Stephan A. Hahn

About 90 percent of human pancreatic carcinomas show allelic loss at chromosome 18q. To identify candidate tumor suppressor genes on 18q, a panel of pancreatic carcinomas were analyzed for convergent sites of homozygous deletion. 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. Potentially inactivating mutations in DPC4 were identified in six of 27 pancreatic carcinomas that did not have homozygous deletions at 18q21.1. These results identify DPC4 as a candidate tumor suppressor gene whose inactivation may play a role in pancreatic and possibly other human cancers.

During tumorigenesis, positive selection is exerted upon those tumor cells that alter rate-limiting regulatory pathways. A corollary of this principle is that mutation of one gene abrogates the need for alteration of another gene in the same pathway and also that the coexistence in a single tumor of mutations in different genes implies their involvement in distinct tumor-suppressive pathways. We studied 42 pancreatic adenocarcinomas for genetic alterations in the K-ras oncogene and the p16, p53, and DPC4 tumor suppressor genes. All of them had the K-ras gene mutated. Thirty-eight % of the tumors had four altered genes, another 38% had three altered genes, 15% had two altered genes, and 8% of the tumors had one altered gene. Interestingly, we noted a high concordance of DPC4 and p16 inactivations (P = 0.007), suggesting that the genetic inactivation of p16 increases the selective advantage of subsequent mutation in DPC4. No statistically significant association was identified between the alteration of these cancer genes and pathological or clinical parameters. This type of multigenic analysis in human tumors may serve to substantiate experimental tumor models and thus increase our understanding of the truly physiologically relevant tumor-suppressive pathways that are abrogated during human tumorigenesis.

We recently identified a novel tumor-suppressor gene, DPC4, at chromosome 18q21.1 and found that both alleles of DPC4 were inactivated in nearly one-half of the pancreatic carcinomas. Here, we analyzed 338 tumors, originating from 12 distinct anatomic sites, for alterations in the DPC4 gene. Sixty-four specimens were selected for the presence of the allelic loss of 18q and were further analyzed for DPC4 sequence alterations. An alteration of the DPC4 gene sequence was identified in one of eight breast carcinomas and one of eight ovarian carcinomas. These results indicate that whereas DPC4 inactivation is prevalent in pancreatic carcinoma (48%), it is distinctly uncommon (< 10%) in the other tumor types examined. The tissue restriction of alterations in DPC4, as in many other tumor-suppressor genes, emphasizes the complexity of rate-limiting checkpoints in human tumorigenesis.