Integration of viral sequences into the c-myc gene in two mammary adenocarcinomas induced by polyomavirus in athymic nude mice.

We report the analysis of polyomavirus (Py) DNA integration into chromosomal DNA of two Py-induced mammary adenocarcinomas of athymic nude mice. Prior observations had established that these tumors had high levels of episomal Py DNA, making analysis of integration sites difficult. Propagation of tumor cells in culture allows the isolation of lines which have lost episomal Py DNA but are still tumorigenic and thus can be used for in situ and Southern analysis of Py sequences. The data reported here support the conclusion that Py DNA integrated into and next to the c-myc gene, adding further importance to this tumor system which, in its modifications of c-myc expression, appears to be similar to some human mammary cancers. In situ hybridization experiments on metaphase chromosomes of tumor cells showed that (i) in both cases, there was a single integration site at the same position on the same chromosome in all cells of a given tumor, and (ii) integration sites were different in the two tumors; in one, it was located on chromosome 15, near the c-myc proto-oncogene, and in the other, it was situated in the distal part of chromosome 1. We have demonstrated a probable rearrangement between chromosome 1 and chromosome 15, in the region of Py insertion, thus suggesting that a specific site on chromosome 15 is involved in tumorigenesis. The discovery that Py DNA was integrated at specific sites in host chromosomes raised the questions of whether such integrations were correlated with the activation of specific oncogenes. The rearrangements of the c-myc proto-oncogene observed on Southern blot analysis for both tumors, along with similar integration patterns of Py sequences, the overexpression of the c-myc gene, and the synthesis of abnormal oversized hybrid transcripts between c-myc and Py genes, favor this hypothesis. Finally, the analysis of episomal Py DNA in various tumors shows viral populations presenting a specific deletion in a part of the Py late region. This deleted region in the episomal virus genome was systematically found integrated in chromosomal DNA, thus arguing for the importance of Py integration in the induction of mammary tumor.