Identification of a specific marker chromosome early in tumor development in gamma-irradiated C57BL/6J mice.

Chromosomal changes such as aneuploidies, translocations, and gene amplification occur in many murine tumors. In this study, we have analyzed the changes in chromosomes at different stages of tumor development in C57BL/6J mice treated with gamma-irradiation or the chemical carcinogen, N-methylnitrosourea. Trisomy 15 occurred in both groups of mice regardless of inducing agent. The frequency of this event differed significantly in radiation-treated animals between stage I and stage II of the disease. A specific marker chromosome occurred only in the gamma-irradiated mice and not in the mice treated with N-methylnitrosourea. This marker consists of a translocation between chromosomes 1 and 5. It occurred in 43% of the gamma-irradiated animals at stage 1 of the disease and did not vary markedly during tumor development. In contrast, trisomy 15 increased in frequency between stages I and II of the disease in the same animals. These results suggest that the translocation event may be an early event in tumor development, whereas trisomy 15 may contribute to tumor progression.