Beyond myoblasts: DUX4 alters the differentiation landscape and promotes a pro-fibrotic phenotype in mesenchymal stromal cells

Facioscapulohumeral muscular dystrophy (FSHD) is caused by aberrant expression of the transcription factor DUX4 in skeletal muscle, leading to progressive muscle degeneration and pathological remodelling of the muscle microenvironment. Although the effects of DUX4 in myogenic cells have been extensively investigated, its potential impact on mesenchymal stromal cells (MSCs), which contribute to tissue homeostasis and regeneration, remains poorly understood. Here, we investigated the consequences of ectopic DUX4 expression in human MSCs and assessed their differentiation potential and transcriptional phenotype. DUX4-expressing MSCs acquired selected features of the myogenic phenotype, including expression of muscle-associated markers and formation of myotube-like structures, although their myogenic differentiation and fusion capacity remained markedly lower than those of bona fide myoblasts. In parallel, DUX4 expression substantially altered MSC lineage potential, reducing adipogenic and osteogenic differentiation. Transcriptomic and functional analyses further revealed prominent activation of extracellular matrix (ECM)-associated and pro-fibrotic programmes, together with increased oxidative stress. These findings indicate that DUX4 profoundly alters the differentiation landscape of MSCs, inducing a complex cell state characterised by partial activation of muscle-associated programmes and a prominent pro-fibrotic/ECM-remodelling phenotype. Our results suggest that DUX4 may influence not only myogenic cells but also mesenchymal populations within the FSHD muscle microenvironment. Further studies are required to determine whether these altered MSC states contribute functionally to fibrosis, impaired muscle regeneration or intercellular communication in vivo. DUX4; FSHD; Mesenchymal stromal cells; differentiation; FAP