Ttc7 is a trans-acting regulator of cell proliferation

Mutations in tetratricopeptide repeat domain 7A (TTC7A) and its mouse orthologue Ttc7 — a ubiquitously expressed protein — result in a life-threatening multisystemic disease, severely affecting epithelial body barriers and the immune system. Patients mainly suffer from intestinal atresias and immunodeficiency. Furthermore, surgical treatment of the atresias is associated with poor outcomes, and the potentially fatal condition progresses despite successful allogeneic hematopoietic stem cell transplantation. There is an ongoing debate of whether TTC7A mutations uniquely dysregulate epithelial cells, or whether a defect in lymphocytes contributes to the pathogenesis of the disease. Therefore, a deeper understanding of mechanisms underlying the multisystemic nature of TTC7A deficiency is needed. The flaky skin mutant mouse (Ttc7fsn/fsn) has been regarded as a model for human psoriasis because of its hyperplastic skin disease. Ttc7fsn/fsn mice present with a severe hyperplastic dermatitis as the main feature of a complex phenotype including a hyperproliferative immune disorder (splenomegaly and peripheral lymphadenopathy), gastric forestomach papillomas, and anemia. The mechanisms leading to the flaky skin phenotype remain largely unknown. In particular, it is unclear whether the hyperplastic skin disease is uniquely due to an intrinsic epithelial dysfunction or if altered immune activation leads to reactive epidermal hyperplasia. To investigate this, we generated double homozygous mutant Rag2-/-Ttc7fsn/fsn mice — lacking adaptive lymphocytes — and triple homozygous mutant mice Rag2-/-IL2rg-/-Ttc7fsn/fsn mice — completely lacking the innate and adaptive lymphocytic compartment. Both immunodeficient mouse strains were not protected from epithelial hyperproliferation and even developed peripheral lymphadenopathy but less severe than observed in the Ttc7fsn/fsn mouse model. Furthermore, the transfer of Ttc7-mutated hematopoietic stem cells did not induce epithelial hyperproliferation or peripheral lymphadenopathy in wild-type animals. Thus, lymphocytes are able to aggravate the severity of epithelial hyperproliferation and inflammation in Ttc7fsn/fsn stroma but cannot elicit the flaky skin phenotype. Serum cytokine profiles revealed high levels of IL-22 in single, double and triple homozygous mutant mice. We herewith identified non-lymphoid Ttc7fsn/fsn cells as the major source of IL-22. ln a xenograft model, Ttc7-mutated mouse fibroblasts markedly increased epithelial proliferation of human keratinocytes. The analyses of transcriptomic data from Ttc7-mutated fibroblasts displayed an mRNA expression pattern that suggested alterations in phosphoinositide-dependent signaling pathways. These pathways play an important role in cytoskeleton regulation and in cell-cell-interactions. In addition, we found an increased expression of IGF-1. Consequently, Ttc7-mutated fibroblasts were identified to critically induce the hyperproliferative behavior of keratinocytes in the presence of the proliferation-inducing factors IL-22 and IGF-1. Our work shows that fibroblasts harmfully alter the dynamic and behavior of epidermal cell maturation, while Ttc7-mutated lymphocytes contribute to epidermal homeostasis. The dysfunctional crosstalk between stromal cells adds an unsuspected new dimension to the pathogenesis of the multisystemic disease associated with TTC7A deficiency. This suggested mechanism reveals why previous HSCT treatments together with intestinal surgery may only partially relieve the multisystemic disease seen TTC7A-deficient patients. Thus, a new targeted therapeutic approach is needed to achieve treatment strategies that intervene in the disturbed interaction between stromal cells.