Clonal variations among multiple primary mammary tumors and within a tumor of individual mice: insertion mutations of int oncogenes.

Laboratory mice infected with the mouse mammary tumor virus (MMTV) often develop multiple mammary tumors. However, no comprehensive studies have been done addressing the question of whether or not different primary tumors of individual mice are related ontogenetically to each other. Further, it is not known to what extent individual tumors vary in their cellular composition. We, therefore, examined intertumor and intratumor patterns of the rearrangements in int-1 (Wnt-1), int-2 (Fgf-3), and int-3 protooncogenes, since mutations in ints caused by MMTV result in the development of mammary tumors in mice and thus provide the most suitable genetic markers for the tumor cells. Our results show that, irrespective of the genetic background of the mice and/or the strain of MMTV, the pattern of MMTV integration in the different tumors of individual mice varies as widely as is found with the tumors of different mice. Of the 79 tumors obtained from 25 mice of different genetic backgrounds, 31 showed insertional mutations in int-1 (39%). By contrast only 9 of the 65 tumors tested had mutations in int-2 (14%). None of the tumors showed mutations in int-3. Interestingly, tumors from individual mice also showed variations in their pattern of int gene mutation, indicating that multiple tumors that develop in a mouse bear no ontogenetical link. The analysis of the pattern of intratumor MMTV integration, int mutation, and int expression revealed that several int mutational events as well as variations in int expression may occur in the same tumor. The diversity of int activation within individual tumors may suggest that the initiation and progression of most mammary tumors, if not all, occur through the concerted action of different mutational events in the same cell or through interactions of different cell populations, each of which acquired different int mutations.