Stiffness and axial pain are associated with the progression of calcification in a mouse model of diffuse idiopathic skeletal hyperostosis

BACKGROUND: Diffuse idiopathic skeletal hyperostosis (DISH) is characterized by progressive calcification of spinal tissues; however, the impact of calcification on pain and function is poorly understood. This study examined the association between progressive ectopic spine calcification in mice lacking equilibrative nucleoside transporter 1 (ENT1(−/−)), a preclinical model of DISH, and behavioral indicators of pain. METHODS: A longitudinal study design was used to assess radiating pain, axial discomfort, and physical function in wild-type and ENT1(−/−) mice at 2, 4, and 6 months. At endpoint, spinal cords were isolated for immunohistochemical analysis of astrocytes (GFAP), microglia (IBA1), and nociceptive innervation (CGRP). RESULTS: Increased spine calcification in ENT1(−/−) mice was associated with reductions in flexmaze exploration, vertical activity in an open field, and self-supporting behavior in tail suspension, suggesting flexion-induced discomfort or stiffness. Grip force during the axial stretch was also reduced in ENT1(−/−) mice at 6 months of age. Increased CGRP immunoreactivity was detected in the spinal cords of female and male ENT1(−/−) mice compared to wild-type. GFAP- and IBA1-immunoreactivity were increased in female ENT1(−/−) mice compared to wild-type, suggesting an increase in nociceptive innervation. CONCLUSION: These data suggest that ENT1(−/−) mice experience axial discomfort and/or stiffness and importantly that these features are detected during the early stages of spine calcification.