Ablation of the FACIT collagen XII disturbs musculoskeletal ECM organization and causes patella dislocation and myopathy

Abstract

Collagen XII, belonging to the fibril-associated collagens with interrupted triple helix (FACIT) family, assembles from three identical -chains encoded by the COL12A1 gene. The trimeric molecule consists of three N-terminal noncollagenous NC3 domains joined by disulfide bonds followed by a short interrupted collagen triple helix at the C-terminus. Collagen XII is expressed widely in the musculoskeletal system and mutations in the COL12A1 gene cause an Ehlers-Danlos/myopathy overlap syndrome, which is associated with skeletal abnormalities and muscle weakness. Our study defines the role of collagen XII in patella development using the Col12a1-/- mouse model. Deficiency in Col12a1 expression causes malformed facies patellaris femoris grooves at an early stage, which leads to patella subluxation and growth retardation. Due to the patella subluxation, more muscle fibers with centralized nuclei occur in the quadriceps than in the gastrocnemius muscles indicating a local effect. To further understand the role of collagen XII in the skeletal tissues single cell RNAseq (scRNA-seq) was performed. Comparison of the gene expression in the tenocyte cell sub-population of wild type and Col12a1-/- mice showed that several matrix genes are altered. Finally, we reinvestigated collagen XII deficient patients and observed a patella instability.