The involvement of chromosome 13 in the X-ray-induced in vitro transformation of mouse m5S cells.

The transformation of an immortalized but nonmalignant near-diploid mouse cell line (m5S) by X-ray irradiation was studied, and the chromosome constitutions of the morphologically transformed foci were analyzed. The frequency of transformation increased in a dose-dependent manner. Chromosome analysis revealed that 17 out of 24 transformants showed a common chromosome change, being characterized by a diminution of the C2-ter region of chromosome 13 as compared with the chromosome constitution of the parental untransformed cells. In these transformants, the diminution of 13(C2-ter) was critical in forming transformed foci, but was not sufficient to engender anchorage independence and tumorigenicity. The agar clonability and tumorigenicity were associated with the further appearance of an extra chromosomal segment seen as an abnormally banded region, probably consisting of tandem repeats of actively functioning genes. This experimental system thus revealed the progressive multistep nature of neoplastic transformation initiated by a loss of suppressive function.