Summary of the 2014 PXE Research Meeting

By Lorna Krabil, Research Intern, PXE International, and
Christine Vocke, Director of Participant Engagement, PXE International

Research Progress in Pseudoxanthoma Elasticum and Related Ectopic Mineralization Disorders

Qiaoli Li, Tamás Arányi, András Váradi, Sharon F. Terry and Jouni Uitto

This recent article, published by several PXE researchers and PXE International’s CEO, describes two recent research meetings on PXE and other ectopic mineralization disorders. One meeting was held in Bethesda, MD, convened and hosted by PXE international in 2014 and another was co-convened in Budapest, Hungary in 2015 by PXE International and by the Hungarian Institute of Enzymology. Generally, there is a PXE research meeting every year, alternating between the USA and Hungary.

Overview

This paper details some of the recent scientific advances on PXE and other heritable ectopic mineralization disorders. The protein associated with PXE seems to produce inorganic pyrophosphate (PPi) and phosphate (Pi), through the breakdown of ATP, and may also maintain the ratio of these substances. The current hypothesis of interest to PXE is that PPi serves to prevent mineral deposition and Pi acts to encourage it.

Progress in Genetic Knowledge

PXE is caused by one of many mutations to the ABCC6 gene, which cause the gene’s product, a transporter protein, to function badly or not at all. PXE International’s public mutation database has about 300 mutations catalogued, including two that have been determined to be the most common. Recently, researchers genotyped the ABCC6 gene of families in China. This was an extensive project that collected and tested the DNA of affected individuals, and resulted in the identification of previously unknown mutations. This new information indicates the importance of continuing to genotype PXEers from all backgrounds (Jin et al., 2015).

The substance (substrate) ABCC6 moves is known to involve ATP and its release, but the substance’s actual identity remains unknown (Pomozi et al., 2013) (Jansen et al., 2013, 2014).

ABCC6 is also being studied in relation to possible consequences for lipid metabolism, including maintaining cholesterol levels. Recent research suggests loss of functional ABCC6 can result in overproduction of cholesterol. ABCC6’s role in this capacity could explain the risk of atherogenesis (the formation of fatty masses in the arteries) and cardiovascular issues in persons with PXE (Kuzaj et al., 2014).

The variation in symptoms and severity of PXE, even between siblings or twins, can be explained by modifying factors, both heritable and environmental. Heritable factors include modifying genes and epigenetic components. Epigenetic factors can be passed down to children but are not encoded in DNA. Changes in one’s epigenetics change how one’s genes can be read, and include rearrangements of proteins that can silence some genes. A person’s epigenetics are affected by one’s environment and the conditions of one’s body. For example, chronic stress can have a measurable effect on health and progression of disease. Environmental factors refer to changes in diet or lifestyle that can affect the severity of the symptoms.

Attempts are now being made to understand the differences between cells with the PXE gene knocked out (a cell in which the function of both copies of the ABCC6 gene has been removed)and those that have a functional gene. The comparative study of these cells may result in the identification of some of these additional factors (Dabisch-Ruthe et al., 2014).

Determining the Unifying Molecular Mechanism of Disease

The researchers who wrote this paper have shed some light on the function of ABCC6 using cells in the lab, called plated cells. Though the exact substance acted on by the protein remains unknown, ABCC6 plays a role in releasing ATP, which is rapidly converted to AMP and PPi. The study of similar mineralization disorders, such as GACI (generalized arterial calcification of infancy,) and ACDC (arterial calcification due to CD57 deficiency) revealed mutations in the ABCC6 gene as well as the ENPP1 gene, explaining some of the similarity in the symptoms and suggests the existence of a complex cellular balance that can result in mineralization when broken. The genes mutated in GACI, PXE, and ACDC can all be seen to play a role in the process that makes PPi and Pi from ATP.

Researchers think that the absence of a functional copy of ABCC6 results in a change in the availability of ATP, an essential small molecule that acts in nearly every pathway in the body. This change would result in lowered PPi levels; meaning mineralization could not be suppressed. This hypothesis is supported by the low PPi levels in the plasma of people with PXE (Jansen et al., 2013, 2014).

A specific type of cell, mesenchymal cells, which have the potential to become bone, muscle, cartilage, or fat cells, have been shown to play a role in PXE, and are affected by a calcified environment as would be present in PXE. However, their precise role and effect is unclear (Boraldi et al., 2014).

New Animal Models

Animal models are an essential tool in the development of a full understanding of a disease and its progression.

The Uitto lab has developed a rat model through direct deactivation of Abcc6 (Li et al., 2016). The development of a rat model was made possible by recent advances in technology, and is significant for several reasons. The cardiovascular system of rats in particular has a higher degree of similarity to humans than mice and other animals (Iannaccone, and Jacob, 2009). In addition, rat livers are larger and so will be more useful for attempting such things as liver transplant, in an effort to understand the effects of PXE on the liver and the liver’s role in PXE overall.

Developing Novel Treatments

Recent studies relating to diet’s effect on symptoms and the progression of mineralization have shown that an “acceleration diet” with high phosphorous content and low magnesium content, has an effect not only on mice themselves but also any offspring borne by pregnant mice. The diet was shown to increase the level of mineralization in the pups. In light of this and other observations, a two-year double blind clinical trial of MgO as a treatment was completed in March of 2015, the results of which have yet to be published.

A potential avenue for treatment involves the administration of PPi which is not available in a stable form, or a known molecule in the bisphosphonate family, that hopefully would mimic PPi. There are serious concerns about the potentially toxic nature of this molecule (Otero et al., 2013). The hope is that providing the body with this would counteract both mineralization and bone resorption. This strategy has shown some promise in mice, but is not ready for clinical application in humans.

Another strategy could be a cellular intervention that would result in the production of functional ABCC6. The use of an FDA approved drug such as PTC124 could make the cell ignore the mutation. PTC124 was tested on cultured cells with varying mutations, and produced significant expression of ABCC6 protein (Zhou et al., 2013). This suggests a possible therapy for individuals with specific mutations in ABCC6 that result in proteins that are not completely formed. This accounts for about 35% of the mutations resulting in PXE. However, the potential therapy must be tested for effectiveness in animal models.

For a different type of mutation, the use of a chaperone ( a small helper molecule) may be effective in putting the ABCC6 protein in the correct locations in the body. A molecule called 4-PBA was tested for this purpose in mice, and may prove an effective treatment strategy for those with specific PXE-causing mutations to ABCC6 (Pomozi et al., 2014).

These meetings helped to explain the consequences of the mutations in PXE and the possibility of therapies in the future. PXE International continues to strive to fund and support good research such as this, and hopes we can move more quickly toward treatments in the coming years.

The complex promineralization/antimineralization network of genetic conditions.
Mutations in specific genes can result in mineralization (deposition of hydroxyapatite crystals) in soft tissues in abnormal mineralization disorders such as PXE, GACI and ACDC. Solid blue circles represent molecules that are currently unidentified and are thought to be transported from inside the cell to outside the cell when ABCC6 is present and functioning.

Boraldi, F., Annovi, G., Bartolomeo, A., Quaglino, D. Fibroblasts from patients affected by Pseudoxanthoma elasticum exhibit an altered PPi metabolism and are more responsive to pro-calcifying stimuli. J Dermatol Sci. 2014;74:72–80.

Dabisch-Ruthe, M., Brock, A., Kuzaj, P., Charbel Issa, P., Szliska, C., Knabbe, C. et al, Variants in genes encoding pyrophosphate metabolizing enzymes are associated with Pseudoxanthoma elasticum. Clin Biochem. 2014;47:60–67.

Iannaccone, P. M. and H. J. Jacob. "Rats!". Disease Models & Mechanisms 2.5-6 (2009): 206-210. Web.

Jansen, R.S., Duijst, S., Mahakena, S., Sommer, D., Szeri, F., Varadi, A. et al, 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;34:1985–1989.

Jansen, R.S., Kucukosmanoglu, A., de Haas, M., Sapthu, S., Otero, J.A., Hegman, I.E. et al, ABCC6 prevents ectopic mineralization seen in pseudoxanthoma elasticum by inducing cellular nucleotide release. Proc Nat Acad Sci U S A. 2013;110:20206–20211.

Jin, L., Jiang, Q., Wu, Z., Shao, C., Zhou, Y., Yang, L. et al, Genetic heterogeneity of pseudoxanthoma elasticum: the Chinese signature profile of ABCC6 and ENPP1 mutations. J Invest Dermatol. 2015;135:1294–1302.

Kuzaj, P., Kuhn, J., Dabisch-Ruthe, M., Faust, I., Gotting, C., Knabbe, C. et al, ABCC6: a new player in cellular cholesterol and lipoprotein metabolism?. Lipids Health Dis. 2014;13:118.

Le Saux, O., Fülop, K., Yamaguchi, Y., Ilias, A., Szabo, Z., Brampton, C.N. et al, Expression and in vivo rescue of human ABCC6 disease-causing mutants in mouse liver. PLoS One. 2011;6:e24738.

Li, Q. et al. "388 A Novel Abcc6 Knockout Rat Model Highlights The Role Of Ppi In The Pathogenesis Of Ectopic Mineralization Process In PXE". Journal of Investigative Dermatology 136.5 (2016): S69. Web.

Otero, J.E., Gottesman, G.S., McAlister, W.H., Mumm, S., Madson, K.L., Kiffer-Moreira, T. et al, Severe skeletal toxicity from protracted etidronate therapy for generalized arterial calcification of infancy. J Bone Miner Res. 2013;28:419–430.

Pomozi, V., Brampton, C., Fülop, K., Chen, L.H., Apana, A., Li, Q. et al, Analysis of pseudoxanthoma elasticum-causing missense mutants of ABCC6 in vivo; pharmacological correction of the mislocalized proteins. J Invest Dermatol. 2014;134:946–953.

Pomozi, V., LeSaux, O., Brampton, C., Apana, A., Ilias, A., Szeri, F. et al, ABCC6 is a basolateral plasma membrane protein. Circ Res. 2013;112:e148–e151.

Zhou, Y., Jiang, Q., Takahagi, S., Shao, C., Uitto, J. Premature termination codon read-through in the ABCC6 gene: potential treatment for pseudoxanthoma elasticum. J Invest Dermatol. 2013;133:2672–2677.