Idiopathic infantile arterial calcification: a surviving patient with renal artery stenosis

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.

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.

Generalized Arterial Calcification of Infancy (GACI): Optimizing Care with a Multidisciplinary Approach

It is very unusual to see evidence of arterial calcification in infants and children, and when detected, genetic disorders of calcium metabolism should be suspected. Generalized arterial calcification of infancy (GACI) is a hereditary disease, which is characterized by severe arterial calcification of medium sized arteries, mostly involving the media with marked intimal proliferation and ectopic mineralization of the extravascular tissues. It is caused by inactivating variants in genes encoding either ENPP1, in a majority of cases (70–75%), or ABCC6, in a minority (9–10%). Despite similar histologic appearances between ENPP1 and ABCC6 deficiencies, including arterial calcification, organ calcification, and cardiovascular calcification, mortality is higher in subjects carrying the ENPP1 versus ABCC6 variants (40% vs 10%, respectively). Overall mortality in individuals with GACI is high (55%) before the age of 6 months, with 24.4% dying in utero or being stillborn. Rare cases show spontaneous regression with age, while others who survive into adulthood often manifest musculoskeletal complications (osteoarthritis and interosseous membrane ossification), enthesis mineralization, and cervical spine fusion. Despite recent advances in the understanding of the genetic mechanisms underlying this disease, there is still no ideal therapy for the resolution of vascular calcification in GACI. Although bisphosphonates with anti-calcification properties have been commonly used for the treatment of CAGI, their benefit is controversial, with favorable results reported at one year and questionable benefit with delayed initiation of treatment. Enzyme replacement therapy with administration of recombinant form of ENPP1 prevents calcification and mortality, improves hypertension and cardiac function, and prevents intimal proliferation and osteomalacia in mouse models of ENPP1 deficiency. Therefore, newer treatments targeting genes are on the horizon. In this article, we review up to date knowledge of the understanding of GACI, its clinical, pathologic, and etiologic understanding and treatment in support of more comprehensive care of GACI patients.

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.

CD73 (Cluster of Differentiation 73) and the Differences Between Mice and Humans

As vascular disease is complex and the various manifestations are influenced by differences in vascular bed architecture, exposure to shear and mechanical forces, cell types involved, and inflammatory responses, in vivo models are necessary to recapitulate the complex physiology and dynamic cellular interactions during pathogenesis. Murine knockout models are commonly used tools for investigators to study the role of a specific gene or pathway in multifaceted disease traits. Although valuable, these models are not perfect, and this is particularly true in regard to CD73 (cluster of differentiation 73), the extracellular enzyme that generates adenosine from AMP. At baseline, CD73-deficient mice do not present with an overt phenotype, whereas CD73-deficient humans present with the complex phenotype of vascular calcification, arteriomegaly and tortuosity, and calcification in small joints. In this review, we highlight the differences between the mouse and human systems and discuss the potential to leverage findings in mice to inform us on the human conditions.

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.

Effects of etidronate on the Enpp1⁻/⁻ mouse model of generalized arterial calcification of infancy

Generalized arterial calcification of infancy (GACI) is an autosomal recessive disorder of spontaneous infantile arterial and periarticular calcification which is attributed to mutations in the ectonucleotide pyrophosphatase/phosphodiesterase 1 (Enpp1) gene. Whilst the bisphosphonate, etidronate, is currently used off-label for the treatment for GACI, recent studies have highlighted its detrimental effects on bone mineralisation. In the present study, we used the Enpp1-/- mouse model of GACI to examine the effects of etidronate treatment (100 µg/kg), on vascular and skeletal calcification. Micro-computed tomography (µCT) analysis revealed a significant decrease in trabecular bone mass, as reflected by the decrease in trabecular bone volume/tissue volume (BV/TV; %), trabecular thickness, trabecular separation, trabecular number and pattern factor (P<0.05) in the Enpp1-/- mice in comparison to the wild-type (WT) mice. Mechanical testing revealed that in the WT mice, treatment with etidronate significantly improved work to fracture and increased work post-failure (P<0.05, in comparison to the vehicle-treated WT mice). This significant increase, however, was not observed in the Enpp1-/- mice. Treatment with etidronate had no effect on bone parameters in the WT mice; however, the Enpp1-/- mice displayed an increased structural model index (SMI; P<0.05). We used a recently developed 3D µCT protocol to reconstruct and quantify the extensive aortic calcification in Enpp1-/- mice in comparison to the WT mice. However, treatment with etidronate did not prevent de novo calcification, and did not arrest the progression of established calcification of the aorta.

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

Generalized arterial calcification (AC) of infancy (GACI) is an autosomal recessive disorder that features hydroxyapatite deposition within arterial elastic fibers. Untreated, approximately 85% of GACI patients die by 6 months of age from cardiac ischemia and congestive heart failure. The first-generation bisphosphonate etidronate (EHDP; ethane-1-hydroxy-1,1-diphosphonic acid, also known as 1-hydroxyethylidene-bisphosphonate) inhibits bone resorption and can mimic endogenous inorganic pyrophosphate by blocking mineralization. With EHDP therapy for GACI, AC may resolve without recurrence upon treatment cessation. Skeletal disease is not an early characteristic of GACI, but rickets can appear from acquired hypophosphatemia or prolonged EHDP therapy. We report a 7-year-old boy with GACI referred for profound, acquired, skeletal disease. AC was gone after 5 months of EHDP therapy during infancy, but GACI-related joint calcifications progressed. He was receiving EHDP, 200 mg/day orally, and had odynodysphagia, diffuse opioid-controlled pain, plagiocephaly, facial dysmorphism, joint calcifications, contractures, and was wheelchair bound. Biochemical parameters of mineral homeostasis were essentially normal. Serum osteocalcin was low and the brain isoform of creatine kinase and tartrate-resistant acid phosphatase 5b (TRAP-5b) were elevated as in osteopetrosis. Skeletal radiographic findings resembled pediatric hypophosphatasia with pancranial synostosis, long-bone bowing, widened physes, as well as metaphyseal osteosclerosis, cupping and fraying, and "tongues" of radiolucency. Radiographic features of osteopetrosis included osteosclerosis and femoral Erlenmeyer flask deformity. After stopping EHDP, he improved rapidly, including remarkable skeletal healing and decreased joint calcifications. Profound, but rapidly reversible, inhibition of skeletal mineralization with paradoxical calcifications near joints can occur in GACI from protracted EHDP therapy. Although EHDP treatment is lifesaving in GACI, surveillance for toxicity is crucial.

Hypophosphatemia, hyperphosphaturia, and bisphosphonate treatment are associated with survival beyond infancy in generalized arterial calcification of infancy

BACKGROUND

Generalized arterial calcification of infancy has been reported to be frequently lethal, and the efficiency of any therapy, including bisphosphonates, is unknown. A phosphate-poor diet markedly increases survival of NPP1 null mice, a model of generalized arterial calcification of infancy.

METHODS AND RESULTS

We performed a multicenter genetic study and retrospective observational analysis of 55 subjects affected by generalized arterial calcification of infancy to identify prognostic factors. Nineteen (34%) patients survived the critical period of infancy. In all 8 surviving patients tested, hypophosphatemia due to reduced renal tubular phosphate reabsorption developed during childhood. Eleven of 17 (65%) patients treated with bisphosphonates survived. Of 26 patients who survived their first day of life and were not treated with bisphosphonates only 8 (31%) patients survived beyond infancy. Forty different homozygous or compound heterozygous mutations, including 16 novel mutations in ENPP1, were found in 41 (75%) of the 55 patients. Twenty-nine (71%) of these 41 patients died in infancy (median, 30 days). Seven of the 14 (50%) patients without ENPP1 mutations died in infancy (median, 9 days). When present on both alleles, the mutation p.P305T was associated with death in infancy in all 5 cases; otherwise, no clear genotype-phenotype correlation was seen.

CONCLUSION

ENPP1 coding region mutations are associated with generalized arterial calcification of infancy in approximately 75% of subjects. Except for the p.P305T mutation, which was universally lethal when present on both alleles, the identified ENPP1 mutations per se have no discernable effect on survival. However, survival seems to be associated with hypophosphatemia linked with hyperphosphaturia and also with bisphosphonate treatment.

Idiopathic infantile arterial calcification: the spectrum of clinical presentations

Idiopathic infantile arterial calcification (IIAC) is a rare disorder characterized by extensive calcification of medium and large arteries. We report the case of a 32-week-old infant with hydrops fetalis and heart failure who died at 4 days of age. At autopsy the infant was found to have cardiomegaly, myocardial infarctions and multifocal calcifications of the aorta and arteries in the lungs, heart, thyroid, spleen, and testis. Calcification extended from the internal elastic lamina into the intima and media and was associated with a giant-cell reaction and smooth muscle proliferation. A search of the English language medical literature identified 161 IIAC case reports. Of these, 48% of cases presented in utero or at birth with hydrops fetalis, maternal hydramnios, heart failure, or respiratory distress and 52% present later, at a median age of 3 months, with sudden onset of fever, vomiting, irritability, or respiratory distress in a previously healthy infant. Significantly, 19 of 22 IIAC survivors presented at less than 2 weeks of age, and 15 survivors were treated with diphosphonates.

Idiopathic infantile arterial calcification: clinical presentation, therapy and long-term follow-up

Idiopathic infantile arterial calcification (IIAC) is a rare disease characterised by extensive depositions of hydroxyapatite in the internal elastic lamina of medium-sized and large arteries, frequently accompanied by periarticular calcifications. We report on three patients with various presenting signs and symptoms. Diagnostic imaging techniques and therapy with bisphosphonates will be discussed. For the first time long-term follow-up of up to 25 years will be reported.

Generalized arterial calcification of infancy: two siblings with prolonged survival

In generalized arterial calcification of infancy (OMIM no. 208000), calcification of the media and proliferation of the intima lead to arterial stenoses. Most affected patients present with untreatable arterial hypertension and die within the first months of life. The disease has recently been linked to mutations in ENPP1. We report two siblings with prolonged survival, both of whom carry the compound heterozygous ENPP1 mutations c.913C>A and c.1164+2T>A. In both siblings, spontaneous regression of arterial calcifications occurred, and antihypertensive treatment could be tapered off gradually. In some patients, the natural course of GACI may be more favourable than previously assumed.

Idiopathic infantile arterial calcification: two case reports, a review of the literature and a role for cardiac transplantation

Idiopathic infantile arterial calcification (IIAC) is a rare, but important, cause of rapidly progressive ischemic heart disease in children. In this paper, we report two recent cases of IIAC seen at tertiary referral hospitals. Both cases presented in infancy with signs of heart failure and, ultimately, died with the diagnosis of IIAC confirmed at postmortem examination. A thorough review of the literature reveals approximately 160 reported cases of IIAC. The clinical outcomes, radiographic findings and pathologic details are summarized. Proposed etiologic mechanisms are reviewed, including promising research into the role of inorganic pyrophosphate as a regulatory factor in the development of IIAC. Because of the typically fatal outcome of IIAC and the lack of proven therapies, the potential role for cardiac transplantation is discussed.

Idiopathic infantile arterial calcification: imaging evaluation and the usefulness of MR angiography

Idiopathic infantile arterial calcification (IIAC) is a rare condition characterized by extensive calcification and stenosis of large and medium-size arteries. The etiology of the disease is unknown. However, the inheritance pattern has been shown to be autosomal recessive. The clinical presentation is variable, including cardiac failure (most common clinical finding), hypertension, and respiratory failure. Plain radiography, sonography and MRI can aid in the diagnosis. We present a case in which contrast-enhanced MR angiography with breath-hold and cardiac gating techniques allowed complete evaluation of the extent of this disease.

Idiopathic infantile arterial calcification: sonographic findings

Idiopathic infantile arterial calcification (IIAC) is a rare disease that is characterized by calcification in the media and fibroproliferative changes in the intima of larger arteries, sometimes resulting in reduced vascular elasticity and blood flow. Although the molecular-genetic basis of the disease is unknown, IIAC is presumed to be acquired by an autosomal recessive mode of inheritance and is associated with a reduction in the levels of enzymes responsible for inorganic phosphate balance, resulting in abnormal deposition of calcium into the vessels. We report the case of a female neonate in whom widespread IIAC was initially diagnosed on postnatal sonographic examination. At birth, the infant experienced cardiac failure and hypertension, and arterial pulsation was absent. Routine prenatal sonographic examinations had not revealed any abnormalities, but postnatal gray-scale and color Doppler echocardiographic and sonographic examinations revealed findings consistent with severe IIAC. The cardiac function improved with treatment, but the neonate died of progressive hepatic failure due to reduced flow in calcified and narrowed hepatic arteries. The common carotid arteries were also grossly affected, resulting in cerebral atrophy at the time of birth. Postnatal gray-scale and color Doppler echocardiographic and sonographic examinations allowed noninvasive diagnosis, assessment of severity, and monitoring of progression.

Isolated pericardial effusion in the human fetus: a report of three cases

OBJECTIVE

Our objective was to determine the possible underlying etiologies and outcome in isolated fetal pericardial effusion.

METHODS

Doppler fetal echocardiography allowed the diagnosis of pericardial effusion in three patients and revealed the etiology in two.

RESULTS

We present the findings in three cases of isolated pericardial effusion. In the first, the pericardial effusion was a manifestation of trisomy 21 associated with a myeloproliferative disorder. In the second, the pericardial fluid collection was the first sign of an autosomal recessive disease, idiopathic infantile arterial calcification. The third case was remarkable because of the spontaneous resolution of a large pericardial fluid collection.

CONCLUSION

Isolated fetal pericardial effusion covers a wide spectrum of etiologies from severe genetic and chromosomal diseases to transient forms.

Bisphosphonates: from grandparents to grandchildren

Bisphosphonates are synthetic analogues of pyrophosphate that inhibit bone resorption by their action on osteoclasts. Bisphosphonates have been extensively used in the elderly with primary and secondary osteoporosis, Paget's disease, and hypercalcemia of malignancy. In recent years, bisphosphonates have been used to treat children acutely for resistant hypercalcemia and chronically for various metabolic bone diseases. The theoretical concerns of possible adverse effects of these drugs on the growing skeleton have not been proven to be true. In the present review, we have critically analyzed the available literature on bisphosphonate therapy in both adult and pediatric clinical trials. Although not yet approved by the FDA for use in children, bisphosphonates, from published experience, demonstrate benefit to the child with no serious adverse effects. Based on the literature analysis the review furnishes detailed recommendations and practical guidelines regarding the use of oral and intravenous bisphosphonates in children. Bisphosphonates might be the first agents to provide the pediatrician with an opportunity to treat mineral and bone disorders of childhood, which until recently did not have satisfactory therapy.

Bisphosphonates for treatment of childhood hypercalcemia

Most clinicians only have a limited experience in treating childhood hypercalcemia with bisphosphonates. We report our experience in the use of intravenous and oral bisphosphonates in a 5-year-old with hypercalcemia secondary to acute lymphocytic leukemia, a 16-year-old with immobilization hypercalcemia, and a 14-year-old with chronic hypercalcemia of unknown cause. Single infusions of 0.5 mg/kg and 1 mg/kg of intravenous pamidronate were administered over 4 hours. No adverse reactions were observed except for hypocalcemia. A dose between 10 and 20 mg of oral alendronate was successfully used to maintain normocalcemia in the patient with chronic hypercalcemia. In our experience, the administration of bisphosphonates has enabled us to achieve normocalcemia in all cases, and in all cases there were no significant side effects. Long-term potential side effects from their use in children during the active phase of growth remain unknown.

Idiopathic arterial calcification and unexpected infant death

Two infants who died unexpectedly and who were found at autopsy to have idiopathic arterial calcification are presented. The first infant died within hours of the sudden onset of shortness of breath. The second infant died suddenly and unexpectedly in hospital where he was being treated for presumed sepsis and cardiac failure. Neither infant had significant past or family histories. Autopsy examination in both infants demonstrated widespread fibrointimal proliferation of elastic and muscular arteries with characteristic calcification of the internal elastic laminae. Kidneys and parathyroid glands were normal. Death in case 1 was attributed to extensive myocardial ischemic damage with right coronary artery ostial stenosis due to idiopathic arterial calcification. Death in case 2 was attributed to saddle pulmonary thromboembolism arising from a right atrial thrombus associated with cardiac failure secondary to idiopathic arterial calcification. These cases demonstrate the variable presentations, causes of death, and autopsy findings that may occur in this uncommon condition.