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

Hexasodium Fytate (SNF472 or CSL525) Inhibits Ectopic Calcification in Various Pseudoxanthoma Elasticum and Calcinosis Cutis Animal Models

Background/Objectives: Ectopic calcification is a pathological condition characterized by the mineralization of soft tissues due to the deposition of calcium phosphate crystals. Hexasodium fytate (CSL525, previously known as SNF472) is a crystallization inhibitor being developed for the treatment of ectopic calcification-related disorders. Our aim was to investigate CSL525 for the treatment of soft-tissue calcification disorders in animal models of pseudoxanthoma elasticum and calcinosis cutis. Methods: In a first study, abcc6-/- zebrafish larvae were exposed to 1 mM CSL525 for 7 days or kept under the same conditions without CSL525, and spinal mineralization was quantified. In a second study, abcc6-/- mice were administered subcutaneously with CSL525 at 15 mg/kg thrice weekly for eight weeks. Vehicle-treated WT (C57BL/6J) and abcc6-/- mice served as controls, and muzzle skin calcification was quantified. In a third study, calcinosis cutis was induced in rats through subcutaneous administration of 0.15 mg FeCl3 at two sites in the thorax. Rats were administered either subcutaneous CSL525 (60 mg/kg) or vehicle (0.9% NaCl), and calcium content was measured in the skin. Results: CSL525 significantly reduced the calcified area (~40%) in abcc6a-/- zebrafish larvae. The abcc6-/- mice receiving CSL525 showed a 57% inhibition of muzzle calcification compared to vehicle-treated abcc6-/- mice. CSL525 inhibited skin calcification development by 60% in the calcinosis cutis rat model. Conclusions: CSL525 may prove beneficial not only in preventing the progression of cardiovascular calcification but also in treating other ectopic calcification conditions, including skin calcification associated with genetic disorders such as PXE.

Metabolites and metabolism in vascular calcification: links between adenosine signaling and the methionine cycle

The global population of individuals with cardiovascular disease is expanding, and a key risk factor for major adverse cardiovascular events is vascular calcification. The pathogenesis of cardiovascular calcification is complex and multifaceted, with external cues driving epigenetic, transcriptional, and metabolic changes that promote vascular calcification. This review provides an overview of some of the lesser understood molecular processes involved in vascular calcification and discusses the links between calcification pathogenesis and aspects of adenosine signaling and the methionine pathway; the latter of which salvages the essential amino acid methionine, but also provides the substrate critical for methylation, a modification that regulates the function and activity of DNA and proteins. We explore the complex and dynamic nature of osteogenic reprogramming underlying intimal atherosclerotic calcification and medial arterial calcification (MAC). Atherosclerotic calcification is more widely studied; however, emerging studies now show that MAC is a significant pathology independent from atherosclerosis. Furthermore, we emphasize metabolite and metabolic-modulating factors that influence vascular calcification pathogenesis. Although the contributions of these mechanisms are more well-define in relation to atherosclerotic intimal calcification, understanding these pathways may provide crucial mechanistic insights into MAC and inform future therapeutic approaches. Herein, we highlight the significance of adenosine and methyltransferase pathways as key regulators of vascular calcification pathogenesis.

Inorganic pyrophosphate plasma levels in patients with GGCX-associated PXE-like phenotypes

Introduction

Pseudoxanthoma elasticum (PXE) is an autosomal recessive ectopic calcification disorder clinically affecting the skin, eyes, and vascular system. Most cases of PXE are caused by inactivating pathogenic variants in the ABCC6 gene encoding a hepatic transmembrane efflux transporter, which facilitates the extracellular release of ATP, the precursor of inorganic pyrophosphate (PPi), a potent endogenous inhibitor of calcification. Pathogenic variants in GGCX, encoding γ-glutamyl carboxylase required for activation of vitamin K-dependent coagulation factors as well as matrix Gla protein (MGP) and Gla-rich protein (GRP), two inhibitors of ectopic calcification, have also been reported to cause cutaneous changes like those seen in PXE. While ectopic calcification in ABCC6 deficiency has been associated with reduced plasma levels of PPi due to loss of ABCC6 transport activity in the liver, plasma PPi levels have not been reported in patients with GGCX-associated phenotypes.

Methods

We analyzed five patients from three unrelated families on their clinical, laboratory, and molecular findings who carry biallelic variants in GGCX and present with phenotypic characteristics associated with PXE. The variants were identified using a next-generation sequencing panel consisting of 29 genes associated with ectopic calcification.

Results and conclusion

This study demonstrates that in addition to ABCC6, GGCX variants can cause the PXE phenotype, expanding PXE and perhaps other heritable ectopic calcification disorders' clinical and genetic heterogeneity. The results also show that the plasma concentrations of PPi in these patients are not reduced compared to healthy control individuals, suggesting that plasma PPi does not govern ectopic calcification in GGCX deficiency.

The pathogenic c.1171A>G (p.Arg391Gly) and c.2359G>A (p.Val787Ile) ABCC6 variants display incomplete penetrance causing pseudoxanthoma elasticum in a subset of individuals

ABCC6 promotes ATP efflux from hepatocytes to bloodstream. ATP is metabolized to pyrophosphate, an inhibitor of ectopic calcification. Pathogenic variants of ABCC6 cause pseudoxanthoma elasticum, a highly variable recessive ectopic calcification disorder. Incomplete penetrance may initiate disease heterogeneity, hence symptoms may not, or differently manifest in carriers. Here, we investigated whether incomplete penetrance is a source of heterogeneity in pseudoxanthoma elasticum. By integrating clinical and genetic data of 589 patients, we created the largest European cohort. Based on allele frequency alterations, we identified two incomplete penetrant pathogenic variants, c.2359G>A (p.Val787Ile) and c.1171A>G (p.Arg391Gly), with 6.5% and 2% penetrance, respectively. However, when penetrant, the c.1171A>G (p.Arg391Gly) manifested a clinically unaltered severity. After applying in silico and in vitro characterization, we suggest that incomplete penetrant variants are only deleterious if a yet unknown interacting partner of ABCC6 is mutated simultaneously. The low penetrance of these variants should be contemplated in genetic counseling.

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.

ENPP1 variants in patients with GACI and PXE expand the clinical and genetic heterogeneity of heritable disorders of ectopic calcification

Pseudoxanthoma elasticum (PXE) and generalized arterial calcification of infancy (GACI) are clinically distinct genetic entities of ectopic calcification associated with differentially reduced circulating levels of inorganic pyrophosphate (PPi), a potent endogenous inhibitor of calcification. Variants in ENPP1, the gene mutated in GACI, have not been associated with classic PXE. Here we report the clinical, laboratory, and molecular evaluations of ten GACI and two PXE patients from five and two unrelated families registered in GACI Global and PXE International databases, respectively. All patients were found to carry biallelic variants in ENPP1. Among ten ENPP1 variants, one homozygous variant demonstrated uniparental disomy inheritance. Functional assessment of five previously unreported ENPP1 variants suggested pathogenicity. The two PXE patients, currently 57 and 27 years of age, had diagnostic features of PXE and had not manifested the GACI phenotype. The similarly reduced PPi plasma concentrations in the PXE and GACI patients in our study correlate poorly with their disease severity. This study demonstrates that in addition to GACI, ENPP1 variants can cause classic PXE, expanding the clinical and genetic heterogeneity of heritable ectopic calcification disorders. Furthermore, the results challenge the current prevailing concept that plasma PPi is the only factor governing the severity of ectopic calcification.

Biallelic inactivating mutations in the ENPP1 gene cause generalized arterial calcification of infancy (GACI), a frequently fatal disease characterized by infantile onset of widespread arterial calcification and/or narrowing of large and medium-sized vessels often resulting in the early demise of affected individuals. Significantly reduced, almost zero plasma levels of a potent and endogenous calcification inhibitor, inorganic pyrophosphate (PPi), is thought to be the underlying cause of vascular calcification in GACI. Mutations in ENPP1 have not been found in patients with pseudoxanthoma elasticum (PXE), another genetic multisystem ectopic calcification disorder caused by mutations in the ABCC6 gene. This study reports that ENPP1 mutations can also cause PXE with more favorable clinical outcomes. In addition, it was previously thought that plasma PPi levels correlate with vascular calcification severity. However, we here show that vascular calcification severity does not correlate with plasma PPi levels. The results suggest that in addition to PPi, the long-believed determinant of ectopic calcification, additional mechanisms may be at play in regulating ectopic calcification.