Background/Purpose: Persistent myofibroblast activation driving progressive fibrosis is the defining hallmark of systemic sclerosis (SSc). Uniquely, fibrosis in SSc affects the skin, heart, lungs and muscles synchronously, suggesting that pathways shared across multiple target organs could be responsible. However, the nature of the fundamental alteration common to distinct organs affected in SSc patients remains unknown. In order to identify novel therapeutic targets, we sought potential novel mechanisms accounting for multi-organ fibrosis.

Methods: We performed next generation sequencing on skin biopsies from patients with SSc (n=19) and matched healthy controls (n=14). Expression of differentially regulated genes in the skin was investigated by immunohistochemistry. Novel knockout mice were generated and at various ages analyzed for fibrosis. Regulation of morphogen signaling and fibrotic responses was examined in cell culture assays using null fibroblasts.

Results: Unbiased RNAseq transcriptome analysis identified SPAG17 as the most differentially-expressed gene in SSc skin biopsies (-4.18 fold; p< 0.0001). SPAG17 is a primary cilia gene with unknown function, previously linked to human height in GWAS. Levels of SPAG17 mRNA in SSc skin biopsies showed negative correlation with the Modified Rodnan Skin Score; low SPAG17 expression was associated with increased morphogen (Hedgehog, Wnt) activity (elevated Smoothened and Dishevelled1) in the skin. Immunofluorescence analysis of SSc skin biopsies demonstrates marked reduction in SPAG17 expression, and absence of SPAG17 in lesional myofibroblasts. SPAG17-deficient myofibroblasts had stunted and dysfunctional primary cilia, and showed constitutive activation of Smad pathways. Moreover, loss of SPAG17 rendered fibroblasts exquisitely sensitive to TGF-ß. Since SPAG17 knockout mice generated in our lab die in the perinatal period due to severe lung involvement, we crossed SPAG17^fl mice with Sox-Cre mice. The offspring survive into adulthood, and are indistinguishable for their wildtype littermates. Remarkably, by 4-6 months of age, SPAG17-deficient mice show notable fibrosis in skin, heart, lungs, kidney and skeletal muscle.

Conclusion: The ciliary gene SPAG17 appears to play a critical role in preventing fibrosis in multiple organs. Loss of SPAG17 renders fibroblasts markedly susceptible to fibrotic stimulation, and is associated with spontaneous scleroderma-like skin changes accompanied by fibrosis in multiple organs. Reduced expression of SPAG17 and consequent ciliary dysfunction and augmented morphogen signaling therefore appear to play a previously unrecognized role in fibrosis and could contribute to the pathogenesis of SSc. SPAG 17 KO mice represent a unique novel disease model phenocopying synchronous fibrosis in multiple organs.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/reduced-spag17-expression-links-dysfunctional-cilia-morphogen-signaling-activation-and-multiple-organ-fibrosis-novel-target-for-systemic-sclerosis/