Mutations In ENPP1 Research Articles

Enpp1 mutations promote upregulation of hedgehog signaling in heterotopic ossification with aging.

Heterotopic ossification of the tendon and ligament (HOTL) is a chronic progressive disease that is usually accompanied by thickening and ossification of ligaments and high osteogenic activity of the surrounding ligament tissue. However, the molecular mechanism of maintaining the cellular phenotype of HOTL remains unclear. We first constructed a model of HOTL, Enpp1flox/flox/EIIa-Cre mice, a novel genetic mouse system. Imaging, histological, and cell-level analyses were performed to investigate the progressive ossification of the posterior longitudinal ligament, Achilles tendons, and degeneration joints caused by Enpp1 deficiency. The results indicate that Enpp1 deficiency led to markedly progressive heterotopic ossification (HO), especially spine, and Achilles tendons, and was associated with progressive degeneration of the knees. The bone mass was decreased in the long bone. Furthermore, fibroblasts from Enpp1flox/flox/EIIa-Cre mice had greater osteogenic differentiation potential following induction by osteogenesis, accompanied by enhanced hedgehog (Hh) signaling. In addition, fibroblast cells show senescence, and aggravation of the senescence phenotype by further osteogenic induction. Our study indicated that with increasing age, mutations in Enpp1 promote ectopic ossification of spinal ligaments and endochondral ossification in tendons and further aggravate knee degeneration by upregulating hedgehog signaling.

The Purinergic Nature of Pseudoxanthoma Elasticum.

Pseudoxanthoma Elasticum (PXE) is an inherited disease characterized by elastic fiber calcification in the eyes, the skin and the cardiovascular system. PXE results from mutations in ABCC6 that encodes an ABC transporter primarily expressed in the liver and kidneys. It took nearly 15 years after identifying the gene to better understand the etiology of PXE. ABCC6 function facilitates the efflux of ATP, which is sequentially hydrolyzed by the ectonucleotidases ENPP1 and CD73 into pyrophosphate (PPi) and adenosine, both inhibitors of calcification. PXE, together with General Arterial Calcification of Infancy (GACI caused by ENPP1 mutations) as well as Calcification of Joints and Arteries (CALJA caused by NT5E/CD73 mutations), forms a disease continuum with overlapping phenotypes and shares steps of the same molecular pathway. The explanation of these phenotypes place ABCC6 as an upstream regulator of a purinergic pathway (ABCC6 → ENPP1 → CD73 → TNAP) that notably inhibits mineralization by maintaining a physiological Pi/PPi ratio in connective tissues. Based on a review of the literature and our recent experimental data, we suggest that PXE (and GACI/CALJA) be considered as an authentic "purinergic disease". In this article, we recapitulate the pathobiology of PXE and review molecular and physiological data showing that, beyond PPi deficiency and ectopic calcification, PXE is associated with wide and complex alterations of purinergic systems. Finally, we speculate on the future prospects regarding purinergic signaling and other aspects of this disease.

Abstract #1399545: Calcification of Left Atrial Appendage in Autosomal Recessive Hypophosphatemic Rickets Type 2 (ENPP1 Mutation)

Autosomal Recessive Hypophosphatemic Rickets type 2 (ARHR2) is a rare genetic condition secondary to a loss-of-function mutation in ENPP1 gene. ARHR2 is associated with renal phosphate wasting and suppressed 1,25-dihydroxyvitamin D due to increased FGF-23 levels and causing abnormal bone mineralization. Furthermore, as ENPP1 is implicated in the generation of PPi (inorganic pyrophosphate) which inhibits hydroxyapatite deposition, some ARHR2 patients present with life-treating generalized arterial calcification of infancy (GACI). We reported the case of a 32-year-old woman with a known diagnosis of ARHR2 since the age of seven. At that time, she presented a growth failure, bowed legs but no GACI history. She was treated from the age of 7 to 18 with phosphate supplements and calcitriol. She had ten surgeries for her leg deformities and had a subtotal parathyroidectomy at age 13 for primary hyperparathyroidism. She was referred to a tertiary centre for management of hypophosphatemia and diffuse bone pain. The alkaline phosphatase was at the upper limit of normal but imagings, including computed tomography scans and bone scintigraphy, did not show pseudofractures. Her chronic pain was associated with osteoarthritis and periarticular calcifications. She was treated for hypertension, and had a history of preeclampsia at 33 weeks of pregnancy. At the physical exam, a 30 mm Hg systolic blood pressure differential was noted between the right and left arms, suspect of severe subclavian artery stenosis. Additional investigations demonstrated a normal cardiac ultrasound (US), an Agatson score of 0 (coronary calcium scan) but extensive calcifications of the left atrial appendage, dense calcifications of the mitral valve and numerous calcifications in the ascending aortic wall. The spectrum of clinical manifestations associated with ENPP1-related conditions (GACI and ARHR2) is still unclear. In some patients who survive GACI, resolution of vascular calcifications has been reported. Furthermore, this case raises the possibility of undetected vascular calcifications in the infancy uncovered only after the diagnosis of ARHR2. Previous cases reported pulmonary stenosis, thickening of the aortic valve, and increased carotid intima-media thickness in patients with ARHR2 phenotype. Left atrial calcifications have been described in mechanical valve replacement, mitral stenosis, post-radiotherapy and in chronic kidney disease. Recommendations of cardiac US and kidney US have been proposed at the diagnosis of AHRH2. We further suggest vascular screening with cardiac scans to examine large vessels, and calcium score in these patients.

PSAT219 Renal Phosphate Wasting Caused by ENPP-1 mutation.

Abstract Background Increased renal phosphate loss can be Fibroblast Growth Factor-23 (FGF-23) dependent or independent. FGF-23 reduces expression of Sodium-Phosphate co-transporters, responsible for phosphate reabsorption; and by inhibition of 1-alpha hydroxylation of 25-OH-D3. Reduced serum concentration of 1, 25-OH2-D3 results in secondary hyperparathyroidism. Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) mutation is one of the rarest causes of hypophosphatemia, which causes FGF-23 dependent hypophosphatemia. Case description 31-year-old female referred to endocrinology for evaluation of recurrent nephrolithiasis despite normalization of PTH and serum Calcium (Ca) after right inferior parathyroid adenoma resection. Repeated renal stone analysis revealed calcium phosphate as the major stone composition. Patient achieved mid parental height, denied history of calcium or phosphate metabolism disorders/ rickets in the family. Physical exam was not suggestive of rickets and skeletal survey showed normal bone mineralization. Initial labs following parathyroid adenoma resection showed normal Ca 9.7 mg/dl, PTH 47 pg/ml, Phosphorus (Ph) 2.3 mg/dl, 25-OH-D3 51 ng/ml, albumin 3.9 g/dl, Mg 1.8 mg/dl. Repeat fasting labs six months later revealed Ca 9 mg/dl, Ph 1.7 mg/dl, 25-OH-D3 52 ng/ml, PTH 67.5 pg/ml and albumin 3.7 g/dl. Persistent low serum phosphate was evaluated further by a 24-hour urine collection which revealed high urine phosphate of 828 mg/day, Ca 181 mg/day, Mg 23 mg/day and creatinine was 1485 mg/day. Tubular reabsorption of phosphorus was 77% (normal: >80%), indicating renal phosphate wasting. Serum FGF-23 on two different days was >180 RU/ml. Next-generation sequencing tested positive for heterozygous ENPP-1 gene mutation variant c1441C>T (p. Arg481Trp). Patient was started on oral Phosphorus supplementation & calcitriol. Repeat labs showed normalization of Ph 2.4 mg/dl. Audiology evaluation suggested mild sensorineural hearing loss. Heart calcium score was zero. Discussion ENPP1 gene mutation's clinical presentations can vary from no symptoms, hypophosphatemic rickets to death in the first six months of life from generalized arterial calcification. ENPP1 is an enzyme, which degrades ATP into adenosine monophosphate (AMP) and pyrophosphate (PPi). PPI is an inhibitor of hydroxyapatite crystallization in vascular tissue and organs, while Ph is a pro-mineralization factor. ENPP1 gene mutation results in a low concentration of PPi, predisposing the patient to pathological calcification. It is speculated that ENPP-1 modulates FGF-23 secretion. FGF-23 elevation in these patients is a protective response, as hypophosphatemia prevents pathological calcification. Treatment target is to keep phosphate at low end of normal. In our patient, ENPP- 1 mutation likely contributed to recurrent renal stones and possible parathyroid adenoma. Evaluation of genetic mutations is necessary to decide for or against FGF-23 antibody/Burosumab treatment. In ENPP-1 mutation. Burosumab should be avoided as it can bind to FGF-23 resulting in pathological calcification. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.

Isolated Hypophosphatemia due to Renal Phosphorous Wasting in Association With ENPP1 Mutations

Isolated Hypophosphatemia due to Renal Phosphorous Wasting in Association With ENPP1 Mutations

Adult Presentation of X-Linked Hypophosphatemia

Adult X-linked hypophosphatemia (XLH) patients present with specific symptoms, including enthesopathies (e.g., ossification of longitudinal ligaments (OPLL), osteophytes around large joints, and enthesopathy in the Achilles tendons), early osteoarthritis, the development of severe secondary and tertiary hyperparathyroidism (SHPT/THPT), and the subsequent progression of chronic kidney disease (CKD). In addition, these patients exhibit the typical phenotypes of osteomalacia, such as pseudofracture and fracture in weight-bearing bones, odontitis, and tooth abscesses. The mechanism underlying enthesopathy development is unknown; however, a common underlying mechanism among XLH and autosomal recessive hypophosphatemic rickets (ARHR1/2) due to mutations in PHEX, DMP1, and ENPP1 is assumed. Clarification of the pathogenesis and drug discovery for this complication is an urgent issue, as many adult XLH patients suffer subsequent debilitating nervous symptoms or impingement syndrome, and existing treatments are ineffective. Severe SHPT and THPT are associated with conventional therapy, including active vitamin D and phosphate supplementation, and complicated and careful adjustment of dosages by experienced clinicians is required to avoid SHPT/THPT. Burosumab is a very effective therapy without risk for the development of SHPT/THPT. However, indications for this drug should be carefully considered, along with cost-effectiveness, guidelines or recommendations, and the health care system of each country.

Mutation update: Variants of the ENPP1 gene in pathologic calcification, hypophosphatemic rickets, and cutaneous hypopigmentation with punctate keratoderma.

ENPP1 encodes ENPP1, an ectonucleotidase catalyzing hydrolysis of ATP to AMP and inorganic pyrophosphate (PPi), and an endogenous plasma protein physiologically preventing ectopic calcification of connective tissues. Mutations in ENPP1 have been reported in association with a range of human genetic diseases. In this mutation update, we provide a comprehensive review of all the pathogenic variants, likely pathogenic variants, and variants of unknown significance in ENPP1 associated with three autosomal recessive disorders-generalized arterial calcification of infancy (GACI), autosomal recessive hypophosphatemic rickets type 2 (ARHR2), and pseudoxanthoma elasticum (PXE), as well as with a predominantly autosomal dominant disorder-Cole disease. The classification of all variants is determined using the latest ACMG guidelines. A total of 140 ENPP1 variants were curated consisting of 133 previously reported variants and seven novel variants, with missense variants being the most prevalent (70.0%, 98/140). While the pathogenic variants are widely distributed in the ENPP1 gene of patientsgen without apparent genotype-phenotype correlation, eight out of nine variants associated with Cole disease are confined to the somatomedin-B-like (SMB) domains critical for homo-dimerization of the ENPP1 protein.

Oral supplementation of inorganic pyrophosphate in pseudoxanthoma elasticum.

Pseudoxanthoma elasticum (PXE; OMIM 264800) is a rare heritable multisystem disorder, characterized by ectopic mineralization affecting elastic fibres in the skin, eyes and the cardiovascular system. Skin findings often lead to early diagnosis of PXE, but currently, no specific treatment exists to counteract the progression of symptoms. PXE belongs to a group of Mendelian calcification disorders linked to pyrophosphate metabolism, which also includes generalized arterial calcification of infancy (GACI) and arterial calcification due to CD73 deficiency (ACDC). Inactivating mutations in ABCC6, ENPP1 and NT5E are the genetic cause of these diseases, respectively, and all of them result in reduced inorganic pyrophosphate (PPi ) concentration in the circulation. Although PPi is a strong inhibitor of ectopic calcification, oral supplementation therapy was initially not considered because of its low bioavailability. Our earlier work however demonstrated that orally administered pyrophosphate inhibits ectopic calcification in the animal models of PXE and GACI, and that orally given Na4 P2 O7 is absorbed in humans. Here, we report that gelatin-encapsulated Na2 H2 P2 O7 has similar absorption properties in healthy volunteers and people affected by PXE. The sodium-free K2 H2 P2 O7 form resulted in similar uptake in healthy volunteers and inhibited calcification in Abcc6-/- mice as effectively as its sodium counterpart. Novel pyrophosphate compounds showing higher bioavailability in mice were also identified. Our results provide an important step towards testing oral PPi in clinical trials in PXE, or potentially any condition accompanied by ectopic calcification including diabetes, chronic kidney disease or ageing.

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.

Insights into dental mineralization from three heritable mineralization disorders

Teeth are comprised of three unique mineralized tissues, enamel, dentin, and cementum, that are susceptible to developmental defects similar to those affecting bone. X-linked hypophosphatemia (XLH), caused by PHEX mutations, leads to increased fibroblast growth factor 23 (FGF23)-driven hypophosphatemia and local extracellular matrix disturbances. Hypophosphatasia (HPP), caused by ALPL mutations, results in increased levels of inorganic pyrophosphate (PPi), a mineralization inhibitor. Generalized arterial calcification in infancy (GACI), caused by ENPP1 mutations, results in vascular calcification due to decreased PPi, later compounded by FGF23-driven hypophosphatemia. In this perspective, we compare and contrast dental defects in primary teeth associated with XLH, HPP, and GACI, briefly reviewing genetic and biochemical features of these disorders and findings of clinical and preclinical studies to date, including some of our own recent observations. The distinct dental defects associated with the three heritable mineralization disorders reflect unique processes of the respective dental hard tissues, revealing insights into their development and clues about pathological mechanisms underlying such disorders.

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

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

Reversion of arterial calcification by elastin-targeted DTPA-HSA nanoparticles

Generalized arterial calcification of infancy (GACI) and pseudoxanthoma elasticum (PXE) are characterized by pathologic calcifications in the media of large- and medium sized arteries. GACI is associated with biallelic mutations in ENPP1 in the majority of cases, whereas mutations in ABCC6 are known to cause PXE. Different treatment approaches including bisphosphonates and orally administered pyrophosphate (PPi) were investigated in recent years, but reversion of calcification could not be achieved. With this study, we pursued the idea of a combination of controlled drug delivery through nanoparticles and active targeting via antibody conjugation to develop a treatment for GACI and PXE.To establish a suitable drug delivery system, the chelating drug diethylenetriamine pentaacetic acid (DTPA) was conjugated to nanoparticles composed of human serum albumin (HSA) as biodegradable and non-toxic particle matrix. To accomplish an active targeting of the elastic fibers exposed through calcification of the affected areas, the nanoparticle surface was functionalized with an anti-elastin antibody. Cytotoxicity and cell interaction studies revealed favorable preconditions for the intended i.v. application. The chelating ability was evaluated in vitro and ex vivo on aortic ring culture isolated from two mouse models of GACI and PXE. The positive results led to the conclusion that the produced nanoparticles might be a promising therapy in the treatment of GACI and PXE.

Human Heterozygous ENPP1 Deficiency Is Associated With Early Onset Osteoporosis, a Phenotype Recapitulated in a Mouse Model of Enpp1 Deficiency.

ABSTRACTBiallelic ENPP1 deficiency in humans induces generalized arterial calcification of infancy (GACI) and/or autosomal recessive hypophosphatemic rickets type 2 (ARHR2). The latter is characterized by markedly increased circulating FGF23 levels and renal phosphate wasting, but aberrant skeletal manifestations associated with heterozygous ENPP1 deficiency are unknown. Here, we report three adult men with early onset osteoporosis who presented with fractures in the thoracic spine and/or left radius, mildly elevated circulating FGF23, and hypophosphatemia. Total hip bone mineral density scans demonstrated osteoporosis (Z‐score < −2.5) and HRpQCT demonstrated microarchitectural defects in trabecular and cortical bone. Next‐generation sequencing revealed heterozygous loss‐of‐function mutations in ENPP1 previously observed as biallelic mutations in infants with GACI. In addition, we present bone mass and structure data as well as plasma pyrophosphate (PPi) data of two siblings suffering from ARHR2 in comparison to their heterozygous and wild‐type family members indicative of an ENPP1 gene dose effect. The skeletal phenotype in murine Enpp1 deficiency yielded nearly identical findings. Ten‐week‐old male Enpp1 asj/asj mice exhibited mild elevations in plasma FGF23 and hypophosphatemia, and micro‐CT analysis revealed microarchitectural defects in trabecular and cortical bone of similar magnitude to HRpQCT defects observed in humans. Histomorphometry revealed mild osteomalacia and osteopenia at both 10 and 23 weeks. The biomechanical relevance of these findings was demonstrated by increased bone fragility and ductility in Enpp1 asj/asj mice. In summary, ENPP1 exerts a gene dose effect such that humans with heterozygous ENPP1 deficiency exhibit intermediate levels of plasma analytes associated with bone mineralization disturbance resulting in early onset osteoporosis. © 2019 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

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

In brief: Generalized arterial calcification of infancy (GACI) is a severe, rare disease characterized by excessive calcification of large and medium sized arteries caused by homozygous loss-of-function mutations in ENPP1. Here, in Enpp1 deficient mice, treatment with recombinant human ENPP1 protein was more effective than bisphosphonates to resolve not only the vascular calcification, but also the hypertension that eventually leads to cardiac failure in GACI patients.

Missense mutations in ENPP1 result in osteoporosis in patients and is recapitulated in the ENPP1 loss of function murine model

Missense mutations in ENPP1 result in osteoporosis in patients and is recapitulated in the ENPP1 loss of function murine model

Generalized arterial calcification of infancy with a novel ENPP1 mutation: a case report

BackgroundGeneralized Arterial Calcification of Infancy (GACI) is a heritable ectopic mineralization disorder resulting in diffuse arterial calcifications and/or stenosis, mostly caused by mutations in the ENPP1 gene. Here we present a case report of GACI in a male infant with a new familial mutation of the ENPP1 gene and the clinical outcome after biphosphonates therapy.Case presentationThe clinical presentation was characterized by a severe early-onset of hypertension refractory to multiple therapy. To investigate this atypical hypertension, a renal Doppler ultra-sonography was performed and diffuse echo-bright arteries were detected; then a low-dose whole-body computed tomography demonstrated extensive arterial calcifications, suggesting GACI. A novel homozygous mutation c.784A > G (p.Ser262Gly) was detected in the ENPP1 gene. The infant was administered four courses of bisphosphonates: arterial calcifications were found to decrease but severe refractory hypertension was persistent. Although GACI can be a rapidly fatal illness and frequently results in death in infancy, the patient was 24 months of age at the time of writing this report.ConclusionsThree points of interest: the first one is to remind clinicians of this rare and atypical etiology in neonates with severe hypertension and in fetuses with cardiomyopathy and non-immune hydrops fetalis. The second point is the identification of a novel mutation in the ENPP1 gene associated with a clinical presentation of GACI. The third point is the fairly favourable outcome of our patient after bisphosphonates therapy, with calcifications regression but not hypertension.

208 Targeting plasma pyrophosphate deficiency in mouse models of heritable ectopic mineralization disorders – PXE and GACI

Pathologic mineralization of the skin and vascular connective tissues is the hallmark of certain single gene heritable disorders. Pseudoxanthoma elasticum (PXE), caused in most cases by loss-of-function mutations in the ABCC6 gene, serves as a prototype of heritable multi-system ectopic mineralization disorders with normal calcium and phosphate homeostasis. Generalized arterial calcification of infancy (GACI), caused in most cases by mutations in the ENPP1 gene, has overlapping phenotypic features with PXE. However, it was recently shown that PXE and GACI in some cases can also be caused by mutations in either ENPP1 or ABCC6, respectively. These observations suggested the possibility of shared pathomechanistic pathways for these conditions, and specifically, studies in these diseases and their corresponding mouse models revealed reduced plasma levels of inorganic pyrophosphate, a powerful inhibitor of mineralization. Consequently, the results suggested that insufficient inorganic pyrophosphate in the blood circulation underlies ectopic mineralization in both conditions. Towards development of treatment for these ectopic mineralization disorders, we have performed studies targeting plasma pyrophosphate deficiency through direct supplementation of inorganic pyrophosphate, or administration of stable, nonhydrolyzable pyrophosphate analogues, bisphosphonates, for treatment of these ectopic mineralization disorders in mouse models of PXE and GACI, Abcc6-/- and Enpp1asj. These approaches resulted in reduced ectopic mineralization in these mice, suggesting novel treatment modalities for these currently intractable diseases.

Mutational analysis of PHEX, FGF23 and CLCN5 in patients with hypophosphataemic rickets.

Hypophosphataemic rickets (HR) is a group of rare hereditary renal phosphate wasting disorders caused by mutations in PHEX, FGF23, DMP1, ENPP1, CLCN5 or SLC34A3. To investigate underlying genetic defects in patients with hypophosphataemic rickets. We analysed genomic DNA from nine unrelated families for mutations in the entire coding region of PHEX, FGF23, DMP1, ENPP1, CLCN5 or SLC34A3 by PCR sequencing and copy number analysis. A total of 14 patients were studied. PHEX mutations were identified in 12 patients from seven families. Five of them were novel mutations present in eight patients: c.154G>T (p.E52*), c.401_402insGCCAAA (p.Q134_K135insPK), c.1600C>T (p.P534S), g.22016715_22056805del (40-kb deletion including promoter and exons 1-3) and c.2242_2243delCT (p.L748fs*48). Four patients had previously reported mutations: c.1768+1G>A and c.1807G>A (p.W602*). Novel CLCN5 (c.1205G>A, p.W402*) and FGF23 (c.526C>G, p.R176G) mutations were found in two patients from the remaining two families. Many of the mutations were de novo: c.154G>T and c.2242_2243delCT in PHEX and c.526C>G in FGF23. Furthermore, we characterized the breakpoint of the novel PHEX g.22016715_22056805del and the c.2242_2243delCT, which is 6bp from the stop codon, resulting in a frameshift and extension of the reading frame by 42 amino acids. Novel and de novo mutations are frequent and PHEX mutations are still the most common genetic defects in the Turkish population. Gene copy number analysis should be considered in patients with negative results by conventional PCR-based sequencing analysis. The current study further expands the mutation spectrum underlying HR.

501 Plasma PPi is the major, but not exclusive, cause of ectopic mineralization in pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE), a prototype of heritable ectopic mineralization disorders, affects the skin, eyes and the arterial blood vessels. PXE is caused by inactivating mutations in the ABCC6 gene and the disease is characterized by calcium hydroxyapatite deposition on connective tissues. It was recently discovered that absence of ABCC6-mediated ATP release from the liver and consequently reduced plasma PPi levels underlie PXE. In this study, we examined whether elevated circulating levels of PPi, a powerful mineralization inhibitor, is the sole mechanism by which ABCC6 counteracts ectopic mineralization in PXE. Abcc6-/- mice, a model of PXE, were crossed with transgenic mice with ubiquitous expression of human ENPP1, an ectonucleotidase which generates PPi from ATP. We generated Abcc6-/-mice either wild type or hemizygous for human ENPP1. The Enpp1asj mice, a model for generalized arterial calcification of infancy caused by ENPP1 mutations, were also crossed with human ENPP1 transgenic mice to compensate for the loss of the endogenous mouse protein. Plasma levels of PPi and the degree of mineralization of connective tissue capsule of vibrissae, a reliable biomarker of the mineralization process in these mice, were determined. Overexpression of human ENPP1 in Enpp1asj mice normalized plasma PPi levels to that of wild type mice, and consequently, completely prevented mineralization. In contrast, significantly reduced mineralization was noted in Abcc6-/- mice with overexpression of human ENPP1, however, with small mineralization foci still evident despite increased plasma PPi levels. These results suggest that PPi is the major mediator of connective tissue mineralization in PXE, but there is a second, as-yet unknown, alternative mechanism other than PPi by which ABCC6 prevents ectopic mineralization under physiologic conditions.

Ectopic Mineralization and Conductive Hearing Loss in Enpp1asj Mutant Mice, a New Model for Otitis Media and Tympanosclerosis.

Otitis media (OM), inflammation of the middle ear, is a common cause of hearing loss in children and in patients with many different syndromic diseases. Studies of the human population and mouse models have revealed that OM is a multifactorial disease with many environmental and genetic contributing factors. Here, we report on otitis media-related hearing loss in asj (ages with stiffened joints) mutant mice, which bear a point mutation in the Enpp1 gene. Auditory-evoked brainstem response (ABR) measurements revealed that around 90% of the mutant mice (Enpp1asj/asj) tested had moderate to severe hearing impairment in at least one ear. The ABR thresholds were variable and generally elevated with age. We found otitis media with effusion (OME) in all of the hearing-impaired Enpp1asj/asj mice by anatomic and histological examinations. The volume and inflammatory cell content of the effusion varied among the asj mutant mice, but all mutants exhibited a thickened middle ear epithelium with fibrous polyps and more mucin-secreting goblet cells than controls. Other abnormalities observed in the Enpp1 mutant mice include over-ossification at the round window ridge, thickened and over-calcified stapedial artery, fusion of malleus and incus, and white patches on the inside of tympanic membrane, some of which are typical symptoms of tympanosclerosis. An excessive yellow discharge was detected in the outer ear canal of older asj mutant mice, with 100% penetrance by 5 months of age, and contributes to the progressive nature of the hearing loss. This is the first report of hearing loss and ear pathology associated with an Enpp1 mutation in mice. The Enpp1asj mutant mouse provides a new animal model for studying tympanosclerotic otitis and otitis media with effusion, and also provides a specific model for the hearing loss recently reported to be associated with human ENPP1 mutations causing generalized arterial calcification of infancy and hypophosphatemic rickets.