Mechanisms of human cell neoplastic transformation: X-ray-induced abnormal clone formation in long-term cultures of human diploid fibroblasts.

Early passage cultures of a strain of normal human diploid fibroblasts were exposed to various doses of X-rays. The cells were serially passaged and followed throughout their life span in vitro. G-banded metaphase chromosome preparations were examined at each subculture to determine the presence of abnormal clones, i.e., groups of cells bearing identical chromosomal rearrangements. It was found that X-irradiation induced random chromosomal rearrangements which persisted throughout the life span of the cells. No abnormal clones were observed among the progeny of four nonirradiated cultures, nor in seven of nine cultures exposed to single radiation doses. On the other hand, multiple abnormal clones emerged among the progeny of cells in all five cultures exposed to multiple sequential radiation doses (three doses of 400 or 600 rads each). Evidence of clonal expansion and attenuation and of clonal succession during serial passaging occurred in these populations. In several cases, these clones expanded to include most of the cell population before the cultures became senescent. These findings are discussed in terms of their possible role in the transformation of human diploid cells by radiation.