Missense mutations of MADH4: characterization of the mutational hot spot and functional consequences in human tumors.

The mutational spectrum of MADH4 (DPC4/SMAD4) opens valuable insights into the functions of this protein that confer its tumor-suppressive nature in human tumors. We present the MADH4 genetic status determined on a new set of pancreatic, biliary, and duodenal cancers with comparison to the mutational data reported for various tumor types. Homozygous deletion, followed by inactivating nonsense or frameshift mutations, is the predominant form of MADH4 inactivation in pancreatic cancers. Among the naturally occurring MADH4 missense mutations, the MH2 domain is the most frequent target (77%) of missense mutations in human tumors. A mutational hot spot resides within the MH2 domain corresponding to codons 330 to 370, termed the mutation cluster region (MCR). A relationship was found between the locations of the missense mutations (the MH1 domain, the MH2-MCR, and the MH2 outside of the MCR) and the tumor types, suggesting environmental or selective influences in the development of MADH4 mutations. Immunohistochemical studies for Madh4 protein in nine archival cancers (six pancreatic cancers, two duodenal cancers, and one biliary cancer) with known missense mutations indicated that all mutations within the MH1 or MH2 domain COOH-terminal to the MCR (seven of nine cases) had negative or weak labeling, whereas two cancers with mutations within the MCR had strong positive nuclear labeling for Madh4 protein. These findings have important implications for in vitro functional studies, suggesting that the majority of missense mutations inactivate Madh4 by protein degradation in contrast to those that occur within the MCR.