Genetic alterations of the TGF-[beta] signaling pathway in colorectal cancer cell lines: A novel mutation in Smad3 associated with the inactivation of TGF-[beta]-induced transcriptional activation

To investigate genetic alterations involved in the TGF-β signaling pathway in colorectal cancer, we assayed DNA synthesis rates after treating TGF-β and checked for genetic alterations inTGF-βRII,TGF-βRI,Smad2,Smad3, andSmad4in 12 colorectal cancer cell lines. Eleven lines, except SNU-61, show no significant change in DNA synthesis rate after TGF-β treatment. In these 11 lines, several mutations were found in genes involved in the TGF-β signaling pathway: (i) frameshift deletions in the poly(A)10tract of theTGF-βRIIgene in SNU-407, SNU-769A, SNU-769B, and SNU-1047 cell lines, (ii) a missense mutation ofSmad2(R321Q) in SNU-81, (iii) two missense mutations inTGF-βRI(R487W in SNU-175 and A202V in SNU-1040), and (iv) a monoallelic loss at theSmad4locus in three cell lines. Interestingly, a missense mutation (R373H) inSmad3gene was found in SNU-769A. To our knowledge, this is the first report ofSmad3mutation in human malignancy. This mutation was found to result in the inhibition of translocation of Smad3 protein to the nucleus and a reduction in the activity of Smad3 during TGF-β-induced transcriptional activation. These results indicate that the majority of cell lines, which are insensitive to TGF-β, have alterations in genes involved in the TGF-β signaling pathway in colorectal cancer cell lines.