SMAD3 mutation in LDS3 causes bone fragility by impairing the TGF-β pathway and enhancing osteoclastogenesis

Area of Science:

• Genetics and Molecular Biology
• Skeletal Biology and Disease
• Connective Tissue Disorders

Background:

• Loss-of-function mutations in SMAD3 gene cause Loeys-Dietz syndrome type 3 (LDS3), a rare disorder affecting connective tissues, characterized by vascular and skeletal abnormalities.
• Dysregulation of the TGF-β/SMAD signaling pathway is implicated in abnormal skeletal development and bone fragility, but specific bone histomorphometric and ultrastructural changes in LDS3 remain largely uncharacterized.
• The precise molecular mechanisms underlying the LDS3 phenotype driven by SMAD3 mutations are not fully understood.

Purpose of the Study:

• To investigate the histomorphometric and matrix mineralization characteristics of bone in a human patient with an SMAD3 mutation.
• To explore the cellular defects within the TGF-β/SMAD pathway associated with the SMAD3 mutation in vitro.
• To elucidate the pathophysiology of Loeys-Dietz syndrome type 3 (LDS3) and identify potential therapeutic targets.

Main Methods:

• Bone histomorphometry and quantitative backscattered electron imaging were performed on patient bone samples.
• Functional analysis of the patient's SMAD3 mutation (p.I67S) was conducted in HEK293 cells using plasmid vectors to assess phosphorylation and transcriptional activity.
• Luciferase reporter assays and analysis of TGF-β signaling target genes were performed, alongside assessment of SMAD3 protein stability in patient fibroblasts and osteoclastogenic differentiation in PBMCs.

Main Results:

• Bone histomorphometry revealed significantly reduced cortical thickness (-68%), trabecular thickness (-32%), and bone formation rate (-50%), with delayed mineralization and undermineralized bone matrix.
• The patient's SMAD3 (p.I67S) mutation exhibited reduced phosphorylation and transcriptional activity compared to wild-type SMAD3, and impaired SMAD3 protein stability was observed in patient fibroblasts.
• Increased osteoclastogenic differentiation and upregulation of osteoclast-associated genes were noted in patient PBMCs, correlated with decreased expression of TGF-β signaling target genes.

Conclusions:

• Bone in patients with the SMAD3 (p.I67S) mutation is characterized by reduced bone formation, decreased SMAD3 activation and protein stability, and increased osteoclastogenesis.
• These findings provide critical insights into the pathophysiology of LDS3, highlighting the role of SMAD3 dysfunction in skeletal abnormalities.
• Targeting the TGF-β/SMAD pathway presents a promising therapeutic strategy for LDS3.