Mitotic tethering enables en masse inheritance of a shattered micronuclear chromosome

Chromothripsis, the shattering and imperfect reassembly of one (or a few) chromosome(s)1, is an ubiquitous2 mutational process generating localized complex chromosomal rearrangements that drive genome evolution in cancer. Chromothripsis can be initiated by missegregation errors in mitosis3,4 or DNA metabolism5-7 that lead to entrapment of chromosomes within micronuclei and their subsequent fragmentation in the next interphase or upon mitotic entry6,8-10. Here, we use inducible degrons to demonstrate that chromothriptically produced pieces of a micronucleated chromosome are tethered together in mitosis by a protein complex consisting of Mediator of DNA damage checkpoint 1 (MDC1), DNA Topoisomerase II Binding Protein 1 (TOPBP1), and Cellular Inhibitor of PP2A (CIP2A), thereby enabling en masse segregation to the same daughter cell. Such tethering is shown to be crucial for the viability of cells undergoing chromosome missegregation and shattering after transient inactivation of the spindle assembly checkpoint. Pan-cancer analysis of transcriptome and whole-genome sequencing data revealed that chromothriptic genome rearrangements were accompanied by elevated expression of MDC1, TOPBP1, and CIP2A. Thus, chromatin-bound tethers maintain proximity of fragments of a shattered chromosome enabling their re-encapsulation into, and religation within, a daughter cell nucleus to form heritable, chromothriptically rearranged chromosomes found in the majority of human cancers.