MALIGNANT TRANSFORMATION OF HUMAN FIBROBLASTS BY IONIZING-RADIATION

As one step, in developing an assay for quantifying the induction of m alignant transformation of human cells by ionizing radiation, we expos ed cells from a non-tumorigenic, infinite life span, near-diploid fibr oblast strain MSU-1.1 to 4.35 Gy Co-60 radiation and assayed them for focus formation. The mean frequency of foci in the irradiated popula t ion was 6 x 10 cells assayed. No foci were found in the control cells. Of four focus-derived cell strains studied in detail, two produced ma lignant tumours within 3-7 weeks. The other two did not produce tumour s during the 12-month period of study. The tumours from one strain wer e classified as sarcoma composed exclusively of spindle-shaped cells. Tumours from the other strain were sarcomas consisting of a mixed popu lation of round and spindle cells. Immunoprecipitation analysis of the status of the p53 gene in the focus-derived strains, using a mutant-s pecific antibody (Pab240) and an antibody that recognizes both mutant and wild-type p53 protein (Pab421), showed that the tumorigenic strain s were completely devoid of p53 protein. One non-tumorigenic strain ex pressed wild-type p53 protein, and the other expressed a lower molecul ar weight form of the protein. Karyotypic analysis showed that the tum our-derived cells from one tumorigenic strain had lost one copy of chr omosome 6, 14, 16 and 17. The tumour-derived cells from the second str ain had lost one copy of chromosome 7, 13, 14 and 17 and part of chrom osome 6, as well as part of the other copy of chromosome 7 and 17. The se results suggest that the common loss of one copy of chromosome 14, 17 and part of 6 plays a causal role in the malignant transformation o f these cells. Furthermore, the results indicate that it will be possi ble to develop a system that uses near-diploid human fibroblasts to qu antify radiation-induced malignant transformation.