Genome based cell population heterogeneity promotes tumorigenicity: the evolutionary mechanism of cancer

Abstract

Cancer progression represents an evolutionary process where overall genome level changes reflect system instability and serve as a driving force for evolving new systems. To illustrate this principle it must be demonstrated that karyotypic heterogeneity (population diversity) directly contributes to tumorigenicity. Five well characterized in vitro tumor progression models representing various types of cancers were selected for such an analysis. The tumorigenicity of each model has been linked to different molecular pathways, and there is no common molecular mechanism shared among them. According to our hypothesis that genome level heterogeneity is a key to cancer evolution, we expect to reveal that the common link of tumorigenicity between these diverse models is elevated genome diversity. Spectral karyotyping (SKY) was used to compare the degree of karyotypic heterogeneity displayed in various sublines of these five models. The cell population diversity was determined by scoring type and frequencies of clonal and non-clonal chromosome aberrations (CCAs and NCCAs). The tumorigenicity of these models has been separately analyzed. As expected, the highest level of NCCAs was detected coupled with the strongest tumorigenicity among all models analyzed. The karyotypic heterogeneity of both benign hyperplastic lesions and premalignant dysplastic tissues were further analyzed to support this conclusion. This common link between elevated NCCAs and increased tumorigenicity suggests an evolutionary causative relationship between system instability, population diversity, and cancer evolution. This study reconciles the difference between evolutionary and molecular mechanisms of cancer and suggests that NCCAs can serve as a biomarker to monitor the probability of cancer progression.

Fig. 1

Example of increased levels of NCCAs detected from the late stages of in vitro models coupled with increased tumorigenicity. This figure shows a karyotype comparison between an early stage (p36) (A) and a late stage (p105) (B) of the LNCaP cell line. In addition to sharing all four types of CCAs as indicated by the blue colored boxes, there are more NCCAs detected as indicated by the yellow boxes coupled with increased tumorigenicity.

Fig. 2

Example of increased levels of NCCAs detected from the late stages of in vitro models coupled with increased tumorigenicity. This figure shows the comparison between subline MCF10A-CSC-1 (A) and CSC-3 (B). Both lines share five common types of CCAs as indicated by the blue colored boxes. In line CSC-3 with increased tumorigenicity, in addition to ploidy changes, there were many NCCAs detected as indicated by the yellow colored boxes.

Fig. 3

Example of increased levels of NCCAs detected from the late stages of in vitro models coupled with increased tumorigenicity. This figure shows the comparison between the HOXA1 expressed line and the control line generated from MCF10. Both lines displayed the same karyotypes with two identical CCAs indicated by the blue colored boxes (A). Interestingly, however, the HOXA1 line also displays a much higher level of abnormal mitotic figures (chromosomes) are not well condensed) (indicated by a red arrow) or separated (indicated by blue arrows) (B). These defective mitotic figures are types of NCCAs.

Fig. 4

CSC3-transformed MCF10A cells form tumors in nude mice. The control MCF10A cells did not form tumors in nude mice within 20 days. Only the MCF10A-CSC3 cell line grew and formed palpable tumors in nude mice within 20 days. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Fig. 5

Distribution of structural and numerical NCCAs. A: Distribution of NCCAs across the five cell lines of five in vitro models with the highest tumorigenicity and the five cell lines with the lowest. Bars indicate 95% confidence intervals. The difference between high and low tumorigenicity is significant (P=0.01791, Student's t-test), illustrating the significant relationship between frequencies of NCCAs and tumorigenicity. B–F: Distribution of chromosome number across the five systems analyzed. Graphs represent average chromosome number, bars indicate 95% confidence intervals. Change in chromosome number does not associate with increased tumorigenicity in most lines except MCF10-CSC, possibly due to the ploidy. Passages/cell lines with higher tumorigenicity, however, do tend to show increased confidence interval widths indicating more variance in chromosome number.

Fig. 6

Illustrating the evolutionary mechanism of cancer and its relationship with molecular mechanisms. The evolutionary mechanism of cancer formation is summarized as three key components: 1, system instability; 2, increased system dynamics or population heterogeneity (reflected as an increased probability of a “hit” of a specific pathway or potential pathways); and 3, natural selection at the somatic cell level. There are many different molecular pathways that can trigger system instability, and it is the unstable system that activates different molecular pathways as the response to system instability. The somatic selection process stochastically favors different packages of genome alterations. The lower left box represents a normal stable state that typically generates infrequent NCCAs and when they do occur will likely go extinct. With increased instability, much higher levels of NCCAs occur representing an increasing number of potential genome systems coupled with specific molecular pathways. Each array represents a given molecular pathway, or the so called molecular mechanism. The increased number of pathways (represented by various colored arrows) increases the probability that evolution will proceed at a faster rate progressing much further in selected cell populations with some eventually achieving cancer status (the evolutionary mechanism).