Single-cell RNA-sequencing reveals expansion of vascular associated cells in stiff skin syndrome

Stiff Skin Syndrome (SSS) is a rare scleroderma-like disorder caused by a mutation in the fibrillin-1 (FBN1) gene. Unlike in scleroderma (SSc), SSS patients develop painful, debilitating skin fibrosis without inflammation; nonetheless, SSS patients are often treated with the same immunosuppressants utilized in SSc, with poor results. To better understand SSS pathogenesis and to identify new therapeutic targets, we performed single-cell RNA-sequencing (scRNA-seq) on affected skin biopsies from 3 SSS patients with confirmed FBN1 mutation as well as from 6 healthy controls (HC) and 6 SSc patients. Clustering and annotation of dermal cells revealed a relative decrease of fibroblasts (FBs), neurons, and immune cells in SSS skin compared to HC and SSc and increased endothelial cells (ECs) and pericytes (PRCs). Despite similar fibrosis in SSS and SSc, FB phenotypes differed notably: SSS FBs showed stronger expression of ECM production genes and lacked the inflammatory transcriptional signature of SSc FBs. Further, whereas SSc showed myofibroblast expansion, all FB subsets were decreased in SSS except COL11A1+ (dermal hair sheath) FBs, which in SSS showed evidence of serine/threonine kinase activity, relevant to fibrogenesis. Despite representing a minority of FBs, SSS COL11A1+ FBs were highly active in cell-cell communication as predicted by ligand-receptor analysis, orchestrating an abnormal signaling network with other FB subsets, ECs, and PRCs marked by enhanced angiogenic and WNT signaling that may drive the clinical hypertrichosis of SSS. The expanded PRC population in SSS also exhibited heightened expression of ECM factors including proteoglycans, which underlie skin stiffening in SSS. Together, these results contrast inflammation-driven SSc with immunologically quiescent SSS, which is instead characterized by altered stromal cell communication that causes fibrosis. Targeting aberrant FB-EC-PRC signaling may interrupt pathogenic circuits promoting fibrosis and thereby SSS symptoms.