Figure 3.

Polyploidization protects the abnormal cancer genome from ongoing structural chromosome aberrations and promotes intratumor heterogeneity. (A) Karyotypic analysis in two groups of 15 VA-13 cells, harvested 10 days after exposure to gamma irradiation, reveals significantly lower rates of random structural CIN in the cells that have undergone 1 or 2 rounds of WGD (composed from 104–178 chromosomes) as compared to those undergoing mitosis of the major VA-13 clones (composing of 64–78 chromosomes). Structural CIN was calculated as breakpoints/chromosome/cell. (B) Similar results were obtained in the osteosarcoma Saos-2 cells suffering from DNA replication stress upon prolonged p21 overexpression that duplicates the average structural CIN load. (C) Distribution of chromosome counts in 50 co-dividing VA-13 cells harvested in subsequent passages after 2.4 Gy of gamma irradiation (1 passage = 2–4 days in culture). Red dotted line represents chromosome numbers proximal to major clones. Pink boxes include cells that underwent one round of WGD. Blue boxes include cells that underwent more than one round of WGD. Note that despite the decline in the rates of WGD by time in culture, very few intermediate chromosome counts were recorded between the distinct ploidy indices, suggesting that cells with heavily unbalanced DNA content are less proficient at dividing. (D) M-FISH pseudo colored partial karyotypes of two WGD clones, generated with p21-overexpression-mediated genome reshuffling, display duplicated copies of novel complex recombinant chromosomes (arrows) not observed in non-endoreduplicated cells, suggesting that WGD increases tolerance of novel structural aberrations and thus contributes to genomic heterogeneity.