Smad4 Mutations And Cross-Talk Between Tgf-Beta/Ifn Gamma Signaling Accelerate Rates Of Dna Damage And Cellular Senescence, Resulting In A Segmental Progeroid Syndrome-The Myhre Syndrome

SMAD4 encodes a member of the SMAD family of proteins involved in the TGF-beta signaling pathway. Potentially heritable, autosomal dominant, gain-of-function heterozygous variants of SMAD4 cause a rare developmental disorder, the Myhre syndrome, which is associated with a wide range of developmental and post-developmental phenotypes that we now characterize as a novel segmental progeroid syndrome. Whole-exome sequencing of a patient referred to our International Registry of Werner Syndrome revealed a heterozygous p.Arg496Cys variant of the SMAD4 gene. To investigate the role of SMAD4 mutations in accelerated senescence, we generated cellular models overexpressing either wild-type SMAD4 or mutant SMAD4-R496C in normal skin fibroblasts. We found that cells expressing the SMAD4-R496C mutant exhibited decreased proliferation and elevated expression of cellular senescence and inflammatory markers, including IL-6, IFN gamma, and a TGF-beta target gene, PAI-1. Here we show that transient exposure to TGF-beta, an inflammatory cytokine, followed by chronic IFN gamma stimulation, accelerated rates of senescence that were associated with increased DNA damage foci and SMAD4 expression. TGF-beta, IFN gamma, or combinations of both were not sufficient to reduce proliferation rates of fibroblasts. In contrast, TGF-beta alone was able to induce preadipocyte senescence via induction of the mTOR protein. The mTOR inhibitor rapamycin mitigated TGF-beta-induced expression of p21, p16, and DNA damage foci and improved replicative potential of preadipocytes, supporting the cell-specific response to this cytokine. These findings collectively suggest that persistent DNA damage and cross-talk between TGF-beta/IFN gamma pathways contribute to a series of molecular events leading to cellular senescence and a segmental progeroid syndrome.