SMAD4, SMAD family member 4, is one of eight SMAD proteins in the human genome (Massague 1998). SMAD4 is sometimes referred to as DPC4 (Hahn et al. 1996). SMAD4 is a signal transduction protein that is the central mediator for downstream transcriptional output in the TGF-β family signaling pathways via its interaction with upstream receptors and fellow SMAD transcription factors (Goustin et al. 1986; Tucker et al. 1984a; Tucker et al. 1984b). The TGF-β pathway plays a complex role in cancer development, progression, and metastasis (Bierie and Moses 2006; Elliott and Blobe 2005; Massague 2008; Miyaki and Kuroki 2003).
SMAD4 consists of two domains connected by a linker region (Shi and Massague 2003). MAD homology domain 1 (MH1) is the N-terminal, DNA-binding domain (NCBI Gene Database). MAD homology domain 2 (MH2) is the C-terminal domain that interacts with upstream receptors and mediates oligomerization and transactivation to provide specificity and selectivity (NCBI Gene Database). Smad4 is made up of 11 coding exons.
Mutations in SMAD4 are involved in several hereditary syndromes with cancer predisposition, including juvenile polyposis syndrome and hemorrhagic hereditary telangiectasia (HHT) syndrome. SMAD4 loss or mutation is also seen in approximately 50% of pancreatic tumors and in 10–35% of invasive CRC (Elliott and Blobe 2005; Hahn et al. 1996; Miyaki et al. 1999). The MH2, C-terminal domain of SMAD4 is the target of tumorigenic inactivation, and mutations in this region disrupt RSMAD oligomerization, which interrupts normal signaling pathways (Shi et al. 1997; Shi and Massague 2003).