Epithelial proliferation in inflammatory skin disease is regulated by tetratricopeptide repeat domain 7 (Ttc7) in fibroblasts and lymphocytes

Background Mutations in tetratricopeptide repeat domain 7A (TTC7A) and its mouse orthologue, Ttc7 , result in a multisystemic disease, mostly affecting the epithelial barriers and immune system. Despite successful hematopoietic stem cell transplantation, ongoing progression of gastrointestinal manifestations can be life-threatening in TTC7A-deficient patients. Objective We sought to identify whether TTC7A mutations dysregulate epithelial cells only or whether a cell-intrinsic defect in lymphocytes or other cells contributes to disease manifestations. Methods Ttc7 -mutated (Ttc7 fsn/fsn ) mice were crossed to generate double-mutant (Rag2 −/− Ttc7 fsn/fsn ) and triple-mutant (Rag2 −/− IL2rg −/− Ttc7 fsn/fsn ) mice. These models, together with bone marrow chimeras, were used to explore the role of adaptive and innate lymphocytes in the flaky skin phenotype. The effect of the Ttc7 fsn/fsn mutation on stromal cells was tested in a xenograft model in conjunction with transcriptomic analysis of Ttc7 fsn/fsn fibroblasts. Results We observed that the severity of epithelial hyperproliferation was accentuated by lymphocytes, whereas the phenotype was not induced by transfer of Ttc7 -mutated hematopoietic cells. Furthermore, mice completely lacking the lymphocytic compartment were not protected from epithelial hyperproliferation. Ttc7 -mutated mouse fibroblasts expressed increased transcript levels of insulin-like growth factor 1 (Igf1 ) and the antimicrobial protein regenerating islet-derived protein 3γ (Reg3γ) . In a xenograft model Ttc7 -mutated fibroblasts markedly increased epithelial proliferation of keratinocytes. Thus Ttc7 -mutated fibroblasts were identified as potent instigators of epithelial hyperproliferation. Conclusion Our results reveal a previously unsuspected fundamental cell-extrinsic role of Ttc7. We have identified potential candidates for molecularly targeted treatment strategies that will need to be evaluated in future preclinical studies.