Escape from replicative crisis and immortalization precedes complex genome evolution

Shortening of telomeres restricts the proliferative capacity of human somatic cells by inducing replicative senescence and crisis. During cancerous transformation, premalignant cells must overcome these barriers and acquire replicative immortality. Single-cell DNA strand-specific genome sequencing of crisis cells revealed extreme genome instability characterized by complex chromosomal abnormalities. Unexpectedly, spontaneously emerging postcrisis clones showed relatively few and simple structural variants. However, with accumulating divisions following crisis escape, the genomes of immortal postcrisis clones evolve and amass genomic alterations, including tetraploidization events with subsequent losses and gains of individual chromosomes. Our data point towards a new model of tumorigenesis in which crisis acts as a strong selective barrier against cells with aberrant karyotypes, and in which high karyotypic complexity is acquired after immortalization and crisis escape.