Investigating Age-Dependent Arthropathy in a Circadian Mutant Mouse Model: A Dissertation

Ectopic calcification can cause pain and limit mobility. Studies suggest that circadian genes may play a role in the calcification process. Core circadian genes Clock, Npas2, and Bmal1 are transcription factors that form CLOCK:BMAL1 or NPAS2:BMAL1 transactivator complexes that drive the rhythmic expression of circadian oscillator genes and output genes. Circadian oscillator genes Period1-3 and Cryptochrome1-2 encode proteins that form transcription repressor complexes that feedback to inhibit CLOCK/NPAS2:BMAL1 activity, thus completing the feedback loop that is the basis of the molecular circadian clockwork. Arrhythmic Bmal1 -/mice exhibit site-specific, age-dependent arthropathy. While studying the circadian phenotype of Clock -/;Npas2 m/m double mutant mice, we discovered that these double mutant mice develop site-specific arthropathy similar to the arthropathy described in Bmal1 -/mice. Based on the circadian clockwork mechanism, we hypothesized that CLOCK/NPAS2:BMAL1 transactivator complexes drive the expression of a gene (or genes) that prevents age-dependent arthropathy. To investigate Clock -/;Npas2 m/m double mutant mouse arthropathy, we evaluated mutant mice using X-ray, micro-computed tomography, and histology, and found that Clock -/;Npas2 m/m double mutant mice exhibit age-dependent, site-specific arthropathy that phenocopies that of Bmal1 -/mice. The costosternal junction and calcaneal tendon are most prominently affected, in that calcification of those tissues is detectable as early as 4-5 weeks and 11-12 weeks, respectively. The arthropathic lesions in these tissues consist of calcium phosphate