DIFFERENTIALLY EXPRESSED PROTEINS IN FIBROBLASTS FROM Abcc6-/- AND Abcc6+/+ MICE HIGHLIGHT THE ROLE OF A LOCALLY ALTERED PHOSPHATE METABOLISM IN THE OCCURRENCE OF ECTOPIC CALCIFICATION

Progressive calcification of elastic fibers is typical of Pseudoxanthoma elasticum (PXE), a rare genetic disease due to mutations in the ABCC6 gene.The pathogenesis of mineral- ization is only partially known.1 Previous studies on dermal fibroblasts from PXE patients demonstrated that the calcifica- tion process is associated to impaired carboxylation of matrix- gla-protein that, in its active form, binds to calcium, therefore inhibiting mineralization.2 However, the recent observation that PXE fibroblasts exhibit also a significant upregulation of alka- line phosphatase (TNAP) activity suggested that an abnormal phosphate metabolism may take place within soft connective tis- sues, thus contributing to ectopic calcification.3 To improve the understanding of PXE it was developed a transgenic mouse model by specifically inactivating the Abcc6 gene.4 Consistently, Abcc6-/- mice recapitulate several histopathological findings typ- ical of PXE, including the slow progression of the disease.5 Aim of the present study was to isolate fibroblasts from Abcc6+/+ and Abcc6-/- mice of different ages (i.e. before and after the devel- opment of ectopic calcification) and to investigate proteins con- trolling phosphate levels in the extracellular matrix. Results demonstrate a down-regulation of pyrophosphatase/phosphodi- esterase 1, of progressive ankylosis protein and of osteopontin, whereas bone morphogenetic protein 2 and TNAP activity were up-regulated in fibroblasts from Abcc6-/- animals. These data support the hypothesis that in PXE the unbalanced ratio between factors locally controlling both calcium and phosphate homeostasis are crucial in triggering tissue calcification.