ABSTRACT Pseudoxanthoma elasticum (PXE) is an autosomal recessive disorder of ectopic calcification with considerable morbidity and mortality due to deposition of hydroxyapatite crystals in the connective tissues. Though ABCC6 was identified as the causative gene for PXE 20 years ago, the disease mechanism was just recently unveiled and there is currently still no effective or specific treatment for the pathologic calcification. We have previously developed and characterized mouse models for PXE, and these mice provide the platform for preclinical development of therapeutics for this currently intractable condition. A critical pathological characteristic in PXE is the lack of ABCC6 transport activity in the liver resulting in reduction of circulating levels of inorganic pyrophosphate (PPi), a key endogenous inhibitor of calcification. Therefore, the goal of the research we propose herein is to use our mouse models in preclinical studies to develop safe and effective liver-targeted human ABCC6 gene therapy to prevent ectopic calcification in PXE by replacing the mutated defective mouse ABCC6 protein. To deliver the normal copy of the human ABCC6 gene to the liver we plan to use adeno-associated virus serotype 8 (AAV8) which has liver tropism and been used for successful gene therapy of hemophilia. We have identified ABCC6 and TNAP proteins as key regulators of PPi homeostasis. ABCC6 and TNAP have opposing actions in maintaining extracellular PPi concentrations, the former generating PPi via release of the PPi precursor ATP and the latter hydrolyzing PPi. We have generated recombinant AAV vector with human ABCC6 codon-optimized cDNA and our strong preliminary data demonstrate that this AAV vector raised plasma PPi levels in vitro. Based upon these findings and the known functions of ABCC6 and TNAP, we propose that modulation of plasma PPi, either using an AAV8-hABCC6 vector, TNAP inhibitors, or a combination of both approaches, represents an innovative strategy to prevent the ectopic calcification that arises as a consequence of defective ABCC6 transport activity and PPi deficiency. To test this hypothesis, we propose to utilize genetic and pharmacologic approaches to define mechanisms by which inhibition of TNAP elevates PPi plasma levels from AAV-hABCC6 administration, and subsequently prevents and/or diminishes the ectopic calcification in a mouse model of PXE. Our team has the requisite research expertise in the ABCC6-PPi-TNAP axis and appropriate mouse models to complete these studies. We anticipate that the proposed studies will provide critical translational information from preclinical approaches that will allow development of novel treatments for ectopic calcification in patients with PXE. If successful, our findings will advance clinical management of ectopic calcification broadly, as PPi deficiency plays an important role in development of ectopic calcification in other genetic and acquired disorders.