The DPC4 gene may play a role as a tumor-suppressor gene in a fraction of colorectal cancers; however, while allelic loss at 18q21 is very often seen in colorectal cancers, only a minority show DPC4 mutations, suggesting that there might be another tumor-suppressor gene in this chromosome region.
Twenty-five of 84 tumors had homozygous deletions at 18q21.1, a site that excludes DCC (a candidate suppressor gene for colorectal cancer) and includes DPC4, a gene similar in sequence to a Drosophila melanogaster gene (Mad) implicated in a transforming growth factor-beta (TGF-beta)-like signaling pathway.
RT-PCR sequencing analysis of the HL60 Smad5 remaining allele ruled out the functional inactivation of the gene analogous to that occurring in the Smad5 homologs DPC4 and Smad2 in cases of pancreatic and colorectal cancers.
These data suggest that DPC4 is rarely if ever mutated during prostatic oncogenesis, whereas inactivation of this gene may contribute to the genesis of a subset of colorectal carcinomas.
Other members of the SMAD family are excellent candidates for JPS, especially SMAD2 (which, like SMAD4, is mutated somatically in colorectal cancers), SMAD3 (which causes colorectal cancer when "knocked out" in mice), SMAD5, and SMAD1.
To investigate the potential role of DPC4/SMAD4 gene in colorectal cancers, we examined 73 tumors of clinical stages II or III from Japanese patients, for LOH at 18q21 and also for subtle mutations anywhere within the coding region of DPC4/SMAD4.
Our data show selective up-regulation of receptor-activated Smad proteins in human colorectal cancers and suggest involvement of Smad4 in differentiation and apoptosis of surface epithelial cells of normal crypts.
These data suggest that the carcinogenetic pathways of protruding and superficial depressed colorectal cancers are different, and that alterations of tumor suppressor gene(s) located on 18q21 other than Smad2, Smad4 and DCC might be associated with most superficial depressed colorectal cancers.
Smad4 plays a pivotal role in the TGF-beta signaling pathway and has been identified as a tumor suppressor, being mutated or deleted in approximately 50% of pancreatic carcinomas and 15% of colorectal cancers.
Although SMAD4 mutations have been associated with late-stage or metastatic disease, our combined molecular and cytogenetic data best fit a model in which SMAD4 mutations occur before colorectal cancers become aneuploid/polyploid, but after the MSI(+) and MSI(-) pathways diverge.
Moreover, these results may provide a refinement at the gene level of the clinical relevance of 18q21 deletion, thereby suggesting SMAD4 as a predictive marker in colorectal cancer.
These results indicate that whereas Smad4 point mutations are prevalent in pancreatic carcinoma, they are infrequent in early stages (I-III) of colorectal cancer.
However, thus far few studies have analyzed the impact of numerical abnormalities of chromosomes 17 and 18, which carry the p53 and DCC plus SHAD4/DPC4 genes involved in colorectal cancer, on the clinical and biological behaviors of the disease.
Following the definition of SMAD4 deletion as a negative predictive marker for chemotherapy benefit in patients with CRC, we aimed to evaluate the clinical relevance of the deletion of other SMAD genes clustered in this region: SMAD2 and SMAD7 in 264 CRC biopsies from a previous clinical study.
These data suggest that inactivation of the DPC4 gene contributes to the genesis of colorectal carcinoma through allelic loss whereas mutation in the coding region of the DPC4 gene is infrequently detected in Croatian patients with A, B or C stages of colorectal cancers.
Two missense mutations in the C-terminal domain of Smad4, D351H (Asp351-->His) and D537Y (Asp537-->Tyr), have been described recently in the human colorectal cancer cell lines CACO-2 and SW948 respectively [Woodford-Richens, Rowan, Gorman, Halford, Bicknell, Wasan, Roylance, Bodmer and Tomlinson (2001) Proc.Natl.Acad.Sci.U.S.A. 98, 9719-9723].