POLQ mediates replication-stress induced structural variant formation throughout common fragile sites during mitosis

Genomic structural variants (SVs) greatly impact human health and disease, but much is unknown about their generative mechanisms, especially for the large class of nonrecurrent alterations. Common fragile sites (CFSs) are unstable loci that provide a model for SV formation, especially large deletions, under replication stress. We studied SV junction formation as it occurred in cells by applying error-minimized capture sequencing to CFS DNA harvested during replication stress. SV junctions formed throughout CFS genes at a 5-fold higher rate after cells passed from G2 into M-phase. Neither SV formation nor CFS expression depended on mitotic DNA synthesis (MiDAS), an error-prone form of conservative replication active at CFSs. Instead, analysis of tens of thousands of de novo SV junctions combined with DNA repair pathway inhibition revealed a primary role for DNA polymerase theta (POLQ)-mediated end-joining (TMEJ) in M-phase SV formation. We propose an important role for TMEJ in nonrecurrent SV formation genome wide.