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Inorganic Pyrophosphate Deficiency Syndromes and Potential Treatments for Pathologic Tissue Calcification

2022; Elsevier BV; Volume: 192; Issue: 5 Linguagem: Inglês

10.1016/j.ajpath.2022.01.012

1525-2191

Douglas Ralph, Koen van de Wetering, Jouni Uitto, Qiaoli Li,

Parathyroid Disorders and Treatments

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. 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. Pathologic Calcification—A Major Health ProblemPathologic calcification refers to the deposition of calcium hydroxyapatite in nonskeletal connective tissues, which normally do not mineralize. Pathologic calcification is increasingly recognized as a major health problem in humans of all ages, causing significant morbidity and mortality. Cardiovascular calcification occurs in aortic valve disease, coronary artery disease, atherosclerosis, diabetes, and chronic kidney disease. Vascular calcification is a well-established independent risk factor for premature death in patients with diabetes and chronic kidney disease. Risk of coronary artery calcification increases with age, occurring in 90% of men and 67% of women >70 years of age. Pathologic calcification is also linked to tendonitis, synovitis and arthritis, malignant neoplasms, as well as autoimmune inflammatory diseases that affect the skin and subcutaneous tissue. Although pathologic calcification usually occurs via dysplastic, metabolic, or inflammatory mechanisms in acquired diseases, it also occurs in genetic disorders caused by mutations in distinct genes. There are no specific and effective treatments currently available to prevent or counteract pathologic calcification.The Spectrum of Heritable Diseases of Pathologic CalcificationTremendous progress has been made in the past two decades toward understanding the pathomechanisms of pathologic calcification, mostly owing to studies of genetic disorders with defined gene defects. An increasing body of evidence suggests that pathologic calcification results from an imbalance between procalcification and anticalcification factors that cause an uncontrolled deposition of calcium crystals in soft connective tissues. This review summarizes the clinical spectrum of five monogenic disorders of pathologic calcification, all caused by altered extracellular inorganic pyrophosphate (PPi) metabolism.Pseudoxanthoma ElasticumPseudoxanthoma elasticum (PXE; Online Mendelian Inheritance in Man [OMIM] no. 264800) is the prototype of hereditary pathologic calcification disorders. This autosomal recessive disease is characterized by a late-onset, yet progressive accumulation of calcium hydroxyapatite occurring in elastic fibers of the skin, eyes, and cardiovascular system.1Neldner K.H. Pseudoxanthoma elasticum.Clin Dermatol. 1988; 6: 1-159Abstract Full Text PDF PubMed Scopus (1) Google Scholar PXE is an orphan disease with an estimated prevalence of 1:50,000. PXE is caused by loss-of-function mutations in the ABCC6 gene with modifying effects in other genes.2Luo H. Faghankhani M. Cao Y. Uitto J. Li Q. Molecular genetics and modifier genes in pseudoxanthoma elasticum, a heritable multisystem ectopic mineralization disorder.J Invest Dermatol. 2021; 141: 1148-1156Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar The Abcc6 knockout mouse and rat models recapitulate the clinical and histologic features of human PXE.3Luo H. Li Q. Cao Y. Uitto J. Therapeutics development for pseudoxanthoma elasticum and related ectopic mineralization disorders: update 2020.J Clin Med. 2020; 10: 114Crossref Scopus (12) Google Scholar ABCC6 encodes ATP binding cassette subfamily C member 6 (ABCC6), a transmembrane efflux transporter expressed primarily in the liver. Although the substrate of ABCC6 is unknown, PXE was determined in early studies to be a systemic metabolic disorder with the primary defect of loss of function of ABCC6 in the liver, causing reduced concentrations of an anticalcification factor in blood circulation.4Jiang Q. Endo M. Dibra F. Wang K. Uitto J. Pseudoxanthoma elasticum is a metabolic disease.J Invest Dermatol. 2009; 129: 348-354Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar More recently, ABCC6 was shown to facilitate the release of ATP from hepatocytes.5Jansen R.S. Duijst S. Mahakena S. Sommer D. Szeri F. Varadi A. Plomp A. Bergen A.A. Oude Elferink R.P. Borst P. van de Wetering K. ABCC6-mediated ATP secretion by the liver is the main source of the mineralization inhibitor inorganic pyrophosphate in the systemic circulation-brief report.Arterioscler Thromb Vasc Biol. 2014; 34: 1985-1989Crossref PubMed Scopus (196) Google ScholarGeneralized Arterial Calcification of InfancyGeneralized arterial calcification of infancy (GACI) is an extremely severe, early-onset condition of vascular calcification that is often diagnosed by prenatal ultrasonography revealing calcium deposits in the fetal heart and arteries.6Rutsch F. Ruf N. Vaingankar S. Toliat M.R. Suk A. Hohne W. Schauer G. Lehmann M. Roscioli T. Schnabel D. Epplen J.T. Knisely A. Superti-Furga A. McGill J. Filippone M. Sinaiko A.R. Vallance H. Hinrichs B. Smith W. Ferre M. Terkeltaub R. Nurnberg P. Mutations in ENPP1 are associated with 'idiopathic' infantile arterial calcification.Nat Genet. 2003; 34: 379-381Crossref PubMed Scopus (460) Google Scholar Inherited in an autosomal recessive pattern, GACI is an ultrarare disease with an estimated prevalence of 1:200,000 to 1:390,000. There are two types of GACI based on gene mutation. GACI type 1 (OMIM no. 208000), the more common form of GACI, is caused by mutations in ENPP1, which encodes ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), the principal enzyme that hydrolyzes ATP to AMP and PPi, the latter being the potent physiologic inhibitor of hydroxyapatite formation in soft tissues.6Rutsch F. Ruf N. Vaingankar S. Toliat M.R. Suk A. Hohne W. Schauer G. Lehmann M. Roscioli T. Schnabel D. Epplen J.T. Knisely A. Superti-Furga A. McGill J. Filippone M. Sinaiko A.R. Vallance H. Hinrichs B. Smith W. Ferre M. Terkeltaub R. Nurnberg P. Mutations in ENPP1 are associated with 'idiopathic' infantile arterial calcification.Nat Genet. 2003; 34: 379-381Crossref PubMed Scopus (460) Google Scholar Pathologic calcification in several Enpp1 mutant mice mimics the features of GACI in patients.3Luo H. Li Q. Cao Y. Uitto J. Therapeutics development for pseudoxanthoma elasticum and related ectopic mineralization disorders: update 2020.J Clin Med. 2020; 10: 114Crossref Scopus (12) Google Scholar GACI type 2 (OMIM no. 614473), the less common form of GACI, is caused by inactivating mutations in ABCC6.7Ferreira C.R. Hackbarth M.E. Ziegler S.G. Pan K.S. Roberts M.S. Rosing D.R. Whelpley M.S. Bryant J.C. Macnamara E.F. Wang S. Muller K. Hartley I.R. Chew E.Y. Corden T.E. Jacobsen C.M. Holm I.A. Rutsch F. Dikoglu E. Chen M.Y. Mughal M.Z. Levine M.A. Gafni R.I. Gahl W.A. Prospective phenotyping of long-term survivors of generalized arterial calcification of infancy (GACI).Genet Med. 2021; 23: 396-407Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar,8Ferreira C.R. Kintzinger K. Hackbarth M.E. Botschen U. Nitschke Y. Mughal M.Z. Baujat G. Schnabel D. Yuen E. Gahl W.A. Gafni R.I. Liu Q. Huertas P. Khursigara G. Rutsch F. Ectopic calcification and hypophosphatemic rickets: natural history of ENPP1 and ABCC6 deficiencies.J Bone Miner Res. 2021; 36: 2193-2202Crossref PubMed Scopus (17) Google Scholar Regardless of the type of GACI, affected newborns present with severe cardiovascular involvement, and most children with GACI die of cardiovascular collapse within the first 6 months of life. Surviving individuals with GACI type 1, however, have a high frequency of developing hypophosphatemic rickets later in life.8Ferreira C.R. Kintzinger K. Hackbarth M.E. Botschen U. Nitschke Y. Mughal M.Z. Baujat G. Schnabel D. Yuen E. Gahl W.A. Gafni R.I. Liu Q. Huertas P. Khursigara G. Rutsch F. Ectopic calcification and hypophosphatemic rickets: natural history of ENPP1 and ABCC6 deficiencies.J Bone Miner Res. 2021; 36: 2193-2202Crossref PubMed Scopus (17) Google Scholar The co-occurrence of vascular calcification and hypophosphatemic rickets in the same individual highlights an unexpected imbalance between soft tissue calcification and bone mineralization, which currently lacks an explanation.Arterial Calcification due to Deficiency of CD73In contrast to GACI, arterial calcification due to deficiency of CD73 (ACDC; OMIM no. 211800) is a late-onset autosomal recessive calcification disorder of elderly individuals.9St Hilaire C. Ziegler S.G. Markello T.C. Brusco A. Groden C. Gill F. Carlson-Donohoe H. Lederman R.J. Chen M.Y. Yang D. Siegenthaler M.P. Arduino C. Mancini C. Freudenthal B. Stanescu H.C. Zdebik A.A. Chaganti R.K. Nussbaum R.L. Kleta R. Gahl W.A. Boehm M. NT5E mutations and arterial calcifications.N Engl J Med. 2011; 364: 432-442Crossref PubMed Scopus (326) Google Scholar ACDC has been identified in T (p.G608G), in LMNA. This mutation causes aberrant splicing and synthesis of a farnesylated mutant lamin A/C protein, resulting in aberrant architecture of nuclei, where most of its functions are performed. The phenotypes in humans are similar to those found in a LmnaG609G/+ knock-in mouse.14Osorio F.G. Navarro C.L. Cadinanos J. Lopez-Mejia I.C. Quiros P.M. Bartoli C. Rivera J. Tazi J. Guzman G. Varela I. Depetris D. de Carlos F. Cobo J. Andres V. De Sandre-Giovannoli A. Freije J.M. Levy N. Lopez-Otin C. Splicing-directed therapy in a new mouse model of human accelerated aging.Sci Transl Med. 2011; 3: 106ra7Crossref Scopus (236) Google ScholarPPi Deficiency as the Unifying Pathomechanistic Finding in Heritable Diseases of Pathologic CalcificationThe phenotypic overlap of the above mentioned heritable pathologic calcification disorders suggested the possibility of shared pathomechanistic pathways of their encoded proteins. This review summarizes current knowledge of the proteins that, when mutated, result in dysregulation of PPi metabolism.ENPP1—The Principal Enzyme Generating PPi by Hydrolysis of ATPENPP1 is well established to play a role in regulating both skeletal mineralization and soft tissue calcification. It is a transmembrane protein that primarily exerts its function through the generation of PPi by hydrolysis of ATP in the extracellular space (Figure 1). PPi is a metabolic by-product of many intracellular biosynthetic processes. It was first identified in the 1960s as the key endogenous inhibitor of biomineralization by antagonizing the ability of Pi to crystallize with calcium to form hydroxyapatite, presumably through its occupation on the surface of nascent-growing hydroxyapatite crystals.15Orriss I.R. Arnett T.R. Russell R.G. Pyrophosphate: a key inhibitor of mineralisation.Curr Opin Pharmacol. 2016; 28: 57-68Crossref PubMed Scopus (82) Google Scholar Patients with GACI type 1 and Enpp1 mutant mouse models of GACI have almost zero plasma PPi, indicating that all PPi in plasma is derived from ATP and other nucleoside triphosphates.ABCC6 and ANK—New Players in Extracellular PPi HomeostasisABCC6, initially considered as an efflux transporter, was subsequently identified as a key regulator of extracellular PPi homeostasis. ABCC6-expressing cells release large amounts of nucleoside triphosphates, including ATP, followed by their extracellular conversion into AMP and PPi by ENPP1.16Jansen R.S. Kucukosmanoglu A. de Haas M. Sapthu S. Otero J.A. Hegman I.E. Bergen A.A. Gorgels T.G. Borst P. van de Wetering K. ABCC6 prevents ectopic mineralization seen in pseudoxanthoma elasticum by inducing cellular nucleotide release.Proc Natl Acad Sci U S A. 2013; 110: 20206-20211Crossref PubMed Scopus (167) Google Scholar Additional studies in mice found that ABCC6 facilitates the transmembrane efflux of ATP from hepatocytes to the extracellular space via an as of yet unknown mechanism. Within the liver niche, ATP is rapidly converted by ENPP1 to AMP and PPi.5Jansen R.S. Duijst S. Mahakena S. Sommer D. Szeri F. Varadi A. Plomp A. Bergen A.A. Oude Elferink R.P. Borst P. van de Wetering K. ABCC6-mediated ATP secretion by the liver is the main source of the mineralization inhibitor inorganic pyrophosphate in the systemic circulation-brief report.Arterioscler Thromb Vasc Biol. 2014; 34: 1985-1989Crossref PubMed Scopus (196) Google Scholar In PXE, loss of ABCC6 activity impairs hepatic release of ATP, and consequently, extracellular PPi synthesis is reduced (Figure 1). The role of ABCC6 as a critical player in extracellular PPi metabolism was confirmed by reduced plasma PPi levels in patients with PXE and in Abcc6−/− murine models of PXE, which were approximately 30% to 40% of controls.5Jansen R.S. Duijst S. Mahakena S. Sommer D. Szeri F. Varadi A. Plomp A. Bergen A.A. Oude Elferink R.P. Borst P. van de Wetering K. ABCC6-mediated ATP secretion by the liver is the main source of the mineralization inhibitor inorganic pyrophosphate in the systemic circulation-brief report.Arterioscler Thromb Vasc Biol. 2014; 34: 1985-1989Crossref PubMed Scopus (196) Google Scholar,17Li Q. Kingman J. van de Wetering K. Tannouri S. Sundberg J.P. Uitto J. Abcc6 knockout rat model highlights the role of liver in PPi homeostasis in pseudoxanthoma elasticum.J Invest Dermatol. 2017; 137: 1025-1032Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar Extrahepatic cells release ATP independent of ABCC6, explaining why patients with PXE contain low but detectable levels of plasma PPi.ANK was initially suggested to be a PPi efflux transporter.18Williams C.J. The role of ANKH in pathologic mineralization of cartilage.Curr Opin Rheumatol. 2016; 28: 145-151Crossref PubMed Scopus (17) Google Scholar Recently, it has been documented, however, that, like ABCC6, ANK mediates cellular release of ATP, which also contributes to PPi levels in the circulation, and that plasma levels in mice lacking functional ANK are approximately 25% lower than in wild-type controls.19Szeri F. Niaziorimi F. Donnelly S. Fariha N. Tertyshnaia M. Patel D. Lundkvist S. van de Wetering K. The mineralization regulator ANKH mediates cellular efflux of ATP, not pyrophosphate.J Bone Miner Res. 2022; ([Epub ahead of print]. doi:)10.1002/jbmr.4528Crossref PubMed Scopus (6) Google Scholar Together, ABCC6 and ANK are responsible for approximately 90% of all PPi in the circulation, emphasizing the crucial role of both proteins in PPi synthesis. The major function of ANK resides, however, in its ability to provide extracellular ATP for PPi generation in the synovial fluid of joints, intervertebral disks, and articular cartilage, all tissues or body fluids not in direct contact with the blood circulation (Figure 1). In Ank mutant mice, reduced local levels of PPi explain the extensive calcification seen in their cartilage, intervertebral disk, and joint space.12Ho A.M. Johnson M.D. Kingsley D.M. Role of the mouse ank gene in control of tissue calcification and arthritis.Science. 2000; 289: 265-270Crossref PubMed Scopus (541) Google Scholar Intriguingly, ANK also mediates release of large amounts of malate, succinate, and especially citrate.20Szeri F. Lundkvist S. Donnelly S. Engelke U.F.H. Rhee K. Williams C.J. Sundberg J.P. Wevers R.A. Tomlinson R.E. Jansen R.S. van de Wetering K. The membrane protein ANKH is crucial for bone mechanical performance by mediating cellular export of citrate and ATP.PLoS Genet. 2020; 16: e1008884Crossref PubMed Scopus (20) Google Scholar Ank mutant mice have reduced levels of citrate in plasma, urine, and the mineral phase of bone. Because citrate is known to chelate calcium, future work should delineate whether ANK-mediated citrate release contributes to inhibition of pathologic calcification.CD73-TNAP Crosstalk Regulating PPi MetabolismCD73, encoded by NT5E, is a cell surface 5′nucleotidase that catalyzes the breakdown of AMP to adenosine and Pi. The role of CD73 in pathologic calcification was hypothesized to result from adenosine-mediated inhibition of tissue-nonspecific alkaline phosphatase (TNAP), a PPi-degrading enzyme.9St Hilaire C. Ziegler S.G. Markello T.C. Brusco A. Groden C. Gill F. Carlson-Donohoe H. Lederman R.J. Chen M.Y. Yang D. Siegenthaler M.P. Arduino C. Mancini C. Freudenthal B. Stanescu H.C. Zdebik A.A. Chaganti R.K. Nussbaum R.L. Kleta R. Gahl W.A. Boehm M. NT5E mutations and arterial calcifications.N Engl J Med. 2011; 364: 432-442Crossref PubMed Scopus (326) Google Scholar In ACDC, loss of CD73 results in reduced levels of adenosine, which diminishes adenosine-mediated inhibition of TNAP activity, causing greater breakdown of PPi (Figure 1). Therefore, patients with ACDC hydrolyze PPi more rapidly, thereby leading to pathologic calcification. Although plasma PPi concentrations have not been reported in patients with ACDC, this hypothesis was confirmed by approximately 50% reduced plasma PPi concentrations in the Nt5e-/- mice devoid of the CD73 protein.10Li Q. Price T.P. Sundberg J.P. Uitto J. Juxta-articular joint-capsule mineralization in CD73 deficient mice: similarities to patients with NT5E mutations.Cell Cycle. 2014; 13: 2609-2615Crossref PubMed Scopus (36) Google ScholarCD39 and ENPP1 Competition for the Hydrolysis of ATP, the Precursor of PPiCD39, encoded by ENTPD1, is an ectonucleoside triphosphate diphosphohydrolase and a direct competitor of ENPP1 for hydrolysis of the available ATP substrate (Figure 1). In vascular smooth muscle cells and endothelial cells, CD39 is the main enzyme that hydrolyzes ATP to two molecules of the procalcification molecule Pi.21Villa-Bellosta R. Synthesis of extracellular pyrophosphate increases in vascular smooth muscle cells during phosphate-induced calcification.Arterioscler Thromb Vasc Biol. 2018; 38: 2137-2147Crossref PubMed Scopus (35) Google Scholar The LmnaG609G/+ knock-in mouse model of progeria with excessive vascular calcification has increased CD39 and TNAP activities in the aorta, resulting in 88% reduced ATP availability and 92% reduced PPi plasma levels.13Villa-Bellosta R. ATP-based therapy prevents vascular calcification and extends longevity in a mouse model of Hutchinson-Gilford progeria syndrome.Proc Natl Acad Sci U S A. 2019; 116: 23698-23704Crossref PubMed Scopus (27) Google Scholar,22Villa-Bellosta R. Rivera-Torres J. Osorio F.G. Acin-Perez R. Enriquez J.A. Lopez-Otin C. Andres V. Defective extracellular pyrophosphate metabolism promotes vascular calcification in a mouse model of Hutchinson-Gilford progeria syndrome that is ameliorated on pyrophosphate treatment.Circulation. 2013; 127: 2442-2451Crossref PubMed Scopus (153) Google ScholarCollectively, patients and/or murine models with PXE, GACI, ACDC, ankylosis, and progeria have reduced plasma levels of PPi that result in a lowered PPi/Pi ratio, providing a unifying pathophysiologic mechanism for pathologic calcification in these different genetic disorders (Figure 1). Therefore, these diseases are considered as PPi deficiency syndromes. PPi homeostasis is regulated by a balance among the ABCC6, ANK, and ENPP1 proteins, which promote PPi synthesis, and the opposing action of TNAP-catalyzed degradation of PPi (Figure 1). In addition, CD39 competes with ENPP1 for the hydrolysis of ATP, limiting PPi production and favoring synthesis of Pi. Although we do not exclude the possibility that additional mechanisms may explain the pathologic calcification, the varying degree of reduced plasma PPi concentrations correlates, in general, with the severity of the pathologic calcification phenotypes in PXE (60% to 70% reduction), GACI (approximately 100% reduction), ACDC (50% reduction), ankylosis (25% reduction), and progeria (92% reduction). The PPi concentrations reported in different studies, however, vary somewhat, depending on the assay method used.PPi-Targeted Preclinical and Clinical Therapy Developments to Counteract Pathologic CalcificationThe identification of PPi deficiency as a critical molecular mechanism for pathologic tissue calcification suggested that development of therapies aimed at increasing plasma PPi concentrations could prevent the formation of these lesions. To this end, there are a number of preclinical studies that have tested the efficacy of PPi-based therapy developments; a few of these approaches have progressed to human clinical trials (Table 1).23Bartstra J.W. de Jong P.A. Kranenburg G. Wolterink J.M. Isgum I. Wijsman A. Wolf B. den Harder A.M. Mali W. Spiering W. Etidronate halts systemic arterial calcification in pseudoxanthoma elasticum.Atherosclerosis. 2020; 292: 37-41Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar, 24Kranenburg G. de Jong P.A. Bartstra J.W. Lagerweij S.J. Lam M.G. Ossewaarde-van Norel J. Risseeuw S. van Leeuwen R. Imhof S.M. Verhaar H.J. de Vries J.J. Slart R. Luurtsema G. den Harder A.M. Visseren F.L.J. Mali W.P. Spiering W. Etidronate for prevention of ectopic mineralization in patients with pseudoxanthoma elasticum.J Am Coll Cardiol. 2018; 71: 1117-1126Crossref PubMed Scopus (55) Google Scholar, 25Rutsch F. Boyer P. Nitschke Y. Ruf N. Lorenz-Depierieux B. Wittkampf T. Weissen-Plenz G. Fischer R.J. Mughal Z. Gregory J.W. Davies J.H. Loirat C. Strom T.M. Schnabel D. Nurnberg P. Terkeltaub R. Group G.S. Hypophosphatemia, hyperphosphaturia, and bisphosphonate treatment are associated with survival beyond infancy in generalized arterial calcification of infancy.Circ Cardiovasc Genet. 2008; 1: 133-140Crossref PubMed Scopus (152) Google Scholar, 26Gordon L.B. Kleinman M.E. Massaro J. D'Agostino Sr., R.B. Shappell H. Gerhard-Herman M. Smoot L.B. Gordon C.M. Cleveland R.H. Nazarian A. Snyder B.D. Ullrich N.J. Silvera V.M. Liang M.G. Quinn N. Miller D.T. Huh S.Y. Dowton A.A. Littlefield K. Greer M.M. Kieran M.W. Clinical trial of the protein farnesylation inhibitors lonafarnib, pravastatin, and zoledronic acid in children with Hutchinson-Gilford progeria syndrome.Circulation. 2016; 134: 114-125Crossref PubMed Scopus (94) Google ScholarTable 1Published and Ongoing PPi-Based Clinical Trials for PXE, GACI, ACDC, and ProgeriaTarget diseasePPi-based therapiesRationale/targetParticipants, nClinic site/sponsorReference or ClinicalTrials.gov identifierLast updatePXEEtidronateA stable nonhydrolyzable PPi analog74University of Utrecht, the Netherlands23,24–PPiThe molecule that is reduced in PXE99Centre Hospitalier Universitaire de Nice, FranceNCT04868578July 22, 2021LansoprazoleTNAP inhibitor20Fundación Pública Andaluza para la Investigación de Málaga en Biomedicina y Salud, SpainNCT04660461December 9, 2020INZ-701Recombinant ENPP1 enzyme9Inozyme PharmaNCT05030831September 27, 2021GACIBisphosphonatesStable nonhydrolyzable PPi analogs55, 63Miscellaneous7,25–INZ-701Recombinant ENPP1 enzyme9Inozyme PharmaNCT04686175September 23, 2021ACDCEtidronateA stable no-hydrolyzable PPi analog7NHLBINCT01585402August 25, 2021ProgeriaZoledronic acid, pravastatin, and lonafarnibProtein farnesyltransferase inhibit