Major finding: Mitotic errors produce micronuclear DNA that can be incorporated into the genome.
Mechanism: Defective, asynchronous DNA replication damages chromosomes in micronuclei.
Impact: Whole-chromosome aneuploidy can cause DNA damage and potentially produce mutations.
The means by which whole-chromosome aneuploidy mechanistically contributes to tumorigenesis remain a matter of debate. Chromosomal segregation errors during mitosis can lead to aneuploidy and result in the incorporation of lagging chromosomes into micronuclei, but little is known about the fate of these chromosomes. Crasta and colleagues hypothesized that mitotic errors resulting in micronuclei could lead to DNA damage. To test this theory, the authors tracked micronucleated cells generated by either nocodazole treatment or kinetochore inactivation throughout the cell cycle and assayed for DNA damage foci and DNA breaks. Surprisingly, in a significant proportion of the micronuclei DNA replication persisted into the G2 phase of the cell cycle and was associated with significant DNA damage and defective recruitment of replication and repair proteins. Further analysis of chromosome spreads revealed that micronucleated cells were much more likely to harbor micronuclei-derived DNA fragments arising from the pulverization of a single chromosome. To determine whether structural aberrations acquired in the micronuclei could be reincorporated into the primary nucleus in subsequent mitoses, the authors used cells stably expressing histone H2B fused to a photocontrovertible fluorescent protein that could be converted from green to red upon UV illumination. Selective photoactivation of single micronuclear chromosomes followed by long-term, live-cell imaging indicated not only that micronuclei could be stably inherited but also that chromosomes within micronuclei frequently reincorporated into the primary nuclei of daughter cells. Chromosomal fragmentation caused by defective replication and repair of isolated chromosomes in micronuclei could therefore generate mutations that ultimately integrate into the genome and could provide a mechanistic basis for chromothripsis, a recently characterized phenomenon in cancer cells in which massive genomic rearrangements are restricted to one or several chromosomes.