# RUNX1 is expressed in a subpopulation of dermal fibroblasts and is associated with disease severity of systemic sclerosis

## TL;DR

RUNX1 is linked to severe skin fibrosis in systemic sclerosis and is active in specific fibroblast cells that contribute to the disease.

## Contribution

This is the first study to show RUNX1's role in systemic sclerosis fibrosis and its connection to specific fibroblast subpopulations.

## Key findings

- RUNX1 expression is associated with more severe dermal fibrosis in systemic sclerosis patients.
- RUNX1 is enriched in a subpopulation of fibroblasts linked to fibrosis in systemic sclerosis skin.
- RUNX1 inhibition reduces fibroblast activation and extracellular matrix production.

## Abstract

The activation of runt-related transcription factor 1 (RUNX1) in fibroblasts has been implicated in wound healing and fibrosis; however, the role of RUNX1 in the fibrotic progression of the autoimmune disease systemic sclerosis (SSc) remains known.

Leveraging gene expression, genome-wide DNA methylation, and single-cell resolution data of SSc skin and fibroblast, we analysed the impact of RUNX1 dysregulation in SSc dermal fibrosis. RUNX1 function was subsequently assessed using siRNA, pharmacologic inhibition, and CRISPR knockout in 2-dimensional and 3-dimensional fibroblast cultures.

Analysis of gene expression in multiple cohorts demonstrated an association between the severity of dermal fibrosis and the expression levels of RUNX1 in the skin of patients with SSc. Epigenomic analyses of methylation identified hypomethylated 5-Cytosine-phosphate-Guanine-3 (CpG) sites proximal to the RUNX1 gene, implicating their potential role in the increased expression of RUNX1. Analysis of single-cell RNA-seq data from skin biopsies of individuals with SSc revealed that RUNX1 is higher in subpopulations of fibroblasts enriched in SSc, which are believed to contribute to fibrosis. RUNX1 CRISPR knockout resulted in reduced alpha smooth muscle actin expression. Inhibition of RUNX1 activity caused a reduction in fibroblast activation, contraction, extracellular matrix components, and proliferation rates, including a reduction in SFRP4, LUM, and COL1A1.

This study is the first to demonstrate a potential role for RUNX1 in the pathogenesis of SSc dermal fibrosis. RUNX1 is associated with more severe SSc fibrosis and is associated with a subpopulation of dermal fibroblasts implicated in fibrosis.