501 Plasma PPi is the major, but not exclusive, cause of ectopic mineralization in pseudoxanthoma elasticum

Abstract

Pseudoxanthoma elasticum (PXE), a prototype of heritable ectopic mineralization disorders, affects the skin, eyes and the arterial blood vessels. PXE is caused by inactivating mutations in the ABCC6 gene and the disease is characterized by calcium hydroxyapatite deposition on connective tissues. It was recently discovered that absence of ABCC6-mediated ATP release from the liver and consequently reduced plasma PPi levels underlie PXE. In this study, we examined whether elevated circulating levels of PPi, a powerful mineralization inhibitor, is the sole mechanism by which ABCC6 counteracts ectopic mineralization in PXE. Abcc6-/- mice, a model of PXE, were crossed with transgenic mice with ubiquitous expression of human ENPP1, an ectonucleotidase which generates PPi from ATP. We generated Abcc6-/-mice either wild type or hemizygous for human ENPP1. The Enpp1asj mice, a model for generalized arterial calcification of infancy caused by ENPP1 mutations, were also crossed with human ENPP1 transgenic mice to compensate for the loss of the endogenous mouse protein. Plasma levels of PPi and the degree of mineralization of connective tissue capsule of vibrissae, a reliable biomarker of the mineralization process in these mice, were determined. Overexpression of human ENPP1 in Enpp1asj mice normalized plasma PPi levels to that of wild type mice, and consequently, completely prevented mineralization. In contrast, significantly reduced mineralization was noted in Abcc6-/- mice with overexpression of human ENPP1, however, with small mineralization foci still evident despite increased plasma PPi levels. These results suggest that PPi is the major mediator of connective tissue mineralization in PXE, but there is a second, as-yet unknown, alternative mechanism other than PPi by which ABCC6 prevents ectopic mineralization under physiologic conditions.