Alternative end-joining is suppressed by the canonical NHEJ component Xrcc4/ligase IV during chromosomal translocation formation

Chromosomal translocations in hematologic and mesenchymal tumors form overwhelmingly by nonhomologous end-joining (NHEJ). Canonical NHEJ, essential for the repair of radiation-induced and some programmed double-strand breaks (DSBs), requires the Xrcc4/ligase IV complex. For other DSBs, the requirement for Xrcc4/ligase IV is less stringent, suggesting the existence of alternative end-joining (alt-NHEJ) pathways. To understand the contribution of the canonical and alt-NHEJ pathways, we examined translocation formation in Xrcc4/ligase IV-deficient cells. We find that Xrcc4/ligase IV is not required for, but rather suppresses, translocations. Translocation breakpoint junctions have similar characteristics in wild-type and Xrcc4/ligase IV-deficient cells, including an unchanged bias toward microhomology, unlike what is observed for intrachromosomal DSB repair. Complex insertions in some junctions demonstrate that joining can be iterative, encompassing successive processing steps prior to joining. Our results imply that alt-NHEJ is the primary mediator of translocation formation in mammalian cells.