Loss of function mutation in progressive ankylosis gene causes aberrant mineralization and acquisition of osteoblast-like-phenotype by the cells of the intervertebral disc

Abstract Pathological mineralization of intervertebral disc is debilitating and painful and linked to disc degeneration in a subset of human patients. An adenosine triphosphate transporter, progressive ankylosis (ANK) is a regulator of extracellular inorganic pyrophosphate levels and plays an important role in tissue mineralization. However, the function of ANK in intervertebral disc has not been fully explored. Herein we analyzed the spinal phenotype of Ank mutant mice (ank/ank) with attenuated ANK function. Micro-computed tomography and histological analysis showed that loss of ANK function results in aberrant annulus fibrosus mineralization and peripheral disc fusions with cranial to caudal progression in the spine. Vertebrae in ank/ank mice exhibit elevated cortical bone mass and increased tissue non-specific alkaline phosphatase-positive endplate chondrocytes with decreased subchondral endplate porosity. The acellular dystrophic mineral inclusions in the annulus fibrosus were localized adjacent to apoptotic cells and cells that acquired osteoblast-like phenotype. Fourier transform infrared spectral imaging showed that the mineral in the outer annulus fibrosus had similar chemical composition to that of vertebral bone. Microarray-based transcriptomic analysis of annulus fibrosus and nucleus pulposus tissues showed changes in several pathways associated with mineralization including transforming growth factor β and mitogen-activated protein kinase signaling. The present study provides new insights into the role of ANK in the disc tissue compartments, and highlights the importance of local inorganic pyrophosphate metabolism in inhibiting mineralization of this important connective tissue.