Pseudoxanthoma elasticumban szenvedő betegek multidiszciplináris ellátása

Összefoglaló. A pseudoxanthoma elasticum (PXE, OMIM # 264800) egy autoszomális recesszív módon öröklődő multiszisztémás érintettséggel járó kórkép, melynek háttérében az ABCC6 gén mutációi állnak. A tünetek kialakulásának oka az ektópiás mineralizáció. Kalcium-só kristályok rakódnak le elsősorban a bőrben, a szem Bruch-membránjában és az erek endotheliumában, így a bőrelváltozások mellett a látás csökkenése és cardiovascularis eltérések is jelentkezhetnek. A klinikai tünetek változó súlyosságúak lehetnek, heterogén megjelenésűek. A betegek fenotípusának azonosítása, valamint gondozása multidiszciplináris feladat, bőrgyógyász, szemész, kardiológus és klinikai genetikus együttműködésén alapul. Célunk, hogy bemutassuk a betegségben előforduló tüneteket, melyek ismerete megkönnyíti a kórkép felismerését, illetve hogy felhívjuk a figyelmet a korai diagnózis fontosságára és ismertessük a korszerű diagnosztikai módszereket. A súlyos szisztémás tünetek kialakulása miatt rendkívüli jelentőséggel bír a társszakmák együttműködése, hogy a korai diagnózis által időben megfelelő gondozásban és terápiában részesülhessenek a betegek. Orv Hetil. 2022; 163(18): 702–711.

Summary. Pseudoxanthoma elasticum (PXE, OMIM # 264800) is an autosomal recessive, multisystemic disorder, associated with mutations of the ABCC6 gene. Ectopic mineralization is in the background of the clinical manifestations of the disease. Calcium-salt crystals are deposited primarily in the skin, in the Bruch membrane of the eyes, and in the vascular endothelium. Thus, in addition to the skin lesions, visual impairment and cardiovascular involvement also occur. Clinical symptoms show varying severity and display heterogeneous appearance. The identification of the phenotype and care of the patients require a multidisciplinary perspective based on the collaboration of a dermatologist, ophthalmologist, cardiologist, and clinical geneticist. The aim of our work is to describe the development of symptoms of the disease, in order to facilitate the diagnosis. In addition, we aim to draw attention to the importance of early diagnosis of pseudoxanthoma elasticum, and to present modern diagnostic methods. Considering the development of severe systemic complications, the early diagnosis with the collaboration between related specialists is crucial to provide optimal clinical care and management of the patients. Orv Hetil. 2022; 163(18): 702–711.

Impact of copper and iron binding properties on the anticancer activity of 8-hydroxyquinoline derived Mannich bases

The anticancer activity of 8-hydroxyquinolines relies on complex formation with redox active copper and iron ions. Here we employ UV-visible spectrophotometry and EPR spectroscopy to compare proton dissociation and complex formation processes of the reference compound 8-hydroxyquinoline (Q-1) and three related Mannich bases to reveal possible correlations with biological activity. The studied derivatives harbor a CH2-N moiety at position 7 linked to morpholine (Q-2), piperidine (Q-3), and chlorine and fluorobenzylamino (Q-4) substituents. Solid phase structures of Q-3, Q-4·HCl·H2O, [(Cu(HQ-2)2)2]·(CH3OH)2·Cl4·(H2O)2, [Cu(Q-3)2]·Cl2 and [Cu(HQ-4)2(CH3OH)]·ZnCl4·CH3OH were characterized by single-crystal X-ray diffraction analysis. In addition, the redox properties of the copper and iron complexes were studied by cyclic voltammetry, and the direct reaction with physiologically relevant reductants (glutathione and ascorbic acid) was monitored. In vitro cytotoxicity studies conducted with the human uterine sarcoma MES-SA/Dx5 cell line reveal the significant cytotoxicity of Q-2, Q-3, and Q-4 in the sub- to low micromolar range (IC50 values 0.2-3.3 μM). Correlation analysis of the anticancer activity and the metal binding properties of the compound series indicates that, at physiological pH, weaker copper(ii) and iron(iii) binding results in elevated toxicity (e.g.Q4: pCu = 13.0, pFe = 6.8, IC50 = 0.2 μM vs.Q1: pCu = 15.1, pFe = 13.0 IC50 = 2.5 μM). Although the studied 8-hydroxyquinolines preferentially bind copper(ii) over iron(iii), the cyclic voltammetry data revealed that the more cytotoxic ligands preferentially stabilize the lower oxidation state of the metal ions. A linear relationship between the pKa (OH) and IC50 values of the studied 8-hydroxyquinolines was found. In summary, we identify Q-4 as a potent and selective anticancer candidate with significant toxicity in drug resistant cells.

In vitro transport of methotrexate by Drosophila Multidrug Resistance-associated Protein

Methotrexate (MTX) is a widely used chemotherapeutic agent, immune suppressant and antimalarial drug. It is a substrate of several human ABC proteins that confer multidrug resistance to cancer cells and determine compartmentalization of a wide range of physiological metabolites and endo or xenobiotics, by their primary active transport across biological membranes. The substrate specificity and tissue distribution of these promiscuous human ABC transporters show a high degree of redundancy, providing robustness to these key physiological and pharmacological processes, such as the elimination of toxins, e.g. methotrexate from the body. A similar network of proteins capable of transporting methotrexate has been recently suggested to exist in Drosophila melanogaster. One of the key players of this putative network is Drosophila Multidrug-resistance Associated Protein (DMRP). DMRP has been shown to be a highly active and promiscuous ABC transporter, capable of transporting various organic anions. Here we provide the first direct evidence that DMRP, expressed alone in a heterologous system lacking other, potentially functionally overlapping D. melanogaster organic anion transporters, is indeed able to transport methotrexate. Our in vitro results support the hypothesized but debated role of DMRP in in vivo methotrexate excretion.

Oral administration of pyrophosphate inhibits connective tissue calcification

Various disorders including pseudoxanthoma elasticum (PXE) and generalized arterial calcification of infancy (GACI), which are caused by inactivating mutations in ABCC6 and ENPP1, respectively, present with extensive tissue calcification due to reduced plasma pyrophosphate (PPi). However, it has always been assumed that the bioavailability of orally administered PPi is negligible. Here, we demonstrate increased PPi concentration in the circulation of humans after oral PPi administration. Furthermore, in mouse models of PXE and GACI, oral PPi provided via drinking water attenuated their ectopic calcification phenotype. Noticeably, provision of drinking water with 0.3 mM PPi to mice heterozygous for inactivating mutations in Enpp1 during pregnancy robustly inhibited ectopic calcification in their Enpp1^−/− offspring. Our work shows that orally administered PPi is readily absorbed in humans and mice and inhibits connective tissue calcification in mouse models of PXE and GACI. PPi, which is recognized as safe by the FDA, therefore not only has great potential as an effective and extremely low‐cost treatment for these currently intractable genetic disorders, but also in other conditions involving connective tissue calcification.

Inhibition of DNA methyltransferase leads to increased genomic 5-hydroxymethylcytosine levels in hematopoietic cells

5‐Hydroxymethylcytosine (5hmC) is produced from 5‐methylcytosine (5mC) by Ten‐eleven translocation (TET) dioxygenases. The epigenetic modification 5hmC has crucial roles in both cellular development and differentiation. The 5hmC level is particularly high in the brain. While 5mC is generally associated with gene silencing/reduced expression, 5hmC is a more permissive epigenetic mark. To understand its physiological function, an easy and accurate quantification method is required. Here, we have developed a novel LC‐MS/MS‐based approach to quantify both genomic 5mC and 5hmC contents. The method is based on the liberation of nucleobases by formic acid. Applying this method, we characterized the levels of DNA methylation and hydroxymethylation in mouse brain and liver, primary hepatocytes, and various cell lines. Using this approach, we confirm that the treatment of different cell lines with the DNA methyltransferase inhibitor 5‐aza‐2′‐deoxycytidine leads to a decrease in 5mC content. This decrease was accompanied by an increase in 5hmC levels in cell lines of hematopoietic origin. Finally, we showed that ascorbate elevates the levels of 5hmC and augments the effect of 5‐aza‐2′‐deoxycytidine without significantly influencing 5mC levels.

Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter

ATP binding cassette type-C (ABCC) transporters move molecules across cell membranes upon hydrolysis of ATP; however, their coupling of ATP hydrolysis to substrate transport remains elusive. Drosophila multidrug resistance-associated protein (DMRP) is the functional ortholog of human long ABCC transporters, with similar substrate and inhibitor specificity, but higher activity. Exploiting its high activity, we kinetically dissected the catalytic mechanism of DMRP by using E₂-d-glucuronide (E₂G), the physiologic substrate of human ABCC. We examined the DMRP-mediated interdependence of ATP and E₂G in biochemical assays. We observed E₂G-dependent ATPase activity to be biphasic at subsaturating ATP concentrations, which implies at least 2 E₂G binding sites on DMRP. Furthermore, transport measurements indicated strong nonreciprocal cooperativity between ATP and E₂G. In addition to confirming these findings, our kinetic modeling with the Complex Pathway Simulator indicated a 10-fold decrease in the E₂G-mediated activation of ATP hydrolysis upon saturation of the second E₂G binding site. Surprisingly, the binding of the second E₂G allowed for substrate transport with a constant rate, which tightly coupled ATP hydrolysis to transport. In summary, we show that the second E₂G binding-similar to human ABCC2-allosterically stimulates transport activity of DMRP. Our data suggest that this is achieved by a significant increase in the coupling of ATP hydrolysis to transport.-Karasik, A., Ledwitch, K. V., Arányi, T., Váradi, A., Roberts, A., Szeri, F. Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter.

Functional Rescue of ABCC6 Deficiency by 4-Phenylbutyrate Therapy Reduces Dystrophic Calcification in Abcc6-/- Mice

Soft-tissue calcification is associated with aging, common conditions such as diabetes or hypercholesterolemia, and with certain genetic disorders. ABCC6 is an efflux transporter primarily expressed in liver facilitating the release of adenosine triphosphate from hepatocytes. Within the liver vasculature, adenosine triphosphate is converted into pyrophosphate, a major inhibitor of ectopic calcification. ABCC6 mutations thus lead to reduced plasma pyrophosphate levels, resulting in the calcification disorder pseudoxanthoma elasticum and some cases of generalized arterial calcification of infancy. Most mutations in ABCC6 are missense, and many preserve transport activity but are retained intracellularly. We have previously shown that the chemical chaperone 4-phenylbutyrate (4-PBA) promotes the maturation of ABCC6 mutants to the plasma membrane. In a humanized mouse model of pseudoxanthoma elasticum, we investigated whether 4-PBA treatments could rescue the calcification inhibition potential of selected ABCC6 mutants. We used the dystrophic cardiac calcification phenotype of Abcc6^-/- mice as an indicator of ABCC6 function to quantify the effect of 4-PBA on human ABCC6 mutants transiently expressed in the liver. We showed that 4-PBA administrations restored the physiological function of ABCC6 mutants, resulting in enhanced calcification inhibition. This study identifies 4-PBA treatment as a promising strategy for allele-specific therapy of ABCC6-associated calcification disorders.

ABCC6-mediated ATP secretion by the liver is the main source of the mineralization inhibitor inorganic pyrophosphate in the systemic circulation-brief report

OBJECTIVE

Mutations in ABCC6 underlie the ectopic mineralization disorder pseudoxanthoma elasticum (PXE) and some forms of generalized arterial calcification of infancy, both of which affect the cardiovascular system. Using cultured cells, we recently showed that ATP-binding cassette subfamily C member 6 (ABCC6) mediates the cellular release of ATP, which is extracellularly rapidly converted into AMP and the mineralization inhibitor inorganic pyrophosphate (PPi). The current study was performed to determine which tissues release ATP in an ABCC6-dependent manner in vivo, where released ATP is converted into AMP and PPi, and whether human PXE ptients have low plasma PPi concentrations.

APPROACH AND RESULTS

Using cultured primary hepatocytes and in vivo liver perfusion experiments, we found that ABCC6 mediates the direct, sinusoidal, release of ATP from the liver. Outside hepatocytes, but still within the liver vasculature, released ATP is converted into AMP and PPi. The absence of functional ABCC6 in patients with PXE leads to strongly reduced plasma PPi concentrations.

CONCLUSIONS

Hepatic ABCC6-mediated ATP release is the main source of circulating PPi, revealing an unanticipated role of the liver in systemic PPi homeostasis. Patients with PXE have a strongly reduced plasma PPi level, explaining their mineralization disorder. Our results indicate that systemic PPi is relatively stable and that PXE, generalized arterial calcification of infancy, and other ectopic mineralization disorders could be treated with PPi supplementation therapy.

ABCC6 is a basolateral plasma membrane protein

RATIONALE

ABCC6 plays a crucial role in ectopic calcification; mutations of the gene cause pseudoxanthoma elasticum and general arterial calcification of infancy. To elucidate the role of ABCC6 in cellular physiology and disease, it is crucial to establish the exact subcellular localization of the native ABCC6 protein.

OBJECTIVE

In a recent article in Circulation Research, ABCC6 was reported to localize to the mitochondria-associated membrane and not the plasma membrane. As the suggested mitochondrial localization is inconsistent with published data and the presumed role of ABCC6, we performed experiments to determine the cellular localization of ABCC6 in its physiological environment.

METHODS AND RESULTS

We performed immunofluorescent labeling of frozen mouse and human liver sections, as well as primary hepatocytes. We used several different antibodies recognizing human and mouse ABCC6. Our results unequivocally show that ABCC6 is in the basolateral membrane of hepatocytes and is not associated with the mitochondria, mitochondria-associated membrane, or the endoplasmic reticulum.

CONCLUSIONS

Our findings support the model that ABCC6 is in the basolateral membrane, mediating the sinusoidal efflux of a metabolite from the hepatocytes to systemic circulation.

The high turnover Drosophila multidrug resistance-associated protein shares the biochemical features of its human orthologues

DMRP, an ABC transporter encoded by the dMRP/CG6214 gene, is the Drosophila melanogaster orthologue of the "long" human multidrug resistance-associated proteins (MRP1/ABCC1, MRP2/ABCC2, MRP3/ABCC3, MRP6/ABCC6, and MRP7/ABCC10). In order to provide a detailed biochemical characterisation we expressed DMRP in Sf9 insect cell membranes. We demonstrated DMRP as a functional orthologue of its human counterparts capable of transporting several human MRP substrates like beta-estradiol 17-beta-D-glucuronide, leukotriene C4, calcein, fluo3 and carboxydichlorofluorescein. Unexpectedly, we found DMRP to exhibit an extremely high turnover rate for the substrate transport as compared to its human orthologues. Furthermore, DMRP showed remarkably high basal ATPase activity (68-75 nmol Pi/mg membrane protein/min), which could be further stimulated by probenecid and the glutathione conjugate of N-ethylmaleimide. Surprisingly, this high level basal ATPase activity was inhibited by the transported substrates. We discussed this phenomenon in the light of a potential endogenous substrate (or activator) present in the Sf9 membrane.

The dMRP/CG6214 gene of Drosophila is evolutionarily and functionally related to the human multidrug resistance-associated protein family

ATP-binding cassette (ABC) transporters are involved in the transport of substrates across biological membranes and are essential for many cellular processes. Of the fifty-six Drosophila ABC transporter genes only white, brown, scarlet, E23 and Atet have been studied in detail. Phylogenetic analyses identify the Drosophila gene dMRP/CG6214 as an orthologue to the human multidrug-resistance associated proteins MRP1, MRP2, MRP3 and MRP6. To study evolutionarily conserved roles of MRPs we have initiated a characterization of dMRP. In situ hybridization and Northern analysis indicate that dMRP is expressed throughout development and appears to be head enriched in adults. Functional studies indicate that DMRP is capable of transporting a known MRP1 substrate and establishes DMRP as a high capacity ATP-dependent, vanadate-sensitive organic anion transporter.

The role of multidrug transporters in drug availability, metabolism and toxicity

Multidrug resistance is frequently observed when treating cancer patients with chemotherapeutic agents. A variety of ATP binding cassette (ABC) transporters, localized in the cell membrane, cause this phenomenon by extruding a variety of chemotherapeutic agents from the tumor cells. However, the major physiological role of the multidrug transporters is the protection of our cells and tissues against xenobiotics, and these transporters play a key role in drug availability, metabolism and toxicity. Three major groups of ABC transporters are involved in multidrug resistance: the classical P-glycoprotein MDR1, the multidrug resistance associated proteins (MRP1, MRP2, and probably MRP3, MRP4 and MRP5), and the ABCG2 protein, an ABC half-transporter. All these proteins were shown to catalyze an ATP-dependent active transport of chemically unrelated compounds. MDR1 (P-glycoprotein) and ABCG2 preferentially extrude large hydrophobic, positively charged molecules, while the members of the MRP family can extrude both hydrophobic uncharged molecules and water-soluble anionic compounds. By examining the interactions of the multidrug transporters with pharmacological and toxic agents, a prediction for the cellular and tissue distribution of these compounds can be achieved. Oral bioavailability, entering the blood-brain and blood-CSF barrier, reaching the fetus through the placenta, liver and kidney secretion, cellular entry for affecting intracellular targets, are all questions, which can be addressed by basic in vitro studies on the multidrug resistance proteins. Investigation of the substrate interactions and modulation of multidrug transporters may pave the way for predictive toxicology and pharmacogenomics. Here we show that by using in vitro assay systems it is possible to measure the interactions of multidrug transporters with various drugs and toxic agents. We focus on the characterisation of the MRP1 and MRP3 proteins, their relevance in chemoresistance of cancer and in drug metabolism and toxicity.

Molecular genetic diagnostic program of multiple endocrine neoplasia type 2A and familial medullary thyroid carcinoma syndromes in Hungary

Medullary thyroid carcinoma (MTC) occurs usually in sporadic form, but about a quarter of the cases are hereditary and appear as part of one of the multiple endocrine neoplasia type 2 (MEN2) syndromes. Mutations in the RET protooncogene are known to be the cause of the MEN2A and familial medullary thyroid carcinoma (FMTC) syndromes in the majority of the families. Direct DNA testing allows prophylactic thyroidectomy to be offered to individuals carrying a mutation in the above codons, and in mutation-negative cases it reduces the yearly screening-related burden on family members at risk of the disease. By DNA sequencing and PCR-restriction fragment length polymorphisms, 65 MTC probands were examined for mutations in residues 609, 611, 618, 620 of exon 10, and in residues 634, 768, 804 of exons 11, 13, and 14 respectively of the RET protooncogene. In our study, mutations in the above codons were detected in all of the 14 clinically MEN2A and FMTC families. One of these mutations, TGC609 TCC has not been reported previously. Of the 14 probands with the mutation, 25 relatives also had the identified mutation and 18 relatives proved to be non-carriers. Among the 51 probands with clinically sporadic MTC, none was found to carry a mutation in the above positions even if indirect signs of MTC, pheochromocytoma or hyperparathyroidism could be detected in some families. The frequency of the TGC634AGC mutation is unexpectedly high in our samples, which can probably be attributed to a founder effect. We conclude that screening for mutations in these codons is effective in families fulfilling the strict clinical criteria of MEN2A or FMTC.