Important etiological clues may arise from comparison of spectra and frequency of mutations in oncogenes/ tumor suppressor genes in human and animal tumors. Previously, the systematic evaluation for such gene mutations was hampered by artifact-plagued techniques, and, as a result, our group made significant improvements in the sensitive use of single strand conformation polymorphism (SSCP), developed an analysis with silver staining of bands, devised a method for distinguishing polymerase chain reaction (PCR) artifact from real mutation using a base-mismatch primer approach, and perfected conditions for extraction of DNA for PCR from formalin-fixed tumors. Applying these advances to several studies of human and rodent tumors has resulted in the following observations: 1. In a comparison of human gastric tumors of 3 histologic types, G to A transitions at CpG sites in p53 predominated in all tumor types, a finding consistent with exposure to nitrosating agents. Mutations in p53 were more common in the intestinal and unclassified types than in the diffuse-type. Furthermore, mutations in diffuse-type tumors were found primarily in the more advanced forms. 2. Previously, we reported a high frequency of transforming K-ras mutations in DMN-OMe-induced rat renal mesenchymal tumors. We have found a highly variant pattern of mutations in these tumors that is more typical of human neoplasms than of chemically induced rodent tumors. We also detected variance even in different regions of the same tumor, a finding reported previously only in human tumors. These observations suggest that use of weak methylating agents such as DMN-OMe may more closely reflect the pathologic process characteristic of human tumors. 3. An examination of the integrity of VHL in chemically induced embryonal or adult-type renal tumors, i.e., sarcomas, nephroblastomas, or cortical epithelial tumors, has failed to detect any base substitutions or small deletions, but the possibility of large deletions in this suppressor gene remains to be examined. 4. In a study of different rat strains, the Rb suppressor gene appears to be expressed at high levels in the kidneys of strains susceptible to nephroblastomas, but at low levels in Fisher rats, in which nephroblastomas cannot be induced. We are currently evaluating various renal tumor types for Rb involvement by expression, deletion, or phosphorylation status. 5. An evaluation of nitroglycerin-induced rat liver tumors has revealed base substitutions in codon 12 of K-ras in 40% of the tumors, and the G to T or A mutations in codon 12 are typical of those caused by NO-releasing agents in bacterial systems. 6. Fumonisin, a potent fungal toxin and hepatocellular carcinogen, is a major international problem as a contaminant of corn. It has been shown to block cell cycle progression through G1 in normal cells but not transformed cells. This suggests that genetic alterations in molecules responsible for the G1/S transition, e.g., pRb, cyclin D, cdk4, and p16, may be involved in fumonisin-mediated tumorigenesis. Thus far, it appears that cyclin D is overexpressed early in tumor development as well as in liver neoplasms, suggesting that this sequence may act as an oncogene in these tumors.
Showing the most recent 10 out of 13 publications
