Background:
Cystine accumulation occurs in all tissues in cystinosis and leads to progressive multisystem pathology, including skeletal muscle dysfunction. Cystinosis-associated myopathy can cause muscle weakness, dysphagia, and pulmonary impairment, contributing to reduced quality of life, loss of independence, and feeding difficulties. Despite its clinical significance, the mechanisms underlying muscle involvement remain poorly understood. While myopathy has been well characterised clinically and in mouse models, the presence of a comparable phenotype in the Ctns knockout (KO) rat remains unclear. This study aimed to determine whether the Ctns KO rat develops a skeletal muscle phenotype consistent with cystinosis-associated myopathy.

Methods:
Animals were bred in line with European Regulations and with approval from the HPRA and local Animal Ethics and Experimentation Committee at University College Cork (AEEC). A 2 × 2 factorial study design was used with sex and genotype as fixed factors in six-month-old animals (n = 4-9 per group). Animals were killed by Schedule 1 method and ex vivo isotonic and isometric contractile measurements were performed on isolated soleus and diaphragm muscle strips. Structural analyses were conducted using haematoxylin and eosin and Sirius red staining, while immunohistochemical fibre typing was used for morphometric analysis. Data were analysed using ANOVA in GraphPad Prism.

Results:
Ctns KO animals showed significant reductions in power under 50% load conditions in both soleus and diaphragm. Isometric performance was also impaired, with reduced maximum specific force measured in response to 150Hz. There was no effect of genotype on rate of fatigue. Structural analyses demonstrated preserved muscle architecture, with no evidence of necrosis, inflammation, fibrosis, or fibre type remodelling at 6-months of age.

Conclusion:
Significant functional deficits in isotonic and isometric contractile performance were identified in the soleus and diaphragm muscles of Ctns KO rats despite preserved muscle structure. These findings suggest that functional impairment precedes detectable structural degeneration in cystinosis and support the Ctns KO rat as a valuable model for investigating neuromuscular involvement in this disease. The diaphragm findings are particularly important given the respiratory complications reported in nephropathic cystinosis.