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Tumor-derived C-terminal mutations of Smad4 with decreased DNA binding activity and enhanced intramolecular

Chenzhong Kuang1, Yan Chen

  • 1Department of Medical and Molecular Genetics, Indiana University School of Medicine and the Walther Cancer Institute, Indianapolis, IN 46202, USA.

Oncogene
|December 3, 2003
PubMed

Insights

Smad4 mutations in cancer impair transforming growth factor beta (TGF-beta) signaling by affecting DNA binding and protein interactions. These alterations disrupt TGF-beta

Area of Science:

  • Molecular Biology
  • Cancer Biology
  • Signal Transduction

Background:

  • Smad4 is crucial for transforming growth factor beta (TGF-beta) signaling, frequently mutated in pancreatic and colorectal cancers.
  • Smad4 possesses distinct MH1 (DNA binding) and MH2 (transcriptional activation, complex formation) domains, each vital for its function.
  • Mutations in Smad4 can disrupt TGF-beta signaling pathways critical for cellular processes and cancer development.

Purpose of the Study:

  • To investigate the functional impact of specific Smad4 mutations within the MH1 and MH2 domains on TGF-beta signaling.
  • To elucidate the molecular mechanisms by which Smad4 mutations abrogate its signaling functions.
  • To understand how these mutations contribute to cancer progression.

Main Methods:

  • Analysis of nine Smad4 mutations in MH1 and MH2 domains.
  • In vitro assays using GST fusion proteins to assess DNA binding affinity.
  • GST pull-down assays to evaluate protein-protein interactions (Smad4-Smad2).
  • Reporter gene assays to measure transcriptional activity.
  • Dominant-negative assays to assess TGF-beta signaling antagonism.

Main Results:

  • All nine analyzed Smad4 mutations reduced transcriptional activity of TGF-beta-responsive genes.
  • MH1 domain mutations significantly impaired DNA binding to Smad binding elements.
  • Specific MH2 mutations (R497H, K507Q, R515G) reduced DNA binding capacity.
  • R497H and R515G mutations enhanced intramolecular interaction between MH2 and MH1 domains, potentially reducing DNA binding.
  • R497H mutation demonstrated enhanced inhibition of TGF-beta receptor-mediated transcription and acted in a dominant-negative manner.

Conclusions:

  • Smad4 mutations disrupt TGF-beta signaling through impaired DNA binding and altered protein interactions.
  • Enhanced intramolecular interactions in MH2 mutants may underlie reduced DNA binding.
  • Mutated Smad4 can antagonize TGF-beta signaling, contributing to cancer development.
  • These findings reveal novel mechanisms of Smad4 functional abrogation in cancer.

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