Cell confinement initiates a delayed but heritable loss of chromosomes.

Heritable genetic changes continually arise in cancer, especially in solid tumors where cells sometimes appear compressed. Rare heritable losses of chromosomes in live cells are quantified here with chromosome reporters (ChReporters) that reveal similar levels of loss after imposing a threshold level of confinement. Compression to ~60% of interphase height ruptures few nuclei compared to deeper compression but perturbs mitotic spindles and prolongs pro/metaphase. Chromosome mis-segregation into micronuclei is discovered only release from modest confinement, but arrest and death predominate. All such effects are phenocopied by Nocodazole washout that generates a 'memory' of prolonged mitosis, and effects differ from the rapid induction of micronuclei by a spindle assembly checkpoint inhibitor and by a clinical CDK4/6-inhibitor of cell cycle entry. Single-cell-RNA-sequencing confirms chromosome loss days after confinement and reveals persistence of chromosome segregation pathways. Chromosome losses as mitotic memories of confinement ultimately address knowledge gaps in mechanobiology and cancer evolution.