Role of SMAD proteins in colitis-associated cancer: from known to the unknown

Small mothers against decapentaplegic (SMAD) proteins are a family of signal transduction molecules in transforming growth factor beta (TGF beta) ligand pathways that have been found to have a key role in the pathogenesis of inflammatory bowel disease (IBD). Long standing IBD predisposes individuals to colitis-associated colorectal cancer (CAC), an entity that possess unique characteristics compared to hereditary and sporadic cancer. The ligands of the TGF beta super family along with SMADs have also been implicated in several aspects of colorectal cancer formation. SMAD proteins are shown to be involved in a number of potentially carcinogenic mechanisms such as altering gene transcription, controlling stem cell differentiation to causing epigenetic changes. Modulation of these proteins has emerged as a novel therapeutic intervention for IBD although its effect on carcinogenesis remains elusive. This account reviews available evidence linking SMAD proteins to CAC and explores the potential areas for future research in this area.