The chromosome breakage-fusion-bridge (BFB) cycle is a mutational process that produces gene amplification and genome instability. Signatures of BFB cycles can be observed in cancer genomes alongside chromothripsis, another catastrophic mutational phenomenon. We explain this association by elucidating a mutational cascade that is triggered by a single cell division error—chromosome bridge formation—that rapidly increases genomic complexity. We show that actomyosin forces are required for initial bridge breakage. Chromothripsis accumulates, beginning with aberrant interphase replication of bridge DNA. A subsequent burst of DNA replication in the next mitosis generates extensive DNA damage. During this second cell division, broken bridge chromosomes frequently missegregate and form micronuclei, promoting additional chromothripsis. We propose that iterations of this mutational cascade generate the continuing evolution and subclonal heterogeneity characteristic of many human cancers.

ABSTRACTThe chromosome breakage-fusion-bridge (BFB) cycle is a mutational process that produces gene amplification and genome instability. Signatures of BFB cycles can be observed in cancer genomes with chromothripsis, another catastrophic mutational process. Here, we explain this association by identifying a mutational cascade downstream of chromosome bridge formation that generates increasing amounts of chromothripsis. We uncover a new role for actomyosin forces in bridge breakage and mutagenesis. Chromothripsis then accumulates starting with aberrant interphase replication of bridge DNA, followed by an unexpected burst of mitotic DNA replication, generating extensive DNA damage. Bridge formation also disrupts the centromeric epigenetic mark, leading to micronucleus formation that itself promotes chromothripsis. We show that this mutational cascade generates the continuing evolution and sub-clonal heterogeneity characteristic of many human cancers.