magnesium may be beneficial for other ectopic mineralization diseases, including nephrocalcinosis.

Consensus guidelines on the use of bisphosphonate therapy in children and adolescents

Bisphosphonate therapy is the mainstay of pharmacological intervention in young people with skeletal fragility. The evidence of its use in a variety of conditions remains limited despite over three decades of clinical experience. On behalf of the Australasian Paediatric Endocrine Group, this evidence-based consensus guideline presents recommendations and discusses the graded evidence (using the GRADE system) for these recommendations. Primary bone fragility disorders such as osteogenesis imperfecta are considered separately from osteoporosis secondary to other clinical conditions (such as cerebral palsy, Duchenne muscular dystrophy). The use of bisphosphonates in non-fragility conditions, such as fibrous dysplasia, avascular necrosis, bone cysts and hypercalcaemia, is also discussed. While these guidelines provide an evidence-based approach where possible, further research is required in all clinical applications in order to strengthen the recommendations made.

Hypercementosis Associated with ENPP1 Mutations and GACI

Mineralization of bones and teeth is tightly regulated by levels of extracellular inorganic phosphate (P_i) and pyrophosphate (PP_i). Three regulators that control pericellular concentrations of P_i and PP_i include tissue-nonspecific alkaline phosphatase (TNAP), progressive ankylosis protein (ANK), and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). Inactivation of these factors results in mineralization disorders affecting teeth and their supporting structures. This study for the first time analyzed the effect of decreased PP_i on dental development in individuals with generalized arterial calcification of infancy (GACI) due to loss-of-function mutations in the ENPP1 gene. Four of the 5 subjects reported a history of infraocclusion, overretained primary teeth, ankylosis, and/or slow orthodontic tooth movement, suggesting altered mineral metabolism contributing to disrupted tooth movement and exfoliation. All subjects had radiographic evidence of unusually protruding cervical root morphology in primary and/or secondary dentitions. High-resolution micro-computed tomography (micro-CT) analyses of extracted primary teeth from 3 GACI subjects revealed 4-fold increased cervical cementum thickness ( P = 0.00007) and a 23% increase in cementum density ( P = 0.009) compared to age-matched healthy control teeth. There were no differences in enamel and dentin densities between GACI and control teeth. Histology revealed dramatically expanded cervical cementum in GACI teeth, including cementocyte-like cells and unusual patterns of cementum resorption and repair. Micro-CT analysis of Enpp1 mutant mouse molars revealed 4-fold increased acellular cementum thickness ( P = 0.002) and 5-fold increased cementum volume ( P = 0.002), with no changes in enamel or dentin. Immunohistochemistry identified elevated ENPP1 expression in cementoblasts of human and mouse control teeth. Collectively, these findings reveal a novel dental phenotype in GACI and identify ENPP1 genetic mutations associated with hypercementosis. The sensitivity of cementum to reduced PP_i levels in both human and mouse teeth establishes this as a well-conserved and fundamental biological process directing cementogenesis across species (ClinicalTrials.gov NCT00369421).

Clinical, radiographic and biochemical characteristics of adult hypophosphatasia

In this study, we report on clinical, radiographic and biochemical characteristics of 38 patients with adult hypophosphatasia. High-resolution peripheral quantitative computed tomography showed alterations of bone microstructure in a subgroup of 14 patients. Pyridoxal-5-phosphate levels correlated with the occurrence of fractures and the number of symptoms.

INTRODUCTION

Hypophosphatasia (HPP) is a rare disorder with a wide range of clinical manifestations. A reduced enzymatic activity of alkaline phosphatase (ALP) is the key marker of the disease, causing an accumulation of ALP substrates such as pyridoxal-5-phosphate (PLP). The purpose of this retrospective study was to further characterize adult onset HPP.

METHODS

We assessed clinical, radiographic and laboratory characteristics of 38 adult patients with HPP. Diagnosis of HPP was established by the combination of low-serum ALP, raised PLP levels and typical symptoms and was genetically confirmed in 32 patients. Dual-energy X-ray absorptiometry (DXA) and laboratory data were available in most patients. High-resolution peripheral quantitative computed tomography (HR-pQCT) was performed in 14 patients.

RESULTS

Clinical characteristics included a wide spectrum of symptoms. A history of fracture was present in 15 patients (39%). Twenty-one patients (55%) complained about recurring headaches, 23 patients (61%) had recurring muscle pain, 4 patients (11%) suffered from severe muscle weakness and 18 patients (47%) showed dental abnormalities. Z-scores assessed by DXA were only slightly reduced in most adult HPP patients. HR-pQCT of 14 patients showed microstructural changes of trabecular and cortical bone compared to reference values of healthy subjects. The occurrence of fractures and multiple symptoms (>2 typical HPP symptoms) were associated with significantly elevated levels of PLP.

CONCLUSION

Adult HPP presents with a wide range of clinical symptoms and is not associated with low bone mass in general. PLP seems to be a good marker for disease severity in adult patients as its level is correlated with the occurrence of fractures and number of symptoms.

Hypophosphatasia: An overview For 2017

Hypophosphatasia (HPP) is the inborn-error-of-metabolism that features low serum alkaline phosphatase (ALP) activity (hypophosphatasemia) caused by loss-of-function mutation(s) of the gene that encodes the tissue-nonspecific isoenzyme of ALP (TNSALP). Autosomal recessive or autosomal dominant inheritance from among >300 TNSALP (ALPL) mutations largely explains HPP's remarkably broad-ranging severity. TNSALP is a cell-surface homodimeric phosphohydrolase richly expressed in the skeleton, liver, kidney, and developing teeth. In HPP, TNSALP substrates accumulate extracellularly. Among them is inorganic pyrophosphate (PPi), a potent inhibitor of mineralization. Superabundance of extracellular PPi explains the hard tissue complications of HPP that feature premature loss of deciduous teeth and often rickets or osteomalacia as well as calcific arthropathies in some affected adults. In infants with severe HPP, blocked entry of minerals into the skeleton can cause hypercalcemia, and insufficient hydrolysis of pyridoxal 5'-phosphate (PLP), the major circulating form of vitamin B₆, can cause pyridoxine-dependent seizures. Elevated circulating PLP is a sensitive and specific biochemical marker for HPP. Also, the TNSALP substrate phosphoethanolamine (PEA) is usually elevated in serum and urine in HPP, though less reliably for diagnosis. Pathognomonic radiographic changes occur in pediatric HPP when the skeletal disease is severe. TNSALP mutation analysis is essential for recurrence risk assessment for HPP in future pregnancies and for prenatal diagnosis. HPP was the final rickets/osteomalacia to have a medical treatment. Now, significant successes using asfotase alfa, a mineral-targeted recombinant TNSALP, are published concerning severely affected newborns, infants, and children. Asfotase alfa was approved by regulatory agencies multinationally in 2015 typically for pediatric-onset HPP.

Inherited Arterial Calcification Syndromes: Etiologies and Treatment Concepts

PURPOSE OF REVIEW

We give an update on the etiology and potential treatment options of rare inherited monogenic disorders associated with arterial calcification and calcific cardiac valve disease.

RECENT FINDINGS

Genetic studies of rare inherited syndromes have identified key regulators of ectopic calcification. Based on the pathogenic principles causing the diseases, these can be classified into three groups: (1) disorders of an increased extracellular inorganic phosphate/inorganic pyrophosphate ratio (generalized arterial calcification of infancy, pseudoxanthoma elasticum, arterial calcification and distal joint calcification, progeria, idiopathic basal ganglia calcification, and hyperphosphatemic familial tumoral calcinosis; (2) interferonopathies (Singleton-Merten syndrome); and (3) others, including Keutel syndrome and Gaucher disease type IIIC. Although some of the identified causative mechanisms are not easy to target for treatment, it has become clear that a disturbed serum phosphate/pyrophosphate ratio is a major force triggering arterial and cardiac valve calcification. Further studies will focus on targeting the phosphate/pyrophosphate ratio to effectively prevent and treat these calcific disease phenotypes.

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

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

Pyrophosphate Supplementation Prevents Chronic and Acute Calcification in ABCC6-Deficient Mice

Soft tissue calcification occurs in several common acquired pathologies, such as diabetes and hypercholesterolemia, or can result from genetic disorders. ABCC6, a transmembrane transporter primarily expressed in liver and kidneys, initiates a molecular pathway inhibiting ectopic calcification. ABCC6 facilitates the cellular efflux of ATP, which is rapidly converted into pyrophosphate (PPi), a major calcification inhibitor. Heritable mutations in ABCC6 underlie the incurable calcification disorder pseudoxanthoma elasticum and some cases of generalized arterial calcification of infancy. Herein, we determined that the administration of PPi and the bisphosphonate etidronate to Abcc6^-/- mice fully inhibited the acute dystrophic cardiac calcification phenotype, whereas alendronate had no significant effect. We also found that daily injection of PPi to Abcc6^-/- mice over several months prevented the development of pseudoxanthoma elasticum-like spontaneous calcification, but failed to reverse already established lesions. Furthermore, we found that the expression of low amounts of the human ABCC6 in liver of transgenic Abcc6^-/- mice, resulting in only a 27% increase in plasma PPi levels, led to a major reduction in acute and chronic calcification phenotypes. This proof-of-concept study shows that the development of both acute and chronic calcification associated with ABCC6 deficiency can be prevented by compensating PPi deficits, even partially. Our work indicates that PPi substitution represents a promising strategy to treat ABCC6-dependent calcification disorders.

Hypophosphatasia: Enzyme Replacement Therapy Brings New Opportunities and New Challenges

Hypophosphatasia (HPP) is caused by loss-of-function mutation(s) of the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). Autosomal inheritance (dominant or recessive) from among more than 300 predominantly missense defects of TNSALP (ALPL) explains HPP's broad-ranging severity, the greatest of all skeletal diseases. In health, TNSALP is linked to cell surfaces and richly expressed in the skeleton and developing teeth. In HPP,TNSALP substrates accumulate extracellularly, including inorganic pyrophosphate (PPi), an inhibitor of mineralization. The PPi excess can cause tooth loss, rickets or osteomalacia, calcific arthropathies, and perhaps muscle weakness. Severely affected infants may seize from insufficient hydrolysis of pyridoxal 5'-phosphate (PLP), the major extracellular vitamin B₆ . Now, significant successes are documented for newborns, infants, and children severely affected by HPP given asfotase alfa, a hydroxyapatite-targeted recombinant TNSALP. Since fall 2015, this biologic is approved by regulatory agencies multinationally typically for pediatric-onset HPP. Safe and effective treatment is now possible for this last rickets to have a medical therapy, but a number of challenges involving diagnosis, understanding prognosis, and providing this treatment are reviewed herein. © 2017 American Society for Bone and Mineral Research.

Anesthesia Management of a Child with Osteopetrosis

Osteopetrosis is a rare genetic disorder of osteoclast dysfunction leading to anatomical and physiological disorders. We present the anesthesia management for the femur fracture of a 4-year-old girl with malignant infantile type of osteopetrosis. She had a ventriculoperitoneal shunt, impaired motion, visual disturbance, growth failure, facial deformity, heart murmur of moderate tricuspid regurgitation, and left ventricular heart failure, with splenomegaly and severe anemia.

Persistence of Vascular Calcification after Reversal of Uremia

The extent to which vascular calcification is reversible and the possible mechanisms are unclear. To address this, calcified aortas from uremic mice were transplanted orthotopically into normal mice, and the calcium content, histology, and minerals of the allografts were compared with the nontransplanted donor aorta. Calcium content decreased immediately after transplantation but remained constant thereafter, with 68% ± 12% remaining after 34 weeks. X-ray diffraction showed the presence of apatite in both donor aortas and allografts. Osteoclasts were absent in the allografts and there was no expression of the macrophage marker CD11b, the osteoclast marker tartrate-resistant acid phosphatase, or carbonic anhydrase II. The initial loss of calcium was less in heavily calcified aortas and was associated with an increase in the Ca/P ratio from 1.49 to 1.63, consistent with a loss of nonapatitic calcium. The results indicate that vascular calcification persists after reversal of uremia, because of a lack of active resorption of apatite. This failure to resorb established calcifications may contribute to the severity of vascular calcification and suggests that therapy should be aimed at prevention.

Functional Rescue of ABCC6 Deficiency by 4-Phenylbutyrate Therapy Reduces Dystrophic Calcification in Abcc6-/- Mice

Soft-tissue calcification is associated with aging, common conditions such as diabetes or hypercholesterolemia, and with certain genetic disorders. ABCC6 is an efflux transporter primarily expressed in liver facilitating the release of adenosine triphosphate from hepatocytes. Within the liver vasculature, adenosine triphosphate is converted into pyrophosphate, a major inhibitor of ectopic calcification. ABCC6 mutations thus lead to reduced plasma pyrophosphate levels, resulting in the calcification disorder pseudoxanthoma elasticum and some cases of generalized arterial calcification of infancy. Most mutations in ABCC6 are missense, and many preserve transport activity but are retained intracellularly. We have previously shown that the chemical chaperone 4-phenylbutyrate (4-PBA) promotes the maturation of ABCC6 mutants to the plasma membrane. In a humanized mouse model of pseudoxanthoma elasticum, we investigated whether 4-PBA treatments could rescue the calcification inhibition potential of selected ABCC6 mutants. We used the dystrophic cardiac calcification phenotype of Abcc6^-/- mice as an indicator of ABCC6 function to quantify the effect of 4-PBA on human ABCC6 mutants transiently expressed in the liver. We showed that 4-PBA administrations restored the physiological function of ABCC6 mutants, resulting in enhanced calcification inhibition. This study identifies 4-PBA treatment as a promising strategy for allele-specific therapy of ABCC6-associated calcification disorders.

Asfotase alfa therapy for children with hypophosphatasia

Background. Hypophosphatasia (HPP) is caused by loss-of-function mutation(s) of the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). Consequently, cell-surface deficiency of TNSALP phosphohydrolase activity leads to extracellular accumulation of inorganic pyrophosphate, a natural substrate of TNSALP and inhibitor of mineralization. Children with HPP can manifest rickets, skeletal pain, deformity, fracture, muscle weakness, and premature deciduous tooth loss. Asfotase alfa is a recombinant, bone-targeted, human TNSALP injected s.c. to treat HPP. In 2012, we detailed the 1-year efficacy of asfotase alfa therapy for the life-threatening perinatal and infantile forms of HPP. Methods. Here, we evaluated the efficacy and safety of asfotase alfa treatment administered to children 6-12 years of age at baseline who were substantially impaired by HPP. Two radiographic scales quantitated HPP skeletal disease, including comparisons to serial radiographs from similarly affected historical control patients. Results. Twelve children receiving treatment were studied for 5 years. The 6-month primary endpoint was met, showing significant radiographic improvement. Additional significant improvements included patient growth, strength, motor function, agility, and quality of life, which for most patients meant achieving normal values for age- and sex-matched peers that were sustained at 5 years of treatment. For most, pain and disability resolved. Mild to moderate injection-site reactions were common and were sometimes associated with lipohypertrophy. Low anti-asfotase alfa antibody titers were noted in all patients. No evidence emerged for clinically important ectopic calcification or treatment resistance. Conclusions. Asfotase alfa enzyme replacement therapy has substantial and sustained efficacy with a good safety profile for children suffering from HPP. Trial Registration. ClinicalTrials.gov NCT00952484 (https://clinicaltrials.gov/ct2/show/NCT00952484) and NCT01203826 (https://clinicaltrials.gov/ct2/show/NCT01203826). Funding. Alexion Pharmaceuticals Inc. and Shriners Hospitals for Children.

Dual Effects of Bisphosphonates on Ectopic Skin and Vascular Soft Tissue Mineralization versus Bone Microarchitecture in a Mouse Model of Generalized Arterial Calcification of Infancy

Generalized arterial calcification of infancy (GACI) is an intractable ectopic mineralization disorder caused by mutations in the ENPP1 gene resulting in reduced plasma inorganic pyrophosphate levels. We previously characterized the Enpp1^asj mutant mouse as a model of GACI, and we have now explored the potential efficacy of bisphosphonates, non-hydrolyzable PPi analogs, in preventing ectopic mineralization in these mice. These mice were maintained on either basic diet (control) or diets containing etidronate or alendronate in three different concentrations (experimental). Considering low bioavailability of bisphosphonates when administered orally, subsequent studies tested the mice with subcutaneous injections of etidronate. The treatments were initiated at 4 weeks of age, and the degree of mineralization was assessed at 12 weeks of age by quantitation of calcium deposits in the muzzle skin containing dermal sheath of vibrissae and in aorta. We found that bisphosphonate treatments significantly reduced mineralization in skin and aorta. These changes in treated mice were accompanied with restoration of their bone microarchitecture, determined bymicrocomputed tomography. The inhibitory capacity of bisphosphonates, with mechanistic implications, was confirmed in a cell-based mineralization assay in vitro. Collectively, these results suggest that bisphosphonate treatment may be beneficial by a dual effect for preventing ectopic soft tissue mineralization while correcting decreased bone mineralization in GACI caused by ENPP1 mutations.

Hypophosphatasia - aetiology, nosology, pathogenesis, diagnosis and treatment

Hypophosphatasia is the inborn error of metabolism characterized by low serum alkaline phosphatase activity (hypophosphatasaemia). This biochemical hallmark reflects loss-of-function mutations within the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). TNSALP is a cell-surface homodimeric phosphohydrolase that is richly expressed in the skeleton, liver, kidney and developing teeth. In hypophosphatasia, extracellular accumulation of TNSALP natural substrates includes inorganic pyrophosphate, an inhibitor of mineralization, which explains the dento-osseous and arthritic complications featuring tooth loss, rickets or osteomalacia, and calcific arthopathies. Severely affected infants sometimes also have hypercalcaemia and hyperphosphataemia due to the blocked entry of minerals into the skeleton, and pyridoxine-dependent seizures from insufficient extracellular hydrolysis of pyridoxal 5'-phosphate, the major circulating form of vitamin B6, required for neurotransmitter synthesis. Autosomal recessive or dominant inheritance from ~300 predominantly missense ALPL (also known as TNSALP) mutations largely accounts for the remarkably broad-ranging expressivity of hypophosphatasia. High serum concentrations of pyridoxal 5'-phosphate represent a sensitive and specific biochemical marker for hypophosphatasia. Also, phosphoethanolamine levels are usually elevated in serum and urine, though less reliably for diagnosis. TNSALP mutation detection is important for recurrence risk assessment and prenatal diagnosis. Diagnosing paediatric hypophosphatasia is aided by pathognomic radiographic changes when the skeletal disease is severe. Hypophosphatasia was the last type of rickets or osteomalacia to await a medical treatment. Now, significant successes for severely affected paediatric patients are recognized using asfotase alfa, a bone-targeted recombinant TNSALP.

Large animal models of cardiovascular disease

The human cardiovascular system is a complex arrangement of specialized structures with distinct functions. The molecular landscape, including the genome, transcriptome and proteome, is pivotal to the biological complexity of both normal and abnormal mammalian processes. Despite our advancing knowledge and understanding of cardiovascular disease (CVD) through the principal use of rodent models, this continues to be an increasing issue in today's world. For instance, as the ageing population increases, so does the incidence of heart valve dysfunction. This may be because of changes in molecular composition and structure of the extracellular matrix, or from the pathological process of vascular calcification in which bone-formation related factors cause ectopic mineralization. However, significant differences between mice and men exist in terms of cardiovascular anatomy, physiology and pathology. In contrast, large animal models can show considerably greater similarity to humans. Furthermore, precise and efficient genome editing techniques enable the generation of tailored models for translational research. These novel systems provide a huge potential for large animal models to investigate the regulatory factors and molecular pathways that contribute to CVD in vivo. In turn, this will help bridge the gap between basic science and clinical applications by facilitating the refinement of therapies for cardiovascular disease.

Research Progress in Pseudoxanthoma Elasticum and Related Ectopic Mineralization Disorders

Heritable ectopic mineralization disorders represent a phenotypically diverse group of conditions characterized by deposition of calcium phosphate complexes in soft connective tissues. The prototype of such conditions is pseudoxanthoma elasticum, and related conditions with overlapping clinical features include generalized arterial calcification of infancy and arterial calcification due to CD73 deficiency. Molecular genetic investigations have revealed mutations in the genes physiologically involved in generation of inorganic pyrophosphate and inorganic phosphate, and the findings suggest a unifying pathomechanism relating to reduced inorganic pyrophosphate/inorganic phosphate ratio. This hypothesis is based on the notion that inorganic pyrophosphate serves as a powerful inhibitor of mineralization, whereas inorganic phosphate is a promineralization factor, and an appropriate inorganic pyrophosphate/inorganic phosphate ratio is critical for prevention of ectopic mineralization under homeostatic conditions.

Auricular ossification: A newly recognized feature of osteoprotegerin-deficiency juvenile Paget disease

We report auricular ossification (AO) affecting the elastic cartilage of the ear as a newly recognized feature of osteoprotegerin (OPG)-deficiency juvenile Paget disease (JPD). AO and auricular calcification refer interchangeably to rigid pinnae, sparing the ear lobe, from various etiologies. JPD is a rare Mendelian disorder characterized by elevated serum alkaline phosphatase activity accompanied by skeletal pain and deformity from rapid bone turnover. Autosomal recessive transmission of loss-of-function mutations within TNFRSF11B encoding OPG accounts for most JPD (JPD1). JPD2 results from heterozygous constitutive activation of TNFRSF11A encoding RANK. Other causes of JPD remain unknown. In 2007, we reported a 60-year-old man with JPD1 who described hardening of his external ears at age 45 years, after 4 years of treatment with bisphosphonates (BPs). Subsequently, we noted rigid pinnae in a 17-year-old boy and 14-year-old girl, yet pliable pinnae in a 12-year-old boy, each with JPD1 and several years of BP treatment. Cranial imaging indicated cortical bone within the pinnae of both teenagers. Radiologic studies of our three JPD patients without mutations in TNFRSF11B showed normal auricles. Review of the JPD literature revealed possible AO in several reports. Two of our JPD1 patients had experienced difficult tracheal intubation, raising concern for mineralization of laryngeal elastic cartilage. Thus, AO is a newly recognized feature of JPD1, possibly exacerbated by BP treatment. Elastic cartilage at other sites in JPD1 might also ossify, and warrants investigation.