An intragenic FAT1 regulatory element deleted in muscular dystrophy patients drives muscle and mesenchyme expression during development

Fat1 is an atypical cadherin playing multiple roles that influence tissue morphogenesis. During mouse development Fat1 is required to modulate muscle morphogenesis through complementary activities in myogenic cells, muscle-associated connective tissue, and motor neurons, ablation of which leads to regionalized muscle phenotypes. We previously identified copy number variants (CNV) deleting an intragenic conserved non-coding element (CNE) in the human FAT1 locus, that were enriched among muscular dystrophy patients with symptoms resembling those of Facioscapulohumeral Dystrophy (FSHD), compared to healthy individuals. Since such deletions of a putative cis-regulatory element had the potential to cause tissue-specific depletion of FAT1, they were postulated to act as symptom modifiers. However, activity of this CNE has not been functionally explored so far. To investigate the possible regulatory activity of this Fat1-CNE, we engineered transgenic mice in which it drives expression of a bi-cistronic reporter comprising the CRE-recombinase (Cre) and a myristilated-tdTomato fluorescent protein. The tissue-specific pattern of cre and tomato expression indicates that this enhancer has bipotential character, and drives expression in skeletal muscle and in muscle-associated mesenchymal cells. We extended our analysis of one of the transgenic lines, which exhibits enhanced expression in mesenchymal cells at extremities of subsets of muscles matching the map of Fat1-dependent muscles. This transgenic line exhibits highly selective CRE-mediated excision in scattered cells within the Tomato-positive territory hotspots. This represents a novel tool to genetically explore the diversity of muscle-associated mesenchymal lineages.