High-dose oral pyrophosphate inhibits connective tissue calcification in Abcc6 null mice but affects bone structure

Abstract

Pseudoxanthoma elasticum is a rare inherited disorder marked by abnormal calcium phosphate deposition in soft connective tissues, particularly the skin, arteries, and eyes. It is caused by inactivating mutations in the ABCC6 gene, which encodes a hepatic efflux transporter. Loss of ABCC6 function leads to reduced plasma levels of pyrophosphate, a key inhibitor of calcification, thereby promoting ectopic mineralization. Oral pyrophosphate therapy has emerged as a potential treatment, but its effectiveness is uncertain. Most ingested pyrophosphate is hydrolyzed in the gut to inorganic phosphate, which may worsen calcification. Moreover, its impact on mineralized tissues remains largely unexplored. Abcc6-/- mice closely mimic human pseudoxanthoma elasticum and are widely used in preclinical studies. Although patients are most concerned about ocular complications, eye calcification is rarely assessed in translational studies using Abcc6-/- mice. Using microcomputed tomography we found that ectopic calcification at the ciliary margin is a reliable marker of ocular disease progression in these mice. Administering pyrophosphate in drinking water at concentrations up to 90 mM did not increase calcification in skin or eyes. However, only very high doses effectively prevented ectopic calcification - doses that would equate to an impractical 2.5 g/kg/day of disodium pyrophosphate in humans. These high doses also led to pyrophosphate accumulation in bone and negatively affected bone structure and strength. In summary, only supraphysiological doses of orally administered pyrophosphate inhibited ectopic calcification in Abcc6-/- mice, but these doses are not feasible for human use and may compromise bone function. These data are especially important considering the currently ongoing clinical trial evaluating the safety and efficacy of oral pyrophosphate administration as a treatment for pseudoxanthoma elasticum. LAY SUMMARYPseudoxanthoma elasticum (PXE) is a rare inherited mineralization disorder caused by the absence of functional ABCC6, a liver-expressed protein. This deficiency leads to reduced plasma levels of pyrophosphate, a key inhibitor of mineralization, resulting in abnormal calcium phosphate deposition in the skin, eyes, and blood vessels. Oral pyrophosphate has been proposed as a therapeutic strategy for PXE, and a clinical trial evaluating its efficacy recently began in France. In a PXE mouse model, we show that only very high oral doses of pyrophosphate are effective, but these doses impair bone quality and are not suitable for human use.