Higher frequency of Smad4 gene mutation in human colorectal cancer with distant metastasis

We have previously detected an increased frequency of loss of heterozygosit y (LOH) on chromosome 18q during progression of colorectal carcinomas. To c larify the target of 18qLOH, mutation of Smad4 and Smad2 genes was analysed in 176 colorectal tumors with different stages, including liver metastasis , from 111 sporadic, 52 familial adenomatous polyposis (FAP) and nine hered itary nonpolyposis colorectal cancer (HNPCC) patients. Mutation of other Sm ad gene families in the TGF-beta signaling pathway was also examined. Twent y-one Smad4 mutations and one Smad2 mutation mere detected, whereas mutatio n of Smad3, 6 and 7 genes was not detected. Smad4 mutations included seven frameshift, one inframe deletion, four nonsense and nine missense mutations , 95% of which resulted in alteration of Smad4 protein regions included in homo-oligomer and hetero-oligomer formation. Frequencies of tumors with Sma d4 mutation were 0/40 (0%) in adenoma, 4/39 (10%) in intramucosal carcinoma , 3/44 (7%) in primary invasive carcinoma,without distant metastasis, 6/17 (35%) in primary invasive carcinoma with distant metastasis, and 11/36 (31% ) in distant metastasis (metastatic/non-metastatic: P=0.006 similar to 0.01 ). Loss of the other allele was observed in 19 of 20 (95%) invasive and met astasized carcinomas with Smad4 mutations. In four eases both primary and m etastasized carcinomas in the same patients showed the same mutations. The present results suggest that Smad4 gene is one of true targets of 18qLOH, a nd that its inactivation is involved in advanced stages, such as distant me tastasis, in human colorectal carcinogenesis.