The Purinergic Nature of Pseudoxanthoma Elasticum

Pseudoxanthoma Elasticum (PXE) is an inherited disease characterized by elastic fibers calcification in the eyes, the skin and the cardiovascular system. PXE results from mutations ABCC6 that encodes an ABC transporter primarily expressed in liver and kidneys. It took nearly 15 years after identifying the gene to better understand the etiology of PXE. the ABCC6 functions facilitates the efflux of ATP (and other nucleotides) which is sequentially hydrolyzed by the ectonucleotidases ENPP1 and CD73 into pyrophosphate (PPi) and adenosine, both inhibitors of calcification. PXE, together with General Arterial Calcification of Infancy (GACI caused by ENPP1 mutations) as well as Calcification of Joints and Arteries (CALJA caused by NT5E/CD73 mutations) form a disease continuum with overlapping phenotypes and sharing steps of the same molecular pathway. The explanation of these phenotypes places ABCC6 as an upstream regulator of a purinergic pathway (ABCC6 → ENPP1 → CD73) that notably inhibits mineralization by maintaining a physiological Pi/PPi ratio in connective tissues. Based on a review of the literature and our recent experimental data, we suggest that PXE (and GACI/CALJA) be considered as an authentic “purinergic disease”. In this article, we recapitulate the pathobiology of PXE, review molecular and physiological data showing that, beyond PPi deficiency and ectopic calcification, PXE is associated with wide and complex alterations of purinergic systems. Finally, we speculate on future prospects regarding purinergic signaling and other aspects of this disease.