Disruption of day-to-night changes in circadian gene expression with chronic tendinopathy.

Overuse injury in tendon tissue (tendinopathy) is a frequent and costly musculoskeletal disorder and represents a major clinical problem with unsolved pathogenesis. Studies in mice have demonstrated that circadian clock-controlled genes are vital for protein homeostasis and important in the development of tendinopathy. We performed RNA sequencing, collagen content and ultrastructural analyses on human tendon biopsies obtained 12 hours apart in healthy individuals to establish whether human tendon is a peripheral clock tissue and we performed RNA sequencing on patients with chronic tendinopathy to examine the expression of circadian clock genes in tendinopathic tissues. We found time-dependent expression of 280 RNAs including 11 conserved circadian clock genes in healthy tendons and markedly fewer (23) differential RNAs with chronic tendinopathy. Further, the expression of COL1A1 and COL1A2 was reduced at night but was not circadian rhythmic in synchronised human tenocyte cultures. In conclusion, day-to-night changes in gene expression in healthy human patellar tendons indicate a conserved circadian clock as well as the existence of a night-reduction in collagen I expression. KEY POINTS: Tendinopathy is a major clinical problem with unsolved pathogenesis. Previous work in mice have shown that a robust circadian rhythm is required for collagen homeostasis in tendons. The use of circadian medicine in the diagnosis and treatment of tendinopathy has been stifled by the lack of studies on human tissue. Here, we establish that the expression of circadian clock genes in human tendons is time dependent, and now we have data to corroborate that circadian output is reduced in diseased tendon tissues. We consider our findings to be of significance in advancing the use of the tendon circadian clock as a therapeutic target or preclinical biomarker for tendinopathy. Abstract figure legend The primary goal of this study was to establish whether human tendon tissue is a circadian clock. Limited by obtaining only two patellar biopsies from each subject, we designed a study taking the biopsies 12 hours apart, once in the day and once in the night, and analysed the gene expression by RNAseq. We found 280 time-dependent RNAs between 9 AM and 9 PM biopsies and found night reduction in genes involved in collagen I fibrillogenesis by RT-qPCR. We also took samples to examine the presence of a chronomatrix by transmission electron microscopy but the differences were not statistically significant. In chronic patellar tendinopathy biopsies we found fewer time-dependent genes and no night-reduction in expression of genes involved in collagen I fibrillogenesis. Our data suggests that tendinopathy may be accompanied by a dampened circadian rhythm. This article is protected by copyright. All rights reserved.