The role of vitamin K and its related compounds in Mendelian and acquired ectopic mineralization disorders

Ectopic mineralization disorders comprise a broad spectrum of inherited or acquired diseases characterized by aberrant deposition of calcium crystals in multiple organs, such as the skin, eyes, kidneys, and blood vessels. Although the precise mechanisms leading to ectopic calcification are still incompletely known to date, various molecular targets leading to a disturbed balance between pro- and anti-mineralizing pathways have been identified in recent years. Vitamin K and its related compounds, mainly those post-translationally activated by vitamin K-dependent carboxylation, may play an important role in the pathogenesis of ectopic mineralization as has been demonstrated in studies on rare Mendelian diseases, but also on highly prevalent disorders, like vascular calcification. This narrative review compiles and summarizes the current knowledge regarding the role of vitamin K, its metabolism, and associated compounds in the pathophysiology of both monogenic ectopic mineralization disorders, like pseudoxanthoma elasticum or Keutel syndrome, as well as acquired multifactorial diseases, like chronic kidney disease. Clinical and molecular aspects of the various disorders are discussed according to the state-of-the-art, followed by a comprehensive literature review regarding the role of vitamin K in molecular pathophysiology and as a therapeutic target in both human and animal models of ectopic mineralization disorders

Pseudoxanthoma elasticum (PXE) patients and heterozygous carriers have decreased plasma PPi levels which correlate with the (cardio)vascular phenotype.

Introduction Pseudoxanthoma elasticum (PXE) is an autosomal recessive soft tissue calcification disorder caused by biallelic mutations in ABCC6. In patients decreased levels of inorganic pyrophosphate (PPi) have been linked to the formation of ectopic mineralizations. However, no data has been reported on PPi levels in heterozygous carriers. Moreover, it is unknown whether a correlation exists between PPi levels and the ABCC6 genotype or the phenotype of patients. Methods A total of 127 citrated blood samples (91 from 62 PXE patients, 22 from 21 carriers and 14 from 14 controls) was collected and processed. Plasma PPi levels were measured via ATPase luminescence assays and samples were internally calibrated. The ABCC6 gene was screened for variants. The diagnosis of PXE was confirmed by the presence of 2 (likely) pathogenic variants (scored as C3 to C5 according to Verschuere et al. 2020) and/or histopathological changes. PXE phenotypes were assessed using the Phenodex scoring (PS) system at the time the first blood sample was obtained for a patient. Two sets of data were analyzed using SPSS26. First an analysis was performed where all available samples were considered [Bulk]. Concomitantly, a smaller subset (i.e. the samples obtained at Phenodex scoring [Single]) was analyzed. Differences in age, sex and PPi levels between the 3 cohorts were analyzed via ANOVA and T-Test. Correlations between PPi, PS, age and sex were analyzed via Spearman’s correlation analysis. After samples were subdivided based on 3 mutation archetypes: erroneous mRNA transcript (D: deletion, frameshift, splicing), nonsense (N) and missense (M) variants, genotype-PPi correlations were assessed via multiple ANOVA analyses based on more stringent variant selection (C3 - C5, C3 [Subset Likely Pathogenic] - C5, C4 - C5, C5). Results Compared to controls, patients and carriers had respectively ±49% and ±22% less PPi (P<0.001), though overlap in the sample ranges exists between the groups. Interestingly, PPi levels were lower in males compared to females in PXE patients (Bulk: P=0.011; Single: P=0.071), but not in the other cohorts. No significant differences in age between cohorts were observed; however, in the patient cohort PPi levels increased significantly with age (P=0.0015, r=0.279). Moreover, a significant inverse correlation between PPi levels and cardiac PS scores was identified (P=0.041, r=-0.263) as well as a weak inverse association between PPi and vascular PS (P=0.09). No correlation was found with skin or eye symptoms. A significant difference in PPi levels between D+M and N+M clusters was found during bulk, but not single, analysis (P<0.01). Conclusion PPi plasma levels are significantly decreased in patients and carriers and may thus also contribute to the subclinical or partial phenotype seen in the latter. Importantly, there is a region of overlap between both and with controls, suggesting PPi not to be the only relevant pro-mineralization factor in at least a subset of PXE patients. In PXE patients PPi levels appear to be influenced by sex and - to some extent unexpectedly - significantly increase with age. Whether there is a solid correlation with the ABCC6 genotype remains to be confirmed. Though unrelated to skin and eye disease, the PPi levels are inversely correlated with cardio(vascular) burden in PXE patients, making PPi promising as a cardiovascular biomarker in PXE. Reference Verschuere et al. 2020: PMID 3287393

Various vascular malformations are prevalent in Finnish pseudoxanthoma elasticum (PXE) patients : a national registry study

BACKGROUND: Pseudoxanthoma elasticum (PXE, OMIM# 264800) is an inborn error of metabolism causing ectopic soft tissue calcification due to low plasma pyrophosphate concentration. We aimed to assess the prevalence of PXE in Finland and to characterize the Finnish PXE population. A nationwide registry search was performed to identify patients with ICD-10 code Q82.84. Information was gathered from available medical records which were requisitioned from hospitals and health centers. Misdiagnosed patients and patients with insufficient records were excluded. RESULTS: The prevalence of PXE in Finland was 1:260,000 with equal sex distribution. Patients with high conventional cardiovascular risk had more visual and vascular complications than patients with low risk. Four patients (19%) had at least one vascular malformation. A high proportion (33%) of ABCC6 genotypes were of the common homozygous c.3421C > T, p.Arg1141Ter variant. Nine other homozygous or compound heterozygous allelic variants were found. CONCLUSIONS: The prevalence of diagnosed PXE appears to be lower in Finland than in estimates from other countries. Decreased visual acuity is the most prevalent complication. We suggest that various vascular malformations may be an unrecognized feature of PXE.publishedVersionPeer reviewe

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&gt;A (p.Val787Ile) and c.1171A&gt;G (p.Arg391Gly), with 6.5% and 2% penetrance, respectively. However, when penetrant, the c.1171A&gt;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

Genotype-phenotype correlation in pseudoxanthoma elasticum

Background and aims: Pseudoxanthoma elasticum (PXE) is caused by variants in the ABCC6 gene. It results in calcification in the skin, peripheral arteries and the eyes, but has considerable phenotypic variability. We investigated the association between the ABCC6 genotype and calcification and clinical phenotypes in these different organs. Methods: ABCC6 sequencing was performed in 289 PXE patients. Genotypes were grouped as two truncating, mixed, or two non-truncating variants. Arterial calcification mass was quantified on whole body, low dose CT scans; and peripheral arterial disease was measured with the ankle brachial index after treadmill test. The presence of pseudoxanthoma in the skin was systematically scored. Ophthalmological phenotypes were the length of angioid streaks as a measure of Bruchs membrane calcification, the presence of choroidal neovascularizations, severity of macular atrophy and visual acuity. Regression models were built to test the age and sex adjusted genotype-phenotype association. Results: 158 patients (median age 51 years) had two truncating variants, 96 (median age 54 years) a mixed genotype, 18 (median age 47 years) had two non-truncating variants. The mixed genotype was associated with lower peripheral (13: 0.39, 95%CI:-0.62;-0.17) and total (13: 0.28, 95%CI:-0.47;-0.10) arterial calcification mass scores, and lower prevalence of choroidal neovascularizations (OR: 0.41 95%CI:0.20; 0.83) compared to two truncating variants. No association with pseudoxanthomas was found. Conclusions: PXE patients with a mixed genotype have less severe arterial and ophthalmological phenotypes than patients with two truncating variants in the ABCC6 gene. Research into environmental and genetic modifiers might provide further insights into the unexplained phenotypic variability

Comprehensive validation of a diagnostic strategy for sequencing genes with one or multiple pseudogenes using pseudoxanthoma elasticum as a model

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From membrane to mineralization : the curious case of the ABCC6 transporter

ATP‐binding cassette subfamily C member 6 gene/protein (ABCC6) is an ATP‐dependent transmembrane transporter predominantly expressed in the liver and the kidney. ABCC6 first came to attention in human medicine when it was discovered in 2000 that mutations in its encoding gene, ABCC6, caused the autosomal recessive multisystemic mineralization disease pseudoxanthoma elasticum (PXE). Since then, the physiological and pathological roles of ABCC6 have been the subject of intense research. In the last 20 years, significant findings have clarified ABCC6 structure as well as its physiological role in mineralization homeostasis in humans and animal models. Yet, several facets of ABCC6 biology remain currently incompletely understood, ranging from the precise nature of its substrate(s) to the increasingly complex molecular genetics. Nonetheless, advances in our understanding of pathophysiological mechanisms causing mineralization lead to several treatment options being suggested or already tested in pilot clinical trials for ABCC6 deficiency. This review highlights current knowledge of ABCC6 and the challenges ahead, particularly the attempts to translate basic science into clinical practice

The Role of Vitamin K and Its Related Compounds in Mendelian and Acquired Ectopic Mineralization Disorders

Ectopic mineralization disorders comprise a broad spectrum of inherited or acquired diseases characterized by aberrant deposition of calcium crystals in multiple organs, such as the skin, eyes, kidneys, and blood vessels. Although the precise mechanisms leading to ectopic calcification are still incompletely known to date, various molecular targets leading to a disturbed balance between pro- and anti-mineralizing pathways have been identified in recent years. Vitamin K and its related compounds, mainly those post-translationally activated by vitamin K-dependent carboxylation, may play an important role in the pathogenesis of ectopic mineralization as has been demonstrated in studies on rare Mendelian diseases, but also on highly prevalent disorders, like vascular calcification. This narrative review compiles and summarizes the current knowledge regarding the role of vitamin K, its metabolism, and associated compounds in the pathophysiology of both monogenic ectopic mineralization disorders, like pseudoxanthoma elasticum or Keutel syndrome, as well as acquired multifactorial diseases, like chronic kidney disease. Clinical and molecular aspects of the various disorders are discussed according to the state-of-the-art, followed by a comprehensive literature review regarding the role of vitamin K in molecular pathophysiology and as a therapeutic target in both human and animal models of ectopic mineralization disorders

Pseudoxanthoma elasticum (PXE) patients and heterozygous carriers have decreased plasma PPi levels which correlate with the cardio(vascular) phenotype

Introduction Pseudoxanthoma elasticum (PXE) is an autosomal recessive soft tissue calcification disorder caused by biallelic mutations in ABCC6. In patients decreased levels of inorganic pyrophosphate (PPi) have been linked to the formation of ectopic mineralizations. However, no data has been reported on PPi levels in heterozygous carriers. Moreover, it is unknown whether a correlation exists between PPi levels and the ABCC6 genotype or the phenotype of patients. Methods A total of 127 citrated blood samples (91 from 62 PXE patients, 22 from 21 carriers and 14 from 14 controls) was collected and processed. Plasma PPi levels were measured via ATPase luminescence assays and samples were internally calibrated. The ABCC6 gene was screened for variants. The diagnosis of PXE was confirmed by the presence of 2 (likely) pathogenic variants (scored as C3 to C5 according to Verschuere et al. 2020) and/or histopathological changes. PXE phenotypes were assessed using the Phenodex scoring (PS) system at the time the first blood sample was obtained for a patient. Two sets of data were analyzed using SPSS26. First an analysis was performed where all available samples were considered [Bulk]. Concomitantly, a smaller subset (i.e. the samples obtained at Phenodex scoring [Single]) was analyzed. Differences in age, sex and PPi levels between the 3 cohorts were analyzed via ANOVA and T-Test. Correlations between PPi, PS, age and sex were analyzed via Spearman’s correlation analysis. After samples were subdivided based on 3 mutation archetypes: erroneous mRNA transcript (D: deletion, frameshift, splicing), nonsense (N) and missense (M) variants, genotype-PPi correlations were assessed via multiple ANOVA analyses based on more stringent variant selection (C3 - C5, C3 [Subset Likely Pathogenic] - C5, C4 - C5, C5). Results Compared to controls, patients and carriers had respectively ±49% and ±22% less PPi (P<0.001), though overlap in the sample ranges exists between the groups. Interestingly, PPi levels were lower in males compared to females in PXE patients (Bulk: P=0.011; Single: P=0.071), but not in the other cohorts. No significant differences in age between cohorts were observed; however, in the patient cohort PPi levels increased significantly with age (P=0.0015, r=0.279). Moreover, a significant inverse correlation between PPi levels and cardiac PS scores was identified (P=0.041, r=-0.263) as well as a weak inverse association between PPi and vascular PS (P=0.09). No correlation was found with skin or eye symptoms. A significant difference in PPi levels between D+M and N+M clusters was found during bulk, but not single, analysis (P<0.01). Conclusion PPi plasma levels are significantly decreased in patients and carriers and may thus also contribute to the subclinical or partial phenotype seen in the latter. Importantly, there is a region of overlap between both and with controls, suggesting PPi not to be the only relevant pro-mineralization factor in at least a subset of PXE patients. In PXE patients PPi levels appear to be influenced by sex and - to some extent unexpectedly - significantly increase with age. Whether there is a solid correlation with the ABCC6 genotype remains to be confirmed. Though unrelated to skin and eye disease, the PPi levels are inversely correlated with cardio(vascular) burden in PXE patients, making PPi promising as a cardiovascular biomarker in PXE. Reference Verschuere et al. 2020: PMID 3287393