Clastogenesis by nucleotide lesions requires the completion of two cell cycles.

Damaged DNA nucleotides can trigger genome rearrangements through clastogenesis, a process driven by erroneous repair of double-strand breaks (DSBs) and associated with cancer development. While DSBs are known to arise from endonuclease activity at stalled replication forks, the clastogenic potential of such DSBs has remained uncertain. Here, we identify a previously unrecognized mechanism of clastogenesis using wild-type, nucleotide excision repair (NER)-deficient and translesion synthesis (TLS)-deficient cells, combined with advanced cytogenetic analyses. We demonstrate that, single-stranded DNA (ssDNA) tracts harboring unrepaired lesions rather than DSBs at collapsed replication forks can persist through mitosis. Only during the subsequent S phase, these tracts are converted into a new class of, highly clastogenic, DSBs. Consistent with a role of this mechanism in carcinogenesis, prostate cancers exhibiting extensive genomic rearrangements frequently harbor somatic defects in NER or in error-free homologous recombination-mediated DSB repair. These findings provide critical mechanistic insight and highlight potential implications for routine clastogenicity testing. Graphical abstractNucleotide lesions (light blue triangle) can trigger double-strand breaks (DSBs) through endonucleolytic cleavage at stalled or reversed replication forks. Traditionally, these DSBs were assumed to drive genome rearrangements, a process termed clastogenesis. Here we describe a distinct, delayed, mechanism of clastogenesis. Thus, unreplicated nucleotide lesions within single-stranded (ss) DNA regions persist through mitosis into the next cell cycle. During the subsequent S phase, these ssDNA tracts collapse into DSBs, presumably via replication runoff. These delayed DSBs then promote extensive genomic reshuffling. Supporting this model, prostate cancers with high levels of genomic rearrangements are frequently associated with somatic defects in nucleotide excision repair (NER)--a pathway that normally prevents lesion-induced clastogenesis. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=138 SRC="FIGDIR/small/694654v1_ufig1.gif" ALT="Figure 1"> View larger version (45K): org.highwire.dtl.DTLVardef@7c37dborg.highwire.dtl.DTLVardef@13721aeorg.highwire.dtl.DTLVardef@842e4forg.highwire.dtl.DTLVardef@1fc7c31_HPS_FORMAT_FIGEXP M_FIG C_FIG