Inhibition of Tissue-Nonspecific Alkaline Phosphatase Attenuates Ectopic Mineralization in the Abcc6 -/- Mouse Model of PXE but Not in the Enpp1 Mutant Mouse Models of GACI.

Pseudoxanthoma elasticum (PXE), a prototype of heritable ectopic mineralization disorders, is caused by mutations in the ABCC6 gene encoding a putative efflux transporter ABCC6. It was recently demonstrated that the absence of ABCC6-mediated ATP release from the liver, and consequently reduced PPi levels, underlie the pathogenesis of PXE. Given that tissue-nonspecific alkaline phosphatase (TNAP), encoded by ALPL, is the enzyme responsible for degrading PPi, we hypothesized that reducing TNAP levels either by genetic or pharmacological means would lead to amelioration of the ectopic mineralization phenotype in the Abcc6 mouse model of PXE. Thus, we bred Abcc6 mice to heterozygous Alpl mice that display approximately 50% plasma TNAP activity. The Abcc6Alpl double mutant mice showed 52% reduction of mineralization in the muzzle skin compared to the Abcc6Alpl mice. Subsequently, oral administration of SBI-425, a small molecule inhibitor of TNAP, resulted in 61% reduction of plasma TNAP activity and 58% reduction of mineralization in the muzzle skin of Abcc6 mice. By contrast, SBI-425 treatment of Enpp1 mutant mice, another model of ectopic mineralization associated with reduced PPi, failed to reduce muzzle skin mineralization. These results suggest that inhibition of TNAP might provide a promising treatment strategy for PXE, a currently intractable disease.