Genetic instability of cancer cells is proportional to their degree of aneuploidy

Abstract

Genetic and phenotypic instability are hallmarks of cancer cells, but their cause is not clear. The leading hypothesis suggests that a poorly defined gene mutation generates genetic instability and that some of many subsequent mutations then cause cancer. Here we investigate the hypothesis that genetic instability of cancer cells is caused by aneuploidy, an abnormal balance of chromosomes. Because symmetrical segregation of chromosomes depends on exactly two copies of mitosis genes, aneuploidy involving chromosomes with mitosis genes will destabilize the karyotype. The hypothesis predicts that the degree of genetic instability should be proportional to the degree of aneuploidy. Thus it should be difficult, if not impossible, to maintain the particular karyotype of a highly aneuploid cancer cell on clonal propagation. This prediction was confirmed with clonal cultures of chemically transformed, aneuploid Chinese hamster embryo cells. It was found that the higher the ploidy factor of a clone, the more unstable was its karyotype. The ploidy factor is the quotient of the modal chromosome number divided by the normal number of the species. Transformed Chinese hamster embryo cells with a ploidy factor of 1.7 were estimated to change their karyotype at a rate of about 3% per generation, compared with 1.8% for cells with a ploidy factor of 0.95. Because the background noise of karyotyping is relatively high, the cells with low ploidy factor may be more stable than our method suggests. The karyotype instability of human colon cancer cell lines, recently analyzed by Lengnauer et al. [Lengnauer, C., Kinzler, K. W. & Vogelstein, B. (1997) Nature (London) 386, 623-627], also corresponds exactly to their degree of aneuploidy. We conclude that aneuploidy is sufficient to explain genetic instability and the resulting karyotypic and phenotypic heterogeneity of cancer cells, independent of gene mutation. Because aneuploidy has also been proposed to cause cancer, our hypothesis offers a common, unique mechanism of altering and simultaneously destabilizing normal cellular phenotypes.

Figure 1

The appearance of clonal cultures of chemically transformed, highly aneuploid (A and B) and little aneuploid or near-diploid (C and D) CHE cells and of clones of flat CHE cells with the normal modal chromosome number 22 (E and F). The highly aneuploid clone B6-4 (A) and the near-diploid clone D3-10 (C) consist of spindle-shaped cells. The highly aneuploid clone M11-1 (B) and the little aneuploid clone M8-11 (D) consist of polygonal cells. The flat clone B6-5 (E) consists of spindle-shaped cells, and the flat clone D3-8 (F) consists of polygonal cells.

Figure 2

The relationship between the genetic instability and the pf of human colon cancer cell lines (17). The pf is the quotient of the mn of a cancer line divided by 46, the chromosome number of normal human cells. Genetic instability was determined from the percentage of cells that gained or lost a given chromosome per 25 generations. The karyotypes of cells with near-diploid and tetraploid karyotypes, i.e., with pfs close to 1 and 2, respectively, are stable. The karyotypes of aneuploid cells are unstable, in proportion to their degree of aneuploidy, or to the deviation of their ploidy factors from 1 and 2.