Abcc6 deficiency in the mouse leads to calcification of collagen fibers in Bruch's membrane

Abstract

Pseudoxanthoma elasticum (PXE) is a heritable disorder characterized by mineralization of connective tissue, which leads to pathology in eye, skin and blood vessels. The disease is caused by mutations in ABCC6. To learn more about PXE eye pathology, we analyzed Bruch's membrane (BM) of the eye of an Abcc6 knockout mouse. With age, BM differences between Abcc6−/− and wild type mice became apparent. At two years of age, von Kossa staining indicated clear calcification of BM in Abcc6−/− mice, and not in healthy controls. Electron microscopy revealed BM changes as early as at 10 months of age: Fibrous structures with abnormal high electron-density were present in the central layers of BM of Abcc6−/− mice. EDX (Energy Dispersive X-ray) analysis demonstrated that these structures contained elevated levels of Ca, P and O. Since some of these electron-dense structures showed a banding pattern with periodicity of about 50 nm, they most likely represent calcified collagen fibers. Immunoelectron microscopy showed that the calcified structures were positive for collagen III. Remarkably, the elastic layer of BM appeared to have a normal ultrastructure, even in 2.5 year old Abcc6−/− mice. Our results suggest that Abcc6 deficiency in the mouse causes calcification of BM. While PXE is considered to affect primarily the elastic fibers, we found predominantly mineralization of collagen fibers.