The biology of breast cancer at the cellular level

Two properties seem fundamental to cancer; heterogeneity and progression (Foulds (1975) Academic Press, New York; Heppner et al. (1979) Commentaries on Research in Breast Disease, Vol. 1 (Bulbrook, R. and Taylor, D.J., eds.), pp. 177-191, Plenum Press, New York). Relatively little is understood about the premalignant stages of human breast disease in vivo. When the disease manifests as invasive carcinoma, its behavior exhibits great diversity, sometimes metastasizing rapidly, while in other cases 10-30 years pass before metastases proliferate. Here we review various aspects of breast cancer in vivo and consider how they predict properties of breast cancer found in culture. All of the experiments are consistent with the hypothesis proposed by Nowell (1976) Science 194, 23-28, that a fundamental aspect of malignancy is an increased genetic instability and that many of the cells within tumors are nonviable results of genetic instability. We suggest that most of the viable cells within primary breast carcinomas are diploid and are not yet capable of aspects of metastatic spread. What these cells have attained is an increased propensity for genetic instability which enables them to generate randomly aneuploid but frequently lethal genetic configurations. Occasionally one of these altered genomes is associated with the ability to proliferate at a metastatic site. This hypothesis implies that metastases from various patients may have arisen by divergent pathways and may also be divergent in many other aspects of their physiology, unrelated to malignancy. Such extreme heterogeneity may hamper attempts to understand fundamental aspects of malignancy. Hence we suggest that the less anaplastic and less divergent diploid cells within the primary carcinomas might be an important resource to gain insights into the critical alterations that are responsible for initiating frankly malignant behavior.