The Tsk2/+ mouse fibrotic phenotype is due to a gain-of-function mutation in the PIIINP segment of the Col3a1 gene

Systemic sclerosis (SSc) is a polygenic, autoimmune disorder of unknown etiology, characterized by the excessive accumulation of extracellular matrix (ECM) proteins, vascular alterations, and autoantibodies. The tight skin (Tsk)2/+ mouse model of SSc demonstrates signs similar to SSc including tight skin and excessive deposition of dermal ECM proteins. By linkage analysis, we mapped the Tsk2 gene mutation to less than 3 megabases on chromosome 1. We performed both RNA sequencing of skin transcripts and genome capture DNA sequencing of the region spanning this interval in Tsk2/+ and wild-type littermates. A missense point mutation in the procollagen III amino terminal propeptide segment (PIIINP) of Col3a1 was found to be the best candidate for Tsk2, so both in vivo and in vitro genetic complementation tests were used to prove that this Col3a1 mutation is the Tsk2 gene. All previously documented mutations in the human Col3a1 gene are associated with Ehlers-Danlos syndrome, a connective tissue disorder that leads to a defect in type III collagen synthesis. The Tsk2 point mutation is the first documented gain-of-function mutation associated with Col3a1, which leads instead to fibrosis. This discovery provides insight into the mechanism of skin fibrosis manifested by Tsk2/+ mice. For the transfection experiment, three Col3a1 knockout fibroblast transfected with WT Col3a1 samples and three Col3a1 knockout fibroblast transfected with Tsk2 Col3a1 samples were used. For the 4-week female, three WT mice and three Tsk2 total skin RNA were used.